Cherenkov Phase-matching Method Research Articles

Terahertz generation in a thin GaAs slab in a tapered parallel plate waveguide by femtosecond laser excitation at 1560 nm

A comparative experimental study of terahertz generation by optical rectification of femtosecond laser pulses with a wavelength of 1560 nm in a 100 μm thick slab of GaAs sandwiched between the walls of a tapered metallic waveguide and in a bulk GaAs crystal has been performed. In both cases, the velocity mismatch between the optical and terahertz beams propagating in GaAs was overcome by using the Cherenkov phase-matching method. The dependence of the terahertz time-domain signal amplitude, spectral bandwidth, and dynamic range on the focusing conditions were measured and compared. For both samples, the most efficient generation was observed for the 50 mm focal length lens. A significant improvement in the directionality of the terahertz emission from the thin GaAs slab due to the tapered waveguide was predicted by FDTD simulations and this was also confirmed experimentally.

Generation of single-cycle terahertz pulse using Cherenkov phase matching with 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate crystal

This is the first report of single-cycle terahertz (THz)-wave pulse generation with a wide band (over 6 THz) and a high dynamic range (over 70 dB) using an organic nonlinear optical crystal, 4-dimethylamino-N′-methyl-4′-stilbazolium tosylate (DAST), and the prism-coupled Cherenkov phase-matching method. The prism-coupled approach allowed the use of a DAST crystal as a Cherenkov-type emitter for reduced light absorption by the crystal, resulting in the generation of single- and half-cycle THz pulse shapes, which were detected using dipole and bow-tie antennas, respectively. The resulting THz pulse generation is suitable for various THz-wave application technologies, such as tomography.

Wide Spectrum Terahertz-Wave Generation From Nonlinear Waveguides

We have obtained efficient terahertz (THz)-wave generation from surface-emitting or waveguide propagation using Cherenkov-type radiation with nonlinear optical (NLO) crystals. This approach presented the following advantages: 1) many crystals can be used as THz-wave emitters; 2) it is not necessary to satisfy the phase-matching condition inside the crystal; and 3) THz-wave generation is not suppressed by the absorption in the crystal. We demonstrated that the THz-wave generation was enhanced 50× by suppressing the phase mismatch with a surfing configuration for bulk lithium niobate (LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) crystals. Using the prism-coupled Cherenkov phase-matching method with organic crystal 4-dimethylamino-N-metyl-4-stilbazolium tosylate, we produced tunable THz radiation within ~0.1-10 THz. There was no evidence of significant absorption in the crystal. To show the advantages of the waveguide emitter, we demonstrated THz generation using a ridge-type LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> waveguide with a thickness of 3.8 μm. We obtained THz-wave generation with an ideal temporal half-cycle pulse and a wide tuning range, which is applied to reflection tomography imaging.

Half Cycle Terahertz Pulse Generation by Prism-Coupled Cherenkov Phase-Matching Method

Nonlinear optical terahertz wave generation is a promising method for realizing a practical source with wide frequency range and high peak power. Unfortunately, many nonlinear crystals have a strong absorption in the terahertz frequency region. This limits efficient and widely tunable terahertz wave generation. The Cherenkov phase-matching method is one of the most promising techniques for overcoming these problems. We propose a prism-coupled Cherenkov phase-matching method, in which a prism with a suitable refractive index at terahertz frequencies is coupled to a nonlinear crystal. We demonstrate prism-coupled Cherenkov phase-matching terahertz generation using the DAST and LiNbO3 crystals. With a DAST crystal, we obtain a spectral flat tunability up to 10 THz by difference frequency generation. With a LiNbO3 crystal, we observe a spectral flat broadband terahertz pulse generation up to 5 THz pumped by a femto second fiber laser. The obtained temporal waveform is an ideal half cycle pulse suitable for reflection terahertz tomography.

Nonlinear optical terahertz wave sources

We developed a Cherenkov phase-matching method for monochromatic THz-wave generation using the DFG, process with a lithium niobate crystal, which resulted in both high conversion efficiency and wide tunability. Although THz-wave generation by Cherenkov phase matching has been demonstrated using femtosecond pumping pulses, producing very high peak power, these THz-wave sources are not monochromatic. Our method generates monochromatic and tunable THz, waves using a nanosecond pulsed laser source. We also show that Cherenkov radiation with waveguide structure is an effective strategy for achieving extremely wide tunable THz-wave source. We fabricated MgO-doped lithium niobate slab waveguide and demonstrated difference frequency generation of THz-wave generation with Cherenkov phase matching. Extremely frequency-widened THz-wave generation, from 0.1 to 7 THz was observed.

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.

�?herenkov phase-matched monochromatic THzwave 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.