Ultrafast nonlinear optical response and high density excitation effects of the stacking fault exciton in BiI3

On the exciton states localized at a two-dimensional stacking fault interface in a layered crystal BiI₃, some nonlinear optical phenomena clearly appear reflecting large transition probability. The optical Stark shift and other high density exciton effect on the energy-shift and the spectral broadening are observed with clear separation under intense laser pumping by time-resolved measurements. The optical Stark shift is analyzed based on the dressed exciton model. The ultrafast optical response faster than 3 ps for the pump-laser field is confirmed on the Stark shift. Degenerate four-wave-mixing signals show fairly long dephasing time of 40 ps in this system. The dephasing probability depends linearly on the pump-laser intensity in the same manner as that of the spectral line-broadening reflecting the relaxation process. The dephasing mechanisms are understood by the exciton scattering at high density in parallel with the spectral changes. The blue-shift due to the high density excitons are discussed on the basis of exciton-exciton interaction in connection with a phase-space filling theory in two-dimensional systems.     

Synthesis, effect of capping agents, structural, optical and photoluminescence properties of ZnO nanoparticles

Zinc oxide nanoparticles were synthesized using chemical method in alcohol base. During synthesis three capping agents, i.e. triethanolamine (TEA), oleic acid and thioglycerol, were used and the effect of concentrations was analyzed for their effectiveness in limiting the particle growth. Thermal stability of ZnO nanoparticles prepared using TEA, oleic acid and thioglycerol capping agents, was studied using thermogravimetric analyzer (TGA). ZnO nanoparticles capped with TEA showed maximum weight loss. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for structural and morphological characterization of ZnO nanoparticles. Particle size was evaluated using effective mass approximation method from UV-vis spectroscopy and Scherrer's formula from XRD patterns. XRD analysis revealed single crystal ZnO nanoparticles of size 12-20 nm in case of TEA capping. TEA, oleic acid and thioglycerol capped synthesized ZnO nanoparticles were investigated at room temperature photoluminescence for three excitation wavelengths i.e. 304, 322 and 325 nm, showing strong peaks at about 471 nm when excited at 322 and 325 nm whereas strong peak was observed at 411 for 304 nm excitation.     

The effects of crystal structure on optical absorption/photoluminescence of bis(8-hydroxyquinoline)zinc

In this paper, the transformation processes of two types of bis(8-hydroxyquinoline)zinc: Znq₂ dihydrate and anhydrous (Znq₂)₄ were investigated by X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG). The effects of crystal structure on optical properties of bis(8-hydroxyquinoiline)zinc were analyzed. Znq₂ dihydrate can be transformed into anhydrous (Znq₂)₄ during heating under vacuum. Reversal transformation occurs by the interaction between chloroform and (Znq₂)₄. But (Znq₂)₄ was partially transformed into Znq₂ dihydrate by the interaction between ethanol and (Znq₂)₄. The different molecular structure results in different crystal stacking and electronic structure, thereby affect its optical properties.     

Near band-edge luminescence of semi-insulating undoped gallium arsenide at high levels of excitation

The dependences of the maximum and the half-width of near band-edge photoluminescence of semi-insulating undoped-GaAs crystals at 77 K on the concentration of background acceptor impurities and the level of excitation in the range from 3×10²¹ to 6×10²² quantum/(cm² s) are investigated. The observed dependences are explained by formation of the density tails of states as a result of fluctuations of impurity concentration and participation of localized states of the donor impurity band in radiative transitions. Reduction of many-particle interaction at increasing of N can be connected with increasing of shielding of charge carriers by atoms of impurity.     

Many-body effects in the highly excited state of fullerenes

The highly excited state of fullerenes is characterised by a luminescence output which is dependent on the cube of the input intensity. This nonlinear emission is red shifted from the low-level emission and has a long, intensity dependent lifetime. Under similar irradiation conditions, the photoconductive response is seen to increase with the cube of the input intensity and the photocurrent in the highly excited state is observed to be largely independent of temperature, contrasting sharply with the thermally activated behaviour at low excitation densities. The degree of nonlinearity of the observed phenomena exclude an interpretation in terms of intra-molecular processes and the temperature dependence of the photoconductive response is suggestive of a Mott-like transition. The nonlinear behaviour is compared to that of indirect band-gap semiconductors in which the origin of similar nonlinear phenomena in the highly excited state luminescence and photoconductivity are explained in terms of electron-hole droplet formation. The similarities of the behaviours leads to a consideration of exchange and correlation energies in fullerenes, which are calculated according to a phenomenological model. Estimates of the contributions are consistent with a Mott-like transition at high excitation densities and an excess exchange/correlation energy in the highly excited state of 150 meV.     

High quality GaAs quantum wires grown by flow rate modulation epitaxy

Accepted: 14 October 1997     

Strongly enhanced free-exciton luminescence in microcrystalline CsPbCl3 films produced via the amorphous phase

High-quality CsPbCl₃ films composed of crystallites with narrow size distributions are achieved for various size levels, from microcrystalline to polycrystalline, by a novel heat-treatment method applied to the same amorphous films. Their photoluminescence is dominated by free-exciton emission at every size level without showing trapped-exciton emission in great contrast to the case for single crystals. The microcrystalline state shows more than an order of magnitude stronger free-exciton emission than the polycrystalline state, and exhibits intense stimulated emission under high-power excitation.     

Photoluminescence spectra of nitrogen implanted GaSe crystals

GaSe single crystals were N-implanted along c-axis with ion beams of 10¹⁴ and 10¹⁶ ions/cm² doses having energy values of 60 and 100 keV. The photoluminescence (PL) spectra of undoped and N-implanted GaSe crystals were measured at different temperatures. The PL intensity was observed to decrease with increasing implantation dose while the FWHM of the exciton peaks increased. In heavily doped crystals, due to the interaction with the radiation induced disorders, the wave vector selection rules are satisfied and an indirect exciton PL band is observed 36 meV below the direct exciton states.     

The optical dielectric function of ZnO

In this work, an analytical expression for the complex dielectric function is proposed, which includes both discrete and continuum exciton effects. We have introduced Lorentzian broadening in Elliott’s formula and compare our model to other models based on Elliott’s formula. The results show that our model is superior to other models for the dielectric function of ZnO in the literature. Excellent agreement with experimental data for the dielectric function of ZnO for both perpendicular and parallel polarizations has been obtained using our model. Received: 18 April 2001 / Accepted: 19 December 2001 / Published online: 20 March 2002 / Published online: 20 March 2002 RID="*" ID="*"Corresponding author. (Fax: +852-2559-8738, E-mail: dalek@eee.hku.hk)     

Exchange interaction and acoustical phonon modes in CdTe nanocrystals

Photoluminescence of CdTe nanocrystals (NC) is excited resonantly in the lowest energy absorption peak. The spectrum shows a luminescence line shifted to a lower energy and acoustical and optical phonon replica. The Stokes shift between the luminescence and excitation lines is attributed to the electron-hole exchange energy in the nanocrystal. By tuning the laser line inside the absorption peak, we are able to measure the Stokes shift as a function of the excitation energy. Calculation of the absorption gap and the Stokes shift is done in a tight-binding theory. It allows us to determine the radius R of NC excited at a given wavelength and to compare the experimental and theoretical values of the exchange energy as a function of R. A very good agreement is obtained. The observed size dependence of the acoustical phonon mode energy provides a further confirmation of our analysis.     

TL, EPR and optical absorption in natural grossular crystal

Grossular is one of six members of silicate Garnet group. Two samples GI and GII have been investigated concerning their luminescence thermally stimulated (TL). EPR and optical absorption and the measurements were carried out to find out whether or not same point defects are responsible for all three properties. Although X-rays diffraction analysis has shown that both GI and GII have practically the same crystal structure of a standard grossular crystal, they presented different behavior in many aspects. The TL glow curve shape, TL response to radiation dose, the effect of annealing at high temperatures before irradiation, the dependence of UV bleaching parameters on peak temperature, all of them differ going from GI to GII. The EPR signals around g=2.0 as well as at g=4.3 and 6.0 have much larger intensity in GI than in GII. Very high temperature (>800 °C) annealing causes large increase in the bulk background absorption in GI, however, only very little in GII. In the cases of EPR and optical absorption, the difference in their behavior can be attributed to Fe³⁺ ions; however, in the TL case one cannot and the cause was not found as yet.     

Novel model for the optical function of GaN

In this work we propose an analytical expression for the complex dielectric function which includes both discrete and continuum exciton effects. We have introduced Lorentzian broadening into Elliott’s formula. The introduction of broadening leads to equations for the dielectric function containing only elementary functions. We have applied the proposed model to the dielectric function of wurtzite GaN in the spectral region 1–10 eV. Excellent agreement with the experimental data has been obtained. We show that the Lorentzian-broadened dielectric function decays more slowly than the experimental data for hexagonal GaN at the low-energy side. This indicates that the broadening of the absorption edge in GaN is not purely Lorentzian. The agreement with the experimental data can be improved using adjustable broadening modification. Received: 29 January 2001 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Experimental and theoretical studies of polaron optical properties in KNbO3 perovskite

Time-resolved absorption and luminescence spectra have been measured in KNbO₃ perovskite crystals after pulsed band-gap excitation by 200 fs laser pulses and 10 ns electron pulses. Quantum chemical calculations using the large unit cell periodic model support the interpretation of the observed transient absorption bands at 0.8 and 1.1 eV as the self-trapped electron polarons and bound hole polarons, respectively. The activation energy for the 2.2 eV green luminescence quenching is 0.05 eV. We suggest that the short lifetime (<15 ns) of the luminescence at RT is caused by the radiative recombination of nearest electron and hole polarons.     

Deep bandtail states picosecond intensity-dependent carrier dynamics of GaN epilayer under high excitation

Picosecond carrier dynamics of deep bandtail states (3.1 eV) in an unintentionally n-doped GaN epilayer at room temperature under high excitation densities (i.e., N ₀ = 1.0× 10¹⁹– 1.1× 10²⁰ cm^–3) have been investigated with nondegenerate femtosecond pump–probe (267/400 nm) reflectance ( R/R ₀). All R/R ₀ traces possess a 2 ps buildup time that represents an overall time for the initial non-thermal carrier population to relax towards the continuum extremes and then into the probed tail states. We observe a saturation of R/R ₀ initial (first 10 ps) recovery rate _i at a density of 5– 6×10¹⁹ cm^–3 close to the Mott transition threshold obtained from time-integrated PL measurements. Such a saturation phenomenon has been identified as the trap-bottleneck due to the bandtail states and deep traps. As N ₀ is further increased, _i accelerates due to the onset of Auger recombination as the trap-bottleneck becomes effective. The best fit by the Auger model for N ₀ in the range of the mid-10¹⁹–10²⁰ cm^–3 yields an Auger coefficient of C _a 5.0× 10^–30 cm⁶ s^–1.     

Dependence of the heavy-hole exciton reduced mass on quantum-well size in InGaAs/GaAs heterostructures

We present low temperature photoluminescence investigations of the exciton ground state of In_0.14Ga_0.86As/GaAs quantum wells (QW) in the presence of pulsed magnetic fields up to 50 T. The exciton in-plane reduced mass and the heavy-hole in-plane mass are determined from the best fit of theoretical calculations to the magnetic field dependence of PL peaks. When the QW thickness decreases, their masses increases due to valence-band mixing effect.     

Microscopic theory of optical dephasing in semiconductors

Dephasing, i.e. the decay of the optical interband polarization in a semiconductor results from destructive interference effects between different microscopic contributions. For a system without any disorder it is shown that the many-body Coulomb correlations lead to excitation-induced dephasing which becomes increasingly important at elevated excitation levels. The effect of disorder-induced dephasing is analyzed for low excitation levels, where the combined influence of excitonic, biexcitonic, and disorder scattering contributions lead to a temporal decay of the four-wave-mixing signal. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Kinetic effects in recombination of optical excitations in disordered quantum heterostructures: Theory and experiment

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.     

Photoluminescence detection of impurities introduced in silicon by dry etching processes

We report on a photoluminescence study of silicon samples subjected to different dry etching processes. Several luminescence lines, known from defects produced by high-energy irradiation, manifest damage of the crystalline material. Noble gas ion beam etching (using Ne⁺, Ar⁺, Kr⁺, and Xe⁺) with ion energies as low as 400 eV produces characteristic luminescence lines which correspond to defects within a 200–300 Å thick surface layer. Incorporation of carbon during CF₄ reactive ion etching produces the familiar G-line defect. The G-line photoluminescence intensity in our samples is directly correlated with the substitutional carbon concentration, as determined by infrared absorption measurements before the etch process; we therefore suggest that a simple method to determine the substitutional carbon concentration in a crystalline silicon sample is a standard dry etching process and a comparison of the resulting G-line photoluminescence intensity to a calibrated sample. The sensitivity of this method seems to be better than 10¹⁴ carbon atoms/cm³.     

SrS:Ce/ZnS:Mn-A di-band phosphor for near-UV and blue LED-converted white-light emitting diodes

The SrS:Ce/ZnS:Mn phosphor blends with various combination viz 75:25, 50:50 and 25:75 were assign to generate the white-light emission using near-UV and blue-light emitting diodes (LED) as an excitation source. The SrS:Ce exhibits strong absorption at 427 nm and the corresponding intense emission occurs at 480 and 540 nm due to electron transition from 5d(²D)−4f(²F_5/2, _7/2) of Ce³⁺ ion as a result of spin-orbit coupling. The ZnS:Mn excited under same wavelength shows broad emission band with λ_max=582 nm originates due to 3d (⁴G−⁶S) level of Mn²⁺. Photoluminescence studies of phosphor blend excited using near-UV to blue light confirms the emitted radiation varies from cool to warm white light in the range 430-600 nm, applicable to LED lightings. The CIE chromaticity coordinate values measured using SrS:Ce/ZnS:Mn phosphor blend-coated 430 nm LED pumped phosphors in the ratio 75:25, 50:50 and 25:75 are found to be (0.235, 0.125), (0.280, 0.190) and (0.285, 0.250), respectively.