CC BY
0Terahertz photonics of nonlinear crystals
N. Nikolaev
Institute of Automation and Electrometry SB RAS, Novosibirsk, Russia nazar@iae.nsk.su
One of the current issues that needs to be resolved in order to create powerful compact THz sources appropriate for a variety of real-world applications is the search for nonlinear optical crystals (NOCs) that can efficiently convert the frequencies of intense lasers into terahertz radiation. Here we provide an overview of NOCs, divided into four hypothetical classes: dielectric, ferroelectric, semiconductor, and molecular. Single-stage conversions are commonly employed in NOCs and can be described by the formalism of optical rectification of ultrashort laser pulses or by difference frequency generation. However, their efficiency is proportional to the square of the product of the frequency and the effective nonlinear coefficient n ~ (radeff)2 [1], and therefore tends to small values when converting frequencies of the visible and near-IR ranges into the THz region, which is from 2 to 3 orders of magnitude lower in frequency.
The solution may be a cascaded (multistage) conversion. It is based on the generation of infrared radiation in the NOC's local transparency windows, which are close to strong optical phonons, and the subsequent downconversion of that radiation into the THz range. In this instance, the phonon contribution may lead to a considerable increase in the quadratic susceptibility (df), which will produce a more effective frequency conversion than a single-stage procedure. Efficient narrow-band radiation generation near or between phonon frequencies has been reported in BBO at a frequency close to 10.6 THz [2], in KTP at 5.6 THz [3] and in GUHP crystal at 1.5 THz [4]. The development of the proposed approach requires the next step, which is a thorough study of the optical properties of NOCs in the phonon region, and finding the conditions for further frequency downconversion. In conclusion, we discuss the prospects of the proposed approach and the nonlinear crystal optics of the terahertz range in general.
The work is carried out with the support of the Russian Science Foundation, project #24-22-00442.
[1] Y.R. Shen, The Principles of Nonlinear Optics (Wiley), (2002).
[2] D.A. Valverde-Chavez, D.G. Cooke, Multi-cycle terahertz emission from p-barium borate, Journal of Infrared, Millimeter, and Terahertz Waves, vol. 38, pp. 96-103, (2017).
[3] M.H. Wu, W.C. Tsai, Y.C. Chiu, Y.C. Huang, Generation of ~100 kW narrow-line far-infrared radiation from a KTP off-axis THz parametric oscillator, Optica, vol. 6(6), pp. 723-730, (2019).
[4] A. Sinko, P. Solyankin, A. Kargovsky, V. Manomenova, E. Rudneva, N. Kozlova, A. Shkurinov, A monoclinic semiorganic molecular crystal GUHP for terahertz photonics and optoelectronics, Scientific reports, vol. 11(1), p. 23433, (2021).
