Takayuki Shibuya

Extremely frequency-widened terahertz wave generation using Cherenkov-type radiation

Terahertz (THz) wave generation based on nonlinear frequency conversion is promising way for realizing a tunable monochromatic bright THz-wave source. Such a development of efficient and wide tunable THz-wave source depends on discovery of novel brilliant nonlinear crystal. Important factors of a nonlinear crystal for THz-wave generation are, 1. High nonlinearity and 2. Good transparency at THz frequency region. Unfortunately, many nonlinear crystals have strong absorption at THz frequency region. The fact limits efficient and wide tunable THz-wave generation. Here, we show that Cherenkov radiation with waveguide structure is an effective strategy for achieving efficient and extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide with 3.8 μm of thickness and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7.2 THz, without no structural dips successfully obtained. The tuning frequency range of waveguided Cherenkov radiation source was extremely widened compare to that of injection seeded-Terahertz Parametric Generator. The tuning range obtained in this work for THz-wave generation using lithium niobate crystal was the widest value in our knowledge. The highest THz-wave energy obtained was about 3.2 pJ, and the energy conversion efficiency was about 10-5 %. The method can be easily applied for many conventional nonlinear crystals, results in realizing simple, reasonable, compact, high efficient and ultra broad band THz-wave sources.

Čherenkov phase-matched monochromatic THz-wave generation using difference frequency generation with a lithium niobate crystal

We demonstrated a Cherenkov phase-matching method for monochromatic THz-wave generation using the difference frequency generation process with a lithium niobate crystal, which resulted in high conversion efficiency and wide tunability. We successfully generated monochromatic THz waves across the range 0.2-3.0 THz. We obtained efficient energy conversion in the low frequency region below 0.5 THz, and achieved a flat tuning spectrum by varying the pumping wavelength during THz-wave tuning.

THz-Wave Spectroscopy Applied to the Detection of Illicit Drugs in Mail

A series of experiments in the field of THz-wave spectroscopy is described. The measurement of absorption spectra of solid samples, powders, and liquids is demonstrated using various optical setups. One of the sources we used is a widely tunable coherent THz-wave generator relying on an optical parametric process: nanosecond Q-switched Nd : YAG laser light scattering from the polariton mode of a MgO-doped LiNbO3 crystal. For the measurement of the absorption of THz waves in liquids, a train of waves is allowed to oscillate inside a silicon prism. A liquid placed on a total internal reflection surface lowers the quality factor of the resonator, which allows determining the complex refraction index of the sample by frequency scanning. We have also developed a technique for THz chemical imaging, by introducing the component spatial pattern analysis. The spatial distribution of the chemicals, such as illicit drugs concealed in an envelope, is obtained from terahertz multispectral transillumination images and absorption spectra. To compensate for the low imaging speed, a prescreening step is proposed, by first detecting the presence of powders in the envelopes, which is achieved by measuring in real time the amount of scattering produced by the envelopes. Details and examples are provided.

Tunability enhancement of a terahertz-wave parametric generator pumped by a microchip Nd:YAG laser

We have enhanced the tunability of a terahertz-wave parametric generator using MgO:LiNbO3 pumped by a subnanosecond, passively Q-switched, microchip Nd:YAG laser. This pump source allows high-intensity pumping without damage to the nonlinear crystal and generates a narrow linewidth and tunable terahertz wave with injection seeding by an external cavity diode laser for the idler wave. The high-intensity pumping causes a gain curve broadening of the terahertz-wave parametric generation, especially in the high-frequency region. We obtained an output terahertz wave with a tuning range of 0.9-3 THz (100-330 microm), a peak power of 100 mW, and a linewidth of less than 10 GHz. This room temperature operated, palm-sized source promises to be a widely tunable terahertz-wave source that is suited to a variety of applications.

Prism-coupled Cherenkov phase-matched terahertz wave generation using a DAST crystal.

Terahertz (THz) wave generation based on nonlinear frequency conversion is a promising method for realizing a tunable monochromatic high-power THz-wave source. Unfortunately, many nonlinear crystals have strong absorption in the THz frequency region. This limits efficient and widely tunable THz-wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. Here, we propose a prism-coupled Cherenkov phase-matching (PCC-PM) method, in which a prism with a suitable refractive index at THz frequencies is coupled to a nonlinear crystal. This has the following advantages. Many crystals can be used as THz-wave emitters; the phase-matching condition inside the crystal does not have to be observed; the absorption of the crystal does not prevent efficient generation of radiation; and pump sources with arbitrary wavelengths can be employed. Here we demonstrate PCC-PM THz-wave generation using the organic crystal 4-dimethylamino-N-metyl-4-stilbazolium tosylate (DAST) and a Si prism coupler. We obtain THz-wave radiation with tunability of approximately 0.1 to 10 THz and with no deep absorption features resulting from the absorption spectrum of the crystal. The obtained spectra did not depend on the pump wavelength in the range 1300 to 1450 nm. This simple technique shows promise for generating THz radiation using a wide variety of nonlinear crystals.

Efficient Cherenkov-Type Phase-Matched Widely Tunable Terahertz-Wave Generation via an Optimized Pump Beam Shape

We demonstrated a Cherenkov-type phase-matching method for monochromatic THz-wave generation using the difference frequency generation process with a lithium niobate crystal. In Cherenkov-type phase-matching process, when the bulk lithium niobate crystal is used, the beam diameter of pump waves causes the phase mismatching for generating THz-wave. We decreased the phase-mismatch by optimizing the pump beam shape with cylindrical lenses, and ensured high conversion efficiency especially in the high-frequency region. We successfully generated monochromatic THz waves across the 0.2- to 4.0-THz range with a simple configuration.

Cherenkov phase matched THz-wave generation with surfing configuration for bulk Lithium Nobate crystal

We demonstrated a Cherenkov phase matched THz-wave generation with surfing configuration for bulk lithium niobate crystal. THz-wave output was enhanced about 50 times by suppressing phase mismatching for THz-wave propagation direction. The suppression was achieved by combining two pumping waves with dual wavelength with finite angle, and THz-frequency was controllable by changing the angle within 2.5 degrees range. Higher frequency THz-wave generation at around 4.0 THz was successfully obtained by the method.

Pulsed High Peak Power Millimeter Wave Generation via Difference Frequency Generation Using Periodically Poled Lithium Niobate

We successfully demonstrated pulsed high-peak-power millimeter wave generation using periodically poled lithium niobate (PPLN) under collinear phase matching. We fabricated 0.5-mm-thick PPLN with a 40 mm interaction length for generating 250 and 300 GHz waves. Pump waves with very close dual wavelength output from a KTiOPO4 (KTP) optical parametric oscillator had a pulse energy of 2 mJ, pulse duration of 15 ns, and wavelength around 1300 nm. Millimeter waves were generated successfully, and the highest peak power obtained was estimated to be about 0.1 mW. This value is sufficiently high for envelope detection using a Schottky barrier diode. An active imaging system in the millimeter wave range could be realized using such a pulsed emitter and a Schottky barrier diode array.

Widely Tunable Monochromatic Cherenkov Phase-Matched Terahertz Wave Generation from Bulk Lithium Niobate

We report an extremely widely tuneable monochromatic Cherenkov phase-matched terahertz (THz) wave generator using a bulk lithium niobate crystal. This was achieved by optimizing the incident conditions of the pump beams, and the surface-emitting condition was produced by total internal reflection of the pump beam at the THz wave output surface of the nonlinear optical crystal. Effective generation at higher frequencies was realized by reducing the absorption inside the crystal, and as a result, we achieved a tuning range from 0.2 to 6.5 THz with a very simple configuration.

A broad-band THz radiation detector using a Nb-based superconducting tunnel junction

We have proposed and been developing a new broad-band terahertz (THz) radiation detector using a Nb-based superconducting tunnel junction (STJ). The STJs were fabricated on LiNbO/sub 3/ and LiTaO/sub 3/ mono-crystal substrates. We radiated monochromatic THz pulses in the range 1-2 THz with the pulse repetition rate of 49 Hz, and successfully detected the corresponding periodic signals via a charge-sensitive preamplifier. The current signal obtained by the derivative of the output signal gave the time response of about 15-20 microseconds. The frequency response was compared with that of a conventional pyro-electric sensor. We found their frequency response showed the similar shape in frequency, demonstrating the flat response of the STJ detector in 1-2 THz. These results show that the detector can be used as a broad-band THz and far-infrared radiation detector above the frequency corresponding to the energy gap of the superconductor used for the base electrode.