High Levels Of Optical Excitation Research Articles

Band gap renormalization of the InSe by laser radiation

Nonlinear absorption of light and its time evolution in InSe under the influence of picosecond laser excitation have been investigated experimentally. It was shown that the decrease in exciton absorption in InSe at high levels of optical excitation is due to the exciton–exciton interaction and screening of the Coulomb potential by free carriers, generated by laser light. Observations of induced absorption in the energy region between the exciton level and the edge of the conduction band are associated with the appearance of a continuum of states due to a shift in the edge of the energy band.

Nonlinear Optical Absorption in GaSe upon Laser Excitation

The nonlinear absorption in GaSe crystals at high optical excitation levels is experimentally studied. A Nd:YAG laser (first and second harmonics, 1064 and 532 nm) and a liquid laser (tuning range 594–643 nm) were used as excitation sources. It is shown that the features observed in the exciton resonance region of the absorption, luminescence, and photoconductivity spectra of GaSe crystals at high optical excitation levels can be explained by exciton–exciton interaction and Coulomb interaction screening by free carriers.

Нелинейное оптическое поглощение в GaSe при лазерном возбуждении

: Nonlinear light absorption in GaSe crystals at high levels of optical excitation have been investiqated experimentally. Nd:YAG laser (1st, 2nd harmonic-1064 nm, 532 nm) and a liquid laser (radiation wavelength tuning range of 594-643 nm) were used as radiation sources. It is shown that the observed features in the absorption, luminescence, and photoconductivity spectra in the exciton resonance region in GaSe crystals at high levels of optical excitation can be explained by the interaction of excitons and the screening of the Coulomb interaction by free carriers.

Reliability of Blue-Emitting Eu2+-Doped Phosphors for Laser-Lighting Applications.

This paper investigates the reliability of blue-emitting phosphors for Near-UV (NUV) laser excitation. By means of a series of thermal stress experiments, and of stress under high levels of optical excitation, we have been able to identify the physical process responsible for the degradation of Eu2+-activated alkaline-earth halophosphate phosphors under typical and extreme operating conditions. In particular, for temperatures equal to or greater than 450 °C the material exhibited a time-dependent drop in the Photo-Luminescence (PL), which was attributed to the thermally induced ionization of the Eu2+ optically active centers. Several analytical techniques, including spatially and spectrally resolved PL, Electron Paramagnetic Resonance (EPR) and X-ray Photo-emission Spectroscopy (XPS) were used to support this hypothesis and to gain insight on the degradation process. By means of further tests, evidence of this degradation process was also found on samples stressed under a relatively low power density of 3 W/mm2 at 405 nm. This indicated that the optically (and thermally) induced ionization of the optically active species is the most critical degradation process for this family of phosphorescent material. The operating limits of a second-generation Eu-doped halophosphate phosphor were also investigated by means of short-term stress under optical excitation. The experimental data showed that a threshold excitation intensity for continuous pumping exists. Above this threshold, decay of the steady-state PL performance and non-recoverable degradation of the material were found to take place. This behavior is a consequence of the extremely harsh excitation regime, mainly due to the thermal management capabilities of the substrate material employed for our experimental purposes rather than from intrinsic properties of the phosphors.

Features of laser-induced luminescence and photoconductivity of layered Cu3In5S9 crystals

Luminescence and photoconductivity of layered Cu3In5S9 crystals at high levels of optical excitation are studied experimentally. A pulsed nanosecond Nd:YAG laser with built-in second and third harmonic generators to generate 1064-, 532-, and 355-nm radiation is used as a light source. It is found that the photoluminescence spectra exhibit two emission bands due to zone–acceptor level and impurity donor–impurity acceptor transitions. It is shown that the photoconductivity in Cu3In5S9 is monopolar. The waveform of the photoconductivity consists of fast and slow components associated with two channels of recombination.

Self-formation of ultrahigh-density (1012 cm−2) InAs quantum dots on InAsSb/GaAs(001) and their photoluminescence properties

InAs quantum dots (QDs) with an ultrahigh density of 1 × 1012 cm−2 were fabricated on a 1.25-monolayer-thick InAsSb wetting layer on a GaAs(001) substrate by molecular beam epitaxy. QD formation was initiated by small two-dimensional InAsSb islands. Coalescence and ripening effects involving neighboring QDs were suppressed. Photoluminescence spectra of the QDs shifted continuously to higher energies with increased optical excitation power. This was attributed to the filling of inhomogeneous ground states via tunneling between QDs. Indirect transitions in a type-II band structure were observed for small QDs. In large QDs, direct transitions were also observed at high optical excitation levels.

Bleaching in the region of exciton resonance of layered GaSe crystals

Light absorption in the region of exciton resonance of GaSe crystal is studied experimentally at high levels of optical excitation. A picosecond YAG:Nd3+ laser emitting 30-ps light pulses and a dye laser with a pulse width of ∼3 ns tunable within the range 594–643 nm were used as light sources. It was found that, at high levels of optical excitation, the exciton absorption line of the GaSe crystal disappeared, which was attributed to increasing exciton density with arising mechanisms of their decay: exciton-exciton interactions and screening of excitons by the free charge-carrier plasma. It is shown that these mechanisms are also responsible for the arising new emission band in the long-wavelength region of the photoluminescence spectrum.

Picosecond kinetics of the electron‐hole layers formation in wide‐bandgap II‐VI type‐II heterostructures

AbstractConsiderable slowdown of luminescence kinetics of the direct optical transition was discovered in ZnSe/BeTe type‐II heterostructures under high levels of optical pumping. The effect is attributed to forming of a potential barrier for holes in the ZnSe layer due to band bending at high densities of spatially separated carriers. That results in a longer time of the photoexcited holes energy relaxation to their ground state in the BeTe layer.The decrease of overlapping of electron and hole wavefunctions in the ZnSe layer in thick ZnSe/BeTe structures at high levels of optical excitation reveals an additional important effect, that leads to sufficient retardation of radiative recombination time for photoexcited carriers (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoconductivity and luminescence in GaSe crystals at high levels of optical excitation

The photoconductivity and luminescence of GaSe layered crystals at high levels of optical excitation are studied experimentally. The specific features observed in the photoconductivity and photoluminescence spectra are controlled by the nonlinear optical absorption in the region of excitonic resonance.

Radiative recombination in type-II ZnSe/BeTe heterostructures at high densities of free carriers

We have studied in detail the photoluminescence in type-II ZnSe/BeTe structures with large band offsets at high optical excitation levels. Our goal is a qualitative and quantitative description of the effect of a high carrier density on the spectral and kinetic properties of the photoluminescence in such structures. We have established that a high photoexcitation density leads to a significant spectral rearrangement of the spatially indirect radiative recombination, which also depends on the thickness of the structure layers. The spectral and kinetic characteristics of the indirect optical transitions in narrow and wide structures have been found to differ qualitatively. Our numerical calculations of the effect of macroscopic electric fields on the energy spectrum of free carriers at high densities of spatially separated charges, n > 1013 cm−2, give a qualitative and quantitative description of the experimentally observed photoluminescence properties. The calculations also show that the degree of localization of the above-barrier hole states grows significantly with n under conditions of increasing band bending, which should cause a weakening of the charge separation, especially in wide structures. In an experiment, the effect manifests itself in an enhancement of the spatially direct transition at a simultaneous weakening of the indirect transition both with increasing structure width and with increasing n.

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E ⊥ c the photoconductivity is observed in the hν < E g and hν > E g regions. In the case of hv < E g , the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E | | c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton–exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.

Transit-time considerations in metal–semiconductor–metal photodiode under high illumination conditions

A two-dimensional self-consistent time-dependent simulation technique has been used to investigate electron–hole transport processes in the active region of metal–semiconductor–metal photodiode structures (MSM-PD) and to analyze their high-speed response at different energy levels of the optical illumination. Charge accumulation and screening of the dark electric field at high optical excitation levels greatly modify the drift conditions of the photogenerated electrons and holes in the active region of the MSM-PD. This effect gives rise to impulse response distortion and reduced bandwidth and efficiency. Some ways of improving the high-speed response of the MSM-PD are analyzed and discussed. The conditions under which screening of the internal field has no effect on the MSM-PD response are formulated. It is shown that large active area MSM-PDs have considerable advantages over smaller area devices in their ability to detect high energy levels of optical radiation without saturation and distortions.

A comparative study of AlGaN- and GaN-based lasing structures for near- and deep-UV applications

ABSTRACTWe report a comprehensive study on the optical properties of GaN- and AlGaN-based lasing structures at high-levels of optical excitation (carrier densities of 1017−1020 cm−3) and identify critical issues necessary for the development of near- and deep-UV light emitting devices. We successfully achieved room temperature stimulated emission (SE) with emission wavelengths ranging from 351 nm to 373 nm in a variety of samples. Through an analysis of the temperature-dependent lasing characteristics, combined with absorption and time-resolved photoluminescence measurements, we estimated the carrier density required to achieve the SE threshold in GaN epilayers. We found that in AlGaN epilayers, the onset of SE (∼1019 cm−3) occurs at carrier densities one order of magnitude higher than in thick GaN epilayers, indicating that an electron-hole plasma is the dominant gain mechanism over the entire temperature range studied (10 K to 300 K). A remarkably low lasing threshold was observed in GaN/AlGaN heterostructures over the temperature range of 10 K to 300 K. Our experimental results indicate that GaN/AlGaN heterostructures could be used to efficiently generate laser emission with wavelengths shorter than 373 nm. The implications of this study on the development of UV laser diodes is discussed.

Influence of photoluminescence and trapping on the photovoltage at the por-Si/p-Si structure

The influence of photoluminescence (PL) and trapping on photovoltage (PV) transients has been compared for as-anodized and for oxidized in air por-Si/p-Si structures. Measurements were carried out with time-resolved surface photovoltage and with contact potential difference techniques. Light sources with wavelengths from the infrared to the ultraviolet regions were used for excitation. It was shown that PV based on PL light can be used for the estimation of the PL efficiency. Electron trapping was found to modify the band bending at the por-Si/p-Si interface of the oxidized in air sample. Further it was shown that non-monotonous PV transient features took place on oxidized por-Si/p-Si structures at high levels of optical excitation.

The energy profile of polymorphous PbTe films. II. Recombination in heterophase PbTe films at high levels of optical excitation

For pt.I see ibid., vol.4, p.8997 (1992). Picosecond photoconductivity measurements of polymorphous PbTe films have been performed. Excess carriers were excited in the films by means of a picosecond YAG:Nd laser with pulse duration t=12 ps. The instantaneous relaxation times of photoconductivity were obtained as a function of the excess carrier concentration Delta N from the kinetic analysis. The heterophase junction nature of the comparatively thin (d approximately=1 mu m) films was confirmed by their high photosensitivity. The junction thickness was evaluated from the interpretation of the instantaneous lifetime dependence on the excess carrier concentrations.

Expansion of the Electron‐Hole Liquid Plasma at GaAs Surface

AbstractThe spectral and planar distribution of the luminescence intensity of the electron‐hole plasma (EHP) created by laser excitation in the subsurface region of a direct‐band semiconductor like GaAs in contact with a dielectric medium (Si3N4) is studied. High optical excitation levels (L ≈ ≈ 5 MW/cm2) make possible to detect: (1) the appearance of stimulated radiation mainly from the regions of the sample edges; (2) the expansion of EHP from the excited region predominantly along the GaAs‐Si3N4 interface up to a distance (l ≈ 50 to 100 μm), which is several times greater than the corresponding bulk value. The assumption is made that the electron‐hole liquid is created not only at the “tails” of the expanded plasma, but also within the excited regions (in the latter case after a partial cooling of EHP, when in the process of the relaxation the electron‐hole pairs concentration achieves the EHP‐condensation value n ≈ 1016 cm−3). The mechanism of a long‐wave range tail formation in EHL radiation, which is caused by the energy losses on non‐equilibrium plasmons, is discussed.

Time evolution of nonequilibrium photoexcited plasma in polar semiconductors

The nonequilibrium thermodynamics and kinetics of evolution of relaxation processes in polar semiconductors under high levels of optical excitation is studied. This is done using a first-principles theory that allows for the determination of the nonlinear transport equations which describe the irreversible processes that develop in the media in typical pump-probe experiments. Numerical calculations are presented which permit a comprehensive discussion of measurements of ultrafast-time-resolved optical spectra of GaAs.

Time-relaxation approach in far from equilibrium semiconductor plasma

The dynamics of evolution of the LO-phonon population in semiconductors under high levels of optical excitation is determined. Collision operators are written in a Boltzmann-like form, with the relaxation-times being a functional of the nonequilibrium thermodynamic parameters which describe the instantaneous macroscopic state of the whole system. The expected behavior of the relaxation channels under different experimental constraints is discussed.

Recombination in Pb0.83Sn0.17Te at high levels of optical excitation

AbstractThe photoconductivity and the photomagnetoelectric current in low‐concentration, high‐lifetime Pb0.83Sn0.17Te samples are measured at strong optical excitation using a Q‐switched ruby laser. At quantum flux Q &gt; 2 × 1021 cm−2s−1 nonlinear recombination is observed. From the characteristics measured the coefficient of Auger recombination C = (4.5 ± 1) × 10−27 cm6s−1 is obtained. The results are confirmed by lifetime data obtained at low excitation levels.

Recombination in Tellurium at High Optical Excitation

AbstractPhotoconductivity (p.c.) and photomagnetoelectric effect (p.m.e.) are observed in tellurium at high levels of optical excitation by means of a Q‐switched ruby laser. The characteristics observed for the stationary case are analysed in terms of a spatial distribution of carriers governed by diffusion and interband Auger recombination. The coefficient of Auger recombination in tellurium C = 1.5 × 10−28 cm6 s‐l is obtained.