Next-generation Light Sources Research Articles

Denoising an X-ray image by exploring the power of its physical symmetry

Next-generation light source facilities offer extreme spatial and temporal resolving power, enabling multiscale, ultra-fast and dynamic characterizations. However, a trade-off between acquisition efficiency and data quality needs to be made to fully unleash the resolving potential, for which purpose powerful denoising algorithms to improve the signal-to-noise ratio of the acquired X-ray images are desirable. Yet, existing models based on machine learning mostly require massive and diverse labeled training data. Here we introduce a self-supervised pre-training algorithm with blind denoising capability by exploring the intrinsic physical symmetry of X-ray patterns without requiring high signal-to-noise ratio reference data. The algorithm is more efficient and effective than algorithms without symmetry involved, including an supervised algorithm. It allows us to recover physical information from spatially and temporally resolved data acquired in X-ray diffraction/scattering and pair distribution function experiments, where pattern symmetry is often well preserved. This study facilitates photon-hungry experiments as well as in situ experiments with dynamic loading.

Translucent ceramic enabling next-generation lasing-driven near-infrared light source

Translucent ceramic enabling next-generation lasing-driven near-infrared light source

Spatial and time-resolved properties of emission enhancement in polar/semi-polar InGaN/GaN by surface plasmon resonance

Abstract Light-emitting diodes (LEDs) are widely used as next-generation light sources because of their various advantages. However, their luminous efficiency is remarkably low at the green-emission wavelength. The luminous efficiencies of InGaN/GaN quantum wells (QWs) significantly decrease with increasing indium content in the green wavelength region, mainly owing to the quantum-confined Stark effect (QCSE). This green gap problem can be solved using QWs grown on semi-polar GaN substrates, such as the {11–22} planes, to reduce the QCSE. We propose that the use of surface plasmons (SPs) is a promising way to improve the light emission efficiency of light-emitting materials such as InGaN/GaN QWs. SP resonance increases the spontaneous emission rates of the excited states, causes a relative reduction in non-radiative relaxation, and ultimately increases the internal quantum efficiencies. In this study, the light emissions of InGaN/GaN QWs grown on polar and semi-polar GaN were investigated using micro-photoluminescence (PL). We successfully enhanced the light emission of semi-polar GaN via SP resonance. The PL peak intensities and wavelengths were mapped and compared to determine the underlying mechanisms. We also measured the emission lifetimes by time-resolved PL and interpreted the detailed mechanism of SP-enhanced emissions. It was found that SP resonances can control not only the emission efficiency but also the exciton dynamics, such as exciton localization effects, QCSE screening, and defect level saturation. We conclude that the green gap problem can be solved by SP-enhanced light emission in semipolar InGaN/GaN.

Hidden imaging in thin polymer films with embedded fluorescent peptide nanodots.

Fluorescent (FL) encrypting nanostructures, such as quantum dots, carbon dots, organic dyes, lanthanide nanocrystals, DNA, and more, are effective tools for advanced applications in high-resolution hidden imaging. These applications include tracking, labeling, security printing, and anti-counterfeiting drug technology. In this work, what we believe to be a new FL encoding nanostructures has been proposed, which consists of recently discovered nanometer-scale peptide dots. When refolded into a beta-sheet peptide secondary structure, these biocompatible nanoparticles exhibit a strong and tunable FL effect. The biophotonic FL covers the entire visible spectrum, making the peptide dots next-generation nanoscale light sources with a quantum yield of 30%. Our studies demonstrate that these FL bio-nanodots also exhibit a significant irreversible photo-bleaching effect associated with the light-induced destruction of noncovalent intermolecular hydrogen bonds of the peptide dots' highly stable beta-sheet secondary structure. We present what we believe is a new approach for achieving high-resolution long-term optical memory by tailoring various hidden images in the developed thin polyvinyl alcohol (PVA) polymer films with an embedded dense array of FL peptide nanodots. The technology enables recording photo-bleached patterns, barcodes, and high-resolution images.

Light-Emitting Plasmonic Tunneling Junctions: Current Status and Perspectives.

Quantum tunneling, in which electrons can tunnel through a finite potential barrier while simultaneously interacting with other matter excitation, is one of the most fascinating phenomena without classical correspondence. In an extremely thin metallic nanogap, the deep-subwavelength-confined plasmon modes can be directly excited by the inelastically tunneling electrons driven by an externally applied voltage. Light emission via inelastic tunneling possesses a great potential application for next-generation light sources, with great superiority of ultracompact integration, large bandwidth, and ultrafast response. In this Perspective, we first briefly introduce the mechanism of plasmon generation in the inelastic electron tunneling process. Then the state of the art in plasmonic tunneling junctions will be reviewed, particularly emphasizing efficiency improvement, precise construction, active control, and electrically driven optical antenna integration. Ultimately, we forecast some promising and critical prospects that require further investigation.

Plasma-based particle sources

High-brightness particle beams generated by advanced accelerator concepts have the potential to become an essential part of future accelerator technology. In particular, high-gradient accelerators can generate and rapidly accelerate particle beams to relativistic energies. The rapid acceleration and strong confining fields can minimize irreversible detrimental effects to the beam brightness that occur at low beam energies, such as emittance growth or pulse elongation caused by space charge forces. Due to the high accelerating gradients, these novel accelerators are also significantly more compact than conventional technology. Advanced accelerators can be extremely variable and are capable of generating particle beams with vastly different properties using the same driver and setup with only modest changes to the interaction parameters. So far, efforts have mainly been focused on the generation of electron beams, but there are concepts to extend the sources to generate spin-polarized electron beams or positron beams.The beam parameters of these particle sources are largely determined by the injection and subsequent acceleration processes. Although, over the last decade there has been significant progress, the sources are still lacking a sufficiently high 6-dimensional (D) phase-space density that includes small transverse emittance, small energy spread and high charge, and operation at high repetition rate. This is required for future particle colliders with a sufficiently high luminosity or for more near-term applications, such as enabling the operation of free-electron lasers (FELs) in the X-ray regime. Major research and development efforts are required to address these limitations in order to realize these approaches for a front-end injector for a future collider or next-generation light sources. In particular, this includes methods to control and manipulate the phase-space and spin degrees-of-freedom of ultrashort plasma-based electron bunches with high accuracy, and methods that increase efficiency and repetition rate. These efforts also include the development of high-resolution diagnostics, such as full 6D phase-space measurements, beam polarimetry and high-fidelity simulation tools.A further increase in beam luminosity can be achieve through emittance damping. Emittance cooling via the emission of synchrotron radiation using current technology requires kilometer-scale damping rings. For future colliders, the damping rings might be replaced by a substantially more compact plasma-based approach. Here, plasma wigglers with significantly stronger magnetic fields are used instead of permanent-magnet based wigglers to achieve similar damping performance but over a two orders of magnitude reduced length.

Prediction of Emission Level Energy of Mn4+ in Oxides Based on the Local Structure Using First-Principles Calculations and Machine Learning

The white LED is an important next-generation light source. Most of the high color rendering white LEDs that have become popular in recent years consist of blue LEDs combined with green and red phosphors. For example, Eu2+:SrxCa1-xAlSiN3 (SCASN) and Mn4+:K2SiF6 (KSF) are used as red phosphors. However, considering the production cost and the stability against high temperature and high humidity, novel red phosphors obtained by adding Mn4+ to oxide crystals are still desired. To realize a Mn-doped oxide red phosphor for white LEDs, it is indispensable to clarify the relationship between the multiplet energies and the local structures. Semi-empirical methods based on experimental data are generally used to analyze luminescent materials, but these methods cannot be applied to unknown or hypothetical materials. In this work, we analyzed the relationship between the local structure and the energy levels of hypothetical Mn4+-doped oxides by performing first-principles electronic structure calculations using the discrete-variational multi-electron (DVME) method. The local structures around Mn4+ were systematically changed by changing the bond distances and angles and the multiplet energies corresponding to each local structure were calculated non-empirically. Then the relationship between the energy and the local structure was visually represented by creating energy-structure maps. Using these systematic theoretical values as the training data, a machine learning model for the prediction of the first-principles calculation results based only on the structural parameters was also created. The results of this study would provide guidelines for the development of novel phosphors based on oxide crystals doped with Mn4+.

Dephasingless laser wakefield acceleration in the bubble regime

Laser wakefield accelerators (LWFAs) have electric fields that are orders of magnitude larger than those of conventional accelerators, promising an attractive, small-scale alternative for next-generation light sources and lepton colliders. The maximum energy gain in a single-stage LWFA is limited by dephasing, which occurs when the trapped particles outrun the accelerating phase of the wakefield. Here, we demonstrate that a single space–time structured laser pulse can be used for ionization injection and electron acceleration over many dephasing lengths in the bubble regime. Simulations of a dephasingless laser wakefield accelerator driven by a 6.2-J laser pulse show 25 pC of injected charge accelerated over 20 dephasing lengths (1.3 cm) to a maximum energy of 2.1 GeV. The space–time structured laser pulse features an ultrashort, programmable-trajectory focus. Accelerating the focus, reducing the focused spot-size variation, and mitigating unwanted self-focusing stabilize the electron acceleration, which improves beam quality and leads to projected energy gains of 125 GeV in a single, sub-meter stage driven by a 500-J pulse.

First Light for a Next-Generation Light Source

First Light for a Next-Generation Light Source

Advanced manufacturing of microscale light-emitting diodes and their use in displays and biomedicine

Thin-film, microscale light-emitting diodes (micro-LEDs)-based III–V compound semiconductors are emerging as a promising technology for next-generation light sources with various applications including lighting, display and biomedicine. With this perspective, we provide an overview and highlight our own progress on the recent developments in advanced manufacturing techniques and potential applications of micro-LEDs. The combination of epitaxial lift-off and transfer printing techniques have produced ultraminiaturized, highly luminous, and energy-efficient thin-film micro-LEDs, with a wide range of applications. To achieve full-color displays, several approaches have been developed, including horizontally and vertically assembled red-green–blue (RGB) pixels, and quantum dot conversion. For neuroscience studies, these micro-LEDs can be integrated in flexible probes as implantable light sources, which facilitate light delivery into the deep brain tissue for optogenetic stimulations and biological sensing. Additionally, micro-LEDs have the potential to function as wireless biosensors to detect changes in the biological environment, through their luminescence or in combination with other devices. Overall, advanced manufacturing techniques produce micro-LEDs that promise significant applications not only in displays but also in various biomedical fields.

Luminescent Silicon Nanosheet Paracrystals from Elemental- and Hydride-Based Syntheses of CaSi2 Precursors: Implications for Photonic and Optoelectronic Applications

Layered silicon nanosheets (SiNSs) have attracted considerable attention owing to their unique combination of chemical and physical properties, which makes them an exciting candidate for next-generation on-chip light sources and lasers. Despite over 150 years of research on SiNSs, the effects of the CaSi2 precursor quality on SiNSs have not been studied. Here, we report a comparison of CaSi2 (and SiNSs derived therefrom) synthesized from two reaction pathways: (1) melting Ca and Si (elemental melting, or EM-CaSi2) and (2) the less-explored reaction between CaH2 and Si (hydride synthesis, or HS-CaSi2). We demonstrate that both reaction pathways lead to CaSi2, but the HS-CaSi2 pathway requires only a single step without the need to melt the CaSi2 product and at a temperature below the peritectic decomposition of CaSi2. We find that the EM-CaSi2 exhibits grains that lay flat against the substrate, whereas the HS-CaSi2 has little preferred orientation. We deintercalated both EM- and HS-CaSi2 with HCl at −35 °C to yield hydrogen-terminated SiNSs. We characterized the SiNSs and found that the HS-SiNSs and EM-SiNSs exhibit properties that are nearly identical, with the exception that the morphology of the precursor is imparted to the SiNSs. These results provide the community with a one-step method to synthesize CaSi2 and demonstrate that the morphology of CaSi2 and SiNSs can be controlled with different synthetic techniques.

Optimization of transformer ratio and beam loading in a plasma wakefield accelerator with a structure-exploiting algorithm

Plasma-based acceleration has emerged as a promising candidate as an accelerator technology for a future linear collider or a next-generation light source. We consider the plasma wakefield accelerator (PWFA) concept where a plasma wave wake is excited by a particle beam and a trailing beam surfs on the wake. For a linear collider, the energy transfer efficiency from the drive beam to the wake and from the wake to the trailing beam must be large, while the emittance and energy spread of the trailing bunch must be preserved. One way to simultaneously achieve this when accelerating electrons is to use longitudinally shaped bunches and nonlinear wakes. In the linear regime, there is an analytical formalism to obtain the optimal shapes. In the nonlinear regime, however, the optimal shape of the driver to maximize the energy transfer efficiency cannot be precisely obtained because currently no theory describes the wake structure and excitation process for all degrees of nonlinearity. In addition, the ion channel radius is not well defined at the front of the wake where the plasma electrons are not fully blown out by the drive beam. We present results using a novel optimization method to effectively determine a current profile for the drive and trailing beam in PWFA that provides low energy spread, low emittance, and high acceleration efficiency. We parameterize the longitudinal beam current profile as a piecewise-linear function and define optimization objectives. For the trailing beam, the algorithm converges quickly to a nearly inverse trapezoidal trailing beam current profile similar to that predicted by the ultrarelativistic limit of the nonlinear wakefield theory. For the drive beam, the beam profile found by the optimization in the nonlinear regime that maximizes the transformer ratio also resembles that predicted by linear theory. The current profiles found from the optimization method provide higher transformer ratios compared with the linear ramp predicted by the relativistic limit of the nonlinear theory.

Achieving High Quantum Efficiency Broadband NIR Mg4 Ta2 O9 :Cr3+ Phosphor Through Lithium-Ion Compensation.

Ultra-efficient broadband near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) are urgently needed to improve the detection sensitivity and spatial resolution of current smart NIR spectroscopy-based techniques. Nonetheless, the performance of NIR pc-LED has severely limited owing to the external quantum efficiency (EQE) bottleneck of NIR light-emitting materials. Herein, a blue LED excitable Cr3+ -doped tetramagnesium ditantalate (Mg4 Ta2 O9 , MT) phosphor is advantageously modified through lithium ion as a key efficient broadband NIR emitter to achieve high optical output power of the NIR light source. The emission spectrum encompasses the 700-1300nm electromagnetic spectrum of first biological window (λmax = 842nm) with a full-width at half-maximum (FWHM) of ≈2280 cm-1 (≈167nm), and achieves a record EQE of 61.25% detected at 450nm excitation through Li-ion compensation. A prototype NIR pc-LED is fabricated with MT:Cr3+ , Li+ to evaluate its potential practical application, which reveals an NIR output power of 53.22mW at a driving current of 100mA, and a photoelectric conversion efficiency of 25.09% at 10mA. This work provides an ultra-efficient broadband NIR luminescent material, which shows great promise in practical applications and presents a novel option for the next-generation high-power compact NIR light sources.

K2LiScF6:Cr3+/Mn4+ fluoride phosphor with enhanced broadband emission for NIR pc-LEDs.

Trivalent chromium ion-activated broadband near-infrared (NIR) luminescence materials have shown great application prospects as next-generation NIR light sources, but improvement of the luminescence efficiency remains a challenge. Herein, novel K2LiScF6:Cr3+ and K2LiScF6:Cr3+/Mn4+ broadband fluoride NIR phosphors are designed and prepared by a combination of hydrothermal and cation exchange methods for the first time. The crystal structure and photoluminescence (PL) properties of K2LiScF6:Cr3+ are studied in detail, which shows strong absorption in the blue light region (λex = 432 nm) and broadband NIR emission (λem = 770 nm) with a PL quantum efficiency of 77.6%. More importantly, the NIR emission of Cr3+ can be enhanced by co-doping with Mn4+, which may provide an alternative way for improving the PL intensity of Cr3+-activated broadband NIR phosphors. Finally, a NIR phosphor-converted LED (pc-LED) device is fabricated using the as-prepared NIR phosphor and its application in bio-imaging and night vision has been evaluated.

Performance Analyses of Photonic-Crystal Surface-Emitting Laser: Toward High-Speed Optical Communication

This study conducts comprehensive performance analyses of a commercial photonic-crystal surface-emitting laser (PCSEL) via small-signal measurement and the bit-error-rate test. Meanwhile, the radio frequency characteristics of the PCSEL are unveiled for the first time. Compared to the vertical-cavity surface-emitting lasers, the PCSEL shows great potential for a broader optical bandwidth that is benefited from the high optical-confinement factor. A maximum bandwidth of around 2.32 GHz is experimentally observed when the PCSEL was biased at 340 mA. Moreover, a theoretical calculation was applied to shed light on the characteristics of the small-signal measurement, providing a deep insight into the corresponding intrinsic response model. The signal transmission capability of the PCSEL was investigated as well. The maximum bit rate and corresponding rise time transmitted at 500 Mbps are 1.2 Gbps and 186.16 ps, respectively. Thus, a high-speed PCSEL can be realised with a shrunk form factor, serving as a promising candidate for the next-generation light sources in high-speed optical communication.

Thermally stable and conductive nickel-incorporated gallium oxide thin-film electrode for efficient GaN microscale light-emitting diode arrays

Microscale light-emitting diodes (µLEDs) have attracted considerable attention as next-generation solid-state lighting sources owing to their reliable performance and attractive properties, such as easy miniaturization and stable operation in various movements. However, the quantum efficiency of μLEDs is lower than that of larger-size LEDs, which hinders their use in high-performance μLED display applications. Herein, a thermally stable and highly conductive GaOx thin-film as a p-type contact electrode is demonstrated by using electric-field-induced doping treatment (EDT) to achieve high-performance GaN µLEDs. The proposed GaOx electrode exhibits high transmittance (92%) and low specific contact resistance (3.5 × 10-3 Ωcm2), along with high thermal stability (over 10 years at 77 °C). Transmission electron microscopy analyses show that conductive channels are formed in the GaOx electrode based on the diffusion of metallic Ni species from the top metal because of EDT, thereby facilitating efficient hole injection into the μLED pixels with little spreading to the passivation layers. Consequently, the µLED array with the GaOx electrode exhibited a 12% higher light output power and 57% higher current level than those of a µLED array with conventional ITO electrodes. The results of this study can guide efforts dedicated to further improving the performance of ITO-based optoelectronic devices, including µLEDs.

Inorganic Chain Mediated Excitonic Properties in One-Dimensional Lead Halide Hybrid Perovskites.

Semiconductors that emit intrinsic white light are considered next-generation lighting sources. Herein, the broadband emission of one-dimensional (1D) lead halide perovskites, TMAPbBr3-xIx (x = 0, 1, 1.5, 2, 3; TMA+ = tetramethylammonium), is systematically investigated. Lattice distortion causes the conversion of dark excitons to bright self-trapped excitons. Owing to its strongly localized exciton recombination and high absorption probability, TMAPbBr3 is the most viable in this family. A delocalized hole increases the nonradiative recombination rate of excitons in TMAPbBr3-xIx alloys. In 1D TMAPbBr3-xIx perovskites, the vibration mode of the Pb-X bond stretching of the PbX6 octahedra contributes more to the effect on exciton-phonon coupling than the mode of the X-Pb-X angle bending. Pb-X bond stretching and spontaneous polarization can tune exciton binding energy. This systematic study of excitonic behavior in 1D compounds relates the nature of ground states to the unknown excited states and provides the rational design of materials with stable and efficient broadband emission.

Enhancing thermal and optical properties for liquid quantum-dot-based self-loop system toward laser illumination by controlling the layer location

White light sources based on laser diodes are considered to be the next-generation light sources owing to their high luminous efficiency, high luminous flux, and excellent directivity. Quantum dots (QDs) are new fluorescent materials that have been widely used in lighting and displays. However, when used for high-power illumination, QDs excitation and emission are accompanied by obvious attenuation problems owing to their reabsorption effect and low stability in high temperature environment. To solve these problems, this study proposes three types of QD-based self-loop systems for laser illumination and analyzes their thermal and optical properties. White light is obtained by exciting red and green QDs using a blue laser. The three types of self-loop converters are green-QD-front type (structure I), red-QD-front type (structure II), and hybrid type (structure III). The average temperatures of the excitation region of structures I and II were maintained at approximately 37.3 °C and 37.0 °C, respectively, which were both lower than that of the hybrid structure (46.7 °C). The radiation flux of structure I only attenuated by 15.6% after laser irradiation for 120 min. Furthermore, the shift in color coordinates was the smallest. When the laser driving current was changed from 500 mA to 1500 mA, the correlated color temperature (CCT) drift was only 474 K. We believe that these results will provide guidelines for QD-based laser-driven illumination.

Cell proliferation effect of deep-penetrating microcavity tandem NIR OLEDs with therapeutic trend analysis

Long wavelengths that can deeply penetrate into human skin are required to maximize therapeutic effects. Hence, various studies on near-infrared organic light-emitting diodes (NIR OLEDs) have been conducted, and they have been applied in numerous fields. This paper presents a microcavity tandem NIR OLED with narrow full-width half-maximum (FWHM) (34 nm), high radiant emittance (> 5 mW/cm2) and external quantum efficiency (EQE) (19.17%). Only a few papers have reported on biomedical applications using the entire wavelength range of the visible and NIR regions. In particular, no biomedical application studies have been reported in the full wavelength region using OLEDs. Therefore, it is worth researching the therapeutic effects of using OLED, a next-generation light source, and analyzing trends for cell proliferation effects. Cell proliferation effects were observed in certain wavelength regions when B, G, R, and NIR OLEDs were used to irradiate human fibroblasts. The results of an in-vitro experiment indicated that the overall tendency of wavelengths is similar to that of the cytochrome c oxidase absorption spectrum of human fibroblasts. This is the first paper to report trends in the cell proliferation effects in all wavelength regions using OLEDs.

Ligand Effects on Photoluminescence and Electroluminescence of Silicon Quantum Dots for Light-Emitting Diodes

Colloidal silicon quantum dots (SiQDs) may potentially minimize the environmental impact of commercial LEDs and advance next-generation light sources. Many studies have investigated the optical properties of SiQDs prepared by chemical synthesis, but the essential features of surface ligands have not fully been understood. Characterizing surface ligands should have a significant impact on optoelectronic research and ensuing applications. In this study, colloidal SiQDs were synthesized by pyrolyzing hydrogen silsesquioxane, followed by thermal hydrosilylation with 1-decene. Decyl-terminated SiQDs exhibited photoluminescence (PL) in a wavelength of 730 nm and PL quantum yields (QYs) of up to 38%. Seven decyl-terminated SiQDs with different ligand coverages were synthesized by varying the reaction time of hydrosilylation between 10 min and 9 h, and then these SiQDs were assembled into LEDs. The PL spectra, PLQYs, and performance of the SiQD LEDs were evaluated as a function of the decyl-ligand coverage. The PL properties (i.e., peak wavelength and PLQY) were insensitive to changes in decyl-ligand coverage, whereas the LED performance changed significantly. In particular, a 2-fold difference in decyl-ligand coverage exhibited a 4-fold difference in electroluminescence (EL) turn-on voltage and a 17-fold difference in EL external quantum efficiencies. In addition, the LED performance was characterized by quantifying the relationship between ligand coverage, the number of bonding sites, and the surface areas of the ligands. At greater than 25% coverage, the total surface area of the decyl-ligands was significantly larger than that of a single SiQD, and when decyl-ligands and Si–O groups covered 50% of the surface, the insulation effect impaired the LED performance. Therefore, ligand coverage significantly affected the performance of SiQD LEDs. Although this study was limited to decyl-terminated SiQDs, the same method can be applied to other ligands to further improve LED efficiency of next-generation light sources in displays, lighting, and biomedical imaging.