Femtosecond Oscillator-Driven Terahertz Emitter With Conical Totally Internally Reflective Radiation Extractor

In this article, we propose and experimentally demonstrate an efficient Cherenkov-type terahertz emitter for optical-to-terahertz conversion of femtosecond laser pulses from an optical oscillator. The emitter consists of a tens of microns thick layer of LiNbO $_{3}$ sandwiched between two Si semicones. The semicones serve for bilateral extraction of the generated terahertz radiation from the LiNbO $_{3}$ layer and its collimation into a single parallel beam by total internal reflection at the semicones' lateral surfaces. The emitter can be operated with pump lasers of different wavelengths. Using a fiber laser (1.55- $\mu$ m wavelength) as a pump, the laser pulses of 2.5-nJ energy and 90-fs duration were converted to terahertz radiation with the efficiency of $\sim \!3\times 10^{-5}$ . Using a Ti:sapphire laser (800-nm wavelength) with the pulse energy of 7 nJ and duration of 90 fs, we achieved a conversion efficiency of $\sim \!5\times 10^{-4}$ , which exceeds the efficiency of the conventional collinear scheme with a ZnTe crystal by two orders of magnitude. The proposed emitter can be used as a universal radiation source for terahertz time-domain spectroscopy.