Minoru Niigaki

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%.

GaN-based photocathodes with extremely high quantum efficiency

We have fabricated phototubes with photocathodes consisting of the Mg-doped GaN films. The spectral shapes of the response and the quantum efficiency (QE) strongly depend on the Mg-doping concentration. The calibrated QE of the photocathode is maximized to be 71.9% at a photon energy of 5.4 eV by a Mg-doping concentration of 3.0×1019cm−3. Consequently, a phototube with the GaN-based photocathode is realized to demonstrate a very high QE, more than 50% and sharp cutoff characteristic over three orders of magnitude.

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.

Development of UV image intensifier tube with GaN photocathode

We developed an UV image intensifier tube with a GaN photocathode in semi-transparent mode. In UV spectroscopy and low-light-level UV-imaging applications, there are strong demands for improved detectors which have higher quantum efficiency, low dark current, sharper wavelength cut-off response, and stable and robust characteristics. III-Nitrides semiconductor is one of the promising candidate materials to meet these demands. We developed a GaN photocathode which is epitaxially grown by MOCVD method. It has flat and high quantum efficiency from 200 nm to 360 nm. The cathode is incorporated into an image intensifier tube, which shows good gating performance and fine imaging resolution. With these improved performances, the UV image intensifier tube with GaN photocathode will expand its application fields to include UV spectroscopy and UV-imaging in low light.

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.

Electron diffusion length and escape probabilities for cesiated and hydrogenated polycrystalline diamond photocathodes

Electron diffusion lengths and escape probabilities from cesiated and hydrogenated polycrystalline diamond photocathodes were estimated by comparing the photoemission spectral response characteristics between the experimental and calculated data. As a result, the diffusion length for the polycrystalline diamond film used in this study was estimated to be as long as approximately 50 nm. Estimated escape probabilities of 0.8 and 0.2 were also obtained for cesiated and hydrogenated surfaces, respectively. The results suggest that the cesiated surface has a true negative electron affinity while the hydrogenated surface has an effective negative electron affinity.

Near-Infrared Photocathode Using Surface Plasmon Resonance

We have fabricated a novel photocathode with a subwavelength-size emission area for the detection of near-infrared light. The photocathode exhibits a p/n diode structure loaded with a grating-structured electrode with a window for electron emission in the central part of the grating. The sensitivity of the photocathode shows a sharp peak at the surface plasmon resonant wavelength determined by the grating period. This indicates that the surface grating electrode acts as a surface plasmon antenna for collecting light into a small emission area. The light collection feasibility due to the surface plasmon effect would be utilized to realize a photocathode with an extremely low dark current.

Novel field-assisted photocathodes with nanoscale grating antennas

The authors present novel field-assisted photocathodes exhibiting an enhanced photoemission by nanoscale metallic grating antennas fabricated on the surface. The antennas serve to guide the incident photon fluxes into the nanoscale apertures of the photocathodes by the surface plasmon resonance. They demonstrate the comparisons of the spectral response characteristics between the calculated and experimental results. The nanoscale antennas would be utilized to realize a photocathode with extremely low dark current especially in the near infrared region.

Efficient Operation of a Cesium-Vapor Laser Longitudinally Pumped by a Fine-Tunable Bandwidth-Narrowed Laser-Diode Bar

We report on a high-power cesium-vapor (Cs-vapor) laser longitudinally pumped by a tunable external-cavity broad-area laser-diode bar. The external cavity consisting of a beam-transformation system and an off-axis volume Bragg grating forced the high-power laser-diode bar to emit a fine-tunable narrow-bandwidth beam. Using the tunable narrowed-bandwidth beam, we effectively pumped a Cs-vapor laser with a flat-concave cavity and obtained strong emission (output peak power, 7 W; slope efficiency, 14%).