R. Tomašiūnas

Orientational relaxation of three different dendrimers in polycarbonate matrix investigated by optical poling

We report on all-optical poling relaxation of E-O active dendrimers photo-patterned in the polycarbonate matrix. Three kinds of polyester type dendrimers G0, G1 and G2 containing azobenzene in the core were used as second-order nonlinear optical active chromophores to create noncentrosymmetry inside the material. We used a guest–host type system of these structures spin-coated on a glass substrate. These samples were poled by using an all-optical poling method, where the polar electric field of electromagnetic waves enables us to orient dendrimer molecules creating a quasi-phase-matched grating of nonlinear second-order susceptibility χ(2). Molecular orientation and subsequent dark relaxation kinetics in the samples with different poling beam applied energies were measured. Theoretical modeling of the poling relaxation kinetic revealed a set of characteristic time constants, which were interpreted by different relaxation mechanisms.

A systematic study of light extraction efficiency enhancement depended on sapphire flipside surface patterning by femtosecond laser

We report on the investigation of light extraction efficiency enhancement at the flip-side surface of sapphire patterned by using femtosecond laser pulse irradiation. The enhancement was measured by means of fluorescence and confocal microscopies. Theoretical simulation (FDTD) gave the results. The variation of patterning geometry provided by different gap and pit size, by triangle and square array configuration revealed compactness of the pit network as an optimum condition for the enhancement. Maximum enhancement over 40% was obtained for the triangle array configuration. The appearance of ripples at the bottom of the pit corresponded to the enhancement. A deeper insight about consequences to radiative effects influenced by the ripples is presented.

Photoisomerization of Azophenylcarbazole Self-assembled Monolayer on GaN Studied by Kelvin Probe

Self assembled monolayers of two fabricated azophenylcarbazoles on GaN epitaxial layers were obtained. Isomerization of the azo derivatives has been accomplished by illumination with wavelength of 440 nm, which coincides with their absorption maxima. The process has been observed in situ by Kelvin probe. A few parameters for azophenylcarbazoles (V-650 and V-728) of trans-cis switching such as difference between work function in trans and cis state and time constant for trans-cis and cis-trans isomerizations have been determined.

Picosecond reflectance recovery dynamics of porous silicon multilayer

Light-induced reflectance dynamics of a porous silicon multilayered structure via the picosecond pump-probe technique is investigated. Fast recovery dynamics is inspected for a range of excitation energy densities (2.4-24 mJ/cm2), incident angles (0°-25°), and polarizations. Modeling performed using the transfer matrix method revealed the photonic origin of the phenomena observed. The spectral shift of the reflectance spectrum, i.e., photonic bandgap, responsible for the positive signal originates from the change of photoinduced carrier concentration with linear and nonlinear character for different porosity layers. The influence of temperature on the refractive index in this thermally nonconductive material is taken into account.

Influence of metalorganic precursors flow interruption timing on green InGaN multiple quantum wells

The paper reports on fully strained green light emitting InGaN/GaN multiple quantum wells, grown by metalorganic vapor phase epitaxy, using metal precursor multiple flow interruptions during InGaN quantum well growth. Optimization of the interruption timing (pulse t 1 = 20 s, pause t 2 = 12 s) lets us reach the integrated photoluminescence enhancement for the growth at temperature 780 oC. The enhancement, as a function of pause duration, appeared to be pulse duration dependent: a lower enhancement can be achieved using shorter pulses with optimized relatively shorter pauses. Indium evaporation during the interruption time was interpreted as the main issue to keep the layers intact. Quantum wells revealing the highest photoluminescence enhancement were inspected for interface quality, layer thickness, growth speed, strain, surface morphology and roughness by TEM, XRD and AFM techniques, and compared with the one grown in the conventional mode.

Photoinduced Optical Functionalism of Branched Azophenylcarbazoles in Molecular Glass Matrices

A new guest-host composition of 3-(4-nitrophenyl)azo-9-(2-ethyl)hexyl-2-(2-ethyl) hexyloxycarbazole as chromophore and 2,2-bis{4-[2-hydroxy-9-(carbazol-9-yl)-5-(carbazol-9-methyl)-4,7-dioxanonyloxy]phenyl}propane (molecular glass) as host for all-optical poling application was developed and investigated. A comparison between molecular glass and commonly used bisphenol Z polycarbonate as host showed a slightly better ability for the molecular glass to keep the azophenylcarbazole molecules oriented. Erasure while reading the non-centrosymmetry created is demonstrated and explained. Using an adopted model orientational diffusion coefficient, lifetime of cis isomer and probabilities of redistribution were evaluated.