V. A. Yakovlev

Raman scattering enhancement in porous silicon microcavity

We have measured an enhancement factor of Raman signal up to 30 times using a Fabry-Perot 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.

An FTIR study of porous silicon layers exposed to humid air with and without pyridine vapors at room temperature

Oxidation processes can strongly influence the properties of porous silicon (PS) material. In this paper, we present the results of FTIR studies for the oxidation process of PS exposed to humid air alone or together with saturated pyridine vapor at room temperature in the time range from 0.1 to 10,000 min. The oxidation has been followed by recording the time evolution of the IR absorption intensities of the main species involved in the process, namely: Si-H x , O y Si-H x , Si OH and Si-O-Si. We found that in the presence of pyridine the oxidation is much faster. We show that the asymptotic value of silicon oxide amount is the same in both cases (with or without pyridine) and is strictly related to the original content of SiH surface species present in the sample. The role of the pyridine in the oxidation process is discussed.

Quick Oxidation of Porous Silicon in Presence of Pyridine Vapor

The oxidation of porous silicon (PS) is an important issue to be considered for a better understanding of the physical and chemical properties of this material in view of its possible application in several technological fields (optoelectronics, optical filters, environmental monitoring, etc.). In this work the quick oxidation of mesoporous PS induced by pyridine vapor in air has been studied by infrared spectroscopy. The logarithmic dependences of the oxidation degree, representing the fraction of Si converted into silicon oxide, on the exposure time to pyridine vapor have been measured. Values of this quantity up to 0.9, depending on the PS porosity and on the exposure time, have been obtained. It has been found that the oxidation continues, although at lower rate, even when pyridine treatment is finished. The possible role of pyridine in this chemical oxidation treatment is suggested.

Giant enhancement of sum-frequency yield by surface-plasmon excitation

We show experimentally that the radiation generated in infrared–visible sum-frequency mixing at an air–silver 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 102 for the prism and 104 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.

Mode dispersion in the optical-phonon spectra of mixed crystals ZnS1−x Sex

The IR reflection and the Raman spectra of ZnS1−xSex crystals (0≤ x ≤ 1) are measured. The mode dispersion of the solid solutions is found to deviate from that calculated using an isodisplacement model. The reasons behind this deviation are discussed. Two additional modes are found in the frequency range between the ZnS-like TO and LO modes. It is assumed that one of them is a line of the second-order spectrum amplified by the Fermi resonance and the other is linked to the resonance (additional local) mode of Se impurity atoms. The latter conclusion is confirmed by calculating the spectrum of a Se impurity in a ZnS crystal in terms of the microscopic lattice dynamics theory in the low impurity concentration approximation. The oscillator strengths of the main and additional optical phonons in ZnS1−xSex solid solutions are discussed.

Sum frequency generation spectroscopy of thin organic films on silver using visible surface plasmon generation

Abstract We present vibrational spectra of thin films of copper phthalocyanine and hexamethylenetetramine on silver obtained by a visible–infrared sum frequency generation (SFG). Excitation of a surface plasmon on the silver layer was used to enhance the efficiency of the SFG process. The free electron laser FELIX was used as a source of a broadband infrared and a pulsed Nd:YLF laser as a source of a visible radiation.

Giant enhancement of sum-frequency generation upon excitation of a surface plasmon-polariton

The generation of the sum frequency of visible (0.5235 μm) and IR (10 μm) radiation on smooth and corrugated silver surfaces is investigated. The sum-frequency signal obtained with a visible-range surface plasmon-polariton excited on a corrugated silver-air interface is found to be more than four orders of magnitude stronger than the signal from a smooth silver surface.

Quantum size effects in the optical conductivity of ultrathin metal films

Abstract Optical and electrical properties of ultrathin (0.15–7 nm) metal films (Nb, Cu, Mo, W, Ni and Ti) deposited by radio-frequency sputtering have been studied. Infrared (IR) reflectivity, transmittance as well as direct current (DC) conductivity measurements have been used to characterise these films. The periodic thickness dependence of the optical and DC conductivity was found. The periods of oscillations for different metals are of the order of the electron Fermi wavelength. Thus it was proposed that these oscillations are due to quantum size effects.

Structure and Lattice Dynamics in PLZT 8/65/35 Ceramics Irradiated by High-Current Pulsed Electron Beam

The comprehensive study of high-current pulsed electron irradiation effect on the structure and lattice dynamics as well as optical properties of PLZT 8/65/35 ceramics have been performed. X-ray powder diffraction studies shown the transformation of the rhombohedral perovskite structure R3m (Z = 1) into orthorhombic Pmmm (Z = 1) for the sample irradiated by one pulse (dose 4 × 1014 electrons/cm2) and, as a consequence, the changes in number of phonon modes position, oscillators strength and damping in IR and Raman spectroscopy data. However neutron powder diffraction measurements of multiple irradiated samples have not been revealed any structural distortions. Dielectric parameters have shown the increasing of real and imaginary parts of complex dielectric permittivity especially in high-temperature range (T > 100°C). In the optical properties, an essential increasing of the coefficient absorption for irradiated sample in the range from 1 to 2.5 eV and the shift of the edge of fundamental absorption to lower photon energy about 0.09 eV take place. The possible mechanisms of pulsed electron irradiation effect in relaxor PLZT ceramics are discussed.

Structural, optical, and dielectric investigations of the relaxor PLZT 9,75/65/35 ceramics irradiated by high-current pulsed electron beam

A comprehensive study of high-current pulsed electron irradiation effects on the structural, optical and dielectric properties of relaxor (Pb (1 m x ) La x )(Zr 0.65 Ti 0.35 ) 1 m x /4 O 3 ceramics with x = 9,75 at.% has been performed for the first time. The electron beam had the following parameters: energy E = 250 keV, current density I = 1000 A/cm 2, pulse duration = 300 ns, beam density m 10 15 electrons/cm 2 per pulse. Infrared reflectivity spectra in the region of 100 1 2000 cm m 1 were obtained in virgin, irradiated by 1500 pulses and annealed up to 500C ceramics. The reconstruction of perovskite ABO 3 structure in irradiated samples has been studied by combined use of X-ray diffraction, neutron scattering and IR spectroscopy techniques. Changes in the unit cell parameters, transverse and longitudinal phonon modes, oscillators strength and damping of modes have been revealed. Radiation effects on temperature behaviour of dielectric permittivity l in the region of phase transition have been studied. Possible mechanisms of pulsed electron irradiation effects in relaxor PLZT ceramics are discussed.