Increasing Excitation Power Research Articles

Lasing from Glassy Ge Quantum Dots in Crystalline Si.

Semiconductor light-emitters compatible with standard Si integration technology (SIT) are of particular interest for overcoming limitations in the operating speed of microelectronic devices. Light sources based on group IV elements would be SIT-compatible, but suffer from the poor optoelectronic properties of bulk Si and Ge. Here we demonstrate that epitaxially grown Ge quantum dots (QDs) in a defect-free Si matrix show extraordinary optical properties if partially amorphized by Ge-ion bombardment (GIB). In contrast to conventional SiGe nanostructures, these QDs exhibit dramatically shortened carrier lifetimes and negligible thermal quenching of the photoluminescence (PL) up to room temperature. Microdisk resonators with embedded GIB-QDs exhibit threshold behavior as well as a superlinear increase of the integrated PL intensity with concomitant line width narrowing as the pump power increases. These findings demonstrate light amplification by stimulated emission in a fully SIT-compatible group IV nanosystem.

Influence of excitation power on temperature-dependent photoluminescence of phase-separated InGaN quantum wells

Temperature-dependent photoluminescence (PL) of phase-separated InGaN quantum wells is investigated over a broader excitation power range. With increasing excitation power from 0.5 μW to 50 mW, the In-rich quasi-quantum dot (QD)-related PL peak disappears at about 3 mW, while temperature behavior of the InGaN matrix-related PL peak energy (linewidth) gradually evolves from a strong “S-shaped” (“W-shaped”) temperature dependence into a weak “S-shaped” (an approximately “V-shaped”), until becoming an inverted “V-shaped” (a monotonically increasing) temperature dependence. This indicates that, with increasing excitation power, the carrier localization effect is gradually reduced and the QD-related transition is submerged by the significantly enhanced InGaN matrix-related transition, while the carrier thermalization effect gradually increases to become predominant at high excitation powers.

Anomalous photoconductive behavior of a single InAs nanowire photodetector

We report on a bare InAs nanowire photodetector which exhibits an anomalous photoconductive behavior. Under low-power illumination, the current is smaller than the dark current, and monotonously decreases as the excitation power increases. When the excitation power is high enough, the current starts to increase normally. The phenomenon is attributed to different electron mobilities in the “core” and “shell” of a relatively thick nanowire originating from the surface effect, which result in a quickly dropped “core current” and slowly increased “shell current” under illumination.

Below‐gap emission bands in undoped GaN and its excitation density dependence

AbstractThe below‐gap emission bands of undoped GaN grown by MOCVD on sapphire substrate have been investigated by photoluminescence technique. The intensity of yellow luminescence decreases due to the saturation of deep states with increasing excitation power that reflects the contribution of deep states at actual device operating condition. The excitation induced blue‐shift of emission energy rationalizes the DAP characteristics. The integrated photoluminescence intensity of each band is analysed experimentally and theoretically by a power law model, and the reasonable agreement between experiment and model calculation justify our model consideration. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Controlled Deposition of ZnS Shell on the Photostability of CdTe Quantum Dots as Studied by Conventional Fluorescence and FCS Techniques.

The effect of one and two monolayers of ZnS shells on the photostability of CdTe quantum dots (QDs) in aqueous and nonaqueous media has been studied by monitoring the fluorescence behavior of the QDs under ensemble and single-molecule conditions. ZnS capping of the CdTe QDs leads to significant enhancement of the fluorescence brightness of these QDs. Considerable enhancement of the photostability of the shell-protected QDs, including the suppression of photoactivation, is also observed. Fluorescence correlation spectroscopy measurements reveal an increase in the number of particles undergoing reversible fluorescent on-off transitions in the volume under observation with increasing excitation power; this effect is found to be more pronounced in the case of core-only QDs than for core-shell QDs.

Influence of the InGaN/GaN quasi-superlattice underlying layer on photoluminescence in InGaN/GaN multiple quantum wells

Photoluminescence (PL) properties of two different InGaN/GaN multiple quantum well (MQW) structures, without and with an InGaN/GaN quasi-superlattice (QSL) underlying buffer layer, were investigated. The results show that inserting a QSL between the n-GaN and MQWs can release the strain in the MQW region, since the sample with a QSL shows a smaller excitation power-dependent blue-shift of its peak energy than that without. Meanwhile, inserting a QSL enhances the localization effect of the carriers inferred from an unusual red-shift of the peak energy with increasing excitation power in low excitation range, and from a more obvious “S-shaped” temperature-dependent behavior of the peak energy characteristic: the strain release facilitates the slight composition fluctuation or phase separation of the InGaN well layers. The reduction of the quantum-confined Stark effect and enhancement of the localization effect of the MQWs induced by the strain release, greatly enhance the radiative recombination rate of the MQWs.

Shielding Upconversion by Surface Coating: A Study of the Emission Enhancement Factor.

Surface coating is a commonly used strategy to enhance upconversion emissions by shielding the luminescent core from surface quenching. In this work, we provide insights into the effect of surface coating on upconversion by investigating NaYF4 :Yb/Er nanoparticles and the corresponding NaYF4 :Yb/Er@NaYF4 core-shell nanoparticles, as a function of dopant concentration of Yb(3+) and excitation power. We observe declining emission enhancement factors with decreasing Yb(3+) concentration and increasing excitation power. Our mechanistic investigations suggest that the phenomenon originates from stepwise excitation in the upconversion process, as well as energy hopping among the Yb(3+) dopants. This increased understanding of the effect of surface coating on upconversion should be important towards the rational design of lanthanide-doped core-shell nanoparticles for various applications.

Study of GaAsSb/GaAs type-II quantum well with top InAs quantum dot layer using complementary spectroscopy techniques

The optical characterization of GaAsSb/GaAs type-II quantum wells (QWs) with top InAs quantum dot (QD) layer composite structures was carried out using the complementary surface photovoltage (SPV) and photoluminescence (PL) spectroscopies. The obtained SPV and PL spectra revealed that the features originated from InAs QDs, modulated potential wells in GaAsSb QWs, the wetting layer (WL) as well as the GaAs cap layer/barrier at room temperature. The optical transition from the modulated potential wells in GaAsSb QWs in the composite structures showed a redshift when the spacer layer was narrowed from 10 to 5 nm. This is attributed to the lower modulated potential minimum in GaAsSb QWs caused by the strain exerted by InAs QDs in the composite structures with the narrower spacer layer. The power-dependent PL measurement showed that the luminescences from the GaAsSb/GaAs heterostructure blue-shifted with increasing excitation power owing to the type-II band alignment. These results demonstrated that SPV and PL are useful techniques for the nondestructive optical characterization of GaAsSb QWs with top InAs QD composite structures.

Reduced Stark shift in three-dimensionally confined GaN/AlGaN asymmetric multi-quantum disks

The optical transitions of the three-dimensionally confined GaN/AlGaN asymmetric multi quantum disks were characterized by micro photoluminescence and time-resolved photoluminescence. Several fine emission lines, originating from the wide and narrow quantum disks, were observed around 3.7 eV from a single nanocolumn dispersed on a patterned SiO2 substrate. The photoluminescence from the wide quantum disk shifts a little with increasing excitation power, while that from the narrow quantum disk does not shift. This effect can be explained by carrier tunneling for the 3-dimensionally confined quantum disks. Kelvin probe force microscopy results confirm that the GaN/AlGaN multiquantum disks are surrounded by a GaN shell, which has a higher potential than core GaN.

Power-Dependent Luminescence of CdSe/ZnS Nanocrystal Assembled Layer-by-Layer on a Silver Nanorod Array**Supported by the National Natural Science Foundation of China under Grant Nos 11174229, 11374236 and 11204221, and the National Basic Research Program of China under Grant No 2011CB922201.

We investigate the power-dependent luminescence of CdSe/ZnS semiconductor quantum dots closely packed layer-by-layer in the proximity of a silver nanorod array cavity. It is found that the emission peak shifts significantly to the longer wavelengths as the excitation power increases, especially when the longitudinal surface plasmon resonance of the Ag nanorod array cavity is adjusted to be close to the emission wavelength. The equivalent gain varies with the coating layer of CdSe/ZnS semiconductor quantum dots and the excitation power is also studied to explain this interesting spectrum-shifting effect. These findings could find applications in the dynamic information processing of active plasmonic and photonic nanodevices.

Emission mechanisms in Al-rich AlGaN/AlN quantum wells assessed by excitation power dependent photoluminescence spectroscopy

The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.

Parametric relaxation in whispering gallery mode exciton-polariton condensates

Polariton condensation of one-dimensional multimode whispering gallery mode exciton polaritons is investigated in hexagonal ZnO microwires at cryogenic temperatures. At threshold, stimulated scattering is fed by the resonant emission from highly populated defect-bound excitons. With further increasing excitation power, condensates relax within the multimode whispering gallery mode polariton ladder, leading to their effective evaporative cooling. The relaxation is a parametric polariton-polariton scattering process evidenced by its intensity, energy, and momentum evolution. The experimental observations are in good agreement with numerical simulations based on a model for polariton-polariton scattering extended to the multimodal whispering gallery mode system.

Random lasing in Eu³⁺ doped borate glass-ceramic embedded with Ag nanoparticles under direct three-photon excitation.

We report the observation of random lasing from Eu(3+) doped borate glass ceramic films embedded with Ag nanoparticles through three-photon absorption at room temperature. Under 1179 nm ultrashort femtosecond pulse excitation, discrete sharp peaks with linewidth ∼0.4 nm emerge randomly from a broad emission band with peak wavelength at ∼612 nm. In addition, the number of sharp peaks increases with the increase of excitation power. We also show that the emission spectrum varies with different observation angles and the corresponding lasing threshold is dependent on the excitation area. Hence, we verify unambiguously that the Eu(3+) doped borate glass ceramic film supports random lasing action via three-photon absorption excitation. In addition, Ag nanoparticles, which act as light scatterers, allow the formation of random microcavities inside the bulk film.

Study on photoluminescence properties of 1.05 eV InGaAsP layers grown by molecular beam epitaxy

The photoluminescence properties of InGaAsP films with a bandgap energy of 1.05 eV for quadruple-junction solar cells grown by molecular beam epitaxy (MBE) are investigated. We make the excitation intensity and temperature dependence of continuous-wave photoluminescence (cw-PL) measurements. The PL peak position is 1.1 eV at 10 K, and almost independent of the excitation power, but the integrated intensity of the PL emission peaks is roughly proportional to the excitation power. The shift of peak position with temperature follows the band gap shrinking predicted by the well-known Varshni's empirical formula. These results indicate that the intrinsic transition dominates the light emission of the InGaAsP material. In addition, we also make the time-resolved photoluminescence (TRPL) measurements to determine the carrier luminescence relaxation time in InGaAsP. PL spectra suggest that the relaxation time is 10.4 ns at room temperature and increases with increasing excitation power, which demonstrates the high quality of the InGaAsP material. However, the relaxation time shows an S-shape variation with increasing temperature: it increases at temperatures lower than 50 K, and then decreases between 50–150 K, and increases again when temperature is over 150 K. According to the effect of temperature and the non-radiative recombination center concentration on the carrier relaxation time, the recombination mechanism of S-shape variation can be explained by the carrier relaxation dynamics.

Color tunability in green, red and infra-red upconversion emission in Tm3+/Yb3+/Ho3+ co-doped CeO2 with potential application for improvement of efficiency in solar cells

The preparation of Tm3+/Yb3+/Ho3+ co-doped CeO2 prepared by the precipitation method using ammonium hydroxide as a precursor is presented. By X-ray diffraction the materials show the phase-type of fluorite structure and the crystallite sizes were calculated by the Scherrer׳s equation. No other phase was observed evincing that the rare earth ions were inserted into the fluorite phase as substitutional or interstitial dopants. The microstrain calculated by the Williamson–Hall method do not show significant changes in their values, indicating that the inclusion of rare earths does not causes structural changes in the CeO2 used as a host matrix. All material showed intense upconversion emission at red and green region under excitation with diode laser at 980nm. The color of emission changes from green to red with increasing excitation power pump. The materials showed suitable photoluminescent properties for applications as a laser source, solar cells, and great emitter at 800nm.

Photoluminescence studies of individual and few GaSb/GaAs quantum rings

We present optical studies of individual and few GaSb quantum rings embedded in a GaAs matrix. Contrary to expectation for type-II confinement, we measure rich spectra containing sharp lines. These lines originate from excitonic recombination and are observed to have resolution-limited full-width at half maximum of 200 μeV. The detail provided by these measurements allows the characteristic type-II blueshift, observed with increasing excitation power, to be studied at the level of individual nanostructures. These findings are in agreement with hole-charging being the origin of the observed blueshift.

Nonlinear photoluminescence in atomically thin layeredWSe2arising from diffusion-assisted exciton-exciton annihilation

We studied multi-exciton dynamics in monolayer WSe2 using nonlinear photoluminescence (PL) spectroscopy and Monte Carlo simulations. We observed strong nonlinear saturation behavior of exciton PL with increasing excitation power density, and long-distance exciton diffusion reaching several micrometers. We demonstrated that the diffusion-assisted exciton-exciton annihilation model accounts for the observed nonlinear PL behavior. The long-distance exciton diffusion and subsequent efficient exciton-exciton annihilation process determined the unusual multi-exciton dynamics in atomically thin layered transition metal dichalcogenides.

Temperature of broadband anti-Stokes white emission in LiYbP4O12: Er nanocrystals

The up-conversion emission of LiYb0.99Er0.01P4O12 nanocrystals was investigated in vacuum upon the focused beam of infrared laser excitation. The intense white broad band emission was observed above the threshold power of 800 mW. An increase of excitation power results in enhancement of temperature of emitting nanocrystals. The temperature of nanocrystals was determined following the thermally equilibrated f-f emission transitions of Er3+ ions. The highest measured temperature of white emission was determined to be about 900 K. It is concluded that the black body radiation is not responsible for the white broad band emission.

GaP-ZnS pseudobinary alloy nanowires.

Multicomponent nanowires (NWs) are of great interest for integrated nanoscale optoelectronic devices owing to their widely tunable band gaps. In this study, we synthesize a series of (GaP)(1-x)(ZnS)(x) (0 ≤ x ≤ 1) pseudobinary alloy NWs using the vapor transport method. Compositional tuning results in the phase evolution from the zinc blende (ZB) (x < 0.4) to the wurtzite (WZ) phase (x > 0.7). A coexistence of ZB and WZ phases (x = 0.4-0.7) is also observed. In the intermediate phase coexistence range, a core-shell structure is produced with a composition of x = 0.4 and 0.7 for the core and shell, respectively. The band gap (2.4-3.7 eV) increases nonlinearly with increasing x, showing a significant bowing phenomenon. The phase evolution leads to enhanced photoluminescence emission. Strikingly, the photoluminescence spectrum shows a blue-shift (70 meV for x = 0.9) with increasing excitation power, and a wavelength-dependent decay time. Based on the photoluminescence data, we propose a type-II pseudobinary heterojunction band structure for the single-crystalline WZ phase ZnS-rich NWs. The slight incorporation of GaP into the ZnS induces a higher photocurrent and excellent photocurrent stability, which opens up a new strategy for enhancing the performance of photodetectors.

Carrier redistribution between different potential sites in semipolar (202¯1) InGaN quantum wells studied by near-field photoluminescence

Scanning near-field photoluminescence (PL) spectroscopy at different excitation powers was applied to study nanoscale properties of carrier localization and recombination in semipolar (202¯1) InGaN quantum wells (QWs) emitting in violet, blue, and green-yellow spectral regions. With increased excitation power, an untypical PL peak energy shift to lower energies was observed. The shift was attributed to carrier density dependent carrier redistribution between nm-scale sites of different potentials. Near-field PL scans showed that in (202¯1) QWs the in-plane carrier diffusion is modest, and the recombination properties are uniform, which is advantageous for photonic applications.