WO2009146561A1

WO2009146561A1 - Dual mode terahertz spectroscopy and imaging systems and methods

Info

Publication number: WO2009146561A1
Application number: PCT/CA2009/000804
Authority: WO
Inventors: Daryoosh Saeedkia; Safieddin Safavi-Naeini
Original assignee: T-Ray Science Inc.
Priority date: 2008-06-06
Filing date: 2009-06-05
Publication date: 2009-12-10

Abstract

A terahertz radiation transmission and detection system includes a dual mode light source operable in a continuous wave mode and in a pulsed wave mode, wherein the dual mode light source is configured for generating a continuous wave light beam when operated in the continuous wave mode, and for generating a pulsed light beam when operated in the pulsed wave mode. The system also includes a beam selector for generating a pump beam comprising either the continuous wave light beam or the pulsed light beam, a terahertz transmitter configured for receiving the pump beam and for generating terahertz radiation, wherein the terahertz radiation comprises continuous wave terahertz radiation when the pump beam comprises the continuous wave light beam, and wherein the terahertz radiation comprises pulsed terahertz radiation when the pump beam comprises the pulsed light beam, and a terahertz receiver configured for receiving the continuous wave terahertz radiation and the pulsed terahertz radiation. The dual mode light source may comprise a continuous wave light source for generating a continuous wave light beam, and a pulsed light source for generating a pulsed light beam.

Description

TITLE: DUAL MODE TERAHERTZ SPECTROSCOPY AND IMAGING SYSTEMS

AND METHODS

FIELD

[0001] The present invention relates to systems and methods for transmitting and receiving terahertz radiation, and in particular, to terahertz spectroscopy and imaging systems and methods of using such systems.

BACKGROUND

[0002] There exist pulsed terahertz radiation spectroscopy and imaging systems, in which a train of terahertz (THz) pulses with the duration of each pulse less than a few picoseconds is generated by using either a nonlinear electro-optic crystal or a THz photoconductive antenna excited by a train of femto-second laser pulses. The generated THz pulses interact with the sample under test and are detected using either a nonlinear electro-optic crystal or an optically gated THz photoconductive antenna excited by a pulsed wave laser. A THz pulsed imaging system can be used to create a three dimensional image of a sample, where the third dimension of the image (in depth) is constructed using time-of-flight information of the received THz pulse. Pulsed THz systems provide the spectroscopic information of the sample under test over a wide range of the THz spectrum in one shot. However, the frequency resolution of the spectroscopic information generated by pulsed THz systems is limited to a few GHz.

[0003] There also exist continuous wave (CW) terahertz spectroscopy and imaging systems, in which the CW THz radiation is generated by mixing two CW laser beams in a first THz photoconductive antenna, where the frequency of the generated THz radiation can be swept in a few megahertz (MHz) steps by tuning the wavelength of one or both of the CW lasers at their central wavelengths. The generated THz radiation interacts with a sample under test and can be detected either by known means such as a bolometer or by using a second THz photoconductive antenna excited by a laser probe beam. The frequency resolution of the spectroscopic information generated from CW terahertz radiation can be a few MHz, which is orders of magnitude higher than that which is achievable using THz pulsed systems. However, acquiring spectroscopic information by using a CW THz system to scan over a wide THz frequency range is difficult and time consuming.

[0004] Accordingly, there is a need for improved terahertz spectroscopy and imaging systems, which do not have at least some of the disadvantages associated with known terahertz systems.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to a dual mode THz radiation transmission and detection system, in which the user may switch between pulsed and CW input laser sources to operate the system under either pulsed generation mode or CW generation mode. In its CW generation mode, the system can generate CW terahertz radiation and may operate in different scanning configurations depending on the required frequency resolution. For higher frequency resolution, the scanning time may be longer. [0006] The THz radiation transmission and detection system of the present invention includes a dual mode light source, which may comprise a pulsed laser source comprising a femto-second pulsed laser and a CW laser source comprising two CW lasers, where one or both of the two CW lasers are tunable around their central wavelengths by a wavelength tuning controller or other known means. The system can be changed between the pulsed generation mode and CW generation mode, by changing the input laser source between the pulsed laser source and the CW laser source. In some embodiments, the rest of the system is not changed. In other embodiments, the first and second photoconductive antenna could be interchanged, by using a multi-element THz integrated circuit mounted on a translational stage. With an interchangeable configuration, one can switch between the THz integrated circuits optimized for pulse or CW modes and also optimized to cover a particular portion of the THz spectrum.

[0007] One of the applications of the subject dual mode terahertz system is spectroscopy, in which the frequency range of interest is a small range around a central frequency, determined by the absorption signature of the material/sample under test. The dual mode THz system of the present invention may be used in a pulsed THz radiation generation mode to quickly find the portion of the THz spectrum where the absorption signature of a particular material under test is located. After that, one can switch to the CW THz radiation generation mode to perform a high resolution scan over a small range of frequency around the absorption frequency determined by the pulsed THz system.

[0008] According to one aspect of the invention, there is provided a terahertz radiation transmission and detection system comprising a dual mode light source operable in a continuous wave mode and in a pulsed wave mode, wherein the dual mode light source is configured for generating a continuous wave light beam when operated in the continuous wave mode, and for generating a pulsed wave light beam when operated in the pulsed wave mode. The system also comprises a beam selector for generating a pump beam comprising either the continuous wave beam or the pulsed light beam, a terahertz transmitter configured for receiving the pump beam and for generating terahertz radiation, and a terahertz receiver for receiving the terahertz radiation. The dual mode light source may comprise a continuous wave light source for generating the continuous wave light beam, a pulsed wave light source for generating the pulsed light beam.

[0009] The continuous wave light source may comprise a first continuous wave laser for generating a first continuous wave laser beam at a first frequency and a second continuous wave laser for generating a second continuous wave laser beam at a second frequency, the first and the second frequency differing by a difference frequency suitable for generating terahertz waves, and a combiner for combining the first continuous wave beam and the second continuous wave beam to form a combined continuous wave laser beam having a component with the difference frequency. The pulsed light source may comprise a pulsed laser that generates a pulsed laser beam with a pulse width suitable for generation of terahertz radiation having a range of frequencies.

[0010] In some embodiments, the beam selector may comprise at least one flip mirror positioned in an optical path of the combined continuous wave light beam and the pulsed wave light beam, wherein the flip mirror is configured to be flipped back and forth between a first position and a second position upon receiving user input, and wherein when the flip mirror is flipped in the first position the combined continuous wave light beam is reflected towards the terahertz transmitter and the pulsed wave light beam is directed away from the terahertz transmitter, and when the flip mirror is flipped in the second position, the pulsed wave light beam is reflected towards the terahertz transmitter and the combined continuous wave light beam is directed away from the terahertz transmitter. In other embodiments, the beam selector may comprise a first optical fiber for carrying the continuous wave light beam and a second optical fiber for carrying the pulsed wave light beam, and a fiber optic connector coupled to the terahertz transmitter, wherein the combined continuous wave light beam can be selected as the pump beam by connecting the first optical fiber to the fiber optic connector, and the pulsed light beam can be selected as the pump beam by connecting the second optical fiber to the fiber optic connector.

[0011] The terahertz transmitter may comprise a transmitting photoconductive antenna, having a voltage bias is applied thereto, for receiving the pump beam and for generating the terahertz radiation. When the pump beam comprises the combined continuous wave laser beam, the terahertz radiation comprises continuous wave terahertz radiation having a frequency equal to the difference frequency of the combined continuous wave laser beam, and when the pump beam comprises the pulsed laser beam, the terahertz radiation comprises pulsed terahertz radiation having a range of frequencies generally inversely related to the pulse width of the pulse wave laser beam. The terahertz receiver may comprise a receiving photoconductive antenna for receiving the terahertz radiation, having a conductance that is modulated when the antenna is coupled to either the combined continuous wave laser beam or the pulsed wave laser beam such that a time varying voltage is induced in the receiving photoconductive antenna by the received terahertz radiation for generating a time varying current. [0012] In some embodiments of the invention, the dual mode light source may comprise a quasi continuous wave light source for generating the continuous wave light beam over a short time period, and a pulsed light source for generating the pulsed light beam, wherein the quasi continuous wave light source comprises a first pulsed laser for generating a first pulsed laser beam, a second pulsed laser for generating a second pulsed laser beam, a beam splitter for splitting the second pulsed laser beam into a first split second pulsed laser beam and a second split second pulsed laser beam, and a pulse stretcher for stretching the first pulsed laser beam and the first split second pulsed laser beam into quasi continuous wave laser beams, and wherein the pulsed light source comprises the second split second pulsed laser beam. The pulsed lasers may comprise two femto-second pulsed lasers, where one or both of the pulsed lasers are tunable around their central wavelengths. To operate the system in its pulsed generation mode, one or both of the femto-second pulses are directly used to generate and detect the THz pulsed radiation. For CW generation mode operation, the femto-second pulses may be stretched to a few picoseconds into quasi Continuous wave laser inputs by passing through the pulse stretcher before impinging onto the first transmitting photoconductive antenna for generating a quasi CW terahertz radiation. Similarly, the detection may be done either in CW or pulsed mode using the stretched quasi- Continuous wave laser inputs as probe beam or the non-stretched pulses to gate the second receiving photoconductive antenna. [0013] A further aspect of the invention is a method for analyzing a sample having a terahertz spectrum, comprising the steps of: a) providing a dual mode spectrometer having a pulsed terahertz wave generation mode that generates pulsed terahertz radiation and a continuous wave terahertz generation mode that generates continuous wave terahertz radiation; b) operating the dual mode spectrometer in pulsed terahertz wave generation mode so as to transmit or reflect the pulsed terahertz radiation through or off the sample; c) identifying a portion of the terahertz spectrum in which an absorption signature of the sample is located, and d) switching the dual mode spectrometer to the continuous wave terahertz radiation generation mode; and e) performing a high resolution scan over a small range of frequencies around the absorption signature. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention will now be described, by way of example only, with reference to the following drawings, in which:

[0015] Figure 1 is a schematic diagram of a dual mode terahertz radiation transmission and detection system made in accordance with an exemplary embodiment of the present invention;

[0016] Figure 2 is a schematic diagram of a dual mode terahertz radiation transmission and detection system made in accordance with another embodiment of the invention;

[0017] Figure 3 is a schematic diagram of a dual mode terahertz radiation transmission and detection system made in accordance to a further embodiment of the invention;

[0018] Figure 4 is a schematic top plan view of an exemplary photoconductive antenna for use with the terahertz systems of the present invention, depicting a first pattern of conducting electrodes; [0019] Figure 5 is a cross-sectional view of the exemplary photoconductive antenna shown in Figure 4;

[0020] Figure 6 is a schematic top plan view of another exemplary photoconductive antenna, depicting a second pattern of conducting electrodes;

[0021] Figure 7 is a graph representing in time domain a detected pulsed THz radiation obtained in pulsed wave generation mode;

[0022] Figure 8 is a graph representing in frequency domain a detected pulsed THz radiation obtained in pulsed wave generation mode;

[0023] Figure 9 is a graph representing in time domain a detected continuous wave THz radiation obtained in continuous wave generation mode; [0024] Figure 10 is a flow chart illustrating a method of analyzing a sample having a terahertz spectrum, using a dual mode spectrometer made in accordance with an embodiment of the present invention;

[0025] Figure 11 is a flow chart illustrating a method of analyzing a sample having a terahertz spectrum, in accordance with another embodiment of the present invention;

[0026] Figure 12 is a schematic diagram of a dual mode terahertz radiation transmission and detection system made in accordance with an alternative embodiment of the invention; [0027] Figure 13 is schematic diagram of a dual mode terahertz radiation transmission and detection system made in accordance with another alternative embodiment of the invention; and

[0028] Figure 14 is a schematic diagram of an exemplary transmitter or receiver chip that may be used in the system of the present invention shown in Figure 13.

DETAILED DESCRIPTION

[0029] Referring to Figure 1 , illustrated therein is a dual mode terahertz radiation transmission and detection system 10 made in accordance with an exemplary embodiment of the invention. The terahertz system 10 comprises a dual mode light source 20 that selectively provides either a CW light beam or a pulsed wave light beam, a terahertz transmitter 40 for receiving the CW or pulsed light beam and for generating and transmitting terahertz radiation 14, a terahertz detector 50 for receiving the transmitted terahertz radiation 14a and generating a time varying current 58 correlatable therewith. A sample 12 to be analyzed may be placed between the terahertz transmitter 50 and the terahertz detector 40 in presence of the generated terahertz radiation 14, for applications such as imaging and spectroscopy.

[0030] Dual mode light source 20 comprises a continuous wave light source

21 , a pulsed wave light source 22, and a beam selector 26 for selecting between the continuous wave light source 21 and the pulsed wave light source 22. The continuous wave light source 21 comprises a first CW laser 23 for generation of a CW laser beam 33 at a first frequency and a second CW laser 24 for generation of a CW laser beam 34 at a second frequency. The first CW laser beam 33 and second CW laser beam 34 are combined and split to form combined CW laser beam 37a and 37b by means such as a power combiner and splitter 38.

[0031] The pulsed wave light source 22 comprises a pulsed wave laser 25 for generating a pulsed wave laser beam 35 with a pulse width generally in the femtosecond range. The pulsed wave laser beam 35 is split to form split pulsed wave laser beams 35a and 35b by means such as a beam splitter 31.

[0032] The beam selector 26 may comprise a first beam selector 26a for receiving the combined CW laser beam 37a and the split pulsed wave laser beam 35a for selectively providing either one as pump beam 36 to the transmitter 40 with a first optional variable time delay, and a second beam selector 26b for receiving the combined CW laser beams 37b and the split pulsed laser beam 25b for selectively providing either one as probe beam 39 to the detector 50 with a second optional variable time delay.

[0033] Terahertz transmitter 40 may comprise a first photoconductive antenna 42 having electrodes 44, and a voltage source 46 for providing a voltage bias to the electrodes 44, wherein the first photoconductive antenna 42 receives pump beam 36 output from dual mode light source 20 to modulate its conductance in order to generating terahertz radiation 14. When the pump beam 36 impinges onto the first photoconductive antenna 42, the conductivity of the photoconductive antenna 42 will increase, thus generating a current that results in terahertz radiation 14. The frequency of the radiation 14 depends on the mode and configuration of the pump beam 36 provided by the dual mode light source 20. The applied voltage bias 46 may be in form of alternating current (AC) or direct current (DC). Photoconductive antennas 42, 52 may have various electrode patterns as discussed in more detailed hereinbelow (see Figure 4-6). [0034] Terahertz detector 50 may comprise a second photoconductive antenna 52 configured to receive probe beam 39 output from the dual mode light source 20, which modulates its conductance in order to generate time varying current 58. A sample-influenced time varying voltage 56 is induced in the second photoconductive antenna 52 upon receiving terahertz radiation 14a. The received terahertz radiation 14a will be sample-influenced and possesses additional information relating to the sample 12 in the case where a sample is used. The sample-influenced time varying current 58 is collected from the electrodes 54 and correlated to the sample-influenced induced time varying voltage 56 and the modulated conductance of the second photoconductive antenna 52.

[0035] In spectroscopy or imaging applications where a sample 12 is targeted, the terahertz radiation 14 is used to non-invasively probe the sample 12, which results in generating the sample-influenced terahertz radiation 14a, which is received by the second photoconductive antenna 52. The probe beam 39 is used to excite the photoconductive antenna 52 and modulate its conductance. Upon receiving the sample-influenced terahertz radiation, a time varying voltage v(t) 56 is induced across the electrodes 54 and a corresponding time varying current i(t) 58 is measured. A time varying electric field E(t) may be computed from the measured i(t) and a Fourier transform may be done to derive the frequency response F(s) of E(t). The system output for further processing may be in the form of the above mentioned frequency response F(s), time varying electric field E(t), or the time varying current i(t).

[0036] The first and the second photoconductive antennas 42, 52 may each be individually excited in continuous wave (CW) or pulsed modes. In the "CW generation mode", the two CW lasers are used to excite the first photoconductive antenna 42 for generating CW terahertz radiation. In the "CW detection mode", the two CW lasers are used to excite the second photoconductive antenna 52 for detecting terahertz radiation. In the "pulsed generation mode", the pulsed wave laser is used to excite the first photoconductive antenna 42 for generating pulsed terahertz radiation. In the "pulsed detection mode", the pulsed laser is used to excite the second photoconductive antenna 52 for detecting terahertz radiation. The operator may select the modes of terahertz generation and detection based on sampling requirements such as resolution and frequency range.

[0037] In the case of the CW generation mode (using the two CW lasers 23 and 24 for exciting the first photoconductive antenna), the two CW laser beam are configured to have a difference between their central frequencies that falls in the terahertz spectrum, and are preferably configured so that combined CW beam impinges onto the first photoconductive antenna 42 over free space. The generated terahertz radiation 14 will generally have a frequency equal to the difference frequency of the combined CW beam. The conductivity of the photoconductor antenna 42 will increase due to the excitation of electrons moving from a valence band to a conduction band from the laser excitation. This will be explained in more detail below.

[0038] In the case of pulsed generation mode (using pulsed wave laser 25 for exciting the first photoconductive antenna), a pulse that contains a range of frequencies (according to the Fourier synthesis of a pulse waveform) is used to modulate the conductance of the photoconductive antenna at a range of frequencies. In turn, the generated terahertz radiation 14 will contain a wide spectrum of terahertz frequencies. The actual range of the frequencies may be controlled by varying the pulse width of the pulsed wave laser. This approach offers generally lower resolution than the CW generation mode, since the CW generation mode can generate terahertz radiation of a narrower spectrum at high power whereas the pulsed generation mode will generate a wide spectrum, low resolution terahertz radiation. However, it may save time in comparison to the CW generation mode because the operator need not repeatedly adjust the frequencies of the CW lasers (or the frequency of one of the two CW lasers) in order to sweep across a desirable spectrum to obtain a complete frequency response.

[0039] In a preferred embodiment, the dual mode system 10 is configured so that the same CW or pulsed lasers are used for both the pump beam for photoconductive antenna 42 and the probe beam for photoconductive antenna 52 for synchronization and correlation purposes. Alternatively, the system 10 could be configured so that different lasers are used for the exciting the first and second photoconductive antennas 42, 52. For example, an operator may use the two CW lasers 23 and 24 for generating the pump beam for exciting the first photoconductive antenna 42 for terahertz radiation, while using two different CW lasers (not shown) for the probe beam to excite the second photoconductive antenna 52 for terahertz detection. The operator may use optional time delay elements to control the timing for providing the probe beam and the pump beam to the photoconductive antennas for coherent generation and detection.

[0040] Overall, the dual mode terahertz system of the present invention, having both a pulsed mode of operation and CW mode of operation, provides a number of advantages over prior art systems. For example, an operator can conduct a first round of testing using the pulsed generation mode to obtain a low- resolution, wide spectrum response of the sample. Then, after analyzing the system output from the first round of testing, the user may focus on a more relevant frequency range with the continuous wave laser set up in a second round of testing in CW generation mode to obtain a narrow spectrum but high resolution response of the sample.

[0041] Referring now to Figure 2, illustrated there is a dual mode terahertz radiation transmission and detection system 80 made in accordance with another embodiment of the invention. The dual mode system 80 comprises two continuous wave (CW) lasers 81 and 82 for generating CW laser beam 105 and 106 generally operating at f1 and f2, respectively, and a wavelength tuning controller 84 for adjusting the wavelength of one of the two CW laser beams. The two laser beams 105 and 106 are routed by fiber optics into a fiber power combiner 85 where the two CW laser beams 105 and 106 are combined and split to form two combined CW laser beam 108a and 108b that are coupled into two output fibers. One of the two combined CW laser beams 108a is used for selectively exciting a first photoconductive antenna 94, depending on user selection. The other combined CW laser beam 108b is fed into a fiber power splitter 87 where the combined CW laser beam 108b is split again into two combined CW laser beams 109a and 109b for an optical spectrum analyzer 104 and for selectively exciting a second photoconductive antenna 96 respectively. Fiber beam collimators 86 are used to couple the combined CW laser beams 108a and 109a from the optical fibers into free space by collimating the light rays.

[0042] Dual mode system 80 also comprises a pulsed wave laser 83 for generating a pulsed laser beam 107 that selectively generate and detect terahertz radiation in pulsed mode. The pulsed laser beam 107 is split into two split pulsed laser beams 107a and 107b by a beam splitter 89. The beam 107b fed to a flip mirror 88a and the beam 107a is fed to a flip mirror 88b. The flip mirrors 88a and 88b can move back and forth between a first and second position upon receiving user input. For example, the flip mirrors 88a and 88b may be hingedly connected to a fixture for a user to either leave the flip mirror 88a and 88b in the position as shown or to flip it out of the position currently shown. In this case, terahertz radiation 98 will be generated in CW mode if flip mirror 88a is left in its current position because the split pulsed wave laser beam 107b will be blocked while the combined CW laser beam 107a will be reflected and eventually impinged onto the first photoconductive antenna 94 as a pump beam for generating CW terahertz radiation. However, if the user flips mirror 88a out of its current position as shown, the split pulsed wave laser beam 107b will be reflected upward and used as a pump beam to generate pulsed terahertz radiation instead. Similarly for terahertz radiation detection, a user may select between CW detection mode and pulsed detection mode by manipulating the position of the flip mirror 88b.

[0043] Short focal length lenses 91 are optionally used for focusing the pump and probe laser beams onto the photoconductive antennas 94 and 96 respectively. A mirror 90 may be used to redirect the probe beam optical path to direct the probe laser beam onto the lens 91. A motorized translational stage 99 controlled by a computer 103 and a retro-reflector mirror 100 are used to change the optical path delay in the probe beam path for coherent detection of incident THz wave 98 by the photoconductive antenna 96. This can be done by increasing the probe beam optical path by moving the retro-reflector mirror 100 further away from the direction of the incoming probe beam. By using the motorized translational stage 99 to introduce delay in the probe beam path, an operator can bring the probe beam to the receiver photoconductive antenna 96 with different time delays with respect to the incident THz wave 98, which makes it possible to record the samples of the incident THz wave 98 at the lock-in at sub-picosecond time intervals and reconstruct the THz electric field. A chopper 92 may be used to modulate the incident pump beam for lock-in detection with a lock-in amplifier 102. Alternatively, one can use a modulated bias voltage on the transmitter photoconductive antenna 94 and remove the chopper from the dual mode system 80.

[0044] The photoconductive antenna 94 requires a DC voltage bias and a pump beam in order to generate terahertz radiation 98. The terahertz radiation 98 may be collimated by a hyper-hemisphere silicon lens 95 and routed to the second photoconductive antenna 96 using a plurality of off axis mirrors 97. The photoconductive antenna 96 will induce a time varying voltage upon receiving the terahertz radiation 98, in addition to receiving the excitation of a probe laser beam, the photoconductive antenna 96 will generate a time varying current. This will be explained in detail below. (See Figure 5) The time-varying current is amplified by a current amplifier 101 and then fed through the lock-in amplifier 102 before being analyzed by a computer 103. In some embodiments, the computer 103 may receive user input and control the position of the flip mirrors 88a and 88b by known means such as servomotors and stepper motors (not shown in Figure 2). [0045] Referring now to Figure 3, illustrated there is a dual mode terahertz radiation transmission and detection system 110 made in accordance with a further embodiment of the invention. The system 110 is different from the system 80 shown in Figure 2 in that system 110 does not include a chopper 92 for modulating the incident pump beam for lock-in detection. A power supply 114 for modulating the required voltage bias is used instead. The power supply 114 may be a signal generator which can generate a voltage signal with varying frequency and varying amplitude. Furthermore, there is provided only one flip mirror 88 in system 110 for user to select from either a) CW generation and detection modes using the CW pump and probe beam or b) pulsed wave generation and detection modes using the pulsed wave pump and probe beam. System 110 may offer less flexibility but also uses fewer components. Other arrangements and design variations can be made following the fundamental principles, known technologies and design requirements.

[0046] Referring now to Figures 4 and 5, illustrated therein is an exemplary photoconductive antenna 120 that could be used with the dual mode terahertz systems of the present invention. Photoconductive antenna 120 has a single dipole structure, comprising two electrodes 121 and 122 having a gap 123 between for receiving the laser excitation. Varying the gap dimensions may vary the performance and other design measurements.

[0047] As shown in Figure 5, the photoconductive antenna 120 comprises a substrate 128, a heat spreader epilayer 127, a photoconductor layer 126, a first electrode 121 , and a second electrode 122. The substrate 128, heat spreader epilayer 127 and the photoconductor layer 126 may together be referred to as the photoconductive substrate. A photoconductive substrate may not include a heat spreader epilayer 127 for covering underneath the photoconductor layer 126 and dissipating heat. In operation, suitable laser excitation as described above, such as two CW laser beams 124 and 125 as shown, can be used to impinge onto the electrode gap 123 between the two electrodes 121 and 122 in order to modulate the conductivity of the photoconductor antenna 120, causing the conductivity of the photoconductor layer 126 to increase due to the excitation of electrons moving from a valence band to a conduction band. Current may then be generated which results in the generation of terahertz radiation, due to the presence of an electric field 129 caused by a voltage bias as shown in Figure 4, where electrode 121 holds a positive charge and electrode 122 holds a negative charge. The photoconductive material used for making the photoconductor layer 126 and the substrate 128 may be GaAs, InxGai-xAs ion implanted GaAs, low-temperature-grown GaAs, low- temperature-grown InxGai-xAs, or other known photoconductive materials.

[0048] Figure 6 illustrates another exemplary photoconductive antenna 140 for use with the terahertz systems of the present invention, which is generally similar to photoconductive antenna 120, except that the photoconductive antenna 140 comprises a pair of electrodes 141 and 142 comprising an array of dipole antenna structures, having a gap 143 formed therebetween. Photoconductive antennas having various other types of electrode structures, such as a pair of interdigitated electrodes or a pair of spiral electrodes, small or large aperture antennas in the form of single element or array configuration, or dipole or spiral antennas in the form of single element or array configuration, or other known photoconductive antenna configurations, could also be used as the photoconductor antennas of the subject terahertz systems.

[0049] In some embodiments, the terahertz transmitter and the terahertz receiver could each comprise a single photoconductive antenna, which is configured to be suitable for receiving either the CW or pulsed pump or probe beams, and generating either continuous wave terahertz radiation or pulsed terahertz radiation. In other embodiments, the terahertz transmitter and the terahertz receiver could each comprise multiple interchangeable photoconductive antennas in the form of THz integrated circuits mounted on a translational stage, which can be selectively placed into or out of the path of the pump or probe beam. With such an interchangeable configuration, the operator can switch between the THz integrated circuits optimized for pulse or CW modes and/or optimized to cover a particular portion of the THz spectrum, by moving the translational stage.

[0050] Figure 7, 8 and 9 illustrate the terahertz waves generated by a dual mode terahertz radiation transmission and detection system made in accordance with an embodiment of the subject invention. In this embodiment of the invention, the transmitter and receiver photoconductive antennas are two identical dipole antennas with 80 μm arm length and 6 μm gap between the two electrodes, each antenna similar to the antenna shown in Figure 4. Each antenna may be manufactured without interdigitated sub-micron finger structures using a relatively simple fabrication process at a relatively low cost. The minimum feature size is 6 μm, which can be realized by a simple lithography method. To obtain the measurement, a pulsed wave light source comprising a femto-second pulsed laser is introduced to the system to generate and detect a THz pulse with a wide spectrum extending up to 3.5 THz. Then, the light source is switched to a CW light source comprising two CW lasers, and without changing the transmitter and receiver modules, and without changing the alignment of other components, a CW THz signal was generated and detected.

[0051] Figure 7 and 8 illustrate the electric field E(t) of a detected pulsed THz radiation in time and frequency domain respectively, obtained in pulsed generation mode. The first transmitting photoconductive antenna has an 80 μm dipole with 6 μm gap with ±5 VDC modulated bias and 14 mW of pump power. The second receiving photoconductive antenna is a 80 μm dipole with 6 μm gap with 13 mW probe power. With 5 V DC voltage bias applied to the first transmitting photoconductive antenna, the DC photocurrent generated at the transmitting photoconductive antenna is 0.36 μA which renders a current of 0.96 μA at the receiving photoconductive antenna. The DC current measurements are done for alignment purpose, and for that a 5 VDC bias and laser power for excitation is placed on the photoconductive antennas to maximize the DC photocurrent. This is done for both the transmitter and the receiver photoconductive antennas to make sure the input laser beams are properly focused on the electrode gap. In this measurement, the maximum DC photocurrent at the transmitter photoconductive antenna is 0.36 μA for 14 mW of pump optical power before pump beam modulation. The maximum DC photocurrent at the receiver end is 0.96 μA for 13 mW of probe optical power before probe beam modulation. Note that modulation devices such as a chopper may be used for both the transmitter and the receiver photoconductive antennas for alignment measurements. The difference in two DC photocurrents can be caused by differences in the responsiveness of the photoconductive antennas. The stage speed is 20 μm/s and lock-in time constant is 100 ms. A THz peak radiation is measured to be 16.9 nA (329mV on lock-in with 50 nA/V gain on current amplifier). The spectrum contains electromagnetic energy up to 3 THz with all water absorption lines appearing as narrow dip lines over the spectrum. The water lines are crowded above 1 THz as shown in Figure 8. The signal goes under the noise level above 3 THz.

[0052] Referring now to Figure 9, shown therein is the electric field E(t) of a detected CW THz radiation generated in CW generation mode. The first transmitting photoconductive antenna is an 80 μm dipole with 6 μm gap with ±5 VDC modulated bias and 28 mW pump power. The receiving photoconductive antenna is a 80 μm dipole with 6 μm gap with 20 mW probe power. With 5 V DC voltage bias applied to the first transmitting photoconductive antenna, the DC photocurrent generated at the transmitting photoconductive antenna is 1.6 μA which causes a current of 1.75 μA at the receiving photoconductive antenna. Similarly, for alignment purposes, note that for a 5 VDC bias, the maximum DC photocurrent in transmitter photoconductive antenna is 1.6 μA for 28 mW of pump optical power before pump beam modulation, and the maximum DC photocurrent is 1.75 μA at the receiver photoconductive antenna for 20 mW of probe optical power before probe beam modulation. The stage speed is 10 μm/s and lock-in time constant is 1 s. The two CW lasers used operate at around λ=780 nm wavelength and the wavelength difference of the two CW lasers is Δλ=0.66 nm, which is equivalent to a 330 GHz beat frequency. The period of a 330 GHz signal is around 3.03 ps, which coincides with the period of the detected terahertz radiation shown in Figure 9. [0053] Referring now to Figure 10, illustrated therein is a method 130 for analyzing a sample having a terahertz spectrum, according to one embodiment of the invention. The method beings at step 132 where a dual mode terahertz spectrometer having a pulsed terahertz wave generation mode and a continuous wave terahertz generation mode is provided. In step 134, the dual spectrometer is operated in the pulsed terahertz wave generation mode, generating pulsed terahertz radiation and transmitting it through the sample. The method 130 then proceeds to step 136 where the transmitted pulsed terahertz radiation is received and analyzed to identify a portion of the terahertz spectrum in which an absorption signature of the sample is located. In step 138, the dual mode spectrometer is switched to continuous wave terahertz radiation generation mode. In step 140, a high-resolution scan is performed over a small range of frequencies around the absorption signature determined in step 136.

[0054] Referring now to Figure 11 , illustrated therein is a method 150 for analyzing a sample, using a dual mode terahertz spectrometer made in accordance with an embodiment of the present invention. [0055] The method 150 begins at step 152 wherein dual mode terahertz spectrometer, comprising a continuous wave light source, and a pulsed wave light source, is provided. The continuous wave light source comprises a first continuous wave laser operating at a first frequency and a second continuous wave laser operating at a second frequency, wherein the first and second frequency differ by a difference frequency. The pulsed wave light source comprises a pulsed wave laser wherein the pulse width is generally in the femto-second range.

[0056] The method proceeds to step 154 where wide-spectrum pulsed terahertz radiation is generated, using the pulsed wave light source. The wide- spectrum pulsed terahertz radiation may be generated by applying a voltage bias to a first dual-mode photoconductive antenna and coupling the voltage biased antenna to the pulse wave light source to generate a wide-spectrum terahertz radiation at a range of terahertz frequency generally inversely related to the pulse width of the pulse wave light source and related to material properties of the photoconductive material and radiation properties of the antenna structure.

[0057] At step 156, the wide-spectrum pulsed terahertz radiation is transmitted through or reflected off a sample to be analyzed, and at step 158, the transmitted or reflected wide-spectrum pulsed terahertz radiation is received by a second photoconductive antenna and a sample-influenced time-varying voltage is induced.

[0058] At step 160, a low-resolution, wide-spectrum, sample-influenced time varying current correlated to the time-varying voltage is generated by coupling the time varying voltage biased second photoconductive antenna either to the continuous wave light source to increase its conductance or to the tunable pulse wave light source to increase its conductance.

[0059] The low-resolution, wide-spectrum, sample-influenced time varying current is analyzed in step 162 to determine narrower ranges of frequencies of interest.

[0060] Once the frequencies of interest is determined, the method 150 proceeds to step 164 where the tunable continuous wave light source is adjusted such that the difference frequency of the first and second continuous wave laser generally cover the previously determined narrower ranges of frequencies of interest.

[0061] A narrow-spectrum continuous wave terahertz radiation is then generated at step 166 by coupling the voltage biased first dual-mode photoconductive antenna to the tunable continuous wave light source to generate the narrow-spectrum terahertz radiation. The generated radiation generally tends to be at the difference frequency of the first and second continuous wave lasers.

[0062] At step 168, the generated narrow-spectrum continuous terahertz radiation is transmitted or reflected off through the sample, and at step 170, the transmitted or reflected narrow-spectrum continuous terahertz radiation is received by the second photoconductive antenna to induce a sample-influenced time-varying voltage.

[0063] The method 150 then proceeds to step 172 where a high-resolution, narrow-spectrum, sample-influenced time varying current is generated by coupling the time varying voltage biased second dual-mode photoconductive antenna either to the tunable continuous wave light source to increase its conductance or to the tunable pulse wave light source to increase its conductance.

[0064] The desirable high-resolution, narrow-spectrum, sample-influenced time varying current is analyzed at step 174.

[0065] Referring now to Figure 12, illustrated therein is a dual mode terahertz radiation transmission and detection system 60 made in accordance with an alternative embodiment of the invention. The system 60 is generally similar to the system 10 except that the continuous wave light source comprises a quasi- continuous wave light source 61 that provides a continuous wave light beam over a short time period. The quasi CW light source comprises a first pulsed wave laser 63 for generating a first pulsed wave laser beam 73, a second pulsed wave laser 64 for generating a second pulsed wave laser beam 74, a beam splitter 87, and a pulse stretcher 65. The system 60 further comprises a beam combiner/splitter 85, and beam selector 66 comprising a first selector 66a and a second selector 66b with optional time delay elements as shown in the figure. In operation, the second pulsed wave laser beam 74 is split by beam splitter 87 into a first split second pulsed laser beam 74a, a second split second pulsed laser beam 74b, and a third split second pulsed laser beam 74c. The pulse stretcher 65 receives the first pulsed laser beam 73 and the first split second pulsed laser beam 74a. The pulse stretcher 65 stretches the pulsed laser beams 73 and 74a, which may typically have femtosecond pulse width, into long pulsed laser beams having pulse width generally in the pico-second range. This results in the generation of two quasi-continuous laser beams (not shown in figure) with time periods in the pico-second range, which may be treated like CW laser beams. The two quasi-continuous laser beams are combined to form a combined quasi-continuous wave beam having a difference frequency in the terahertz range, and then split to form two combined quasi-CW laser beams 75a and 75b, by combiner/splitter 85. The combined quasi-CW laser beams 75a and 75b are routed to the first selector 66a and the second selector 66b, respectively. The other two split second pulsed laser beams 74b and 74c are routed to the first selector 66a and the second selector 66b, respectively, in order for the user to select between the quasi-CW mode with the combined quasi-CW laser beams 75a and 75b, and the pulsed mode with the pulsed wave laser beams 74b and 74c as shown. Note that for the pulsed mode, any suitable pulsed laser beam may be used. For example, pulsed wave laser beam 73 may be split into three for connecting to the pulse stretcher 65, the first selector 66 and the second selector 67 instead of the second pulsed wave laser beam 74. A plurality of separate pulsed wave laser sources may be used.

[0066] Referring now to Figure 13, illustrated therein is a dual mode terahertz radiation transmission and detection system 200 made in accordance with another alternative embodiment of the invention. The system 200 differs from the system 80 shown in Figure 2, in that the laser beams from the continuous wave lasers and the short pulse laser are carried by optical fibers to the point where the laser beams can be interchangeably coupled to the terahertz transmitter and the terahertz receiver modules through fiber optic connectors. The pulsed laser beam from short pulse laser 83 is carried by optical fiber 201 to splitter 87 where the beam is split into a pulsed pump beam and a pulsed probe beam. The pulsed pump beam is carried by optical fiber 203 to dispersion compensator 220 and by optical fiber 207 to connector 230 of transmitter head 250. The pulsed probe beam is carried by optical fiber 206 to dispersion compensator 222 and by optical fiber 208 to connector 232 of the receiver head 252. The continuous wave laser beams from CW lasers 81 , 82 are carried by optical fibers 211 , 212 to combiner/splitter 85, where the beams are combined and split into a CW pump beam and a CW probe beam. The CW pump beam is carried by optical fiber 217 to connector 230 of transmitter head 250, and the CW probe beam is carried by optical fiber 213 to splitter 87 and by optical fiber 218 to connector 232 of receiver head 252.

[0067] The optical fiber connectors 230, 232 may have varying grades of polish. For example, the connectors 230, 232 could be "FC/PC" designated connector for physical contact connector or "FC/APC" designated connector for angle polished connectors. [0068] The dispersion compensation modules 220, 222 can be used to compensate for the dispersion and pulse broadening effects of the optical fibers carrying the pump and probe laser beams. Dispersion compensation optical fibers may also be used for the same purpose.

[0069] The system 200 also differs from the system 80 in that the terahertz receiver head 252 and the off-axis parabolic mirror 242 (mirror#2) that focuses the terahertz beam on the receiver head 252 are placed on a motorized translation stage 245. By moving the stage 245, the path difference between the pump beam and the probe beam changes, which results in a delay change between the probe beam and the received terahertz signal. Alternatively, the transmitter head 250 and the corresponding off-axis mirror 241 (mirror #1) can be placed on a motorized translation stage (not shown) to change the delay between the probe beam and the received terahertz signal.

[0070] Referring now to Figure 14, the receiver head 250 and the transmitter head 252 may take the form of a fiber coupled receiver/transmitter module 260, which may be used to generate and transmit or receive terahertz frequency radiation. The receiver/transmitter module 260 comprises a transmitter/receiver chip 268 for generating and receiving terahertz radiation, which is housed in a housing 261 having an optical fiber connector 262 for connecting to an optical fiber connector head 265 of an optical fiber 263. The module 260 also comprises a laser beam collimating lens 264 for collimating the laser beam carried by the optical fiber 263, a laser beam focusing lens 266 for focusing the collimated laser beam on the transmitter/receiver chip 268, a connector 270 for connecting the transmitter/receiver chip 268 to the power supply 93 or the current amplifier 101 shown in Figure 13, and a silicon lens 272 through which the terahertz radiation may be transmitted or received by the chip 268. The transmitter/receiver chip 268 may comprise one of the photoconductive antennas shown in Figures 4 - 6, or small or large aperture antennas in the form of single element or array configuration, dipole or spiral antennas in the form of single element or array configuration, or other known photoconductive antenna configurations. The connector 262 may alternatively comprise a FC switch having a plurality of inputs that can be connected to a plurality of optical fibers.

[0071] In use, when module 260 is configured to function as the transmitter head 252, the operator can selectively connect either the optical fiber 207 carrying the pulsed pump beam or the optical fiber 217 carrying the continuous wave pump beam to the optical fiber connector 262. The pump beam carried by the optical fiber 207 or 217 is collimated through collimating lens 264, and focused on the transmitter/receiver chip 268. Terahertz radiation 274 is then generated and transmitted by the chip through the silicon lens 272.

[0072] If the module 250 is configured to function as the receiver head 252, the operator can selectively connect either the optical fiber 208 carrying the pulsed probe beam or the optical fiber 218 carrying the continuous wave probe beam to the optical fiber connector 262. Terahertz radiation 276 is received through silicon lens

272 by the transmitter/receiver chip 268. A time varying current is induced by the received terahertz radiation 276 within the chip 268, using the probe beam carried by the optical fiber 208 or 218. The induced current may then been connected to a current amplifier 101 through the connector 270. [0073] While the above description includes a number of exemplary embodiments, it should be apparent to those skilled in the art that changes and modifications can be made to these embodiments without departing from the present invention, the scope of which is defined in the appended claims.

Claims

CLAIMS:

1. A terahertz radiation transmission and detection system, comprising: a) a dual mode light source operable in a continuous wave mode and in a pulsed wave mode, wherein the dual mode light source is configured for generating a continuous wave light beam when operated in the continuous wave mode, and for generating a pulsed light beam when operated in the pulsed wave mode; b) a beam selector for generating a pump beam comprising either the continuous wave light beam or the pulsed light beam; c) a terahertz transmitter configured for receiving the pump beam and for generating terahertz radiation, wherein the terahertz radiation comprises continuous wave terahertz radiation when the pump beam comprises the continuous wave light beam, and wherein the terahertz radiation comprises pulsed terahertz radiation when the pump beam comprises the pulsed light beam; and d) a terahertz receiver configured for receiving the continuous wave terahertz radiation and the pulsed terahertz radiation.

2. The system defined in claim 1 , wherein the dual mode light source comprises a continuous wave light source for generating the continuous wave light beam and a pulsed light source for generating the pulsed light beam.

3. The system defined in claim 2, wherein the continuous wave light source comprises a first continuous wave laser for generating a first continuous wave laser beam at a first frequency and a second continuous wave laser for generating a second continuous wave laser beam at a second frequency, the first and the second frequency differing by a difference frequency suitable for generating terahertz waves, and a combiner for combining the first continuous wave beam and the second continuous wave beam to form a combined continuous wave laser beam having a component at the difference frequency.

4. The system defined in claim 3, wherein at least one of the first continuous wave laser and the second continuous wave laser is tunable around a central wavelength.

5. The system defined in claim 3, wherein the pulsed light source comprises a pulsed laser that generates a pulsed laser beam with a pulse width suitable for generation of terahertz radiation having a range of frequencies.

6. The system defined in claim 2, wherein the beam selector comprises at least one flip mirror positioned in an optical path of the combined continuous wave light beam and the pulsed wave light beam, wherein the flip mirror is configured to be flipped back and forth between a first position and a second position upon receiving user input, and wherein when the flip mirror is flipped in the first position the combined continuous wave light beam is reflected towards the terahertz transmitter and the pulsed wave light beam is directed away from the terahertz transmitter, and when the flip mirror is flipped in the second position, the pulsed wave light beam is reflected towards the terahertz transmitter and the combined continuous wave light beam is directed away from the terahertz transmitter.

7. The system defined in claim 2, wherein the beam selector comprises a first optical fiber for carrying the continuous wave light beam and a second optical fiber for carrying the pulsed wave light beam, and a fiber optic connector coupled to the terahertz transmitter, wherein the combined continuous wave light beam can be selected as the pump beam by connecting the first optical fiber to the fiber optic connector, and the pulsed light beam can be selected as the pump beam by connecting the second optical fiber to the fiber optic connector.

8. The system defined in claim 5, further comprising a beam splitter for splitting the combined continuous wave laser beam into a continuous wave pump beam and a continuous wave probe beam, and a second beam splitter for splitting the pulsed laser beam into a pulsed pump beam and a pulsed probe beam, wherein the terahertz receiver is configured for receiving the continuous wave pump beam and the continuous wave probe beam.

9. The system defined in claim 8, wherein the beam selector comprises a first selector for generating the pump beam, wherein the pump beam comprises either the continuous wave pump beam or the pulsed pump beam, and a second beam selector for generating a probe beam, wherein the probe beam comprises either the continuous wave probe beam or the pulsed probe beam.

10. The system defined in claim 9, wherein the first beam selector and the second beam selector are operable such that when the pump beam comprises the continuous wave pump beam, the probe beam comprises the continuous wave probe beam, and when the pump beam comprises the pulsed pump beam, the probe beam comprises the pulsed probe beam.

11. The system defined in claim 5, wherein the terahertz transmitter comprises a transmitting photoconductive antenna having a voltage bias applied thereto for receiving the pump beam and for generating the terahertz radiation, the transmitting photoconductive antenna being configured such that when the pump beam comprises the combined continuous wave laser beam, the terahertz radiation comprises continuous wave terahertz radiation having a frequency equal to the difference frequency of the combined continuous wave laser beam, and when the pump beam comprises the pulsed laser beam, the terahertz radiation comprises pulsed terahertz radiation having a range of frequencies generally inversely related to the pulse width of the pulsed laser beam.

12. The system defined in claim 11 , wherein the terahertz receiver comprises a receiving photoconductive antenna for receiving the terahertz radiation, the receiving photoconductive antenna having a conductance that is modulated when the antenna is coupled to either the combined continuous wave laser beam or the pulsed wave laser beam such that a time varying voltage is induced in the receiving photoconductive antenna by the received terahertz radiation for generating a time varying current.

13. The system defined in claim 9, further comprising a time delay device for selectively delaying the pump beam or the probe beam.

14. The system defined in claim 13, wherein the time delay device comprises a mirror mounted on a motorized translation stage placed in an optical path of the pump beam or the probe beam, the translation stage being configured move the mirror along the optical path of the beam so as to change the optical path of the beam.

15. The system defined in claim 1 , further comprising a modulator in the optical path of the pump beam, the modulator being configured to modulate the pump beam.

16. The system defined in claim 15, wherein the modulator comprises a beam chopper.

17. The system defined in claim 11 , wherein the transmitting photoconductive antenna comprises a substrate, a photoconductive layer applied to the substrate, a first electrode applied to the photoconductive layer, and a second electrode applied to the photoconductive layer, the second electrode being spaced from the first electrode so as to form a gap between the first electrode and the second electrode, the gap being configured for receiving the pump beam and for generating both the continuous wave terahertz radiation and the pulsed terahertz radiation.

18. The system defined in claim 17, wherein the first electrode and the second electrode comprise dipole structures, each of the dipole structures having a transversely extending arm portion of pre-selected length, the arm portion being positioned so as to define a gap between the electrodes of pre-selected length, the gap being configured for receiving the pump beam and for generating both the continuous wave terahertz radiation and the pulsed terahertz radiation.

19. The system defined in claim 18, wherein the arm portion of each of the first electrode and the second electrode has a length of about 80 microns, and the gap is about 6 microns.

20. A terahertz radiation transmission and detection system, comprising a) a continuous wave light source for generating a continuous wave light beam, wherein the continuous wave light source comprises a first continuous wave laser for generating a first continuous wave laser beam at a first frequency and a second continuous wave laser for generating a second continuous wave laser beam at a second frequency, the first and the second frequency differing by a difference frequency suitable for generating terahertz waves, and a combiner for combining the first continuous wave laser beam and the second continuous wave laser beam to form a combined continuous wave laser beam having a component with the difference frequency; b) a pulsed wave light source for generating a pulsed light beam, wherein the pulsed wave light source comprises a pulsed laser that generates a pulsed laser beam with a pulse width generally in the femto-second range for generation of terahertz radiation having a range of frequencies; c) a first beam splitter for splitting the combined continuous wave laser beam into a continuous wave pump beam and a continuous wave probe beam; d) a second beam splitter for splitting the pulsed laser beam into a pulsed pump beam and a pulsed probe beam; e) a first beam selector for generating a pump beam comprising either the continuous wave pump beam or the pulsed pump beam; f) a terahertz transmitter for receiving the pump beam and for generating terahertz radiation, wherein the terahertz transmitter comprises a transmitting photoconductive antenna having a voltage bias applied thereto for receiving the pump beam and for generating the terahertz radiation, the transmitting photoconductive antenna being configured such that when the pump beam comprises the combined continuous wave laser beam, the terahertz radiation comprises continuous wave terahertz radiation having a frequency equal to the difference frequency of the combined continuous wave laser beam, and when the pump beam comprises the pulsed laser beam, the terahertz radiation comprises pulsed terahertz radiation having a range of frequencies generally inversely related to the pulse width of the pulsed laser beam; g) a second beam selector for generating a probe beam comprising either the continuous wave probe beam or the pulsed probe beam; and h) a terahertz receiver for receiving the probe beam and the continuous terahertz radiation and the pulsed terahertz radiation.

21. The system defined in claim 1 , wherein the dual mode light source comprises a quasi continuous wave light source for generating the continuous wave light beam over a short time period, and a pulsed light source for generating the pulsed light beam, wherein the quasi continuous wave light source comprises a first pulsed laser for generating a first pulsed laser beam, a second pulsed laser for generating a second pulsed laser beam, a beam splitter for splitting the second pulsed laser beam into a first split second pulsed laser beam and a second split second pulsed laser beam, and a pulse stretcher for stretching the first pulsed laser beam and the first split second pulsed laser beam into quasi continuous wave laser beams, and wherein the pulsed light source comprises the second split second pulsed laser beam.

22. The system defined in claim 21 , also comprising a beam combiner for combining the quasi continuous wave laser beams into a combined quasi continuous laser beam having a difference frequency in the terahertz range, wherein the beam selector generates a selected beam comprising either the combined quasi continuous wave beam or the second split second pulsed light beam.

23. A terahertz radiation transmission and detection system, comprising a) a pulsed laser source consisting of a first femto-second pulsed laser for generating a first pulsed light beam, and a second pulsed laser for generating a second pulsed light beam, wherein at least one of the first pulsed laser and the second pulsed laser is tunable around a central wavelength; b) a pulse stretcher for stretching the first pulsed laser beam into a quasi continuous wave laser beam; c) a beam selector for generating a selected beam comprising either the quasi continuous wave beam or the pulsed light beam; d) a terahertz transmitter for receiving the selected beam and for generating terahertz radiation; and e) a terahertz receiver for receiving the terahertz radiation.

24. A method for analyzing a sample having a terahertz spectrum, comprising the steps of: a) providing a dual mode spectrometer having a pulsed terahertz wave generation mode that generates pulsed terahertz radiation and a continuous wave terahertz generation mode that generates continuous wave terahertz radiation; b) operating the dual mode spectrometer in pulsed terahertz wave generation mode so as to transmit or reflect the pulsed terahertz radiation through or off the sample; c) identifying a portion of the terahertz spectrum in which an absorption signature of the sample is located, and d) switching the dual mode spectrometer to the continuous wave terahertz radiation generation mode; and e) performing a high resolution scan over a small range of frequencies around the absorption signature.

25. A method for analyzing a sample having a terahertz spectrum, comprising the steps of: a) providing a dual mode terahertz spectrometer comprising a continuous wave light source having a first continuous wave laser operating at a first frequency and a second continuous wave laser operating at a second frequency, wherein the first frequency and the second frequency differ by a difference frequency, and a pulsed wave light source having a pulsed wave laser wherein the pulse width is generally in the femto-second range; b) generating wide-spectrum pulsed terahertz radiation by coupling a voltage biased first photoconductive antenna to the pulsed wave light source to generate a wide-spectrum terahertz radiation at a range of terahertz frequency generally inversely related to the pulse width of the pulse wave light source and related to material properties of the photoconductive material and radiation properties of the antenna structure; c) transmitting or reflecting the wide-spectrum pulsed terahertz radiation through or off a sample to be analyzed; d) receiving the transmitted or reflected wide-spectrum pulsed terahertz radiation by a second photoconductive antenna to induce a sample- influenced time-varying voltage; e) generating a low-resolution, wide-spectrum, sample-influenced time varying current correlated to the time-varying voltage by coupling the time varying voltage biased second photoconductive antenna either to the continuous wave light source to increase its conductance or to the tunable pulse wave light source to increase its conductance; f) analyzing the low-resolution, wide-spectrum, sample-influenced time varying current for determining narrower ranges of frequencies of interest; g) adjusting the tunable continuous wave light source such that the difference frequency of the first and second continuous wave laser generally cover the previously determined narrower ranges of frequencies of interest; h) generating a narrow-spectrum continuous wave terahertz radiation by coupling the voltage biased first dual-mode photoconductive antenna to the tunable continuous wave light source to generate the narrow-spectrum terahertz radiation generally at the difference frequency of the first and second continuous wave lasers; i) transmitting or reflecting the narrow-spectrum continuous terahertz radiation through or off the sample; j) receiving the transmitted/reflected narrow-spectrum continuous terahertz radiation by the second photoconductive antenna to induce a sample- influenced time-varying voltage; k) generating a high-resolution, narrow-spectrum, sample-influenced time varying current by coupling the time varying voltage biased second dual-mode photoconductive antenna either to the tunable continuous wave light source to increase its conductance or to the tunable pulse wave light source to increase its conductance; and

I) analyzing the desirable high-resolution, narrow-spectrum, sample influenced time varying current.