A multimode scanning near-field optical microscopy technique that allows the mapping of surface morphology, photoluminescence (PL) spectra in illumination and illumination-collection modes, and PL dynamics, all in one scan, has been developed along with a method to use it for evaluation of carrier diffusion. The method allows measuring diffusion lengths as small as similar to 100 nm and their anisotropy and spatial distribution, parameters remaining inaccessible to conventional far-field techniques. The procedure has been applied to study ambipolar carrier diffusion in a nonpolar m-plane InGaN/GaN quantum well. The diffusion was found to be highly anisotropic with diffusion coefficients along and perpendicular to the wurtzite c axis equal to 0.4 and 1.9 cm(2)/s, respectively. The large diffusion anisotropy confirms band structure calculations that suggest that the topmost valence band in an m-plane InGaN quantum well is highly anisotropic.

Scanning near-field optical microscopy was used to study the influence of the surface morphology on the properties of light emission and alloy composition in InGaN epitaxial layers grown on GaN substrates. A strong correlation between the maps of the photoluminescence (PL) peak energy and the gradient of the surface morphology was observed. This correlation demonstrates that the In incorporation strongly depends on the geometry of the monolayer step edges that form during growth in the step-flow mode. The spatial distribution of nonradiative recombination centers-evaluated from PL intensity maps-was found to strongly anticorrelate with the local content of In atoms in the InGaN alloy. 

The paper reviews our recent achievements in developing a multimode scanning near-field optical microscopy (SNOM) technique. The multimode SNOM apparatus allows us to simultaneously measure spatial variations of photoluminescence spectra in the illumination and illumination-collection modes, their transients and sample surface morphology. The potential of this technique has been demonstrated on a polar InGaN epitaxial layer and nonpolar InGaN/GaN quantum wells. SNOM measurements have allowed revealing a number of phenomena, such as the band potential fluctuations and their correlation to the surface morphology, spatial nonuniformity of the radiative and nonradiative lifetimes, as well as the extended band nature of localized states. The combination of different modes enabled measurements of the ambipolar carrier diffusion and its anisotropy.

Multimode scanning near-field photoluminescence spectroscopy was developed and applied to study carrier localization and dynamics in m-plane InGaN quantum wells. The study showed that localized hole states maintain properties of extended bands, radiative and nonradiative carrier lifetimes are spatially nonuniform, and hole diffusion is anisotropic.

To assess the impact of random alloying on the optical properties of the InGaN alloy, high-quality InxGa1-xN (0 < x < 0.18) epilayers grown on c-plane free-standing GaN substrates are characterized both structurally and optically. The thickness (25-100 nm) was adjusted to keep these layers pseudomorphically strained over the whole range of explored indium content as checked by x-ray diffraction measurements. The evolution of the low temperature optical absorption (OA) edge line-width as a function of absorption energy, and hence the indium content, is analyzed in the framework of the random alloy model. The latter shows that the OA edge linewidth should not markedly increase above an indium content of 4%, varying from 17 meV to 30 meV for 20% indium. The experimental data initially follow the same trend with, however, a deviation from this model for indium contents exceeding only similar to 2%. Complementary room temperature near-field photoluminescence measurements carried out using a scanning near-field optical microscope combined with simultaneous surface morphology mappings reveal spatial disorder due to growth meandering. We conclude that for thick high-quality pseudomorphic InGaN layers, a deviation from pure random alloying occurs due to the interplay between indium incorporation and longer range fluctuations induced by the InGaN step-meandering growth mode.

A multimode scanning near-field optical microscopy technique allowing to map surface morphology, photoluminescence (PL) spectra in illumination and illumination-collection modes, as well as PL dynamics, all in one scan, has been developed along with a method to use it for evaluation of carrier diffusion. The method allows to measure diffusion lengths as small as ~100 nm, their anisotropy and spatial distribution, parameters remaining inaccessible to conventional far-field techniques. The procedure has been applied to study ambipolar carrier diffusion in a nonpolar m-plane InGaN/GaN quantum well. The diffusion was found to be highly anisotropic with diffusion coefficients along and perpendicular to the wurtzite c axis equal to 0.4 and 1.9 cm²/s, respectively. The large diffusion anisotropy confirms band structure calculations that suggest that the top-most valence band in m-plane InGaN quantum well is highly anisotropic

The report on a study of laser emission from a conceptually new organic material based on transparent wood (TW) with embedded dye Rhodamine 6G molecules is presented in this paper. The lasing performance is compared to a reference organic material containing dye in a poly-methyl-methacrylate matrix. From experimental results, one can conclude that the optical feedback in dye-TW material is realized within cellulose fibers, which play the role of tiny optical resonators. Therefore, the output emission is a collective contribution of individual resonators. Due to this fact, as well as low Q-factor of the resonators/fibers and their length variation, the spectral line of laser emission is broadened up to several nanometers.

Gallium phosphide nanowaveguide arrays, designed to fulfill the phase matching conditions and field-overlap, are characterized by second-harmonic generation. The bandwidth of 30nm with maximum conversion efficiency of 10-3 is measured for 150fs optical pulses.

We demonstrate enhanced second-harmonic generation in arrays of nanowaveguides satisfying modal-phase-matching condition, both theoretically and experimentally. The overlap of interacting fields defined by the fundamental mode of the pump and the second-order mode of the second-harmonic wave is enhanced by the longitudinal component of the nonlinear polarization density. For guided modes with significant longitudinal electric field components, the overlap of fields is comparable to that obtained in the quasi-phase-matching technique leading to higher conversion efficiencies. Thus, the presented method is preferable to achieve higher conversion efficiency in second-order nonlinear processes in nanowaveguides.

We present a polarization, spectrally, and spatially resolved near-field photoluminescence (PL) measurement technique and apply it to the study of wide m-plane InxGa1-xN/GaN quantum wells grown on on-axis and miscut GaN substrates. It is found that PL originates from localized states; nevertheless, its degree of linear polarization (DLP) is high with little spatial variation. This allows an unambiguous assignment of the localized states to InxGa1-xN composition-related band potential fluctuations. Spatial PL variations, occurring due to morphology features of the on-axis samples, play a secondary role compared to the variations of the alloy composition. The large PL peak wavelength difference for polarizations parallel and perpendicular to the c axis, the weak correlation between the peak PL wavelength and the DLP, and the temperature dependence of the DLP suggest that effective potential variations and the hole mass in the second valence-band level are considerably smaller than that for the first level. DLP maps for the long wavelength PL tails have revealed well-defined regions with a small DLP, which have been attributed to a partial strain relaxation around dislocations.