Optical properties of borate family nonlinear crystals and their application in sources of intense terahertz radiation

Abstract

Nonlinear crystals of the borate family are efficient harmonic generators for intense laser sources because of their high laser-induced damage threshold at near-infrared wavelengths. Recent studies have shown that they exhibit relatively low absorption coefficients at sub-terahertz frequencies, which could enable them to generate terahertz radiation. Based on this assumption, we compare terahertz sources based on the frequency down-conversion of the radiation from a titanium-sapphire amplifier in crystals of barium beta-borate (β-BaB2O4), lithium triborate (LiB3O5), and lithium tetraborate (Li2B4O7). The calculation of collinear three-wave interactions, which provide the generation of the subterahertz difference frequency, is carried out considering the previously studied dispersion of the main components of the terahertz refractive index of these crystals. The phase-matching conditions and the corresponding coherence lengths are determined for each of the crystals. Taking into account the quadratic susceptibility tensors, the coefficients of the effective nonlinearity are calculated, and the terahertz generation efficiency in crystals with different cuts is evaluated and compared. The down-conversion in the β-BaB2O4 crystal is numerically shown to be three and five orders of magnitude more efficient than in the LiB3O5 and Li2B4O7 crystals, respectively. Thus, terahertz generation in a sample of β-BaB2O4 crystal with a cut that provides phase-matching for a frequency of 0.3 THz (θ = 5°) has been studied experimentally using radiation from a titanium-sapphire amplifier. The comparison of the experimental data and the numerical results leads to the conclusion that the main contribution to the generation process is given by the o – e → e, e – e → o, and o – o → o types of interaction. The peak terahertz power reaches 20 kW. The data obtained in this work will be useful for the development of intense sub-terahertz radiation sources based on the energy conversion of high-power laser sources. It is estimated that tens of GW of peak terahertz power can be achieved by increasing the intensity of the optical fields to pre-threshold values for the β-BaB2O4 crystal. A source of this intensity can be used in systems for sounding the atmosphere as well as in charged particle accelerators.

Keywords

• nonlinear crystals
• terahertz frequency generation
• phase matching
• refractive index
• absorption coefficient