Shigehiko Mori

Investigation of the organic solar cell characteristics for indoor LED light applications

We report an experimental study on the current–voltage characteristics of organic solar cells (OSCs) under indoor light illumination. A daylight color light-emitting diode (LED) was used as the indoor light source. We investigated the short circuit current density, open circuit voltage, and fill factor of the OSC under LED irradiation and compared them with those for a crystal silicon solar cell (c-Si-SC), which occupy a large part of the solar cell market. We found that compared with the c-Si-SC, the OSC had higher power conversion efficiency (PCE). We also derived the maximum feasible PCE of an OSC for indoor applications and calculated that a PCE value of 21.3% could be obtained under daylight color LED illumination at approximately 200 lx. From the results of the calculation, it became apparent that the open circuit voltage plays an important role in achieving a high PCE from OSCs, indicating they are promising as electrical energy harvesting module for indoor applications.

Preservation of photon indistinguishability after transmission through surface-plasmon-polariton waveguide.

We experimentally demonstrated preservation of indistinguishability between two photons via mode conversions, namely, photon-to-plasmon and plasmon-to-photon conversions. A two-photon interference experiment was carried out using a broadband photon pair generated through a spontaneous parametric downconversion process. We observed the so-called Hong-Ou-Mandel dip with an interferometer including a 1-mm-long surface-plasmon-polariton (SPP) waveguide. The photon indistinguishability of 92.4% was retained after propagation in the SPP waveguide.

Quantum key distribution over an installed multimode optical fiber local area network.

We have investigated the possibility of a multimode fiber link for a quantum channel. Transmission of light in an extremely underfilled mode distribution promises a single-mode-like behavior in the multimode fiber. To demonstrate the performance of the fiber link we performed quantum key distribution, on the basis of the BB84 four-state protocol, over 550 m of an installed multimode optical fiber local area network, and the quantum-bit-error rate of 1.09 percent was achieved.

Single-Photon Source for Telecommunication Wavelengths Using Nondegenerate Photon Pairs from a Periodically Poled LiNbO3 Waveguide

We report on single-photon generation at an optical telecommunication wavelength using nondegenerate photon pairs generated by a periodically poled lithium niobate waveguide and a conventional single-photon detector. The probability that a single photon was present was 0.16, and the second-order correlation function was 0.19. In addition, the photon statistics of the photon source were studied.

ANALYSIS OF THE GENERATION OF PHOTON PAIRS IN PERIODICALLY POLED LITHIUM NIOBATE

The process of spontaneous parametric down-conversion (SPDC) in nonlinear crystals makes it fairly easy to generate entangled photon states. It has been known for some time that the conversion efficiency can be improved by employing quasi-phase-matching in periodically poled crystals. Using two single-photon detectors, we have analyzed the photon pairs generated by SPDC in a periodically poled lithium niobate crystal pumped by a femtosecond laser. Several parameters could be varied in our setup, allowing us to obtain data in close agreement with both thermal and Poissonian photon-pair distributions.

Interference of Photon Emitted by a Slit–Groove Structure after the Conversion of Photon to Surface Plasmons

We report on an observation of an interference pattern from a slit–groove structure that was fabricated in a Au film. Although we had not used a double-slit, a Young-like interference pattern was observed. The origin of the interference pattern can be attributed to the fact that one cannot distinguish between the photon reemitted from the slit and that reemitted from the groove after the conversion of the photon to surface plasmons (SPs). We have investigated the modulation of the fringe visibility by varying the geometric parameters (slit–groove distance and depth of the grooves) in order to explore the properties of SPs. In our experiment, a visibility of 24% was obtained in the case of a slit–groove distance of 9.7 µm and a groove depth of 120 nm.

Optical Transmission Properties of Subwavelength Slit Structures in Visible Wavelength Range

We report an experimental study of the optical transmission of polarized light through subwavelength slit structures in Au films. We investigated the transmission properties by varying the slit number, period, and width. The transmission spectra were strongly depend on the polarization of the incident light. Transmission minima were observed for transverse magnetic (TM) polarization at the wavelength of surface plasmon (SP) resonance at the air–Au interface. We measured the transmission normalized by the slit area at a wavelength of 780 nm by varying the slit width, and observed that the normalized transmission increased with the narrowing of the slit.

Interference of light emitted by a slit and a groove coupled by surface plasmons

We report on a method to generate an interference pattern from a slit-groove structure. Although we have not used a double-slit, a Young like interference pattern is observed due to the effect of surface plasmons.

Quantum key distribution over an installed multimode optical fiber local area network

Transmission of light in an under-filled mode distribution promises a single-mode like behavior in the multi-mode fiber. We performed the quantum key distribution over a 550-m installed multi-mode fiber with quantum-bit-error rate of 1.09%.

Single-photon source at the telecommunication wavelength using non-degenerate photon-pairs from a periodically poled LiNbO/sub 3/ waveguide

Using nondegenerate photon pairs from a periodically poled LiNbO/sub 3/ waveguide, a single photon source emitting at the telecommunication wavelength is proposed.