Volume 92, Issue 12, 15 December 2002
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS (PACS 42)
92(2002); http://dx.doi.org/10.1063/1.1516264View Description Hide Description
The relaxation kinetics of the photoinduced surface relief grating (SRG) formed on an azopolymer film after stopping laser beam writing has been systematically investigated. In addition to a photoinduced increase of the diffraction efficiency in the SRG, an anomalous further enhancement of the efficiency was observed even without light irradiation, after the recording light beam was switched off. This anomalous relaxation process consists of two components with short and long relaxation times, and strongly depends on the temperature and polarization of the probe beam used for the diffraction measurement. At lower temperatures the anomalous growth is more effective and the fast component is dominant. From the results of the polarization dependence of the diffraction efficiency and optical absorption, it has been manifested that the fast component of the relaxation originates from the cis-trans thermal isomerization of azobenzene in the side chain. The origin of the slow decay is also discussed in terms of the order parameter change of the azomolecules caused by the reorientational relaxation.
92(2002); http://dx.doi.org/10.1063/1.1515127View Description Hide Description
Macroporous silicon with two-dimensional periodicity has been produced by electrochemicaletching, using a p-type dopedsilicon substrate. The structure shows photonicenergy gaps in the infrared region, as demonstrated by variable angle reflectance measurements. The agreement between measurement and band calculations confirms the high quality of the samples. The use of an optimized electrolyte allows the fabrication of very high quality samples, with high aspect ratio and low roughness both at the surface and on the pore walls. The best results are obtained with aprotic and protophilic solvents.
92(2002); http://dx.doi.org/10.1063/1.1513190View Description Hide Description
A model for laser-assisted particle removal was developed. The model was applied to micrometer-sized particles of gold,copper, and tungsten on silicon substrates. A two-dimensional heat transfermodel is used to calculate the temperature profiles and the thermal expansion for the metallic particles and the silicon substrate irradiated with ultraviolet nanosecond laser pulses. The particle/substrate system was modeled as a simple spring oscillator under the sudden thermal expansion excitation pulse and a concept of degree of coupling between the laser induced thermal expansion associated frequency and the particle/substrate system is introduced. In terms of this degree of coupling a simple method to determine the most appropriate laser pulse duration is proposed. The theoretical results were compared with previously reported experimental observations. The experimental cleaning efficiency for the different particle materials and size distributions, as well as the possibility of substrate contamination due to metal evaporation are well accounted by the theoretical model.
92(2002); http://dx.doi.org/10.1063/1.1519945View Description Hide Description
We demonstrate near-field enhanced Raman spectroscopy with the use of a metallized cantilever tip and highly p-polarized light directed onto the tip with side illumination optics using a long working distance objective lens. The highly p-polarized light field excites surface plasmonpolaritons localized at the tip apex, which results in the enhanced near-field Raman scattering. In this article, we achieved an enhancement factor of 4000 for Rhodamine 6G molecules adsorbed on a silver island film. The side illumination is also applicable to an opaque sample and to near-field photolithography.
92(2002); http://dx.doi.org/10.1063/1.1522816View Description Hide Description
We report photoconductivity measurements in a binary system of naphthalene-based liquid crystals. Under ultraviolet (365 nm) illumination we observe an anomalous increase in the photocurrent for the mixtures. For a particular concentration, the photocurrentanisotropy defined as the ratio of photocurrents orthogonal to and along the director in the Crystal E phase reaches a value of 850, the highest achieved under steady-state illumination conditions from a conventional source. We argue that when using such mixtures, it is possible to reach the limiting values predicted by models based on conduction dictated by charge-carrier hopping.