Function Of Excitation Power Research Articles

Exciton behavior in GaAs/AlGaAs coupled double quantum wells with interface disorder

In this work, we present a detailed study on the optical properties of two GaAs/Al 0.35Ga 0.65As coupled double quantum wells (CDQWs) with inter-well barriers of different thicknesses, by using photoluminescence (PL) spectroscopy. The two CDQWs were grown in a single sample, assuring very similar experimental conditions for measurements of both. The PL spectrum of each CDQW exhibits two recombination channels which can be accurately identified as the excitonic e 1-hh 1 transitions originated from CDQWs of different effective dimensions. The PL spectra characteristics and the behavior of the emissions as a function of temperature and excitation power are interpreted in the scenario of the bimodal interface roughness model, taking into account the exciton migration between the two regions considered in this model and the difference in the potential fluctuation levels between those two regions. The details of the PL spectra behavior as a function of excitation power are explained in terms of the competition between the band gap renormalization (BGR) and the potential fluctuation effects. The results obtained for the two CDQWs, which have different degrees of potential fluctuation, are also compared and discussed.

Formation of Lateral Low Density In(Ga)As Quantum Dot Pairs in GaAs Nanoholes

In this work we present a growth procedure to form lateral In(Ga)As quantum dot pairs by using a low density, 2 × 10 8 cm -2 , GaAs nanohole template previously formed in situ by droplet homoepitaxy. In particular, by changing the arsenic pressure at which InAs is grown on the template, we demonstrate the possibility to select the formation of single quantum dots (QD) or QD pairs inside each of the nanoholes. In the case of QD pairs, the ensemble photoluminescence (PL) as a function of excitation power and temperature reveals spectral signatures typical of laterally coupled nanostructures.

Luminescence properties and upconversion of Er3+ doped and Er3+/Yb3+ co-doped LaF3 particles

Er3+ doped and Er3+/Yb3+ co-doped superfine LaF3 powders are prepared using co-precipitation method. The sample shows spherical particles with size of about 250 nm. The quantum efficiencies of 4S3/2 and 4F9/2 levels in the prepared Er3+ doped LaF3 powders are calculated to be 67.0% and 71.9%, respectively. With the enhancement of the Yb3+ doping concentration, The upconvertion intensity increases as a function of excitation power more slowly due to the enhancement of the sensitization of Yb3+ to Er3+. The Er3+-Er3+ interaction is also reinforced as the Yb3+ concentration increases. This leads to the increase of the ratio of the green emission to the red one with the enhancement of Yb3+ concentration.

Optical Properties of Cu2ZnSnS4 Thin Films Prepared by Sol–Gel and Sulfurization Method

Cu2ZnSnS4 (CZTS) thin films were prepared by a sol–gel sulfurization method. One sample (CZTS1) was stoichiometric (Cu : Zn : Sn : S = 2.0 : 1.0 : 1.0 : 4.0) and another sample (CZTS2) was Cu poor and Zn rich (Cu : Zn : Sn : S = 1.87 : 1.15 : 1.00 : 4.00). Photoluminescence from the CZTS thin films was studied as a function of excitation power and temperature. The CZTS1 and CZTS2 thin films showed a broad luminescence between 1.1 and 1.4 eV and between 1.1 and 1.45 eV respectively. The peak photon energy of the photoluminescence was shifted to higher energy side when the excitation power was increased. The origin of the photoluminescence was attributed to donor-acceptor pair recombination with activation energy of 39 and 59 meV for CZTS1 and CZTS2 respectively. Transmittance spectra for the thin films were studied as a function of temperature. The band gap energy shift was smaller than by 10 meV with increasing temperature from 22 to 300 K.

Size-Dependent Upconversion Luminescence in Er3+/Yb3+-Codoped Nanocrystalline Yttria: Saturation and Thermal Effects

In this paper, the upconversion luminescent properties of Y2O3:Er3+(1%)/Yb3+(4%) nanoparticles with different sizes (13−55 nm) and its corresponding bulk material as a function of excitation power were studied under 978-nm excitation. Red (4F9/2 → 4I15/2), green (2H11/2, 4S3/2 → 4I15/2), and blue (2H9/2 → 4I15/2) transitions were observed. The results indicated that the relative intensity of the blue as well as the red to the green increased gradually with decreasing particle size. As a function of excitation power, the slope in the ln−ln plot for the red emission changed between 2.0 and 1.0 and gradually decreased with increasing particle size, which was attributed to competition between linear decay and upconversion processes for the depletion of the intermediate excited states. As the particle size decreased to 13 nm, a three-photon populating process occurred for the green emission. As the excitation power varied in different paths, gradually increasing or gradually decreasing, a hysteresis loop appeared in the power dependence of emission intensity, which was mainly caused by a local thermal effect induced by laser irradiation. The intensity ratio of 2H11/2 → 4I15/2 to 4S3/2 → 4I15/2 (RHS) varied complicatedly with excitation power, which was theoretically explained considering the thermal distribution and relaxation processes. Two novel cross-relaxation paths were proposed on the basis of the variation of RHS under excitation at different wavelengths, 488 and 978 nm.

Determination of conduction band offsets in type-IIIn0.27Ga0.73Sb∕InxAl1−xAsySb1−yheterostructures grown by molecular beam epitaxy

Low-temperature photoluminescence (PL) has been performed on a set of specially designed ${\mathrm{In}}_{0.27}{\mathrm{Ga}}_{0.73}\mathrm{Sb}∕{\mathrm{In}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{As}}_{y}{\mathrm{Sb}}_{1\ensuremath{-}y}$ multiple quantum well (MQW) heterostructures grown by molecular beam epitaxy in order to provide a measure of the conduction band offsets in this material system. These alloys are of interest for the development of high-speed heterojunction bipolar transistors (HBTs) that show promise for operation at lower power dissipation than in $\mathrm{GaAs}$- and $\mathrm{InP}$-based HBTs. Excitation power studies revealed evidence for strong electron-hole recombination at $0.56\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ within the $\mathrm{InGaSb}$ layers of the type-I MQW structure with $(x=0.52,y=0.25)$, while several weaker indirect transitions involving electrons in the ${\mathrm{In}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}{\mathrm{As}}_{y}{\mathrm{Sb}}_{1\ensuremath{-}y}$ and holes in the $\mathrm{InGaSb}$ layers were observed between 0.38 and $0.53\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ from the nominally type-II MQW samples with $(x,y)=(0.67,0.39)$ and (0.69,0.41). Neglecting small corrections $(\ensuremath{\sim}15\phantom{\rule{0.3em}{0ex}}\mathrm{meV})$ due to the electron and hole confinement energies, we estimate conduction band offsets of $\ensuremath{\sim}120--150\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ in these type-II structures. The general trends of the PL features as a function of excitation power have been reproduced from modeling of the quantum well electron and hole subband energies, including effects due to band bending at the heterointerfaces.

Fabrication and characterization of microdisk resonators with In(Ga)As/GaAs quantum dots

AbstractWe report on the fabrication and characterization of GaAs‐based microdisk resonators containing lowsurface‐density self‐assembled In(Ga)As quantum dots. Microdisks of different sizes are obtained by optical and electron‐beam lithography followed by a two‐step wet chemical etching. The use of wet etching results in smooth disk edges. The optical properties of the quantum dots and microdisks are studied by microphotoluminescence spectroscopy as a function of excitation power. Disks with quality factors larger than 12000 (resolution limited spectral width) are easily obtained with the described approach even with optical lithography. Laser heating of microdisks is also discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time resolved measurements on low‐density single quantum dots at 1300 nm

AbstractWe present time integrated and time resolved measurements on single In/As quantum dots (QD) emitting at 1300 nm, at 10 K, embedded in a planar microcavity. We clearly identify a standard spectroscopic signature from single QDs and to compare the exciton line width and biexciton (BX) binding energy for several QDs. We present this data for QDs spatially selected by etched mesas and metallic apertures. Time resolved photoluminescence (PL) from single electron‐hole recombination in the ground state of the QD is investigated as a function of excitation power and temperature. The measurements reveal the presence of a background emission that delays the PL of excitons in the ground state. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical losses in YVO4: RE (RE = Nd3+, Er3+, Tm3+) laser crystals

The relevance of processes contributing to depletion of pump and upper laser levels has been assessed based on experimental data obtained during measurement of excited state absorption, steady state emission and dynamics of excited states as a function of excitation power and activator concentration. It has been concluded that the excited state absorption in YVO4: Nd and YVO4: Er is not significant except for that from the 4 I 11/2 level of Er3+. In these systems, the interionic processes are dominant. In particular, the reported decrease of the YVO4: Er laser slope efficiency when the Er3+ concentration increased from 0.5 to 1 at % is due mainly to the up-conversion by energy transfer from the pump level and upper laser level. Excited state absorption cannot contribute to depletion of excited states involved in the 3 F 4-3 H 6 laser operation near 1800 nm in the YVO4: Tm crystal. However, the heavy doping required to enhance the cross-relaxation process which feeds the upper laser level brings about the migration-accelerated energy transfer to energy sinks.

Upconversion luminescence, intensity saturation effect, and thermal effect in Gd2O3:Er3,Yb3+ nanowires

In this paper, the upconversion luminescent properties of Gd2O3:Er3+,Yb3+ nanowires as a function of Yb concentration and excitation power were studied under 978-nm excitation. The results indicated that the relative intensity of the red emission (4F(9/2)-4I(15/2)) increased with increasing the Yb3+ concentration, while that of the green emission (4S(3/2)/2H(11/2)-4I(15/2)) decreased. As a function of excitation power in ln-ln plot, the green emission of 4S(3/2)-4I(15/2) yielded a slope of approximately 2, while the red emission of 4F(9/2)-4I(15/2) yielded a slope of approximately 1. Moreover, the slope decreased with increasing the Yb3+ concentration. This was well explained by the expanded theory of competition between linear decay and upconversion processes for the depletion of the intermediate excited states. As the excitation power density was high enough, the emission intensity of upconversion decreased due to thermal quenching. The thermal effect caused by the exposure of the 978-nm laser was studied according to the intensity ratio of 2H(11/2)-4I(15/2) to 4S(3/2)-4I(15/2). The practical sample temperature at the exposed spot as a function of excitation power and Yb3+ concentration was deduced. The result indicated that at the irradiated spot (0.5 x 0.5 mm2) the practical temperature considerably increased.

Anomalous optical transitions in AlInGaN/GaN heterostructures grown by metalorganic chemical vapor deposition

Using temperature-dependent photoluminescence (PL) measurements, we report a comprehensive study on optical transitions in Al y In x Ga 1− x− y N epilayer with target composition, x=0.01 and y=0.07 and varying epilayer thickness of 40, 65 and 100 nm. In these quaternary alloys, we have observed an anomalous PL temperature dependence such as an S-shape band-edge PL peak shift and a W-shape spectral broadening with an increase in temperature. With an increase in excitation power density, the emission peak from the AlInGaN epilayers shows a blue shift at 100 K and a substantial red shift at room temperature. This is attributed to the localization of excitons at the band-tail states at low temperature. Compared to 40 and 65 nm thick epilayers, the initial blue shift observed with low excitation power from 100 nm thick AlInGaN epilayer at room temperature is caused by the existence of deeper localized states due to confinement effects arising from higher In and Al incorporation. The subsequent red shift of the PL peak can be attributed by free motion of delocalized carriers that leads to bandgap renormalization by screening. Due to competing effects of exciton and free carrier recombination processes, such behavior of optical transitions leads to two different values of exponent ‘ k’ in the fitting of PL emission intensity as a function of excitation power.

Excitation Dependence of Steady-State Photoluminescence in CdSe Nanocrystal Films

The excitonic and deep-level photoluminescence (PL) in CdSe nanocrystal (NC) films (wurtzite type) was studied under continuous-wave excitation as a function of excitation power, temperature, and time of photoaging. It was shown that the intensity-power dependencies are identical for excitonic and deep-level emissions in a wide temperature range. At low temperatures (80-100 K), both emissions were saturated at the laser power used, which generates more than one exciton per nanocrystal. A transition point from the linear to the saturated region was dependent on the temperature, size, and quality of the NCs. A clear inverse dependency between the intensities of excitonic and deep-level emissions was revealed at 80 K over the entire sample area. At room-temperature, the quantum yield dropped significantly and a higher laser power was needed to reach PL saturation. An increase in temperature led to worsening of the reverse dependence between excitonic and deep-level emissions, and at room-temperature, they became uncorrelated. These results can be explained by Auger recombination and also by an increase of nonradiative recombination in the surface states with increasing temperature.

Emission stimulation in a directional band gap of a CdTe-loaded opal photonic crystal.

An anisotropic photonic crystal light source has been realized by impregnation of a thin film latex opal with brightly luminescent CdTe nanocrystals. Its photoluminescence along a given direction has been studied as a function of excitation power. An increase of the photoluminescence saturation threshold in the directional photonic band gap has been observed. This result is interpreted as a stimulation of emission coupled to specific eigenmodes of a directional band gap. A theoretical model exploiting the low group velocity eigenmodes has been elaborated to explain the resonance character of the emission with band gap frequencies.

Structural Defect-Related Photoluminescence in GaN

ABSTRACTBroad, low temperature photoluminescence (PL) peaks near 3.4–3.42 eV in GaN have previously been associated with basal-plane stacking faults. Recently, we observed unusually sharp and highly structured PL peaks in this region in high quality bulk GaN grown from a Na/Ga flux, some of which display characteristic shifts and narrowing as a function of excitation power. Here, we study these peaks as a function of excitation intensity and crystal polarity, and compare them to those observed in GaN grown on off-axis SiC or sapphire (0001) substrates by metalorganic chemical vapor deposition (MOCVD). In the off-axis material on either substrate, similar peaks are observed to those in the bulk samples. In addition, a low energy peak near 3.21 eV is observed, which does not occur in the bulk material.

Photon avalanche upconversion inTm3+-doped fluoroindogallate glasses

In this work, a study of photon avalanche at 476 nm(1G4→3H6) in Tm-doped fluoroindogallate glasses is reported. TransientTm3+ emissions at 476, 450 nm (1D2→3F4) and transmission at the pumping wavelength (645 nm)were monitored as a function of excitation power and Tm3+concentration. We propose a theoretical treatment based on a set offive non-linear coupled rate equations, which include sequentialtwo-photon absorption (TPA), two cross-relaxations among Tm3+ions, and energy migration at the 3F4 metastable state. Itis demonstrated that energy migration in highly doped samples playsa fundamental role in the photon avalanche process, and may evensuppress it. Additionally, sequential TPA must be considered inorder to explain the transient behaviour of the 450 nm emission inthe photon avalanche regime.

Photoluminescence of self-assembled InSb quantum dots grown on GaSb as a function of excitation power, temperature, and magnetic field

We report measurements of photoluminescence (PL) from self-assembled InSb quantum dots (QD's) grown by metal-organic vapor-phase epitaxy in a matrix of GaSb as a function of excitation power, temperature, and magnetic field. PL is observed in the region 1.7--1.8 \ensuremath{\mu}m from InSb quantum dots. For low excitation power the PL is dominated by the lowest quantum dot transition energy. When the excitation power is increased the quantum dot transition increases in energy by \ensuremath{\sim}11 meV, and further transitions are observed from the wetting layer, bulk acceptor, and free excitons. Magneto-PL is used to calculate the in-plane dot confinement energies by fitting the data to the ground state of a Fock-Darwin set of energy levels. The in-plane confinement energy deduced increases from \ensuremath{\sim}6 to \ensuremath{\sim}18 meV as the excitation power is increased. This is similar to the increase in the quantum dot transition energy, and suggests that this is due to a progressive population of a distribution of strongly communicating dots with decreasing lateral sizes. Further support for this picture comes from the temperature dependence of the quantum dot transition energy, which is also found to increase by a relatively similar amount as the temperature is raised from 11 to 50 K, following a correction for the temperature dependence of the bulk energy band gaps.

Synthesis and optical properties of Nd3+-doped Y3Al5O12 nanoceramics

The Nd-doped yttrium aluminum garnet (Nd:YAG) nanocrystallites were prepared by the sol–gel method. The preparation technology for semi-transparent ceramics is described. Their structure and morphology were studied by XRD. The emission properties of Nd:YAG measured for nanopowders and ceramics are reported. In particular the spectra were measured as a function of excitation power. It was found that with increasing excitation power, there was increased emission from the 4F5/2 state. The effect of grain size on luminescence properties was observed.

Optical Characterization of Self-Organized PbSe/Pb1-xEuxTe Quantum Dot Superlattices

Photoluminescence (PL) experiments have been performed on self-organized PbSe/Pb1—xEuxTe (x = 0.05 to 0.1) quantum dot superlattices grown by molecular beam epitaxy (MBE). At a temperature of 1.7 K, several PL lines were observed in the spectral range from 200 to 230 meV with a relatively narrow width of typically 1 to 2 meV. These lines emerge on top of a weak, broader band. Comparison of the luminescence photon energy with that of bulk PbSe (146 meV) shows the strong influence of carrier confinement. Studies of the temperature and magnetic field dependence demonstrate that the narrow lines are not significantly shifted up to 50 K and 7 T, respectively, whereas the broader band exhibits a shift comparable to that of bulk PbSe material. Studies of the PL intensity as a function of excitation power and the preferred emission of radiation from the sample edge indicate stimulated emission.

Broad-band photoluminescence from ZnGeP2

The near-infrared photoluminescence from single crystals of bulk ZnGeP2 was studied as a function of excitation power, excitation wavelength, sample temperature, and polarization. The nature of this broad-band luminescence from large single crystals grown for nonlinear optical applications is established. Two distinct bands with quite different polarization, power, and temperature behaviors were resolved. At 5 K, these broad bands have peaks in intensity near 1.58 and 1.36 eV. The 1.58 eV band is partially polarized perpendicular to the crystal’s c axis, has a relatively small thermal quenching activation energy (45 meV), and excitation spectra show a resonance in intensity associated with a shallow level approximately 90 meV below the minimum conduction band. The 1.36 eV band is partially polarized parallel to c, has a much larger quenching activation energy (220 meV), and its excitation spectrum includes two weak resonances corresponding to the A′ and B′ n=1 excitons. The high-energy band was enhanced in crystals containing relatively larger concentrations of phosphorus vacancies (measured by electron paramagnetic resonance).

Multi-exciton spectroscopy on a self-assembled InGaAs[formula omitted]GaAs quantum dot

We report about spatially resolved magneto-optical experiments on a self-assembled InGaAs quantum dot. Using electron beam lithograpy for patterning a metal shadow mask we can isolate a single dot. This allows us to study the optical response of a single dot as a function of excitation power and magnetic field. We investigate the influence of many body interaction in the emission spectra for different exciton occupation numbers of the dot. The diamagnetic/orbital shift as well as Zeeman splitting in a magnetic field can be fully resolved and are used to identify the observed emission lines. Further we report on absorption properties of the quantum dot as a function of magnetic field. We analyse in detail the phonon-assisted absorption process connected with the GaAs LO-phonon 36 meV above the single-exciton ground state.