V. S. Dneprovskii

Third‐order optical nonlinearity and all‐optical switching in porous silicon

The third‐order optical nonlinearity χ(3) of porous silicon has been measured using the Z‐scan technique. Intensity dependent absorption was observed and attributed to a resonant two photon absorption process. The real and imaginary parts of χ(3) have been measured at 665 nm and found to be 7.5×10−9 esu and −1.9×10−9 esu, respectively. This constitutes a significant enhancement over crystalline silicon. All optical switching based on nonlinear absorption is demonstrated.

Time-resolved luminescence of CdSe microcrystals

Abstract Abrupt intensity-dependent decrease of the recombination time of nonequilibrium charge carriers was observed in CdSe (120 A radius) microcrystals. Such behaviour was assigned to the transition from linear radiative recombination to the nonradiative Auger process.

Strong optical nonlinearities and laser emission of semiconductor microcrystals

Changes in the transmission of quasi-zero-dimensional CdSe microcrystals (quantum dots) induced by ultrashort laser pulses were investigated with picosecond time resolution. The transitions between the energy levels of electrons and holes spatially confined within microcrystals were observed as bleaching bands in nonlinear transmission spectra. Gain registered at the transitions between the lowest levels of size quantization in microcrystals allowed to achieve the regime of laser generation in Fabry-Perot resonator made of glass containing semiconductor microcrystals.

Nonlinear-transmission spectra of porous silicon: Manifestation of size quantization

Changes in the transmission of porous silicon layers induced by ultrashort laser pulses are studied by using picosecond pump and probe measurements. Bleaching bands attributed to the saturation of optical transitions in silicon wires or/and crystallites with nanometer dimensions are observed in time‐resolved differential transmission spectra. The measured nonlinearity is characterized by a relatively high value of the third‐order nonlinear susceptibility (≊10−8 esu) and by fast relaxation dynamics (transmission recovery time is 30–40 ps).

Linear and nonlinear optical properties of excitons in semiconductor-dielectric quantum wires

The characteristic features of the absorption, luminescence and photoluminescence excitation spectra of InP nanocrystals crystallized in chrysotile asbestos nanotubes and CdS nanocrystals crystallized in hollow channels of a dielectric template have been explained in terms of exciton transitions in semiconductor–dielectric quantum wires. In these structures, the dielectric confinement effect leads to a considerable increase of the exciton binding energy—the Coulomb attraction between electron and hole is considerably enhanced as a result of the difference between the permittivities of the semiconductor and insulator. The kinetics of porous InP photoluminescence at high excitation by picosecond laser pulses has been explained by the slowing down of the intraband energy relaxation, collective exciton–exciton (electron) interaction and Auger recombination in nanostructures.

Nonlinear optical absorption of GaAs quantum wires

Abstract Nonlinear optical absorption at discrete frequencies (bleaching bands) has been observed for GaAs quantum wires in chrysotile asbestos nanotubes with average diameter ≈ 6 nm. The induced decrease of absorption has been explained by filling of the size-quantized energy levels of quantum wires with nonequilibrium carries (saturation effect). Strong ( χ (3) ≈ 10 −8 e.s.u.) and fast (relaxation time ≤ 50 ps) third order optical nonlinearity has been determined.

Excitons in CdS and CdSe Semiconducting Quantum Wires with Dielectric Barriers

Features of the photoluminescence spectra observed for various polarizations and intensities of the pumping radiation and the kinetics of photoluminescence of the CdS and CdSe nanocrystals grown in hollow nanochannels of an Al2O3 matrix are explained in terms of exciton transitions in semiconducting quantum wires with dielectric barriers. The observed exciton transition energies coincide with the values calculated with an allowance for the effects of quantum confinement and the “dielectric enhancement” of excitons. The latter effect is manifested by a significant increase in the Coulomb attraction between electrons and holes (the exciton binding energy exceeds 100 meV) due to a difference between the permittivities of semiconductor and insulator. It is shown that the exciton transition energy remains constant when the quantum wire diameter varies within broad limits. This is related to the fact that a growth in the one-dimensional bandgap width of the quantum wire caused by a decrease in the diameter is compensated by an increase in the exciton binding energy.

Two-dimensional dynamic photonic crystal creation by means of three non-coplanar laser beams interference in colloidal CdSe/ZnS quantum dots solution

We demonstrated a simple way to create dynamic photonic crystals with different lattice symmetry by interference of three non-coplanar laser beams in colloidal solution of CdSe/ZnS quantum dots. Two-dimensional dynamic photonic crystal was formed due to the periodical changing of refraction and/or absorption of resonantly excited excitons in CdSe/ZnS quantum dots. The formation of dynamic photonic crystal was confirmed by the observed diffraction of the beams that have excited photonic crystal at the angles equal to that calculated for the corresponding two-dimensional lattice (self-diffraction regime).

Self-diffraction of laser beams in the case of resonant excitation of excitons in colloidal CdSe/ZnS quantum dots

Self-diffraction of two types has been discovered in the case of resonant excitation of excitons in CdSe/ZnS quantum dots (highly absorbing colloidal solution) by powerful beams of mode-locked laser with picosecond pulse duration. I. The bleaching of exciton transition provokes the creation of transparency channel and laser beam’s self-diffraction at the induced circular aperture. II. Self-diffraction arises for two laser beams intersecting in the cell with colloidal CdSe/ZnS quantum dots due to the induced transient diffraction grating. Nonlinear optical properties responsible for the observed self-action effects in CdSe/ZnS quantum dots are discussed and a method for estimating laser pulse duration is suggested.

The features of nonlinear absorption during resonance one- and two-photon excitation of the basic exciton transition in colloidal CdSe/ZnS quantum dots

The features of the nonlinear absorption of CdSe/ZnS quantum dots (colloidal solution) in the case of resonant one- and two-photon excitation of the basic exciton transition by powerful ultra-short laser pulses were determined. In one-photon excitation, with an increasing intensity of impulses, a decrease in absorption (bleaching) is relayed by an increase in absorption, which is associated with the process of the filling of the states (saturation) of a two-level system with the lifetime of the excited state depending on the light intensity. The arising Fresnel or Fraunhofer diffraction of the laser ray that pass through a colloidal solution with a high concentration of quantum dots is associated with the formation of the transparency channel and self-diffraction of laser ray on an induced diaphragm. In two-photon excitation, the features of the nonlinear absorption and luminescence tracks (the dependence of luminescence intensity on distance) were explained by the influence, in addition to the two-photon absorption, of the processes that are responsible for the slower growth of nonlinear absorption and luminescence quenching at high intensities of laser pulses.