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.

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.

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.

Thorough subcells diagnosis in a multi-junction solar cell via absolute electroluminescence-efficiency measurements.

World-wide studies on multi-junction (tandem) solar cells have led to record-breaking improvements in conversion efficiencies year after year. To obtain detailed and proper feedback for solar-cell design and fabrication, it is necessary to establish standard methods for diagnosing subcells in fabricated tandem devices. Here, we propose a potential standard method to quantify the detailed subcell properties of multi-junction solar cells based on absolute measurements of electroluminescence (EL) external quantum efficiency in addition to the conventional solar-cell external-quantum-efficiency measurements. We demonstrate that the absolute-EL-quantum-efficiency measurements provide I-V relations of individual subcells without the need for referencing measured I-V data, which is in stark contrast to previous works. Moreover, our measurements quantify the absolute rates of junction loss, non-radiative loss, radiative loss, and luminescence coupling in the subcells, which constitute the "balance sheets" of tandem solar cells.

Transient gain analysis of gain-switched semiconductor lasers during pulse lasing.

We analyzed the transient gain properties of three gain-switched semiconductor lasers with different materials and cavity structures during pulse lasing. All the semiconductor lasers were pumped with impulse optical pumping, and all the generated gain-switched output pulses were well described by exponential functions in their rise parts, wherein the transient gains were derived according to the rate-equation theoretical model. In spite of the different laser structures and materials, the results consistently demonstrated that a higher transient gain produces shorter output pulses, indicating the dominant role of higher transient gain in the generation of even shorter gain-switched pulses with semiconductor lasers.

Atomically engineered metal-insulator transition at the TiO2/LaAlO3 heterointerface.

We demonstrate that the atomic boundary conditions of simple binary oxides can be used to impart dramatic changes of state. By changing the substrate surface termination of LaAlO3 (001) from AlO2 to LaO, the room-temperature sheet conductance of anatase TiO2 films are increased by over 3 orders of magnitude, transforming the intrinsic insulating state to a high mobility metallic state, while maintaining excellent optical transparency.

Direct generation of 2-ps blue pulses from gain-switched InGaN VCSEL assessed by up-conversion technique.

Ultra-short pulses in blue region generated from compact and low-cost semiconductor lasers have attracted much attention for a wide variety of applications. Nitride-based vertical-cavity surface-emitting lasers (VCSELs), having intrinsic high material gain and short cavities, favor the generation of ultra-short blue pulses via a simple gain-switching technique. In this study, we fabricated a single-mode InGaN VCSEL consisting of 10-period InGaN/GaN quantum wells (QWs). The output pulses were evaluated accurately with an up-conversion measurement system having time resolution of 0.12 ps. We demonstrated that ultra-short blue pulses, as short as 2.2 ps at 3.4 K and 4.0 ps at room temperature, were generated from the gain-switched InGaN VCSEL via impulsive optical pumping, without any post-processing. The gain-switched pulses we obtained should greatly promote the development of ultra-short blue pulse generation. In addition, this successful assessment demonstrates the up-conversion technique's usefulness for characterizing ultra-short blue pulses from semiconductor lasers.

Mode imaging and loss evaluation of semiconductor waveguides.

An imaging and loss evaluation method for semiconductor waveguides coupled with non-doped quantum wells is presented. Using the internal emission of the wells as a probe light source, the numbers and widths of the modes of waveguides with various ridge sizes were evaluated by CCD imaging, and the obtained values were consistent with effective index method calculation. Waveguide internal losses were obtained from analyses of the Fabry-Pérot fringes of waveguide emission spectra. We quantified the quality of 29 single-mode waveguide samples as an internal loss and variation of 10.2 ± 0.6 cm(-1).

Spectral dynamics of picosecond gain-switched pulses from nitride-based vertical-cavity surface-emitting lasers.

Short pulses generated from low-cost semiconductor lasers by a simple gain-switching technique have attracted enormous attention because of their potential usage in wide applications. Therein, reducing the durations of gain-switched pulses is a key technical point for promoting their applications. Therefore, understanding the dynamic characteristics of gain-switched pulses is highly desirable. Herein, we used streak camera to investigate the time- and spectral-resolved lasing characteristics of gain-switched pulses from optically pumped InGaN single-mode vertical-cavity surface-emitting lasers. We found that fast initial components with ultra-short durations far below our temporal resolution of 5.5 ps emerged on short-wavelength sides, while the entire pulses were down-chirped, resulting in the simultaneous broadening of the spectrum and pulse width. The measured chirp characteristics were quantitatively explained using a single-mode rate-equation model, combined with carrier-density-dependent gain and index models. The observed universal fast short-wavelength components can be useful in generating even shorter pulses from gain-switched semiconductor lasers.

Gain-switching dynamics in optically pumped single-mode InGaN vertical-cavity surface-emitting lasers.

The gain-switching dynamics of single-mode pulses were studied in blue InGaN multiple-quantum-well vertical-cavity surface-emitting lasers (VCSELs) through impulsive optical pumping. We measured the shortest single-mode pulses of 6.0 ps in width with a method of up-conversion, and also obtained the pulse width and the delay time as functions of pump powers from streak-camera measurements. Single-mode rate-equation calculations quantitatively and consistently explained the observed data. The calculations indicated that the pulse width in the present VCSELs was mostly limited by modal gain, and suggested that subpicosecond pulses should be possible within feasible device parameters.

Intrinsic radiative lifetime derived via absorption cross section of one-dimensional excitons.

Intrinsic radiative lifetime is an essential physical property of low-dimensional excitons that represents their optical transition rate and wavefunction, which directly measures the probability of finding an electron and a hole at the same position in an exciton. However, the conventional method that is used to determine this property via measuring the temperature-dependent photoluminescence (PL) decay time involves uncertainty due to various extrinsic contributions at high temperatures. Here, we propose an alternative method to derive the intrinsic radiative lifetime via temperature-independent measurement of the absorption cross section and transformation using Einstein's A-B-coefficient equations derived for low-dimensional excitons. We experimentally verified our approach for one-dimensional (1D) excitons in high-quality 14 × 6 nm(2) quantum wires by comparing it to the conventional approach. Both independent evaluations showed good agreement with each other and with theoretical predictions. This approach opens a promising path to studying low-dimensional exciton physics.

Dynamics of short-pulse generation via spectral filtering from intensely excited gain-switched 1.55-µm distributed-feedback laser diodes.

Picosecond-pulse-generation dynamics and pulse-width limiting factors via spectral filtering from intensely pulse-excited gain-switched 1.55-µm distributed-feedback laser diodes were studied. The spectral and temporal characteristics of the spectrally filtered pulses indicated that the short-wavelength component stems from the initial part of the gain-switched main pulse and has a nearly linear down-chirp of 5.2 ps/nm, whereas long-wavelength components include chirped pulse-lasing components and steady-state-lasing components. Rate-equation calculations with a model of linear change in refractive index with carrier density explained the major features of the experimental results. The analysis of the expected pulse widths with optimum spectral widths was also consistent with the experimental data.

Gain-switched pulses from InGaAs ridge-quantum-well lasers limited by intrinsic dynamical gain suppression.

Gain-switched pulses of InGaAs double-quantum-well lasers fabricated from identical epitaxial laser wafers were measured under both current injection and optical pumping conditions. The shortest output pulse widths were nearly identical (about 40 ps) both for current injection and optical pumping; this result attributed the dominant pulse-width limitation factor to the intrinsic gain properties of the lasers. We quantitatively compared the experimental results with theoretical calculations based on rate equations incorporating gain nonlinearities. Close consistency between the experimental data and the calculations was obtained only when we assumed a dynamically suppressed gain value deviated from the steady-state gain value supported by standard microscopic theories.

Sub-5-ps optical pulse generation from a 1.55-µm distributed-feedback laser diode with nanosecond electric pulse excitation and spectral filtering.

This paper reports generation of sub-5-ps Fourier-transform limited optical pulses from a 1.55-µm gain-switched single-mode distributed-feedback laser diode via nanosecond electric excitation and a simple spectral-filtering technique. Typical damped oscillations of the whole lasing spectrum were observed in the time-resolved waveform. Through a spectral-filtering technique, the initial relaxation oscillation pulse and the following components in the output pulse can be well separated, and the initial short pulse can be selectively extracted by filtering out the short-wavelength components in the spectrum. Short pulses generated by this simple method are expected to have wide potential applications comparable to mode-locking lasers.

Biexciton gain and the Mott transition in GaAs quantum wires.

Optical gain and the Mott transition in GaAs quantum wires were studied via simultaneous measurements of absorption and photoluminescence (PL). We observed well-separated PL peaks assigned to excitons (X) and biexcitons (XX) even at densities where optical gain existed. A sharp optical gain first appeared when the XX peak overtook the X peak, indicating the gain origin of biexciton-exciton population inversion. The XX peak eventually changed to a broad peak of plasma, and a broad gain due to plasma was observed as the Mott transition was completed.

One-dimensional band-edge absorption in a doped quantum wire.

Low-temperature photoluminescence-excitation spectra are studied in an n-type modulation-doped T-shaped single quantum wire with a gate to tune electron densities. With a nondegenerate one-dimensional (1D) electron gas, the band-edge absorption exhibits a sharp peak structure induced by the 1D density of states. When the dense 1D electron gas is degenerate at a low temperature, we observe a Fermi-edge absorption onset without many-body modifications.