Volume 87, Issue 11, 01 June 2000
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
87(2000); http://dx.doi.org/10.1063/1.373431View Description Hide Description
Two tunable pulse lasers are used to selectively photoionize Ni atoms from an atomic beam containing mixtures of Ni and Cu, by a two-step resonant exitation process. Considering the bandwidth of of the lasers used, it is expected that pure Ni atoms are separated unless accidental resonant excitations and multiphoton ionizations of Cu atoms occur. The photoionized Ni atoms are extracted by means of an electric field and deposited on a Ag substrate as a pure film. Using a alloy as a starting material, a purified film of which was measured by an inductively coupled plasma-mass spectrometry was obtained after laser purification, though no Cu impurities were detected when an electron probe microanalysis was used. The Cu impurity level of determined from the ICP-MS is too high, contrary to the expectation of high selectivity of this laser method. The drifting vaporized Cu atoms that contribute to the background pressure are considered to be one of the main sources of the contamination in the purified film.
87(2000); http://dx.doi.org/10.1063/1.373432View Description Hide Description
We experimentally examined the threshold of the spectral collapse in dye embedded in a strongly scattering medium as a function of the excitation beam diameter and the transport mean free path in order to find a condition to minimize the threshold. We found a critical transport mean free path, below which the threshold pulse energy is almost independent of the mean free path. Experimental observations are well explained by a theoretical model based on the rate and diffusion equations, which shows that the luminescence spectra are also dependent on the position in the medium.
87(2000); http://dx.doi.org/10.1063/1.373433View Description Hide Description
We envisage the enhanced effect of harmonic generations from aperiodic optical superlattices (AOSs) achieved by inverting poled ferroelectric domains in samples. The search of optimal AOSs structures belongs to solving a difficult inverse source problem in nonlinear optics. We employ the simulated annealing method to successfully deal with this problem. We present the analyses of the design idea in the real-space representation and carry out several model designs. The constructed AOSs implement multiple wavelength second-harmonic generation and the coupled third-harmonic generation with an identical effective nonlinear coefficient at the preassigned wavelengths. The simulations show that the harmonic generations in the constructed AOSs can approach the prescribed goal better than those with the Fibonacci optical superlattice. The effective nonlinear coefficients versus the optical wave propagating distance from the impinging surface of incident light in the sample exhibit monotonically increasing behavior. This clearly infers that the contribution from each and every block to the optical parametric processes is in the constructive interference state. We also investigate the influence of the random fluctuation of the thickness of blocks on the performance of the constructed AOSs in detail for simulating practical fabrications and testing the performance stability of the sample.
87(2000); http://dx.doi.org/10.1063/1.373434View Description Hide Description
We present a theoretical and numerical study of the scattering of a diffusive wave by an object embedded in a semi-infinite substrate. We derive exact integral equations for the scattered wave, using Green’s theorem and appropriate Green’s functions. We show that two methods can be used, leading either to a purely surface-integral formalism or to a formalism involving a volume integral and a surface term. In the first case, we derive an extinctiontheorem for diffusive waves and present an efficient numerical procedure to solve exactly the scattering problem. In the second formalism, physically more explicit, we apply an improved Born approximation, and study its range of validity by comparison with rigorous numerical results. Our approach also suggests a simple way to determine the depth of the object. In this article, we focus on thermal waves. Yet the formalism can be applied to photon-density waves.