Excitation Power Research Articles

Temperature‐ and Excitation Power Density‐Resolved Photoluminescence of AlGaN‐Based Multiple Quantum Wells Emitting in the Spectral Range of 220–260 nm

Internal quantum efficiency (IQE) of AlGaN‐based multiple quantum wells (MQWs) on face‐to‐face‐annealed sputter‐deposited AlN templates is examined by photoluminescence spectroscopy. The excitation power density dependence of IQE is evaluated as a function of temperature under the selective excitation of the quantum wells. The temperature dependences of the maximum IQE and the corresponding excitation power density (EPD) are analyzed based on the rate equation models for carriers and excitons. The decrease of the maximum IQE and increase of the corresponding EPD are mainly due to the increase in the nonradiative recombination rate via nonradiative recombination centers. Furthermore, the nonradiative recombination rate for the MQW with an emission wavelength around 220 nm is activated at a lower temperature than the other samples, which is expected to lead to the lower IQE of the MQW with an emission wavelength around 220 nm.

Raman evidence for the oxidation of amorphous As2(SexS1–x)3 film surfaces under visible light

Abstract Raman spectra of thermally evaporated As2Se3 and Se-rich (above 50 at. %) ternary As–Se–S films measured at high (above 1 MW cm−2) excitation power density reveal new narrow peaks emerging during the measurements which are attributed to crystalline arsenolite As2O3. The latter is formed on the As–Se–S film surface due to thermal decomposition of the film and oxidation of arsenic in ambient air. Contrary to As2S3, for which the photoassisted oxidation of the film surface requires UV light, for narrower-gap As–Se–S films this effect occurs under illumination by visible light.

Highly efficient upconversion luminescence in narrow-bandgap Y2Mo4O15.

Lanthanide-doped upconversion (UC) materials have been extensively investigated for their unique capability to convert low-energy excitation into high-energy emission. Contrary to previous reports suggesting that efficient UC luminescence (UCL) is exclusively observed in materials with a wide bandgap, we have discovered in this study that Y2Mo4O15:Yb3+/Tm3+ microcrystals, a narrowband material, exhibit highly efficient UC emission. Remarkably, these microcrystals do not display any four- or five-photon UC emission bands. This particular optical phenomenon is independent of the variation in doping ion concentration, temperature, phonon energy, and excitation power density. Combining theoretical calculations and experimental results, we attribute the vanishing emission bands to the strong interaction between the bandgap of the Y2Mo4O15 host matrix (3.37 eV) and the high-energy levels (1I6 and 1D2) of Tm3+ ions. This interaction can effectively catalyze the UC emission process of Tm3+ ions, which leads to Y2Mo4O15:Yb3+/Tm3+ microcrystals possessing very strong UCL intensity. The brightness of these microcrystals outshines commercial UC NaYF4:Yb3+,Er3+ green phosphors by a factor of 10 and is 1.4 times greater than that of UC NaYF4:Yb3+,Tm3+ blue phosphors. Ultimately, Y2Mo4O15:Yb3+/Tm3+ microcrystals, with their distinctive optical characteristics, are being tailored for sophisticated anti-counterfeiting and information encryption applications.

Effect of Pr3+ concentration in luminescence properties & upconversion mechanism of triple doped NaYF4: Yb3+, Er3+, Pr3+

Lanthanide-doped fluoride nanocrystals (NCs) exhibit excellent optical features, including upconversion and downconversion luminescence (UCL and DCL), that can be utilized in a variety of applications. In this study, we have successfully demonstrated the photoluminescence behavior of triple-doped NaYF4: Yb3+, Er3+, Pr3+ NCs in the Vis-NIR region. Herein, highly monodisperse hexagonal phase NaYF4: Yb0.2, Er0.02, Prx nanocrystals in various Pr3+ (x = 0, 0.1, 0.5, and 1 mol %) concentration with ∼22 nm diameter synthesized by thermal decomposition technique. The photoluminescence studies for all samples were performed under 980 nm laser excitation. The luminescence intensity of Er3+ including blue (407 nm), green (520 and 540 nm), red (654 nm), and near-infrared (845 nm and 1530 nm) emissions was significantly quenched and Pr3+ emission intensity at 1290 nm (Pr3+:1G4→3H5) changes irregularly upon doping with Pr3+ ions. Furthermore, we performed the excitation power dependence and decay time analysis to investigate the energy transfer and upconversion mechanisms of samples. These findings indicate that the presence of praseodymium strongly reduces emission intensities due to abundant cross-relaxation channels. In addition, particle size is an efficient factor, shedding light on the influence of Pr3+ on the energy transfer and upconversion mechanisms of the fluorides.

Triplet-triplet annihilation photon upconversion-mediated photochemical reactions.

Photon upconversion is a method for harnessing high-energy excited states from low-energy photons. Such photons, particularly in the red and near-infrared wavelength ranges, can penetrate tissue deeply and undergo less competitive absorption in coloured reaction media, enhancing the efficiency of large-scale reactions and in vivo phototherapy. Among various upconversion methodologies, the organic-based triplet-triplet annihilation upconversion (TTA-UC) stands out - demonstrating high upconversion efficiencies, requiring low excitation power densities and featuring tunable absorption and emission wavelengths. These factors contribute to improved photochemical reactions for fields such as photoredox catalysis, photoactivation, 3D printing and immunotherapy. In this Review, we explore concepts and design principles of organic TTA-UC-mediated photochemical reactions, highlighting notable advancements in the field, as well as identify challenges and propose potential solutions. This Review sheds light on the potential of organic TTA-UC to advance beyond the traditional photochemical reactions and paves the way for research in various fields and clinical applications.

Development of excitation power-responsive anti-stokes emission wavelength switching and their energy saving induced by localized surface plasmon resonance

We designed an external stimulus-responsive anti-Stokes emission switching using dual-annihilator-based triplet–triplet annihilation upconversion systems. This system, which was constructed by incorporating a palladium porphyrin derivative as a sensitizer and 9,10-diphenylanthracene (DPA) and 9,10-bis(triisopropylsilyl)ethynylanthracene (TIPS) as annihilators into polymer thin films, produced TIPS- and DPA-based anti-Stokes emission under low and high excitation powers, respectively. The mechanism involves the following: under low excitation power, triplet energy transfer from triplet-excited PdOEP to DPA is induced, followed by relay to TIPS. This results in the generation of triplet-excited TIPS, and the subsequent triplet–triplet annihilation between them produces TIPS-based anti-Stokes emission. Conversely, under high excitation power, the high-density triplet-excited DPA, generated through triplet energy transfer from PdOEP, undergoes triplet–triplet annihilation among themselves, resulting in the generation of DPA-based anti-Stokes emission. Additionally, we achieved energy savings by reducing the required excitation power for switching through the utilization of plasmonic metal nanoparticles. The strong local electromagnetic fields associated with the localized surface plasmon resonance of metal nanoparticles enhance the photoexcitation efficiency of PdOEP, subsequently increasing the density of triplet-excited DPA. As a result, anti-Stokes emission switching becomes feasible at lower excitation powers.

Space–Charge‐Limited Photocurrent as a Possible Cause for Low Power Conversion Efficiency in GaInN/GaN‐Based Optoelectronic Semiconductors

This study attempts to understand the cause of low power conversion efficiency (PCE) in III‐nitride optoelectronic semiconductors under optical operation. For this purpose, a GaInN/GaN heterojunction semiconductor is fabricated, and the photoexcited current–voltage (PEJV) curves are carefully measured depending on the optical excitation power and temperature. The results show unexpected excitation power‐ and temperature‐dependent behaviors, that is, the PCE decreases with increasing excitation power and increases with increasing temperature. To understand this, the space–charge‐limited photocurrent (JPh,SCL) theory (also referred to as Goodman and Rose theory) is employed, where the accumulated charge carriers in the active layer play a significant role. The conduction of JPh,SCL is ascertained by analyzing the PEJV curves. The conduction of JPh,SCL is investigated as a possible cause of the low PCE, revealing that the conduction of JPh,SCL could limit the high‐power operation of the device.

Comparison of temperature and renal tissue thermal damage by holmium laser with different energy parameters during lithotripsy: in vitro porcine kidney model.

Holmium laser percutaneous nephrolithotripsy was simulated by porcine kidney calculus model in vitro to investigate thermal damage of renal tissue by different energy parameters of the holmium laser. We placed human kidney calculus specimen in fresh vitro porcine kidney, then insert thermocouple temperature probes into the submucosa of the renal pelvis and reheated in a 37°C water bath. A percutaneous nephrological sheath was used to penetrate the renal parenchyma with a moderate irrigation rate of 30ml/min at 18℃. The Holmium laser was used to fragment the stones under a nephroscope, and the temperature was recorded. The four independent models were lithotripsy with 30W and 60W laser for 5 and 10min, respectively; the mean temperature of 30W vs. 60W within 5min was 36.06°C vs. 39.21°C (t = 5.36, P < 0.01) and the highest temperature was 43.60°C vs. 46.60°C; the mean temperature of 30W vs. 60W within 10min was 37.91°C vs. 40.13℃ (t = 5.28, P < 0.01), maximum temperature 46.80℃ vs. 49.20℃. Pathologically, each kidney was observed to have different degrees of thermal damage lesions, and the higher power and longer time the more severe the injury, but the injury was mainly limited to the uroepithelial and subepithelial tissues, with rare damage to renal tubules. The higher laser excitation power and longer duration raised the intrarenal temperature significantly and caused a certain degree of thermal damage to the kidney tissue, but overall it was found to be safe and reliable. Urologists can avoid further side effects through surgical expertise.

Orientation Dependent Interlayer Coupling in Organic–Inorganic Heterostructures

AbstractOrganic–inorganic 2D heterostructures combine the high optical absorption of organic molecules with exciton‐dominated optical properties in layered transition metal dichalcogenides (TMDs) such as MoS2. Critical to the interaction and the optical response in such hybrid systems is the electronic band alignment at the interface between the two species. Here, the coupling of monolayers of perylene derivatives is investigated with bilayer MoS2. In particular, variation in the perylene orientation on the MoS2 surface is identified using Raman spectroscopy and scanning probe microscopy. Low‐temperature optical spectroscopy reveals orientation‐dependent interlayer exciton formation. Furthermore, power‐dependent photoluminescence measurements provide insight into the modified interlayer charge transfer in these heterostructures. A saturation of interlayer states is found under high excitation power when the perylene molecules are in a perpendicular orientation to the surface that leads to electron accumulation in the MoS2, whereas parallel alignment of the perylene molecules leads to enhanced populations of organic–inorganic interlayer excitons. This work provides insights into the optimization of organic–inorganic heterostructures, with particular relevance to applications for optoelectronic and excitonic devices.

Diagnosis of multiple gases using a multi-pass ring cavity to enhance Raman scattering

Spontaneous Raman scattering is widely used to enable simultaneous concentration measurements of all relevant species with overcomes whose signal is weak and susceptible to fluorescence interference. A multi-pass ring-cavity scheme is proposed to enhance Raman scattering signal. The Raman spectra of CO2, N2, O2 and H2O in the air are acquired under condition of low excitation power which is 200 mW laser of 532 nm. Compared with the results obtained by single collection without ring cavity, the Raman signal level is enhanced 40 times and the signal-to-noise ratio is improved by 43 times at maximum. Power of pump is taken into consideration to analyze its impact on signal. A limit of detection (3σ criterion standard) of 83 ppm for CO2 is calculated for the multi-pass ring-cavity. The limit of detection of 14 ppm for CH4 is also estimated by the theoretical ratio of cross section values of CO2 and CH4. When power of excitation is low, high level of signal of multi-species gases is acquired with fine detection sensitivity through ring cavity.

A Small-Signal Stability Analysis Method Based on Minimum Characteristic Locus and Its Application in Controller Parameter Tuning

A small-signal stability analysis method based on minimum characteristic locus (MCL) was recently proposed by the authors. The method is found to be particularly useful in analyzing various oscillation modes in a unified way. The present work continues this line of research. After providing a brief review of the basic principle of MCL method, the paper proceeds to show the mathematical relationship between the stability margin obtained using an MCL method, and that obtained by traditional modal analysis method. Next, a sensitivity analysis procedure is proposed to study quantitatively the effects of control parameters on the stability of the system. Third, the properties of the so-called exciter mode are explained in detail, in particular, explicit formulas of exciter mode frequency and power system stabilizer (PSS) critical gain are derived. Finally, taking PSS4B as an example, a controller parameter tuning method is proposed. The effectiveness and practicability of the proposed method are verified by case studies of Kundur's four-machine-two-area system and China's Yunnan power grid.

Design and realization of plasmonic nanostructures with large third-order optical nonlinearity

We designed an optical structure comprising plasmonic and dielectric materials that exhibits large third-order optical susceptibility and low-power consumption. Finite-difference time-domain simulation results were used to select tightly-packed gold nanosphere (AuNS) film, indium tin oxide (ITO) films, and gold (Au) thin film to construct AuNS/ITO/Au sandwiched nanostructures. Local-field enhancement factors up to 425 and large third-order optical susceptibility (χ(3)) with an amplitude order of 4.4 × 10−2 esu were achieved under an excitation power of 0.1 mW. These χ(3) results are exceptionally high, thereby rendering AuNS/ITO/Au a promising material for future applications in optical switches and related nonlinear optical devices.

Nonlinear erasing of propagating spin-wave pulses in thin-film Ga:YIG

Nonlinear phenomena are crucial for magnon-based information processing, but the nonlinear interaction between two spin-wave signals requires their spatiotemporal overlap, which can be challenging for directional processing devices. Our study focuses on a gallium-substituted yttrium iron garnet film, which exhibits an exchange-dominated dispersion relation and, thus, provides a particularly broad range of group velocities compared to pure YIG. Using time- and space-resolved Brillouin light scattering spectroscopy, we demonstrate the excitation of time-separated spin-wave pulses at different frequencies from the same source, where the delayed pulse catches up with the previously excited pulse and outruns it due to its higher group velocity. By varying the excitation power of the faster pulse, the outcome can be finely tuned from a linear superposition to a nonlinear interaction of both pulses, resulting in a full attenuation of the slower pulse. Therefore, our findings demonstrate the all-magnonic erasing process of a propagating magnonic signal, which enables the realization of complex temporal logic operations with potential application, e.g., in inhibitory neuromorphic functionalities.

Nonequilibrium spinor exciton-polariton condensates in a magnetic field

A complete-nonequilibrium model is proposed to investigate spinor exciton-polariton condensates (SEPCs) in a magnetic field. Due to the Zeeman splitting (ZS), the loss rates of two spin components in a magnetic field are distinct and affected by the magnetic field. Therefore, the individual spin component of a SEPC has its own threshold pump power. We investigate the ZS and degree of circular polarization of the SEPC as a function of the excitation power and magnetic field. Above both spin-dependent threshold pump powers, we find an increasing degree of circular polarization with the magnetic field and a quenching of the ZS below a critical strength of the magnetic field. The ZS of a SEPC shows a reverse sign and its magnitude is increasing significantly compared to the uncondensed exciton-polartion ZS. The effect of a magnetic field on excitation spectra of SEPCs are also investigated. We find that the excitation dispersions are Bogoliubov excitations if the pump power is low. The nonequilibrium fluctuations in a SEPC are reduced when a magnetic field is applied.

Radiative‐Recombination Thermodynamics in Boron‐Doped Diamond

AbstractBoron‐doped diamond is a wide‐bandgap semiconductor with excellent semiconductor properties, which is widely utilized in various applications. Till now, the thermodynamic rules governing the carrier transitions in this material have not been fully understood. In this research, the different luminescence behaviors of boron‐doped diamond under 193 nm pulse laser with high‐ and low‐ power density excitation are analyzed. Under high‐power density excitation (@≈63 kW cm−2), the emission intensity of excitons is nearly ten times higher than that of defect luminescence. Conversely, under low‐power density excitation (@≈1.4 kW cm−2, different emission peaks exhibit similar intensities, indicating a competitive relationship among them. Based on experimental and theoretical analyses, the difference is attributed to the thermodynamic distribution of carriers at varying excitation powers. Specifically, high excitation power brings an independent behavior of each emission peak, and the exciton emission is described by a phonon‐assisted radiation model; different from that, the competition among different emission centers under the condition of low excitation power cannot be neglected, and now the thermodynamic evolution of each emission peak is described by a carrier trapping‐dissociation model.

Scalable Bright and Pure Single Photon Sources by Droplet Epitaxy on InP Nanowire Arrays.

High-quality quantum light sources are crucial components for the implementation of practical and reliable quantum technologies. The persistent challenge, however, is the lack of scalable and deterministic single photon sources that can be synthesized reproducibly. Here, we present a combination of droplet epitaxy with selective area epitaxy to realize the deterministic growth of single quantum dots in nanowire arrays. By optimization of the single quantum dot growth and the nanowire cavity design, single emissions are effectively coupled with the dominant mode of the nanowires to realize Purcell enhancement. The resonance-enhanced quantum emitter system boasts a brightness of millions of counts per second with nanowatt excitation power, a short radiation lifetime of 350 ± 5 ps, and a high single-photon purity with g(2)(0) value of 0.05 with continuous wave above-band excitation. Finite-difference time-domain (FDTD) simulation results show that the emissions of single quantum dots are coupled into the TM01 mode of the nanowires, giving a Purcell factor ≈ 3. Our technology can be used for creating on-chip scalable single photon sources for future quantum technology applications including quantum networks, quantum computation, and quantum imaging.

A pulsed bipolar current-mode power supply with high power factor in a single stage for dielectric barrier discharge application

Purpose Dielectric barrier discharge (DBD) is widely used in the treatment of skin disease, surface modification of material and other fields of electronics. The purpose of this paper is to design a high-performance power supply with a compact structure for excimer lamps in electronics application. Design/methodology/approach To design a high-performance power supply with a compact structure remains a challenge for excimer lamps in electronics application, a current-source type power supply in a single stage with power factor correction (PFC) is proposed. It consists of an excitation voltage generation unit and a PFC unit. By planning the modes of the excitation voltage generation unit, a bipolar pulse excitation voltage with a high rising and falling rate is generated. And a high power factor (PF) on the AC side is achieved by the interaction of a non-controlled rectifier and two inductors. Findings The experimental results show that not only a high-frequency and high-voltage bipolar pulse excitation voltage with a high average rising and falling rate (7.51GV/s) is generated, but also a high PF (0.992) and a low total harmonic distortion (5.54%) is obtained. Besides, the soft-switching of all power switches is realized. Compared with the sinusoidal excitation power supply and the current-source power supply, the proposed power supply in this paper can take advantage of the potential of excimer lamps. Originality/value A new high-performance power supply with a compact structure for DBD type excimer lamps is proposed. The proposed power supply can work stably in a wide range of frequencies, and the smooth regulation of the discharge power of the excimer lamp can be achieved by changing the switching frequency. The ideal excitation can be generated, and the soft switching can be realized. These features make this power supply a key player in the outstanding performance of the DBD excimer lamps application.

Study of high-power breakdown accompanied by sliding discharge mode in a three-dimensional sliding arc plasma igniter

In order to improve the ignition reliability of a small-thrust rocket engine, a three-dimensional sliding arc igniter discharge characteristic research experimental system was constructed. The work gas flow rate and excitation parameters were changed, the plasma discharge characteristic and spectral characteristic parameters were measured, and the characteristics of the igniter discharge modes were investigated. The influence of the excitation parameters on the discharge modes was explored, and the effect of the discharge modes on the ignition performance of the igniter was analyzed as well as the mechanism of the discharge modes. The results show that the igniter has three different discharge modes during the discharge process, i.e., breakdown with sliding mode, high-power breakdown with sliding mode, and the transition state between the two modes, the shift of the discharge mode is mainly affected by the excitation power, and the igniter is in the breakdown with sliding mode when the excitation power is low, and in the high-power breakdown with sliding mode when the power is high. High-power breakdown with sliding mode has both breakdown with sliding and stable sliding characteristics in the early stage of the sliding arc movement for the breakdown with sliding and then transformed into sliding. Compared with the breakdown with sliding mode and high-power mode, the sliding arc discharge area is wider, the concentration of O * than the breakdown with sliding mode increased by 2.86 times, the electron temperature increased by 31.76%, and it can significantly improve plasma ignition effect, reduce the activation energy and enhance the performance of the igniter.

Electronic excitation of high-order spoof surface plasmons on metallic grating at terahertz frequencies

High-order spoof surface plasmon (SSP) mode on corrugated metallic surfaces can find many interesting applications, such as in imaging, sensing, transmission and enhanced radiation source, etc. In this paper, an efficient excitation method of the high-order SSP mode by using an injected electron beam on the uniform rectangular metallic grating is proposed and investigated numerically. Based on the matched wave momentum between the SSP mode and the electron beam, both the fundamental and high-order SSP modes can be excited on the structure by using a single injected electron beam. Numerical simulation results indicate that the maximum electric field intensity of the generated high-order SSP mode is about two orders higher than that of the fundamental SSP mode. In addition, the peak power of the excited high-order SSP mode is almost two times that of the fundamental SSP mode power by the same energy electron beam, which demonstrates the obvious advantage of the high-order SSP electronic excitation approach compared to the previous fundamental SSP mode excitation on the structure. The central working frequency of high-order SSP power spectrum is about three times that of the fundamental SSP power spectrum. Moreover, the influences of the injected electron beam energy on the excited SSP power spectrum are analyzed specifically. It is shown that the generated SSP power spectrum demonstrates a blue shift with the decreased working voltage of the electron beam simultaneously, with its peak power increasing. However, the working bandwidth is narrowed with decreased beam voltage, which further reveals its working mechanism of presented electronic excitation of the SSP mode. The presented studies provide a new method to excite a high-order SSP mode on the metallic grating, which can find some potential applications in high-sensitivity sensing, deep sub-wavelength waveguide, and many others in terahertz spectra.

An efficiently excited Eu3+ luminescent site formed in Eu,O-codoped GaN

For the development of III-nitride-semiconductor-based monolithic micro-light-emitting diode (LED) displays, Eu,O-codoped GaN (GaN:Eu,O) is a promising material candidate for the red LEDs. The luminescence efficiency of Eu-related emission strongly depends on the local atomic structure of Eu ions. Our previous research has revealed that post-growth thermal annealing is an effective method for reconfiguring luminescent sites, leading to a significant increase in light output. We observed the preferential formation of a site with a peak at ∼2.004 eV by the annealing process. In this study, we demonstrate that it is a previously unidentified independent site (OMVPE-X) using combined excitation–emission spectroscopy and time-resolved photoluminescence measurements. In addition, we perform excitation power-dependent photoluminescence measurements and show that this OMVPE-X site dominates the emission at a low excitation power region despite its small relative abundance, suggesting a high excitation efficiency. Most importantly, applying our annealing technique to an LED exhibits a reasonably increased electroluminescence intensity associated with OMVPE-X, confirming that this site has a high excitation efficiency also under current injection. These results demonstrate the importance of OMVPE-X as a notable luminescent site for brighter and more efficient GaN:Eu,O-based LEDs.