MULTILAYER OPTICS PROVIDES LASER-PLASMA-COUPLING INSIGHT

The Herpin theorem permits the reflectance of an arbitrary medium to b e reproduced with a substrate of one complex refractive index overlaid with a coating him of a second complex refractive index and an approp riate thickness. The reflectances of a variety of laser-plasma-couplin g experiments can be matched with a simple and physically meaningful p rocedure for choosing the fitting parameters. For short-pulse experime nts, s-light coupling is dominated by substrate interactions at electr on densities above critical and p-light coupling is dominated by reson ance at the critical density. The weak dependence of resonance absorpt ion on damping is the result of the harmonic-oscillator resonance form . As the plasma scale length becomes vanishingly small, the reflectanc e is that of the Fresnel equations, but surprisingly small plasma scal e lengths have dramatic effects on the p-light reflectance. Film-subst rate and WKB models predict similar resonance absorption, where both m odels are valid approximations and both should agree with multilayer a nd Helmholtz wave-optics calculations. For plasma scale lengths well b elow a wavelength, the film-substrate and the vacuum-heating models ar e closely related.