We have measured an enhancement factor of Raman signal up to 30 times using a Fabry-Pérot structure made of porous silicon (PS) layers of different porosity. The obtained enhancement was due to the coupling of the laser radiation and Stokes photons of porous silicon with the microcavity mode at the optimal laser beam incidence and scattering angles. Our results provide a way to increase the sensitivity of Raman spectroscopy for studying the species inside porous silicon which can considerably influence the properties of this material and hence of PS based devices.

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We show experimentally that the radiation generated in infrared-visible sum-frequency mixing at an airsilver interface can be greatly enhanced when the visible input beam excites a surface plasmon-polariton at the interface. With either a prism or a grating used to couple the visible radiation with the surface polariton, the sum-frequency-generation yield is observed to be enhanced by a factor of 10 2 for the prism and 10 4 for the grating for counterpropagating infrared and visible input beams. The result for the prism configuration can be simply understood in terms of the field enhancement associated with the surface polariton excited by the visible input beam. For the grating configuration there is an additional effect in that the nonlinear polarization at the sum frequency can also couple with a surface polariton. As a result the effective interaction length of the sum-frequency-mixing process is sizably increased. The experimental results are in good agreement with estimates based on this model.

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The conditions of the existence and transformations of surface electromagnetic waves (SEWs) on metals (surface plasmons) and dielectrics (phonon-polaritons) are discussed. Interferometric SEW experiments provide the possibility for the direct determination of the real and imaginary parts of the dielectric constants at the frequencies in the tuning range of a free electron laser (FEL) without any preliminary models. The important role of the outstanding facilities of FEL—namely, the broad tuning range, high power, narrow bandwidth of emission, and well-collimated beam—in SEW experiments is outlined. It is demonstrated by the examples of the infrared absorption spectra of a Langmuir-Blodgett film on metal, of a metal oxide film, and of polymeric films of nanometer thicknesses on metals. Free surfaces of single-crystal CaF2 and the thin polymeric film were studied by an interferometric experiment for the optical constant determination. Nonlinear spectroscopic applications of the SEW-FEL technique to studies of a second harmonic generation (SHG)—the frequency dependence of efficiency, the angle dependence of SHG, and the influence of a thin-film deposition on a quartz surface—are described.

We have studied vibrational spectra of thin films of copper phthalocyanine and fullerene of different thicknesses on silver obtained by a visible‐infrared sum‐frequency generation (SFG). The free electron laser FELIX was used as a source of broadband infrared and the pulsed Nd:YLF laser system as a source of visible radiation. Excitation of a surface plasmon on silver has allowed to enhance the efficiency of the SFG process. The SFG signal has appeared to be almost independent of the film thickness demonstrating the exclusive contribution of the interfacial layer to the SFG signal.

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