RESEARCH ACTIVITIES   Surface Plasmon Enhanced Luminescence from Silicon Nanocrystals   123. Yugo Mochizuki, Minoru Fujii, Shinji Hayashi, Takaaki Tsuruoka, and Kensuke Akamatsu “Enhancement of Photoluminescence from Silicon Nanocrystals by Metal Nanostructures made by Nanosphere Lithography,” Journal of Applied Physics, Vol. 106, 013517, pp. 1-5 (2009).  The effect of metal nanostructures prepared by nanosphere lithography on photoluminescence (PL) properties of silicon nanocrystals (Si-ncs) is studied. By placing Ag nanotriangles or Au nanovoids on SiO2 films containing Si-ncs, the PL intensity is enhanced. For the sample having Ag nanotriangles, the largest PL enhancement is obtained when the excitation wavelength coincides with the absorption band of Ag nanotriangles. This suggests that the enhancement of the incident field by surface plasmon polariton (SPP) excitation is responsible for the PL enhancement. On the other hand, for the sample having Au nanovoids, the PL enhancement is mainly made by the enhancement of effective radiative decay rate of Si-ncs by efficient excitation and scattering of SPPs.   95. Toshihiro Nakamura, Minoru Fujii, Satoru Miura, Masaki Inui, and Shinji Hayashi, "Spontaneous Emission Rate of Si Nanocrystals on Thin Au Film,” Japanese Journal of Applied Physics, Vol. 46, No. 10A, pp. 6498-6502, October (2007). The effect of Au films with different thicknesses (20 and 100 nm) on the luminescence decay rate of Si nanocrystals was studied in a wide wavelength range. For the sample with the Au thickness of 100 nm, the luminescence decay rate oscillated depending on the distance between the Si nanocrystals and the Au film owing to the modification of the photonic mode density at the position of the Si nanocrystals. On the other hand, when the thickness of the Au film was 20 nm, the oscillation was not observed and a strong enhancement of the decay rate appeared in a small distance range. The degree of enhancement depended on the emission wavelength. These effects are considered to arise from a strong coupling between the electronic excitation of Si nanocrystals and the surface plasmon polariton modes supported by the rough Au surface.   P57. Nobuyuki Ishikura, Minoru Fujii, Masaki Inui, and Shinji Hayashi "Photoluminescence Properties of Si Nanocrystals near Rough Au Films,” Transactions of the Materials Research Society of Japan, Vol. 33, No. 1, pp. 141-144 (2008). (Proceedings of 18th Materials Research Society of Japan Academic Symposium, Nihon University, Tokyo, December 9 (2007))  We studied metal-enhanced photoluminescence (PL) from Si nanocrystals (Si-ncs) placed near rough Au films. Au films with different degree of roughness were prepared by electroless Au plating. We found that the PL intensity and the decay rate increase as the roughness becomes large. The PL excitation spectra revealed that the PL enhancement is the largest when the excitation wavelength corresponds to that of the surface plasmon resonance of rough Au films. The result combined with the emission wavelength dependence of PL enhancement factors suggest that the PL enhancement is caused by the enhancement of an electric field of incident light due to the excitation of surface plasmons supported by the rough Au films and also the increase of radiative decay rate of Si-ncs.   93. Eiji Takeda, Minoru Fujii, Toshihiro Nakamura, Yugo Mochizuki, and Shinji Hayashi, "Enhancement of Photoluminescence from Excitons in Silicon Nanocrystals via Coupling to Surface Plasmon Polaritons,” Journal of Applied Physics, Vol. 102, pp. 023506-1-6, July (2007).  The enhancement of photoluminescence (PL) is demonstrated from silicon nanocrystals (Si-ncs) by strong coupling of excitons to surface plasmon polaritons (SPPs) supported by a Au thin film. SPPs excited via excitons in Si-ncs were Bragg scattered to photons by one- or two-dimensional gratings, and strong and directional PL was obtained. From the angular dependence of PL spectra, dispersion relations of electromagnetic modes involved in the light emission process were obtained. The overall agreement between experimentally obtained and theoretically calculated dispersion relations confirmed that the strong and directional PL is mediated by SPPs. The PL decay rate of Si-ncs increased by placing a Au thin film on top and the wavelength dependence of the rate enhancement agreed well with that of the calculated SPP excitation rate. This suggests that the observed PL enhancement is due to efficient energy transfer from excitons to SPPs followed by efficient scattering of SPPs to photons, resulting in the enhancement of luminescence quantum efficiency.   86. Eiji Takeda, Toshihiro Nakamura, Minoru Fujii, Satoru Miura, and Shinji Hayashi, "Surface Plasmon Polariton Mediated Photoluminescence from Excitons in Silicon Nanocrystals,” Applied Physics Letters, Vol. 89, 101907, pp. 1-3, September (2006). Surface plasmon polaritons (SPPs) of a metal film can efficiently be excited when a light emitter is placed nearby. The excited SPPs are converted to photons by compensating for the momentum mismatch. The authors study SPP-mediated emission from excitons in Si nanocrystals (Si-nc’s) by placing an organic grating on a thin Au film placed near Si-nc’s. The dispersion relation is obtained from angle-resolved photoluminescence measurements, and all the observed modes are well explained by model calculation. The results indicate that excitons in Si-nc’s can efficiently excite SPPs in thin metal films and directed photoluminescence can be realized.   85. Kenji Imakita, Minoru Fujii, Toshihiro Nakamura, Satoru Miura, Eiji Takeda and Shinji Hayashi, "Enhancement of Radiative Recombination Rate of Excitons in Si Nanocrystals on Au Film,” Japanese Journal of Applied Physics, Vol. 45, No. 8A, pp. 6132-6136, August (2006). We investigate the time-resolved photoluminescence (PL) spectra of Si-nanocrystal (Si-nc)-doped SiO2 on Au thin films. It is shown that PL intensity within several tenth of µs after excitation is increased in the presence of Au films. The data suggest that the radiative recombination rate of excitons in Si-nc's is increased, and the degree of increase depends strongly on the emission photon energy. We show that the enhancement is caused by the modification of the local photonic mode density in the presence of Au thin films.