The most difficult problem in mastering the terahertz range is development of effective methods of generation of coherent terahertz radiation. It is difficult to apply the physical principles of generation of optical and microwave radiation well developed over the past half century in the terahertz range that is located in the spectral region between the infrared and microwave bands.
The progress of femtosecond laser technology in the last decade has greatly advanced the development of compact terahertz sources. A conventional approach to generation of terahertz radiation is to expose photoconducting antennas, electro-optical and gas environment to femtosecond laser pulses. However, the optical-to-terahertz energy conversion efficiency is very low, about 0.001—0.01%. Consequently, the development of new, more efficient methods of conversion of femtosecond laser pulses into terahertz radiation is still a topical problem.

Scheme of terahertz radiation generation in a sandwich structure (left) and efficiency of optical-to-terahertz conversion as a function of pump energy for different laser pulse durations (right); in the insert — terahertz pulse spectrum

Theoretical and experimental studies aimed at finding effective schemes of generating terahertz radiation are carried out by R. A. Akhmedzhanov,
S. B. Bodrov, N. V. Vvedensky, E. V. Suvorov, and other IAP RAS researchers. These schemes are based, in particular, on the phenomena of electromagnetic field transformation in a dense laser plasma formed at ionization of various gases (including atmospheric air) by intense femtosecond laser pulses. Original methods of frequency tuning, controlling polarization and directi-vity pattern of the generated terahertz radiation, as well as new methods of enhancing generation efficiency and peak power of terahertz pulses were proposed and investigated. As a result, reasonably compact and relatively easy to implement laser plasma schemes were developed that are capable of providing stable generation of a train of megawatt terahertz pulses with very high (from tens of hertz to hundreds of kilohertz) repetition rate. Also, a scheme based on the Cherenkov radiation in a special sandwich structure consisting of a 30-micrometer layer of lithium niobate (LiNbO3), a silicon prism and a metal substrate with variable air gap was proposed and implemented. Record optical-to-terahertz conversion efficiency of 0.25% and the possibility of tuning the emission spectrum of terahertz radiation by changing the width of the air gap were demonstrated in experiments using this scheme. High efficiency of the optical-to-terahertz conversion is achieved by means of laser pulses with a moderate energy of 1—10 µJ and duration of 50—200 fs, which allows using compact and efficient fiber lasers for optical pumping.
Another effective method of generating terahertz radiation is based on the use of special laser pulses with intensity front tilted relative to the phase fronts. These pulses permit obtaining the phase-matching condition in crystals where the phase velocity of terahertz radiation is less than the group velocity of the laser pulse. Research aimed at optimizing this method is currently conducted at IAP RAS. The generation of THz radiation with 0.2% efficiency in cryogenically cooled (to 77 K) LiNbO3 was demonstrated. The theory of generation that agrees well with experiment and predicts possible ways of efficiency enhancement was developed.

Scheme of terahertz generation by a laser pulse with tilted intensity front in a cooled LiNbO3 crystal (left); efficiency of optical-to-terahertz conversion as a function of pump energy (in the center), and temporal dynamics of spatial intensity distribution of terahertz beam (right)

Spectra of mutual beats of backward wave tube at a frequency of ~100 GHz and the 1092-th component of terahertz comb with (bottom) and without (top) stabilization.

In addition to works on the generation of terahertz radiation, search of new applications of such radiation is carried out. Broadband terahertz pulses are widely used for spectroscopy of various materials and study of the dynamics of fast processes. For example, IAP RAS was a pioneer in using terahertz pulses for diagnosing decay of a plasma filament formed in air at filamentation propagation of a high-power femtosecond laser pulse. The dynamics of electron density in a plasma channel was studied experimentally by the method of terahertz scattering on plasma in external electric field. It was shown that the plasma density reduces by two orders of magnitude in ~ 2 ns, and the decay slows down about twice in the 8 kV/сm electric field.

Generation of broadband terahertz radiation using a train of femtosecond laser pulses enables producing a frequency comb in the range from zero to a few terahertz with a step between the components equal to the laser pulse repetition rate. When the repetition rate is stabilized, such a terahertz comb has a unique spectral purity of components. Frequency stabilization of a backward wave tube of millimeter and submillimeter ranges by means of phase-locked-loop frequency control by one of the components of the terahertz comb formed by a Ti:Sa femtosecond laser in a GaAs Schottky diode was first proposed and demonstrated experimentally at IAP RAS. The stability of radiation frequency was better than 10 Hz. The results obtained open up a possibility of creating basically new frequency synthesizers possessing a record narrow emission spectrum (M. Yu. Tretyakov, A. P. Shkaev, A. M. Kiselev, S. B. Bodrov).