US20150285728A1

US20150285728A1 - Detection of nano-scale particles with a self-referenced and self-heterodyned raman micro-laser

Info

Publication number: US20150285728A1
Application number: US14/659,427
Authority: US
Inventors: Sahin Kaya Ozdemir; Lan Yang
Original assignee: Washington University in St Louis WUSTL
Current assignee: Washington University in St Louis WUSTL
Priority date: 2009-12-11
Filing date: 2015-03-16
Publication date: 2015-10-08
Also published as: US11131619B2; US20180306696A1

Abstract

A system and method for is a micro-laser based nano-scale object detection system and method using frequency shift and/or mode splitting techniques. The system and method can provide highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman micro-laser. The system and method also provides for nano-particle size measurement.

Description

CROSS REFERENCE

This Application Is A Continuation-In-Part Of And Claims The Benefit Of Application Ser. No. 13/460,170 Entitled SYSTEMS AND METHODS FOR PARTICLE DETECTION, Filed Apr. 30, 2012, Which Said Application Is Incorporated Herein By Reference In Its Entirety.

U.S. GOVERNMENT RELATED CONTRACT(S)/GRANT(S)

National Science Foundation under Grants 0954941 and 1264997;
US Army Research Office under Grant W911NF-12-1-0026

BACKGROUND

1. Field
This technology relates generally to nano-scale sized particle detection, and, more particularly, to detection of nano-scale sized particles using frequency shift and/or mode splitting techniques.
2. Background Art
With recent progress in nanotechnology, nanoparticles of different materials and sizes have been synthesized and engineered as key components in various applications ranging from solar cell technology to the detection of biomolecules. Meanwhile, nanoparticles generated by vehicles and industry have become recognized as potential threats to health and environment. Microscopy and spectroscopy techniques have played central roles in single nanoparticle/molecule detection. However, their widespread use has been limited by bulky and expensive instrumentation, long processing time, and/or the need for labeling. Light scattering techniques, while suitable for label-free detection, are hindered by the extremely small scattering cross-sections of single nanoparticles.
Interest in nanoparticle detection and characterization techniques has increased with the increasing awareness of the potential benefits and risks of the continuously generated byproduct or massively synthesized nano-particles. Nano-particles of special interests range from biological agents and virions to specially synthesized semiconductor, metal, and polymer nanoparticles. Detection and characterization of biological agents and virions is important for biodefense applications and early detection of pandemic outbreaks, while detection and characterization of synthesized nanoparticles is important for a broad range of applications in nanotechnology.
At least some known particle detection systems use conventional microscopic techniques which, despite their high sensitivity and resolution, may not be suitable for field measurements due to their expensive and bulky constructions, long processing times, and the necessity of pretreatment (labeling with fluorescent dyes, etc.) of the particles. Further, at least some known optical particle counters use light scattering measurements to allow field measurements and detect and count individual particles or groups of particles. These counters generally require off-axis detectors for the collection of the scattered light, bulky configurations, and relatively sophisticated signal processing components.
There is a growing interest for nanoparticle detection using nano and micro-scale sensors, which, with relatively high sensitivity, also have the potential for in-situ sensing. Some nano/micro-scale sensors detect particles by monitoring resonance frequency changes caused by additional effective mass of binding particles, while resonator-based micro/nano-optical resonator sensors rely on either resonance frequency shift or mode splitting due to changes in the effective polarizability of the resonator system upon particle binding. Resonator-based sensors have shown to detect and count individual nanoparticles having a radius as small as radius 30 nanometers (nm) This high sensitivity is attributed to the resonance-enhanced interaction between the particle and the evanescent tail of the light field due to tight light confinement and extended interaction time provided by the resonator. These sensors generally require a fiber taper to couple the light into and out of the resonator from a tunable laser, whose wavelength is continuously scanned to monitor the changes in the resonance modes, thus making these highly compact and sensitive sensors relatively expensive.
Optical whispering-gallery-mode resonators (WGMRs) have emerged as promising platforms for label-free detection of nano-objects. The ultimate sensitivity of WGMRs is determined by the strength of the light-matter interaction quantified by quality factor/mode volume, Q/V, and the resolution is determined by Q. To date, in order to improve the sensitivity and the precision of detection, WGMRs have been either doped with rare-earth ions to compensate for losses and increase Q; or plasmonic resonances have been exploited for their superior field confinement and lower V. In addition to rare-earth ions, previous whispering gallery mode (WGM) micro-laser based particle detection methods utilized quantum dot(s) or optical dye(s) as dopant(s) into the WGM resonator. Use of dopants make the fabrication process complicated (i.e., one has to find ways of doping the resonators), costly (rare-earth ions, quantum dots and dyes are expensive, and new fabrication processes add to the cost) and introduce biocompatibility issues.
For example, silica is a biocompatible material; however, rare-earth-ions are not biocompatible. Therefore, doping silica WGM resonator with a rare-earth-ion destroys biocompatibility. Moreover, each rare-earth ion, quantum dot or optical dye can be used only for a specific wavelength band (each has its own pump laser wavelength band and emission band). For each different wavelength band of operation a different rare-earth ion and a different pump laser should be used.
Whispering-gallery-mode (WGM) micro-resonators with their high quality factor, Q, and small mode volume, V, are known to enhance light-matter interactions and have extraordinary sensitivities to changes and perturbations in their structure or proximity. They have been of great interest for sensing biomarkers, DNA, and medium-size proteins at low concentrations, as well as for detecting viruses and nanoparticles at single-particle resolution. A particle or molecule entering the mode volume of a resonator or binding onto its surface induces a net change in the polarizability of the resonator-surrounding system and perturbs its optical properties. This manifests itself as a shift of the resonance frequency, broadening of the resonance linewidth, or formation of a doublet via mode splitting depending on the interaction strength and the scattering and absorption properties of the binding particle or the molecule.
In WGM sensors, the fundamental limit of sensitivity is determined by Q/V, which quantifies the strength of the interaction between the particle and the cavity field. Thus, it can be improved by decreasing V or increasing Q. One can increase Q by compensating for the losses and decrease V by shrinking the size of the WGM resonator (WGMR). However, decreasing the resonator size below a critical value inevitably increases bending losses and eventually decreases Q. Instead, hybrid systems combining high-Q WGMs with highly confined (small-V) localized plasmons have been demonstrated, achieving detection of single proteins and very small viruses. Q enhancement of WGM resonances by compensating losses via optical gain has also been demonstrated in silica micro-toroids doped with rare-earth ions such as erbium (Er3+) and ytterbium (Yb3+). Resonators with optical gain are referred to as active resonators.
When such a WGMR is optically pumped above lasing threshold, the resultant laser has a narrower linewidth than the cold cavity and thereby improves the detection limit and sensitivity beyond what can be achieved by the passive (no optical gain-providing mechanism) or by the active resonator below the lasing threshold. However, fabricating WGM-plasmon hybrids and active WGMRs with dopants introduces additional processing steps and costs. For example, WGM-plasmon hybrids require preparation and adsorption of plasmonic nano-structures onto the resonator surface, and active resonators suffer from the fact that most rare-earth ions are not biocompatible and that for each different wavelength band of operation a different rare-earth ion and a different pump laser should be used. A better system and method for leveraging the favorable characteristics of WGMRs is needed.

BRIEF SUMMARY

The invention is a micro-laser based nano-scale object detection system and method using frequency shift and/or mode splitting techniques. The system and method can provide highly sensitive detection of nanoparticles with a self-referenced and self-heterodyned whispering-gallery Raman micro-laser.
As indicated in application Ser. No. 13/460,170 Entitled SYSTEMS AND METHODS FOR PARTICLE DETECTION, Filed Apr. 30, 2012, which is incorporated herein in its entirety by reference, and for which this application is a continuation-in-part, in one aspect, a particle detection system is provided. The particle detection system can include at least one tapered optical fiber, a light source configured to transmit light through the at least one tapered optical fiber, a photodetector configured to measure a characteristic of the light being transmitted through the at least one optical fiber, and a computing device coupled to the photodetector and configured to determine whether a nanoparticle is present within an evanescent field of the at least one tapered optical fiber based on the measured light characteristic. In another aspect, a method for detecting nanoparticles is provided. The method includes transmitting light through a tapered optical fiber, measuring a characteristic of the light being transmitted through the tapered optical fiber, and determining whether a nanoparticle is present within an evanescent field of the tapered optical fiber based on the measured light characteristic. In yet another aspect, a method of assembling a particle detector is provided. The method includes coupling a tapered optical fiber to a light source. The light source is configured to transmit light through the tapered optical fiber. A photodetector is coupled to the tapered optical fiber, wherein the photodetector is configured to measure a characteristic of the light being transmitted through the tapered optical fiber. A computing device is coupled to the photodetector. The computing device is configured to determine whether nanoparticles are present within an evanescent field of the tapered optical fiber based on the measured light characteristic.
The technology as disclosed and claimed herein demonstrates enhanced detection of single nano-particle induced mode splitting in a silica WGMR via Raman gain-assisted loss compensation and WGM Raman micro-laser. Raman gain is optical gain (e.g. amplification) arising from stimulated Raman scattering. Raman gain can occur in transparent solid media (e.g. optical fibers), liquids and gases under the influence of intense pump light, and is used in Raman amplifiers and Raman lasers. The technology as disclosed herein can be implemented utilizing a micro-toroid WGMR constructed of a silica material. However, instead of a WGMR, the technology can be implemented with photonic crystals, and further instead of a micro-toroid configuration, the technology can be implemented using a micro-ring, micro-sphere, micro-disk, micro-bottle or other configuration. Also, instead of silica, silicon, titanium or other materials having comparable characteristics in key areas can be used. Its magnitude depends on the optical frequency offset between pump and signal wave, to some smaller extent on the pump wavelength, and on material properties. Compared with laser gain (e.g. in rare-earth-doped gain media), Raman gain requires higher pump intensities and/or longer interaction lengths, has substantially different saturation characteristics, and a gain spectrum which depends on the wavelength of the pump light.
The nonlinear response of a transparent optical medium to the optical intensity of light propagating through the medium is very fast, but not instantaneous. In particular, a non-instantaneous response is caused by vibrations of the crystal (or glass) lattice. When these vibrations are associated with optical phonons, the effect is called Raman scattering. When e.g. two laser beams with different wavelengths (and normally with the same polarization direction) propagate together through a Raman-active medium, the longer wavelength beam (called the Stokes wave) can experience optical amplification at the expense of the shorter wavelength beam. In addition, lattice vibrations are excited, leading to a temperature rise. The Raman gain for the longer wavelength beam can be exploited in Raman amplifiers and Raman lasers. The Raman gain can be substantial if the Stokes shift corresponds to a frequency difference of several terahertz.
In the Raman scattering process, one pump photon is converted into one lower-energy signal photon, and the difference of photon energies is carried away by a phonon (a quantum of the lattice vibrations). In principle, it is also possible that an already existing phonon interacts with a pump photon to generate one higher-energy photon, belonging to an anti-Stokes wave at a shorter wavelength. That process, however, is usually weak, particularly at low temperatures. When the intensity of the generated Stokes wave becomes sufficiently high, that wave may again act as the pump for a further Raman process. Particularly in some Raman lasers, it is possible to observe several Stokes orders (cascaded Raman lasers). Raman scattering can also occur within the broad optical spectrum of, e.g. effectively shifting the spectral envelope of the pulse towards longer wavelengths. Some typical Raman-active media are solid-state media such as glass fibers or certain crystals, e.g. barium nitride ═Ba(NO₃)₂, various others such as KGd(WO₄)₂═KGW and KY(WO₄)₂═KYW, and synthetic diamond.
In particular, the use of the Raman micro-laser provides a dopant-free, self-referenced, and self-heterodyned scheme with a detection limit ultimately determined by the thermos-refractive noise. Notably, the technology as disclosed herein has been demonstrated to detect and count individual nanoparticles with polarizabilities down to 3.82=10-6 μm³by monitoring a heterodyne beat-note signal, without using plasmonic effects, passive or active stabilization, or frequency locking. The interference between two independent beams of light is often referred to as heterodyne detection. This level of sensitivity is achieved without exploiting plasmonic effects, external references, or active stabilization and frequency locking. Single nanoparticles are detected one at a time; however, their characterization by size or polarizability can be obtained by an ensemble of measurements and statistical averaging. A beat note is a signal with the difference of the optical frequencies.
The self-heterodyne method is a heterodyne technique, which can be used to measure the linewidth (width of the optical spectrum) of a laser, particularly a single-frequency laser. One portion of the laser beam can be sent through a long optical fiber which provides some time delay. Another portion is sent through an acousto-optic modulator, which is driven with a constant frequency (typically some tens of megahertz) and shifts all the optical frequency components by that frequency. Both beams are finally superimposed on a beam splitter, and the resulting beat note (centered at the acouto-optic modulator frequency) is recorded with a photodetector (typically a photodiode). From this signal, the laser linewidth can then be calculated.
The present technology as disclosed provides a dopant-free scheme, which retains the inherited biocompatibility of silica and can have widespread use for sensing in biological media. The Raman laser and operation band of the sensor can be tailored for the specific sensing environment and the properties of the targeted materials by changing the pump laser wavelength. This scheme also opens the possibility of using intrinsic Raman or parametric gain for loss compensation in other systems where dissipation hinders progress and limits applications.
There is an increasing demand for new technologies to detect small molecules, nano-particles, and airborne species. In the past decade there is an increase in the number of label-free detection techniques with varying levels of sensitivities. Techniques relying on electrical conductance, light scattering and interferometry, surface and localized plasmonic resonance, nano-mechanical resonators, and optical resonances have been demonstrated.
The technology as disclosed herein utilizes a fundamentally different physical process to increase Q/V and thereby the fundamental sensitivity limit, as well as the detection limit. Instead of embedding rare-earth ions as the gain medium in a silica micro-toroid resonator, the technology as disclosed leverages the Raman gain in silica to achieve loss compensation and highly sensitive nanoparticle detection. The technology as disclosed does not require any dopant or additional fabrication complexities.
The technology as disclosed demonstrates Raman gain-induced Q enhancement (linewidth narrowing via loss compensation), Raman gain-enhanced detection of mode splitting in the transmission spectra, and splitting in Raman lasing for single nanoparticle detection and counting. As demonstrated by test results, the technology as disclosed can detect NaCl nanoparticles of radii 10 nm that have smaller polarizabilities than polystyrene and gold nanoparticles of the same size. This level of sensitivity can be achieved without using plasmonic enhancement or any laser stabilization or noise cancelation schemes. However, integrating plasmonics and stabilization techniques into the technology scheme will further enable significant improvement in the sensitivity and detection limit.
The approach utilized by the technology as disclosed replaces the traditional rare-earth ion-doped WGM micro-resonator/micro-laser with WGM silica Raman micro-resonator/micro-laser for mode splitting-based nanoparticle detection realizes various fundamental improvements. The technology as disclosed realizes an intrinsically self-referenced (i.e., no need for an external reference to eliminate or suppress thermal drifts and laser noise) and self-heterodyned sensor (i.e., nanoparticle-induced splitting of a Raman lasing line creates a doublet that when detected by a photodetector generates a beat note signal whose frequency corresponds to the amount of mode splitting).
The technology further realizes a higher sensitivity and a lower detection limit at single-particle resolution using WGMRs pumped below the lasing threshold (i.e., active resonators have much narrower linewidth and better sensitivity than a passive resonator) or above the lasing threshold (i.e., microlaser). The technology as disclosed also realizes a dopant-free low-threshold WGM micro-resonator/micro-laser for sensing applications, which retains the inherent biocompatibility of silica. The technology realizes faster detection due to the elimination of the need for scanning the wavelength of a tunable laser around a resonance to obtain the amount of splitting.
A WGM sensor with significantly lower cost can be achieved because the technology as disclosed eliminates the need for narrow linewidth tunable lasers and does not require dopants or plasmonic structures (i.e. in silica micro-toroids, Raman lasing with a fundamental linewidth as narrow as 4 Hz has been reported, which is reported to be much narrower than the commercially available tunable lasers). The technology also realizes the ability to use the same WGMR as a micro-laser with emission in different spectral bands just by changing the wavelength of the pump laser or by using a broadband pump.
In WGM micro-lasers with rare-earth-ion dopants, one should not only change the dopant but also the pump to obtain emission in different spectral windows. However, the present technology exploits the Raman gain, which enables one to operate the same WGMR at different wavelengths and loosens the requirement of a specific wavelength for pump lasers. The technology also introduces a method, which can be used to estimate the size of particles-this method can assign an average size to an ensemble of particles. WGM sensors can benefit from this in various ways, as demonstrated by the test data provided herein.
Stimulated Raman scattering is a nonlinear optical process that provides optical gain in a broad variety of materials. The Raman process generates photons at a frequency that is up-or down-shifted (anti-Stokes or Stokes) from the frequency of the incident photons by an amount equivalent to the frequency of an internal oscillation of the material system, such as vibration, rotation stretching, or translation. Raman gain has found many applications in biology, material science, sensing, environmental monitoring, optical communication, laser science, and spectroscopy.
However, in many of the materials, such as silica, silicon, and CaF₂, Raman gain is very small (of the order of 10⁻¹³m/W), requiring high-intensity pump lasers to drive the system above its lasing threshold. This is overcome by field confinement and resonant enhancement in waveguides and resonators.
Raman lasing has been observed in silicon waveguide cavities, silicon waveguides within fiber ring cavities, silicon photonic crystal cavities, and WGMRs such as silicon micro-ring, silica microspheres, silica micro-toroids, glycerol-water droplets, and CaF2 disks. However, the technology as disclosed herein implements a different approach than previously seen by using Raman gain or Raman lasing for loss compensation to enhance optical detection capabilities at single-particle resolution.
WGM micro-toroidal silica resonators are ideally suited for Raman laser applications because they can be mass fabricated on a silicon chip such that different spectral bands can be covered on a single chip. They have high Q and micro-scale V, which make it easier to achieve high intra-cavity powers to enhance nonlinear effects and obtain low threshold lasing (P_threshold˜V/Q²). They are also compatible with optical fibers and can be readily integrated into existing optical fiber networks. The Raman gain spectra for silica is given in FIG. 1C, Inset, which depicts that Raman gain takes place within a band of 5-30 THz with the highest gain at about 10-15 THz away from the pump frequency. This translates to 15-23 nm for a pump in 660-nm band, 33-51 nm for a pump in 980-nm band, 74-113 nm for a pump in 1,450-nm band, and 85-130 nm for a pump in the 1,550-nm band, suggesting that the spectral band where Raman gain contributes is pump wavelength-dependent. This broad spectrum of Raman gain is due to the rapid dephasing of phonons. In addition to the spectral band to which it contributes, the Raman gain itself is wavelength dependent and varies inversely with wavelength. For a pump with wavelength 1.55 μm, the peak Raman gain g_R˜10⁻¹³m/W of silica occurs for a shift of 13.2 THz, whereas for a pump of 526 nm the peak Raman gain of silica is reported as 1.49×10-13 m/W at a shift of 10.1 THz.
As demonstrated by testing, the technology as disclosed utilizing fabricated silica WGM micro-resonators in toroidal shapes showed Raman gain and Raman lasing and thereby demonstrated nanoparticle detection using silica micro-toroid resonators both below and above the Raman lasing threshold. As demonstrated, below the lasing threshold, mode splitting in the transmission spectra can be used for detection; and above the lasing threshold, heterodyning of split laser lines can be used, and beat frequency can be monitored. Testing of the technology demonstrated that NaCl particles can be detected at 10 nm. As noted previously, the configuration of the technology does not rely on plasmonic effects and do not use active or passive stabilization or frequency locking techniques. Therefore, those skilled in the art will readily recognize the improvement over prior systems and techniques for particle detection.
There is a demand for micro or nanoscale sensors with high sensitivity and lower detection limits, to detect, count and identify nano-scale objects (including but not limited to nanoparticles, aerosols, biomolecules, viruses, virions, etc) one-by-one in an environment (including but not limited to air, water, serum, blood, saliva, urine etc). It is also key that these sensors are biocompatible so that they can be used in a biological medium. Simplified signal processing and fabrication techniques are also important. The particle detection sensor should be versatile in the sense that it should be able to operate it at different wavelength bands, and in different medium and environments. The present technology as disclosed herein addresses these concerns.
These and other advantageous features of the present invention will be in part apparent and in part pointed out herein below.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference may be made to the accompanying drawings in which:

FIGS. 1A-1C are an illustration of one implementation of the technology, and a setup for measuring detection sensitivity, and a graphical illustration of the transmission spectra;

FIGS. 2A and 2B are a graphical illustration of a normalized transmission spectra of a Raman gain-enhanced detection;

FIGS. 3A-3D is a graphical illustration of an emission spectra of Raman lasing at different bands;

FIGS. 4A, 4B and 4C are a graphical illustration of detection of scatterer-induced mode splitting using Whispering Gallery Mode Raman lasing;

FIGS. 5A-5F is a graphical illustration of detection of NaCl nano-particles using the scatter induced splitting of WGM Raman laser;

FIGS. 6A-6D is a graphical illustration of noise analysis measuring beat note frequency;

FIGS. 7A-7D is a graphical illustration of noise analysis measuring beat note frequency; and

FIGS. 8A-8B is a graphical illustration of a measurement of an ensemble of nano-particles using scatterer induced beat note frequency changes of a WGM Raman laser.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description presented herein are not intended to limit the invention to the particular embodiment disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF INVENTION

According to the embodiment(s) of the present invention, various views are illustrated in FIG. 1-8 and like reference numerals are being used consistently throughout to refer to like and corresponding parts of the invention for all of the various views and figures of the drawing. Also, please note that the first digit(s) of the reference number for a given item or part of the invention should correspond to the Fig. number in which the item or part is first identified.
One embodiment of the present technology comprising whispering gallery mode resonator based nano-particle detection teaches a system and method for effectively detecting nano-sized particles. The details of the technology as disclosed and various implementations can be better understood by referring to the figures of the drawing. Referring to FIG. 1A, a diagram of one implementation of the photo-detection system is provided with a photodiode (PD) array monitor and optical spectral analyzer (OSC) setup is shown. A wavelength division multiplexer (WDM) can be utilized to multiplex the wavelengths of the output from the waveguide prior being sensed by the photodiodes. A computer can be used to receive the results. The micro-toroids used can be 40 μm in major diameter and 8 μm in minor diameter. An optical microscope image (the upper left inset of the diagram) of a typical resonator that can be used is shown in FIG. 1A. External cavity lasers with polarization controlled (PC) emission in the 660-, 980-, 1,450-, and 1,550-nm band can be used as pump lasers. A tapered fiber coupling (FC) can be utilized as a waveguide. Optical spectra of the pump and the resulting Raman laser can be measured by an optical spectrum analyzer with 0.1-nm resolution in order to monitor and verify the intended performance. To obtain the transmission spectra of the resonators in different bands, the lasers can be scanned repeatedly through a frequency range of 30 GHz around a single WGM and the transmitted power can measured.
FIG. 1B shows a typical resonance spectra obtained for a silica micro-toroid in the 1,450-nm pump band and 1,550-nm Raman gain band. The quality factors of these resonance modes are 4×10⁷and 3.5×10⁷, respectively. Referring to FIG. 1, one implementation of a demonstration setup and measurement method is shown. FIG. 1A provides on implementation of a setup of the technology as disclosed for Raman gain-enhanced detection of single nanoparticles using mode splitting. A differential mobility analyzer (DMA) with a nozzle can be used to deposit nanoparticles into the mode volume of the resonator one by one. However, other devices and methods can be utilized to introduce nano-particles to the particle detection device. Also, nano-particles can also be introduced to the particle detection device simply by a nano-particles movement, unaltered by the technology as disclosed, either in free-space, a gas or liquid. Light from a pump laser can coupled to the WGM of a micro-toroid resonator by a fiber-taper coupler (FC). The residual pump, Stokes photons, and the Raman laser can be out-coupled from the resonator with the same fiber taper.
A silicon chip with silica micro-toroids can be placed on a 3D nano-positioning stage to precisely tune the distance between the fiber taper and the micro-toroid. A fiber polarization controller (PC) can be used to change the polarization state of the pump laser to maximize the coupling efficiency. The pump light and the Raman laser light (probe light in case of below lasing threshold operation) can be separated from each other using a wavelength division multiplexer (WDM) and detected with photodetectors (PD) connected to an oscilloscope (OSC). The top view of a fiber-taper-coupled silica micro-toroid resonator taken by an optical microscope is provided as an inset in the upper left of FIG. 1A. FIG. 1B provides a transmission spectra of the silica micro-toroid obtained in the 1,450-nm and 1,550-nm bands at low optical power. The resonances in these bands are separated from each other by 11.67 THz, which lies within the Raman gain spectra given in FIG. 1C, as insets.
FIG. 1C provides a transmission spectra obtained for the system with the pump operating in the 1,450-nm band and Raman gain and the laser in the 1,550-nm band when the wavelength of the pump laser (top line) is scanned in time. The saw-tooth like waveform is due to the thermal broadening of the resonance line. Moving along the saw-tooth-like form in time, more pump light is coupled into the resonator, and the cavity power build-up is sufficient to produce Raman gain and lasing can be achieved as seen in the increased signal in the transmission obtained in the 1,550-nm band (bottom line). As seen in the graphical inset of FIG. 1C, the Raman process and the Raman gain spectra for silica, the Raman gain is provided within a band of 5-30 THz away from the pump light.
A differential mobility analyzer (DMA) can be utilized accompanied by a nozzle to deposit nanoparticles onto the WGMR. Nano-particles can be carried out from their colloidal solution using a collision atomizer. After the evaporation of the solvent in poly-disperse droplets, the solid particles can be neutralized to maintain a well-defined charge distribution. Then they can be sent to the DMA, which classifies them according to their electrical mobility. The output slit of the DMA allows only the particles within a narrow range of sizes to exit and land on the WGMR via the nozzle. The flow rate and the concentration of the colloidal solution can be set low to ensure deposition of particles one by one. Transmission spectra obtained for the pump in the 1,450-nm band and the Raman signal in the 1,550-nm band as the pump wavelength was scanned are given in FIG. 1C. As a result of the high Q and strong pump power, the cavity power build-up in the pump mode becomes so high that a strong thermal broadening is clearly seen in the 1,450-nm band as the wavelength of the pump laser was up-scanned from shorter to longer wavelengths. This thermal broadening helped the pump light to stay on resonance long enough to achieve Raman gain and Raman lasing in the 1,550-nm band.
The technology as disclosed therefore demonstrates that Raman gain in silica can be used to compensate for a portion of the optical losses in a micro-toroid and thereby improve the sensitivity of the mode-splitting technique. It is known that WGMRs support two counter-propagating modes (clockwise, CW and counterclockwise, CCW) at the same resonance frequency ω, and that a scattering center (e.g., a nanoparticle a virus, or a molecule) can lift this degeneracy, leading to the splitting of the single resonance mode into two modes, by mediating a scattering-induced coupling between the CW and CCW modes.
Mode splitting can then be resolved in the transmission spectra of the WGMR if the amount of splitting 2g=−αf²ω/V is larger than the total loss of the system, quantified by the strict condition |2g|>Γ+ω/Q for well-resolved mode-splitting. Here f is the field distribution of the WGM, α=4πR³(n_p ₂)/(n_p ²+2) is the polarizability of the particle of radius R and refractive index n_pwith the surrounding medium as air, Γ=(8/3)gπ²α/λ³is the additional loss induced by the scatterer, and ω/Q is the linewidth of the resonance (quantifying loss before the scatterer is introduced).
For very small particles we have Γ<<ω/Q, thus the strict condition reduces to 2g>ω/Q. In practice, satisfying this strict condition is in general difficult, and the split modes overlap with each other. In principle, splittings as small as ω/NQ can be resolved, where N is a number in the range 10-50 depending on the system and signal processing capabilities. The dependence of 2g on f²and a implies that if the overlap between the mode field and the scatterer is not high enough or if the particle is too small the induced mode splitting may be so small that it cannot be resolved within the background noise. In such cases, providing optical gain to increase the Q and hence to reduce the linewidth of the resonance will help to resolve the mode splitting.
Referring to FIG. 2, a graphical illustration Raman gain-enhanced detection of scatterer-induced mode splitting is provided. One implementation of the technology can be performed by a pump-and-probe method. A pump laser operating in the 1,450-nm band can be used to provide Raman gain in the probe band of 1,550 nm. As the pump power is increased, Raman gain increases from bottom to top in FIGS. 2A and 2B, where the spectra in the bottom panels were obtained when the pump was turned off (no gain). The power of the probe laser is set at 1.2 μW and 2.0 μW in FIGS. 2A and 2B, respectively, and thermal broadening is not seen for the probe mode in the 1,550-nm band. Referring to FIG. 2A, initially there is unseen mode splitting in the transmission spectra (Bottom, pump is off), which becomes visible (Top, pump power was 151 μW) due to the narrowing of resonance linewidth as the pump power was increased. As reflected in the middle graph the pump power is 87 μW, which is enough to induce linewidth narrowing for the probe mode but not sufficiently high to help resolve the splitting. Note that at these high pump powers there is thermal broadening in the 1,450-nm band.
Referring to FIG. 2B, initially mode splitting (Bottom, pump was off) is barely seen, but becomes much clearer and well-resolved (Middle and Top, pump power was set as 69 μW and 97 μW, respectively) as the pump power is increased. Moreover, the split resonances become deeper, implying that the provided Raman gain, which compensates intrinsic losses of the resonator, moves the taper-resonator systems from the under coupling regime to closer to the critical coupling regime. Note that the distance between the resonator and the taper can be kept fixed, thus there is no change in the coupling loss. In FIGS. 2A and 2B, test data obtained for the transmission spectra is provided, and the best theoretical fit to the experimental data is provided. γ denotes the linewidth of resonance modes and is obtained from the fitting curve.
Two sets of test data help to verify the present technology as disclosed in that the Raman gain-assisted Q enhancement via loss compensation and hence improved detection of mode splitting. The test intentionally included inducing a small mode splitting using a fiber tip such that mode splitting could not be resolved by a low-Q resonance in the 1,550-nm band. When pumping the silica micro-toroid using a laser in the 1,450-nm band, the transmission spectrum can be monitored in the 1,550-nm band by a probe laser whose power is set so low that no thermal broadening is observed in the transmission spectra. As the pump power is increased, generated Stokes photons compensated for the losses, leading to narrowing of the linewidth of the resonance (See FIG. 2) in the probe band. As a result, initially unobservable mode splitting becomes clear (See FIG. 2A).
In another demonstration of the technology, the position of the fiber tip can be adjusted in the mode volume such that it introduces a very small amount of mode splitting. The taper resonator system can be set to the under coupling regime so that the features of the mode splitting are barely seen when there is no pump. When the pump laser is turned on and its power is increased, a clear mode splitting of 1.5 MHz is observed in the transmission spectrum. This is an indication of the enhancement of the Q of the probe mode in the 1,550-nm band. Also observed is that the gain shifted the taper-resonator system from an under coupling regime to close-to-critical coupling. This can be understood as follows. In the under coupling regime, coupling losses quantified by κ_extis much smaller than the intrinsic losses κ_o(i.e., κ_o>>κ). During the test the distance between the fiber taper and the resonator is kept fixed,thus K_extstayed the same. The induced gain reduced κ_oand brought it closer to κ_ext, and thus shifted the system from an under coupling regime to critical coupling where κ_o=κ_ext.
This is reflected in the transmission spectra as a transition from a close-to-unity transmission to close-to-zero transmission at resonance and better resolution in detecting the splitting (See FIG. 2B). The system can move from one coupling regime to another in at least two different ways, including keeping κ_ofixed and varying κ_extvia tuning the distance between the resonator and the fiber-taper; or by keeping κ_extconstant (i.e., taper-resonator distance is fixed) and varying κ_o, which can be done either by introducing extra losses or by compensating the losses. The system as disclosed demonstrated the second approach.
Once the pump power exceeds a threshold value, lasing can be observed at frequencies shifted relative to the pump frequency. By fine tuning the pump power and the taper resonator coupling strength, single and multimode lasing can be obtained in the same micro-toroid. Therefore, one skilled in the art can conclude that a single WGMR can be used to generate lasing at different colors covering a large range of the spectrum and hence can be used for optical detection and sensing in all bands as indicated (See FIG. 3).
Referring to FIG. 3, an emission spectra is shown of Raman lasing obtained in the same silica micro-toroid resonator at different bands of the spectrum covering visible to near-IR. The pump lasers can operate in various bands as indicated in FIG. 3A—660-nm, FIG. 3B—980-nm, FIG. 3C—1,450-nm and FIG. 3D 1,550-nm bands. Higher-order cascaded Raman Stokes lasing can be seen in all of the spectra. The single-mode operation of the Raman lasers can be obtained by tuning the pump power and the coupling condition.
The lowest lasing thresholds demonstrated by the test data are 147.2 μW for the 1,550-nm pump, 169.5 μW for the 1,450-nm pump, 92.1 μW for the 980-nm pump, and 79.3 μW for the 660-nm pump. At much higher pump powers the test data shows that the spectra evolved from single mode (FIG. 3, Insets) to a spectrum with multiple Raman lasing peaks separated by the free spectral range, as well as cascaded Raman lasing lines separated by the Raman shift. The first-order Raman lasing in the resonator serves as the pump for the second order Raman lasing, so multiple second-order Raman lasing lines for the pumps can be observed in the bands of 660 nm, 980 nm, and 1,450 nm.
The ability to operate the same WGM sensor in different spectral bands has specific industrial applicability. It is noted that optical losses associated with the operating medium (i.e., aqueous solution, serum, air, etc.) are wavelength-dependent. Light sources and WGM resonances in the near-Infra-Red (near-IR) and IR bands are not preferable for operation in water due to strong absorption. Shifting the operating wavelength to the visible band minimizes losses, leading to higher Q and easy excitation of WGM lasing. Due to the pump wavelength dependence of the Raman gain, the same dopant-free resonator can be used in many different media for lasing and sensing applications by choosing the proper pump wavelength and WGM.
It is also noted that loss induced by a binding particle scales as R⁶/λ⁴, where R is the radius of the particle and λ is the wavelength of the light. Thus, operating the sensors in longer wavelengths will help to minimize particle-induced losses and enable detecting and characterizing larger particles. The same Raman WGM sensor can still be operated at shorter wavelengths for detecting smaller particles whose detection is limited mostly by the resonance linewidth. Thus, WGM sensors using Raman gain will have a larger operating range.
When using the technology as disclosed, different resonances and lasing modes in the same resonator can have different spatial field distributions; therefore, their responses to a binding particle/molecule/protein are different. A nanoparticle inducing splitting or frequency shift in one lasing mode may not be able to induce a resolvable splitting in a different lasing mode in the same WGM micro-laser. Therefore, the ability to have multiple wavelength lasing can avoid missing a binding nanoparticle/molecule or decrease the probability of a binding particle's going undetected. Thus, having lasing in the same resonator at multiple wavelengths will help to improve detection efficiency and decrease the number of binding events gone undetected. Raman gain allows multi-wavelength lasing in different bands and is suitable for various applications.
The present technology leverages a Raman process, which allows one to generate lasing at many different spectral bands, which without the present technology is not presently a commercially available laser. For situations in which there exists no laser covering the bands where a particle has its maximum absorption or scattering cross-section, a Raman micro-laser can be very useful to detect and discriminate particles by monitoring their responses (absorption, scattering, etc.) to light at different wavelengths. Similarly, in situations where high absorption of a binding particle at a specific wavelength band significantly degrades Q, interfering with lasing conditions or even preventing lasing, Raman gain can be useful because one can tune the operation wavelength far from the absorption band of the particles. Thus, the ability to work at different spectral bands with the same WGM sensor using Raman gain may help one choose the proper operating band according to the properties of the particle/molecule/analyte and the surrounding, as well as to use specific wavelength dependent responses of the particles/molecules and the medium for improving the operating range, detection efficiency, and sensitivity. Therefore, the present technology as disclosed has specific industrial applicability.
The generation of a beat-note signal due to scatterer induced mode splitting can be confirmed by introducing a nanofiber tip into the mode volume of a Raman WGM micro-laser and monitoring the self-heterodyne beat-note signal in response to its position (See FIG. 4). Referring to FIG. 4, detection of scatterer-induced mode splitting using WGM Raman lasing in two different bands in the same silica micro-toroid is graphically illustrated. A nano-fiber tip introduced into the mode volume of the resonator can be used to simulate scatterers within the mode volume. Test data is graphically illustrated to correspond with the pump laser in the 980-nm and 1,450-nm bands. Referring to FIG. 4A, Optical spectra of Raman lasers with pumps at 980-nm (Upper) and 1,450-nm (Lower) bands is illustrated.
Referring to FIG. 4B, change in the beat-note signal (See Inset Graph In FIG. 4B) and its frequency obtained using fast Fourier transform (FFT) when the nanofiber tip was within the mode volume is illustrated. Referring to FIG. 4C, changes in the beat-note frequency can be observed as the fiber tip repeatedly enters and exits the mode volume, each time at a different position and with different effective tip size in the mode volume. The response of the lasing modes at different bands are different. Splitting of the lasing modes (beat-note frequency) may increase or decrease depending on how the scatterers are distributed within the mode volume of each mode. Splitting may increase or decrease for both lasing modes in different bands or may decrease for one lasing mode and increase for the other lasing mode.
Using different lasing lines in the same resonator reveals that the beat-note signal and its frequency are not only affected by the size of the nano-fiber within the mode volume but also by its spatial overlap with the fields of the lasing lines. At a fixed location of the nanofiber, the amount of splitting experienced by Raman lasers at different spectral bands is different (See FIG. 4). Splitting may increase or decrease for all lasing modes in different bands or may decrease for some lasing modes but increase for the others. The amount of change in the splitting is different for different lasing modes. This is attributed to the facts that the spatial overlap between the nanofiber and the fields of different lasing modes are different and that mode splitting scales inversely with the wavelength. The present technology as disclosed provides measurements at multiple wavelengths or spectral bands, which enable detecting scattering centers (e.g., nanofiber) that could have gone undetected if only a single lasing mode were used.
Testing of the technology as disclosed evaluated the performance of the WGM Raman micro-laser and the mode-splitting method to detect nanoparticles with single-particle resolution. The technology can be tested using Au, polystyrene (PS), and NaCl nanoparticles. As discussed previously above, particle binding to the WGM micro-laser led to the splitting of a lasing line into two, which eventually gave a self-heterodyne beat-note signal when mixed at a photodetector. The beat-note frequency corresponds to the amount of splitting. Each consecutive nanoparticle binding event leads to a discrete change in the beat-note frequency. The frequency may increase or decrease depending on the location of each particle with respect to the field distribution of the lasing modes and the position of the particle with respect to previously deposited particles in the mode volume.
Referring to FIG. 5, a graphical illustration of data is provided demonstrating detection of NaCl nanoparticles using the scatterer-induced splitting of WGM Raman laser. Each discrete upward or downward jump in the beat-note frequency spectra corresponds to a binding event of one nanoparticle with radius R=15 nm (FIG. 5A), R=20 nm (FIG. 5C), and R=25 nm (FIG. 5E). Histograms of the beat frequency changes for each nanoparticle binding event for NaCl nanoparticles of size R=15 nm (FIG. 5B), R=20 nm (FIG. 5D), and R=25 nm (FIG. 5F), which demonstrates a correlation between the size of the particle and the width (SD or rms) of the distribution.
FIG. 5 graphically illustrates the change in beat frequency and hence the splitting of the lasing mode as NaCl nanoparticles of size R=15 nm (FIG. 5A), 20 nm (FIG. 5C), and 25 nm (FIG. 5E) are continuously deposited onto the WGM Raman laser. With each particle binding event, a discrete up or down jump in the beat frequency can be observed. The histograms shown in FIGS. 5B, 5D, and 5F reveal that the larger the particles, the wider the distribution of the changes in the beat-note frequency. To estimate the reproducibility of the measured beat-note frequency, nanoparticle deposition can be stopped at some point, while continuously measuring beat frequency for an extended duration.
Referring to FIG. 6A, a change in the measured beat-note frequency as a function of time is graphically illustrated. FIG. 6B provides a histogram of the measured beat-note frequency. FIG. 6C provides an Allan deviation (the square root of Allan Variance, which is a measure/estimate of frequency stability due to noise processes) of measured frequency as a function of time. FIG. 6D, provides a beat-note frequency measured for NaCl nanoparticles of size R=10 nm.
Referring to FIGS. 6A and 6B, a depiction is provided of the beat frequency as a function of time and the histogram of measured frequencies, respectively. The measured frequencies stayed within 100 kHz of the mean frequency. Allan deviation can be calculated, which is a commonly used technique to estimate frequency stability. The Allan deviation for the beat frequency can be obtained using segments (integration time) from 0.1 to 200 s. The result is shown in FIG. 6C. FIG. 6D graphically illustrates the measurement results for NaCl nanoparticles of R=10 nm. It is seen that whereas some of the particle-binding events led to resolvable changes in the beat frequency, the changes in some others were not very clear.
The technology as disclosed can resolve the binding events even at the present noise level without any active or passive stabilization procedure. Based on the test data there is detection of the WGM Raman micro-laser down to 10 nm for NaCl particles. This corresponds to a polarizability of 3.82 ×10-6 μm3, which is 100-fold smaller than that of the gold nano-rods detected with a silica micro-toroid stabilized using the Pound-Drever-Hall technique.
It should be noted that WGM-type sensors respond to the changes in the effective polarizability; therefore, they measure the polarizability of a particle/molecule entering the mode volume. Size or volume measurement is possible when the refractive index of the nanoparticle is known. Two particles in the same environment having the same volume (size) will have different polarizability if their refractive indexes are different; the one with higher refractive index has higher polarizability. Metallic nanoparticles (e.g., Au, Ag, etc.) with or without plasmonic enhancement have higher refractive index than dielectric particles (e.g., PS, NaCl, KCl, or silica). Thus, with the same sensor and under the same measurement conditions the size of the smallest detectable nanoparticles by plasmonic enhancement is always smaller than the size of the smallest detectable dielectric nanoparticle where plasmonic effects are not valid. Therefore, detecting particles with smaller volume does not necessarily mean better sensitivity.
The test data to demonstrate the technology as disclosed provides that the raw noise and hence the sensitivity is at a level of 100 kHz (FIG. 6A), which translates into an SD of 26.4 kHz. To estimate the frequency noise floor (i.e., the uncertainty in the beat frequency) and hence the sensitivity limit, we performed cross-correlation δv(t)=(2/T) ∫^T/2−_T/2v(t)h(t+τ)dτ between the raw beat-frequency data v(t) and a step function h(t) of duration T (FIG. 7A, Inset).
To demonstrate the noise floor, the particle deposition can be stopped after the first particle binding event and the beat frequency can be monitored over the time window during which the particle flow is stopped. As seen in FIG. 7A, the uncertainty in the beat-frequency change decreases as the duration of the step function increases (i.e., more data is included). The uncertainty observed in the measured beat frequency drops from ˜23.5 kHz for T=0.2 s to ˜16 kHz for T=0.5 s, reaching a noise floor of 14 kHz for T=1 s. For step functions longer than 4 s the noise floor can be determined by the fluctuations of the coupling conditions. In the case that a binding event takes place within the duration of the step function, the noise floor increases significantly. For similar cases in which binding events take place at random time instants and the arrival times of the particles cannot be controlled, the duration T of the step function should be chosen by taking into account the low-frequency noises.
Referring to FIG. 7, a graphical illustration of a noise analysis is provided. FIG. 7A, illustrates uncertainty in the beat-note frequency as a function of the duration of the step function (Inset of FIG. 7A) used in the cross-correlation. FIGS. 7B-7D, illustrate a beat-note frequency as a function of time for NaCl particles of radius R=20 nm (FIG. 7B), R=15 nm (FIG. 7C), and R=10 nm (FIG. 7D). The curves that graphically illustrate raw data of beat-note frequency as demonstrated by testing is shown in FIGS. 7B, 7C, and 7D. The curves beginning a time T=550 with beat-note above 7 Mhz (FIG. 7B); T=300 with beat-note below 8.8 Mhz; and T=1200 with beat-note below 16.7 Mhz are illustrative of raw data of beat-note frequency. The other curves denote the beat-frequency changes captured by the cross-correlation method using step functions of duration T=2 s, (C) T=2 s, and (D) T=4 s.
FIG. 7B-D depicts the results of the cross-correlation method together with the raw beat frequency data. Clearly, when the duration of the step function is properly chosen, the cross-correlation method captures the binding events that lead to beat-frequency changes. In FIGS. 7C and 7D, an increase is seen in the raw noise as well as the noise floor at time intervals 343-410 s and 791-837 s, respectively. These correspond to time intervals between consecutive binding events that are clearly resolved by the cross correlation method. The measured noise levels of ˜13.2 kHz and ˜9.7 kHz, respectively, for T=2 s and T=4 s during these time intervals are clearly higher than the noise floors ˜11.4 kHz and ˜9.4 kHz estimated from FIG. 7A. The increase in the noise in these intervals can be due to hidden and unresolved binding events that take place between two consecutive resolved detection events, within the duration of the step functions used in the cross-correlation method.
Each discrete change in the beat-note frequency corresponds to a nanoparticle binding and detection event. In order to extract the size or polarizability of each detected nanoparticle directly from these changes, due to the amount of the changes is determined not only by the polarizability of the detected particle but also by its location within the mode volume as well as by its location with respect to previously deposited nanoparticles, statistical analysis can be performed to assign an average polarizability or size to a particle ensemble.
As shown in FIG. 5, each discrete change of the beat-note frequency signals a nanoparticle landing in the mode volume. Different heights for the same particle size are due to different amounts of overlaps between the WGM and the detected particles. The distribution of these discrete jumps contains information on the particle size. The larger the particles, the wider the distribution is for beat-note frequency jumps (See FIGS. 5B, 5D, and 5F). The same particle delivery system can be utilized, for example a system that has a nozzle of inner diameter 80 μm and an output air cone covering a much larger area than the micro-toroid resonator can be utilized, thus its effect on distribution of the particles on the micro-toroid is negligible. One skilled in the art can reasonably conclude from the data that the distributions of particle positions in the resonator mode volume and the shape of the distribution function for the beat-note frequency jumps are the same for different particle sizes.
Referring to FIG. 8, a graphical illustration of a measurement of an ensemble of nanoparticles using scatterer induced beat-note frequency changes of a WGM Raman micro-laser is provided. FIG. 8A illustrates the distribution of the discrete changes in Raman laser beat-note frequency. Larger particles induce larger changes with wider distribution. However, the shapes of beat-note frequency-change distributions are the same. FIG. 8B illustrates the relations between Δ_th/δ and Δ_thfor detected NaCl particles of radii 15, 20, and 25 nm (from left to right). At the same Δ_th/δ ratio, the corresponding Δ_thvalue gives the estimated width of the distribution of beat-note frequency changes.
To extract the size information, the rms (denoted as δ here) of the beat-note frequency changes that are above a threshold value Δ_thcan be calculated. For different particle sizes, the distributions of the beat-frequency changes follow the same statistical model, and thus the ratio Δ_th/δ should be equal when Δ_this at the same position with respect to each distribution (See FIG. 8A). Therefore, by plotting Δ_th/δ for different Δ_thvalues, one can estimate the effective width of the respective jump distributions.
FIG. 8B shows the curves of Δ_th/δ versus Δ_thfor three different sizes of NaCl particles. By comparing the scaling of the horizontal axis for all three cases, the ratio of the widths of the distributions can be extracted. This suggests that by using measurement results from an ensemble of particles with known sizes one can use a referencing scheme to assign an average size to a given ensemble of particles. For the reference ensemble and the ensemble of the particle of interest, one first obtains Δ_th1/δ₁and Δ_th2/δ₂, respectively, as a function of δ₁and δ₂from the measured data, and then finds Δ_th1and Δ_th2, which satisfy Δ_th1/_δ1=Δ_th2/δ₂.
In FIG. 8, Δ_th1/δ₁=Δ_th2/δ₂=0.4 is utilized. The discrete jump heights are related to particle polarizability, which is proportional to R₃, therefore one can estimate the size ratio between the reference ensemble and the ensemble of the particles of interest using Δ_th1/Δ_th2=(R₁/R₂)3. Using this method, the size ratios among the three different ensembles of NaCl particles can be estimated to be 30.6: 39.3: 50.0, which represents less than 3% error compared with nominal values.
As demonstrated by the test results of the technology as disclosed, Raman gain in silica WGMRs can be used to compensate losses, to enhance Q, and to enable gain-enhanced detection and characterization of nanoparticles at single nanoparticle resolution using the mode splitting method. When the Raman gain in the WGMR is below lasing threshold, loss compensation increases Q and hence enables detection of smaller splitting. When the WGMR is pumped above lasing threshold, split lasing modes induced by a binding particle leads to a beat-note frequency that changes abruptly with each binding event. By monitoring the changes in the beat-note frequency one can count the number of particles binding to the sensor. Multiple measurements and histograms can be used to assign an average polarizability to the ensemble of detected particles in order to extract polarizability. In both of the cases, mode splitting provides a self-referencing scheme immune to laser frequency noise and thermal drift of resonances. This is an intrinsic property of the present technology as disclosed, in contrast to other schemes where external reference interferometers are used to subtract the probe laser noise by offline signal processing or noise was minimized by frequency stabilization techniques.
It is noted herein that when the particles are deposited onto the resonator they may or may not fall onto a location that overlaps with the spatial mode of the WGM. Particles that do not land on the mode volume may go undetected. For a particle landing on a location within the mode volume, particle polarizability (or size) and the intensity of the WGM field at the location of the particle are the parameters for determining the amount of change in mode splitting. A large particle overlapping with a weak field may cause smaller splitting than a smaller particle overlapping with a stronger field.
Although the test data demonstrating the technology as disclosed, as presented herein, has been performed in a dry environment, recent demonstrations of particle induced mode splitting and WGM Raman lasing in a liquid environment imply that the techniques developed here can be extended to loss compensation of these devices in a liquid environment and bio-sensing in biological fluids. Moreover, similar to what has been demonstrated here for a silica micro-toroid (Raman gain in silica for loss compensation and for improving the detection limit of WGM resonators), Raman gain in materials that are used to fabricate photonic crystals, plasmonic and metamaterial structures, and as well as other types of WGMRs can also be used to compensate for losses and enhance their performance by eliminating the drawbacks associated with dopants. For example, Raman gain in silicon can be used for loss compensation in silicon micro-rings and silicon photonic crystal structures. The technology as disclosed can be extended to parametric gain in silica and other materials for loss compensation.
The technology as disclosed that is a dopant-free loss compensation technique can have applications in other photonic devices and can be effectively used to improve their performance and enhance the sensitivity and the detection limits of sensors based on resonance effects. Achieving the detection of nanoparticles down to 10 nm in size and counting them one by one is within the operation of the technology as disclosed. It should be noted that plasmonic effects, laser frequency stabilization, and noise suppression techniques can be integrated into the schemes of the present technology as disclosed to further enhance the sensitivity and lower detection limit.
The various implementations of the technology as shown above illustrate a system and method for detection nano-particles. A user of the present technology may choose any of the above implementations, or an equivalent thereof, depending upon the desired application. In this regard, it is recognized that various forms of the subject particle detection system and could be utilized without departing from the spirit and scope of the present invention.
As is evident from the foregoing description, certain aspects of the present invention are not limited by the particular details of the examples illustrated herein, and it is therefore contemplated that other modifications and applications, or equivalents thereof, will occur to those skilled in the art. It is accordingly intended that the claims shall cover all such modifications and applications that do not depart from the sprit and scope of the present invention.
Other aspects, objects and advantages of the present invention can be obtained from a study of the drawings, the disclosure and the appended claims.

Claims

What is claimed is:

1. A particle detection system comprising:

a mode-splitting based micro-resonator implemented micro-laser, where the micro-resonator is constructed to effect a Raman scattering based Raman gain;

a pump laser having a light emission coupled into the micro-resonator, where said light emission is sufficient to effect Raman scattering based Raman gain of the micro-resonator to emit a Raman laser light transmission.

2. The particle detection system as recited in claim 1, further comprising:

a wavelength division multiplexer adapted to separate the polarization controlled light emission of the pump laser and the Raman laser light transmission; and

a photodetector optically coupled to the wavelength division multiplexer to sense the Raman laser light transmission and said photodetector having a signal output representative of the Raman laser light transmission.

3. The particle detection system as recited in claim 2, where said micro-resonator is placed in one or more of a free air, gas or liquid environment containing a nano-particle such that the nano-particle is introduced to the micro-resonator.

4. The particle detection system as recited in claim 3, further comprising:

a frequency analyzer coupled to the signal output of the photo detectors for detecting mode splitting.

5. The particle detection system as recited in claim 4, further comprising:

a computer coupled to the frequency analyzer configured to determine if the signal output of the photo detector is indicative of a presence of the nano-particle by resolving mode-splitting.

6. The particle detection system as recited in claim 1, further comprising a three dimensional nano-positioning stage having the micro-resonator placed thereon and having a range of motion to precisely tune the distance between the fiber coupled waveguide and the micro-resonator.

7. The particle detection system as recited in claim 1, further comprising:

a broad band pump configured with various frequency components, which can couple into the micro-resonator to thereby effect Raman gain, where the broad band pump is tunable to one or more of the various frequency components if a Raman laser at a specific spectral band is desired.

8. A method for particle detection comprising:

pumping a light emission and coupling the light emission to the micro-resonator constructed to effect a Raman scattering based Raman gain, where said light emission is sufficient to effect Raman scattering based Raman gain of the micro-resonator to emit a Raman laser light transmission;

inducing mode-splitting when a nanoparticle is detected; and

resolving mode splitting with Raman gain assisted loss compensation.

9. The method for particle detection as recited in claim 8, further comprising:

introducing a nano-particle to the micro-resonator by placing the micro-resonator in an environment containing nano-particles.

10. The method for particle detection as recited in claim 9, further comprising:

heterodyning of split laser lines; and

monitoring beat frequency for detecting the nano-particle.

11. The method for particle detection as recited in claim 9, further comprising:

nanoparticle-induced splitting of a Raman lasing line creating a doublet; and

detecting the splitting of the Raman lasing line with a photodetector, which generates a beat note signal whose frequency corresponds to the amount of mode splitting.

12. The method for particle detection as recited in claim 11, further comprising:

changing the wavelength of the controlled light emission into different spectral bands.

13. The method for particle detection as recited in claim 11, further comprising:

scanning repeatedly the pump laser a frequency to obtain a transmission spectra of the micro-resonator; and

measuring the transmitted power.

14. The method for particle detection as recited in claim 13, further comprising:

increasing the power of the pumping of the light emission through a fiber coupled waveguide thereby narrowing the resonance linewidth to thereby render mode-splitting as detectable.

15. The method for particle detection as recited in claim 9, further comprising:

adjusting the micro-resonator from a first position to a second position with a three dimensional nano-positioning stage having the micro-resonator placed thereon and having a range of motion to precisely tune the distance between the fiber coupled waveguide and the micro-resonator.

16. The method for particle detection as recited in claim 8, further comprising:

introducing one or more of a plurality of nano-particles to the micro-resonator by placing the micro-resonator in an area proximate to the plurality of nano-particles and positioned to effect introduction of the one or more of the plurality of nano-particles to the micro-resonator.

17. The method for particle detection as recited in claim 16, further comprising:

measuring the one or more of the plurality of nanoparticles by monitoring a scatterer induced width of distribution of a beat-note frequency change of the micro-resonator, where larger particles induce larger changes with wider distribution;

calculating the route-mean-square of the beat-note frequency changes that are above a threshold value, where for different particle sizes and the distributions of the beat-frequency changes follow a same statistical model; and

plotting the width of distribution of beat-note frequency changes to estimate an effective width of respective jump distributions;

estimating a size ratio between a reference plurality of nano-particles and the plurality of the particles of interest.

18. The method for particle detection as recited in claim 17, further comprising:

monitoring the changes in the beat-note frequency and counting the number of particles binding; and

taking multiple measurements and assigning an average polarizability to a plurality of detected particles.