Takuya Akiba

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.

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.

Terahertz-Wave Generation Using a 4-Dimethylamino-N-methyl-4-stilbazolium tosylate Crystal Under Intra-Cavity Conditions

We succeeded in generating terahertz waves using difference-frequency generation (DFG) in an organic 4-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) crystal under intra-cavity conditions. We constructed a dual wavelength output optical parametric oscillator (OPO) with two potassium titanium oxide phosphate (KTP) crystals for the DFG pumping source, and placed the DAST crystal inside the OPO cavity to act as a nonlinear wavelength conversion element. Our experimental measurements confirmed that intra-cavity generation was possible and that the output level was approximately the same as that of normal excitation using the external OPO output.

Behavior of three waves in Cherenkov phase matched monochromatic terahertz wave generation investigated by numerical analysis

The dynamical behavior of electric fields in Cherenkov phase matched terahertz wave generation was numerically analyzed. The effect of diffraction of the terahertz wave was the dominant factor for efficiency of terahertz wave generation. Tightly-focused pump and signal beams radiated efficient terahertz wave drastically in the transverse direction. Propagating terahertz wave at the Cherenkov angle separated from the pump and signal beams; thus destructive interaction of the three waves under phase mismatched conditions no longer occurred. The effect resulted in continuous pump depletion and efficient energy conversion from the pump wave to the terahertz wave. Our results indicate that complete pump depletion could be achieved with Cherenkov phase matching.