Maximum Light Output Research Articles

Low-threshold limaçon-shaped micro-cavity semiconductor lasers with an approximate ring-shaped top contact

Based on the whispering gallery mode (WGM) characteristics in the limaçon-shaped micro-cavity, we designed a micro-cavity laser with an approximate ring-shaped top electrical contact, which theoretically realized the annular current injection and low temperature rise features in the active gain region. Then we fabricated micro-cavity lasers with approximate ring-shaped top contact at different ring widths and compared them with conventional full disk-shaped top contact lasers, and observed a lower operating injection threshold current as a consequence of this relatively straightforward and feasible technique. The threshold current density of the approximate ring-shaped top contact laser is ∼0.41 kA/cm2, which is 16% lower than that of the conventional laser. Furthermore, we obtain a maximum light output peak power of 133 mW with directional emission at a far-field divergence angle of ∼38° at a full width of half maximum (FWHM).

The composited high reflectivity p-type electrodes with patterned ITO for AlGaN-based ultraviolet light emitting diodes

Composited p-type electrodes with high reflectivity have been investigated in AlGaN-based ultraviolet light emitting diodes (UV-LEDs) to improve the light extraction efficiency, which are composed of a patterned ITO layer and an Al reflector. It is verified that the patterned ITO with a thickness of 30 nm can not only well form Ohmic contact with p-GaN capping layer, but also be nearly 90% transparent to ultraviolet light, and thus presenting a reflectivity of 73% at 280 nm when combined with an Al reflector. Further experimental efforts confirm that the performance of the UV-LEDs is dramatically improved with such p-type electrodes. The maximum light output power and wall plug efficiency in the current range of 0–100 mA are severally increased by 49.8% and 54.2% compared to the device with traditional Ni/Au electrodes.

Influence of {Ce3+ – Mg2+} pairs on the photo- and thermally stimulated luminescence of Mg2+ co-doped Gd3(Ga,Al)5O12:Ce single crystals

Single crystals of Gd3(Ga,Al)5O12:Ce,Mg with different concentration of Mg2+ ions varying from 0 to 0.029 at.% are investigated by the X-ray diffraction, steady-state and time-resolved photoluminescence, and thermally stimulated luminescence (TSL) methods in a wide temperature range (77–510 K). The origin and structure of the luminescence centers are considered. The influence of Mg2+ ions on the structure of the Ce3+-related excited state, the processes taking place in this state, and the TSL characteristics is clarified. At low temperatures, the photoluminescence optical quenching in close {Ce3+ - Mg2+} pairs is found to be responsible for the light yield reduction accompanied with the acceleration of the luminescence decay kinetics. At room temperature, the luminescence quenching can be additionally caused by the thermal ionization of the 5d1 excited state of Ce3+ ion, whose probability increases with the increasing Ga content. The conclusion is made that in order to create the fast scintillator with the maximum light yield, the concentration of intrinsic crystal lattice defects in Gd3(Ga,Al)5O12:Ce,Mg should be minimized, the optimum composition of the host should be chosen, and the balance between the concentrations of the doped Mg2+ and Ce3+ ions should be optimised.

High performance deep-ultraviolet light-emitting diodes with transverse electron injection

Conventional structure deep-ultraviolet light-emitting diodes (DUV-LEDs) at short wavelengths typically suffer from electron overflow and low light-emitting efficiency. For solar-blind communication application, conventional DUV-LEDs are struggling to simultaneously implement high external quantum efficiency (EQE) and high modulation bandwidth. In this paper, we report that transverse structure LEDs can overcome abovementioned problems and achieve high EQE and high modulation bandwidth. The n-AlGaN free transverse structure LEDs have been demonstrated to yield high crystal quality multiple quantum wells (MQWs). Due to the transverse electron injection, transverse structure LEDs can confine electrons well within the MQWs and avoid electron overflow. To enhance the electron injection and light extraction characteristics, the narrow and long mesa designs were optimized by both experimental and theoretical-fit results. Finite-difference time domain simulation indicates that the n+-GaN layer of the transverse LEDs with the transverse MQWs mesa design does not degrade light extraction. The transverse structure LEDs attain the maximum light output power and EQE recorded to date for DUV-LEDs below 250 nm, respectively 14 mW and 1.72 %. Furthermore, the −3 dB modulation bandwidth can be achieved up to 416 MHz.

Optical and scintillation properties of Pr3+-doped (La, Y)2Si2O7 single crystals

Pr-doped scintillators are attracting attention because of their potential applications in medical devices. In this study, we focused on Pr-doped (La, Y)2Si2O7 scintillators to grow single crystals by the micro-pulling-down method and to evaluate their optical and scintillation properties. From the powder XRD results, the crystal structure and the space group were identified to be monoclinic and P21/n, respectively. The results of optical and scintillation characterization show that the luminescence from the Pr3+ 5d1–4f transition increases with increasing Pr3+ concentration. The maximum light yield was also obtained for the sample with the highest Pr3+ concentration. The Pr-doped (La, Y)2Si2O7 single crystal shows promising properties for application in radiation detection.

Extraction of the electron excess temperature in terahertz quantum cascade lasers from laser characteristics

Abstract We propose a method to extract the upper laser level’s (ULL’s) excess electronic temperature from the analysis of the maximum light output power (P max) and current dynamic range ΔJ d = (J max − J th) of terahertz quantum cascade lasers (THz QCLs). We validated this method, both through simulation and experiment, by applying it on THz QCLs supporting a clean three-level system. Detailed knowledge of electronic excess temperatures is of utmost importance in order to achieve high temperature performance of THz QCLs. Our method is simple and can be easily implemented, meaning an extraction of the excess electron temperature can be achieved without intensive experimental effort. This knowledge should pave the way toward improvement of the temperature performance of THz QCLs beyond the state-of-the-art.

Monolithic integrated all-GaN-based µLED display by selective area regrowth.

This work demonstrates an all-GaN-based µLED display with monolithic integrated HEMT and µLED pixels using the selective area regrowth method. The monochrome µLED-HEMT display has a resolution of 20 × 20 and a pixel pitch of 80 µm. With the optimized regrowth pattern, the µLED-HEMT achieves a maximum light output power of 36.2 W/cm2 and a peak EQE of 3.36%, mainly due to the improved crystal quality of regrown µLED. TMAH treatment and Al2O3 surface passivation are also performed to minimize the impact of nonradiative recombination caused by the dry etching damage. With a custom-designed driving circuit board, images of "HKUST" are successfully shown on the µLED-HEMT display.

Red InGaN Micro-LEDs on Silicon Substrates: Potential for Multicolor Display and Wavelength Division Multiplexing Visible Light Communication

Red micro light-emitting diodes (micro-LEDs) on silicon substrates are crucial for the realization of large-scale, high-quality, low-cost micro-LED displays, and are also beneficial for high-speed visible light communication (VLC). In this letter, micro-LEDs with different sizes were fabricated. The maximum light output power density of the 20 μm pixel can reach 3.36 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which is 14 times higher than that of the largest pixel (150 μm). By adjusting the pixel size and injection current, a direct color shift from red to green can be observed. Moreover, multicolor emission with uniform brightness can be realized by adjusting duty cycle, which has broad application prospect in monolithic, multicolor displays. Despite the significant blue shift phenomenon, the peak wavelength of all pixels is still greater than 630 nm at the current density of 100 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , which means that red light can be emitted in a large current density range to meet various application scenarios requiring different driving conditions. We have realized red, yellow and green micro-LED transmitters on Si substrate by changing the driving current density and modulation bandwidth up to 533.15 MHz is achieved when emitting green light. This is the report of red-emission micro-LEDs on Si substrates for visible light communication for the first time. And we proposed a proof-of-concept monolithic, multicolor wavelength division multiplexing scheme that achieved a total allowable transmission data rate of 2.35 Gbps. Such micro-LED is expected to be applied in multicolor displays, high-speed multi-channel VLC, color-tunable light sources, etc. In particular, due to its high integration and miniaturization, it is expected to be used in future practical application scenarios such as wearable communication devices and smartwatches.

The Influence of Halide Ion Substitution on Energy Structure and Luminescence Efficiency in CeBr2I and CeBrI2 Crystals.

This study aims to determine the optimum composition of the CeBr1-xIx compound to achieve the maximum light output. It is based on calculations of the band energy structure of crystals, specifically taking into account the characteristics of the mutual location of local and band 5d states of the Ce3+ ions. The band energy structures for CeBr2I and CeBrI2 crystals were calculated using the projector augmented wave method. The valence band was found to be formed by the hybridized states of 4p Br and 5p I. The 4f states of Ce3+ are located in the energy forbidden band gap. The conduction band is formed by the localized 5d1 states, which are created by the interaction between the 5d states of Ce3+ and the 4f0 hole of the cerium ion. The higher-lying delocalized 5d2 states of Ce3+ correspond to the energy levels of the 5d states of Ce3+ in the field of the halide Cl0 (Br0) hole. The relative location of 5d1 and 5d2 bands determines the intensity of 5d-4f luminescence. The bottom of the conduction band is formed by localized 5d1 states in the CeBr2I crystal. The local character of the bottom of the conduction band in the CeBr2I crystal favors the formation of self-trapped Frenkel excitons. Transitions between the 5d1 and 4f states are responsible for 5d-4f exciton luminescence. In the CeBrI2 crystal, the conduction band is formed by mixing the localized 5d1 and delocalized 5d2 states, which leads to quenching the 5d-4f luminescence and a decrease in the light output despite the decrease in the forbidden band gap. CsBr2I is the optimum composition of the system to achieve the maximum light output.

Influence of fluorescent dopants on the vat photopolymerization of acrylate-based plastic scintillators for application in neutron/gamma pulse shape discrimination.

Plastic scintillators, a class of solid-state materials used for radiation detection, were additively manufactured with vat photopolymerization. The photopolymer resins consisted of a primary dopant and a secondary dopant dissolved in a bisphenol A ethoxylate diacrylate-based matrix. The absorptive dopants significantly influence important print parameters, for example, secondary dopants decrease the light penetration depth by a factor > 12 ×. The primary dopant 2,5-diphenyloxazole had minimal impact on the printing process even when loaded at 25 % by mass of the resin. Working curve measurements, which relate energy dose to cure depth, were performed as a function of feature size to further assess the influence of dopants. Photopatterns smaller than 150 μm width had apparent increases in critical energy dose compared to larger photopatterns, while all resins maintained printed features in line gratings with 50 μm of separation. Printed scintillator monoliths were compared to scintillators cast by traditional molding, demonstrating that the layer-by-layer printing process does not decrease scintillation response. A maximum light output of 31 % of a benchmark plastic scintillator (EJ-200) and successful pulse shape discrimination were achieved with 20 % by mass 2,5-diphenyloxazole as the primary dopant and 0.1 % by mass 9,9-dimethyl-2,7-distyrylfluorene as the secondary dopant in printed scintillator samples.

Simulation of Electron-Hole Pairs Created by γ-Rays Interaction in Scintillation Crystals Using Geant4 Toolkit

Understanding the electron-hole pair (e-h pair) production process in scintillators is essential for further studies of the scintillation mechanism. The number of e-h pairs can be derived from theoretical and empirical approaches. However, simulation approaches have been chosen to study all the related physical processes and spatial information in scintillation crystals. Due to the complexity of particle interaction and physical properties of solid-state, the simulation approach could be better than the analytical approach. This work aims to develop a Geant4-based simulation toolkit for studying scintillation mechanisms in single-crystalline scintillators since Geant4 can accurately calculate particle energy loss. Due to the lack of a solid-state physic model for insulators, the plasmon creation and phonon loss process for electrons are implemented in the Geant4 toolkit. The electrons, which are created by γ-rays interaction in crystals, can make further ionization to generate secondary electrons or create plasmons, or lose energy to phonons. Then the created plasmons decay into e-h pairs. These electrons and holes are tracked with information on energy and position. Therefore, the number of created e-h pairs and their spatial distribution can be extracted. In this work, we will present the result of the creation process of e-h pairs per MeV in scintillation crystals induced by γ-rays interaction. The number of e-h pairs created by γ ray in some popular inorganic crystals has been calculated. Assuming maximum conversion efficiency from the e-h pair to the luminescence light, we can calculate the maximum light yield of the intrinsic scintillator.

Kinetics of narrow-spectrum LED glow under pulsed power

The results of experimental investigations of the glow kinetics of narrow-spectrum LEDs based on InGaN-GaN 450 and 520 nm and AlGaInP-GaAs 625 nm structures are presented. The increase and decrease of the light flux intensity under pulsed power are described by exponential dependences containing fast and slow components. The time constants of both components decrease with the increase of the pulse frequency for all three types of LED samples. The time constant of the slow component decreases with the increase of the current and voltage pulse amplitudes. The maximum light output on the frequency dependences of LED energy characteristics is observed at the frequency of 75…100 kHz. Further frequency increase results in the decrease of the LED energy efficiency. The obtained results are explained based on the LED equivalent electrical and energy circuits.

Ce3+ doped Lu3Al5O12 ceramics prepared by spark plasma sintering technology using micrometre powders: Microstructure, luminescence, and scintillation properties

Ce3+ doped Lu3Al5O12 (Ce:LuAG) ceramics were fabricated by the solid-state reaction method through spark plasma sintering (SPS) from 1350 °C to 1700 °C for 5 min at a pressure of 50 MPa using micro powders. The average grain size of the SPSed ceramics gradually grew from 0.42 µm (1400 °C) to 1.55 µm (1700 °C), which is nearly one order of magnitude lower than that of vacuum sintered (VSed) Ce:LuAG ceramics (~24.6 µm). Characteristic Ce3+ emission peaking at around 510 nm appeared and 92% photoluminescence intensity of room temperature can be reserved at 200 °C revealing excellent thermal stability. The maximum radioluminescence intensity reached around 3 times of VSed Ce:LuAG ceramics and 7.8 times of BGO crystals. The maximum scintillation light yield under γ-ray (137Cs) excitation reached 9634 pho/MeV @ 2 μs. It is concluded that SPS technology is a feasible way to develop Ce:LuAG ceramics and further optical enhancement can be expected.

Wide Concentration Range of Tb3+ Doping Influence on Scintillation Properties of (Ce, Tb, Gd)3Ga2Al3O12 Crystals Grown by the Optical Floating Zone Method.

To obtain a deeper understand of the energy transfer mechanism between Ce3+ and Tb3+ ions in the aluminum garnet hosts, (Ce, Tb, Gd)3Ga2Al3O12 (GGAG:Ce, Tb) single crystals grown by the optical floating zone (OFZ) method were investigated systematically in a wide range of Tb3+ doping concentration (1–66 at.%). Among those, crystal with 7 at.% Tb reached a single garnet phase while the crystals with other Tb3+ concentrations are mixed phases of garnet and perovskite. Obvious Ce and Ga loss can be observed by an energy dispersive X-ray spectroscope (EDS) technology. The absorption bands belonging to both Ce3+ and Tb3+ ions can be observed in all crystals. Photoluminescence (PL) spectra show the presence of an efficient energy transfer from the Tb3+ to Ce3+ and the gradually quenching effect with increasing of Tb3+ concentration. GGAG: 1% Ce3+, 7% Tb3+ crystal was found to possess the highest PL intensity under excitation of 450 nm. The maximum light yield (LY) reaches 18,941 pho/MeV. The improved luminescent and scintillation characteristics indicate that the cation engineering of Tb3+ can optimize the photoconversion performance of GGAG:Ce.

Semipolar {202¯1} GaN Edge-Emitting Laser Diode on Epitaxial Lateral Overgrown Wing

Edge-emitting laser diodes (LDs) were fabricated on a reduced dislocation density epitaxial lateral overgrown (ELO) wing of a semipolar {202¯1} GaN substrate, termed an ELO wing LD. Two types of facet feasibility studies were conducted: (1) “handmade” facets, wherein lifted-off ELO wing LDs were cleaved manually, and (2) facets formed on wafers through reactive ion etching (RIE). Pulsed operation electrical and optical measurements confirmed the laser action in the RIE facet LDs with a threshold current of ~19 kAcm−2 and maximum light output power of 20 mW from a single uncoated facet. Handmade facet devices showed spontaneous, LED-like emission, confirming device layers remain intact after mechanical liftoff.

Improving light output power of AlGaN-based deep-ultraviolet light-emitting diodes by optimizing the optical thickness of p-layers

In this study, we investigated the relationship of light output power with the optical thickness of the p-layers in AlGaN-based deep ultraviolet light-emitting diodes with a transparent high-Al-composition p-AlGaN clad layer, a thin p-GaN contact layer, and a reflective p-type electrode. By adjusting the thickness of the transparent high-Al-composition p-AlGaN clad layer, we observed a marked change in light output power. A maximum light output power of 385 mW at 1500 mA, a maximum external quantum efficiency of 15.7% at 10 mA, and a maximum wall-plug efficiency of 15.3% at 10 mA were obtained at an emission wavelength of 275 nm.

Monte Carlo simulation of the passage of γ-rays and α-particles in CsI

Theoretical and computational methods for simulating the creation of ionization tracks by fast ions in solids were applied to the passage of α-particles in CsI, an inorganic scintillator commonly used for radiation detection. The methods were implemented in a Monte Carlo program to simulate the interaction of α-particles, with incident energies of up to 1 MeV, with CsI. The simulations followed the fate of individual electron-hole pairs and included the formation of Frenkel pairs and thus allowed for a detailed description of the microscopic structure of ionization and defect tracks created by incident radiation. Simulations were also performed with γ-rays of the same energy to compare and contrast the ionization tracks obtained with both types of particles. Intrinsic properties such as the mean energy per electron-hole pair, Fano factor, maximum theoretical light yield, and spatial distributions of electron-hole pairs were computed for both α-particles and γ-rays. α-Particles created cylindrical tracks that were initially aligned with the incident direction and with initial radii of a few tens of nanometers, whereas γ-rays showed significant scattering, resulting in probability distributions with lower intensities and much greater radial extents.

Improved light extraction efficiency of AlGaN deep-ultraviolet light emitting diodes combining Ag-nanodots/Al reflective electrode with highly transparent p-type layer.

Enhancement of light extraction efficiency (LEE) of AlGaN-based deep-ultraviolet (DUV) light emitting diodes (LEDs) has been attempted by adopting Ag-nanodots/Al reflective electrodes on a highly transparent complex p-type layer. By thinning the p-GaN to several nm, highly DUV transparent p-type layer is achieved, making it meaningful for the application of reflective electrodes composed of Ag-nanodots and Al film to allow most light emitted upward to be reflected back to the sapphire side. By this approach, the maximum light output power and external quantum efficiency of the DUV-LEDs with optimized Ag nanodots/Al electrodes are severally increased by 52% and 58%, respectively, compared to those with traditional Ni/Au electrodes when the current is below 200 mA.

Carrier Velocity Modulation by Asymmetrical Concave Quantum Barriers to Improve the Performance of AlGaN-Based Deep Ultraviolet Light Emitting Diodes

Carrier velocity modulation by asymmetrical concave quantum barriers to improve the performance of AlGaN-based deep-ultraviolet light emitting diodes has been investigated. The concave structure is realized by inserting a low Al composition AlGaN layer in the barrier, and thus forming a concave region on energy band. The measured device performance shows a significant improvement under 0-250 mA operation current, with the maximum external quantum efficiency and light output power of 1.02% and 8.0 mW, which are more than double (2.32 and 2.35 times) of that for the conventional one, respectively. Simulation confirms that the concave quantum barriers make it possible for electrons to be scattered, which reduce the electron energy before being injected into quantum wells, and thus the capability of quantum wells to capture electrons is enhanced. Meanwhile, the vertical transport of holes within the active region is also enhanced because the asymmetrical setting of the concave layer in the quantum barriers helps holes accelerate under the polarization field, and then get more energy to overcome the barrier.

Aperture diameter dependences in GaN-based vertical-cavity surface-emitting lasers with nano-height cylindrical waveguide formed by BCl3 dry etching

We demonstrated GaN-based vertical-cavity surface emitting lasers (VCSELs) with 5–30 μm wide nano-height cylindrical waveguide formed by BCl3 etching. A 5 nm-depth etching with BCl3 showed the most efficient current blocking at the interface of the etched p++-GaN and an ITO electrode among the cases with BCl3, Ar, or O2, which could be due to not only etching damages but also diffused B atoms into the etched surface. While room-temperature continuous-wave operations of the VCSELs with the large apertures were demonstrated, maximum light output power values of the large aperture VCSELs seemed limited by nonuniform current injection and device thermal resistances.