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<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> sandwiched between two Si semicones. The semicones serve for bilateral extraction of the generated terahertz radiation from the LiNbO<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula> 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-<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim \!3\times 10^{-5}$</tex-math></inline-formula>. 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 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\sim \!5\times 10^{-4}$</tex-math></inline-formula>, 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.