Koji Suizu

Random Frequency Accessible Broad Tunable Terahertz-Wave Source Using Phase-Matched 4-Dimethylamino-N-methyl-4-stilbazolium Tosylate Crystal

Random frequency accessible, ultra-broad-band (1.5–37 THz) THz-wave generation was demonstrated using difference frequency generation (DFG) in an organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal. Such DAST crystals are promising materials for efficient and high-power THz-wave generation because of their very high nonlinearity and low refractive index dispersion between the near-infrared region and the THz-wave region. We can use the highest nonlinear component of DAST, d11 (about 230 pm/V), to generate THz waves using DFG because the collinear phase-matching condition of the Type 0 configuration is satisfied. We constructed a dual-wavelength optical parametric oscillator (OPO) with two KTP crystals pumped by a frequency-doubled Nd:YAG laser. Each KTP crystal was set on a galvano scanner. The angle of each crystal was controlled independently. The OPO is tunable at 1300–1900 nm, giving an ultra-broad tunable range of the THz wave. We generated an ultra-broad tunable THz wave using only one DAST crystal without any change of the experimental setup aside from the computer-controlled galvano-scanner angle change. The highest THz-wave energy of 10 nJ was obtained at around the 26 THz region under 2 mJ of pumping energy. Also, the THz-wave source can access a desired THz frequency at every pulse (50 Hz at present). The galvano scanner has 1 kHz of response, with 1-ms frequency access speed.

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.

Broadband-Frequency-Tunable Sub-Terahertz Wave Generation Using an Optical Comb, AWGs, Optical Switches, and a Uni-Traveling Carrier Photodiode for Spectroscopic Applications

We present a monochromatic sub-terahertz signal generation technique using an optical comb signal, arrayed waveguide gratings (AWGs), and a uni-traveling carrier photodiode (UTC-PD) for spectroscopic applications. This scheme offers random or continuous frequency tuning in the range between 100 GHz and up to 1 THz. In addition, since a RF synthesizer is employed as a reference signal source of the photonic frequency multiplier, frequency locking with external instruments and reliable operation are offered. Highly coherent optical comb signal for the photonic frequency multiplication provides a narrow linewidth and very low phase noise in the generated sub-terahertz signal. For 125 GHz, the phase noise is approximately -92 dBc/Hz at the offset frequency of 10 kHz. This is larger than that of the 25-GHz RF source by about 13 dB and agrees well with the theory regarding phase noise multiplications due to frequency multiplication. For generating monochromatic signals, unwanted spurious signals are suppressed in the optical domain over a wide range with two AWGs, and the suppression ratio is expected to be better than 46 dBc. Utilizing the implemented sub-terahertz signal generator with a J-band UTC-PD module, absorption lines of N2O were measured in the frequency range between 240 and 360 GHz and compared with theoretical calculations.

High-power terahertz-wave generation using DAST crystal and detection using mid-infrared powermeter

The exact power output of a table-top-sized terahertz (THz)-wave source using a nonlinear optical process has not been clarified because detectors for these experiments [Si bolometer, deuterated triglycine sulfate (DTGS), etc.] are not calibrated well. On the other hand, powermeters for the mid-infrared (mid-IR) region are well established and calibrated. We constructed a high-power dual-wavelength optical parametric oscillator with two KTP crystals as a light source for difference frequency generation. The obtained powers of dual waves were 21 mJ at ~1300 nm, ten times higher than that of the previous measurement. The device provides high-power THz-wave generation with ~100 times greater output power than that reported in previous works. A well-calibrated mid-IR powermeter at ~27 THz detected the generated THz wave; its measured energy was 2.4 microJ. Although the powermeter had no sensitivity in the lower-frequency range (below 20 THz), the pulse energy at such a low-frequency region was estimated in reference to the output spectrum obtained using a DTGS detector: the energy would be from about the submicrojoule level to a few microjoules in the THz-wave region.

High-resolution time-of-flight terahertz tomography using a femtosecond fiber laser

High-resolution tomographic imaging is demonstrated using a reflection-type terahertz time-domain spectroscopy. To realize a practical system for general use, a robust all-fiber laser was used as the pump light source. Broadband terahertz waves were generated with the combination of optical pulses compressed to 17 fs using optical fibers and a DAST crystal. Using deconvolution signal processing, the wideband spectrum of the generated terahertz waves provided high-axial resolution leading to successful imaging of a multilayered structure containing a 2-microm-thin GaAs layer. To our knowledge, this is the first demonstration of terahertz tomographic imaging of such a thin layer.

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.

Surface-emitted terahertz-wave generation by ridged periodically poled lithium niobate and enhancement by mixing of two terahertz waves

Surface-emitted terahertz- (THz-) wave generation by difference-frequency mixing with ridge-shaped periodically poled lithium niobate (PPLN) was demonstrated. The PPLN had a ridge height of 300 microm, a thickness of 20 microm, and an interaction length of 35 mm. The ridge behaves as a slab waveguide for optical pump beams. The PPLN gives rise to THz waves in opposite directions, perpendicular to the pump-beam direction. Reflecting the THz wave on one side and overlapping it with the THz wave on the other side increased the total THz-wave intensity approximately 2.7 times compared with that without reflection and mixing.

Direct observation of the topological charge of a terahertz vortex beam generated by a Tsurupica spiral phase plate

A terahertz (THz) spiral phase plate with high transmission (>90% after Fresnel correction) and low dispersion has been developed based on the Tsurupica olefin polymer. Direct observations of the topological charge (both magnitude and sign) of a THz vortex beam are performed by using a THz camera with tilted lens focusing and radial defect introduction. The vortex outputs with a topological charge of ±1 (or ±2) are obtained at a frequency of 2 (or 4) THz.

Terahertz Sensing of Thin Poly(ethylene Terephthalate) Film Thickness Using a Metallic Mesh

A metallic mesh, i.e., a metallic thin film with regularly spaced openings, works as a band-pass filter in the terahertz region. The filters transmittance is far higher than the open-area ratio and its transmission spectrum is affected by the index of refraction in and above the openings. Therefore, metallic-mesh films can be used as high-sensitivity sensing or imaging conduits if samples adhere to the metal surface. Here, we report on our success in detecting 1-µm-thickness differences in poly(ethylene terephthalate) (PET) films and recording two-dimensional (2D) images using metallic-mesh sensing and imaging applications.

Monochromatic-Tunable Terahertz-Wave Sources Based on Nonlinear Frequency Conversion Using Lithium Niobate Crystal

We review widely tunable terahertz (THz)-wave generation by optical parametric processes using lithium niobate crystal. Applying the parametric oscillation of or a MgO-doped crystal pumped by a nanosecond Q-switched Nd:YAG laser, we realized coherent THz-wave sources with a simple configuration widely tunable in the range of 0.7-3 THz. For efficient coupling of the THz wave, we used a monolithic grating coupler or a Si-prism array coupler. In addition, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A line width of about 110 MHz (0.003 cm-1) was assured by measuring the absorption spectrum of low-pressure water vapor. Using the difference frequency generating method with a periodically poled crystal, we achieved higher conversion efficiency and realized continuous THz-wave generation. This room temperature operated, tabletop system promises to be a widely tunable THz-wave source suited to a variety of applications.