Sergei Novikov

Optical gain in Si/SiO2 lattice: Experimental evidence with nanosecond pulses

Experimental evidence of population inversion and amplified spontaneous emission was found for Si nanocrystallites embedded in SiO2 surrounding under pumping with 5 ns light pulses at 380, 400, and 500 nm. As an important property, our experiments show a short lifetime of the population inversion allowing a generation of short (a few nanosecond) amplified light pulses in the Si/SiO2 lattice. The estimate for optical gain in the present samples is 6 cm−1 at 720 nm.

Systematic correlation between Raman spectra, photoluminescence intensity, and absorption coefficient of silica layerscontaining Si nanocrystals

The correlation between optical, structural, and light-emitting properties of annealed Si-rich silica samples containing different amounts of Si nanocrystals is studied. The intensity of the 1.6eV emission band weakens when the Raman signal coming from the Si nanocrystals gets stronger. On the contrary, the absorption coefficient follows the increase of the Raman intensity with the Si nanocrystal density. The decrease of the photoluminescence is accompanied with the increase of tensile stress, which is suggested by the Raman spectra. Possible explanations of the observed dependencies are discussed.

Thermal annealing of Si/SiO2 materials: Modification of structural and photoluminescence emission properties

We comparatively study two conventional types of Si/SiO2 materials, Si/SiO2 superlattices (SLs) and Si-rich silica (SiOx) films, prepared with a molecular beam deposition method. Raman scattering, photoluminescence (PL), ultraviolet-visible-infrared absorption, and x-ray photoelectron spectroscopies are employed to characterize the samples. The results show clear parallelism in microstructure and emitting properties of Si/SiO2 SLs and SiOx films. The as-grown material is amorphous, and disordered Si areas are seen in Raman spectra for samples with higher Si contents. Annealing at 1150 °C in nitrogen atmosphere leads to ordering of the Si grains and the typical crystalline size is estimated to be 3–4 nm. For all samples, an annealing-induced increase of PL at ∼1.6 eV is observed, and its resulting position is quite independent of the initial sample architecture. Furthermore, this PL is practically identical for continuous wave and pulsed excitation at 488 nm as well as for pulsed excitation at various wave...

Efficient wavelength-selective optical waveguiding in a silica layer containing Si nanocrystals

The optical properties of a silica layer containing Si nanocrystals deposited onto fused quartz are studied with emphasis on optical waveguiding (WG) of the photoluminescence (PL). The WG layer was estimated to be 4.3 μm thick with a refractive index of 1.67. We observed efficient (long-distance) propagation of the PL light in the layer, the losses being below 1 cm−1 at 1000 nm. Efficient narrowing of the PL spectrum (down to 8 meV) was detected demonstrating spectral filtering by the waveguide.

Raman scattering from very thin Si layers of Si/SiO2 superlattices: Experimental evidence of structural modification in the 0.8–3.5 nm thickness region

Raman study of very thin (⩽3.5 nm) Si layers constituting Si/SiO2 superlattices and grown by molecular beam epitaxy is described. The Raman spectra show systematic dependence on thickness of the Si layers, which highlights the variety of disordered microstructures in the Si/SiO2 superlattices. A clear change in the vibrational properties is found to occur in the 0.8–3.5 nm thickness region. In particular, the Raman spectra are typical for amorphous silicon for the thicker layers, and the characteristic phonon band disappears for the thinner layers, presumably representing another form of Si coordination with a small Raman scattering cross section. In addition, absorption of the material changes essentially with the Si-layer thickness. Photoluminescence is detected from the Si/SiO2 superlattices, the superlattices with 1.2 and 1.8 nm Si layers being the most efficient emitters among our samples, and the photoluminescence is blueshifted with the decrease of the Si-layer thickness. The Raman spectra show no ...

Giant Raman gain in silicon nanocrystals

Nanostructured silicon has generated a lot of interest in the past decades as a key material for silicon-based photonics. The low absorption coefficient makes silicon nanocrystals attractive as an active medium in waveguide structures, and their third-order nonlinear optical properties are crucial for the development of next generation nonlinear photonic devices. Here we report the first observation of stimulated Raman scattering in silicon nanocrystals embedded in a silica matrix under non-resonant excitation at infrared wavelengths (~1.5 μm). Raman gain is directly measured as a function of the silicon content. A giant Raman gain from the silicon nanocrystals is obtained that is up to four orders of magnitude greater than in crystalline silicon. These results demonstrate the first Raman amplifier based on silicon nanocrystals in a silica matrix, thus opening new perspectives for the realization of more efficient Raman lasers with ultra-small sizes, which would increase the synergy between electronic and photonic devices.

Laser-controlled stress of Si nanocrystals in a free-standing Si∕SiO2 superlattice

We report laser manipulations with stress at the nanoscale level. The continuous-wave Ar+ laser radiation melts Si nanocrystals in a free-standing Si∕SiO2 superlattice. Silicon crystallization from the liquid phase leads to a compressive stress, which can be accurately tuned in the 3GPa range using laser annealing below the Si melting temperature and then recovered by laser annealing above the melting temperature. This allows investigations of various phenomena as a function of stress and makes a case of Si-nanocrystal memory with very long retention time, which can be written, erased, and read by optical means.

Optics of Si/SiO2 superlattices: Application to Raman scattering and photoluminescence measurements

A quantitative model of a Si/SiO2 superlattice (SL) is developed and applied to Raman and photoluminescence (PL) measurements. By analyzing the experimental reflection spectra of Si/SiO2 SLs on Si and Al substrates, we obtained optical parameters of amorphous Si layers with thickness below 4 nm. Both refractive index and extinction coefficient are found to decrease with Si-layer thickness, and this behavior reflects interaction of the Si network and the oxide surrounding. Interference-induced modification of Raman scattering and PL is quantitatively studied for Si/SiO2 SLs on Si and Al substrates, and the developed optical model describes well all observed features. PL spectra of the Si/SiO2 SLs are found to change under Ar+-laser irradiation, and this effect of laser annealing becomes stronger for thinner Si layers. For 1-nm-thick Si layers, a prolonged laser exposure decreases the PL intensity at 550 nm by a factor of 10 and red-shifts its maximum by about 50 nm, which indicates essential reorganization...

Light-emission mechanism of thermally annealed silicon-rich silicon oxide revisited: What is the role of silicon nanocrystals?

The experimental data obtained with Raman, x-ray photoelectron, and continuous-wave and time-resolved photoluminescence spectroscopies on silicon-rich silicon oxide samples with different O/Si ratios and annealing temperatures are analyzed. It is shown that Si grains, with atoms bound as in bulk Si, are not the direct emitting phase, which is in agreement with the model of localized light-emitting centers. These oxygen-defect centers probably locate in suboxide structures and the excitation migrates to them from the absorbing Si grains. The photoluminescence quantum yield strongly increases for samples with larger O/Si ratios, suggesting a nonradiative capture of migrating excitons in Si grains.

Free-standing silica film containing Si nanocrystals: Photoluminescence, Raman scattering, optical waveguiding, and laser-induced thermal effects

A Raman and photoluminescence study of a thermally annealed free-standing film of silica containing Si nanocrystals is reported. The laser-induced thermal effects are observed, including the increase of the absorption coefficient and the strongly nonlinear rise of the light emission. The light emission measured at high excitation powers is probably blackbody radiation, and the Raman measurements confirm high laser-induced temperatures. The Ar+ laser annealing strongly increases the crystalline Raman peak showing that thermal annealing at 1150 °C does not finish structural reorganization of the SiOx material. In the waveguiding detection geometry, the spectral narrowing of the photoluminescence is observed.