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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.

We characterized a temporal profile of sub-nanosecond terahertz (THz)-wave pulses from an injection-seeded THz-wave parametric generator by using an optical up-conversion-based cross-correlation in a MgO-doped lithium niobate 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.

We experimentally demonstrate that the photovoltaic responsivity in an InP-based dual-grating-gate HEMT plasmonic THz detector with the gate-readout configuration is significantly enhanced by a positive gate bias application due to the nonlinear rectification effect at the InGaAs/InAlAs heterobarrier.

We studied cascaded backward terahertz-wave parametric oscillation (BW-TPO) to enhance the output THz-wave power. The pump beam converted to the photon pair of counter-propagating THz-wave and co-propagating idler beam. The idler beams also acted as a pump beam for the cascaded THz-wave generation. The oscillation threshold of cascaded process was reduced 77% compared with the primary one thanks to the spectrum overlaps of counter-propagating THz waves. Furthermore, optical injection seeding to the first idler beam provided threshold reduction of the primary one. Output peak power of THz-wave reached 205 W with the pulse width of 0.78 ns at the frequency of 0.3 THz.

We report on fast, sensitive terahertz (THz) detection in an asymmetric dual-grating-gate graphene channel field-effect transistor (ADGG-GFET). Bias-dependent photoresponse is measured to clearly identify the plasmonic and photo-thermoelectric rectification mechanisms that could co-exist while preserving picosecond response speed.

The photonic carrier frequency downconverter is an active device that directly converts digital coherent optical data signals to the digital coherent wireless data signals and furthermore to the intermediate frequency (IF) signals, too. Its key functionality is called photonic double-mixing, consisting of a photomixing of an optical data signal with an optical subcarrier signal to generate a down-converted terahertz (THz) wireless data signal and an rf-mixing of the generated THz data signal and a local (LO) signal to generate an IF signal. It also works to mix the THz wireless data signal with a LO signal generated by photomixing two optical subcarrier signals to generate an IF signal. At the present, various photomixers as well as several photonic carrier frequency downconverters have been developed [1] - [2] . However, their operation frequency range is limited by the electron transit time effect. In this study, we demonstrate the 1-THz band photonic double-mixing operation by using plasmonic technology. Plasmons, the quanta of the collective oscillation of electrons, are expected to overcome the electron transit time limitation [3] .

Terahertz (THz) electromagnetic waves hold great potentialities to be utilized for applications such as large-capacity, ultrahigh-speed wireless communications beyond 5G as well as non-destructive inspection of hazardous substances. Plasmonic THz devices based on two-dimensional (2D) plasmons in transistor channels have been studied as potential candidates of on-chip, room-temperature operating, high-speed THz detectors [1] . In particular, graphene Dirac plasmons are promising to break through the limit on operating frequency and sensitivity performance in existing technology [2] . In this paper, we propose our original graphene-based device structure and demonstrate its capability of fast, sensitive THz 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.

The capability for actual measurements—not just simulations—of the dynamical behavior of THz electromagnetic waves, including interactions with prevalent 3D objects, has become increasingly important not only for developments of various THz devices, but also for reliable evaluation of electromagnetic compatibility. We have obtained real-time visualizations of the spatial evolution of THz electromagnetic waves interacting with a single metal micro-helix. After the micro-helix is stimulated by a broadband pico-second pulse of THz electromagnetic waves, two types of anisotropic re-emissions can occur following overall inductive current oscillations in the micro-helix. They propagate in orthogonally crossed directions with different THz frequency spectra. This unique radiative feature can be very useful for the development of a smart antenna with broadband multiplexing/demultiplexing ability and directional adaptivity. In this way, we have demonstrated that our advanced measurement techniques can lead to the development of novel functional THz devices., 微小金属らせんとテラヘルツ光との相互作用を可視化 --次世代超高速移動通信などにおける高性能アンテナへ応用--. 京都大学プレスリリース. 2021-02-09.

We demonstrated novel phase-matching conditions of a tunable backward optical parametric down-conversion generating Terahertz-wave in a periodically poled lithium niobate. The cascading generation and threshold reduction by injection seeding provides high-brightness THz- wave emission.

We experimentally demonstrate the photovoltage from an InGaAs-channel dual-grating-gate HEMT THz detector in the gate-readout configuration is significantly enhanced by the positive gate bias application due to the nonlinear rectification effect at the InGaAs/InAlAs heterobarrier.

Over the last decade, there has been a growing interest in the development of semiconductor-based THz-wave detectors with high-speed responses for many applications, such as non-destructive sensing and wireless communications.

We propose the use of the gate electrode as a readout port of photovoltage of a grating-gate plasmonic THz detector. We fabricate the device and experimentally demonstrate the scaling of the photovoltage with the size of active area for the gate-readout and an excellent impedance matching to a 50-Ω interconnection system, both of which are impossible for the drain-readout.

This paper presents examination of a backward Terahertz-wave (THz-wave) parametric oscillator (BW - TPO) with wide tunability in the sub-THz wave region (~1 THz). The simple and compact device has been demonstrated under a special optical parametric down-conversion found in a periodically poled lithium niobate. The potential property of the BW - TPO is expected to pave the way for THz-wave applications, especially a swept-source THz OCT with a higher depth-resolution because of a dynamically wide frequency-tuning range.

We have characterized the sensitivity and noise equivalent power (NEP) of resonant-tunneling-diode (RTD) oscillators as terahertz (THz)-wave detectors. Using a continuous-wave (CW) frequency-tunable THz-wave photomixer source, we observed that sensitive detection was achieved by RTD oscillators around its oscillation frequencies. The quasi-optical RTD device oscillating at 0.78 THz showed a maximum current responsivity of 7.3 A/W near the peak point of the negative differential conductance (NDC) region and a minimum NEP of 7.7 pW/√Hz within the NDC region, respectively, at room temperature.

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.

We identified eight nonlinear crystals enabling THz emission from quadratic phase-matched Difference-Frequency-Generation: YCOB, BNA, LBO, CSP, AGS, CdSe, ZnO and GaP. For all these crystals, we performed Time-Domain Spectroscopy in the same conditions to determine their absorption spectra in polarized light as well as their principal refractive indices as a function of wavelength in the 0.5-2.0 THz range. By combining previous data with the Sellmeier equations valid in their visible and infrared transparency ranges, we calculated the coherence length of Difference-Frequency-Generation associated to all possible configurations of polarization and found interesting and complementary phase-matching conditions in the eight studied crystals.

Using terahertz time-domain spectroscopy, we report the principal values of absorption coefficients and refractive indices as a function of wavelength between 0.5 and 2.0 THz of eight attractive nonlinear crystals: YCOB, BNA, LBO, CSP, AGS, CdSe, ZnO and GaP. From these data, we calculated the coherence length and phase-matching conditions associated to the emission of a THz wave from a second-order difference-frequency generation.

Terahertz-wave security screening system has been developed by using an injection-seeded terahertz-wave parametric generator with intense-power and narrowband emission. Methanol gas in atmosphere as a phantom was measured within one second without dismpting foot traffic.

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.

We measured the terahertz spectra of five powdered vitamins, nicotinamide, nicotinic acid, ascorbic acid, pyridoxine hydrochloride, and biotin, to investigate intermolecular hydrogen bonding. These vitamins have various weak to moderate hydrogen bonds between molecules and thus are good candidates for the investigation of various hydrogen bond stretching. Using dispersion-corrected density functional theory calculations on a periodic system, we assigned the observed THz peaks with excellent accuracy. The stretching vibrations of the intermolecular hydrogen bonds are mixed with the intermolecular translational and rotational modes. Depending on the strength of the hydrogen bond and the mass of the moiety linked via the hydrogen bond, the wavenumber of each type of hydrogen bond is located within a limited region. According to the present study, the stretching vibration of a weak hydrogen bond located at approximately 60–105 cm−1 is summarized as having a sharp full width at half maximum of 1–2 cm−1 and the molar extinction coefficient of 4–10 M−1 cm−1. The anharmonicity of the potential energy curve was found in modes containing weak intermolecular NH⋯N and NH⋯O stretching vibrations. Present research on the specific properties of hydrogen bond stretching vibrations observed in the THz region provides useful information for quickly estimating the existence of intermolecular hydrogen bonds.

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.

We report on efficient terahertz-wave generation in organic and inorganic crystals by nonlinear wavelength conversion approach using a 3.3 μm femtosecond pulse laser. Experimental results reveal the relation between pump power and terahertz-wave output power, which is proportional to the square of the pump power at the range of mega- to tera-watt cm−2 class even if the pump wavelength is different. Damage threshold of organic and inorganic crystals are recorded 0.6 and 18 tera-watt cm−2 by reducing several undesirable nonlinear optical effects using mid-infrared source.

Injection-seeded terahertz (THz)-wave parametric generation pumped by subnanosecond pulses was demonstrated as an exploration of the effective pump pulse width. In experiments, pump sources with different pump widths of 35–850 ps were used. The relationship between the undesirable effect of stimulated Brillouin scattering (SBS) and the THz-wave output in LiNbO3 was analyzed. The transient effect on SBS was not actualized around the acoustic phonon lifetime of LiNbO3. Pump widths below 420 ps were found to provide THz-wave generation with an energy conversion efficiency of 10−4. With a pump width of 700 ps, which is around five phonon lifetimes, the conversion efficiency was decreased about one order of magnitude from that of the pump width of 420 ps. Additionally, the peak power of the THz-wave output was maintained at about 15 kW with an actual range of 10–20 kW below 200 ps. A continuous change in the THz-wave pulse energy was obtained for six-fold energy scaling from 100 to 600 nJ.

We report on the performance characterization of resonant tunneling diode (RTD) devices as a terahertz (THz)-wave detector. The frequency-dependent sensitivity of quasi-optical RTD detectors has been calibrated by using a frequency-tunable, sub-ns pulsed THz-wave source. The 0.78-THz, slot-antenna-integrated RTD device shows the peak responsivity of 80 V/W at 0.76 THz with the full-width at half-maximum bandwidth of 70 GHz.

Electromagnetic wave experiences phase changing both in propagation and resonance. Extreme phase changing is interesting and has its applications. Here we report a phase singularity phenomenon in double-layer metamaterial.

Arbitrary control of high-power THz polarization and frequency is studied by the numerical calculation based on the coupled wave approach. THz wave generation using organic crystals via optical rectification provides high conversion efficiency. To overcome the limitation of the generated THz field direction which comes from their optical anisotropy, we propose the synthesis of THz wave generated from two crystals whose optical axis are orthogonal each other. By the calculation, the linearly and circularly polarized THz wave generation by changing the time delay of two pump pulses is demonstrated.

Micro helical antennas whose resonance correspond to THz frequency region can be fabricated from biological algae spirulina via electroless metal coating. Radiation characteristics of the micro helical antenna are investigated by using THz near-field microscope. Both normal and end-fire mode radiations with different frequency characteristics can be successfully observed in real time.

We demonstrate a frequency-domain differential absorption spectroscopy in the terahertz (THz) range for gas sensing applications. Our system is based on an injection-seeded THz-wave parametric generator driven by a multi-furcated Nd:YAG microchip laser. Within a single excitation cycle of the laser, the is-TPG generates up to three narrowband THz-wave pulses that are separated from each other in time by 78 μs and in frequency by 11 GHz, which is due to the spatial hole burning effect in the laser cavity. These pulses can be directly used to measure differential absorption signals, and first and second derivative spectra of target gas molecules without measuring reference spectra.

An injection seeded terahertz (THz) parametric generator (is-TPG) is a high power and widely tunable THz-wave source which is promising instrument for a swept-source THz coherence tomography. In this paper, the depth resolution and the depth range are characterized using a narrow-linewidth is-TPG. The resolution of 0.2 mm is obtained in measurement and the depth range of15 mm is calculated.

We developed a security passengers gate using by terahertz-wave screening system. The security gate was based on a gas sensing system with an injection-seeded terahertz-wave parametric generator and was demonstrated with methanol gas in atmosphere detected as a phantom. The measurement was done within one second without disrupting foot traffic.

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.

There are many compelling characteristics of signals in the terahertz band (100GHz to 10THz) located between the millimeter wave band and the infrared band. In particular, terahertz waves have higher spatial resolution than mm-waves. Moreover, they can transmit through various substances such as plastics, fibers and paper, and can detect hazardous substances. Because of these features, interest in terahertz applications such as security screening is rising. However, there is a paucity of low-cost terahertz detectors. The Si-CMOS process technology is low-cost and highly integratable with readout electronics and on-chip signal processors. A key consideration for many of the terahertz-detector technologies is the need for additional process steps to make them compatible with CMOS technologies [1]. Recent antenna-type pixel detectors have shown success in high-speed operation and do not require extra process steps [2]; however, power consumption of each pixel and the sequential read-out architecture offsets the speed advantage.