Edgaras Jelmakas

Photoelectrolysis of water: Solar hydrogen–achievements and perspectives

Thermodynamic analysis of energy conversion from light-to-chemical, light-to-electric and electric-to-chemical is presented by the case study of water photoelectrolysis on TiO(2) surface. It is demonstrated that at the current state-of-the-art energy conversion efficiency of water photoelectrolysis can be increased approximately 17 times by separating the processes of solar-to-electric and electric-to-chemical energy conversion and optimizing them independently. This allows to mitigate a high overvoltage of oxygen evolution reaction with respect to thermodynamic E(0)(O(2)/H(2)O) = 1.23 V potential as well as spectrally narrow absorbtivity of solar light by TiO(2) which determine the low efficiency (approximately 1.0%) of direct light-to-chemical energy conversion. Numerical estimates are provided illustrating practical principles for optimization of the solar energy conversion and storage processes.

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.

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.

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.

InGaAs quantum dot 1050 nm saturable absorber mirror: Investigation under high excitation condition

In this paper, semiconductor saturable absorber mirror designed for the near-IR spectra and based on InGaAs quantum dots was investigated. Absorption saturation, absorption femtosecond recovery kinetics measured at various close to resonance quantum energies revealed the wavelength ∼ 1050 nm as best performing absorption bleaching (highest value of 2 % for reflectivity change) for this sample. Nonlinear absorption, carrier thermalization appeared to govern carrier generation, their dynamics in the quantum dots.