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.

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.

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.

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.

Absorption Spectra for Firefly Bioluminescence Substrate Analog: TokeOni in Various pH Solutions.

AkaLumine hydrochloride, named TokeOni, is one of the firefly luciferin analogs, and its reaction with firefly luciferase produces near-infrared (NIR) bioluminescence. Prior to studying the bioluminescence mechanism, basic knowledge about the chemical structures, electronic states, and absorption properties of TokeOni at various pH values of solution has to be acquired. In this paper, the absorption spectra for TokeOni and AkaLumine at pH 2-10 were measured. Density functional theory (DFT) calculations, time-dependent DFT calculations, and the vibrational analyses were carried out. The absorption spectra indicate that the chemical forms of TokeOni in solutions are same as those of AkaLumine. The peaks at pH 7-10 in the absorption spectra correspond to the excitation from the ground state of a carboxylate anion of AkaLumine, the peak at pH 2 corresponds to the excitation from the ground state of a carboxylate anion with an N-protonated thiazoline ring and N-protonated dimethylamino group of AkaLumine, and the peak at pH 4 corresponds to the excitation from the ground state of a carboxylate anion with an N-protonated thiazoline ring of AkaLumine.

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 immunohistochemistry using an antibody-fused bioluminescent protein.

We established a quantitative detection method for immunohistochemistry based on a reference standard light-emitting diode, protein microarray and antibody-fused bioluminescent protein. In this procedure, we calibrated the bioluminescence imaging system and prepared the calibration curve between antigen and antibody-fused bioluminescent protein using a protein microarray. Then we converted the detecting light signal to antigen count via absolute photon number in the bioluminescent images; there was a resulting threefold difference in the target antigen number between normal and cancerous tissues. Our technique can easily compare immunohistological images and evaluate tumor progression in quantitative pathological diagnosis.

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.

Theoretical Study of the Wavelength Selection for the Photocleavage of Coumarin-caged D-luciferin.

The equilibrium structures and optical properties of the photolabile caged luciferin, (7-diethylaminocoumarin-4-yl)methyl caged D-luciferin (DEACM-caged D-luciferin), in aqueous solution were investigated via quantum chemical calculations. The probable conformers of DEACM-caged D-luciferin were determined by potential energy curve scans and structural optimizations. We identified 40 possible conformers of DEACM-caged D-luciferin in water by comparing the Gibbs free energy of the optimized structures. Despite the difference in their structures, the conformers were similar in terms of assignments, oscillator strengths and energies of the three low-lying excited states. From the concentrations of the conformers and their oscillator strengths, we obtained a theoretical UV/Vis spectrum of DEACM-caged D-luciferin that has two main bands of shape nearly identical to the experimental UV/Vis spectrum. The absorption bands with maxima ~ 384 and 339 nm were attributed to the electronic excitations of the caged group and the luciferin moiety, respectively, by analysis of the theoretical UV/Vis spectrum. Furthermore, the analysis showed that DEACM-caged D-luciferin is excited in the caged group only by light of wavelength ranging within 400-430 nm, which is in the long-wavelength tail of the 384 nm band. This should be tested to lower damage upon photocleavage.

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.

Photoabsorption Spectra of Aqueous Oxyluciferin Anions Elucidated by Explicit Quantum Solvent.

Experimental photoabsorption spectra of three possible isomers (phenolate-keto, phenolate-enol, and phenol-enolate) of oxyluciferin anions in aqueous solution were reproduced by first-principles time-dependent density functional theory simulations in which the entire system including the oxyluciferin anion and 64 water molecules were modeled by full quantum mechanics (full QM), unlike the conventional hybrid method, where the surrounding water molecules are modeled by molecular mechanics (MM) or a continuum solvent model. The full QM photoabsorption spectra were calculated from 1000 structures that had been obtained using the first-principles Born-Oppenheimer molecular dynamics simulations, which included the van der Waals correction, to take into account the effect of dynamical fluctuations of the hydration structure. The full QM calculation with CAM-B3LYP functional, which is the most elaborate one and is apparently the most consistent with experiment, is compared to others obtained with different levels of the functional and the solvent model. The amount of charge leakage from the oxyluciferin anions to the aqueous solution is found to differ significantly between the ground and excited states and is strongly dependent on the simulation method. The conventional solvent models do not take this into account, but the QM/MM can do it appropriately when including more than 10 water molecules into the QM region.

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.

Intrinsic and extrinsic drops in open-circuit voltage and conversion efficiency in solar cells with quantum dots embedded in host materials.

We systematically analyzed the detailed-balance-limit-conversion efficiency of solar cells with quantum dots (QDs) embedded in host materials. We calculated their open-circuit voltage, short-circuit current, and conversion efficiency within single-photon absorption conditions, both in the radiative limit and in other cases with non-radiative recombination loss, using modeled absorption band with various absorptivities and energy widths formed below that of the host material. Our results quantitatively revealed the existence of intrinsic and significant drops in the open-circuit voltage and conversion efficiency of QD solar cells, in addition to extrinsic drops due to degraded material quality.

Absolute bioluminescence imaging at the single-cell level with a light signal at the Attowatt level.

Bioluminescence imaging (BLI) demonstrates cellular events as a light signal at the single-cell level using a highly sensitive, cooled CCD camera. However, BLI signals are relative values and thus, images taken on different days or using different equipment cannot be compared directly. We established a reference LED light source that was characteristic of the total flux and light distribution and calibrated the BLI system as an absolute light signal. This calibrated BLI system revealed that the average light signal of beetle luciferase was at an attowatt level per sec at the single cell level.

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.

Light-emitting-diode Lambertian light sources as low-radiant-flux standards applicable to quantitative luminescence-intensity imaging.

Planar-type Lambertian light-emitting diodes (LEDs) with a circular aperture of several tens of µm to a few mm in diameter were developed for use as radiant-flux standard light sources, which have been in strong demand for applications such as quantitative or absolute intensity measurements of weak luminescence from solid-state materials and devices. Via pulse-width modulation, time-averaged emission intensity of the LED devices was controlled linearly to cover a wide dynamic range of about nine orders of magnitude, from 10 µW down to 10 fW. The developed planar LED devices were applied as the radiant-flux standards to quantitative measurements and analyses of photoluminescence (PL) intensity and PL quantum efficiency of a GaAs quantum-well sample. The results demonstrated the utility and applicability of the LED standards in quantitative luminescence-intensity measurements in Lambertian-type low radiant-flux level sources.

Broadband tunable integrated CMOS pulser with 80-ps minimum pulse width for gain-switched semiconductor lasers.

High power pulsed lasers with tunable pulse widths are highly favored in many applications. When combined with power amplification, gain-switched semiconductor lasers driven by broadband tunable electric pulsers can meet such requirements. For this reason, we designed and produced a low-cost integrated CMOS pulse generator with a minimum pulse width of 80 ps and a wide tuning range of up to 270 ns using a 40-nm microelectronic process technique. We used this pulser to drive a 1.3-µm semiconductor laser diode directly, and thereafter investigated the gain-switching properties of the laser system. The optical pulses consist of a spike followed by a steady state region. Tuning the width of the electrical pulse down to approximately 1.5 ns produces optical pulses consisting only of the spike, which has a minimum pulse-width of 100 ps. Moreover, the duration of the steady state can be tuned continuously by tuning the electrical pulse width, with a peak power of approximately 5 mW. The output voltage of the electric pulser has a tuning range of 0.8-1.5 V that can be used to directly drive semiconductor laser diodes with wavelengths in the near-infrared spectrum, which are suitable for power amplification with rare-earth doped fiber amplifiers.

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.