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Nonlinear optical reflection spectroscopies for surface studies often suffer from their relatively weak signals. To enhance the signal strength, femtosecond input pulses are generally preferred, but their intensity is usually limited by supercontinuum (SC) generation in a bulk medium. We show here that the problem can be largely resolved by means of spatiotemporal focusing (STF) of the input that effectively suppresses the SC, permits much higher input intensity on a medium, and greatly enhances the nonlinear output signal. We use second-harmonic generation (SHG) on oxide as examples and demonstrate that, with STF, the input intensity for the onset of the SC can be increased by ∼30 times in comparison with conventional focusing, and the reflected SHG can be increased by about three orders of magnitude.

We show that a monolayer grating of CO on Ni(111) can be generated by laser desorption using two interfering laser pulses and detected and characterized in situ by surface second-harmonic generation.

We show that optical second-harmonic generation can also be used as an effective surface probe with a submonolayer sensitivity for media without inversion symmetry. The adsorption of Sn on GaAs is used as an example. A detailed summary of all accessible elements of the surface nonlinear susceptibility tensor χ(2) for the various crystal faces of GaAs is provided.

We demonstrate that monolayer detection by optical second-harmonic generation is possible even with a 20-mW cw diode laser. Using a focused scanning laser beam, we also demonstrate a new method of monolayer surface microscopy. The same experiments suggest the possibility of an optical monolayer memory device.

A Fabry-Perot interferometer filled with an 83-microm-thick nematic-liquid-crystal film exhibits multiple-order optical bistability and self-oscillation. The oscillation is shown to be due to two competing mechanisms with different response times contributing to the laser-induced refractive-index change.

We have used a molecular-beam photoionization technique to probe the vibrational-energy distribution of SF(6) molecules excited by an intense CO(2) laser at energy fluences close to and below the dissociation threshold. We observe a significant bottleneck at low laser intensities. Even at laser fluences close to the dissociation threshold, most of the molecules remain in the discrete levels, and only a small fraction (approximately 10-25%) are excited into the quasi-continuum.

Electrical poling induces polar ordering of molecules in a grating that has been holographically inscribed on a thin film of polymer with azobenzene side chains. The resulting χ(2) grating, seen by second-harmonic-generation (SHG) near-field scanning optical microscopy, can have a periodic structure that is significantly different from the topographical image. The far-field linear and SHG diffraction patterns correlate well with the grating structures. Poling of the thin-film grating, which presumably has photodriven nonuniform material properties within each period, leads to the more complex structure of the χ(2) grating.

Giant enhancement of optical reorientation in pure nematic liquid crystals (LCs) was observed with a near-resonant input beam. The enhancement observed through reduction of the threshold of the optical Fréedericksz transition in a 28- microm-thick homeotropic LC cell was almost 700 times in comparison with the case of an off-resonant input beam. Wavelength dependence of the enhancement followed the LC absorption spectrum very well, confirming that the enhancement arose from the excitation of LC molecules.

We have shown that optically active second-harmonic generation is allowed in a uniaxial fluid medium. A homeotropically aligned chiral smectic-A liquid crystal was used as an example. Phase matching was achievable by angle tuning. Chiral nonlinear susceptibility for the liquid crystal was deduced. The signal dropped precipitously as the sample underwent the transition from smectic-A to isotropic.

We present what are to our knowledge the first measurements of the phase of the infrared-visible sum-frequency generation signal. We introduce two techniques and apply the measurements to the determination of the polar orientation of a selected group of atoms within an adsorbed molecule.

We have observed second-harmonic generation by counterpropagating surface-plasmon waves. The output is in the form of a well-collimated beam along the surface normal. The results are in excellent agreement with theoretical prediction.

We have studied the characteristics of a nonlinear Fabry–Perot filled with Kerr liquids CS2 and nitrobenzene and found excellent agreement between theory and experiment. The transient behavior due to cavity-field buildup is discussed.

The wave-front curvature of an incident laser beam can modify the spatial phase modulation of the beam, and hence the diffraction of the beam, traversing a liquid-crystal film. It is shown both theoretically and experimentally that this explains the anomalous fine structure in the diffraction ring pattern observed with laser intensities above the Freederickscz transition threshold.