We present results of our research on generation of THz radiation in liquid nitrogen. We used a dual-frequency scheme when emissions of the main laser frequency and its second harmonic are mixed in the same medium. The research showed a possibility of effective conversion of optical radiation into THz radiation.
This work is partially supported by RFBR grant №18-29-20104.
The source of electromagnetic radiation in the THz band on the basis of laser spark was firstly presented in many years ago. An experiment in which a liquid, namely, water was used for the conversion of femtosecond radiation into the THz one is also described. Water is a polar liquid which has high absorption in the THz frequency range and the authors of previously published works have to use for the experiments the very thin water films. Unlike water, considerable absorption both in THz and NIR ranges is absent in liquid nitrogen. In our experiments the laser beam ω is directed vertically from top to bottom with the help of a set of mirrors and is focused by a lens inside the LN in Dewar vessel. The Dewar vessel is mounted on a microscopic translation stage and can be moved vertically, which enables the regulation of the position of the lens focus regarding the surface level of the LN. THz radiation generated in the beam-waist is reflected from a flat aluminum mirror and is collimated by the parabolic mirror.
First of all, we obtained the THz radiation with the use of a dual frequency scheme in an experimental set-up without liquid nitrogen, from a routine optical air breakout. After that, the laser beams on the fundamental and the second harmonic was focused into LN and the THz radiation was also observed. IN the talk we show the dependences of the THz pulse energy on the lens focus position regarding the surface of liquid nitrogen (the laser beam waist is located inside the liquid if z < 0 and in the air if z > 0). The intensity of the generation changes exponentially as the beam-waist position varies and a and a leap in the level is observed when the level of the surface is passed.
Also we have studied how THz yield scales with laser pulse duration and its energy, angle of rotation of the BBO crystal and measured spectra of the THz radiation.
The present work describes broadband generation of THz radiation first obtained in liquid gas. Unlike previous attempts to generate THz radiation in water, in our experiments we used liquid nitrogen, and the results enabled us to suggest a physical mechanism of this process.