No abstract

We demonstrated the separation of the component spatial patterns of chemical samples in transillumination terahertz (THz) images using known spectral curves. The images and spectral data were measured between 1.3 and 1.8 THz, using a widely tunable coherent THz-wave parametric oscillator source. This method could be effective for analyzing spatial patterns and the concentrations of components with various chemical compositions.

The hottest frequency region in terms of research currently lies in the ‘frequency gap' region between microwaves and infrared: terahertz waves. Although new methods for generating terahertz radiation have been developed, most sources cannot generate high-brightness terahertz beams. Here we demonstrate the generation of ultrabright terahertz waves (brightness ~0.2 GW/sr·cm2, brightness temperature of ~1018 K, peak power of >50 kW) using parametric wavelength conversion in a nonlinear crystal; this is brighter than many specialized sources such as far-infrared free-electron lasers (~1016 K, ~2 kW). We revealed novel parametric wavelength conversion using stimulated Raman scattering in LiNbO3 without stimulated Brillouin scattering using recently-developed microchip laser. Furthermore, nonlinear up-conversion techniques allow the intense terahertz waves to be visualized and their frequency determined. These results are very promising for extending applied research into the terahertz region, and we expect that this source will open up new research fields such as nonlinear optics in the terahertz region.

Using room-temperature parametric oscillation of a LiNbO3 crystal pumped by a Q-switched Nd:YAG laser with a simple configuration, we have realized a widely tunable coherent terahertz- (THz-) wave source in the range between 1 and 3 THz. Inasmuch as the THz wave is affected by total internal reflection at the crystal edge, we used a Si prism coupler to couple out the THz wave. We introduce an arrayed Si-prism coupler that increases the efficiency and decreases the diffraction angle. By use of the arrayed-prism coupler, there is a sixfold increase in coupling efficiency and a 40% decrease in the far-field beam diameter, compared with the use of a single-prism coupler. We discuss the negative effect of the free carriers at the Si-prism surface that is excited by the scattered pump beam, and the positive effect of cavity rotation on the unidirectional radiation of the THz wave from a Si prism.