R. Tomasiunas

Optical characteristics of two-dimensional photonic crystals in anodic aluminum oxide films

The optical properties of two-dimensional photonic crystals (PhCs) in anodic aluminum oxide (AAO) films obtained using a simple and low-cost pre-patterning procedure are described. The prepatterning of the initial Al film surface was carried out by imprinting with an optical diffraction grating; the anodization of the prepatterned sample led to the formation of a good quality, large-area PhC with a triangular lattice of air holes (lattice period a=0.48 µm, hole radius r=0.2 µm) in an AAO film. The optical transmission spectra of the sample were measured at visible wavelengths in the range of 0.4–1.0 µm for various incidence angles and linear polarizations of the probing light. The detailed analysis of the transmission data indicates a photonic band gap in the 0.9–1.0 µm wavelength range for light waves linearly polarized in the direction perpendicular to the axes of PhC pores.

Photoinduced absorption saturation dynamics of InGaAs quantum dot structure dedicated for wavelength 1070 nm

In this paper, results of investigation of InGaAs quantum dot structure designed as a semiconductor saturable absorber for near-IR are presented. Photoinduced absorption bleaching spectroscopy performed confirm the dedicated wavelength 1070 nm for absorption bleaching of the ground state of quantum dots. Transitions including states from the wetting layer and the ground states have been interpreted to define absorption recovery dynamics on femto- and picosecond time scale. Fast recovery time of absorption was measured for the upper excited energy levels, while the ground state dynamics show relative longer times. Non-degenerate pump-probe technique was applied to investigate the interstate transition dynamics.

All-optical poling kinetics of azophenylcarbazoles influenced by attached anchor

Second order non-linear optical features of anchor attached azophenylcarbazole based derivative 9-(2,3-epoxy)propyl-2-(2,3-epoxy)propoxy-3-(4-nitrophenyl)azo-9H-carbazole dispersed in polycarbonate was investigated. Using all-optical poling technique writing and reading kinetics of second order susceptibility gratings were measured. Theoretical modelling of the experimental results revealed one cis-trans thermoisomerization rates k1= 1.44×10-3 s-1, which is twice or more times less than for the branched donors, depended on branching. Two arguments have been raised to explain the anchor influenced decrease in cis isomer flexibility. The investigated input of orientational diffusion confirmed the conclusion.

Blue and green LED structures investigated in frequency domain using violet LD as an excitation source

In this work carrier lifetime investigations by applying photoluminescence frequency domain technique to investigate blue and green light emitting diode (LED) structures, thus representing wide spectra for the InGaN LEDs were performed. Similar measurements have been carried out in GaN at extremely low excited carrier densities [1], as low as 1 mW/cm2 by using UV LED as an excitation source. For these measurements we applied laser diode (LD), to excite resonantly the MQWs in the sample investigated with modulated light. This allowed us to cover a broad range of excitation power density (1 – 500 mW/cm2). To analyze the activation mechanisms measurements down to 10 K temperatures were performed. The non-equilibrium charge carrier lifetime dynamics was investigated by applying a model with a superposition of exponential and stretch exponential decay depended on the sample and on the measurement condition. The uniqueness of this method is the possibility to study the transient processes in structures under very low to low non-equilibrium charge carrier densities at an un-saturated recombination channel condition.

Photo-Switching dynamics of azo-phenylcarbazoles in polycarbonate around β transition temperature

Second order non-linear optical features of azophenylcarbazole based derivative 9-(2,3-epoxy)propyl-2-(2,3-epoxy)propoxy-3-(4-nitrophenyl)azo-9H-carbazole in polycarbonate investigated at temperature 240 K, which is close to β transition for polycarbonate, are presented. Using all-optical poling technique writing and reading kinetics of second order susceptibility gratings in the azophenylcarbazole derivative were measured. Theoretical modelling of the experimental results indicates two cis-trans thermoisomerization rates in the range of k₁ = (2.5-3.3)×10^-3 s^-1, k₂ = (6.3-7.14)×10^-4 s^-1 depending on writing laser beam intensities (I_ω= 0.75-1.43 J/cm², I_2ω= 0.5-0.96 mJ/cm²); with slightly lower rates for the lower writing beam fluence. The orientational diffusion rate 1.43·10^-5 s^-1 does not depend on the writing conditions. The result indicates that the range of writing conditions used has very minor or even no influence over the intermolecular interaction observed during the all-optical poling process.

Optical poling of azo-compounds for applications in nanophotonics

Azo-compounds in polymers will likely play an increasingly important role in engineering of silicon photonics and plasmonic nano-devices for E-O modulation, parametric generation, switching and sensing. In this paper, we present a phenomenological theory of all-optical poling and photo-assisted electric field poling of azo-dye doped polymers and compare theoretical simulation results with experimental all-optical poling transients at varying temperatures obtained for azophenylcarbazole type azo-dyes in polycarbonate polymer matrix.

Azochromophore isomerization dynamics: From monomers to polymers

The synthesis and second order optical features of azophenylcarbazole based monomer 9-(2,3-epoxy)propyl-2-(2,3-epoxy)propoxy-3-(4-nitrophenyl)azo-9H-carbazol and polymeric azo dye AZAP are described. Relative higher optical poling efficiency was measured for the polymer, however, better stability, i.e. less orientational diffusion and thermal relaxation ability, were observed for the monomer. Modeling revealed parameters describing probability for molecules to rotate, orientational diffusion and thermal relaxation.

Picosecond reading of optically poled azophenylcarbazole

We report on reading process of poled media determined by duration and fluence of reading light. The poled media - oriented azophenylcarbazole molecules - was obtained by all-optical poling using nanosecond YAG laser radiation. While comparing picosecond and nanosecond pulses used for reading, it turned out that the power of light is dominating, if considering the erasure decay of poled media.

Photoinduced transmittance at 1250 nm of InAs/InGaAs quantum dot based semiconductor optical amplifier measured via waveguiding configuration

New results on investigation of InAs/InGaAs quantum dot structure designed as a waveguide are presented. Photoinduced transmission and absorption recovery measurements using femtosecond pump-probe experiment in a waveguiding configuration, when exciting/probing from the edge of waveguide, were performed at a dedicated wavelength 1250 nm covering the ground state levels of a group of chirped quantum dots. The results of absorption saturation fit well electroluminescence results giving qualitative insight about density of states. From the absorption recovery kinetics picosecond lifetimes for the ground state were considered.

Photoinduced molecular polar alignment in E-O polymers by all-optical poling holographic methods

A phenomenological model of photoinduced poling dynamics is developed on the basis of the experimental results. Given the intensities of the fundamental field and the second harmonic field used in the poling process, generation and relaxation of χ(2) and χ(3) nonlinear susceptibilities are described by this model. The cis and trans orientational order parameters up to the fourth order are used in the calculations. The fact that the laser used in experiments operates in pulses, rather than a continuous wave mode, is also taken into account in this model. The model could eventually be used to determine the optimal intensities of poling fields for efficient generation of nonlinear optical structures. We present results on all-optical poling kinetics of electro-optic active polyester type dendrimers Gl photo-patterned in the polycarbonate matrix.