[en] Porous silicon is excited using near-infrared femtosecond pulsed and continuous wave radiation at an average intensity of ∼10⁶ W/cm² (8x10¹⁰ W/cm² peak intensity in pulsed mode). Our results demonstrate the presence of micron-size regions for which the intensity of the photoluminescence has a highly nonlinear threshold, rising by several orders of magnitude near this incident intensity for both the pulsed and continuous wave cases. These results are discussed in terms of stimulated emission from quantum confinement engineered intrinsic Si-Si radiative traps in ultrasmall nanocrystallites, populated following two-photon absorption. (c) 1999 American Institute of Physics

[en] The linear and nonlinear dynamical susceptibilities of a two-level system are calculated as it undergoes a transition to a decoherent state. Analogously to the Glover-Tinkham-Ferrell sum rule of superconductivity, spectral weight in the linear susceptibility is continuously transferred from a finite frequency resonance to nearly zero frequency, corresponding to a broken symmetry in the thermodynamic limit. In this respect, the behavior of the present model (the Mermin model) differs from the spin-boson model. The third order nonlinear susceptibility, corresponding to two-photon absorption, has an unexpected nonmonotonic behavior as a function of the environmental coupling, reaching a maximum within the decoherent phase of the model. Both linear and nonlinear susceptibilities may be expressed in a universal form. (c) 2000 The American Physical Society

[en] A bichromatic sodium guide star based on two-photon coherent excitation followed by cascade emission is considered. Results of density matrix computations, including Doppler averaging, are presented for both a three-level model and the full hyperfine structure of the sodium 3S , 3P , and 4S levels. These results indicate that the proposed scheme for a bichromatic guide star is feasible. (c) 2000 Optical Society of America

[en] One-photon detachment and two-photon nonresonant excess photon detachment of electrons from the H^- ion (outer-electron binding energy = 0.7542 eV) are observed with 1.165 eV laser pulses from a Nd:YAG laser (where YAG denotes yttrium aluminum garnet). A Penning ion source produces a pulsed 8 μA, 35 keV H^- beam that intersects a laser beam cylindrically focused down to a 17 μm full width at half maximum waist in the ion beam direction, creating a high-intensity interaction region with peak intensities of up to 10¹¹ W/cm². The interaction time is 7 ps. The detached electrons are detected by a time-of-flight apparatus enabling us to detect a very small two-photon signal in the presence of a very large signal from single photon detachments. By rotating the linear polarization angle, we study the angular distribution of the electrons for both one- and two-photon detachments. The spectra are modeled to determine the asymmetry parameters and one- and two-photon cross sections. We find β₂ to be 2.54+0.44/-0.60 and β₄ to be 2.29+0.07/-0.31, corresponding to a D state of 89+3/-12% of the S wave and D wave detachments for the two-photon results. The relative phase angle between the S and D amplitudes is measured to be less than 59 degree sign . The measured cross sections are found to be consistent with theoretical predictions. The one-photon photodetachment cross section is measured to be (3.6±1.7)x10^-17 cm². The two-photon photodetachment generalized cross section is (1.3±0.5)x10^-48 cm⁴ sec, consistent with theoretical calculations of the cross section. The three-photon generalized cross section is less than 4.4x10^-79 cm⁶ sec². (c) 1999 The American Physical Society

[en] We present a generalization of the variationally stable method of Gao and Starace [B. Gao and A. F. Starace, Phys. Rev. Lett. 61, 404 (1988); Phys. Rev. A 39, 4550 (1989)] for two-electron atoms and ions that incorporates a coupled-channel adiabatic hyperspherical approach. Using this approach, we report results for two-photon detachment of H^-, in which we have coupled one, two, three, and four adiabatic hyperspherical channels within each term level of the initial, intermediate, and final states. We present results also for the dynamic polarizability of H^- as well as for the one-photon detachment cross section. Comparisons are given with results of prior work. (c) 2000 The American Physical Society

[en] We calculate the two-photon ionization of clusters for photon energies near the surface plasmon resonance. The plasmon is described in a schematic jellium-RPA model assuming a separable residual interaction between electrons determined so that the plasmon energy is reproduced. The ionization rate of a double plasmon excitation is calculated perturbatively. In Na₉₃⁺ clusters we find an ionization rate of the order of at most 0.05-0.10 fs^-1. This rate is used to determine the ionization probability in an external field in terms of the number of absorbed photon pairs and the duration of the field. We discuss the dependence of the results on the choice of our empirical separable force. The number of emitted electrons per pair of absorbed photons is found to be small, in the range 10^-5 to 10^-3. (c) 2000 The American Physical Society

[en] We report crossed molecular beam measurements of state-resolved differential cross sections for NO+NO collisions at relative kinetic energy of 442 cm^-1. The initial state (NO²Π_1/2v=0,j≤2.5) was prepared in both beams by pulsed supersonic expansion of pure NO gas. Scattered products were detected by resonance enhanced two-photon ionization. NO product distributions were measured in both scattering angle and final rotational state. Intensity maxima were found in both types of scans, and comparable populations were found in both of the spin-orbit manifolds (²Π_1/2 and ²Π_3/2). The results obtained here are compared to previous NO+Ar scattering experiments and theory, NO dimer studies, and NO+NO bulb kinetics experiments. copyright 1997 American Institute of Physics

[en] Linear and nonlinear [second harmonic generation (SHG)] optical functions of cubic and hexagonal BN, AlN, GaN, and InN have been studied by using the first-principles full-potential linearized augmented plane-wave method. Equilibrium lattice constants are determined from the total-energy minimization method. The calculated spectra of the second-order optical susceptibility show pronounced structures related to the two-photon resonances. In materials with heavy metals there are remarkable contributions from the single-photon transitions. Line shapes of the linear and particularly the nonlinear optical spectra of GaN and InN crystals are very sensitive to the interactions between the conduction bands and metallic d states. Studies of the nonlinear optical susceptibilities in both wurtzite and cubic crystals show high sensitivity of the SHG spectra to the changes of atomic structure. (c) 2000 The American Physical Society