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By using absolute electroluminescence measurement, we characterized current-dependent external radiative efficiency of GaInAs/GaAsP multiple quantum well (MQW) single-junction solar cells with and without a back distributed Bragg reflector (DBR). Internal radiative efficiency ( η int) under illuminated condition was quantified for analyzing their difference in photovoltaic performances. It was revealed that MQWs showed an advantage of improved η int compared with a bulk layer, and that a back DBR indeed improved conversion efficiency via double-pass absorption of sun light, but improvement via reduction of rear radiative emission loss toward substrate was small. Efficiency add-on via improved material quality, or η int, in the same structures was predicted.

Using heat that otherwise would be lost is important for efficient use of solar energy, but continues to be held back for lack of an ultrathin photovoltaic absorber that permits both sufficient absorption of light and ultrafast extraction of ``hot'' carriers before they lose their kinetic energy. This article presents a concept to enable heat recovery in a solar cell $w\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}u\phantom{\rule{0}{0ex}}t$ the need for ultrafast carrier extraction. This thermoelectric approach should impact engineering solutions for further improvement in silicon solar cells, beyond the Shockley-Queisser limit.

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evolves into a singular enhancement at the absorption edge in the high-density region where the exciton picture is no longer valid.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.

We report local wavelength tuning of quantum-dot single-photon sources integrated on a CMOS silicon chip as well as in-situ spectral matching between two dissimilar sources. All the necessary heterogeneous integration processes were performed in a simple pick-and-place manner based on transfer printing, largely facilitating the fabrication of complicated device structures.

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.

The possibility of evaluating minority-carrier mobility and lifetime by measuring fast photovoltage dynamics of completed cells via time-resolved photoemission spectroscopy is investigated. A numerical simulation based on drift-diffusion model is performed to investigate how mobility and lifetime can be extracted. A simulation assuming a constant lifetime shows that photovoltage decay reflects carrier lifetime over two orders of magnitude of the photo-injection level while photoluminescence decay does not. The possibility of a detailed analysis on recombination mechanisms is discussed. The discrepancy in the magnitude of photovoltage observed in the previous measurement was investigated. The contribution from surface band-bending is discussed.

An attempt to apply quantum-dot (QD) intermediate band solar cell (IBSC) as bottom cell in 4-terminal tandem solar cell instead of a conventional bulk bottom cell is presented. Considering the inferior lattice qualities in QD cells, we introduce a parameter η IC = 1 ~ 0.0001 to represent slow or fast conduction-band-to-dots non-radiative relaxation rate respectively corresponding to strong or weak phonon-bottleneck effects, aiming to develop a more general IBSC model. Our results exhibit a well-designed QD solar cell can more sufficiently utilize the below-1.41eV photons passing through the GaAs top cell, which would be a better candidate as bottom cell. Compared with the highest efficiency (11.15% absolute) achieved by the best bulk bottom cell (E g = 0.93eV), QD bottom cell with E g = 1.74eV and E 1 = 1.04eV will raise an over 3% absolute of efficiency boost, when phonon bottleneck effects are running well (η IC ≥0.1). If use a GaAs-host QD bottom cell with E 1 = 0.98eV, 1.5% absolute of efficiency add-on can still be achieved, which attributes to a significant voltage boost by the separation of quasi-Fermi levels between IB and CB. Even for those QD bottom cells with inferior lattice qualities (η IC 1 .

Transition metal compounds sometimes exhibit beautiful colors. We report here on a new oxychloride Ca3ReO5Cl2 which shows unusually distinct pleochroism; that is, the material exhibits different colors depending on viewing directions. This ple-ochroism is a consequence of the fact that a complex crystal field splitting of the 5d orbitals of the Re6+ ion in a square-pyramidal coordination of low-symmetry occurs accidentally in the energy range of the visible light spectrum. Since the rele-vant d-d transitions possess characteristic polarization dependences according to the optical selection rule, the orbital states are "visible" in Ca3ReO5Cl2., 6 pages, 5 figures

In this study, Sb2(S,Se)3 thin films are fabricated using the vapor transport deposition (VTD) method, with Sb2Se3 and Sb2S3 mixed powders as the evaporation source. The performance of the corresponding glass/ITO/CdS/Sb2(S,Se)3/Au solar cells are found to be correlated to the mass ratio between Sb2S3 and the overall powder mixture. The properties of the Sb2(S,Se)3 thin films and cell devices adopting four different Sb2S3 mass ratios (x = 0.1, 0.25, 0.5 and 0.75) are compared. Further, the electrical properties – from dark and light J-V measurements; structural properties – from X-ray diffraction and scanning electron microscope measurements; and carrier-recombination rates at the buffer/absorber interface in the space-charge region (SCR) and in the quasi-neutral region – from temperature-illumination-dependent open-circuit voltage (VOC) measurements – are compared. It is found that a Sb2(S,Se)3 solar cell with a Sb2S3 mass ratio of 0.25 had optimal crystallinity, the lowest density of deep traps and the smallest carrier-recombination rates at the interface, leading to a high efficiency of 7.31%.

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.

Via absolute electroluminescence measurements, we characterized degradations of internal luminescence quantum yield or internal radiative efficiency ( $\eta _{{\rm int}i}$ ) in respective subcells in GaInP/GaAs/Ge triple-junction and GaInP/GaAs double-junction solar cells after irradiations of protons and electrons with different energy and fluence ( $\phi$ ). Compared with typical open-circuit-voltage characterizations, $\eta _{{\rm int}i}$ turned out to be a sensitive, high-dynamic-range, quantitative, and fair indicator of radiation damage, since it purely represents material-quality change due to radiation damage, independently from small differences in bandgap energy due to alloy composition fluctuations and in other cell structures. A detailed fluence-dependence study has shown that the data of $\eta _{{\rm int}i}$ versus $\phi$ in moderate and high $\phi$ regions are very similar and almost independent of subcell materials, while the difference in beginning-of-life qualities of GaInP and GaAs materials causes dominant difference in subcell sensitivity to the low-radiation damages.

All-inorganic perovskite micro/nanolasers are emerging as a class of miniaturized coherent photonic sources for many potential applications, such as optical communication, computing, and imaging, owing to their ultracompact sizes, highly localized coherent output, and broadband wavelength tunability. However, to achieve single-mode laser emission in the microscale perovskite cavity is still challenging. Herein, we report unprecedented single-mode laser operations at room temperature in self-assembly CsPbX 3 microcavities over an ultrawide pumping wavelength range of 400–2300 nm, covering one- to five-photon absorption processes. The superior frequency down- and upconversion single-mode lasing manifests high multiphoton absorption efficiency and excellent optical gain from the electron–hole plasma state in the perovskite microcavities. Through direct compositional modulation, the wavelength of a single-mode CsPbX 3 microlaser can be continuously tuned from blue-violet to green (427–543 nm). The laser emission remains stable and robust after long-term high-intensity excitation for over 12 h (up to 4.3 × 10 7 excitation cycles) in the ambient atmosphere. Moreover, the pump-wavelength dependence of the threshold, as well as the detailed lasing dynamics such as the gain-switching and electron–hole plasma mechanisms, are systematically investigated to shed insight into the more fundamental issues of the lasing processes in CsPbX 3 perovskite microcavities.

Obtaining whispering gallery mode (WGM) lasing with prominent stability and high quality optical cavities is extremely desirable but still a great challenge. Despite the recent surge of reports on microspheres, nanowires and nanoplatelets metal halide crystals, there has few reports on the morphology of Hemispheres (HSs) in the form of uniform, component-tunable microcrystals. Here, CsPbX3 (X = Cl/Br) HSs with smooth surface and regular geometry structure were synthesized on silicon substrates by chemical vapor deposition (CVD) method, and the wavelength of laser can be continuously tuned from blue to green (450–540 nm) at room temperature by varying the halide composition. WGM laser with polarization ratio of 0.70 and cavity quality factor Q of 2485 were realized in the CsPbBr3 HS cavity. Besides, the crystal cavity suggested a high photonic and thermal stability for the superb capability of continuous stable lasing output under ambient conditions. All these results illustrate that the synthesis of cesium lead halide perovskite HSs may provide an alternative solution for fundamental researches and applications in optical devices.

Silicon quantum photonics provides a promising pathway to realize large-scale quantum photonic integrated circuits (QPICs) by exploiting the power of complementary-metal-oxide-semiconductor (CMOS) technology. Toward scalable operation of such silicon-based QPICs, a straightforward approach is to integrate deterministic single-photon sources (SPSs). To this end, hybrid integration of deterministic solid-state SPSs, such as those based on InAs/GaAs quantum dots (QDs), is highly promising. However, the spectral and spatial randomness inherent in the QDs pose a serious challenge for scalable implementation of multiple identical SPSs on a silicon CMOS chip. To overcome this challenge, we have been investigating a new hybrid integration technique called transfer printing, which is based on a pick-and-place operation and allows for the integration of desired QD SPSs on any locations on the silicon CMOS chips at will. Nevertheless, even in this scenario, in-situ fine tuning for perfect wavelength matching among the integrated QD SPSs will be required for interfering photons from the dissimilar sources. Here, we demonstrate in-situ wavelength tuning of QD SPSs integrated on a CMOS silicon chip. To thermally tune the emission wavelengths of the integrated QDs, we augmented the QD SPSs with optically driven heating pads. The integration of all the necessary elements was performed using transfer printing, which largely simplified the fabrication of the three-dimensional stack of micro/nanophotonic structures. We further demonstrate in-situ wavelength matching between two dissimilar QD sources integrated on the same silicon chip. Our transfer-printing-based approach will open the possibility for realizing large-scale QPICs that leverage CMOS technology.

Silicon photonics is a powerful platform for implementing large-scale photonic integrated circuits (PICs), because of its compatibility with mature complementary-metal-oxide-semiconductor (CMOS) technology. Exploiting silicon-based PICs for quantum photonic information processing (or the so-called silicon quantum photonics) provides a promising pathway for large-scale quantum applications. For the development of scalable silicon quantum PICs, a major challenge is integrating on-silicon quantum light sources that deterministically emit single photons. In this regard, the use of epitaxial InAs/GaAs quantum dots (QDs) is a very promising approach, because of their capability of deterministic single-photon emission with high purity and indistinguishability. However, the required hybrid integration is inherently difficult and often lacks the compatibility with CMOS processes. Here, we demonstrate a QD single-photon source (SPS) integrated on a glass-clad silicon photonic waveguide processed by a CMOS foundry. Hybrid integration is performed using transfer printing, which enables us to integrate heterogeneous optical components in a simple pick-and-place manner and thus assemble them after the entire CMOS process is completed. We observe single-photon emission from the integrated QD and its efficient coupling into the silicon waveguide. Our transfer-printing-based approach is fully compatible with CMOS back-end processes, and thus will open the possibility for realizing large-scale quantum PICs that leverage CMOS technology., 15 pages, 5 figures

Femtosecond semiconductor lasers are ideal devices to provide the ultrashort pulses for industrial and biomedical use because of their robustness, stability, compactness and potential low cost. In particular, gain-switched semiconductor lasers have significant advantages of flexible pulse shaping and repetition rate with the robustness. Here we first demonstrate our laser, which is initiated by very strong pumping of 100 times the lasing threshold density, can surpass the photon lifetime limit that has restricted the pulse width to picoseconds for the past four decades and produce an unprecedented ultrashort pulse of 670 fs with a peak power of 7.5 W on autocorrelation measurement. The measured phenomena are reproduced effectively by our numerical calculation based on rate equations including the non-equilibrium intraband carrier distribution, which reveal that the pulse width is limited by the carrier–carrier scattering time, instead of the photon lifetime. Femtosecond lasers are used for a vast variety of applications where super resolution is required. The authors present gain-switched semiconductor-laser operations using an extreme optical pump allowing them to generate ultrashort, high power pulses.

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.

Owing to the improvement of device design and material properties, the conversion efficiencies of state-of-the-art solar cells are approaching the theoretical limit. However, identification of the remaining losses is still difficult. Therefore, the quantitative analysis of losses is important for further optimization of the conversion efficiencies. In this study, we propose a method to quantitatively analyze the losses in open-circuit voltage of multi-junction solar cells. This method employs absolute electroluminescence and external quantum efficiency measurements including uncertainties. We analyze the voltage output losses for three triple-junction solar cells and discuss possible improvements of multi-junction solar cells.

A contactless measurement of the picosecond dynamics of photovoltage in two GaAs solar cells, one with n-layer on the top and the other with p-layer, was performed using time-resolved photoemission spectroscopy. We found that the shift of photoelectron spectra occurs in opposite directions for the two samples. This agrees with the interpretation that the spectral shift is caused by photovoltage at the p-n junction. We also summarize the limiting factors on the present technique including the degradation of the temporal resolution caused by propagation effect, and the deformation of spectra originating from the work-function cutoff, and discuss the possible solutions.

Via absolute electroluminescence imaging measurement, we systematically examine the drops in open-circuit voltage (V OC ) and conversion efficiency for an InAs quantum dot (QD) solar cell, in contrast to a similar geometric GaAs bulk solar cell. Our results quantitatively reveal that apart from an extrinsic (V OC ) drop of 137mV stemming from the inferior lattice quality, an intrinsic drop of 115mV also co-exist in QD cell caused by the practical band edge extending towards lower energy. Moreover, the extrinsic and intrinsic drops in conversion efficiency are respectively evaluated as 5.6% and 1.6%, indicating even without lattice deterioration, such QD cell still cannot exceed the conversion efficiency of the bulk-host-material solar cell, if the boost in photocurrent cannot compensate the large intrinsic voltage drop induced by the narrow-gap QD material. This study could contribute to clarify the empirical characteristics of realistic QD solar cells, and also to verify the validity of the present theoretical QD-cell models.

We propose a new concept of a solar cell that enables heat recovery to improve its conversion efficiency, namely “heat recovery (HERC) solar cell”. The two key elements are an absorber hotter than electrodes and carrier-energy selecting potential barriers placed between the absorber and electrodes, whose bandgap is larger than that of the absorber. HERC solar cell operating out of equilibrium can have high efficiency exceeding the detailed-balance limit. Unlike hot carrier solar cells, it does not require fast carrier extraction within the thermalization time, which largely improves the feasibility. HERC solar cell can be realized with a silicon absorber.

Strong interaction of a terahertz (THz) wave with excitons induces nonperturbative optical effects such as Rabi splitting and high-order sideband generation. Here, we investigated coherent properties of THz-induced sideband emissions from GaAs/AlGaAs multiquantum wells. With increasing THz electric field, optical susceptibility of the THz-dressed exciton shows a redshift with spectral broadening and extraordinary phase shift. This implies that the field ionization of the $1s$ exciton modifies the THz-dressed exciton in the nonperturbative regime.

The ideal solar cell conversion efficiency limit known as the Shockley-Queisser (SQ) limit, which is based on a detailed balance between absorption and radiation, has long been a target for solar cell researchers. While the theory for this limit uses several assumptions, the requirements in real devices have not been discussed fully. Given the current situation in which research-level cell efficiencies are approaching the SQ limit, a quantitative argument with regard to these requirements is worthwhile in terms of understanding of the remaining loss mechanisms in current devices and the device characteristics of solar cells that are operating outside the detailed balance conditions. Here we examine two basic assumptions: (1) that the photo-generated carriers lose their kinetic energy via phonon emission in a moment (fast thermalization), and (2) that the photo-generated carriers are extracted into carrier reservoirs in a moment (fast extraction). Using a model that accounts for the carrier relaxation and extraction dynamics, we reformulate the nonequilibrium theory for solar cells in a manner that covers both the equilibrium and nonequilibrium regimes. Using a simple planar solar cell as an example, we address the parameter regime in terms of the carrier extraction time and then consider where the conventional SQ theory applies and what could happen outside the applicable range., Comment: 25 pages, 15 figures

Magnetic field effect on enzymatic reactions is under intensive study in the past decades. Recently, it was reported that firefly bioluminescence was suppressed and red-shifted significantly when exposed to external magnetic field. However in this work, by means of selective excitation, we confirmed that emission properties of firefly light emitter “oxyluciferin” are completely immune to external magnetic field of up to 53 T. These findings pose strong contrast to existing relevant results. Potential reasons for the discrepancies found and the underlying physics towards the understanding of firefly bioluminescence were discussed.

We calculated the oxygen 1s X-ray absorption spectra (XAS) of acetone and acetic acid molecules in vacuum by utilizing the first-principles GW+Bethe-Salpeter method with an all-electron mixed basis. The calculated excitation energies show good agreement with the available experimental data without an artificial shift. The remaining error, which is less than 1% or 2-5 eV, is a significant improvement from those of time-dependent (TD) density functional methods (5% error or 27-29 eV for TD-LDA and 2.4-2.8% error or 13-15 eV for TD-B3LYP). Our method reproduces the first and second isolated peaks and broad peaks at higher photon energies, corresponding to Rydberg excitations. We observed a failure of the one-particle picture (or independent particle approximation) from our assignment of the five lowest exciton peaks and significant excitonic or state-hybridization effects inherent in the core electron excitations.

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.

Absolute electroluminescence (EL) measurements have been becoming essential methods not only to evaluate internal properties of individual subcells in multi-junction solar cells but also to characterize performance of series-connected solar-cell arrays. A photodetector-contact method., in which a spectral-sensitivity-calibrated photodetector is proximately placed on a sample surface in face-to-face geometry, is a simple and useful quantitative method for characterizing the absolute EL intensities of the solar cells. In this work, we investigated collection efficiencies of emission for this method by numerical calculations based on a ray-tracing calculation model and evaluated accuracy in the absolute EL measurements of the solar-cell devices using this method.

Via detailed-balance analysis, we systematically studied conversion efficiency limit, short-circuit current, and open-circuit voltage $(V_{oc})$ of quantum dot (QD) solar cells for various absorptivity and bonding energy of QD below host-material absorption band. Our results quantitatively revealed that drops of $V_{oc}$ and conversion efficiency in QD solar cells were caused by the intrinsic consequence of mid-gap absorption band, and that current gain in intermediate-band QD solar cells via multi-step infrared-light absorption must be large enough to compensate at least the intrinsic drop of $V_{oc}$ to improve final conversion efficiency.

Aiming at the realization of high efficiency and low cost simultaneously, we modeled light absorption by subcells for a substrate-free thin-film multi-junction cell with a rear texture. The model evaluated subcell photocurrents and the requisite thicknesses at current matching. For InGaP/GaAs/lnGaAs metamorphic 3-junction solar cells, the rear texture significantly enhanced light absorption in the InGaAs bottom cell and GaAs middle subcells, and the requisite thickness of InGaAs is almost less than 10% of that with flat rear reflector, even GaAs is also less than half of that without rear texture, if InGaP-subcell is not greater than 600 nm. The reduced lower subcell thickness by the texture rear surface will mitigate the difficulty of metamorphic growth and contribute to the cost reduction of multi-junction solar cells.

Absolute electroluminescence (EL) measurements have been becoming essential methods to evaluate internal properties of individual subcells in multi-junction (MJ) solar cells. To establish methodology of absolute EL for standardization of MJ solar-cell characterizations, we developed independently calibrated absolute EL methods and verified accuracy and uncertainties via round-robin studies measuring EL emission rates for InGaP/GaAs/Ge triple-junction and single-junction solar cells. The results showed good quantitative agreements within 10% discrepancy among the individually calibrated methods, which corresponds to less-3-meV uncertainties in the estimation of the open-circuit voltages. This enables accurate comparison of the deviations between the open-circuit voltages via the transport experiment and via the absolute EL methods and detailed characterization of the internal properties of MJ solar cells.

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.

We performed optical pump-THz probe spectroscopy on bulk GaAs to investigate the nature of exciton Mott transition. The behavior of excitonic correlation in the proximity of the Mott transition density is elucidated through the resonant excitation of 1s excitons with using a nonlinear terahertz spectroscopy technique. We discuss the anomalous charge carrier dynamics of the metallic phase on the verge of Mott transition that appears only at low temperatures.

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.

We evaluated the pK(a) values of oxyluciferin and its conjugate acids and bases theoretically with the help of experimental correction values, from which free energies for the first excited and the ground states of all the species were estimated. On the basis of these results, we calculated pH-dependent absorption spectra, where the relative absorption intensities of various species strongly depend on photoexcitation energy, and we further analyzed the photoluminescence pathways of oxyluciferin in aqueous solutions with various pH. In the case of 350 nm photoexcitation, in particular, experiments have shown that dominant emission color is green and it attenuates with pH decreasing, while blue (3 < pH < 8) and red (pH < 3) emissions appear. Our present results clarify the pathways of these photoluminescence depending on the pH values and thus should be useful in further analyses of photoluminescence pathways for other photoexcitation wavelength in comparison with experiments.

Low threshold lasing at room temperature was achieved in optically pumped GaN-based vertical-cavity surface-emitting lasers (VCSELs) with sub-nanometer roughness polishing. The cavity region sandwiched by two dielectric distributed Bragg reflectors, incorporating InGaN/GaN multiquantum wells, a p-type AlGaN layer, and n- and p-type GaN layers, is a typical structure for electrically driven VCSELs. We observed lasing at a wavelength of 431.0 nm with a low threshold pumping energy density of ~ 3.2 mJ/cm2 and a high spontaneous emission coupling factor of ~ 0.09. These results were attributed to the significant reduction of the internal cavity loss by the removal of the high-dislocation GaN region, the reduction of cavity length, and the achievement of sub-nanometer level surface roughness (root mean square roughness of 0.3 nm) via inductively coupled plasma etching and chemical mechanical polishing. The loss mechanism is discussed and loss is quantitatively calculated in this letter.

We have investigated the electrical and optical properties of p–i–n junction structures in which un-doped GaNAs/GaAs multiple quantum wells (MQWs) were sandwiched by p- and n-doped GaAs layers. The samples were formed on the GaAs (001) substrates by plasma assisted molecular beam epitaxy (RF-MBE) using the modulated N radical beam method. We have prepared several samples for various GaNAs MQW structures. The electroluminescence (EL) measurements showed slightly different spectra to those of photoluminescence (PL) and the EL intensities were almost proportional to the applied currents. The room temperature EL measurement revealed strong electron confinement of GaNAs/GaAs MQW.

The excitation and emission energies of oxyluciferin are calculated by the TDDFT method and the absorption spectra in the aqueous solutions with various pH values are assigned using the relative absorption intensity estimated from the theoretical pKa values. The results suggest that the peak at 414 nm in the experimental absorption spectra at pH 8.3 corresponds to the excitation to the S1 state of (6′O−, 4O−) and that the peak at 371 nm in the strong acidic solution at pH 5.4 corresponds to the excitation to the S1 state of (6′OH, 4OH).

The coherent interaction of light with matter imprints the phase information of the light field on the wave function of the photon-dressed electronic state. A driving electric field, together with a stable phase that is associated with the optical probe pulses, enables the role of the dressed state in the optical response to be investigated. We observed optical absorption strengths modulated on a subcycle time scale in a GaAs quantum well in the presence of a multicycle terahertz driving pulse using a near-infrared probe pulse. The measurements were in good agreement with the analytical formula that accounts for the optical susceptibilities caused by the dressed state of the excitons, which indicates that the output probe intensity was coherently reshaped by the excitonic sideband emissions.

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.

In-situ characterization is one of the most powerful techniques to improve material quality and device performance. Especially in view of highly efficient tandem solar cells this is an important issue for improving the cost-performance ratio. Optical techniques are suitable characterization methods, since they are non-destructing and contactless. In this work, we measured the power dependence of photoluminescence (PL) from the InGaP and GaAs subcells of an industry-standard triple-junction solar cell. High luminescence yields enhance the luminescence coupling, which was directly verified by time-resolved PL measurements. We present a new method to determine the internal luminescence efficiencies of InGaP and GaAs subcells with the aid of luminescence coupling. High luminescence efficiencies of 90% for GaAs and more than 20% for InGaP were found, which suggest that the material quality of the grown GaAs layer is excellent while the intrinsic luminescence limit of InGaP is still not reached even for high excitation conditions. The PL method is useful for probing the intrinsic material properties of the subcells in flat band condition, without influence of transport. Since no calibration of absolute PL is required, a fast screening of the material quality is possible, which should be extremely helpful for the solar cell industry.