The injection-seeded terahertz wave parametric generator (is-TPG) technology presented here is expected to enable measurements through thick shielding due to its wide dynamic range of over 120 dB in combination with detection methods that up-convert the terahertz wave to a near-infrared beam. Furthermore, recently machine learning has been applied to the fingerprint spectral analysis of reagents, which has enhanced identification accuracy. Real-time measurement has also been achieved by multi-wavelength generation and detection with image recognition of detection Stokes beams. We will present these improvements in is-TPG technology for non-destructive testing.
We achieved to extract terahertz (THz)-waves generated by cascade process (i.e., higher-order THz-waves) in an injection-seeded THz parametric generator (is-TPG), and realized higher-power output.
Because of the angular phase-matching condition, the higher-order THz-waves are generated deep in a crystal and extraction of them from the crystal was difficult due to the large absorption loss. In our setup, we used total reflection of the pump beam on the crystal surface to reduce absorption loss, and optimized the THz focusing lens and the position of Si-prism coupler. As a result, we succeeded in extracting higher-order THz waves and improved the output power.
In this work, we demonstrated real-time spectroscopy using a discreetly and fast wavelength tunable injection-seeded THz wave parametric generator (is-TPG). In our is-TPG, pump and seed beams shine the MgO:LiNbO3 crystal to generate the THz wave. At that time, the wavelength was tuned by changing the wavelength of the seed beam. Therefore, in order to achieve real-time measurement, we introduced fast wavelength switchable ECDL that we developed recently using a digital micromirror device (DMD) as a wavelength selector. As a result of this improvement, we achieved to identify the reagents hidden in cardboard box in real-time.
We developed a fast wavelength switchable external cavity diode laser (ECDL) using a DMD as a seed laser for an injection-seeded terahertz (THz) wave parametric generator (is-TPG). Generally, the wavelength of an ECDL is selected by the angle of the mirror or grating; however, in this case, a DMD is used as the wavelength selector. A power output of 300 mW and a tunable range of 26 nm were obtained. Furthermore, high-rate modulation at 6.55kHz was realized. This laser makes it possible to switch the THz wavelengths of is-TPG rapidly to match the absorption spectrum of a reagent.
We have been developing an injection-seeded terahertz (THz) parametric generator (is-TPG), detector, and amplifier. This THz generation and detection scheme have allowed us to develop a wide-dynamic range THz wave spectrometer which can be used for nondestructive inspection of illicit drugs under thick envelopes. In this work, we have improved the sensitivity of the THz parametric detector drastically using a multistage configuration, which consists of the up-conversion (pre-amplifier) and the main amplifier parts. Using this new configuration, we were able to suppress the broadband noise, and only an upconverted detection beam can be measured using the NIR detector. As a result of this improvement, the minimum sensitivity reached 130 zJ (zJ=10-21J), which equals to about 90 photons at 1.05 THz. Moreover, the principle of this multistage detection can be used for high gain THz amplification which has more than 6 billion-fold amplification factor.
An injection-seeded THz-wave parametric generator (is-TPG) uses a near-infrared (NIR) pump beam and seed beam input to the MgO:LiNbO3 crystal. In contrast, the THz parametric detector uses a THz wave as the seed beam instead of a NIR beam. In the detection configuration, when the pump beam and THz wave are input into the crystal, the THz wave is upconverted to the NIR idler beam by parametric processes in the crystal that are measured using a NIR detector. This THz generation and detection scheme has allowed us to develop a high dynamic range THz wave spectroscopic system that can be used for spectroscopic imaging of saccharides hidden in thick envelopes. Recently, we have improved the sensitivity of the THz parametric detector drastically using a multistage configuration in order to suppress the spontaneous THz emission and enhance the gain. In our new system, the THz parametric detectors were divided into two parts, i.e., for the up-conversion (pre-amplifier) and for the main amplifier. THz waves were upconverted to a NIR idler beam in the first part of the set-up. An iris positioned behind the upconverted region passed the idler beam and blocked the broadband spontaneous THz emission. Thus, in the amplification region, only the desired idler beam was clearly amplified. The amplified detection idler beam was then measured by a NIR beam profiler.
The pharmaceutical industry requires a simple nondestructive quantitative inspection technique that does not change the characteristic properties of the target of inspection. Terahertz (THz) spectroscopy has potential as a nondestructive inspection technique because of the high transmittance of inspected materials and the absorption peaks related to some active pharmaceutical ingredients (APIs) in THz frequency regions. We have developed a compact terahertz spectroscopy system that uses an injection-seeded terahertz parametric generator (is-TPG) as a THz light source for the inspection and quantitative analysis of pharmaceutical tablets. Using this system, acetylsalicylic acid, acetaminophen, tranexamic acid, loxoprofen sodium, and caffeine included in over-the-counter (OTC) medicine tablets were identified as APIs by their specific absorption peaks. We also conducted a quantitative analysis of acetaminophen in one of the medicines by performing multivariate analyses. The root-mean-squared error of cross-validation (RMSECV) was 0.297 wt%. The limit of detection (LOD) and limit of quantitation (LOQ) were 0.990 wt% and 3.00 wt%, respectively. These results indicate that this is-TPG system could be applied as an inspection and quantitative analysis technique for OTC medicine tablets.
Recently, some studies have demonstrated that the sweat ducts present in the skin play a significant role in terahertz (THz) wave interaction with human beings. It was reported that the sweat ducts act as a low-Q-factor helical antenna due to their helical structure, and resonate in the sub-terahertz frequency range according to their structural parameters, such as helix diameter and helix length. According to the antenna theory, a helical antenna resonates in two different modes of operation known as normal mode and axial mode and the dimension of the helix plays a key role to determine the frequency of resonance. Therefore, here we performed the optical coherence tomography (OCT) of number of human subjects on their palm and foot to investigate the density, distribution and morphological features of sweat ducts. Moreover, we calculated the dielectric properties of human skin using terahertz time domain spectroscopy. Based on the structural parameters of human sweat ducts and its THz dielectric properties of surrounding medium, we computed the frequency of resonance of sweat duct in different modes of operation and we found that these ducts resonate in subterahertz frequency region. We believe that these findings will facilitate further investigation of the THz-skin interaction and provide guidelines for safety levels with respect to human exposure to electromagnetic waves at these frequencies.
A non-invasive, unstaining tissue measurement method is expected to be an important tool for regenerative medical. As THz wave do not affect biological tissue because of their low energy, THz measurement method is expected to become a new modality of biomedical analysis as a noninvasive diagnosis. Due to the fact that biological tissues possess high level of hydration, it results in strong absorption at terahertz frequencies. A ridge waveguide LiNbO3 based nonlinear terahertz generator was used to achieve high output power for THz time domain spectroscopy (THz-TDS). A ridge waveguide was designed for high efficiency emission from the LiNbO3 crystal by the electro-optic Cherenkov effect. This THz-TDS system has realized six orders of dynamic range, and the bandwidth of the spectrum reaches 7 THz in the upper limit. We measured a reflected terahertz pulse shape at the interface between a plastic culture dish and biological tissues. By studying the gradient of phase spectroscopy of reflected THz pulse, we have been able to differentiate between human fibroblast tissue and cancer tissue. We demonstrate the application of terahertz time domain spectroscopy pulse in reflection geometry for the non-distractive measurement of biological tissues cultured on a plastic culture dish. These results demonstrate the potential of terahertz phase information for the study of biological tissues.
Recently, some studies reported that the sweat ducts act as a low-Q-factor helical antenna due to their helical structure, and resonate in the terahertz frequency range according to their structural parameters. According to the antenna theory, when the duct works as a helical antenna, the dimension of the helix plays a key role to determine the frequency of resonance. Therefore, the accurate determination of structural parameters of sweat duct is crucially important to obtain the reliable frequency of resonance and modes of operations. Therefore, here we performed the optical coherence tomography (OCT) of human subjects on their palm and foot to investigate the density, distribution and morphological features of sweat ducts. Moreover, we measured the dielectric properties of stratum corneum using terahertz time domain spectroscopy and based upon this information, we determined the frequency of resonance. We recruited 32 subjects for the measurement and the average duct diameter was 95±11μm. Based upon this information on diameter of duct and THz dielectric properties of stratum corneum (ε=5.1±1.3), we have calculated the frequency of resonance of sweat duct. Finally, we determined that the center frequency of resonance was 442±76 GHz. We believe that these findings will facilitate further investigation of the THz-skin interaction and provide guidelines for safety levels with respect to human exposure. We will also report on the EEG measurement while being shined by micro watt order THz waves.
We have suggested a wide range of real-life applications using novel terahertz imaging techniques. A high-resolution
terahertz tomography was demonstrated by ultra short terahertz pulses using optical fiber and a nonlinear organic crystal.
We also report on the thickness measurement of very thin films using high-sensitivity metal mesh filter. Further we have
succeeded in a non-destructive inspection that can monitor the soot distribution in the ceramic filter using
millimeter-to-terahertz wave computed tomography. These techniques are directly applicable to the non-destructive
testing in industries.
Metal meshes work as band-pass filters in the terahertz (THz) region, with their transmission spectra acutely affected
by the refractive index of the material inside and above the metal mesh openings. We used a metal mesh for
high-sensitivity observations by focusing on the "dip", that is, a sudden change in transmittance that only appeared when
the THz wave was obliquely incident onto the metal mesh. Here we report a measurement of stratum corneum to inspect
the feasibility of applying the metal mesh sensor to observations of human skin.
We have suggested a wide range of real-life applications using novel terahertz imaging techniques. A high-resolution terahertz tomography was demonstrated by ultra short terahertz pulses using optical fiber and a nonlinear organic crystal. We also describe a non-destructive inspection system that can monitor the soot distribution in the ceramic filter using millimeter-to-terahertz wave computed tomography. Further we report on the thickness measurement of very thin films using high-sensitivity metal mesh filter. These techniques are directly applicable to the non-destructive testing in industries.
Using the high penetrability of the terahertz waves and the characteristic absorption spectra in this frequency range, we
built a non-invasive mail inspection system targeting drugs and explosives. The system is composed of two stages; in
the first stage, the scattering of a continuous terahertz wave is used for selecting mail that contains concealed powder; in
the second stage, the absorption spectrum of the suspicious mail is measured and the material is identified using a
terahertz spectrum database. We evaluated the performance and the limits of the inspection system.
After more than a dozen years of basic research into the submillimeter and far infrared range, terahertz (THz) wave
research has finally come into its own, and is recognized by the world scientific community as a new frontier. While
femtosecond laser pumped THz wave sources have opened up a new vista in applied research, the ideal THz wave source
will likely require high coherence and wide tunability. When this level of quality is finally made available in a userfriendly
device, there is little doubt that applied research efforts into the THz region will enjoy a true renaissance. In this
direction we have developed a widely tunable injection seeded THz-wave parametric generator (is-TPG) that operates at
room temperature. The spectral resolution is the Fourier transform limit of the nanosecond THz wave pulses. In our
laboratory, THz-waves continue to broaden their range of applications as following. We have developed a basic
technology for THz imaging which allows detection and identification of drugs concealed in envelopes by introducing
the component spatial pattern analysis. On the other hand, for inspecting electrical failures in large scale integration
circuit, we developed the laser-THz emission microscope, which records the map of THz emission amplitude in a sample
upon excitation with fs laser pulses.
We introduce several types of terahertz- (THz) wave parametric sources. THz-waves can be generated by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals. Using parametric oscillation of MgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd:YAG laser, we have realized broadband sources as well as coherent (narrow band) and widely tunable THz-wave sources. The THz-wave Parametric Generator (TPG) generates a broadband THz wave using a simple configuration; the THz-wave Parametric Oscillator (TPO) and the injection seeded THz-wave Parametric Generator (is-TPG) are two sources that generate coherent, widely tunable THz radiation by suitably controlling the idler wave. We report the characteristics of the oscillation and the radiation including linewidth and tunability. Further, we show the recent progress about these THz-wave parametric sources. We developed two new kinds of TPG by using compact pump sources. One TPG includes a flash-lamp-pumped multimode Nd:YAG laser with a top-hat beam profile, that allows generating high energy, broadband THz waves. Fitting in a space as small as 12 cm × 22 cm (including the pump source) this TPG outputs more than 100 pJ/pulse, which is about 100 times higher than the best results previously reported for TPG. The other has a potential to be a narrow-linewidth injection-seeded TPG, based on an laser-diode-pumped single-mode microchip Nd:YAG laser. The pump laser linewidth is below 0.009 nm and its size is 105×30×32 mm3. This allowed us to achieve a narrow-linewidth compact injection-seeded terahertz-wave parametric generator.
Recent progress in the field of terahertz (THz) imaging is overviewed. First, various THz-wave sources developed and recently improved in our group are described. Second, imaging of samples can be achieved in different modes, of which we discuss here the transmission mode and the reflection-scattering mode. An emphasis in placed on the latter, which can be used to detect and determine for example the distribution of powders inside THz-transparent containers and packages. One-frequency or wide-spectrum imaging can be extended to chemical imaging, a technique by which images acquired at different THz frequencies can be combined to allow the identification of the chemical composition of the target at each spatial position. Other THz imaging applications are also discussed.
We have studied the generation of terahertz (THz) waves by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals. Using parametric oscillation of LiNbO3 or MgO-doped LiNbO3 crystal pumped by a nano-second Q-switched Nd:YAG laser, we have realized a widely tunable coherent THz-wave sources with a simple configuration. We report the detailed characteristics of the oscillation and the radiation including tunability, spatial and temporal coherency, uni directivity, and efficiency. A Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. Further, we have developed a spectroscopic THz imaging system using a TPO, which allows detection and identification of drugs concealed in envelopes, by introducing the component spatial pattern analysis. Several images of the envelope are recorded at different THz frequencies and then processed. The final result is an image that reveals what substances are present in the envelope, in what quantity, and how they are distributed across the envelope area. The example presented here shows the identification of three drugs, two of which illegal, while one is an over-the-counter drug.
A method to detect production faults in plastic packages using terahertz (THz) radiation is presented. Relying on the large difference between the absorption coefficients of plastic and water (for water-filled defects), and on the refraction index difference between plastic and air (for air-filled defects), our technique consists of focusing and scanning a terahertz beam on the sealed area of the package, followed by the detection of the transmitted signal. Compared to previous methods such as visual and ultrasound inspection, our technique can be applied for optically opaque packages and does not require immersion in a matching liquid. We tested our terahertz system on defects simulated by water-filled and air-filled round channels imbedded in polyethylene films, with diameters ranging from 10 to 100 µm. The results show that detection is possible down to 30 µm for water-filled and 40 µm for air-filled channels. The results are the same for both transparent and opaque packages.
We have studied the generation of terahertz (THz) waves by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals. Using parametric oscillation of MgO-doped LiNbO3 crystal pumped by a nano-second Q-switched Nd:YAG laser, we have realized a widely tunable coherent THz-wave sources with a simple configuration. We have also developed a novel basic technology for THz imaging, which allows detection and identification of chemicals by introducing the component spatial pattern analysis. The spatial distributions of the chemicals were obtained from terahertz multispectral trasillumination images, using absorption spectra previously measured with a widely tunable THz-wave parametric oscillator. Further we have applied this technique to the detection and identification of illicit drugs concealed in envelopes. The samples we used were methamphetamine and MDMA, two of the most widely consumed illegal drugs in Japan, and aspirin as a reference.
We have proposed and demonstrated a nondestructive and non-contact inspection method for electrical faults using laser-Terahertz (THz) emission microscopy (LTEM). By measuring the position dependence of the amplitude of the THz emission from integrated circuits (IC) excited with femtosecond (fs) laser pulses, it is possible to investigate the electrical faults in IC. By improving the spatial resolution of the system, we successfully observed the THz emission
image of a microprocessor on standby mode. The LTEM system has a spatial resolution about 3µm and it can localize electrically defective sites in the chip to within a ten square microns.
We have developed a novel basic technology for terahertz (THz) imaging, which allows detection and identification of chemicals by
introducing the component spatial pattern analysis. The spatial distributions of the chemicals were obtained from terahertz
multispectral transillumination images, using absorption spectra previously measured with a widely tunable THz-wave parametric
oscillator. Further we have applied this technique to the detection and identification of illicit drugs concealed in envelopes. The
samples we used were methamphetamine and MDMA, two of the most widely consumed illegal drugs in Japan, and aspirin as a reference.
High-sensitivity terahertz direct detectors using superconducting tunnel junctions were fabricated. They were designed for detecting terahertz radiation in the frequency range of 0.4 and 0.65 THz with the fractional bandwidth of above 10 percent. The results of their performance evaluation of five detector elements are presented. We show the results of the frequency response as well as that the absolute efficiency ranged from 10 to 30 percent and that the the
sensitivity was 1.9 x 10-16 W Hz-0.5 in noise equivalent power.
This paper presents a new method for surface-emitted difference frequency generation (DFG) in planar optical waveguides based on non-ferroelectric materials. The main thrust of this paper is based on the application of a slotted grating to the surface of a waveguide cover. The grating has equally spaced slots at distances equivalent tot eh wavelength of anon-linear polarization wave. DFG power emitted in the direction normal to the surface of the step- index planar waveguide is calculated. It is shown that the efficiency of frequency conversion for a surface emitting geometry is greater than that in a common collinear geometry case. This is especially true for the high-absorption wavelength region of nonlinear materials. According to our estimation, surface-emitted DFG will enable the design of compact solid-state THz-wave sources with a few mW output power.
Recently, we grew organic ionic-salt crystals of 4- dimethylamino-N-methyl-4-stilbazolium-tosylate (DAST) with extremely large nonlinearity, and also realized dual wavelength oscillation of an electronically tuned Ti:Sapphire laser. In this report, the generation of a coherent THz-wave from DAST crystal was demonstrated for the first time by the difference frequency generation of a dual- wavelength oscillating Ti:Sapphire laser.
A compact tunable terahertz (THz)-wave source that operates at room temperature has been realized by introducing a novel configuration of a laser-pumped parametric oscillator using a trapezoidal LiMbO3 crystal. We used total reflection for the pump and resonated idler waves under noncollinear phase-matching conditions by using a trapezoidal LiNbO3 crystal, so that the interacting position was located at the THz-wave exit surface, and direct radiation could be produced without any coup;ling devices. Continuously tunable coherent THz-wave radiation was successfully demonstrated at wavelengths from 130-310 micrometers with a maximum output of 45pJ/pulse. In addition, the THz-wave had an excellent, circular Gaussian-like beam profile with a divergence of 2.4 degrees. Out parametric method has several advantages over other methods. This easy to use compact system has wide tunability, coherency, a relatively high peak power, and a single fixed wavelength pump source. These features will be useful for a wide range of applications.
We had recently demonstrated a room temperature operated widely tunable THz-wave generation (frequency: 0.9 - 2.1 THz, wavelength: 140 - 310 micrometer) introducing a Si prism coupler onto a LiNbO3 crystal which was pumped by a Q- switched Nd:YAG laser. The process involved is an optical parametric oscillation (OPO) utilizing the polariton mode scattering of LiNbO3. This tunable THz-wave source was applied to the problem of differential imaging. In a proof-of- concept experiment, we optically tagged objects embedded in a shade and measured the difference between transmittance at two wavelengths. The image of a tagged object was emphasized in comparison with that of an untagged objects. Differential THz imaging has not been reported previously, to our knowledge, mainly because of the lack of convenient tunable THz-wave sources. It seems possible to use dual-wavelength differential transmittance spectroscopy in the THz-wave region to monitor the gases in the industry.
A couple of decades ago, the efficient and wide tunable THz wave generation had been reported by a group of Stanford Univ.1-3 based on both parametric and Raman effects. Although the interaction was highly efficient, most of the generated THz wave was absorbed or totally reflected inside the crystal due to a large absorption coefficient, as well as large refractive index (≈5.2 at THz range). They used a crystal one of whose corner was cut and polished at the proper angle to allow the signal radiation to emerge approximately normal to the exit surface.
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