GB2530500A

GB2530500A - Photon detector

Info

Publication number: GB2530500A
Application number: GB1416787.8A
Authority: GB
Inventors: Gueorgui Horhe-Karlos Barreto; Nicola Andrea Tyler; Mark Thompson
Original assignee: University of Bristol
Current assignee: University of Bristol
Priority date: 2014-09-23
Filing date: 2014-09-23
Publication date: 2016-03-30
Anticipated expiration: 2034-09-23
Also published as: GB201416787D0; GB2530500B

Abstract

A photon detector 2 comprises a resonant cavity 4 that allows input light to circulate within and an elongate superconductor element 6 to absorb photons within the cavity, the element arranged such that it is elongate in the direction of photon propagation. The detector may conduct electrons along the superconducting element, tune the optical resonances within the cavity, and be used with a cryostat system. The resonator may be an integrated optic waveguide, may be a ring resonator in a racetrack configuration, and may have a rectangular cross section. There may be an integrated input waveguide 22 that evanescently couples light into the resonator. The superconductor may be in contact with the resonator waveguide and may comprise a superconducting nanowire single photon detector. There may be a plurality of detectors optically coupled to a common waveguide, at least two of which support different resonant wavelengths. The superconductor may be cooled, and have electrical current below a threshold value input through it. It may be determined when the element has absorbed a photon by monitoring electrical current diverted away from the element.

Description

Photon detector The present invention is related to the detection of photons, particularly the detection of single photons.

Optical detectors are commonly used throughout many technology fields. Optical detectors typically utilise transducers which convert one or more photons into a different physical parameter that is more easily measured. Several different detectors exist, however most work on the principle of converting one or more photons into electrical current and monitoring changes in the electrical current. Detectors typically have a particular material that is able to absorb light within a range of wavelengths and create photoelectrons.

One type of material class that is known to be used to detect photons is superconducting materials. Superconducting materials are materials the can be configured to give zero electrical resistance. To exhibit superconductivity, such materials need to be cooled below a critical temperature TCRIT and not to sustain a magnetic field or carry a value of electrical current above certain threshold values known as HcRIT and 1cRIT Patent document W02014026724 describes a single photon spectrometer using a nanophotonic circuit to separate different wavelengths into different output waveguides.

Each output waveguide was terminated at one end with an on chip Superconducting Single Photon Detector (SSPD). This device was not a resonator and therefore the optical signals had to be absorbed by a single pass of the SSPD element.

One type of superconducting optical detector is described in "Nanowire single-photon detector with an integrated optical cavity and antireflection coating", K. M. Rosfjord et al, Optics express 527, vol. 14, no.2,23 January2006. The detector was a Superconducting Nanowire Single-Photon Detector (SNSPD) included an optical cavity with an NbN nanowire formed in a tight meandering pattern. Photons are described to be incident upon an antireflection coated sapphire substrate before entering an NbN/hydrogen silsesquioxane/mirror cavity engineered so that light reflected from the mirror interfered destructively with the reflection from the NbN/sapphire interface. The path way of the wire was shown to be perpendicular to the direction of propagation of the light in the cavity.

A similar type of detector is described in "Superconducting single photon detectors designed for operation at 1.55-m telecommunication wavelength", I Milostnava et al., Journal of physics conference series 43, 1334-1337, 7th European conference on applied superconductivity, 2006. This publication shows a device with an NbN meander-structure formed upon a sapphire substrate. An optical micro-cavity consisting of a one mirror resonator consisting of a dielectric Si02 layer and a metallic mirror was formed on-top of the meander structure. Light is shown incident through the bottom of the sapphire substrate such that the meander structure was perpendicular to the direction of propagation of the light in the resonator.

In both of the above publications, the superconducting elements of the devices described had limited interaction with the propagating light wave in the cavity and utilised long meandering paths.

Another type of detector is described in "High quantum efficiency photon-number resolving detector for photonic on-chip information processing", B. Calkins et al., Optics Express, 22657, Vol. 21, no. 19,23 September2013. Unlike detectors using SNSPD/SSPD based structures; this detector used a transmission edge sensor (TES). Three such TES detectors were placed one after another along a single waveguide patterned using direct UV writing.

Weak Bragg gratings were co-patterned with the waveguides to obtain in-situ waveguide loss and detector absorption measurements. The TES device was formed by a 10 pit x 10 pm square of tungsten that was 4Onm thick. The Tungsten square was formed above a portion of the waveguide. Electrical bonding pads were electrically connected to the Tungsten square such that current transferred between the pads through the TES was perpendicular to the direction of the waveguide. TES detection based devices are configured to monitor temperature change and typically require an operating temperature below 0.5K which in some circumstances can be difficult to achieve.

The paper'Waveguide Integrated Superconducting Single Photon Detectors Implemented as Coherent Perfect Absorbers' by Mohsen K. Akhlaghi etAl., (arXiv:1409:1962v1, physics.ins-det) describes a waveguide-integrated superconducting nanowire single photon detector. An 8.5pm long and narrow 8x35nm U-shaped NbTiN nanowire atop a silicon-on-insulator waveguide is turned into an absorber by etching an asymmetric nanobeam cavity around it.

According to a first aspect of the present invention, there is provided a photon detector comprising: an optical resonator comprising a resonant cavity; the resonator configured to: receive input light; and, allow the input light to circulate within the resonant cavity; an elongate superconductor element configured: to absorb one or more photons propagating within the resonant cavity; and, such that at least a portion of the elongate length of the element is elongate in a direction substantially parallel to a propagation direction of the light within the cavity.

The first aspect may be modified in any suitable way as disclosed herein including but not limited to any one or more of the following.

The photon detector may be configured such that the detector is configured to conduct electrons along the elongate length of the superconducting element.

The photon detector may comprise: an optical resonator comprising a resonant cavity; the resonator configured to: receive input light; and, allow the input light to propagate within the resonant cavity; an elongate superconductor element configured: to absorb one or more photons propagating within the resonant cavity; and, such that at least a portion of the elongate length of the element is elongate in a direction substantially parallel to a propagation direction of the light within the cavity; and, the detector being configured to conduct electrons along the elongate length of the superconducting element.

The photon detector may be configured such that the optical resonator is configured to circulate the input light within the resonator cavity.

The photon detector may be configured such that the optical resonator complises an integrated optic waveguide.

The photon detector may be configured such that the resonator comprises a ring resonator.

The photon detector may be configured such that the ring resonator comprises a racetrack configuration.

The photon detector may further comprise an integrated optic input waveguide, and wherein the detector is configured to couple light from the input waveguide to the resonator.

The photon detector may be configured such that the detector is configured to evanescently couple light into the resonator.

The photon detector may be configured such that the superconducting element is in contact with the optical resonator integrated optic waveguide.

The photon detector may be configured such that: the integrated optic resonator comprises a rectangular cross section comprising four faces; and, the superconducting element is in contact with at least one of the faces of the said cross section.

The photon detectol may be configuied such that the elongate supeiconductor element comprises a superconducting nanowire single photon detector.

The photon detector may be configured such that each cross sectional dimension of the cross section of the superconductor element along the elongate direction, is less than l5Onm.

The photon detector may be configured such that the superconductor element comprises a strip of superconducting material.

The photon detector may be configured such that the strip comprises a cross sectional height between 3-bnm.

The photon detector may be configured such that the strip comprises a cross sectional width between 40-l5Onm.

The photon detector may be configured such that the strip comprises a cross sectional width between 8O-lOOnm.

The photon detector may be configured to tune the optical resonances within the cavity.

According to a second aspect of the present invention, there is provided a photon detector system comprising a plurality of photon detectors as described in the first aspect and optional any of the optional features of the first aspect, wherein the plurality of photon detectors are optically coupled to a common optical waveguide.

The second aspect may be modified in any suitable way as disclosed herein including but not limited to any one or more of the following.

The photon detector system may be configured such that at least two of the plurality of photon detectors are configured to support different resonant wavelengths.

According to a third aspect of the present invention, there is provided a detection system comprising a cryostat system and a photon detector or photon detector system as described in the first and second aspects above, and optionally any of the optional features described for the first and second aspects.

According to a fourth aspect of the present invention, there is provided a method of detecting a photon using a photon detector, or photon detector system as described in any of the above aspects and their optional further features, the method comprising the steps of: inputting electrical current through the superconducting element; the current value not exceeding the critical current value at which the superconducting element becomes non-superconducting; determining when the superconducting element has absorbed a photon by monitoring electrical current diverted away from the superconducting element.

The fourth aspect may be modified in any suitable way as disclosed herein including but not limited to any one or more of the following.

In certain operating environments the superconducting element will not be at a temperature to exhibit superconductivity, therefore the detector may need cooling, therefore the method of detecting a photon may comprise the step of: cooling the photon detector such that the superconductor element becomes superconducting.

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings, in which: Figure 1 shows an example of a ring resonator detector as described herein; Figure 2 shows a graph of detector efficiency as a function of detector length and cavity coupling strength; Figure 3 shows a further example of a ring resonator detector as described herein; Figure 4 shows another further example of a ring resonator detector as described herein Figure 5 shows an example of a ring resonator detector similar to figure 4 with a bridging structure for the feeding the superconducting detector wire onto the ring; Figure 6 shows an example of a multi-wavelength detection device with multiple detection rings; Figure 7 shows an example of a detector as described herein with a resonant cavity with two reflecting ends; Figure 8 shows a cross section through an integrated optic waveguide suitable for use with detectors described herein; Figure 9 shows an example of a detector with a schematic of an electronic circuit configured to provide an electrical signal upon absorption of a photon; Figure 10 shows an example of a ring resonator type detector with a racetrack configuration; Figure 11 shows an example of a process for manufacturing a detector.

There is provided a photon detector 2 comprising an optical resonator 4 and an elongate superconductor element 6. The optical resonator 4 comprises a resonant cavity and is configured to receive input light and allow the input light to propagate within the resonant cavity. In some examples the resonator 4 is configured to circulate light within the cavity. The elongate superconductor element 6 is configured to absorb one or more photons propagating within the resonant cavity. At least a portion of the elongate length of the element 6 is elongate in a direction substantially parallel to a propagation direction of the light within the cavity. In some examples the detector 2 is configured to conduct electrons along the elongate length of the superconducting elementS. Figure 1 shows an example of

S

a detector 2 using a ring resonator 8 as the optical resonator 4. The detector 2 may be used to detect photons of any wavelength in principle, but preferably in the optical communication C-Band of 1530-1 565 nm.

Operation The operation of the detector 2 is as follows. Photons intended for detection are coupled into the resonant cavity of the detector 2 wherein they are allowed to propagate within the cavity.

This may be back and forth within the cavity or circulating around a cavity. As the photons traverse the cavity they pass a region where at least a part of the optical mode they are in interacts with the superconducting element 6. The superconducting element 6 has the possibility of absorbing one or more photons. The absorption of photons by the element 6 generates photoelectrons. The superconducting element 6 is typically required to be cooled to the point that it becomes superconducting. The preferred temperature depends on the geometry of the nano-wire and the superconducting material. In the case of Niobium-Nitride superconducting elements as described herein, the preferred temperature is below 4 degrees Kelvin. In the case of Tungsten-Silicide, that temperature is typically at most 2 degree Kelvin. A preferred temperature is between 0.1 to 4 Kelvin, a further preferred temperature range is between 1-4 Kelvin.

As the photon travels along the optical resonator 4 in the vicinity of the superconducting element 6 there is a chance that it may be absorbed or not absorbed by the element 6. If the photon is not absorbed on that particular pass along the resonator 4, the superconducting element 6 has a further chance of absorbing the photon upon the next successive pass, and every other further pass the photon makes within the cavity. This entails that the elongate superconducting element 6 of the detector 2 described herein can be made of a shorter length than a similar element that does not allow the photon to successively pass the same element. This can make the whole detection system smaller. A smaller detector 2 gives rise to increased numbers of detectors 2 per manufacturing run, for example if the detectors 2 are manufactured using integrated optic technology where multiple detectors 2 may be formed on the same substrate. A smaller detector 2 also lends to the ability to manufacture more complex optical systems on-chip where the detector 2 is integrated with other optical components and functionality.

Furthermore, a long length of superconducting elementS is prone to manufacturing errors, for example a malformed part of the element 6 rendering the detector 2 unusable. Restricting the length of the superconducting element 6 reduces the likelihood of defects occurring in the element 6, hence increases the manufacturing yield of the detector 2.

An optical resonant cavity allows only certain tesonant' wavelengths of light to successfully propagate within the cavity via constructive interference. The detector 2 is therefore also wavelength selective.

In any of the examples described herein the detector, or any of the detectors, may be configured to tune the resonant wavelength of the cavity. This may be accomplished in any number of ways. One method is to change the refractive index of at least a portion of the cavity by applying an electric field, for example applying an electric field to a portion of the resonant cavity that comprises a material with a x2 nonlinearity (for example Lithium Niobate). Another example of tuning the resonant wavelength in the cavity includes changing the refractive index of at least a portion of the cavity using carrier injection. This may be accomplished by having at least a portion of a P-O)-N junction within the cavity and introducing charge carriers to the P-(i)-N junction to locally change the refractive index. The refractive index may also be changed in the cavity using carrier depletion.

In any of the tuning examples above the cavity may be configured to give effect to the refractive index changes. Another example of changing the resonant wavelength of the cavity is by an opto-mechanical feature such as a MEMS what can be operated to mechanically change the optical path length of the cavity. The detector 2 is preferably operated by passing an electrical current through the superconducting element 6. The current level is preferably at a level below 1CRIT such that when one photon gets absorbed by the superconducting element 6, the photoelectron or photoelectrons created cause the superconducting element 6 to momentarily change into a non-superconducting state. During this period, the superconducting element 6 attains a resistance which can be used to determine the presence of a detected photon.

Figure 9 shows an example of a detector system 18 wherein an electrical source 10 is connected to the elongate superconducting element 6. Current flowing from the electrical source lOis allowed to flow down two parallel electrical paths 12, 14 back to the electrical source 10. The first path 12 directs current along the elongate superconducting element 6, whilst the second alternative path 14 directs current to a device 16 for measuring/monitoring current change, such as but not limited to an ammeter. When the superconducting element 6 is in a superconducting state, it has zero resistance, therefore substantially all the electrical current from the source 10 will flow through the first path 12 (i.e. through the superconducting element 6). When the superconducting element 6 absorbs a photon and ceases to be in a superconducting state, it gains an electrical resistance which leads to at least a portion of the current originally flowing through the superconducting element 6 to be diverted into the second electrical path 14 which is detected by the current monitoring device 16.

The detection system 18 may utilise any electronic circuitry and components to determine that a photon has been detected.

The detector Figure 1 shows a plan view of an example of a detector 2 configured to circulate light within the cavity. The detector 2 in this example is a ring resonator 8. Figure 1 shows this ring being circular, however any type of circulating resonant cavity may be used, for example a racetrack configuration 20 (see figure 10), an ellipse or a disc. The ring resonator 8 is optically coupled to an input waveguide 22 (also referred to as a bus guide'). In figure 1 this is shown as an evanescent coupling (directional coupler configuration), however any optical coupling of photons to the resonator 8 may be used, including an MMI coupler.

Both the input waveguide 22 and the ring resonator 8 in the example shown in figure 1 are integrated optic waveguides. The waveguides are preferably single mode for the wavelengths of the light intended to be propagated in the waveguides. Preferably the waveguides are single mode for both TE and TM polarisations however the waveguides may be single mode for a single polarisation. Preferably, the waveguide couples one of a fundamental TE or TM mode.

Light is coupled into the bus guide 22 using any mechanism in principle including but not limited to end-on coupling to an optical fibre or lensed optical fibre or a grating coupler 24 (not shown in figure 1) formed in the bus guide 22. Light can also be generated in the bus guide 22 by different methods such as Four-Wave Mixing, Spontaneous Down-Conversion, Spontaneous Up-Conversion or any other non-linear optical process.

An example of a cross section of a waveguide structure suitable for use with detectors 2 is shown in figure 8. A core waveguide 26 is located upon a cladding layer 28 (also called undercladding) which in turn is located upon a substrate 30. The core 26, cladding 28 and substrate 30 may be formed from any one or more materials, however, the core 26 is preferably silicon, the cladding 28 is preferably silica and the substrate 30 is preferably silicon. The waveguide structures may be formed using any technique in principle, preferably deposition, growth and etching techniques. In principle any waveguide cross sectional geometry may be used including but not limited to buried core waveguides where cladding material completely surrounds the core. Integrated optic waveguides are preferably rib or ridge waveguides wherein the top of the core 26 may be air-clad or have a protective (over-clad) layer located on top, such as but not limited to a polymer or glass layer). This layer may be located on top of any of: the resonatorwaveguide 8, portions of the resonator waveguide with current carrying (typically metal) tracks 32 and/or superconducting element 6 formed upon the resonator waveguide 8; the bus guide 22.

The preferred materials for the core waveguide 26 comprise materials with a refractive index greater than 3 at the wavelength region of intended use. Such cores 26 may be formed from any one or more of (but not limited to) un-doped or doped silicon, gallium arsenide, lithium niobate and indium phosphide.

When using a silicon core 26, or equivalent material with a similar refractive index above 3, the cross section of the core has the preferred dimensions of: a width between 450nm and 800nm and a height between lOOnm and 250nm. More preferable the core has a width between 600-700nm and a height between 2lOnm-230nm. A further preferred cross section is a width of 700nm and a height of 220nm. The term height' refers to the dimension extending substantially perpendicular away from the undercladding surface 34 contacting the core 26. The term width' corresponds to the dimension substantially perpendicular to the height dimension (i.e. that substantially parallel to the plane of the surface contacting the core).

Preferably the minimum bend radius of waveguides used in the detector 2 (using silicon as a core 26 or another material with a refractive index at 1550nm of 3.5) is 10p.m. Such a bend radius is substantially loss-less in the absence of an over-cladding layer or where an over-cladding layer, with a refractive index of less than 2, is included above the core 26. The length of the loop of a circulating optical resonator 4 used herein (using silicon as a core 26 or another material with a refractive index at 1 550nm of 3.5) is preferably greater than jim, and preferably smaller than 150 jim, more preferably between 80 p.m -100 jim.

If a racetrack configuration 20 is used, then the straight part 21 of the racetrack 20 preferably has a minimum length of Sjim. Preferably the straight part 21 of the racetrack 20 is spaced apart from and runs parallel with the bus guide 22 to form the directional coupling region.

The elongate superconducting element 6 is configured to be able to absorb one or more photons propagating within the resonator 4. This absorption may arise through evanescent coupling between the optical mode propagating in the resonator 4 and the superconductor element 6. In figure 1, the superconducting element 6 is located directly upon a surface (i.e. contacting) the resonator 4 such that the superconducting element 6 intersects part of the electric field of the mode that crosses that surface boundary.

At least a portion of the superconducting element 6 substantially follows (in its elongate dimension) a direction that the light propagates within the optical resonator 4. This direction may be straight or not straight (such as a curved path of the ring resonator 8 shown in figure 1). Preferably over 50% of the length of superconducting element 6 follows the propagation path.

The superconducting elementS in figure 1 is shown to be located on the top surface 36 of the ring resonator 8, i.e. the surface directly opposite the core surface contacting the undercladding top surface 34. However the superconducting element 6 may be located on one or more of any of the surfaces of the resonator 8, for example any one oi more of but not limited to: the top surface of the core 26 the bottom suiface of the core 26 contacting the undercladding top surface 34; one of the side surfaces.

In principle at least a portion the superconducting element 6 may be located inside the core 26 (such that the core 26 surrounds the superconducting element 6). Additionally or alternatively at least a portion of the superconducting element 6 may be located adjacent to but not contacting the core 26 (for example by having one or more intermediate layers between a surface of the core 26 and the superconducting element 6). For such an intermediate layer arrangement where the one or more intermediate layers aie sandwiched between a surface of the core 26 and the superconducting element 6, the superconducting elementS should be sufficiently close to the core 26 so that a significant amount of the modal tail existing outside of the core 26 intersects the superconducting element 6 and allows for photons to be absorbed.

The superconducting element 6 shown in figure 1 is electrically connected to two electrical connections or tracks 32 configured to allow current to flow from an electrical source 10 (not shown in figure 1) such as a current or voltage source, to the superconducting element 6.

Figure 1 shows arrows denoting the path of current flow to and from the electrical tracks 32.

These tracks 32 may be formed from the same superconducting material used to form the elongate superconducting element 6, one or more different materials, or a combination of both. Preferred the track materials may be any of Au/Ti, Cu and Al.

The elongate superconducting element 6 may be formed from any suitable material that can become superconducting, including any one or more of, but not limited to: niobium, niobium nitride, niobium titanium nitride, niobium-germanium, tungsten, tungsten silicide, magnesium diboride.

The elongate superconducting element 6 preferably has a thickness (i.e. the cross sectional dimension of the element 6 perpendicular to the surface contacting the top of the ring 8) of less than Snm, preferably between 3 and 5 nm, more preferably 4nm. The elongate superconducting element 6 preferably has a width (i.e. the cross sectional dimension of the element 6 perpendicular to the height) between 4Onm-l5Onm, more preferably 80-lOOnm, more preferably 9Onm.

A superconducting element 6 with a width greater than l5Onm or a thickness greater than bnm may have a lower efficiency for detecting single photons because the element 6 has a greater chance of continually staying superconducting when a single photon is absorbed. A superconducting element 6 with a width less than SOnm may be difficult to fabricate. These values depend on the superconducting material and the operating temperature.

As shown in figure 1, the detectors 2 described herein may use optical resonant cavities that allow light to circulate. In other words the cavity does not have any ends but is a continually joined loop. One such type of circulating optical resonator 4 is a ring resonator 8 which, the theory of which is briefly described below.

Ring resonators Ring resonators 8 support resonant optical modes or particular wavelengths according to the specific design of the optical cavity.

The wavelength resonant spacing zX of a single ring resonator 8 coupled to a bus guide 22 can be shown to be given by the following equation: [1] / A2 Where N is the group index of the mode in the ring 8 and L is the ring resonator length. The transmission' for such a system refers to the light which continually propagates along the bus guide 22 after the interaction with the ring resonator 8, i.e. the combination of the light not coupled into the ring 8 from the bus guide 22 together with the light coupled back out of the ring 8 into the bus guide 22 after one or more passes around the ring 8. Which wavelengths of light actually continue to propagate along the bus guide 22, away from the ring 8, is determined by the ring design and the ring/bus guide coupling design as follows.

It can be shown that for a single ring resonator 8 coupled to a straight bus' waveguide 22 similar to the input waveguide 22 shown in figure 1, the intensity transmittance of the optical ring resonator 8, as a function of wavelength is: [2] (1-x2)(1 -y2) = (1-F' --xy)2 + 4xysin2(/2))I where: [3] 1 p x = (1-y)exp(-L) [4] y = cos(cO [5] 2ff cb=GL=TL S Where ic is the mode coupling coefficient determined primarily by the bus 22 and ring 8 cross sections and their proximity to each other, i. is the coupling length of the ring 8 and bus guide 22, y is the intensity insertion loss coefficient (i.e. the inherent loss associated with presence of the ring 8 perturbing the optical mode in the bus guide 22), p is the intensity attenuation coefficient of the mode propagating around the ring 8, fi is the propagation constant of the mode.

For photons travelling up the bus guide 22 towards the ring coupling point, the ring resonator 8 can theoretically be designed such that for particular wavelengths, photons (shown by the solid curved arrowed line in figure 1) are coupled into the ring 8 as shown by the straight arrows in figure 1 and destructively interfered on their exit from the ring 8 with the bus guide 22. Therefore no optical modes at that wavelength are propagated further down the bus guide 22 beyond the coupling point of the ring resonator 8; hence light at the resonant wavelength is coupled into the ring resonator 8.

As shown above the coupling strength and length of the ring/bus coupling is a determining factor for the transmission minima of the bus/ring coupling system (i.e. the probability that light at the resonant wavelength stays within the ring 8 once coupled in).

Figure 2 shows a theoretical graph of detector efficiency for detectors 2 comprising a racetrack 20 configuration with a cavity length of 90j.tm. The graph shows how the detector efficiency varies with cavity coupling strength (i.e. representing, upon an amplitude basis, the proportion of light coupled into the ring) and the of the length superconducting element 6 (termed "detector length" in figure 2). [RI] The length of the superconducting element 6 can be any length in principle. A preferred length of the superconducting element 6 is between 0.2.tm and 5pm, more preferably S between 1 pm and 2pm.

The coupling strength of the ringlbus coupling (i.e. upon an amplitude basis, the proportion of light coupled across from the bus 22 into the ring 8) can be any value in principle but is preferably between 1% and 50%, more preferably between 1% and 30 %, more preferably between 1% and 10%.

SuDerconductinci element configurations The superconducting element 6 may take any in-plan design in principle. Preferably the element 6 is a strip of superconducting material.

Figure 1 shows a detector 2 with a superconducting element 6 that starts at one point 38 on the ring 8, follows the curved path of the ring 8 in one direction, then turns and doubles back upon itself following the curved path of the ring 8 in the opposite direction, past the starting point 38, the doubles back again following the ring in the initial direction until it gets close to, but not contacting, the starting point 38, thus terminating at an end point 40. The element 6 therefore forms a continuous electrical path from its start 38 and end points 40. Electrical tracks 32 are provided to and from the start 38 and end points 40 of the element 6. The superconducting element 6 has a width that is thin enough to allow the element 6 to run parallel with itself along the same portion of the ring 8 (similar to a tramline like configuration), whilst being on the same surface of the ring 8.

Figure 3 shows a further example of a detector 2 with a superconducting element 6 where the element 6 starts at one point 38 on the ring 8, follows the curved path of the ring 8 until it gets to an end point, without doubling back on itself.

Figures 4 and 5 show a further example of a detector 2 with a superconducting element 6 where the element 6 starts at one point 38 on the ring 8, follows the curved path of the ring 8 until it gets to a point where it doubles back on itself and runs substantially parallel to but not contacting the portion of the element 6 in the opposite direction until it passes the original start point 38 and reaches an end point 40.

The top of figure 4 shows a cross section of the ring 8 and bus guide 22 taken though the line denoted A-A in the figure. This shows that the electrical tracks 32 leading and connecting the start and end points of the superconducting element 6 run horizontally along the top of the ring 8 until the ring edge, then run down the side of the ring and then continue horizontally along the top of the undercladding 34 (not shown) away from the ring 8.

Figure 5 shows a similar arrangement as to figure 4 except that a bridging structure 42 is shown connected to the ring 8 that allows the tracks 32 to run away from the ring 8 and start/end points 38/40 of the superconducting element 6 without having to vertically run down the side of the ring 8. This configuration is advantageous because the tracks 32 are simpler to form. The bridging structure 42 in this example is formed from a solid material comprising a height (above the top cladding surface 34) that is substantially the same as the height of the ring 8 above the top cladding surface 34, so that the electrical tracks can run off the ring 8 without changing their vertical position. Preferably the bridging structure 42 is formed from the same material and preferably the same deposition and fabrication steps as the ring 8. In this case, the bridging structure 42 is preferably designed to minimise perturbations on the propagation of light in the ring 8.

Any of the features described in figures 3-5, such as the element structure, tracks and bridging structure may be used in any of the examples described herein.

Figure 6 shows a detector system 18 comprising a plurality of ring resonator detectors 2 coupled to a common bus guide 22 (superconducting elements 6 and tracks 32 not shown).

In this example, each detector 2 is configured to couple in (hence detect) photons of a particular wavelength due to the different resonances determined by the differing ring radii.

The detector system 18 may therefore detect, independently, a plurality of defined wavelengths in parallel. In principle, at least two of the plurality of photon detectors may be configured to support different resonant wavelengths. The detectors are arranged sequentially along the length of the bus guide 22 and are spaced apart so that the optical modes propagating within one cavity are not optically coupled to a nearby cavity. The optical cavities may be coupled to either side of the bus guide 22, for example alternately coupled either side as shown in figure 6.

Any of the detectors 2 described herein may be used or incorporated into this detection system 18. The detection system 18 may include other features of detection systems 18 described herein, including but not limited to one or more electrical sources and corresponding detection circuitry. In principle, multiple rings 8 of the same radii may be incorporated into the same detection system 18, for example along the same bus guide 22, to try and ensure that all photons of a particular wavelength are detected by the detection system 18. This may be needed because not all of the photons may be coupled into the first ring 8 having that particular resonant wavelength. Any of the detectors used in the system may have a tune-able resonant cavity as described above.

A detection system 18 may comprise one or more detectors 2 as described herein and a cryostat.

One or more rings 8 may be included to detect different polarisations of the same wavelength. A resonant structure may have a different refractive index for the TE and TM polarisations, therefore difference resonance peaks. A different resonant structure may therefore be required to detect the different polarisations.

Figure 7 shows a further example of a detector 2. In this example, the optical resonator 4 is not in a loop configuration but has a single path 44 having two separate end points 46, 48 that reflect light back along the path it originally came from. This may be a single length of waveguide with a reflective element 50 on each end, such as but not limited to a Bragg grating which is configured to reflect only particular resonant wavelengths back into the resonant cavity. The waveguide may be similar to any such waveguide described herein.

Other features described herein, including features in other examples (such as the track, element, and bridging structure configurations) may be used in this example.

Method of manufacture The detectors 2 described herein may be manufactured using any suitable fabrication process including, but not limited to, the following fabrication process. The following process describes making a detector 2 described herein using grating couplers 24 to couple light into the bus guide 22. The example process 1000 is shown diagrammatically in figure 11.

Step 1002 of the process begins by providing a silicon core layer 1016 formed upon a silica undercladding layer 1018 which in turn is formed upon a silicon substrate 1020. A layer of niobium nitride (NbN) 1022 (eventually forming the superconducting element 6) is deposited on top of the silicon core layer. A layer of PMMA (Poly(methyl methacrylate)) 1024 is then deposited on top of the NbN layer 1022 and subsequently patterned (using, for example electron-beam lithography) to leave an exposed area in the shape of the electrical tracks 32.

At step 1004, a layer of Au/Ti 1026 is then deposited over the electrical track shaped areas to form the electrical tracks 32 and the remaining PMMA 1024 is removed (in the process also known as lift-off).

At step 1006, a further step of PMMA deposition and patterning 1028 is then performed to define exposed areas of the NbN layer, that when etched away, allow the remaining portions of the layer defining the elongate superconducting element 6.

A step 1008, the NbN is etched to form the elongate superconducting element 6.

At step 1010, a further layer 1030 of PMMA is deposited and patterned to define exposed areas of the silicon core layer 1016, that when etched away, allow the remaining portions of the layer 1016 defining the grating 24 on the bus guide 22 and at least partially defining the ring 8 and bus guides 22.

A step 1012, the silicon layer is etched (for example using Fluorine etch) partially through the layer.

At step 1014, a further layer of PMMA 1032 is then deposited and patterned to reside over and protect the previously formed grating coupler 24 region, allowing exposed areas of the silicon core layer 1016 that when etched away, allow the remaining portions of the silicon layer 1016 defining the ring Sand bus guides 22. The exposed portions of the core layer 1016 are etched to the silica layer 1018.

It is understood that the PMMA may be any suitable masking material and any suitable layer depositing, patterning, etching and layer removal techniques may be used.

Embodiments of the present invention have been described with particular reference to the examples illustrated. However, it will be appreciated that variations and modifications may be made to the examples described within the scope of the present invention.

Claims

Claims 1. A photon detector comprising: I) an optical resonator comprising a resonant cavity; the resonator configured to: A) receive input light; and, B) allow the input light to circulate within the resonant cavity; II) an elongate superconductor element configured: C) to absorb one or more photons propagating within the resonant cavity; and, 0) such that at least a portion of the elongate length of the element is elongate in a direction substantially parallel to a propagation direction of the light within the cavity.
2. A photon detector as claimed in claim 1 wherein the detector is configured to conduct electrons along the elongate length of the superconducting element.
3. A photon detector comprising: I) an optical resonator comprising a resonant cavity; the resonator configured to: A) receive input light; and, B) allow the input light to propagate within the resonant cavity; II) an elongate superconductor element configured: C) to absorb one or more photons propagating within the resonant cavity; and, 0) such that at least a portion of the elongate length of the element is elongate in a direction substantially parallel to a propagation direction of the light within the cavity; and, the detector being configured to conduct electrons along the elongate length of the superconducting element.
4. A photon detector as claimed in claim 3 wherein the optical resonator is configured to circulate the input light within the resonator cavity.
5. A photon detector as claimed in any preceding claim wherein the optical resonator comprises an integrated optic waveguide.
6. A photon detector as claimed in claim 5 wherein the resonator comprises a ring resonator.
7. A photon detector as claimed in claim 6 wherein the ring resonator comprises a racetrack configuration.
8. A photon detector as claimed in any preceding claim further comprising an integrated optic input waveguide, and wherein the detector is configured to couple light from the input waveguide to the resonator.
9. A photon detector as claimed in claim 8 wherein the detector is configured to evanescently couple light into the resonator.
10. A photon detector as claimed in any of claims 5-9 as dependent upon claim 5 wherein the superconducting element is in contact with the optical resonator integrated optic waveguide.
11. A photon detector as claimed in claim 10 wherein: I) the integrated optic resonator comprises a rectangular cross section comprising four faces; and, II) the superconducting element is in contact with at least one of the faces of the said cross section.
12. A photon detector as claimed in any preceding claim wherein the elongate superconductor element comprises a superconducting nanowire single photon detector.
13. A photon detector as claimed in any preceding claim wherein each cross sectional dimension of the cross section of the superconductor element along the elongate direction, is less than lSOnm.
14. A photon detector as claimed in any preceding claim wherein the superconductor element comprises a strip of superconducting material.
15. A photon detector as claimed in any preceding claim wherein the superconductor element comprises a strip of superconducting material.
16. A photon detector as claimed in claim 15 wherein the strip comprises a cross sectional height between 3-5nm.
17. A photon detector as claimed in claims 15 or 16 wherein the strip comprises a cross sectional width between 40-l5Onm.
18. A photon detector as claimed in claim 15 or 16 wherein the strip comprises a cross sectional width between 80-lOOnm.
19. A photon detector as claimed in any preceding claim, the detector being configured to tune the optical resonances within the cavity.
20. A photon detector system comprising a plurality of photon detectors as claimed in any preceding claim wherein the plurality of photon detectors are optically coupled to a common optical waveguide.
21. A photon detector system as claimed in claim 20 wherein at least two of the plurality of photon detectors are configured to support different resonant wavelengths.
22. A detection system comprising a cryostat system and a photon detector or photon detector system as claimed in any preceding claim.
23. A method of detecting a photon using a photon detector, or photon detector system as claimed in any preceding claim, the method comprising the steps of: I) inputting electrical current through the superconducting element; the current value not exceeding the critical current value at which the superconducting element becomes non-superconducting; II) determining when the superconducting element has absorbed a photon by monitoring electrical current diverted away from the superconducting element.
24. A method of detecting a photon as claimed in claim 23 comprising the step of: I) cooling the photon detector such that the superconductor element becomes superconducting.
25. A detector substantially as shown and/or described herein with reference to any one or more of Figures 1, 3, 4, 5, 6, 7 and 9 of the accompanying drawings.AMENDMENTS TO CLAIMS HAVE BEEN FILED AS FOLLOWSClaims 1. A photon detector comprising: I) an optical resonator comprising a resonant cavity; the resonator configured to: A) receive input light; and, B) allow the input light to piopagate within the resonant cavity; II) an elongate superconductor element configured: C) to absorb one or more photons propagating within the resonant cavity; and, 0) such that at least a portion of the elongate length of the element is elongate in a direction substantially parallel to a propagation direction of the light within the cavity.2. A photon detector as claimed in claim 1 wherein the optical resonator is configured to circulate the propagating input light within the resonator cavity.(315 3. A photon detector as claimed in claim 1 or 2 wherein the detector is configured to 0 conduct electrons along the elongate length of the superconducting element.4. A photon detector as claimed in any preceding claim wherein the optical resonator comprises an integrated optic waveguide.5. A photon detector as claimed in claim 4 wherein the resonator comprises a ring resonator.6. A photon detector as claimed in claim 5 wherein the ring resonator comprises a racetrack configuration.7. A photon detector as claimed in any preceding claim further comprising an integrated optic input waveguide, and wherein the detector is configured to couple light from the input waveguide to the resonator.8. A photon detector as claimed in claim 7 wherein the detector is configured to evanescently couple light into the resonator.9. A photon detector as claimed in any of claims 4-8 as dependent upon claim 4 wherein the superconducting element is in contact with the optical resonator integrated optic waveguide.10. A photon detector as claimed in claim 9 wherein: I) the integrated optic resonator comprises a rectangular cross section comprising four faces; and, II) the superconducting element is in contact with at least one of the faces of the said cross section.11. A photon detector as claimed in any preceding claim wherein the elongate superconductor element comprises a superconducting nanowire single photon detector.12. A photon detector as claimed in any preceding claim wherein each cross sectional dimension of the cross section of the superconductor element along the elongate direction, is less than l5Onm.13. A photon detector as claimed in any preceding claim wherein the superconductor element comprises a strip of superconducting material.14. A photon detector as claimed in any preceding claim wherein the superconductor element comprises a strip of superconducting material.(3 15. A photon detector as claimed in claim 14 wherein the strip comprises a cross 0 sectional height between 3-Snm.16. A photon detector as claimed in claims 14 or 15 wherein the strip comprises a cross sectional width between 40-l5Onm.17. A photon detector as claimed in claim 14 or 15 wherein the strip comprises a cross sectional width between 80-lOOnm.18. A photon detector as claimed in any preceding claim, the detector being configured to tune the optical resonances within the cavity.19. A photon detector system comprising a plurality of photon detectors as claimed in any preceding claim wherein the plurality of photon detectors are optically coupled to a common optical waveguide.20. A photon detector system as claimed in claim 19 wherein at least two of the plurality of photon detectors are configured to support different resonant wavelengths.21. A detection system comprising a cryostat system and a photon detector or photon detector system as claimed in any preceding claim.22. A method of detecting a photon using a photon detector, or photon detector system as claimed in any preceding claim, the method comprising the steps of: I) inputting electrical current through the superconducting element; the current value not exceeding the critical current value at which the superconducting element becomes non-superconducting; II) determining when the superconducting element has absorbed a photon by monitoring electrical current diverted away from the superconducting element.23. A method of detecting a photon as claimed in claim 22 comprising the step of: I) cooling the photon detector such that the superconductor element becomes superconducting.24. A detector substantially as shown and/or described herein with reference to any one or more of Figures 1, 3, 4, 5, 6, 7 and 9 of the accompanying drawings. IC)CD (4