Experimental and theoretical investigation of third-harmonic generation in phase-matched dye solutions

The phase-matched third-harmonic light generation in dye solutions is studied experimentally and theoretically. In the experiments picosecond light pulses of a passive mode-locked Nd-glass laser are converted to the third-harmonic frequency. A third-harmonic conversion efficiency of up to 4×10^–4 was achieved for one of the dyes investigated (1,3,3,1,3,3-hexamethylindocarbocyanine iodide in hexafluoroisopropanol). The theoretical calculations determine the influence of various dye and solvent parameters on the conversion efficiency. The conversion efficiency is found to be limited by excited-state absorption of pump laser light and third-harmonic light from the S₁-state to higher singlet states. The S₁-state is mainly populated by two-photon absorption. Amplified spontaneous emission may reduce the limiting effects of excited-state absorption. Phase changes caused by the non-linear refractive index and the refractive index dispersion within the spectral bandwidth of the laser pulses reduce the conversion efficiency. Under ideal conditions conversion efficiencies up to 10% may be achieved.