Pressure-induced degenerate frequency resonance in four-wave light mixing.

Phase-matched four-wave mixing in sodium vapor with a helium buffer gas is carried out in a noncoplanar geometry. The directions of the four light beams remain nondegenerate even if all frequencies are equal. A sharp zerodifference frequency resonance in the intensity of the parametrically generated fourth beam by three incident beams is observed that is proportional to the square of the pressure of the buffer gas. A theoretical interpretation of this effect caused by elimination of destructive interference between two coherent pathways is presented.

Quantitative characteristics of pressure-induced degenerate frequency resonance in four-wave mixing with continuous-wave laser beams.

The linewidth and intensity of the four-wave degenerate mixing signal in Na vapor have been observed as functions of He buffer-gas pressure with cw single-mode dye-laser beams. The width is independent of He pressure, and a quantitative comparison with theory is made. We also report the observation of a new effect, a four-wave mixing Raman-type resonance between two equally populated hyperfine levels.

Two-photon absorption coefficients in uv window and coating materials.

The two-photon absorption coefficients of some uv transmitting window and coating materials are measured at 355 nm and 266 nm. In materials with band gaps larger than 2homega, the nonlinear absorption is below the detection limit of 3 x 10(-6) cm/MW. CdF(2) has a two-photon absorption coefficient 1.6 x 10(-3) cm/MW at 266 nm, which is comparable with that of the alkali-halides. The newly developed window material LiYF(4) has no detectable nonlinear loss at 266 nm.

Role of cracks, pores, and absorbing inclusions on laser induced damage threshold at surfaces of transparent dielectrics.

The concentration of the electric field strength in the neighborhood of micropores and cracks may lower the nominal external intensity for electric avalanche breakdown by a factor 2-100 depending on the geometry of the crack and the dielectric constant. The presence of absorbing inclusions at the edge of microcracks will often be the dominant mechanism giving the lowest surface damage threshold. Inclusions and cracks with characteristic dimensions less than about 10(-6) cm will not lower the breakdown threshold appreciably.