Coherent UV light sources can be utilized for various chemical, medical and industrial applications. Third-harmonic generation (THG) of solid-state lasers using a single nonlinear optical (NLO) medium, via direct third-order and cascaded second-order NLO processes, is an appealing approach for generating coherent UV light, which has several advantages over other UV light sources, such as compact size, easy maintenance and chemical safety. Although efficient single-medium THG has been realized recently by using $β-BaB_2O_4$ (BBO) and $KTiOPO_4$ (KTP), the single-medium THG have not yet been fully characterized. In this thesis, theoretical and experimental aspects of single-medium THG in $CsLiB_6O_10$ (CLBO) crystals are exploited.
Third-order NLO susceptibility was derived by using a classical anharmonic oscillator model. The result reveals a new process that gives third-order NLO effect, which is attributed to cascading of second-order coupling at the same local site. The characteristics of various processes related to third-order NLO effect were predicted.
Theoretical analysis for single-medium THG leads to an amplitude solution for third-harmonic electric field under undepleted-pump approximation. An overall effective third-order susceptibility was newly defined for single-medium THG and calculated for phase-matched THG of 1.064㎛ in CLBO. The characteristics of the overall effective third-order susceptibility were discussed in association with optimal phase-matching (PM) conditions.
A full characterization of single-medium THG in CLBO crystals was made using a Nd:YAG laser. PM properties and the overall effective third-order susceptibility were examined. The results show a number of unique features of single-medium THG, such as distortion of phase-mismatched behavior and asymmetric intensity distributions with respect to polarized angle, which is in excellent agreement with the theoretical analysis. Third-harmonic intensities were measured as a function of fundamental beam intensity and the expected cubic dependence was confirmed. A conversion efficiency of 0.20% was achieved at a fundamental intensity of 8.4 GW/㎠ This demonstrates that CLBO is a promising material for UV light generation via single-medium THG.
