Motoichi Ohtsu

Infrared–visible light conversion using DCM dye micrograins embedded in a resin sheet and application to an IR sensor card

We report high-efficiency visible light emission (λ=600–690 nm) from aggregated 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) dye micrograins excited by infrared light (λex = 805 nm). The light-emitting regions were localized at the surfaces and edges of the micrograins, where the optical near-field was more intense. This has been theoretically explained by exciton−phonon polaritons, i.e., the dressed photon model, in which an electron is excited due to a multistep transition via an intermediate phonon state coupled with a localized exciton polariton (a dressed photon). The lifetime (about 1 ps) of the intermediate state has been measured by the pump–probe method. We prepared a resin sheet containing the DCM and identified the origin of the intermediate state as an O-H stretching vibration mode, by time-resolved Raman spectroscopy using a 10 fs pulsed laser as an excitation light source. As the molecules aggregated, the emission efficiency with visible light excitation (λ<550 nm) decreased, i.e., concentration quenching, whereas the emission efficiency with infrared excitation increased. This is because a broad O-H phonon band with strong oscillator strength is created by hydrogen bonds among the molecules, acting as an efficient intermediate phonon state for the multistep transition. An IR sensor card was also demonstrated.We report high-efficiency visible light emission (λ=600–690 nm) from aggregated 4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran (DCM) dye micrograins excited by infrared light (λex = 805 nm). The light-emitting regions were localized at the surfaces and edges of the micrograins, where the optical near-field was more intense. This has been theoretically explained by exciton−phonon polaritons, i.e., the dressed photon model, in which an electron is excited due to a multistep transition via an intermediate phonon state coupled with a localized exciton polariton (a dressed photon). The lifetime (about 1 ps) of the intermediate state has been measured by the pump–probe method. We prepared a resin sheet containing the DCM and identified the origin of the intermediate state as an O-H stretching vibration mode, by time-resolved Raman spectroscopy using a 10 fs pulsed laser as an excitation light source. As the molecules aggregated, the emission efficiency with visible light excitation (λ<550 nm) decr...

1.3μm-Band Si photodetectors with optical gains fabricated by dressed photon assisted annealing

We fabricated a novel photodetector by subjecting a Si crystal having a p–n homojunction to phonon-assisted annealing. The photosensitivity of this device for incident light having a wavelength of 1.16 μm or greater was about three-times higher than that of a reference Si-PIN photodiode. The photosensitivity for incident light with a wavelength of around 1.32 μm was increased by applying a forward current. When the forward current density was 10 A/cm2, the device showed photosensitivities of 3.1 A/W at a wavelength of 1.14 μm and 0.10 A/W at 1.32 μm. The photosensitivity at 1.32 μm is at least 4000-times higher than the zero-bias photosensitivity. This remarkable increase was due to the manifestation of optical amplification cause by the forward current injection. For a forward current density of 9 A/cm2, the small-signal gain coefficient of the optical amplification was 2.2 × 10−2, and the saturation power was 7.1 × 102 mW.We fabricated a novel photodetector by subjecting a Si crystal having a p–n homojunction to phonon-assisted annealing. The photosensitivity of this device for incident light having a wavelength of 1.16 μm or greater was about three-times higher than that of a reference Si-PIN photodiode. The photosensitivity for incident light with a wavelength of around 1.32 μm was increased by applying a forward current. When the forward current density was 10 A/cm2, the device showed photosensitivities of 3.1 A/W at a wavelength of 1.14 μm and 0.10 A/W at 1.32 μm. The photosensitivity at 1.32 μm is at least 4000-times higher than the zero-bias photosensitivity. This remarkable increase was due to the manifestation of optical amplification cause by the forward current injection. For a forward current density of 9 A/cm2, the small-signal gain coefficient of the optical amplification was 2.2 × 10−2, and the saturation power was 7.1 × 102 mW.