Teruo Hiruma

Cesium vapor laser pumped by a volume-Bragg-grating coupled quasi-continuous-wave laser-diode array

The experimental demonstration of a broad-area Quasi-continuous-wave (QCW) laser-diode array pumped cesium vapor laser is reported in this letter according to our experimental results. We used a volume-Bragg grating to narrow the spectral linewidth for acquirement of enough resonance transitions. A glass cylindrical cell filled with cesium/buffer gas was set in an end-pumped flat-concave cavity and the gas pressure was set lower than 1atm. Using the QCW drive mode with the repetitive rate and pulse width of 1kHz and 50μs, respectively, 13.45μJ pulse energy has been achieved with the slope efficiency of about ∼1.8%.

Room-Temperature 1.56 µm Electroluminescence of Highly Oriented β-FeSi2/Si Single Heterojunction Prepared by Magnetron-Sputtering Deposition

β-FeSi2/Si light-emitting devices were prepared by growing continuous and highly oriented β-FeSi2 films on Si (111) substrates using RF magnetron-sputtering deposition with a Fe target. In-plane X-ray diffraction reveals that the epitaxial relationship is such that β-FeSi2 // Si, indicating a (110) orientation in the growth direction. Post annealing was performed at 830°C and Al electrodes were deposited on both p-β-FeSi2 and n-Si sides. Room-temperature 1.56 µm electroluminescence was clearly observed from such a simple structure for the first time. Compared with the previously reported results for β-FeSi2 balls or β-FeSi2 precipitates buried in Si, the injection current density is more than one order of magnitude lower, indicating that the nonradiative recombination was effectively suppressed for the junction consisting of a continuous film.

Field-assisted photoemission from InP/InGaAsP photocathode with p/n junction

A high quantum efficiency of photoemission to a 1.35 μm threshold has been achieved from an externally biased InP/InGaAsP photocathode which has a p/n junction instead of a Schottky contact. The quantum efficiency at 1.3 μm is 5% (electron per incident photon) with the photocathode cooled to −80 °C. The photocathode consists of a n+InP contact layer, a p−InP photoelectron-emitting layer and a p−InGaAsP photon-absorbing layer on a p+InP substrate.

Electroluminescence and Response Characterization of β-FeSi2-Based Light-Emitting Diodes

β-FeSi2-based light-emitting devices were prepared by growing continuous and highly oriented β-FeSi2 films on Si(111) substrates using RF magnetron sputtering deposition with an Fe target. Response characterization of electroluminescence (EL) was carried out by measuring the rise time of the EL signal at room temperature, giving a characteristic time of approximately 15 ns for the device size of 1.5 mm ×1.5 mm. Temperature-dependent EL measurement was carried out from 10 K to 300 K and it was found that a 1.5 µm band EL signal underwent thermal quenching with an active energy of 0.11±0.01 eV, whereas a 1.1 µm band EL signal appeared at around 200 K and its intensity increased with temperature, indicating a spatial change in emission area. The use of n-type β-FeSi2 as an active layer is proposed based on the consideration of current injection mode.

Time-Resolved 1.5 µm-Band Photoluminescence of Highly Oriented β-FeSi2 Films Prepared by Magnetron-Sputtering Deposition

Temporal decay characteristics of highly (110)-oriented β-FeSi2 films were explored at 77 K and at room temperature. The 1.5 µm-band photoluminescence (PL) decayed as fast as that of the laser pulse, giving a characteristic decay time of sub-nanosecond, which is four orders faster than that of β-FeSi2 precipitates previously reported. In addition, the PL intensity of our β-FeSi2 films suffered a much weaker thermal quenching from 77 K to room temperature. These results indicated possible promising applications in light-emitting and high-speed response devices.

Full-field quantitative phase imaging by white-light interferometry with active phase stabilization and its application to biological samples

We report a Koehler-illumination-based full-field, actively stabilized, low-coherence phase-shifting interferometer, which is built on a white-light Michelson interferometer. By using a phase-stepping technique we can obtain full-field phase images of the sample. An actively stabilized phase-lock circuit is employed in the system to reduce phase noise. An application to human epithelial cells (HeLa cells) is achieved in our experiment. The advancement of this technique rests in its ability to take images of unstained biological samples quantitatively and on a nanometer scale.

High-Efficiency 894-nm Laser Emission of Laser-Diode-Bar-Pumped Cesium-Vapor Laser

We report a high-efficiency cesium-vapor laser with a high-gas-pressure (~3-atm helium and 0.49-atm ethane) cell pumped by a high-power external-cavity laser-diode bar. Peak laser power of 12.1 W at 894 nm was obtained, when the absorbed peak pump power was 23.1 W. The achieved slope efficiencies with the incident pump power and the absorbed pump power were 33 and 81.7%, respectively.

EXTREMELY HIGH QUANTUM PHOTOYIELD FROM CESIATED POLYCRYSTALLINE DIAMOND FILMS

An extremely high quantum photoyield, as high as 70% at the photon energy of 10 eV, was observed from cesiated polycrystalline diamond films. The threshold photon energy of 5.5 eV or less was observed. The results suggest that the cesiated polycrystalline diamond surface has a true negative electron affinity. In contrast, a quantum photoyield of 17% at the photon energy of 10 eV was observed for a hydrogenated polycrystalline diamond film. The threshold photon energy of 5.5 eV or less was also observed, as in the cesiated one. It seems that the hydrogenated polycrystalline diamond surface has an effective negative electron affinity.

Single-wavelength 5.6 kW direct diode laser with a high-efficiency beam combination

Our group developed a new optical system equipped with a slit mirror to combine the output power from two LD stack modules (30 bars each) and focus them into a single beam. The new device realized a CW output power of more than 3 kW with a transfer efficiency of 92%. Furthermore, by combining beams from two added stack modules to the main optical path using polarization coupling, we have achieved a CW output power of 5.6 kW with a single wavelength of 808 nm. The focused spot size was 2.4 mm×0.5 mm (the power intensity was 467 kW/cm2) at a practical focal length of 100 mm. The transfer efficiency from the LD output to the focal spot was estimated to be 86.5%. The total energy efficiency against the input electricity was approximately 45%. This value is the highest ever reported in a high-power LD applied to material processing, and a full threefold higher than that of the diode-pumped Nd:YAG laser, a device heretofore known to be highly efficient (at around 15%).

Time Integrated Spectroscopy of Turbid Media Based on the Microscopic Beer–Lambert Law: Application to Small-Size Phantoms Having Different Boundary Conditions

Continued work on time-integrated spectroscopy (TIS) is presented to quantify absorber concentrations in turbid media. We investigated the applicability of the TIS method to small-size media that have different boundary conditions by measuring two 20×20×50 mm3 cuboid liquid tissue-like phantoms at various absorption levels (absorption coefficients of the phantom from 2.5×10-3 to 4.4×10-2 mm-1 at 782 nm and from 3.1×10-3 to 2.7×10-2 mm-1 at 831 nm). The scattering and absorbing solution was filled into ordinary and black-anodized aluminum containers to provide different boundary conditions. By means of a single equation, the absorber concentrations have been recovered within errors of a few percent in both cases. This demonstrates that the TIS method can quantify absorbers in small-size media having different boundary conditions.