Search

All inorganic CsPbBr3 perovskite quantum dots (QDs) are potential emitters for electroluminescent display. We demonstrated an easy hot-injection method to partially replace the toxic Pb2+ with the highly stable Sn4+. Meanwhile, the absolute photoluminescence (PL) quantum yield of CsPb1-xSnxBr3 with Sn(IV) substitution increased from 45% to 83%. Based on femtosecond transient-absorption (TA), time-resolved PL, and single-dot spectroscopies, TA dynamics in undoped CsPbBr3 and CsPb0.67Sn0.33Br3 QDs at various excitation fluences., providing a clear evidence for the suppression of trion generation. These highly luminescent CsPb0.67Sn0.33Br3 QDs exhibit emission wavelength of 517 nm. The highest device performance exhibited a luminescence of 12,500 cd/m2, a current efficiency (CE) of 11.63 cd/A, an external quantum efficiency (EQE) of 4.13%, a power efficiency (PE) of 6.76 lm/w, and a low turn-on voltage of 3.6 V, which are the highest values among reported Sn-based perovskite QLEDs.

All inorganic CsPbBr3 perovskite quantum dots (QDs) are potential emitters for electroluminescent displays. We have developed a facile hot-injection method to partially replace the toxic Pb2+ with highly stable Sn4+ . Meanwhile, the absolute photoluminescence quantum yield of CsPb1-x Snx Br3 increased from 45 % to 83 % with SnIV substitution. The transient absorption (TA) exciton dynamics in undoped CsPbBr3 and CsPb0.67 Sn0.33 Br3 QDs at various excitation fluences were determined by femtosecond transient absorption, time-resolved photoluminescence, and single-dot spectroscopy, providing clear evidence for the suppression of trion generation by Sn doping. These highly luminescent CsPb0.67 Sn0.33 Br3 QDs emit at 517 nm. A device based on these QDs exhibited a luminance of 12 500 cd m-2 , a current efficiency of 11.63 cd A-1 , an external quantum efficiency of 4.13 %, a power efficiency of 6.76 lm w-1 , and a low turn-on voltage of 3.6 V, which are the best values among reported tin-based perovskite quantum-dot LEDs.

We characterize the radiation-induced damage of InGaP/GaAs/Ge solar cells for various proton irradiation energies and fluences using conventional current-voltage (I-V) measurements, external quantum efficiency, and a noncontact time-resolved photoluminescence (PL) technique. From the I-V curves, we obtain the conversion efficiency of the entire device. The external quantum efficiency showed that the short-circuit current is only determined by the top InGaP subcell. To obtain accurate information about the point of maximum power, a new PL technique is introduced. The PL time decays of the InGaP and GaAs subcells are measured to obtain the characteristic decay time constants of carrier separation and recombination. We empirically verify that the time-resolved PL method can be used to predict the electrical conversion efficiency of the subcells. We find that the limiting subcell at the point of maximum power is different from that for short-circuit current. Radiation damage in unexpected regions of the device is revealed using this optical method.

The realization of high-quality quantum emitters that can operate at room temperature is important for accelerating the application of quantum technologies, such as quantum communication, quantum information processing, and quantum metrology. In this work, we study the photon-antibunching properties on room-temperature emission from individual colloidal quantum dots (CQDs) using superconducting-nanowire single-photon detectors and temporal filtering of the photoluminescence decay curve. We find that high single-photon purities and high photon-generation rates can be simultaneously achieved by removing the signals originating from the sequential two-photon emission of biexcitons created by multiple excitation pulses. We successfully demonstrate that the ultrahigh performance of the room-temperature single-photon sources showing g(2)(0) ≪ 10−2 can be confirmed by the ultralow-dark-count detection of the temporally purified single photons. These findings provide strong evidence for the attractiveness of CQDs as candidates for high-quality room-temperature quantum light sources.

Colloidal quantum dots (CQDs) are attractive material for single-photon sources (SPSs) that can be utilized at room temperature. In this work, we performed simultaneous measurements of photoluminescence (PL) decay curve and second-order photon correlation, g(2), on single CQDs to study the purity of their single photons, which is determined by the value of g(2)at the time origin. We demonstrate that the single-photon purity can be improved through time-gated g(2) analysis, where a part of the PL signal is excluded by applying a temporal filter to the PL decay curve. We show that by using a superconducting nanowire single-photon detector, it is possible to observe highly-purified single photons that exhibited g(2) at the time origin ranging from 0.01 to 0.02.

Recently, metal-halide perovskite semiconductors have been extensively studied because of their potential use in solar-cell, photodetector, light-emitting-diode, and laser applications. In particular, perovskite nanocrystals (NCs) are gaining more and more attention because their optical spectra can be manipulated by changing the NC size and the halogen composition [1]. In contrast to the conventionally structured NCs such as colloidal CdSe NCs, high-quality perovskite NCs with unique cubic shapes and large sizes can be fabricated. However, their intrinsic photocarrier recombination dynamics have not yet been understood. Especially, the recombination dynamics of exciton complexes including biexcitons and charged excitons (trions) are complicated due to the fast Auger recombination [2]. Thus, a combination of different optical techniques for ensemble and single NC samples should be performed for a deep understanding of multiexciton dynamics in perovskites.

Charge carrier dynamics in perovskite CH3NH3PbI3 solar cells were studied by means of microscopic photoluminescence (PL) and photocurrent (PC) imaging spectroscopy. The PL intensity, PL lifetime, and PC intensity varied spatially on the order of several tens of micrometers. Simultaneous PL and PC image measurements revealed a positive correlation between the PL intensity and PL lifetime, and a negative correlation between PL and PC intensities. These correlations were due to the competition between photocarrier injection from the CH3NH3PbI3 layer into the charge transport layer and photocarrier recombination within the CH3NH3PbI3 layer. Furthermore, we found that the decrease in the carrier injection efficiency under prolonged light illumination leads to a reduction in PC, resulting in light-induced degradation of solar cell devices. Our findings provide important insights for understanding carrier injection at the interface and light-induced degradation in perovskite solar cells.

Time-resolved photoluminescence (PL) is very useful for analyzing carrier dynamics and electrical properties of subcells which cannot be contacted directly. We investigate the excitation spot-size dependence of the PL decay time constants which are observed at high excitation powers. The excitation spot-size dependence is approximately linear and almost the same for both time constants. The origin of the spot-size dependence and the method for obtaining the subcell carrier-collection efficiencies are discussed.

We measured the cavity length dependence of the internal differential quantum efficiency ηint and the internal optical loss αint in 1500-nm-wavelength laser diodes (LDs). By evaluating αint directly from gain/absorption spectra for various injection current densities and measuring the external differential quantum efficiency ηext, we obtained the ηint and αint values of all the LDs with different cavity lengths. The obtained ηint and αint respectively showed strong and weak cavity length dependences, and were both very different from those derived by the widely used method of plotting ηext-1 against the cavity length.

A one-dimensional active region consisting of 20-period T-shaped GaAs/AlGaAs quantum wires of 14×6 nm2 combined with a well-defined current-confinement scheme has been utilized to obtain a low-threshold-current high-efficiency laser. A threshold current as low as 0.27 mA, a single-mode cw output power of 0.13 mW at 1 mA injection current, and a differential quantum efficiency of 12% were achieved at 30 K using a quantum-wire laser with a 500-µm-long Fabry–Perot cavity coated by high-reflectivity thin metals.

We performed microscopic imaging of photoluminescence (PL) and photocurrent (PC) on InAs nanostructures including disklike structures (nanodisks) and quantum dots (QDs). The correlation between PL and PC images indicates that the major fraction of upconverted carriers originates from nanodisks. By analyzing the excitation spectra, we find evidence that nanodisks and QDs need to be spatially separated to enhance PC generation via upconversion. The efficient simultaneous use of both QDs and nanodisks is an alternative approach to intermediate-band solar cells, where low-energy photons are upconverted in the QDs and high-energy photons are efficiently upconverted in the nanodisks, resulting in enhanced carrier generation yields. With spatially resolved upconverted PL, we show that PC generation in nanodisks is due to ejection of both electrons and holes.

We have systematically investigated the time-evolution of transient electron transport in semiconductor superlattices (SLs) under dc electric fields for various initial electron distributions created by femtosecond laser pulses. It is found that the terahertz (THz) waveforms are very insensitive to the pump photon energy and that the transient current starts from its maximum as damped cosω B t. We will show that the unique electron distribution under WS localization of electron wave functions in the translational symmetry is essential for the observed phase of the photoinduced current. Furthermore, as a first step toward realization of Bloch oscillators, a preliminary result on the effects of planar photonic crystal electrode on the THz emission will be briefly presented.

Gain characteristics of optically-excited semiconductor quantum wires are studied theoretically and experimentally at various carrier densities to clarify the correlation effects in electron-hole plasma. The profiles of gain spectra are significantly modified by the Coulomb interactions, by which serious suppression of peak gain occurs at high carrier densities.

CW single-mode lasing from 30 K to 70 K with the lowest threshold current of 0.27 mA has been demonstrated in 20-period T-shaped quantum-wire laser diodes with parallel p and n doping layers.

Conventional external quantum-efficiency measurement of solar cells provides charge-collection efficiency for approximate short-circuit conditions. Because this differs from actual operating voltages, the optimization of high-quality tandem solar cells is especially complicated. Here, we propose a contactless method, which allows for the determination of the voltage dependence of charge-collection efficiency for each subcell independently. By investigating the power dependence of photoluminescence decays, charge-separation and recombination-loss time constants are obtained. The upper limit of the charge-collection efficiencies at the operating points is then obtained by applying the uniform field model. This technique may complement electrical characterization of the voltage dependence of charge collection, since subcells are directly accessible.

Semiconducting nanocrystal quantum dot (QD) films with high multiple exciton generation (MEG) efficiencies and high exciton dissociation efficiencies are appealing as solar cell materials. Here, we report the strong enhancement of the photocurrent in close-packed QD films, and discuss the importance of surface ligands on MEG and charge transport in QD films. Both exciton-recombination and charge-transport properties are influenced by the phenomenon of carrier tunnelling between neighboring QDs in thin films. These results can have important implications in the design of chemical treatments to control the electronic interactions between QDs in solution-processed solar cells. We demonstrate that the inorganic- and small sized-ligand passivation of PbS QDs by potassium thiocyanate causes a significant photocurrent enhancement.

We demonstrated a new method of radiation thermometry at cryogenic temperatures on the basis of the detailed balance relation between absorption and photoluminescence in a small n-type-modulation-doped GaAs quantum-well sample. This method has unique advantages such as being non invasive. It also does not need to be calibrated and possibly offers high spatial resolution. We measured temperatures of a sample mounted on a sapphire substrate attached to a copper cold finger at 7–101 K in a cryostat. The temperatures measured with this method were consistent with those obtained from a separate calibrated traditional temperature sensor. The estimated uncertainties were ±2 K below 50 K and ±8 K below 100 K.

We have investigated the terahertz (THz) emission from a semiconductor superlattice placed in a waveguide structure with a photonic crystal surface electrode of a honeycomb lattice pattern. For uniformly applying bias voltages, we inserted a semitransparent Ti film between the superlattice layer and the photonic crystal electrode. It is found that optimization of the sheet resistivity of the Ti layer is crucial to suppress the coupling between the waveguide mode and the surface leaky mode and obtain a clear photonic band gap. When the Ti layer was 40 nm, a large extinction ratio of ∼120 in the THz emission intensity was obtained at the midgap frequency around 1.7 THz.

We report on the electronic structure, efficient carrier injection, and quantitative lasing characteristics of T-shaped GaAs/AlGaAs quantum-wire-laser diodes with parallel p- and n-doping layers grown by a cleaved-edge-overgrowth method with molecular-beam epitaxy. Continuous single-mode lasing from the ground subband of the quantum wires was demonstrated between 30 and 70 K in laser diodes with high-reflectivity Au coating on both cavity facets. The lowest threshold of 0.27 mA and the highest differential quantum efficiency of 12% were achieved at 30 K. Micro-photoluminescence measurements demonstrated the high optical quality of the quantum wires with narrow linewidth of 0.9 meV and provided electronic structures of surrounding layers. Microscopic electroluminescence (EL) imaging measurements demonstrated the efficient carrier injection into the quantum wires at 30 K. These two factors, i.e., high material quality and efficient carrier injection, contribute to the low threshold current and high efficien...

The authors measured the temperature dependence of the lasing properties of current-injection T-shaped GaAs∕AlGaAs quantum-wire (T-wire) lasers with perpendicular p- and n-doping layers. The T-wire lasers with high-reflectivity coatings on both cleaved facets achieved continuous-wave single-mode operation between 5 and 110K. The lowest threshold current was 2.1mA at 100K. The temperature dependences of differential quantum efficiency and threshold current were attributed mainly to that of current-injection efficiency.

One of the most important problems in the design of the thickness vibration resonator is how to suppress the anharmonic higher-order spurious mode which is generated near the main resonant frequency. It is easy to realize a single resonant characteristic by the design based on the trapped energy theory, but the area of the electrode and, consequently, the inductance, are limited. This paper proposes a method which improves this point and increases the degrees of freedom in the design, suppressing the anharmonic spurious mode by adding a mass to the edge of the electrode and by cancelling the charge. In other words, a modified circular electrode structure is proposed in which the peripheral part (ring) of the electrode has a larger thickness. The third overtone resonator is analyzed in detail, and it is shown theoretically that the dominant (3, 3, 1) spurious mode is suppressed by the charge cancellation, thereby realizing a resonator with an essentially single resonant characteristic over a wide frequency range. It is noted that the charge cancellation characteristics for the spurious mode are strongly dependent on the platebacking of the ring electrode. The data for the optimum design are summarized in a table which will be utilized in the future design.

We measured absorption spectra of T-shaped quantum wires at 5 K by laser-waveguide transmission experiment. The absorption coefficient at one-dimensional-exciton peak is 130 cm-1 and material absorption coefficient is 6times104 cm-1