High Excitation Power Research Articles

Experimental and Numerical Studies of a Tunable Plasma Antenna Sustained by RF Power

In this work, a custom plasma antenna was created, in which the pressure and gas composition could be varied. The reflection coefficient, deposited power, and the antenna transmission and reception gains were measured for an argon plasma at 0.3 torr sustained by radio frequency power up to 150 W at an excitation frequency of 98 MHz. These measurements were performed through a single RF connector with the help of a directional coupler circuit that isolated the high-power excitation signal from the low-power probing ones. It was found that the resonant frequency of this plasma antenna could be tuned over a span of 80 MHz by varying the applied power in the range of 10–150 W. The antenna gain could also be varied with applied power. At power levels exceeding 40 W, the gain approached, within 3 dB, that of a standard reference antenna. The experimental setup was modeled using ANSYS HFSS software. The simulation and experimental results were found to be in good agreement. This study shows the viability of creating tunable plasma antennas with gain comparable to that of metallic antennas.

Heat Dissipation Enhancement of Phosphor-Converted White Laser Diodes by Thermally Self-Managing Phosphor-in-Glass

Laser-driven white lighting has been considered as a next-generation high-brightness white lighting source. Thermally stable color converter is an essential down-conversion material for phosphor-converted white laser diodes (pc-wLDs), which inevitably endures high-density laser excitation and huge heat shock from conversion loss. In this work, we proposed a thermally self-managing phosphor-in-glass (PiG) converter to enhance the heat dissipation of reflective pc-wLDs. The PiG converter was prepared by printing and sintering a phosphor glass film on the cold side of thermoelectric cooler (TEC). By controlling the input current of TEC, the heat generated from phosphors under high-power laser excitation was effectively and initiatively dissipated into the ambient environment due to the Peltier effect of TE materials. As the laser power (LP) increases from 0.73 to 5.6 W, the working temperature of PiG film slowly increases from 2.3°C to 6.1°C. The maximum temperature of PiG film is reduced by 61.1 °(~91%) at the LP of 5.6 W. Furthermore, the PiG-TEC-based pc-wLD achieves a cool white light and stable light quality under various LPs. At the LP of 4.5 W, the correlated color temperature (CCT) of pc-wLD is 6449 K, and the chromaticity coordinate of pc-wLD is (0.3139, 0.3274). The results indicate that the proposed PiG-TEC converter enhances the thermal performance of high-brightness laser-driven white lighting.

Encapsulation Narrows and Preserves the Excitonic Homogeneous Linewidth of Exfoliated Monolayer MoSe2

The excitonic homogeneous linewidth of an exfoliated monolayer MoSe$_2$ encapsulated in hexagonal boron nitride is directly measured using multidimensional coherent spectroscopy with micron spatial resolution. The linewidth is 0.26 $\pm$ 0.02 meV, corresponding to a dephasing time $T_2 \approx$ 2.5 ps, which is almost half the narrowest reported values for non-encapsulated MoSe$_2$ flakes. We attribute the narrowed linewidth to Coulomb screening by the encapsulated material and suppression of non-radiative processes. Through direct measurements of encapsulated and non-encapsulated monolayers, we confirm that encapsulation reduces the sample inhomogeneity. However, linewidths measured using photoluminescence and linear absorption remain dominated by inhomogeneity, and these linewidths are roughly 5 times larger than the homogeneous linewidth in even the highest-quality encapsulated materials. The homogeneous linewidth of non-encapsulated monolayers is very sensitive to temperature cycling, whereas encapsulated samples are not modified by temperature cycling. The nonlinear signal intensity of non-encapsulated monolayers is degraded by high-power optical excitation, whereas encapsulated samples are very resilient to optical excitation with optical powers up to the point of completely bleaching the exciton.

Ternary solid solution phosphors Ca1-x-yLixAl1-x-ySi1+x+yN3-yOy:Ce3+ with enhanced thermal stability for high-power laser lighting

The discovery of thermally robust phosphors is essential to achieve efficient and stable luminescence in solid-state lighting devices, especial in high-power devices. Here, a novel ternary solid solution phosphor enabling to address this challenge is designed by introducing isostructural LiSi2N3 and Si2N2O into CaAlSiN3 lattice. The compound, Ca1-x-yLixAl1-x-ySi1+x+yN3-yOy:Ce3+ (CALSON:Ce), exhibits an excellent thermal stability with an integrated emission loss of only 9% at 200 °C, far smaller than that of the parent CaAlSiN3:Ce3+ (25%). The enhanced thermal stability is assigned to the broadened band gap (5.21 → 5.50 eV) upon the formation of solid solutions. Moreover, the new phosphor presents a broadband (full width at half maximum of 135 nm) yellow-orange emission peaking at 580 nm and shows a high external quantum efficiency of 41.5%. The excellent photoluminescence properties and thermal stability of CALSON:Ce thus enable it to be used under high-power laser excitation, and to produce high-brightness white light having a luminous flux of 392 lm (luminous emittance of 781 lm mm−2), a low correlated color temperature of 4349 K, and a high color rendering index of 70. This work suggests that creating multi-component solid solutions would be a general strategy for designing novel phosphors.

Inhibition of Phase Segregation in Cesium Lead Mixed-Halide Perovskites by B-Site Doping.

SummaryThe emergence of all-inorganic halide perovskites has shown great potential in photovoltaic and optoelectronic devices. However, the photo-induced phase segregation in lead mixed-halide perovskites has severely limited their application. Herein, by real-time monitoring the photoluminescence (PL) spectra of metal mixed-halide perovskites under light irradiation, we found that the photo-induced phase transition can be significantly inhibited by B-site doping. For pristine mixed-halide perovskites, an intermediate phase of CsPbBrxI3-x can only be stabilized under low excitation power. After introducing Sn2+ ions, such intermediate phase can be stabilized in nitrogen atmosphere under high excitation power and phase segregation can be started after the exposure in oxygen due to oxidization of Sn2+. Replacing Sn2+ by Mn2+ can further improve the intermediate phase's tolerance to oxygen proving that B-site doping in perovskites structure by Sn2+ or Mn2+ could effectively minimize the light-induced phase segregation and promote them to serve as promising candidates in photovoltaic and light-emitting devices.

High-Brightness Red-Emitting Phosphor La3(Si,Al)6(O,N)11:Ce3+ for Next-Generation Solid-State Light Sources.

A novel high-brightness red-emitting phosphor, La3(Si,Al)6(O,N)11:Ce3+ (LSA), which can potentially be used as a laser-excited light source, is demonstrated. Laser-excited phosphor system has great potential for use as a white-light source, as it is orders of magnitude brighter than white LEDs. Although conventional yellow-green phosphors show excellent luminescent properties even under high-power laser excitation, red-emitting phosphors, which are essential to achieve a high color-rendering index and low color-temperature, show quantum efficiency quenching. This limits the output power in multiphosphor excitation systems. Ce3+ should successfully tolerate high-power excitation due to the shortest emission lifetime seen in rare-earth ions, caused by the 5d1-4f1 spin-allowed transition; however, a red-emitting Ce3+-doped phosphor of practical use has not been realized. LSA is described by the crystal-field modification of a yellow-emitting phosphor, La3Si6N11:Ce3+, with substitution of Al in Si sites. LSA shows 640 nm red emission together with tolerance for high-power excitation and thermal quenching, suggesting its significant potential for industrial applications that require ultrahigh brightness.

Multiphysics Structured Eddy Current and Thermography Defects Diagnostics System in Moving Mode

Eddy current testing (ET) and eddy current thermography (ECT) are both important nondestructive testing methods that have been widely used in the field of conductive materials evaluation. Conventional ECT systems have often employed to test static specimens even though they are inefficient when the specimen is large. In addition, the requirement of high-power excitation sources tends to result in bulky detection systems. To mitigate these problems, in this article, a moving detection mode of multiphysics structured ET and ECT is proposed in which a novel L-shape ferrite magnetic yoke circumambulated with array coils is designed. The theoretical derivation model of the proposed method is developed which is shown to improve the detection efficiency without compromising the excitation current by ECT. The specimens can be speedily evaluated by scanning at a speed of 50–250 mm/s while reducing the power of the excitation current due to the supplement of ET. The unique design of the excitation-receiving structure has also enhanced the detectability of omnidirectional cracks. Moreover, it does not block the normal direction visual capture of the specimens. Both numerical simulations and experimental studies on different defects have been carried out and the obtained results have shown the reliability and detection efficiency of the proposed system.

Photoswitchable Photon Upconversion from Turn-on Mode Fluorescent Diarylethenes

Photon upconversion (UC) is one kind of anti-Stokes shift process, which generally requires a combination of two or more low-energy photons to produce one high-energy photon. However, another inter...

Excitation power dependent Coulomb induced recombination dynamics in magnetically doped type-II quantum dots

We observe that the wavefunction overlap of the carriers in type-II quantum dots (QDs) can be controlled by magnetic doping and strongly depends on the excitation power density. We study two different II-VI magnetic systems; ZnTe/(Zn, Mn)Se QDs with magnetic dopants in the matrix surrounding the dots, and (Zn, Mn)Te/ZnSe QDs doped in the dot core. Both magnetic systems, regardless of the location of the dopant magnetic ions, show a stark contrast in their emission with high excitation power densities (Pex) when compared to nonmagnetic ZnTe/ZnSe QDs. Using time-resolved photoluminescence (TRPL), we observe a saturation in the blue shift for the magnetic systems at a lower Pex, while additionally exhibiting a limited lifetime shortening over the entire range of Pex, when compared to the nonmagnetic QDs. The results for the two magnetic systems are very similar, showing no dependence on the location of the magnetic impurities. This suggests that the behavior observed is an effect of the magnetic polaron on the band bending in the high Pex regime. The ability to use magnetic ions to quickly saturate the charge concentration and control band bending in QDs could potentially aid in optimizing optoelectronic devices which are sensitive to high charge variations.

Dipolar interactions between localized interlayer excitons in van der Waals heterostructures.

Although photons in free space barely interact, matter can mediate interactions between them resulting in optical nonlinearities. Such interactions at the single-quantum level result in an on-site photon repulsion, crucial for photon-based quantum information processing and for realizing strongly interacting many-body states of light. Here, we report repulsive dipole-dipole interactions between electric field-tuneable, localized interlayer excitons in the MoSe2/WSe2 heterobilayer. The presence of a single, localized exciton with an out-of-plane, non-oscillating dipole moment increases the energy of the second excitation by ~2 meV-an order of magnitude larger than the emission linewidth and corresponding to an inter-dipole distance of ~7 nm. At higher excitation power, multi-exciton complexes appear at systematically higher energies. The magnetic field dependence of the emission polarization is consistent with the spin-valley singlet nature of the dipolar molecular state. Our finding represents a step towards the creation of excitonic few- and many-body states such as dipolar crystals with spin-valley spinor in van der Waals heterostructures.

Modeling and Verification of a Parasitic Nonlinear Energy Storage Effect Due To High-Power Electromagnetic Excitation

The analysis of nonlinear effects in electromagnetic compatibility can reveal unexpected system behavior. This article examines the response of nonlinearly loaded receiving structures with respect to high-power electromagnetic pulses. It is observed that induced voltages may exhibit dc components, which are retained after the decay of the usual transient oscillations. These dc components are attributed to stored electric field energy and can considerably be increased by the application of several pulses. This a priori surprising effect can both be predicted by modeling and confirmed by measurement.

Photoluminescence properties of N–B Co-doped fluorescent SiC

N–B co-doped SiC, as an excellent candidate for white LEDs, were prepared by the physical vapor transport growth method. Doping type and concentrations of fluorescent SiC crystals were examined by Raman spectroscopy and secondary ion mass spectroscopy measurements. The band-to-band excitation mechanism of donor and acceptor pairs (DAPs) luminescence in f-SiC has been clarified based on excitation spectroscopic measurements. The luminescence intensity of DAPs decreases with the increase of excitation source energy due to the absorption when excitation source energy is greater than 3.7 eV. Moreover, the threshold energy and the optimal excitation wavelength did not change with the carrier concentration, while the photoluminescence emission intensity increased with the decrease of carrier concentration. When the excitation power increased from 2 to 44 mW, the luminescence intensity can be well fitted by the power dependence I = aPγ, the higher the B doping concentration, that is, the lower the carrier concentration, the intensity is more sensitive to the excitation power change. At same time, the luminescence peaks of high doping concentration samples shift to the high energy with high excitation power. The shifts and the underlying mechanism have been discussed.

Enhancing the generating and collecting efficiency of single particle upconverting luminescence at low power excitation

Abstract Upconverting luminescent nanoparticles are photostable, nonblinking, and low chemically toxic fluorophores that are emerging as promising fluorescent probes at the single molecule level. High luminescence intensity upconversion nanoparticles (UCNPs) have previously been achieved by doping with high amounts of rare-earth ions using high excitation power (>2.5 MW/cm2). However, such particles are inadequate for in vitro live-cell imaging and single-particle tracking, as high excitation power can cause photodamage. Here, we compared UCNP luminescence intensities with different dopant concentrations and presented more efficient (about seven times) UCNPs at low excitation power by increasing the concentrations of Yb3+ and Tm3+ dopants (NaYF4: 60% Yb3+, 8% Tm3+) and adding a core-shell structure.

High quantum efficiency Ce:(Lu,Y)3(Al,Sc)2Al3O12 transparent ceramics with excellent thermal stability for high-power white LEDs/LDs

Transparent ceramics (TCs) with high quantum efficiency and excellent thermal stability are essential for the applications in high-power remote excitation white light-emitting diode or laser diode (LED/LD) lighting devices as color convertors.

Laser on erbie glass with diode pumping

The results of studies aimed at the development and creation of an erbium-diode-laser laser with diode pumping and providing an output energy of several millijoules in pulses of 20-30 ns duration at a repetition rate of 0.5-1.0 hertz are presented.The interest around this laser is due to the fact that its radiation falls into the spectral region, which is relatively safe for vision. In this spectral region there is a transparency window in the earth's atmosphere. Thus, half-micron radiance can be used efficiently for location aims and rangefinder. This is facilitated by the presence of highly sensitive uncooled photo detectors. In addition, the spectral region of 1.5-1.6 μm matches with the region of maximum transparency of quartz optical fibers used in fiber-optic communication lines.The experimental sample of ytterbium-erbium laser emitting at a wave length of 1, 54 μm was designed and manufactured. Pumping of active element was done according to the transverse scheme by two laser diode rulers with a power of 100 W each, which has been put in line. The selected pump scheme provides a high excitation power and eliminates the occurrence of a temperature gradient in the active element. To increase the pumping efficiency, a cylindrical active laser element was placed in a sapphire cylinder, the outer surface of which had a reflective coating. During free generation regime, we implemented the optimization of laser characteristics, depending on the Q-factor of resonator and excitation conditions. To obtain a giant impulse, a passive Q-switch based on aluminum-magnesium spinel with cobalt was used in the work. In the Q-switched mode, single pulses with the energy of 6 mJ were obtained with a duration of less than 30 ns at a pulse repetition rate of one hertz.

Live‐Cell Localization Microscopy with a Fluorogenic and Self‐Blinking Tetrazine Probe

AbstractRecent developments in fluorescence microscopy call for novel small‐molecule‐based labels with multiple functionalities to satisfy different experimental requirements. A current limitation in the advancement of live‐cell single‐molecule localization microscopy is the high excitation power required to induce blinking. This is in marked contrast to the minimal phototoxicity required in live‐cell experiments. At the same time, quality of super‐resolution imaging depends on high label specificity, making removal of excess dye essential. Approaching both hurdles, we present the design and synthesis of a small‐molecule label comprising both fluorogenic and self‐blinking features. Bioorthogonal click chemistry ensures fast and highly selective attachment onto a variety of biomolecular targets. Along with spectroscopic characterization, we demonstrate that the probe improves quality and conditions for regular and single‐molecule localization microscopy on live‐cell samples.

Live-Cell Localization Microscopy with a Fluorogenic and Self-Blinking Tetrazine Probe.

Recent developments in fluorescence microscopy call for novel small‐molecule‐based labels with multiple functionalities to satisfy different experimental requirements. A current limitation in the advancement of live‐cell single‐molecule localization microscopy is the high excitation power required to induce blinking. This is in marked contrast to the minimal phototoxicity required in live‐cell experiments. At the same time, quality of super‐resolution imaging depends on high label specificity, making removal of excess dye essential. Approaching both hurdles, we present the design and synthesis of a small‐molecule label comprising both fluorogenic and self‐blinking features. Bioorthogonal click chemistry ensures fast and highly selective attachment onto a variety of biomolecular targets. Along with spectroscopic characterization, we demonstrate that the probe improves quality and conditions for regular and single‐molecule localization microscopy on live‐cell samples.

Azobenzenes as Light-Activable Carrier Density Switches in Nanocrystals

Control of carrier density in colloidal quantum dots is a major challenge for their integration into optoelectronic devices. Several chemical methods have been proposed to reach this goal including introduction of impurities, nonstoichiometric compounds, introduction of redox molecules as ligands, and surface gating obtained by tuning the dipole associated with surface ligands. None of these techniques provide post synthesis tunability. Alternatively, optical pumping requires high excitation power which may heat and finally damage the sample. Here, we propose a new procedure based on the grafting of azobenzenes (AZBs) on the nanocrystal surface. The AZBs have two conformations (cis and trans), which are associated with strongly different dipole moments. The transition from one conformation to the other can be activated using UV or visible light at low intensities (<100 mW·cm-2). Grafting the AZBs on the nanocrystal surface leads to a light-tunable surface dipole, which shifts the nanocrystal bands and leads to a tunable carrier density. We apply this method to p-type HgTe and degenerately n-doped HgSe nanocrystals. We demonstrate, thanks to transport measurements, a change of the carrier density corresponding to a band shift up to 40 meV.

Ultrafast and low-power optoelectronic infrared-to-visible upconversion devices

Photon upconversion with transformation of low-energy photons to high-energy photons has been widely studied and especially applied in biomedicine for sensing, stimulation, and imaging. Conventional upconversion materials rely on nonlinear luminescence processes, suffering from long decay lifetime or high excitation power. Here, we present a microscale, optoelectronic infrared-to-visible upconversion device design that can be excited at low power (1–100&#x2009;&#x2009;mW/cm2). By manipulating device geometry, illumination position, and temperature, the device luminescence decay lifetime can be tuned from tens to hundreds of nanoseconds. Based on carrier transportation and circuit dynamics, theoretical models are established to understand the transient behaviors. Compared with other mechanisms, the optoelectronic upconversion approach demonstrates the shortest luminescence lifetime with the lowest required excitation power, owing to its unique photon&#x2013;electron conversion process. These features are expected to empower the device with essential capabilities for versatile applications as high-performance light emitters.

Highly-efficient upconversion via direct one-photon absorption of xanthene-based chromophores

A series of xanthene-based chromophores, including rhodamines (Rh6G, RhA, RhB and RhBS) and luciferin derivatives (fluorescein, eosin and rose bengal), were firstly reported to obtain the structure-performance correlations in the upconversion via one-photon absorption (OPA-UC). These xanthene derivatives capped with different aryl groups contain the varying tautomeric donor-acceptor pairs at both ends. Electron cloud density calculations show that attaching different tautomeric donor-acceptor pairs and aryl group can change the intramolecular charge-transfer (ICT) strength and the restricted intramolecular rotation (RIR) degree. Under the excitation of CW diode laser (655 nm, < 2 W cm−2), all the chromophore solutions (5 × 10−5 M) radiate yellow upconversion (UC), but the UC efficiency (ΦUC) showed a difference of more than two orders of magnitude, ranging from 7.56% for RhBS to 0.01% for fluorescein. The results show that strong ICT and large RIR lead to highly-efficient OPA-UC. Finally, the OPA-UC microscopy imaging and OPA-UC detection were briefly investigated under the conditions of neither deaerating nor the high-power excitation light, suggesting that OPA-UC of this system possesses practical applications.