Generation of the second harmonic of femtosecond pulses on reflection from a metal surface: amplification due to periodic modulation of the relief

It is observed experimentally that the optical second-harmonic generation signal produced when femtosecond laser pulses are reflected from a metal surface increases more than 100-fold when a specially chosen periodic relief that gives quasiresonance excitation of a surface electro-magnetic wave is depoited on the surface. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 155–159 (10 February 1996)     

Phase matching of second-harmonic generation in birefringent porous silicon

Optical second-harmonic generation is investigated in free-standing (110) porous silicon films having strong in-plane birefringence. Based on the measured values of the refractive indices, the conditions of phase matching are calculated and verified in the experiments on angular and polarization dependences of second-harmonic generation. Filling pores with a transparent dielectric liquid allows phase matching to be achieved and the second-harmonic signal to be increased by two orders of magnitude. The results of these experiments suggest an opportunity to form new-type nonlinear-optical media consisting of birefringent porous silicon as a phase-matching matrix for the materials embedded in the pores. Received: 30 April 2001 / Published online: 7 June 2001     

Observation of a compressed state of the quantum wire material in porous silicon by the method of Raman scattering

An experiment on Raman scattering in porous silicon containing a regular system of quantum wires reveals a shift of the phonon frequencies of the silicon lattice toward higher energies. The effect is interpreted as a manifestation of a compressed state of the material in the quantum wires. It is shown that the observed deformation plays a large role in the formation of the structure of porous silicon. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 10, 695–700 (25 November 1996)     

Anomalous polarization of Raman scattering spectra from porous silicon

A violation of the polarization selection rules for Raman scattering is observed in porous silicon. This effect is caused by a weak disorientation of the quasi-one-dimensional silicon wires, with the crystal structure of the wires themselves and the macroscopic homogeneity of the material in optical experiments remaining intact. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 2, 95–100 (25 January 1998)     

Efficient nonlinear-optical frequency conversion in periodic media in the presence of diffraction of the pump and harmonic fields

It has been predicted by Shelton and Shen [Phys. Rev. A 5, 1867 (1972)] and observed by Kajikawa et al. [Jpn. J. Appl. Phys. Lett. 31, L679 (1992)] and Yamada et al. [Appl. Phys. B 60, 485 (1995)] that the efficiency of nonlinear-optical frequency conversion increases significantly in a nonlinear periodic medium and, accordingly, the intensity of the generated harmonic increases as the fourth power of the sample thickness, as opposed to the square law observed in homogeneous media. In this paper it is shown that the same enhancement of the efficiency of nonlinear-optical frequency conversion in a nonlinear periodic medium can be achieved using an ordinary pump wave in the form of a plane wave when both the pump wave and the harmonics are diffracted by the periodic structure of the nonlinear medium. The phenomenon is analyzed quantitatively in the example of second-harmonic generation. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 793–799 (25 December 1999)     

Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

Second harmonic generation induced by mechanical stresses in silicon

The optical second-harmonic generation induced by the external mechanical tension of the (001 ) silicon surface has been detected. A change in the spectrum of the second harmonic, which is caused by the modification of the band structure of silicon near the critical points E’_o and E₁ under mechanical tension, has been observed. The direct contribution to the optical second harmonic from changes in the band structure of silicon under tension is experimentally separated from the electric-field-induced co ntribution associated with the redistribution of the surface charge under the action of mechanical stresses.     

Distinctive features of time-resolved photoluminescence of porous silicon coated with a layer of diamondlike carbon

Time-resolved photoluminescence from porous silicon coated with a diamondlike carbon film is investigated. The intensity of the photoluminescence from the carbon film is obserd to increase after deposition, and there is an accompanying change in the intensity and a shortwavelength shift of the photoluminescence band of porous silicon that depends on the porosity of its original layers. These changes are explained by the formation of carbon nanoclusters on the surface of the silicon filaments. Zh. Tekh. Fiz. 68, 83–87 (April 1998)     

Optical properties of multilayered Period-Doubling and Rudin-Shapiro porous silicon dielectric heterostructures

To investigate the optical properties in quasi-regular porous-silicon-based dielectric Period-Doubling and Rudin-Shapiro multilayer systems, we study here the reflection of light from these structures. The Period-Doubling and Rudin-Shapiro structures are fabricated in such a way that the optical thickness of each layer is one quarter of 600 and 640 nm respectively. We find that porous silicon Period-Doubling dielectric multilayers could demonstrate the optical properties similar to the classical periodic Febry–Perot interference filters with one or multiple resonant peaks, but with an advantage of having total optical thickness much lesser than the periodic structures. Additionally, light propagation in porous silicon Rudin-Shapiro structures is investigated for the first time, both theoretically and experimentally. The reflectance spectra of the structures exhibit photonic band gaps centered at predetermined wavelengths. In both cases, numerical simulation of light transmission is performed using transfer matrix method.     

Formation of a two-dimensional periodic structure of local melting regions on a silicon surface upon pulsed light irradiation

The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures is discussed. Zh. Tekh. Fiz. 67, 97–99 (December 1997)     

Modification of porous silicon in ultrahigh vacuum and contribution of graphite nanocrystallites to photoluminescence

A replacement of the adsorbate in porous silicon is carried out in ultra-high vacuum. The photoluminescence line is shifted and quenched as the products of anodization of silicon — silicon hydrides and atomic and molecular hydrogen — undergo thermal decomposition and desorption. Adsorption of molecular chlorine restores the 560 nm photoluminescence band, which we identified as radiation from graphite nanoparticles. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 2, 106–111 (25 January 1996)     

On the nature of the spectral shift caused by photoluminescence fatigue in porous silicon

Luminescence spectra of porous silicon with a regular columnar-layered structure have been studied. A substantial narrowing of the luminescence band in samples of this type and a considerable shift of the band induced by fatigue have been established. An explanation for the spectral shift of the luminescence band resulting from fatigue relaxation is proposed for the first time. Fiz. Tverd. Tela (St. Petersburg) 39, 2137–2140 (December 1997)     

Investigation of fatigue relaxation of luminescence in porous silicon by time-resolved spectroscopy

This paper describes the first investigations of how the intensities of various time-resolved components of the luminescence from porous silicon relax with time. A paradoxical correlation is observed between the macro-and microtemporal relaxation of luminescence from porous silicon under pulsed photoexcitation: namely, a relative increase in the rate of macrorelaxation for the slower components of the luminescence. Spectral investigations show that the difference in the rates of fatigue relaxation “tiredness” is maximum at the long-wavelength edge of the luminescence band. We propose a model that allows us to explain the observed effects starting from the assumption that photoexcitations drift toward radiative recombination centers. Fiz. Tverd. Tela (St. Petersburg) 39, 1165–1169 (July 1997)     

Matched second-harmonic generation of ultrashort laser pulses in photonic crystals

It is shown that one-dimensional photonic bandgap structures are capable of simultaneously satisfying the phase and group-velocity matching conditions for second-harmonic generation involving extremely short light pulses. When these conditions are satisfied, an optical frequency doubler utilizing photonic bandgap structures provides a means for increasing the rate of growth of the second-harmonic signal as a function of the nonlinear-optical interaction length relative to structures designed for quasi-matched interactions and affords possibilities for enhancing the frequency doubling efficiencies independently of the matching length in the bulk nonlinear material. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 800–805 (25 December 1999)     

Diffractional enhancement of the Kerr magnetooptic effect

The results of investigations of the transverse Kerr effect on an array of thin magnetic strips deposited on a silicon substrate are reported. The periodic structure of the sample gives rise to diffraction. It is observed that under certain experimental conditions the magnitude of the effect measured in diffracted beams is much greater than the maximum value for a sample with a uniform surface. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 7, 466–469 (10 October 1997)     

Low refractive index contrast porous silicon omnidirectional reflectors

We report on the fabrication and characterization of a porous silicon omnidirectional reflector formed by periodic substructures stacked together. For these substructures, a low refractive index contrast has been used, resulting in substructures without omnidirectional reflectivity band. The use of a low refractive index contrast involves the reduction of the requirements to obtain omnidirectional reflectors. We demonstrate the existence of an omnidirectional reflectivity range with a high gap-to-midgap ratio by means of reflectivity spectra measurements for a range of incidence angles. The results are in good agreement with a theoretical model of the reflector. The fabricated structure is the first reported porous silicon reflector suitable for 1.55 μm applications.     

Influence of physical and chemical surface treatment on the photoluminescence of porous silicon

It is shown that surface treatment of porous silicon in inorganic acids and solutions of metal chlorides leads to an increase in the intensity of photoluminescence of this material. In the case of chlorides, a short-wavelength shift of the photoluminescence maximum is also observed. The effect of a brief high-temperature anneal in vacuum on the photoluminescence of porous silicon is investigated. Such treatment is observed to cause partial degradation. Zh. Tekh. Fiz. 69, 133–134 (January 1999)     

Backscattering of low-energy (0–8 eV) electrons by a silicon surface

An experimental apparatus and method for investigating elastic and inelastic backscattering (180°) of low-energy (0–8 eV) monoenergetic electrons by a solid surface are described and the first results are presented for the reflection of electrons by samples of pure single-crystalline silicon with a polished surface (Si), doped p-type single-crystalline silicon with a porous surface (Si-p) as well as H₂O and H₂O₂ passivated porous samples, Si-p + H₂O and Si-p + H₂O₂. A structure due to the excitation of surface plasmons has been observed for the first time in the loss spectra. Features corresponding to a resonance excited state of molecular nitrogen adsorbed on the surface of porous silicon have been observed in the constant residual energy spectra. Zh. Tekh. Fiz. 67, 103–108 (May 1997)     

Photoelectric instability in oxide glass

The spatially periodic modulation of optical anisotropy (MOA) induced in oxide glass by mutually coherent light beams with different frequencies (ω and 2ω) is unstable under illumination with monochromatic light with frequency ω. Disturbances with small amplitudes intensify and disturbances with large amplitudes relax. Irrespective of its initial degree, the MOA reaches the same steady-state level, which depends on the illumination intensity. Intensification of MOA is accompanied by the appearance of second-harmonic radiation whose intensity grows in time to a steady-state level. The instability of the anisotropy is due to degenerate three-wave mixing and to feedback arising as a result of the coherent photogalvanic effect. A hypothesis that takes into account the observed giant growth (by three orders of magnitude) of light absorption in the MOA region is proposed to explain the stabilization of the instability and the formation of stationary periodic refractive-index gratings. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 771–776 (25 December 1997)     

Instability of the photoluminescence of porous silicon

Results of studies of the photoluminescence of porous silicon with different prehistories have revealed the mechanism and nature of the instability of the luminescence properties of freshly prepared samples. It was established that the initial quenching and subsequent rise of the photoluminescence is attributable to the intermediate formation of silicon monoxide (photoluminescence degradation) and subsequent additional oxidation to form SiO₂ (photoluminescence rise). Ultraviolet laser irradiation accelerates this process by a factor of 200–250 compared with passive storage of the samples in air. Plasma-chemical treatment in an oxygen environment merely results in a subsequent rise in the photoluminescence as a result of the formation of monoxide on the porous silicon surface. A kinetic model is proposed for this process. Zh. Tekh. Fiz. 69, 135–137 (June 1999)