Yujie J. Ding

Efficient, tunable, and coherent 0.18–5.27-THz source based on GaSe crystal

Continuously tunable and coherent radiation in the wide range 56.8-1618 mum (0.18-5.27 THz) has been achieved as a novel and promising terahertz source based on collinear phase-matched difference frequency generation in a GaSe crystal. This source has the advantages of high coherence, simplicity for tuning, simple alignment, and stable output. The peak output power for the terahertz radiation reaches 69.4 W at a wavelength of 196 mum (1.53 THz), which corresponds to a photon conversion efficiency of 3.3%. A simple optimization of the design can yield a compact terahertz source.

A monochromatic and high-power terahertz source tunable in the ranges of 2.7–38.4 and 58.2–3540 μm for variety of potential applications

Based on phase-matched collinear difference-frequency generation in a single GaSe crystal, continuously tunable and coherent radiation in the extremely wide ranges of 2.7–38.4 and 58.2–3540 μm has been achieved. This unique source has the additional advantages of high coherence (narrow linewidth) and simple alignment. The peak output power for the terahertz radiation reaches 209 W at the wavelength of 196 μm (1.53 THz), which corresponds to a power conversion efficiency of 0.055%. Moreover, the terahertz transmission spectra on DNA macromolecules and protein were directly measured, demonstrating some potential and important applications of this terahertz source.

Experimental verification of the rainbow trapping effect in adiabatic plasmonic gratings

We report the experimental observation of a trapped rainbow in adiabatically graded metallic gratings, designed to validate theoretical predictions for this unique plasmonic structure. One-dimensional graded nanogratings were fabricated and their surface dispersion properties tailored by varying the grating groove depth, whose dimensions were confirmed by atomic force microscopy. Tunable plasmonic bandgaps were observed experimentally, and direct optical measurements on graded grating structures show that light of different wavelengths in the 500–700-nm region is “trapped” at different positions along the grating, consistent with computer simulations, thus verifying the “rainbow” trapping effect.

Continuously tunable and coherent terahertz radiation by means of phase-matched difference-frequency generation in zinc germanium phosphide

Continuously tunable and coherent radiation has been obtained in the ranges of 66.5–300 μm and 72.7–237 μm for two configurations of phase-matched difference-frequency generation, respectively, in zinc germanium phosphide. The highest output peak powers are measured to be 36 and 19 W for the two configurations, respectively.

Tunable terahertz waves generated by mixing two copropagating infrared beams in GaP

By mixing two copropagating coherent beams near 1 microm in a zinc blende GaP crystal, we have efficiently generated coherent terahertz (THz) waves. Such efficient conversion is made possible by use of a rest-strahlen band in the THz region to achieve phase matching in an isotropic crystal. A tuning range as wide as 71.1-2830 microm (0.106-4.22 THz) was achieved, whereas the highest output peak power reached 15.6 W at 173 microm. To obtain such a tuning range we continuously tuned the wavelength of one coherent infrared beam within a bandwidth of approximately 15.3 nm.

Backward second-harmonic generation in periodically poled bulk LiNbO(3).

We experimentally demonstrate backward second-harmonic generation in periodically poled LiNbO(3) with a 3.3- microm domain period. We observed higher-order phase matching near 1490, 1600, and 1700 nm (fundamental) for the 19th, 18th, and 17th orders, respectively, with a maximum conversion efficiency of 0.02%.

Compact and portable terahertz source by mixing two frequencies generated simultaneously by a single solid-state laser

By mixing two frequencies generated from a single Q-switched Nd:YLF laser in a GaSe crystal, an average terahertz output power reaches 1 μW within a bandwidth of 65 GHz at 1.64 THz.

Plasmonic surface-wave splitter

The authors present an analysis of a plasmonic surface-wave splitter, simulated using a two-dimensional finite-difference time-domain technique. A single subwavelength slit is employed as a high-intensity nanoscale excitation source for plasmonic surface waves, resulting in a miniaturized light-surface plasmon coupler. With different surface structures located on the two sides of the slit, the device is able to confine and guide light waves of different wavelengths in opposite directions. Within the 15 mu m simulation region, it is found that the intensity of the guided light at the interface is roughly two to eight times the peak intensity of the incident light, and the propagation length can reach approximately 42 and 16 mu m and at the wavelengths of 0.63 and 1.33 mu m, respectively. (c) 2007 American Institute of Physics.

High-Power Tunable Terahertz Sources Based on Parametric Processes and Applications

We summarize our progress made on developing widely tunable monochromatic terahertz sources. They have been implemented based on difference-frequency generation (DFG) in GaSe, ZnGeP, and GaP crystals. Using a GaSe crystal, the output wavelength was tuned continuously in the range of 66.5 to 5664 m (from 150 to 1.77 cm with the peak power reaching 389 W. Such a high peak power corresponds to a conversion efficiency of 0.1% (a photon conversion efficiency of 19%). A further optimization on the terahertz beam parameter may result in higher output powers and conversion efficiencies. Our experimental results indicate that within the range of 100-250 mum, the output peak powers were higher than 100 W. On the other hand, based on DFG in a ZnGeP crystal, the output wavelength was generated to be tunable in the ranges of 83.1-1642 mum and 80.2-1416 mum for two phase-matching configurations. The output power reached 134 W. Using a GaP crystal, the output wavelength was tuned in the range of 71.1-2830 m, whereas the highest peak power reached 15.6 W. GaP offers an advantage for tuning the output wavelength compared with GaSe and ZnGeP since crystal rotation is no longer required. Instead, one just needs to tune the wavelength of one mixing beam within a narrow bandwidth of 15.3 nm. Based on power scaling, a shoe box-sized tunable terahertz source is feasible. We also review our recent results obtained following the investigation of backward DFG and feasibility studies on backward parametric oscillation. The terahertz radiations produced by DFG have pulse durations of about 5 ns, wide tuning ranges, and narrow linewidths, which are quite different from the broadband terahertz pulses. We also describe a few important applications that were realized by taking advantage of the wide tuning range and narrow linewidths of terahertz pulses such as chemical sensing and differentiation of isotopic variants by measuring the rotational spectra of gases and terahertz imaging.

Backward optical parametric oscillators and amplifiers

Degenerate backward optical parametric oscillators and amplifiers have been considered, for the first time, to the best of our knowledge. When the pump intensity is four times the threshold pump intensity, the conversion efficiency reaches a maximum value. On the other hand, for nondegenerate optical parametric oscillators, the conversion efficiency always increases as the pump intensity increases. This behavior is different from those for forward optical parametric oscillators. In either configuration, the oscillation can occur without an external feedback. There is, however, a distributed feedback provided through the opposite propagation directions of the signal and idler. The threshold pump intensities for the oscillators can be achieved by the lasers currently available based on quasi-phase matching in several structures. As the input intensity for the backward parametric wave increases, the gain for this wave decreases dramatically if the pump intensity is on the order of the threshold or higher. When the input intensity is much larger than the threshold pump intensity, there is almost no gain regardless of the level of the pump intensity.