Spontaneous Emission Quantum Efficiency Research Articles

Analysis of the structure and luminescence properties of Zn2 GeO4 :Eu3+ according to Judd-Ofelt theory.

The preparation and characteristic of nanorod-like Zn2 GeO4 doped with Eu3+ or zinc germanate (ZGO):xEu3+ (x = 0 ÷ 0.05), which was synthesized using the hydrothermal method, are described. The influence of Eu3+ -doping ions on the structure and the optical properties of ZGO was also investigated. According to the photoluminescence spectra, ZGO:xEu3+ nanophosphors gave a red emission due to the 5 D0 →7 F2 emission of Eu3+ ions. In accordance with Judd-Ofelt theory, the intensity parameters for f-f transitions from the emission and absorption spectrum were determined. At the 5 D0 excited state of Eu3+ , total spontaneous emission probabilities (AR ), lifetimes (τR ), branching ratios (βR ), and quantum efficiency (η) were calculated. The ZGO:xEu3+ (x = 0.02, 0.03, 0.04) phosphor showed the branch ratio β (5 D0 →7 F2 ) > 60%, indicating that the phosphors prepared here have a promising potential as laser light. The sample with a concentration of 0.04Eu3+ achieved the highest quantum efficiency of 84%, suggesting that it has potential light-emitting diode applications.

High-Temperature Spontaneous Emission Quantum Efficiency Analysis of Different InGaN MQWs for Future Power Electronics Applications

The commercial light-emitting diode (LED) epitaxy materials-blue (i.e., InGaN/gallium nitride (GaN) multiple quantum wells (MQWs)) for lighting and display applications and green (i.e., InGaN/GaN MQWs) for display applications-are evaluated with a varied range of temperatures (10-800 K) for future applications in high-density power electronic modules. Six different InGaN/GaN MQW structures (peak wavelengths λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</sub> = 448, 467, and 515 nm) are studied and compared to evaluate whether they can satisfy the light output requirements in the optocouplers at high temperatures. The spontaneous emission quantum efficiency (QE) of these LED epitaxy materials are studied using temperature-dependent and power photoluminescence (PL) spectroscopy. The LED materials, which have no pre-QWs, a higher number of QWs, and GaN buffer layer, show the highest QEs at high temperatures. All six LED materials' exhibit 65% QE at 500 K and stable operation at 800 K were without failure or flickering. At 800 K, the spontaneous emission QEs are between 56% and 60% for blue and green LEDs for displays, respectively. This work evaluates the high-temperature capability of various LED epitaxy materials for future applications in the high-temperature optocouplers as the galvanic isolations in high-density power modules.

Systematic Investigation of Spontaneous Emission Quantum Efficiency Drop up to 800 K for Future Power Electronics Applications

Future high-density power electronics applications may require high-temperature optoelectronic devices for gate drive. Thus, a systematic study of optoelectronic material from 10 to 800 K has been performed to understand the potential of the high-temperature operation of optoelectronic devices. The temperature dependence of the photoluminescence (PL) of indium gallium nitride/gallium nitride multiple quantum wells was studied. The integrated PL intensity dropped by an order of magnitude at 800 K compared to 10 K. The spontaneous emission quantum efficiency was calculated from the power-law relation linking the integrated PL signal and the excitation pump power. The validation of the traditional ABC model for solid-state lighting is extended to 800 K. This paper demonstrates the feasibility of developing high-temperature optoelectronic devices, which have operating temperatures over 500 K.

High Temperature and Power Dependent Photoluminescence Analysis on Commercial Lighting and Display LED Materials for Future Power Electronic Modules

Commercial light emitting diode (LED) materials - blue (i.e., InGaN/GaN multiple quantum wells (MQWs) for display and lighting), green (i.e., InGaN/GaN MQWs for display), and red (i.e., Al0.05Ga0.45In0.5P/Al0.4Ga0.1In0.5P for display) are evaluated in range of temperature (77–800) K for future applications in high density power electronic modules. The spontaneous emission quantum efficiency (QE) of blue, green, and red LED materials with different wavelengths was calculated using photoluminescence (PL) spectroscopy. The spontaneous emission QE was obtained based on a known model so-called the ABC model. This model has been recently used extensively to calculate the internal quantum efficiency and its droop in the III-nitride LED. At 800 K, the spontaneous emission quantum efficiencies are around 40% for blue for lighting and blue for display LED materials, and it is about 44.5% for green for display LED materials. The spontaneous emission QE is approximately 30% for red for display LED material at 800 K. The advance reported in this paper evidences the possibility of improving high temperature optocouplers with an operating temperature of 500 K and above.

Semianalytical modeling of arbitrarily distributed quantum emitters embedded in nanopatterned hyperbolic metamaterials

Nanopatterned hyperbolic metamaterials (NPHMs) have proven to be efficient structures for enhancing the spontaneous emission rate (Γ) and quantum efficiency (η) of quantum emitters (QEs). However, many of the NPHM designs still rely on computationally costly 3D numerical simulations. In this context, we propose a fast, semianalytical method capable of calculating both Γ and η of QEs placed inside a medium bounded by nanopatterned structures. The low computational cost of our approach makes it attractive for optimizing the NPHMs’ geometrical parameters that maximize η for a desired Γ. Furthermore, we suggest a more realistic procedure to calculate the decay behavior of multiple QEs arbitrarily distributed in the NPHM. This calculation is only feasible with the knowledge of Γ and η mapped for all possible positions of the QEs, which is easily achieved with the proposed model. For the validation procedure, we compare the model results with those obtained by the FDTD method. We apply the proposed model to an NPHM composed of nine Ag/SiO2 layers, with the polymer host layer embedded with rhodamine 6G, to maximize η for a specified tenfold increase of Γ. This procedure allowed η to be increased by 69% and 170% for 1D and 2D nanopatterning, respectively. The time required to build the Γ and η maps (used in the calculation of the decay behavior) is reduced by approximately 96% when compared with those numerically calculated via FDTD.

Radiative Recombination Dominated Monocrystalline CdTe/MgCdTe Double-Heterostructures

Monocrystalline CdTe/MgCdTe double-heterostruc-tures (DHs) grown on lattice-matched InSb substrates have achieved remarkable carrier-lifetimes up to 3.6 μs recently. In this paper, external luminescence quantum efficiencies ${\eta _{{\rm{ext}}}}$ of CdTe/MgCdTe DHs are determined using photoluminescence quantum efficiency measurements. The external luminescence quantum efficiency of a 1-μm-thick CdTe/MgCdTe DH with indium-doped n-type at 1 × 1017 cm−3 is measured to be 3.0% under one-sun condition, i.e., a carrier injection current density of ∼30 mA/cm2, which corresponds to a spontaneous emission quantum efficiency ${\eta _q}$ of 90 ± 4%. Such a high efficiency gives an implied open-circuit voltage V oc, or quasi-Fermi-level splitting of 1.13 V. The ${\eta _q}$ of unintentionally doped DH samples with optimum barrier layers can be over 95% at one-sun injection level.

Radiative Enhancement of Plasmonic Nanopatch Antennas

Efficient light manipulation at subwavelength scale is of great interest for solar energy conversion, optical sensing, and nanophotonic devices. Recently, plasmonic nanopatch antennas (PNAs), which consist of plasmonic nanoparticles and metal films with thin layers of dielectric spacers sandwiched between them, have shown promise for directing and enhancing radiation from the dipole emitters at the PNAs. Herein, we apply finite-difference time-domain simulations to comparatively study the radiative enhancement of a series of PNAs consisting of Ag nanoparticles with different geometries, i.e., nanosquare, nanotriangle, nanorod, and nanodisk. We find that the shape of the Ag nanoparticles influences the resonant wavelength of the plasmonic waveguide modes in the spacers, the enhancement of localized electric field, and multiple aspects of the radiation, including spontaneous emission rate, quantum efficiency, and radiative enhancement factor. Nanodisk-based PNAs exhibit both high quantum efficiency (~0.74) and radiative enhancement factor (>20), while nanotriangle-based PNAs show remarkable spontaneous emission rate enhancement (>2500). Furthermore, we examine the effects of dipole emitter locations on the radiative properties. Our results pave the way towards the rational design of PNAs for the optimal plasmonic enhancement of light emission for targeted applications.

Focused-ion beam patterning of organolead trihalide perovskite for subwavelength grating nanophotonic applications

The coherent amplified spontaneous emission and high photoluminescence quantum efficiency of organolead trihalide perovskite have led to research interest in this material for use in photonic devices. In this paper, the authors present a focused-ion beam patterning strategy for methylammonium lead tribromide (MAPbBr3) perovskite crystal for subwavelength grating nanophotonic applications. The essential parameters for milling, such as the number of scan passes, dwell time, ion dose, ion current, ion incident angle, and gas-assisted etching, were experimentally evaluated to determine the sputtering yield of the perovskite. Based on our patterning conditions, the authors observed that the sputtering yield ranged from 0.0302 to 0.0719 μm3/pC for the MAPbBr3 perovskite crystal. Using XeF2 for the focused-ion beam gas-assisted etching, the authors determined that the etching rate was reduced to between 0.40 and 0.97, depending on the ion dose, compared with milling with ions only. Using the optimized patterning parameters, the authors patterned binary and circular subwavelength grating reflectors on the MAPbBr3 perovskite crystal using the focused-ion beam technique. Based on the computed grating structure with around 97% reflectivity, all of the grating dimensions (period, duty cycle, and grating thickness) were patterned with nanoscale precision (&amp;gt;±3 nm), high contrast, and excellent uniformity. Our results provide a platform for utilizing the focused-ion beam technique for fast prototyping of photonic nanostructures or nanodevices on organolead trihalide perovskite.

Ten-fold enhancement of ZnO thin film ultraviolet-luminescence by dielectric microsphere arrays.

Here we report strong enhancement in ultraviolet-photoluminescence (UV-PL) of ZnO thin films (grown on a SiC substrate) covered by monolayer dielectric fused silica or polystyrene microspheres with diameters ranging from 0.5 to 7.5 μm. The excited light scatted in the film is collected by the microspheres to stimulate whispering gallery modes, by which the internal quantum efficiency of spontaneous emission is enhanced. Meanwhile, the microsphere monolayer efficiently couples emitted light energy from the luminescent film to the far-field for PL detection. A UV-PL enhancement up to 10-fold via a 5-µm-diameter microsphere monolayer is experimentally demonstrated in this work. The unique optical property of microsphere in photoluminescence (PL) enhancement makes them promising for high-sensitivity PL measurements as well as design of photoelectric devices with low loss and high efficiency.

Enhanced Single-Molecule Spontaneous Emission in an Optimized Nanoantenna with Plasmonic Gratings

In this study, we theoretically investigated the single-molecule fluorescence enhancement in a slit–groove structure. The excitation field enhancement, modified quantum efficiency, collection efficiency, and position dependence of a single-molecule spontaneous emission coupled to a plasmonic antenna are explored together. Simulation results revealed that the metal gratings play a crucial role in strengthening the local electromagnetic field enhancement in the excitation process and beaming the radiation light into a tiny angular volume in the emission process, whereas the total radiative decay rate and quantum efficiency are mainly determined by the slit cavity. Our findings provide an intuitive guideline to further optimize the plasmonic antenna for single-molecule detection and the results make a promising route to the development of photonic devices for the manipulation of single-molecule spontaneous emission.

Spontaneous emission rate enhancement of nano-structured silicon by surface plasmon polariton

Surface plasmon polariton (SPP) is an attractive candidate to improve internal quantum efficiency (QE) of spontaneous emission (SE) from nano-structured silicon (Si) including nano-porous silicon (NP-Si) and silicon nanocrystal (Si-NC). Since the SPP resonant frequency of common metals, e.g., gold (Au), silver (Ag), copper (Cu), and aluminum (Al), is too high, the SPP resonance has to be engineered to match the luminescence from nanostructured Si. For this purpose, we have proposed and demonstrated three approaches including metal-rich Au(1 − α)-SiO2(α) cermet SPP waveguide (WG), compound layer structure WG and metallic grating. In this paper, those approaches are reviewed and discussed. According to the calculated results, such three methods could effectively enhance SE rate from NP-Si or Si-NCs and show potential in developing high efficiency Si based light sources with electric pump.

Single-Molecule Spontaneous Emission in the Vicinity of an Individual Gold Nanorod

An individual gold nanorod as an optical antenna to modulate single-molecule fluorescence spontaneous emission behaviors is investigated theoretically. 2D and 3D numerical finite-difference time-domain methods are implemented to investigate changes in the excitation rate, spontaneous emission rate, quantum efficiency, and emission spectral shape as functions of the separation between the emitter and nanorod. Our simulations reveal that the 3D relative configuration between the gold nanorod and the single dipole definitely affects the quantum efficiency and emission spectral shape. The orientation of the dipole and the polarization of the excitation light are also investigated to clarify the polarization dependence of the plasmonic-enhanced fluorescence. Furthermore, we calculate the modified quantum efficiency and emission spectral shape of a single Cy5 dye molecule in the vicinity of a single gold nanorod taking experimental requirements into account.

Photoluminescence properties of Eu 3+ in lithium titanate

Lithium titanate ceramics doped with Eu were synthesized and their photoluminescence (PL) and emission spectral characteristics were investigated. PL spectra of the sample showed peaks corresponding to the 5D 0→ 7F J ( J=0, 1, 2, 3 and 4) transitions under 230 nm excitation. The fluorescence lifetimes of Eu 3+ ions were found out to be 645 μs. Strong red emission coming from the hypersensitive 5D 0→ 7F 2 transition of Eu 3+ ion suggested the presence of the dopant ion in highly asymmetric environment. Further analysis of the emission spectrum revealed that the symmetry of the metal ion is very low i.e. C 2. The emission intensity of the sample was compared with a commercial phosphor to get an idea about the commercial utility of the phosphor. Various emission properties for the system namely Judd–Ofelt intensity parameters, spontaneous emission probabilities, branching ratios, radiative lifetimes and quantum efficiency were evaluated for the dopant ion by adopting standard procedure.

Future of group-III nitride semiconductor green laser diodes [Invited

Some research groups have realized green laser diodes (LDs) using InGaN-based nitride semiconductors. However, the performances of their devices, in particular wall-plug efficiency, were still one digit lower than those of established red and blue semiconductor LDs. To achieve high-performance green LDs and apply them to LD displays or micro-projectors, the field should focus on improving the internal quantum efficiency (IQE) of green spontaneous emission from InGaN quantum wells (QWs). Green InGaN QWs have completely different features than blue InGaN QWs in terms of growth and concomitant material quality. The exploration of 100% IQE-quality InGaN materials for green emission continues.

Determination and improvement of spontaneous emission quantum efficiency in GaAs/AlGaAs heterostructures grown by molecular beam epitaxy

AbstractThe injection and temperature dependence of the spontaneous emission quantum efficiency of molecular beam epitaxy grown GaAs/AlGaAs heterostructures is determined using excitation dependent photoluminescence (PL) measurements. Two samples are compared; one grown with Sb as a surfactant and one grown without Sb. The PL measurements were performed at temperatures from 80 to 320 K using a HeNe pump laser with powers ranging from 0.6 to 35 mW. The quantum efficiency is inferred from the power law predicted by the rate equations that links pump power and integrated PL signal. The use of Sb as surfactant improved the extrapolated peak spontaneous emission quantum efficiency from 0.970 to 0.977 at 300 K and from 0.996 to 0.998 at 180 K. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Excitation dependent photoluminescence measurements of the nonradiative lifetime and quantum efficiency in GaAs

The nonradiative lifetime and spontaneous emission quantum efficiency in molecular-beam epitaxy grown bulk GaAs is determined using injection level dependent photoluminescence (PL) measurements. These measurements were performed at temperatures of 300, 230, 100, and 50K using a HeNe pump laser with powers ranging from 0.3to40mW. The quantum efficiency and lifetime is inferred from the power law relation linking pump power and integrated PL signal that is predicted by the rate equations. The nonradiative lifetime for bulk GaAs is determined to be 0.3μs, with an additional temperature dependent component attributed to the AlGaAs barriers that rapidly reduces the nonradiative lifetime at temperatures above 230K. The peak quantum efficiency is &amp;gt;0.96 at 300K and &amp;gt;0.99 at temperatures below 230K.

Effect of radiation trapping on spectroscopic properties of Er3+-doped oxide glasses

The absorption spectra, fluorescence spectra and the lifetimes of 4I13/2 level of Er3+-doped phosphate and tellurite glasses ha ve been measured for sample with different concentrations and thickness. The abs orption cross-section (σa), emission cross-section(σe), spontaneous emission probability(Arad), radiative lifetime(τrad ) and quantum efficiency(η) of fluorescence around 1.53 μm of Er3+-doped phosphate and tellurite glasses were determined. The effect of radia tion trapping on the spectral properties and parameters of Er3+-doped tellurite and phosphate glasses has been investigated. It was found that radiat ion trapping exists generally in erbium-doped glass hosts, even at low Er3+ -doping concentration (0.1 mol% Er2O3). Due to radia tion trapping, the values of the τrad of the Er3+∶4 I13/2 level in tellurite glasses increased about 11%—37％ with the sample thickness and erbium doping concentration, while in phosphate glasse s τrad increased by 6%—17%. The full-width at half maximum (FWHM)of fluorescence in tellurite glasses increased about 15%—64%, with 11%—55% for p hosphate glasses. It caused a high overestimation on the figure of merits (FOM) for amplifier bandwidth (σe×FWHM). The spectral overlap between the emission and absorption spectra in 1.53 μm band is relatively larger in tellur ite than in phosphate glasses, and values of the emission cross-section in tellu rite glasses are larger than that in phosphate glasses. Consequently, the radiat ion trapping in tellurite glasses is more severel than that in phosphate glasses .

Er 3+ to glass matrix energy transfer in Ga–Ge–S:Er 3+ system

Spectroscopic characteristics (spontaneous emission probabilities, radiative lifetimes, oscillator strengths, branching ratios and quantum efficiency) of Er 3+ in Ga 2S 3–GeS 2–Er 2S 3 glassy system are investigated by use of Judd–Ofelt theory. Lifetime measurements 4 S 3/2 level of Er 3+ in Ga 2S 3–GeS 2–Er 2S 3 glasses with various Er 3+ concentrations and optical gap energy are performed. Estimations of the non-radiative relaxation rate and the multiphonon relaxation rate demonstrate the occurrence of energy transfer from Er 3+ ion to the electronic states of the glass matrix. The way of the practical application of this non-radiative energy transfer is suggested.

Ultrahigh spontaneous emission quantum efficiency, 99.7% internally and 72% externally, from AlGaAs/GaAs/AlGaAs double heterostructures

Optically thin AlGaAs/GaAs/AlGaAs double heterostructures, (5000 Å), are floated off their substrates by the epitaxial liftoff technique and mounted on various high reflectivity surfaces. From the absolute photoluminescence intensity, we measure internal and external quantum efficiencies of 99.7% and 72%, respectively. High spontaneous emission quantum efficiency, is important for photon number squeezed light, diode lasers, single-mode light-emitting-diodes, optical interconnects, and solar cells.

Pulsations in AlxGa1−xAs buried heterostructure lasers caused by the heating of defects

The onset of pulsations in buried heterostructure lasers was investigated with a two-pulse method. First, an above threshold pulse was applied to reversibly convert the laser from a nonpulsating to a pulsating state. The pulsating state persists for several hundred nsec. A second pulse, applied 50 nsec or more after the first pulse, was then used to study the pulsations and other associated changes in the laser. In this way, a laser which initially does not even exhibit observable relaxation oscillations was changed into a laser which has sustained pulsations with a period of 1.8 nsec. These studies provide evidence that pulsations are caused by defects locally heated by absorbed laser light. Measurements of spontaneous emission intensity, laser threshold, and quantum efficiency showed that the loss in the laser cavity increased when the laser was converted into the pulsating state. The minimum loss increase observed to cause pulsations was about 13–19 cm−1. The characteristic time for the increase in loss to rise and fall was 100–200 nsec. These changes were nonexponential with both long- and short-lived components. Both the time scale and the nonexponential behavior are expected for the increased absorption loss associated with the heating of nonradiative defects. Calculations of defect heating predicted average temperature rises of 4–12 K, depending on defect size. It is estimated that a temperature rise of 7 K could halve the defect size necessary for pulsations. The increased loss correlated with the laser intensity during the first pulse, indicating that the heating is due primarily to the absorption of laser light, rather than to injected current. Local heating will promote pulsations by increasing the optical absorption and the change in gain with carrier density.