Terahertz Parametric Generator Research Articles

Enhanced picosecond terahertz wave generation based on cascade effects in a terahertz parametric generator

Abstract Enhanced terahertz wave generation via Stokes cascaded process has been demonstrated using a picosecond pulse pumped terahertz parametric generation at 1 kHz repetition rate. Clear cascade-saturation of terahertz output was observed, and the corresponded cascade-Stokes spectra were analyzed. The maximum terahertz wave average power was 22 μW under the pump power of 30 W, whereas the maximum power conversion efficiency was 8×10-7 under the pump power of 21 W. The THz power fluctuation was measured to be about 1% in 20 min. This THz parametric source with a relative stable output is suitable for a variety of practical applications.

Collinear injection-seeded terahertz parametric generator

In a conventional injection-seeded terahertz (THz) parametric generator (is-TPG), a complicated achromatic optical system controlling the angle of incidence of the seed beam is used to ensure tunability because both the pump and seed beams must satisfy non-collinear phase-matching conditions. In this study, we found that a THz output and tunability similar to those characteristics of a conventional is-TPG can be obtained even when the pump and seed beams are coaxially injected into the LiNbO3 crystal. In this new generation mechanism, a weak THz-wave is generated by the difference frequency of optical mixing between the pump and seed beams at the Cherenkov phase-matching angle and then strongly parametrically amplified. Thus, we describe our device as a collinear is-TPG. Simplicity is improved by eliminating any need for achromatic optics.

Wide dynamic range and real-time reagent identification and imaging using multi-wavelength terahertz parametric generation and machine learning

In this study, we propose a technique for identifying and imaging reagents through shielding over a wide dynamic range using a real-time terahertz (THz) spectroscopy system with multi-wavelength THz parametric generation/detection and machine learning. To quickly identify reagents through shielding, the spectral information of the “detection Stokes beam” is used for reagent recognition via machine learning. In general THz wave-based reagent identification, continuous spectra are acquired and analyzed quantitatively by post-processing. In actual applications, however, such as testing for illicit drugs in mail, the technology must be able to quickly identify reagents as opposed to quantifying the amount present. In multi-wavelength THz parametric generation/detection, THz spectral information can be measured instantly using a “multi-wavelength detection Stokes beam” and near-infrared (NIR) camera. Moreover, machine learning enables reagent identification in real-time and over a wide dynamic range. Furthermore, by plotting the identification results as pixel values, the spatial distribution of reagents can be imaged at high speed without the need for post-processing.

Continuously Tunable and Energy-Enhanced Injection Pulse-Seeded Terahertz Parametric Generator Based on KTP Crystal

A continuously tunable, energy-enhanced injection, pulse-seeded THz parametric generator based on a KTP crystal (KTP ips-TPG) has been demonstrated in this paper. Theoretical analysis denotes that improving the initial Stokes intensity is favorable to achieve a continuously tunable, high-energy THz generation in a KTP crystal, which is clarified to originate from the enhancement of stimulated polariton scattering (SPS) process in the photon-like region of polariton and parametric amplification process in the phonon-like region of polariton. In the experiment, the THz continuously tunable range was from 2.93 to 6.21 THz, and the THz energies were 6.27, 2.62, 5.43, 4.59, and 5.88 μJ at the output peaks of 3.33, 3.83, 4.41, 4.85, and 5.69 THz, respectively. Compared with the traditional KTP THz parametric oscillator (KTP TPO) with discontinuous frequency tuning, the KTP ips-TPG has great improvement in terms of continuous frequency tuning range and THz output energy, where the enhancement ratios of THz output energy were 6.88× at 3.36 THz, 5.25× at 4.35 THz, 18.21× at 4.79 THz, and 1.82× at 5.69 THz. This injection pulse-seeded THz parametric source based on a KTP crystal is not only suitable to the practical applications in THz high frequency range, but also provides a technical solution for the KTP-family crystals to achieve a continuously tunable and high energy THz generation in a wide THz region.

Discrimination of Pharmaceutical Tablets Based on the Analysis of Solid-State Structures of Ingredients Using Terahertz Transmission Spectroscopy with the Injection-Seeded Parametric Generation Technique.

A frequency-domain terahertz (THz) spectrometer that uses a tunable source, called an injection-seeded THz parametric generator, was applied to the analysis of solid-state structures of ingredients in pharmaceutical tablets, and its performance on discriminating pharmaceutical products was evaluated. The spectrometer has a dynamic range of 70 dB at 2 THz and is suitable for analyzing materials such as pharmaceutical ingredients that often have characteristic absorption peaks between 0.5 and 2.5 THz. Nine ofloxacin (racemate) and four levofloxacin (levorotatory enantiomer) tablet products commercially available in Japan were used as samples. They contain 8–12 additives in addition to the API. The sample tablets were filed down to a thickness of 1.2 mm (ofloxacin tablets) and 1.6 mm (levofloxacin tablets) to obtain transmission spectra over the wide spectral range of 0.8–2.1 THz. The absorption spectra obtained from the spectrometer were preprocessed by the second derivative; then, principal component analysis (PCA) was conducted on the results. Next, quadratic discriminant analysis (DA) was conducted on the scores of the three PCA components. The accuracy of the DA for all 13 products was 96.1%. In addition to the difference in crystal forms of the active ingredient, the small differences in the formulation were clearly discriminated using the THz absorption spectra. The spectrometer combined with data analysis shows potential for applications such as identifying pharmaceutical tablets, monitoring the stability of production processes, evaluating the stability of formulations during storage, and detecting counterfeit drugs on the market.

Real-time wide dynamic range spectrometer using a rapidly wavelength-switchable terahertz parametric source.

In this study, we demonstrate real-time terahertz (THz) spectroscopy using a rapidly wavelength-switchable injection-seeded THz parametric generator. We developed a wavelength-switchable external cavity diode laser using a digital micromirror device as a seed source for the generator. We realized fast acquisition of THz spectra by switching the wavelength of the laser for each pump beam pulse. This system can rapidly switch wavelengths and easily increase the number of measurement wavelengths, and it also has a wide dynamic range, of more than 75 dB, and high stability. Furthermore, by combining this system with THz parametric detection, all wavelengths can be detected in a single frame using a near infrared camera for real-time reagent measurement.

Nonlinear Optical Up-Conversion in Terahertz Parametric Process: Simulation and Experiment

We present a numerical method to model the nonlinear optical up-conversion in the terahertz (THz) parametric process. Specifically, we introduce the concept of competition between the spontaneous parametric down-conversion (SPDC) idler beam and the up-converted Stokes beam. We simulate the spectra with the absolute power of each frequency component that clearly shows the competition result. By this method, we theoretically explain the suppression effect of injected signal on the SPDC idler background. We also verify the simulation experimentally by using an up-conversion parametric detection system. We show the competition result between the Stokes signal and SPDC idler background, the different levels of suppression caused by different input THz energy. The actual detection ability of the up-conversion system is also evaluated. All these experimental results are in good agreement with theoretical simulations. This research enables a comprehensive understanding of the physics of the up-conversion detection as well as the THz parametric generation and the seed injection.

Six-Billion-Fold Amplification via a Two-Stage Terahertz Parametric Amplifier

We have been using the principles of injection-seeded terahertz parametric generation (TPG) to develop the terahertz parametric amplifier (TPA) since 2014. Although we previously achieved amplification of 55 dB at 1.6 THz, amplification of low-frequency terahertz waves (≤1 THz) or ultraweak inputs was difficult, because broadband terahertz waves (spontaneous TPG) were generated and interfered with amplification. Thus, we herein present a new technique that divides terahertz-wave amplification into two steps: preamplification and main amplification. In this configuration, the terahertz wave is upconverted to a near-infrared (NIR) beam. Then, the noise is removed under the NIR condition and converted again to terahertz waves by the main amplifier. This two-stage TPA affords an extremely high amplification factor (up to six-billion-fold; 98 dB) for an ultraweak input of 500 zJ (zJ = 10−21 J). This is the first amplifier that has such a high gain for sub-aJ order input in the terahertz region.

Terahertz tag identifiable through shielding materials using machine learning.

In recent years, there has been great interest in chipless radio-frequency identification (RFID) devices that work in the terahertz (THz) frequency range. Despite advances in RFID technology, its practical use in the THz range has yet to be realized, due to cost and detection accuracy issues associated with shielding materials. In this study, we propose two types of low-cost THz-tags; one is based on the thickness variation of coated polyethylene and the other on the fingerprint spectra of reagents. In the proposed approach, machine learning, specifically a deep-learning method, is used for high-precision tag identification even with weak signals, or when the spectrum is disturbed by passing through shielding materials. We achieved almost 100% identification accuracy despite using an inexpensive tag placed under thick shielding materials with an attenuation rate of about -50 dB. Furthermore, real-time tag identification was demonstrated by combining a multiwavelength injection-seeded THz parametric generator and a convolutional neural network.

Enhanced Terahertz Wave Generation via Stokes Wave Recycling in Non-Synchronously Picosecond Pulse Pumped Terahertz Source

Enhanced terahertz wave generation via Stokes wave recycling in a non-synchronously picosecond pulse pump configuration has been demonstrated. It is theoretically analyzed that under the condition of high pump peak power density, the oscillation of the strong 1st order Stokes wave could benefit the higher order Stokes wave emission and terahertz wave generation. In the experiment, 5.71 times enhancement of terahertz wave generation was obtained via Stokes wave recycling in the non-synchronously picosecond pulse pumped terahertz parametric generator compared with the conventional single-pass terahertz parametric generation. The maximum terahertz wave average power was 61.7 μW under the pump power of 20 W and the cavity length of 170 mm, while the maximum power conversion efficiency was 3.085 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> .

Terahertz confocal microscopy with an injection-seeded terahertz parametric generator

We report on the implementation of a confocal microscopy system based on an injection-seeded terahertz parametric generator working at 1.76 THz. The system has a fairly reliable long-term stable operation with pulse energy fluctuations <5 % . We experimentally demonstrate a subwavelength lateral resolution of 158 μm, corresponding to 0.93λ. We also demonstrate the capability of resolving overlapping objects at the different longitudinal surfaces by imaging a designed sample. This is the first detailed demonstration of confocal microscopy based on a THz parametric source, which is helpful for the practical application of THz imaging technology.

Perspective: Terahertz wave parametric generator and its applications

The injection-seeded terahertz (THz) parametric generator (is-TPG) is one of the most high-power single-longitudinal mode THz-wave sources. Our system is less influenced by scattering, refraction, and multiple reflections by samples because it is a narrow-linewidth source, and the detection area of the THz parametric detector is large. Thus, it is suitable for nondestructive inspection of practical samples in the real world. In 2003, we reported on the development of a mail inspection system that employed a THz parametric oscillator. However, with a dynamic range of less than four orders of magnitude, this system could only identify reagents through thin envelopes. Recently, we succeeded in developing a high-power, highly sensitive THz-wave spectroscopic imaging system with a dynamic range of 100 dB using the is-TPG and a THz parametric detector. Nondestructive inspection of reagents inside thick envelopes and three-dimensional computed tomography of plastics, which attenuate THz-waves by more than 60 dB, were conducted using this system. More recently, we have focused our efforts on a real-time measurement system using a multiwavelength is-TPG, which gives rise to numerous potential applications, given the significantly shorter measurement times. Thus, this system will facilitate the implementation of THz-wave measurements in real-world applications. In this paper, we report on our recent results and provide a perspective on the is-TPG.

Multiwavelength terahertz-wave parametric generator for one-pulse spectroscopy

In this study, the simultaneous generation of multiwavelength terahertz (THz) waves by an injection-seeded THz parametric generator (is-TPG) was achieved for the first time. The output and stability of the multiwavelength THz waves were equivalent to those of the THz waves generated via a single-wavelength is-TPG. Spatial separation of frequencies and high-sensitivity detection were achieved by converting the THz waves to near-infrared detection beams. Furthermore, one-pulse spectroscopy of saccharides was realized, and a dynamic range of more than 60 dB was obtained. The results demonstrated the possibility of using the is-TPG to significantly shorten the measurement times of spectroscopic systems.

Terahertz parametric generation and amplification from potassium titanyl phosphate in comparison with lithium niobate and lithium tantalate.

We report superior terahertz parametric generation from potassium titanyl phosphate (KTP) over congruent-grown lithium niobate (CLN) and lithium tantalate (CLT) in terms of parametric gain and laser damage resistance. Under the same pump and crystal configurations, the signal emerged first from KTP, 5% Mg-doped CLN, CLN, and then finally from CLT. The signal growth rate in KTP was comparable to that in 5%-Mg-doped CLN, but the signal power from KTP reached a much higher value after all the other crystals were damaged by the pump laser. We further demonstrate seeded terahertz parametric amplification in an edge-cut KTP at 5.74 THz. The THz parametric amplifier (TPA) employs a 17-mm long KTP gain crystal, pumped by a passively Q-switched pump laser at 1064 nm and seeded by a continuous-wave diode laser tuned to the signal wavelength at 1086.2 nm. With 5.8-mJ energy in a 520-ps pump pulse and 100-mW seed signal power, we measured 5-W peak-power THz output from the KTP TPA with 22% pump depletion. In comparison, we measured no detectable THz output power from a similar edge-cut CLN TPA under the same pump power, detection scheme, and crystal configuration, when tuning the seed laser wavelength to 1072.2 nm and attempting to generate a radiation at 2.1 THz.

Beam Dynamics Investigation for the Compact Seeded THz-FEL Amplifier

A compact seeded terahertz (THz) free-electron laser (FEL) amplifier is under development at the Institute of Advanced Energy, Kyoto University. The system consists of a 1.6-cell S-band BNL-type photocathode radio frequency (RF) gun, a focusing solenoid magnet, a magnetic bunch compressor, focusing quadrupoles, a short planar undulator and a THz parametric generator for seeding. The accelerator system will generate a high brightness ultra-short electron bunch injected to the short undulator. Since characteristics of the emitted radiation strongly depend on the electron beam properties, the beam dynamics of the system has been investigated. As the result, the system should be operated at the high accelerating voltage of 80 MV/m to avoid the space-charge effect which causes serious bunch lengthening. The laser with the uniform temporal distribution and the injection phase of 12 degree is suitable for the bunch compression in the chicane. This condition provides the electron beam injected to the undulator with the peak current of more than 300 A and the FWHM bunch length of 0.36 ps at the bunch charge of 200 pC.

Non-destructive drug inspection in covering materials using a terahertz spectral imaging system with injection-seeded terahertz parametric generation and detection.

In 2003, we reported the first-ever development of a spectral imaging system for illicit drugs detection using a terahertz (THz) wave parametric oscillator (TPO) [K. Kawase et al., Opt. Exp. 11(20), 2549 2003]. The system has a dynamic range below four orders of magnitude, which enables it to identify reagents only through thin envelopes using spectral imaging. Recently, we succeeded in developing a high power and high sensitivity THz wave spectral imaging system using injection-seeded THz parametric generation and detection. A dynamic range in excess of 80 dB has been obtained, which is much higher than that of the 2003 system. In this study, the new spectral imaging system successfully identified reagents through thicker material than the thin envelopes used previously.

A High Dynamic Range and Spectrally Flat Terahertz Spectrometer Based on Optical Parametric Processes in LiNbO&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;$_{3}$&lt;/tex&gt;&lt;/formula&gt;

A wideband, wavelength tunable, spectrally flat terahertz (THz) spectrometer with a dynamic range in excess of 70 dB was demonstrated. The emission of THz-wave was achieved using an injection-seeded THz parametric generator (is-TPG), and detection was achieved using nonlinear optical wavelength conversion, both in magnesium-oxide-doped lithium niobate crystal. This system has potential applications in non-destructive sensing and imaging of a wide variety of materials.

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Terahertz parametric generation photonic band gap structure with negligible structural dispersion in the optical range

We present a photonic band gap (PBG) structure (or nonlinear photonic crystal) design for terahertz (THz) wave parametric generation, whose component materials have a small refractive index difference in the near infrared and a large index difference for THz waves. The structural dispersion of such a PBG structure is strong in the THz range but negligible in the optical range. The former allows the phase-matched pump wavelength to be placed in the near infrared to eliminate two-photon absorption of the pump and signal beams. The latter leads to a crystal layer fabrication tolerances of a few micrometers and traditional polishing methods are suitable for device fabrication. The added design flexibility also allows the use of the most efficient crystal orientations.

The effect of injection seeding on terahertz parametric oscillation

AbstractWe report on the use of injection seeding to narrow the terahertz‐wave linewidths of terahertz parametric light sources [a terahertz parametric generator (TPG) and a terahertz parametric oscillator (TPO)] using a LiNbO3 crystal and a Nd:YAG laser (wavelength 1.06 µm) as a pump light source. In this technique, the light of a Yb:fiber laser (wavelength 1.07 µm) operating in CW mode is injected into the LiNbO3 crystal, which brings about the narrowing of the terahertz‐wave spectral line. Our experiments confirm that injection seeding into the TPG not only narrows the terahertz‐wave spectral line but also greatly improves the input‐output characteristics. When we injection‐seeded a TPO pumped by light from a Nd:YAG laser with a single‐longitudinal‐mode spectrum, we obtained a 200‐MHz spectral line width, which approaches the Fourier‐transform limit. That is, our spectral linewidth was limited by the resolution of the measurement system. © 2002 Wiley Periodicals, Inc. Electron Comm Jpn Pt 2, 86(1): 26–35, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.1115