Nanocrystal-size-sensitive third-harmonic generation in nanostructured silicon

Third-harmonic generation (THG) in reflection geometry was studied in nanostructured silicon layers grown by electrochemical porosifying of p-type (110) silicon substrates. An order of magnitude enhancement of the THG efficiency compared to crystalline silicon (c-Si) was observed for the samples prepared on a highly doped substrate, whereas for the samples grown on a low-doped substrate the THG was much less efficient than for c-Si. The finding is discussed in terms of fluctuations of the electric field of the pump-laser irradiation in the layers of anisotropically distributed silicon nanocrystals. Received: 26 July 2002 / Revised version: 9 January 2003 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +7-95/939-1566, E-mail: leo@vega.phys.msu.su     

Water clustering by the surface of a vitreous body/hydroxyapatite nanocomposite

Low-temperature ¹H NMR spectroscopy was used to examine the effect of external factors (temperature and environment) on the hydrate properties of nanocomposite powder materials based on hyaluronic acid and hydroxyapatite. Thermodynamic parameters of layers of strongly and weakly bound water and the nanocompositewater interface energy were determined.     

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.     

Disorder-correlated enhancement of second-harmonic generation in strongly photonic porous gallium phosphide

We report an order of magnitude enhancement of second-harmonic generation (SHG) from porous gallium phosphide relative to SHG in crystalline gallium phosphide. Optical heterodyning measurements of photon free-path length reveal a correlation between SHG enhancement and disorder of the porous material.     

Role of the Polymer Matrix on the Photoluminescence of Embedded CdSe Quantum Dots

The photoluminescence (PL) of CdSe quantum dots (QDs) that form stable nanocomposites with polymer liquid crystals (LCs) as smectic C hydrogen‐bonded homopolymers from a family of poly[4‐(n‐acryloyloxyalkyloxy)benzoic acids] is reported. The matrix that results from the combination of these units with methoxyphenyl benzoate and cholesterol‐containing units has a cholesteric structure. The exciton PL band of QDs in the smectic matrix is redshifted with respect to QDs in solution, whereas a blueshift is observed with the cholesteric matrix. The PL lifetimes and quantum yield in cholesteric nanocomposites are higher than those in smectic ones. This is interpreted in terms of a higher order of the smectic matrix in comparison to the cholesteric one. CdSe QDs in the ordered smectic matrix demonstrate a splitting of the exciton PL band and an enhancement of the photoinduced differential transmission. These results reveal the effects of the structure of polymer LC matrices on the optical properties of embedded QDs, which offer new possibilities for photonic applications of QD–LC polymer nanocomposites.     

Observation of two-step excitation of photoluminescence in silicon nanostructures

Efficient visible-range photoluminescence with photon energy higher than the photon energy of the exciting radiation is observed in nanostructures of porous silicon subjected to heat treatment in vacuum. The photoluminescence intensity is found to be virtually identical for cw and femtosecond excitation by Ti:sapphire laser radiation with the same average power. The results can be explained by a two-step cascade photoluminescence excitation process in which optical passivation of defects of the dangling silicon bond type occurs. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 10, 732–736 (25 November 1998)     

Effect of photonic crystal structure on the nonlinear optical anisotropy of birefringent porous silicon

Anisotropic photonic crystal structures consisting of birefringent porous silicon layers with alternating porosity were fabricated. The in-plane birefringence formed as a result of anisotropic etching in Si(110) results in unique multilayered structures with two distinct photonic bandgaps for orthogonal light polarizations. Nonlinear optical studies based on the third-harmonic generation from these structures demonstrate variation in the symmetry of the nonlinear optical response.     

Problem of information equivalent functional schemes in aided inertial navigation systems

It has already been shown [1] that the problem of aiding inertial navigation systems (INS) by using information supplementing the inertial information can be solved both by estimating the INS errors from its output data and by introducing a feedback in the INS navigation algorithm. In the framework of the linear theory, it was also shown that any combination of these two schemes is information equivalent to one of them if the problem is solved as a pure estimation problem. The procedure of constructing the corresponding algorithms was described. Although the problem has been solved sufficiently completely, our meetings with the designers show that there is no clarity in its understanding. This understanding is especially important if the aiding problem is solved by using coarse sensors of source information, for example, MEMS sensors, which is actual at present. The material presented below should be considered as an additional explantation of the previously presented materials. Here illustrative examples are especially important.     

Dispersion of optical anisotropy in nanostructurized silicon films

Optical characteristics of (110) oriented porous silicon films obtained by electrochemical etching are studied. Dispersion of refractive indices, dichoroism in the visible range of the spectrum, depolarization factor, and porosity of silicon films are measured. It is shown that the results obtained may be described based on the generalized Bruggeman model. Possible causes of the established discrepancies between the experimental data and the model are discussed.     

Effects of light localization in photoluminescence and Raman scattering in silicon nanostructures

The effect of light localization in photoluminescence (PL) and Raman scattering (RS) in silicon nanowires with diameter of 100 nm was investigated. The optical excitation was done by CW radiation of a YAG:Nd laser at 1.064 μm. The PL an RS intensities were found to increase strongly for the samples with Si nanowires in comparison with corresponding values of c-Si substrate. The effect is explained by an increase of the lifetime of photons in silicon nanowire structures.