Gain-switched pulse generation of 5.3 ps from 30†GHz-modulation-bandwidth 1270†nm DFB laser diode: erratum.

We present an erratum to our Letter [Opt. Lett.48, 6344 (2023)10.1364/OL.510237]. This erratum corrects the error of Figs. 3(b) and 4(b) made via incorrect scaling in the horizontal axes. The corrections have no influence on the main text and conclusions of the original Letter.

Continuous-wave quasi-single-mode random lasing in CH3NH3PbBr3 perovskite films on patterned sapphire substrates.

We report intriguing continuous-wave quasi-single-mode random lasing in methylammonium lead bromide (CH3NH3PbBr3) perovskite films synthesized on a patterned sapphire substrate (PSS) under excitation of a 532-nm laser diode. The random laser emission evolves from a typical multi-mode to a quasi-single-mode with increasing pump fluences. The full width at half-maximum of the lasing peak is as narrow as 0.06 nm at ∼547.8 nm, corresponding to a high Q-factor of ∼9000. Such excellent random lasing performance is plausibly ascribed to the exciton resonance in optical absorption at 532 nm and the enhanced optical resonance due to the increased likelihood for randomly scattered light to re-enter the optical loops formed among the perovskite grains by multi-reflection at the perovskite/PSS interfaces. This work demonstrates the promise of single-mode perovskite random lasers by introducing the exciton resonance effect and ingeniously designed periodic nano/micro optical structure.

Observation of Terahertz Spin Hall Conductivity Spectrum in GaAs with Optical Spin Injection.

We report the first observation of the spin Hall conductivity spectrum in GaAs at room temperature. Our terahertz polarimetry with a precision of several µrads resolves the Faraday rotation of terahertz pulses arising from the inverse spin Hall effect of optically injected spin-polarized electrons. The obtained spin Hall conductivity spectrum exhibits an excellent quantitative agreement with theory, demonstrating a crossover in the dominant origin from impurity scattering in the dc regime to the intrinsic Berry-curvature mechanism in the terahertz regime. Our spectroscopic technique opens a new pathway to analyze anomalous transports related to spin, valley, or orbital degrees of freedom.

Regulating the crystal orientation of vapor-transport-deposited GeSe thin films by a post-annealing treatment.

Recently, GeSe has emerged as a highly promising photovoltaic absorber material due to its excellent optoelectronic properties, nontoxicity, and high stability. Although many advantages make GeSe well suited for thin-film solar cells, the power conversion efficiency of the GeSe thin-film solar cell is still much below the theoretical maximum efficiency. One of the challenges lies in controlling the crystal orientation of GeSe to enhance solar cell performance. The two-step preparation of GeSe thin films has not yet been reported to grow along the [111] orientation. In this work, we study the effect of a post-annealing treatment on the GeSe thin films and the performance of the solar cells. It was found that amorphous GeSe films can be converted into polycrystalline films with different orientations by changing the post-annealing temperature. [111]-oriented and [100]-oriented GeSe thin films were successfully prepared on the same substrate by optimizing the annealing conditions. With the structure of Au/GeSe/CdS/ITO cell devices, PCEs of 0.14% and 0.16% were ultimately achieved.

Twisting and Protonation of Retinal Chromophore Regulate Channel Gating of Channelrhodopsin C1C2.

Channelrhodopsins (ChRs) are light-gated ion channels and central optogenetic tools that can control neuronal activity with high temporal resolution at the single-cell level. Although their application in optogenetics has rapidly progressed, it is unsolved how their channels open and close. ChRs transport ions through a series of interlocking elementary processes that occur over a broad time scale of subpicoseconds to seconds. During these processes, the retinal chromophore functions as a channel regulatory domain and transfers the optical input as local structural changes to the channel operating domain, the helices, leading to channel gating. Thus, the core question on channel gating dynamics is how the retinal chromophore structure changes throughout the photocycle and what rate-limits the kinetics. Here, we investigated the structural changes in the retinal chromophore of canonical ChR, C1C2, in all photointermediates using time-resolved resonance Raman spectroscopy. Moreover, to reveal the rate-limiting factors of the photocycle and channel gating, we measured the kinetic isotope effect of all photoreaction processes using laser flash photolysis and laser patch clamp, respectively. Spectroscopic and electrophysiological results provided the following understanding of the channel gating: the retinal chromophore highly twists upon the retinal Schiff base (RSB) deprotonation, causing the surrounding helices to move and open the channel. The ion-conducting pathway includes the RSB, where inflowing water mediates the proton to the deprotonated RSB. The twisting of the retinal chromophore relaxes upon the RSB reprotonation, which closes the channel. The RSB reprotonation rate-limits the channel closing.

Gain-switched pulse generation of 5.3†ps from 30†GHz-modulation-bandwidth 1270†nm DFB laser diode.

We generated gain-switched pulses via electrical pulse excitations in a 1270 nm distributed feedback (DFB) laser diode (LD) with a direct-modulation bandwidth of 30 GHz. The measurements revealed short-pulse widths of 5.3 and 8.8 ps with and without chirp compensation, via a single-mode optical fiber. The 5.3 ps pulses exhibited a spectral width of 0.40 nm (spectral bandwidth of 71 GHz), yielding a time-bandwidth product of 0.38. Although the gain-switched pulses in DFB LDs inherently contain linear and nonlinear chirp, optimized pumping conditions enable generation of nearly transform-limited ps pulses after linear chirp compensation.

Theoretical modeling and ultra-thin design for multi-junction solar cells with a light-trapping front surface and its application to InGaP/GaAs/InGaAs 3-junction.

Light-trapping design is a good strategy to obtain ultra-thin solar cells without sacrificing conversion efficiency. If applied to III-V compound multi-junction solar cells (MJSCs), it not only can greatly reduce the cell cost and weight, but also improve its radiation tolerance when operating in space. This paper formulates all subcell absorptance in an arbitrary N-junction solar cell with an ideal front textured surface and perfect rear mirror, including the effects of complex absorption and luminescence coupling in the stack. Taking the well-known InGaP/GaAs/InGaAs triple-junction solar cell (3J) for instance, the ultra-thin design and the conversion efficiency both in radiative limit and that with subcell internal radiative efficiency below-unity are predicted. Our results show that such front-textured 3J with top-subcell thickness varying from 200 to 500 nm can enhance light absorption so significantly that more than 28% of top-subcell, 56% of middle-subcell, and 90% of bottom-subcell thickness will be cut down when compared with the smooth-surfaced 3J. Typically, (350 nm, 315 nm, 28 nm) is recommended as the optimal design for the front-textured 3J with an experimental efficiency of over 38%. For the same benchmarks on photocurrent of 15.1 mA/cm2 or detailed balance limit of 44%, the minimum total thickness (all subcells only) in the front-textured 3J is only 1453 nm, that is even 71% of that in the rear-textured 3J, quantitatively revealing front texturization has a greater potential for material cut-down than rear texturization. Finally, the impacts of non-ideal scattering texturization on cell performance and ultra-thin design are also discussed. This work provides theoretical guidance for experimental studies on ultra-thin and high-efficient MJSCs with various light-trapping strategies.

Unidirectional output from a quantum-dot single-photon source hybrid integrated on silicon.

We report a quantum-dot single-photon source (QD SPS) hybrid integrated on a silicon waveguide embedding a photonic crystal mirror, which reflects photons and enables efficient unidirectional output from the waveguide. The silicon waveguide is constituted of a subwavelength grating so as to maintain the high efficiency even under the presence of stacking misalignment accompanied by hybrid integration processes. Experimentally, we assembled the hybrid photonic structure by transfer printing and demonstrated single-photon generation from a QD and its unidirectional output from the waveguide. These results point out a promising approach toward scalable integration of SPSs on silicon quantum photonics platforms.

Direct generation of sub-picosecond pulse via multi-section gain switching.

We have directly generated optical pulses having a duration of 0.56 ps with a peak power of 25 W by gain switching of multi-section semiconductor lasers in which the optimized lengths of the absorption and gain regions were 50 and 200 µm, respectively. Even though the experiment was conducted via impulsive optical pumping at a low temperature, we observed that the multi-section gain switching suppresses the low-energy tail and chirping inherent to conventional gain switching in single-section lasers and is useful in direct short-pulse generation.

Quantum-mechanical hydration plays critical role in the stability of firefly oxyluciferin isomers: State-of-the-art calculations of the excited states.

Stabilizing mechanisms of three possible isomers (phenolate-keto, phenolate-enol, and phenol-enolate) of the oxyluciferin anion hydrated with quantum explicit water molecules in the first singlet excited state were investigated using first-principles Born-Oppenheimer molecular dynamics simulations for up to 1.8 ns (or 3.7 × 106 MD steps), revealing that the surrounding water molecules were distributed to form clear single-layered structures for phenolate-keto and multi-layered structures for phenolate-enol and phenol-enolate isomers. The isomers employed different stabilizing mechanisms compared to the ground state. Only the phenolate-keto isomer became attracted to the water molecules in its excited state and was stabilized by increasing the number of hydrogen bonds with nearby water molecules. The most stable isomer in the excited state was the phenolate-keto, and the phenolate-enol and phenol-enolate isomers were higher in energy by ∼0.38 eV and 0.57 eV, respectively, than the phenolate-keto. This was in contrast to the case of ground state in which the phenolate-enol was the most stable isomer.

Quantitative study of electron tunneling dynamics in asymmetric coupled InGaN/GaN quantum wells.

Electron tunneling dynamics in asymmetric coupled triple InGaN/GaN quantum wells (ACQWs) with different well thicknesses of 3.0 nm (QW1), 2.5 nm (QW2), and 2.0 nm (QW3) were quantitatively investigated based on the time- and spectrally-resolved photoluminescence (PL) measurements and the rate-equation theory. Under weak excitation, only the emission peak of the widest well was observed at room temperature due to the effective electron tunneling from a wide to a narrow well, while all three emission peaks of the distinct wells were obtained at a high excitation level. The PL-intensity ratios of the wells in the initial transient spectra differed from those in the time-integrated spectra. With a set of rate equations and the experimental results of PL ratios and decay times, a 2 ns tunneling time from QW2 to QW1 was extracted and was decreased to 0.5 ns with increasing excitation, while the one from QW3 to QW2 was extracted to be ∼170ps. The extracted tunneling times are in good qualitative agreement with the data from the exponential fitting of the PL decay traces, which can be interpreted by the energy mismatches between relevant energy levels in the ACQWs. These results provide not only a better understanding of the carrier recombination and tunneling processes in the ACQW systems but also a useful guidance for high-performance ACQW-based optoelectronic and functional devices.

Light-Driven Electron-Hole Bardeen-Cooper-Schrieffer-Like State in Bulk GaAs.

We investigate the photon-dressed state of excitons in bulk GaAs by optical pump-probe spectroscopy. We reveal that the high-energy branch of the dressed states continuously evolves into a singular enhancement at the absorption edge in the high-density region where the exciton picture is no longer valid. Comparing the experimental result with a simulation based on semiconductor Bloch equations, we show that the dressed state in such a high-density region is better viewed as a Bardeen-Cooper-Schrieffer-like state, which has been theoretically anticipated to exist over decades. Having seen that the dressed state can be regarded as a macroscopic coherent state driven by an external light field, we also discuss the decoherence from the dressed state to an incoherent state after the photoexcitation in view of the Coulomb enhancement in the transient absorption.

Nonlinear propagation effects in high harmonic generation in reflection and transmission from gallium arsenide.

High harmonic spectroscopy in solids is emerging as a new tool to investigate ultrafast electron dynamics in the presence of strong optical fields. However, the observed high harmonic spectra do not usually reflect the microscopic origin of high harmonic generation (HHG) because of nonlinear and/or linear propagation effects. Here, we systematically investigate the HHG in reflection and transmission from gallium arsenide exposed to intense mid-infrared optical pulses. In transmission geometry, we find that the properties of high harmonics are drastically changed by nonlinear effects during the propagation of even tens of micrometers. Especially, the nonlinear absorption and/or nonlinearly induced ellipticity of the drive pulses as well as a cascade nonlinear mixing significantly alter the high harmonic signals in the case of the transmission geometry, making an extraction of the microscopic electron dynamics of gallium arsenide difficult. On the contrary, in reflection geometry, we obtain HHG spectra that are free from propagation effects, opening a general approach for high harmonic spectroscopy.

Effect of interaction between the internal cavity and external cavity on beam properties in a spectrally beam combined system.

A high-power beam can be achieved in spectrally beam combined systems due to the interaction between internal and external cavities. In order to investigate effects of the coupled cavity on the combined beam, we started with the derivation of the equivalent reflectance and transmittance of the external cavity based on a multilayer dielectric grating (MDG). Then, photons in active regions of diode lasers with/without feedback were calculated by utilizing Lang-Kobayashi rate equations. The results indicate that the lens and the MDG have an obvious impact on both the reflectance and transmittance of the external cavity, which determine whether the photons of a laser with feedback are larger or smaller than those without feedback. The photons of the laser with feedback and the transmittance of the external cavity codetermine the combining efficiency. Impacts of the confinement factor and spontaneous emission factor of diode laser emitters on the coupled cavity are similar to those on a common laser internal cavity. Spectral compositions of the combined beam have different brightness from each other.

"Visible" 5d Orbital States in a Pleochroic Oxychloride.

Transition metal compounds sometimes exhibit attractive colors. Here, we report a new oxychloride, Ca3ReO5Cl2, that shows unusually distinct pleochroism; that is, the material exhibits different colors depending on the viewing direction. This pleochroism is a consequence of the coincidental complex crystal field splitting of the 5d orbitals of the Re6+ ion in a square-pyramidal coordination of low symmetry in the energy range of the visible spectrum. Since the relevant d-d transitions show characteristic polarization dependence according to the optical selection rule, the orbital states are "visible" in Ca3ReO5Cl2.

Picosecond tunable gain-switched blue pulses from GaN laser diodes with nanosecond current injections.

We investigated the gain-switching properties of GaN-based ridge-waveguide lasers on free-standing GaN substrates with low-cost nanosecond current injection. It was observed that the output pulses with intense injection consisted of an isolated short pulse with a duration of around 50 ps at the high-energy side and a long steady-state component at the lower energy side independent of the electric pulse duration. The energy separation between the short pulse and steady-state component can be over 30 meV, favoring short-pulse generation with the spectral filtering technique. The duration of the steady-state component can be tuned freely by controlling the duration and voltage of the electric pulse, which is very useful for generating pulse-width-tunable optical pulses for various applications.

Anomalous Metal Phase Emergent on the Verge of an Exciton Mott Transition.

We investigate the exciton Mott transition (EMT) by using optical pump-terahertz probe spectroscopy on GaAs, with realizing the condition of Mott's gedanken experiment by the resonant excitation of 1s excitons. We show that an anomalous metallic phase emerges on the verge of the EMT as manifested by a peculiar enhancement of the quasiparticle mass and scattering rate. From the temperature and density dependence, the observed anomaly is shown to originate from the electron-hole (e-h) correlation which becomes prominent at low temperatures, possibly suggesting a precursor of e-h Cooper pairing.

The effect of dynamical fluctuations of hydration structures on the absorption spectra of oxyluciferin anions in an aqueous solution.

In this study, the effect of hydration on the absorption spectra of oxyluciferin anion isomers in an aqueous solution is investigated for elucidating the influence of characteristic hydration structures. Using a canonical ensemble of hydration structures obtained from first-principles molecular dynamics simulations, the instantaneous absorption spectra of keto-, enol-, and enolate-type aqueous oxyluciferin anions at room temperature are computed from a collection of QM/MM calculations using an explicit solvent. It is demonstrated that the calculations reproduce experimental results concerning spectral shifts and broadening, for which traditional methods based on quantum chemistry and the Franck-Condon approximation fail because of the molecular vibrations of oxyluciferin anions and dynamical fluctuations of their hydration structures. Although the first absorption band associated with the lowest energy excitation corresponds to a π-π* transition for all oxyluciferin anion isomers, the changes in this band upon hydration are different among the isomers. In particular, the bands of enol- and enolate-type of oxyluciferin anions are significantly blue-shifted by hydration, whereas those of the keto-type oxylucifeion anion are shifted relatively less. Thus, the order of the first-peak positions in the aqueous solution changed relative to that in vacuo. We ascribe this to the nature of the oxyluciferin anion being more hydrophobic in the keto form as compared with the enol and enolate isomers.

Theoretical insights into the effect of pH values on oxidation processes in the emission of firefly luciferin in aqueous solution.

To elucidate the emission process of firefly d-luciferin oxidation across the pH range of 7-9, we identified the emission process by comparison of the potential and free-energy profiles for the formation of the firefly substrate and emitter, including intermediate molecules such as d-luciferyl adenylate, 4-membered dioxetanone, and their deprotonated chemical species. From these relative free energies, it is observed that the oxidation pathway changes from d-luciferin → deprotonated d-luciferyl adenylate → deprotonated 4-membered dioxetanone → oxyluciferin to deprotonated d-luciferin → deprotonated d-luciferyl adenylate → deprotonated 4-membered dioxetanone → oxyluciferin with increasing pH value. This indicates that deprotonation on 6'OH occurs during the formation of dioxetanone at pH 7-8, whereas luciferin in the reactant has a 6'OH-deprotonated form at pH 9.

Conversion efficiency limits and bandgap designs for multi-junction solar cells with internal radiative efficiencies below unity.

We calculated the conversion-efficiency limit ηsc and the optimized subcell bandgap energies of 1 to 5 junction solar cells without and with intermediate reflectors under 1-sun AM1.5G and 1000-sun AM1.5D irradiations, particularly including the impact of internal radiative efficiency (ηint) below unity for realistic subcell materials on the basis of an extended detailed-balance theory. We found that the conversion-efficiency limit ηsc significantly drops when the geometric mean ηint* of all subcell ηint in the stack reduces from 1 to 0.1, and that ηsc degrades linearly to logηint* for ηint* below 0.1. For ηint*<0.1 differences in ηsc due to additional intermediate reflectors became very small if all subcells are optically thick for sun light. We obtained characteristic optimized bandgap energies, which reflect both ηint* decrease and AM1.5 spectral gaps. These results provide realistic efficiency targets and design principles.