Optical difference frequency generation of far infrared radiation
- Univ. of California, Berkeley, CA (United States)
Three investigations of difference frequency generation (DFG) of far-infrared radiation by optical mixing are described: a theory of DFG by monochromatic, focused Gaussian pump laser beams, a theory of DFG by a picosecond pump laser pulse, and an experiment using ruby-pumped dye lasers. First, the theory of far-infrared generation by optical mixing of monochromatic, focused Gaussian beams in a uniaxial crystal is developed, taking into account the effects of diffraction, absorption, double refraction, and multiple reflections and total reflection at the boundary surfaces. (Reflection and transmission coefficients of a uniaxial crystal slab are derived by a new matrix technique.) Results of numerical calculations are presented. Focusing the pump beams appreciably enhances the far-infrared output despite the strong far-infrared diffraction. In a 1-cm long crystal, the optimum focal spot size is approximately equal to or smaller than the far-infrared wavelength for output frequencies less than 100 cm-1. Double refraction of the pump beams is relatively unimportant. Both far-infrared absorption and boundary reflections have major effects on the far-infrared output and its angular distribution. The former is often the factor which limits the output power. We show that a simple model treating the nonlinear polarization as a constant lie-radius Gaussian distribution of radiating dipoles adequately describes the effect of pump-beam focusing. We also compare the results of our calculations with those for second-harmonic generation. Second, a theoretical calculation of far-infrared power spectra generated by picosecond pulses in a nonlinear crystal is developed. The results are illustrated with two practical examples: LiNbO3 slabs oriented for rectification of the optical e-ray and for beating of the optical o-ray with the optical e-ray. The former is phase matched at 0 cm-1; the latter, at both the forward-(FCPM) and backward-collinear phase-matching frequencies. The one-dimensional, time-dependent electric field is discussed and then used to explain the origin of the oscillation periods in the power spectra. Finally, a series of experiments using a pair of ruby-pumped dye lasers and a novel dual-frequency dye laser system is described. With these two laser systems, continuously tunable far-infrared radiation in the frequency range 20 to 190 cm-1 was generated. Forward-, backward-, and non-collinear phasematching in LiNbO3 at frequencies between 20 and 160 cm-1 were investigated; 90 °tical FCPM was also observed in ZnO at 190 cm-1, CdS at 180 cm-1, and ZnS at 91 cm-1. The highest peak power ({approx}200 mW) and the broadest tuning range using a single crystal sample (40 to 160 cm-1) were obtained with noncollinear phase matching in LiNbO3.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- US Energy Research and Development Administration (ERDA)
- DOE Contract Number:
- AC02-05CH11231
- OSTI ID:
- 991950
- Report Number(s):
- LBL--6632
- Country of Publication:
- United States
- Language:
- English
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