Mutagenicity assessment of high-power 1.6-THz pulse laser radiation.

The effect of terahertz (THz) radiation has been studied in medicine. However, there is a lack of scientific information regarding its possible mutagenicity. Therefore, the present study aimed to assess the mutagenicity of 1.6 THz laser irradiation. The Ames test was conducted using five bacterial tester strains. The bacteria were subjected to (i) 1.6 THz laser irradiation at 3.8 mW/cm2 for 60 min using a tabletop THz pulse laser system, (ii) ultraviolet irradiation, (iii) treatment with positive control chemicals (positive control) or (iv) treatment with the solvent used in the positive control (negative control). After treatment, the bacterial suspensions were cultured on minimal glucose agar to determine the number of revertant colonies. In addition, the comet assay was performed using fibroblasts (V79) to assess possible DNA damage caused by the THz laser irradiation. The Ames test demonstrated that the THz laser irradiation did not increase the number of revertant colonies compared to that in the negative control group, whereas the ultraviolet irradiation and positive control treatment increased the number of revertant colonies. Thus, 1.6 THz laser irradiation is unlikely to be mutagenic. The comet assay additionally suggests that the THz laser irradiation unlikely induce cellular DNA damage.

Tunable backward terahertz-wave parametric oscillator centered at a high frequency of 0.87 THz with injection seeding.

We report the first demonstration of a frequency tunable backward THz-wave parametric oscillator (BW-TPO) centered at a high frequency of 0.87 THz using a slant-stripe-type magnesium oxide-doped periodically poled lithium niobate (PPLN) crystal as the nonlinear medium. Down-converted THz and idler beams generate upon excitation of the PPLN with a sub-nanosecond pulsed source of λ = 1064.44 nm. The resulting first idler has a wavelength of 1067.75 nm, equivalent to an oscillation frequency of 0.872 THz as per the spectral line separation from the pump. We also present angle tuning of the BW-TPO frequency ranging from 0.836-0.905 THz through PPLN rotation. The threshold pump intensity for BW-TPO is determined to be 5.6 GW/cm2 while obtaining a conversion efficiency as high as 12.3% at a pump energy (intensity) of 15.25 mJ (8.90 GW/cm2). A reduction of the BW-TPO threshold energy and improved pump-to-idler energy conversion efficiency resulted from injection seeding with a CW laser at the same wavelength as the first idler. The THz output is also directly proportional to seed power.

Terahertz Fresnel-zone-plate thin-film lens based on a high-transmittance double-layer metamaterial phase shifter.

Planar diffractive lenses, with metamaterial artificial structures and subwavelength thickness, provide unique and flexible platforms for optical design in the terahertz (THz) regime. Here, we present a metamaterial-based Rayleigh-Wood Fresnel-zone-plate (FZP) thin-film lens designed to focus a monochromatic THz beam at 1.0 THz with a high transmittance of 80%, short focal length of 24 mm, and subwavelength thickness of 48 µm. Specifically, the FZP lens is composed of 8 alternating concentric zones through a polymer film substrate, where odd zones are patterned with double-layer un-split ring resonators (USRRs) that provide a polarization-independent phase shift of π/2 compared to un-patterned even zones. Both simulation and experiment confirm that our FZP lens creates a focused beam at the designed frequency of 1.0 THz by constructive interference through alternating concentric metamaterial-patterned and un-patterned zones, producing a diffraction-limited resolution of 0.6 mm for imaging applications. In contrast to conventional approaches in which the uniform periodic array of metamaterial unit cells has been treated as an effective material, we newly find that double-layer USRRs can work as an independent meta-atom without degradation of its performances, which benefits the behavior of small arrays of double-layer USRRs located in the outer zones of the FZP lens. Such a planar thin-film lens would enable us to realize compact and lightweight THz systems.

Carbon nanotube-based, serially connected terahertz sensor with enhanced thermal and optical efficiencies.

Owing to their high thermal and optical performances, carbon nanotube (CNT) films are used in various photo-thermo-electric (PTE) applications, such as terahertz (THz) sensing and energy harvesting. To improve the performance of PTE devices, a device structure should be designed based on a deep understanding of the thermal and optical responses of the CNT film. However, the optical properties of CNT films in the THz frequency region remain unclear because of the difficulties associated with device processing and measurements. Herein, we report our findings on the thermal and optical characteristics of CNT films. The shape of the CNT film that maximizes the product of the thermal and optical factors (optimal structure of the PTE sensor) depends on the frequency of the irradiating electromagnetic wave. The optimal film thickness and width values for THz irradiation range from 300-600 nm and 50-70 µm, respectively. Subsequently, we fabricated a serially connected, multi-element PTE sensor with an optimal device structure and enhanced the detection sensitivity by approximately 13 times compared with a single-element PTE sensor. In addition, we demonstrated the first THz spectroscopy application using a PTE sensor. The findings of this study, thermal/optical factor enhancement, and micro-sized CNT film processing technology can be used to improve the performance of all CNT-based photothermal devices, including PTE sensors and thermoelectric generators.

Optical up-conversion-based cross-correlation for characterization of sub-nanosecond terahertz-wave pulses.

Using a nonlinear optical mixing known as a frequency up-conversion process, we demonstrate an optical cross-correlation technique for the detection and characterization of sub-nanosecond (sub-ns) terahertz (THz)-wave pulses. A monochromatic THz-wave pulse from an injection-seeded THz-wave parametric generator (is-TPG) was mixed with a near-infrared (NIR) pump pulse to generate a NIR idler pulse in a trapezoidal-prism-shaped MgO-doped lithium niobate crystal under the noncollinear phase-matching condition. By measuring pump-energy and crystal-length dependencies, we show that the frequency up-conversion of sub-ns THz-wave pulses with and without subsequent parametric amplification can be used for sensitive detection and intensity cross-correlation characterization, respectively. Using this cross-correlation technique, we reveal that the temporal profile of THz-wave pulses from the is-TPG driven by a 351-ps 1064-nm pump laser has slightly-frequency-dependent pulse width in the range of 150-190 ps at full width at half-maximum in the tunable range of 0.95-2.00 THz.

Highly sensitive multi-stage terahertz parametric upconversion detection based on a KTiOPO4 crystal.

In this Letter, we demonstrate a highly sensitive multi-stage terahertz (THz) wave parametric upconversion detector based on a KTiOPO4 (KTP) crystal pumped by a 1064-nm pulsed-laser (10 ns, 10 Hz). The THz wave was upconverted to near-infrared light in a trapezoidal KTP crystal based on stimulated polariton scattering. The upconversion signal was amplified in two KTP crystals based on non-collinear and collinear phase matching, respectively, to improve detection sensitivity. A rapid-response detection in the THz frequency ranges of 4.26-4.50 THz and 4.80-4.92 THz was achieved. Moreover, a dual-color THz wave generated from THz parametric oscillator using KTP crystal was detected simultaneously based on dual-wavelength upconversion. The minimum detectable energy of 2.35 fJ was realized with a dynamic range of 84 dB at 4.85 THz, which gives a noise equivalent power (NEP) of the order of 21.3 pW/Hz1/2. By changing the phase-matching angle or the wavelength of the pump laser, it is suggested that the detection of the THz frequency band of interest in a wide range from approximately 1 to 14 THz is possible.

Injection-seeded terahertz-wave parametric generator with timing stabilized excitation for nondestructive testing applications.

An injection-seeded terahertz (THz)-wave parametric generator (is-TPG) with a footprint the size of an A3 paper is presented. We improved the measurement performance of the is-TPG source for nondestructive inspection applications. A high pulse repetition rate up to 70 kHz and a low pulse timing jitter of a few tens of picoseconds, which is approximately one ten-thousandth of the conventional is-TPG, were achieved. THz waves exhibited excellent performance with a maximum average output power of 20 µW, a monochromatic spectrum linewidth of ∼20 GHz, and a frequency tuning range of 1.7-3.0 THz. This was achieved by designing the entire system configuration from the pump laser source to THz-wave generation. A new double-pass all-solid-state optical amplifier was developed with a high gain and low noise using an externally pulse-modulated laser diode (LD) as the master oscillator source. An achromatic optical injection system was developed for the is-TPG with a 40% reduction in the conventional path length. They were housed in a single enclosure in two layers. LDs and optical fiber amplifiers could be rack-mounted, and the outputs were delivered to the housing via optical fibers. The developed THz-wave source performed nondestructive imaging of a human hair sample fixed with Scotch tape on a test pattern in an envelope by irradiating 2.1 THz waves. A clearly recognizable THz-wave image of an enclosed hair with a spatial resolution close to the THz wavelength was obtained.

Security screening system based on terahertz-wave spectroscopic gas detection.

Tunable terahertz (THz)-wave absorption spectroscopy is a promising technique to detect trace gases suspended in ambient air owing to their strong absorption fingerprints in the THz-wave spectral region. Here, we present a THz-wave spectroscopic gas detection platform based on a frequency-tunable injection-seeded THz-wave parametric generator and compact multipass gas absorption cells. Using a 1.8-m-path-length multipass cell, we detected gas-phase methanol (CH3OH) down to a trace concentration of 0.2 ppm at the 1.48-THz transparent atmospheric window. We also developed a transportable walk-through screening prototype using a 6-m-path-length multipass cell to identify suspicious subjects. Our results demonstrate the potential of the proposed system for security screening applications.

Actively tunable THz filter based on an electromagnetically induced transparency analog hybridized with a MEMS metamaterial.

Electromagnetically induced transparency (EIT) analogs in classical oscillator systems have been investigated due to their potential in optical applications such as nonlinear devices and the slow-light field. Metamaterials are good candidates that utilize EIT-like effects to regulate optical light. Here, an actively reconfigurable EIT metamaterial for controlling THz waves, which consists of a movable bar and a fixed wire pair, is numerically and experimentally proposed. By changing the distance between the bar and wire pair through microelectromechanical system (MEMS) technology, the metamaterial can controllably regulate the EIT behavior to manipulate the waves around 1.832 THz, serving as a dynamic filter. A high transmittance modulation rate of 38.8% is obtained by applying a drive voltage to the MEMS actuator. The dispersion properties and polarization of the metamaterial are also investigated. Since this filter is readily miniaturized and integrated by taking advantage of MEMS, it is expected to significantly promote the development of THz-related practical applications such as THz biological detection and THz communications.

Frequency-agile injection-seeded terahertz-wave parametric generation: publisher's note.

This publisher's note contains corrections to Opt. Lett.45, 77 (2020)OPLEDP0146-959210.1364/OL.45.000077.

Characterization of all second-order nonlinear-optical coefficients of organic N-benzyl-2-methyl-4-nitroaniline crystal.

Full elements of second-order nonlinear optical (NLO) tensor can be completely characterized for an organic NLO crystal for the first time. As-grown bulk N-benzyl-2-methyl-4-nitroaniline (BNA) crystal was processed to expose (100) and (010) crystal orientations with fine optical surfaces by using precision lathe and diamond blade. Then, every five nonvanishing second-order NLO coefficient of BNA can be determined quantitatively using the precisely processed crystals based on 1st-kind Maker fringe measurements. Our method makes it possible to clarify uncertain NLO property of any organic materials and to accelerate application study via precise device fabrications even for fragile organic materials.

Tunable Backward Terahertz-wave Parametric Oscillation.

Backward optical parametric oscillation has attracted attention for cavityless spectral narrowband generation based on perfect photon conversion. Few demonstrations have shown its potential from the aspect of nonlinear photonics; therefore, the mechanisms of momentum conservation among interacting light waves have been concealed by the restricted configuration under the phase-matching condition of periodically poled structures. Here, we unveil a tunable mechanism in the terahertz region by active control of the phase-matching condition. The tunability of backward terahertz-wave parametric oscillation is investigated using a quasi-collinear phase-matching model and its frequency range from the sub-terahertz to terahertz region is identified. Transform-limited terahertz-wave pulse is achieved simply by installing a device on the pump propagating line with no cavity. Moreover, the cascading terahertz-wave generation enhances the photon conversion efficiency, thus making nonlinear optics and its applications more promising. The results highlight new capabilities for using modern ferroelectric materials and encourage further research on nonlinear optics.

Semiconductor property imaging on as-grown wafer with monochromatic tunable THz-wave source.

In this paper, we propose the use of a tunable monochromatic terahertz (THz) wave source to measure the carrier properties of semiconductors. We also report on the recent improvement of our measurement system and demonstrate its ability to measure an as-grown sample; our system involves reference mirror-free reflective measurement. In our method, carrier properties, such as resistivity and mobility in semiconductors relating to the carrier density, can be determined by reflective measurement. This method was applied to composite semiconductors-GaN, GaAs, and SiC. In the measurement system, we improved the filter to block the infrared beam co-linearly propagating with the desired THz-wave. Consequently, the signal intensity was 13 times higher than that achieved in our previous work. We demonstrated the measured carrier properties in an as-grown n-type GaN wafer. We observed flat features via our THz-reflective measurement, whereas the sample showed prominent surface roughness in a picture taken with visible light.

Off-resonance and in-resonance metamaterial design for a high-transmission terahertz-wave quarter-wave plate.

This Letter describes a novel metamaterial design by employing off-resonance and in-resonance excitation for a high-transmission terahertz-wave quarter-wave plate (QWP). The device is demonstrated with a thin film metamaterial with double-layer split ring resonators (SRRs). Different from a usual resonant metamaterial device, here we design the work frequency off from the inductor-capacitor (LC) resonance for the TE mode, while in a dipole resonance for the TM mode to obtain the artificial birefringence. Rectangular SRRs in this Letter provide a choice to optimize the off-resonance and in-resonance excitation, to assist the double-layer design for high transmission. Converting a linearly polarized wave to circular polarization with our QWP, the experiment confirms a transmittance of 0.8 and an ellipticity of 0.99 at 0.98 THz. The developed thin film device is flexible and has a thickness of 48 µm (sub-wavelength). This is an advantage for potential integration in systems where overall device compactness is required.

Quadratic nonlinear optical properties of the organic N-benzyl-2-methyl-4-nitroaniline (BNA) biaxial crystal.

We performed the direct measurement of second harmonic generation and sum frequency generation phase-matching directions in the organic N-benzyl-2-methyl-4-nitroaniline crystal over its visible and near-infrared transparency range. The fit of these data allowed us to refine the Sellmeier equations of the three principal refractive indices in this range. With these equations, we improved the calculated tuning curves of terahertz emission from a phase-matched difference frequency process. We also determined the absolute magnitude of the d24 nonlinear coefficient.

Thin terahertz-wave phase shifter by flexible film metamaterial with high transmission.

Thin terahertz (THz)-wave optical components are fundamentally important for integrated THz-wave spectroscopy and imaging systems, especially for phase manipulation devices. As described herein, a thin THz-wave phase shifter was developed using a flexible film metamaterial with high transmission and polarization independent properties. The metamaterial unit structure employs double-layer un-split ring resonators (USRRs) with a designed distance between the two layers to obtain phase retardance of π/2, thus constituting a THz-wave phase shifter. The metamaterial design keeps the transmission coefficient as high as 0.91. The phase shifter also has polarization independence due to the four-fold symmetry of the USRR structure. Because of the subwavelength feature size of the USRR, this shifter can offer benefits for manipulating the spatial profile for the THz-wave phase through design of a binary optics phase plate by arranging a USRR array. The thickness of 48 µm has benefits for developing integrated THz optics and other applications that demand compactness and flexibility. The developed film size of 5 cm × 5 cm from the device fabrication process is suitable for THz lenses or gratings of large optical components.

Nonlinear optical detection of terahertz-wave radiation from resonant tunneling diodes.

The sensitive detection of terahertz (THz)-wave radiation from compact sources at room temperature is crucial for real-world THz-wave applications. Here, we demonstrate the nonlinear optical detection of THz-wave radiation from continuous-wave (CW) resonant tunneling diodes (RTDs) at 0.58, 0.78, and 1.14 THz. The up-conversion process in a MgO:LiNbO3 crystal under the noncollinear phase-matching condition offers efficient wavelength conversion from a THz wave to a near-infrared (NIR) wave that is detected using a commercial NIR photodetector. The minimum detection limit of CW THz-wave power is as low as 5 nW at 1.14 THz, corresponding to 2-aJ energy and 2.7 × 103 photons within the time window of a 0.31-ns pump pulse. Our results show that the input frequency and power of RTD devices can be calibrated by measuring the output wavelength and energy of up-converted waves, respectively. This optical detection technique for compact electronic THz-wave sources will open up a new opportunity for the realization of real-world THz-wave applications.

Temperature Dependence in the Terahertz Spectrum of Nicotinamide: Anharmonicity and Hydrogen-Bonded Network.

We have investigated the terahertz-spectral property of nicotinamide focusing on the temperature dependence in the range of 14-300 K. We observed that almost all peaks in the terahertz spectrum of the nicotinamide crystal showed a remarkable shift with temperature, whereas the lowest-frequency peak at 34.8 cm-1 showed a negligible shift with temperature. By analyzing the terahertz spectrum with the dispersion-corrected density functional theory calculations, we found that the difference in the temperature dependence of the peak shift is well understood in terms of the presence/absence of stretching vibration of the intermolecular hydrogen bond in the mode and the change of cell parameters. The anharmonicity in the dissociation potential energy of very weak intermolecular hydrogen bonding causes the remarkable peak shift with temperature in the terahertz spectrum of nicotinamide. This finding suggests that the assignment and identification of peaks in the terahertz spectrum are systematically enabled by temperature-dependent measurements.

Diffraction-limited real-time terahertz imaging by optical frequency up-conversion in a DAST crystal.

Real-time terahertz (THz) wave imaging has wide applications in areas such as security, industry, biology, medicine, pharmacy, and the arts. This report describes real-time room-temperature THz imaging by nonlinear optical frequency up-conversion in an organic 4-dimethylamino-N'-methyl-4'-stilbazolium tosylate (DAST) crystal, with high resolution reaching the diffraction limit. THz-wave images were converted to the near infrared region and then captured using an InGaAs camera in a tandem imaging system. The resolution of the imaging system was analyzed. Diffraction and interference of THz wave were observed in the experiments. Videos are supplied to show the interference pattern variation that occurs with sample moving and tilting.

Ultrabright continuously tunable terahertz-wave generation at room temperature.

The hottest frequency region in terms of research currently lies in the 'frequency gap' region between microwaves and infrared: terahertz waves. Although new methods for generating terahertz radiation have been developed, most sources cannot generate high-brightness terahertz beams. Here we demonstrate the generation of ultrabright terahertz waves (brightness ~0.2 GW/sr·cm(2), brightness temperature of ~10(18) K, peak power of >50 kW) using parametric wavelength conversion in a nonlinear crystal; this is brighter than many specialized sources such as far-infrared free-electron lasers (~10(16) K, ~2 kW). We revealed novel parametric wavelength conversion using stimulated Raman scattering in LiNbO3 without stimulated Brillouin scattering using recently-developed microchip laser. Furthermore, nonlinear up-conversion techniques allow the intense terahertz waves to be visualized and their frequency determined. These results are very promising for extending applied research into the terahertz region, and we expect that this source will open up new research fields such as nonlinear optics in the terahertz region.