L. A. Golovan

Bright emission from a random Raman laser

Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm−1. Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium.

Ultrashort excitations of surface polaritons and waveguide modes in semiconductors

Polarization-dependent structures have been formed on the silicon surface under the action of femtosecond laser pulses. Some model concepts are proposed to describe changes in the response of the semiconductor surface caused by the generation of a nonequilibrium electron-hole plasma and explain the excitation of surface polaritons and waveguide modes during a femtosecond laser pulse.

Evaporation effect on laser induced solid–liquid phase transitions in CdTe and HgCdTe

Numerical simulation of laser induced phase transitions (melting, solidification, evaporation) in a near-surface region of CdTe and HgCdTe was carried out. The possibility of solidification from the outer surface of the melted layer caused by a vaporization of volatile component (Cd, Hg, etc.) was shown. This allows to explain experimentally observed high defectiveness of the laser-processed layer.

Form birefringence in porous semiconductors and dielectrics: A review

The phenomenon of optical anisotropy in porous semiconductors and dielectrics (porous silicon, gallium phosphide, and alumina) and photonic crystal structures formed on their basis is reviewed. It is shown that anisotropic nanostructuring of initially isotropic media leads to the occurrence of strong birefringence. Applicability of the effective-medium model to description of the form birefringence in porous semiconductors and dielectrics is discussed.

Optical properties of silicon nanowire arrays formed by metal-assisted chemical etching: evidences for light localization effect

We study the structure and optical properties of arrays of silicon nanowires (SiNWs) with a mean diameter of approximately 100 nm and length of about 1–25 μm formed on crystalline silicon (c-Si) substrates by using metal-assisted chemical etching in hydrofluoric acid solutions. In the middle infrared spectral region, the reflectance and transmittance of the formed SiNW arrays can be described in the framework of an effective medium with the effective refractive index of about 1.3 (porosity, approximately 75%), while a strong light scattering for wavelength of 0.3 ÷ 1 μm results in a decrease of the total reflectance of 1%-5%, which cannot be described in the effective medium approximation. The Raman scattering intensity under excitation at approximately 1 μm increases strongly in the sample with SiNWs in comparison with that in c-Si substrate. This effect is related to an increase of the light-matter interaction time due to the strong scattering of the excitation light in SiNW array. The prepared SiNWs are discussed as a kind of ‘black silicon’, which can be formed in a large scale and can be used for photonic applications as well as in molecular sensing.

Generation of the second optical harmonic in porous-silicon-based structures with a photonic band gap

Efficient generation of the second optical harmonic is observed experimentally in a multilayer periodic structure based on porous silicon. The second-harmonic signal is much stronger than the signal from a uniform porous silicon layer or from the single-crystal silicon substrate. The orientational dependence of the second-harmonic signal is isotropic. The second-harmonic intensity as a function of the reflection angle reaches a maximum in the direction corresponding to the minimum phase detuning in a multilayer periodic structure.

Generation of surface electromagnetic waves in semiconductors under the action of femtosecond laser pulses

It is demonstrated that intense photoexcitation of the surface of a semiconductor with femtosecond laser pulses can induce fundamental changes in its optical response and ensure conditions for the generation of surface electromagnetic waves of various types. The connection between electronic processes initiated in the surface layer by photoexcitation and the formation of periodic surface microstructures observed in experiments on irradiation of Si targets is considered. The importance of photoemission in the processes taking place under ultrashort excitation is confirmed.

Phase-matched third-harmonic generation in anisotropically nanostructured silicon

A frequency-tunable laser system based on an optical parametric oscillator is used to implement phase-matched third-harmonic generation in porous silicon films with a strong form birefringence. Phase matching, orientation dependences, and the behavior of the third harmonic as a function of the thickness of an absorbing film are confirmed by calculations and linear-optical measurements.

Efficient second-harmonic generation by scattering from porous gallium phosphide

We experimentally study second-harmonic generation by femtosecond Cr: forsterite-laser radiation scattered on the surface of porous gallium phosphide with characteristic pore sizes and distances between the pores comparable with the second-harmonic wavelength. The intensity of the second-harmonic signal from samples with initial crystallographic surface orientations (110) and (111) is more than an order of magnitude higher than the intensity of the second harmonic generated in reflection from single-crystal gallium phosphide. The efficiency of second-harmonic generation by macroporous gallium phosphide substantially increases as the pump wave-length becomes shorter. The influence of light localization and scattering effects on the enhancement of second-harmonic generation and polarization properties of the second-harmonic is discussed.

Structural properties of silicon nanoparticles formed by pulsed laser ablation in liquid media

Silicon nanoparticles have been formed as a result of the irradiation of single-crystal silicon targets in distilled water and liquid nitrogen, by, respectively, picosecond and femtosecond laser pulses. The main structural properties of these nanoparticles have been investigated by atomic force microscopy, transmission electron microscopy, electron diffraction, Raman scattering, and photoluminescence spectroscopy. These particles are found to be mainly spherical. The presence of crystalline and amorphous silicon phases under picosecond ablation in water is established experimentally. Irradiation by femtosecond pulses in liquid nitrogen can yield nanoparticles smaller than 5 nm in size, which are quantum dots with a characteristic photoluminescence peak near 750 nm.