Increasing Excitation Power Research Articles

Time-resolved ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layer

The ultraviolet photoluminescence of ZnO/ZnGa2O4 composite layer grown by the thermal oxidation of ZnS with gallium was investigated by the time-resolved photoluminescence as a function of measuring temperature and excitation power. With increase of excitation power, the D0X emission is easily saturated than the DAP emission from ZnO/ZnGa2O4 composite layer, and which is dramatically enhanced as compared with that from pure ZnO layer grown without gallium. The radiative recombination process with ultra-long lifetime controlled the carrier recombination of ZnO/ZnGa2O4 composite layer.

Structural and optical properties of Er3+/Yb3+ doped barium titanate phosphor prepared by co-precipitation method

In the present work we have synthesized the Er3+/Yb3+ codoped barium titanate phosphor via co-precipitation method and studied its upconversion emission properties. The prepared BaTiO3 powder was found in cubic phase as a major component and having good crystallinity revealed by the XRD analysis. Optical band gap of the cubic barium titanate was calculated using the diffuse reflectance absorption spectrum. Good green upconversion emission is observed from the samples when excited by 980nm diode laser. The variation in upconversion emission intensity is studied with the increase in excitation power as well as temperature of the sample. It is found that the emission bands centred at 524 and 548nm are thermally coupled and can act as a temperature sensor in the 300–480K temperature range.

Blinking Statistics and Excitation-Dependent Luminescence Yield in Si and CdSe Nanocrystals

ON–OFF intermittency or blinking is a phenomenon observed in single quantum emitters, which reduces their overall light emission. Even though it seems to be a fundamental property of quantum dots (QDs), substantial differences can be found in the blinking statistics of different nanocrystals. This work compares the blinking of numerous single, oxide-capped Si nanocrystals with that of CdSe/ZnS core–shell nanocrystals, measured under the same conditions in the same experimental system and over a broad range of excitation power densities. We find that ON- and OFF-times can be described by exponential statistics in Si QDs, as opposed to power-law statistics for the CdSe nanocrystals. The type of blinking (power-law or monoexponential) does not depend on excitation but seems to be an intrinsic property of the material system. Upon increasing excitation power, the duty cycle of Si quantum dots remains constant, whereas it decreases for CdSe nanocrystals, which is readily explained by blinking statistics. Both ON–OFF and OFF–ON transitions can be regarded as light-induced in Si/SiO2 QDs, while the OFF–ON transition in CdSe/ZnS nanocrystals is not stimulated by photons. The differences in blinking behavior in these systems will be discussed.

Homogeneous in Distribution of InGaN/GaN Quantum Wells in High Performance GaN-Based Light-Emitting Devices

We have investigated the effect of well growth rate on threshold current and slope efficiency of GaN-based laser diodes (LDs). The lasing performance was significantly dependent on optical and crystal quality of In0.08Ga0.92N/ GaN QWs with different well growth rates. The InGaN QWs grown with lower growth rate represented better interface quality and had low surface defects in the InGaN/GaN QW region. In addition, InGaN QWs grown with lower growth rate exhibited the higher optical properties such as the higher PL intensity and the smaller blueshift with increasing excitation power density. The present results suggest that optical and crystal qualities of InGaN/InGaN MQW are significantly improved by lowering well growth rate, resulting in the increase of slope efficiency and the decrease of threshold current density in GaN-based LDs.

Tunable multicolor upconversion luminescence and paramagnetic property of the lanthanide doped fluorescent/magnetic bi-function NaYbF4 microtubes

In this paper, the lanthanide (Ln) doped fluorescent/magnetic NaYbF4 microtubes with hexagonal phase were synthesized via a hydrothermal method using oleic acid as a capping ligand and surface modifier. The as-prepared samples were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), upconversion luminescence (UCL) spectra, and vibrating sample magnetometer (VSM). The TEM and FE-SEM observations revealed that the NaYbF4 microtubes present high quality hexagonal prism shape. Under the excitation of a 980nm laser diode (LD) with a power density of 0.5Wcm−2, the UCL colors can be tuned from purplish blue to blue, greenish white, and further to yellowish green by adjusting the Tm3+ and Ho3+ content in NaYbF4 system. Moreover, the brighter greenish white light can be readily achieved via increasing excitation power. The UCL mechanisms for the white emission were proposed based on the spectral and pumping power dependence analyses. Moreover, these as-prepared NaYbF4 microtubes exhibit excellent paramagnetic property at room temperature. The magnetic mass susceptibility and magnetization are measured to 4.17×10−5emu/gOe and ∼0.84emug−1 at 20kOe, respectively. Therefore, these NaYbF4 microtubes with hexagonal prism shape, tunable multicolor UCL and excellent paramagnetic property imply that NaYbF4 is an excellent host material and may find applications in flat-panel displays, lasers photonics and dual-modal bioprobes.

Effect of mechanical compression on Cu(In,Ga)Se2 films: micro-structural and photoluminescence analysis

Cu(In,Ga)Se2 (CIGS) thin films were deposited by a two-step process on Mo-coated soda-lime glass substrates. The CuInGa (CIG) precursors were prepared in an in-line evaporation system at room temperature, and then selenised at 500 °C. The two-step processed CIGS films were mechanically compressed at 25 MPa to improve their optoelectronic properties, which were verified by photoluminescence (PL). The surface and structural properties were compared before and after compression. The mechanical compression has brought changes in the surface morphology and porosity without changing the structural properties of the material. The PL technique has been used to reveal changes in the electronic properties of the films. PL spectra at different excitation laser powers and temperatures were measured for as-grown as well as compressed samples. The PL spectra of the as-grown films revealed three broad and intense bands shifting at a significant rate towards higher energies (j-shift) with the increase in excitation power suggesting that the material is highly doped and compensated. At increasing temperature, the bands shift towards lower energies, which is a characteristic of the band tails generated by spatial potential fluctuation. The compression increases the intensity of energy bands by an order of magnitude and reduces the j-shift, demonstrating an improvement of the electronic properties.

A Fluorescence Correlation Spectroscopy, Steady-State, and Time-Resolved Fluorescence Study of the Modulation of Photophysical Properties of Mercaptopropionic Acid Capped CdTe Quantum Dots upon Exposure to Light

Light-induced modulation of the fluorescence behavior of mercaptopropionic acid (MPA) capped CdTe quantum dots (QDs) in aqueous solution is studied by a combination of fluorescence correlation spectroscopy (FCS) and steady-state and time-resolved fluorescence techniques. These investigations reveal a dramatic variation in the fluorescence properties of the QDs under exposure to light. In the FCS measurement, a large decrease in amplitude and change in shape of the correlation curves are observed with increasing excitation power. The change in the shape of the correlation curves, particularly at short lag time, e.g., a faster relaxation at high excitation power, is attributed to the increasing contribution of the off state of the QDs. Interestingly, despite this increasing contribution of the off state, which reduces the effective number of emitters in the observation volume and hence should increase the amplitude of the correlation curve, the latter actually decreases at high excitation power. This appare...

Light emission despite doubly-forbidden radiative transitions in AlP/GaP quantum wells: Role of localized states

The GaP/AlP/GaP heterostructure has an indirect gap both in real as well as momentum space, making the first order radiative recombination doubly forbidden. Nevertheless, we have observed relatively efficient emission from these structures. This paper comprehensively studies the origin of this improved light emission through a detailed analysis of the photoluminescence (PL) spectra. Our observations suggest that localized excitons within the acceptor states in GaP close to the heterostructure interface are enough for efficient light emission in these structures, doing away with the need for more complicated structures (superlattices or neighboring confinement structures). This real space localization of holes, close to the interface, apart from increasing the wave function overlap, also relaxes the delta-function momentum selection rule. Independent experimental evidence for this assertion comes from (i) the PL spectrum at high excitation power where transitions from both the localized as well as extended states are independently observed, (ii) the observation that extended states emission has the expected band-bending-induced blue-shift with increase in excitation power, whereas the localized states do not, (iii) observation of phonon replicas for PL from localized states, and (iv) observation of persistent photoconductivity at low temperature. Finally, we propose a simple analytical model that accounts for both the type-II nature as well as the indirect bandgap to explain the improvement of radiative recombination efficiency with increased localization. The experimental observations are reproduced within an order of magnitude. The model is very general and it also provides a framework to study the optical properties of other such (type-II and/or indirect gap) heterostructures.

Effects of exciton localization on internal quantum efficiency of InGaN nanowires

The optical properties of InGaN nanowires with different emission wavelengths of 485, 515, 555, and 580 nm have been studied by means of photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy. The PL peak energy of the nanowires exhibited an anomalous shift to higher energy and then to lower energy with increasing temperature. Analysis of the temperature-dependent variations in the PL peak energy let us evaluate the localization energies of excitons, which increased with increasing indium composition. TRPL measurements also revealed that the PL decay time of the nanowires increased and then became constant with decreasing emission energy, which was typical of localized excitons and enabled us to evaluate the characteristic energies of localized states. The characteristic energy increased with increasing indium composition, indicating that the density of localized states broadened with increasing indium composition. In addition, a correlation was clearly observed between the internal quantum efficiency (IQE) and localization energy of the nanowire: the IQE increased with increasing localization energy. The increase in the IQE was attributed to the increase in the degree of exciton localization as the indium composition of the nanowire increased. Moreover, it was found that with increasing excitation power density, a reduction in the IQE occurred simultaneously with a PL blue shift. This indicated that the reduction in the IQE was associated with saturation of localized states.

Blueshifts of the emission energy in type-II quantum dot and quantum ring nanostructures

We have studied the ensemble photoluminescence (PL) of 11 GaSb/GaAs quantum dot/ring (QD/QR) samples over ≥5 orders of magnitude of laser power. All samples exhibit a blueshift of PL energy, ΔE, with increasing excitation power, as expected for type-II structures. It is often assumed that this blueshift is due to band-bending at the type-II interface. However, for a sample where charge-state sub-peaks are observed within the PL emission, it is unequivocally shown that the blueshift due to capacitive charging is an order of magnitude larger than the band bending contribution. Moreover, the size of the blueshift and its linear dependence on occupancy predicted by a simple capacitive model are faithfully replicated in the data. In contrast, when QD/QR emission intensity, I, is used to infer QD/QR occupancy, n, via the bimolecular recombination approximation (I∝n2), exponents, x, in ΔE∝Ix are consistently lower than expected, and strongly sample dependent. We conclude that the exponent x cannot be used to differentiate between capacitive charging and band bending as the origin of the blueshift in type-II QD/QRs, because the bimolecular recombination is not applicable to type-II QD/QRs.

Enhanced Hot-Carrier Luminescence in Multilayer Reduced Graphene Oxide Nanospheres

We report a method to promote photoluminescence emission in graphene materials by enhancing carrier scattering instead of directly modifying band structure in multilayer reduced graphene oxide (rGO) nanospheres. We intentionally curl graphene layers to form nanospheres by reducing graphene oxide with spherical polymer templates to manipulate the carrier scattering. These nanospheres produce hot-carrier luminescence with more than ten-fold improvement of emission efficiency as compared to planar nanosheets. With increasing excitation power, hot-carrier luminescence from nanospheres exhibits abnormal spectral redshift with dynamic feature associated to the strengthened electron-phonon coupling. These experimental results can be well understood by considering the screened Coulomb interactions. With increasing carrier density, the reduced screening effect promotes carrier scattering which enhances hot-carrier emission from such multilayer rGO nanospheres. This carrier-scattering scenario is further confirmed by pump-probe measurements.

Carrier Density Dependent Localization and Consequences for Efficiency Droop in InGaN/GaN Quantum Well Structures

We report on the observation of a reduction in the depth of the S-shape in the temperature dependence of the photoluminescence peak energy with increasing excitation power density. Over the range of excitation power density where the depth of the S-shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively.

Effect of substrate temperature on optical properties and strain distribution of ZnTe epilayer on (100) GaAs substrates

The photoluminescence properties of the ZnTe epilayers grown on (100) GaAs substrates with various substrate temperatures are investigated. The spectral characteristics of both bound and free excitonic emissions measured at different measurement temperatures and excitation power, show that compared with the too low (390°C) or too high (440°C) substrate temperature, the moderate substrate temperature (around 420°C) is suitable to obtain a high-quality epilayer due to its narrowest spectral width and smallest strain. With increasing excitation power the peak energy of the free excitonic emission decreases linearly, while the spectral width remains an approximate constant. This implies that the decreasing peak energy should be attributed to an increasing strain with increasing the effective excited depth, and the increasing strain only reduces the bandgap, but does not cause much influence on the crystalline quality for each epilayer. Furthermore, the increasing strain also causes the ground state free excitons splitting into the light hole and heavy hole free excitons.

Light-matter coupling in ZnTe-based micropillar cavities containing CdTe quantum dots

We study the coupling of CdTe quantum dots emission with ZnTe-based micropillar cavity modes. Nonresonant cavity mode feeding is reported together with an enhancement of the emission of a quantum dot thanks to resonant coupling with the cavity mode. The coupling is evidenced both in experiments with continuous and pulsed excitation. A theoretical Purcell factor is calculated and an experimental Purcell factor 5.7 is determined confirming the theoretical predictions. Additionally, we discuss the influence of the cascaded emission occurring under increased excitation power on the observed decay time of the excitonic transition.

Optical properties of coupled three-dimensional Ge quantum dot crystals

We report on optical properties of coupled three-dimensional (3D) Ge quantum dot crystals (QDCs). With increasing the vertical periodic number of the QDCs, the photoluminescence (PL) spectral linewidth decreased exponentially, and so did the peak energy blueshift caused by increasing excitation power, which are attributed to the electronic coupling and thus the formation of miniband. In the PL spectra, the relative intensity of the transverse-optical (TO) phonon replica also decreases with increasing the vertical periodic number, which is attributed to the increased Brillouin-zone folding effect in vertical direction and therewith the relaxation of indirect transition nature of exciton recombination. Besides, the optical reflectivity at the interband transition energy was much more reduced for the QDCs than for the in-plane disordered QDs grown with the same parameters, indicating a higher interband absorption of the QDCs due to the miniband formation.

High-power InP quantum dot based semiconductor disk laser exceeding 1.3 W

We demonstrate an optically pumped semiconductor disk laser (OP-SDL) using InP quantum dots (QDs) as active material fabricated by metal-organic vapor-phase epitaxy. The QDs are grown within [(Al0.1Ga0.9)0.52In0.48]0.5P0.5 (abbr. Al0.1GaInP) barriers in order to achieve an emission wavelength around 655 nm. We present optical investigations of the active region showing typical QD behavior like blue shift with increasing excitation power and single emission lines, which show anti-bunching in an intensity auto-correlation measurement. We report maximum output powers of the OP-SDL of 1.39 W at low emission wavelength of ∼654 nm with a slope efficiency of ηdiff=25.4 %.

Temperature Effect of Activation Energy for GaSb Quantum Dots Using Variable Temperature Photoluminescence

The excitation power dependence of radiative transitions in the type-II GaSb/GaAs quantum dots structure has been studied by the photoluminescence (PL) at different temperatures. The QDs' photoluminescence exhibits a strong blue-shift with increasing the excitation power and the peak energy (E(PL)) is proportional to the third root of excitation power. With the increase of excitation power, the nonlinear change for both the PL peak and the intensity was observed, which is attributed to the excited state transition. The thermal process and activation energy (E(a)) of electron-hole pairs at different excitation powers has also been studied for GaSb QDs. As the excitation power increases, the activation energy decreases, while E(PL) +E(a) keeps nearly constant, which is smaller than the band gap of GaAs. The decrease of E(a) is probably caused by the band bending effect.

Formation and Characterization of Multilayer GeSi Nanowires on Miscut Si (001) Substrates

A multilayer of GeSi nanowires separated with Si spacers was readily self-assembled on miscut Si (001) substrates with 8 degrees off toward (110). The nanowires oriented along the miscut direction were very small and compactly arranged on the vicinal surface. Systematic photoluminescence (PL) spectroscopy studies were carried out on the GeSi nanowires. With increasing excitation power, a sublinear power dependent PL intensity and a blue-shift of -7 meV/decade with nearly constant full width of half maximum (FWHM) of PL peaks from multilayer GeSi nanowires was observed. These results indicated a typical type-III band alignment of GeSi nanowires/Si. The blue-shift was attributed to band bending effect with the increase of photon-generated carriers. The nearly independent FWHM of PL peaks with the excitation power was explained in terms of the formation of mini-band due to strong coupling of holes in closely neighboring nanowires. An activation energy of -12 meV was extracted from the temperature-dependent intensity of PL peaks of the nanowires, which was assigned to be the exciton binding energy around the nanowires. Based on Raman spectra, the Ge content in GeSi nanowires was estimated to be -61%.

Investigation of saturation and excitation behavior of 1.5 μm emission from Er3+ ions in SiO2 sensitized with Si nanocrystals

AbstractUsing time‐resolved photoluminescence spectroscopy we investigated the 1.5 μm emission from Er3+ ions embedded in SiO2 matrix sensitized with Si nanocrystals. Temporal characteristics are analyzed as the function of excitation powers and temperatures. We confirm the predominance of two excitation channels – a fast one, with nanosecond dynamics, and a slow one, taking place on the microsecond time scale. Upon increasing excitation power, the fast mechanism saturates and further increase of total photoluminescence intensity is facilitated by the slow energy transfer process. Consequences of this finding on the microscopic modeling of the excitation process are discussed (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Studies of upconversion emission of Yb3+, Er3+:Lu2O3 nanoceramics

The translucent Yb3+ and Er3+ doped Lu2O3 ceramics composed of nanocrystalline grains were sintered using a low temperature and high pressure method. The upconversion emission associated with the 2H11/2→4I15/2, 4S3/2→4I15/2 (green), 4F9/2→4I15/2 (red) and 4I9/2→4I15/2 (IR) transitions was observed upon intense infrared laser excitation. The dependence of the upconversion intensity on incident laser diode power in Yb3+, Er3+:Lu2O3 nanoceramics was investigated. In particular it was found that the green/red intensity ratio of upconversion transitions increased with the power of the incident laser light. This effect was due to heating of the nanoceramics with increasing excitation power and may be utilized for high temperature thermometry.