Abstract

During this poster presentation, we will highlight our recent progress on the development of widely-tunable monochromatic THz sources, based on difference-frequency generation (DFG) in GaSe, ZnGeP₂, and GaP, respectively. Using a GaSe crystal the output wavelength was tuned in the range of from 58.2 µm to 3540 µm (from 172 cm^-1 to 2.82 cm^-1) with the peak power reaching 209 W. Most recently, we significantly improved the output power of our THz source from 209 W to 389 W. This high peak power corresponds to a conversion efficiency of ∼ 0.1%. A further optimization on the THz beam parameter may result in even higher output powers and conversion efficiencies. Our experimental results indicate that within the range of 100-250 µm the output peak powers are higher than 100 W. In addition, we further extended the tuning range to 66.5 µm – 5664 µm (150 – 1.77 cm^-1), i.e. into the mm-wave region. Such a tuning range is the widest ever produced for a continuously-tunable and coherent tabletop THz source. Moreover, the conversion efficiency of 0.1% is also the highest ever achieved for a tabletop system using a commercially-available laser system as the pump sources. The only THz source that could compete with our source is a free-electron laser, which is extremely bulky and consumes a huge electrical power. On the other hand, based on DFG in a ZnGeP₂ crystal the output wavelength was tuned in the ranges of 83.1–1642 µm and 80.2–1416 µm for two phase-matching configurations, respectively. The output power has reached 134 W. Finally, using a GaP crystal the output wavelength was tuned in the range of 71.1−2830 µm whereas the highest peak power was 15.6 W. The advantage of using GaP over GaSe and ZnGeP₂ is obvious: crystal rotation is no longer required for achieving wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within a bandwidth of as narrow as 15.3 nm. Since the above THz radiations have pulse durations of about 5 ns, these THz pulses are quite different from the subpicosecond THz pulses generated by optical rectification, photoconduction, and Cherenkov radiation in terms of tunability, pulse width, peak power, and especially the linewidth. Furthermore, the output wavelengths of our THz sources can be tuned in extremely-wide ranges, unlike the quantum-cascade lasers each of which can only generate a fixed wavelength. Therefore, certain applications which can be realized using our widely-tunable monochromatic THz sources would not be possible if relying on the subpicosecond THz pulses or quantum-cascade lasers instead. Most recently, we also implemented a new scheme for detecting THz waves based on frequency upconversion at room temperature, i.e. by mixing the THz wave with an infrared laser beam, we observed the upconverted signal at the wavelength just slightly longer than that of the infrared laser. To date the detectable signal is only an order of magnitude higher than that by using a bolometer. Such a scheme allows us to measure the peak power, wavelength, linewidth, and pulse width of a THz beam at room temperature.

© 2005 Optical Society of America