Toru Hirohata

Noncollinear parametric fluorescence by chirped quasi-phase matching for monocycle temporal entanglement

Quantum entanglement of two photons created by spontaneous parametric downconversion (SPDC) can be used to probe quantum optical phenomena during a single cycle of light. Harris [Opt. Express 98, 063602 (2007)] suggested using ultrabroad parametric fluorescenc generated from a quasi-phase-matched (QPM) device whose poling period is chirped. In the Harriss original proposal, it is assumed that the photons are collinearly generated and then spatially separated by frequency filtering Here, we alternatively propose using noncollinearly generated SPDC. In our numerical calculation, to achieve 1.2 cycle temporal correlation for a 532 nm pump laser, only 10% -chirped device is sufficien when noncollinear condition is applied, while a largely chirped (50%) device is required in collinear condition. We also experimentally demonstrate an octave-spanning (790-1610 nm) noncollinear parametric fluorescenc from a 10% chirped MgSLT crystal using both a superconducting nanowire single-photon detector and photomultiplier tube as photon detectors. The observed SPDC bandwidth is 194 THz, which is the largest width achieved to date for a chirped QPM device. From this experimental result, our numerical analysis predicts that the bi-photon can be compressed to 1.2 cycles with appropriate phase compensation.

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.

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.

Near-infrared organic light-emitting diodes for biosensing with high operating stability

We demonstrate highly stable NIR organic light-emitting diodes (OLEDs) based on a system using excitonic energy transfer from thermally activated delay fluorescence molecules to NIR fluorophores. The NIR OLEDs showed an electroluminescence peak at 780 nm and robust operational stability with 2% loss of the initial radiant flux after 1000 h under a constant current density of 10 mA/cm2. The variation of hemoglobin oxygen saturation can be detected using the NIR OLEDs as a light source.

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.

High photoresponse in room temperature quantum cascade detector based on coupled quantum well design

We report high photoresponse measured in a room temperature quantum cascade detector (QCD) based on a coupled quantum well design that operates with a peak response wavelength of 5.4 μm. The coupled quantum well design is expected to produce higher photocurrents when compared with device active regions that use a combination of simple quantum wells. The coupled quantum well QCD demonstrated high responsivity of 22 mA/W at room temperature with a commonly used 45° wedge-based light coupling configuration. Application of a waveguide configuration to the proposed QCD yielded an elevated responsivity of ∼130 mA/W and a specific detectivity (D*) of 1.1 × 108 cm W−1 Hz1/2 at room temperature.

A narrow band-rejection filter based on block copolymers

We demonstrate filtering characteristics of a polymer band-rejection filter (PBRF) with highly-ordered microphase-separated structure of block copolymers (BCPs). This PBRF is characterized by an Optical Density > 5 blocking at the center wavelength and narrow blocking full bandwidth of 8 nm. Moreover, the wavelength is easily tuned by blending two BCPs with different molecular-weight. A low frequency Raman shift of 200 cm(-1) are, in fact, detected with a sufficient resolution by using this filter in Raman spectroscopy.

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.

Low-Field Breakdown and Negative Differential Resistance in Semi-Insulating GaAs

In a semi-insulating GaAs (S.I. GaAs) diode with an ohmic cathode, a negative differential resistance (NDR) appears at a field as low as 2 kV/cm. In the NDR region, luminescence with a broad spectrum is observed. The luminescence area is localized just in front of the anode. These results are explained on the basis of the avalanche injection effect in S.I. GaAs material. The electron temperature estimated from the luminescent spectrum is 3200 K. The avalanche multiplication is discussed in terms of the low-field avalanche scheme.