V. Yu. Timoshenko

Strong in-plane birefringence of spatially nanostructured silicon

We report on a strong intrinsic optical anisotropy of Si induced by its dielectric patterning. As a result, an in-plane birefringence for nanostructured (110) Si surfaces is found to be 104 times stronger than that observed in bulk silicon crystals. We found the value of birefringence to be strongly dependent on the dielectric surrounding of the silicon nanoparticles assembling these layers. Beyond numerous potential implications for realization of optical devices and sensors, this gives a favorable route for studying the physics of condensation phenomena in a mesoscopic geometrical scale.

Giant birefringence in anisotropically nanostructured silicon

We performed a study of the in-plane birefringence of anisotropically nanostructured Si layers, which exhibit a greater difference in the main value of the anisotropic refractive index than that of natural birefringent crystals. The anisotropy parameters were found to be strongly dependent on the typical size of the Si nanowires used to assemble the layers. This finding opens the possibility of an application of birefringent Si retarders to a wide spectral range for control of the polarization state of light.

Quantitative analysis of room temperature photoluminescence of c-Si wafers excited by short laser pulses

Effect of surface nonradiative recombination on kinetics and total yield of the interband photoluminescence (PL) of c-Si wafers excited at room temperature by short laser pulses is studied. Numerical simulations show that a correlation of the PL quenching with the surface defect density takes place even at the high excitation level in spite of Auger recombination in the bulk. The quantum yield of PL reaches some percent for Si wafers with low bulk and surface defect concentrations. The calculations are confirmed by the experimental correlation between the PL quenching with the density of interface states (Dit) at the Si/SiO2 interface which has been obtained by conventional capacitance–voltage measurements. The express characterization of the Dit of Si surfaces by the pulsed PL can be carried out for the defect density in the range from 108 to 1014 cm−2 at room temperature.

Silicon Nanocrystals As Photosensitizers of Active Oxygen for Biomedical Applications

Silicon nanocrystals dispersed in water have been used to photosensitize the generation of active oxygen. The photosensitizing efficiency has been estimated through the quenching of the exciton photoluminescence of silicon nanocrystals. Experiments have revealed a strong (up to 80%) decrease in the number of cancer mouse fibroblast cells when they come into contact with photoexcited silicon nanocrystals. The obtained results show that the use of silicon nanocrystals for biomedical applications, in particular, for photodynamic therapy of cancer, is feasible.

Dichroic Bragg reflectors based on birefringent porous silicon

Multilayers of anisotropically nanostructured silicon (Si) have been fabricated and studied by polarization-resolved reflection measurements. Alternating layers having different refractive indices exhibit additionally a strong in-plane anisotropy of their refractive index (birefringence). Therefore, a stack of layers, acting as a distributed Bragg reflector, has two distinct reflection bands, depending on the polarization of the incident linearly polarized light. This effect is governed by a three-dimensional (in-plane and in-depth) variation of the refractive index. These structures can yield optical effects which are difficult to achieve with conventional Bragg reflectors.

In Situ Monitoring of Electrochemical Processes at the (100) p‐Si/Aqueous NH 4 F Electrolyte Interface by Photoluminescence

Photoluminescence (PL) with short N 2 -laser pulses is applied for in situ monitoring of electrochemical processes at the (100) p-Si/aqueous NH 4 F electrolyte interface during anodic oxidation followed by electrochemical hydrogenation at low cathodic potential. The anodic oxidation is carried out in a potential regime where electropolishing with current oscillations occurs. The etch rate of the anodic oxide, which is characterized by the reciprocal oscillation period 10 is changed by the composition of the fluoride solution. At the minimum of the current oscillations the PL intensity increases with decreasing etch rate and anticorrelates with the oxidation rate, which is monitored by the current. The hydrogenation of the Si surface is characterized by the anodic current transient. The PL intensity increases strongly during the decay of this transient.

Comparative study of photoluminescence of undoped and erbium-doped size-controlled nanocrystalline Si∕SiO2 multilayered structures

Spectra and transients of the photoluminescence (PL) of undoped and Er-doped size-controlled nanocrystalline Si∕SiO2 multilayered structures with mean nanocrystal size of 1.5–4.5nm have been comparatively investigated. The Er-doped structures exhibit a strong Er-related PL band at 0.81eV, while the efficiency of the intrinsic PL band of Si nanocrystals at 1.2–1.7eV decreases by several orders of magnitude in comparison with the undoped structures. At low temperature the PL spectra of the Er-doped structures show several dips separated by the energy of Si TO-phonon and bound to the transition energies between the second and third excited states to the ground state of Er3+. The Er-related PL is characterized by lifetimes of around 3–5ms, a weak temperature quenching, and a high efficiency, which is comparable or even stronger than that of the intrinsic PL in the corresponding undoped samples. This efficient sensitizing of the Er-related luminescence is explained by the structural properties of the samples, ...

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.

Porous silicon nanoparticles as sensitizers for ultrasonic hyperthermia

Aqueous suspensions of porous silicon nanoparticles (NPs) with average size ∼100 nm and concentration ∼1 g/L undergo significant heating as compared with pure water under therapeutic ultrasonic (US) irradiation with frequencies of 1–2.5 MHz and intensities of 1–20 W/cm2. This effect is explained by taking into account the efficient absorption of US energy by NPs. The observed US-induced heating of biodegradable NPs is promising for applications in ultrasonic hyperthermia of tumors.

Highly efficient sensitizing of erbium ion luminescence in size-controlled nanocrystalline Si/SiO2 superlattice structures

Comparative studies of photoluminescence (PL) of undoped and Er-doped size-controlled nanocrystalline Si/SiO2 superlattice structures show that the optical excitation of Si nanocrystals can be completely transferred to the Er3+ ions in surrounding SiO2, resulting in a strong PL line at 1.5 μm. The PL yield of the Er-doped structure increases for higher photon energy of excitation and for smaller nanocrystal sizes. This highly efficient sensitizing of the Er-related PL is explained by a strong coupling between excitons confined in Si nanocrystals and neighboring Er3+ ions in their upper excited states.