Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Bismuth-doped Near Infrared Luminescencet Thin Films

		181.　Minoru Fujii, Satoshi Morimoto, Shohei Kitano, Kenji Imakita, Jianrong Qiu, and Hong-Tao Sun, "Low-Temperature Growth of Near-Infrared Luminescent Bi-Doped SiO_xN_y Thin Films", Optics Letters, Vol. 38, Issue 20, pp. 4224-4227 (2013).
		Bi-doped siliconoxynitride (SiON:Bi) thin films were prepared by a sputtering method and the photoluminescence (PL) properties were studied. Without any thermal treatments, broad Bi-related luminescence was observed in the near-infrared (NIR) range. The luminescence efficiency depended strongly on the film composition. It was found that N atoms play a crucial role for the formation of Bi NIR luminescence centers. The effect of annealing on the luminescence efficiency was also studied. The optimum annealing temperature to have the largest number of Bi NIR luminescence centers depended strongly on the film composition and it was lower for films with lower N concentration. The PL excitation spectra revealed that two different Bi NIR luminescence centers exist in the films.

		170.　Kenji Imakita, Yuya Tsuchihashi, Ryo Naruiwa, Minoru Fujii, Hong-Tao Sun, Jianrong Qiu, and Shinji Hayashi, Ultrafast nonlinear optical responses of bismuth doped silicon-rich silica films Applied Physics Letters, Volume 101,Issue 19, 191106, pp. 1-4 (2012).
		Nonlinear optical responses of bismuth (Bi) doped silicon-rich silicon dioxide (Si-rich SiO2) films were studied by a z-scan and an optical Kerr gate method under femtosecond excitation around 800 nm. It was found that the Bi-doping enhances the nonlinear optical response of Si-rich SiO2 films by several orders of magnitudes. The nonlinear refractive index was of the order of 10−11 cm2/W and the response time was shorter than our time resolution of 100 fs. The nonlinear refractive index was independent of the wavelength in the range from 750 to 835 nm, suggesting that virtual transitions are involved in the nonlinear optical processes.

		167.　Satoshi Morimoto, Minoru Fujii, Hong-Tao Sun, Yuji Miwa, Kenji Imakita, Jianrong Qiu, and Shinji Hayashi, Broadband near-infrared emission from bismuth-doped multilayer films Journal of Applied Physics, Vol. 112, 073511, pp. 1-4 (2012).
		Multilayer structures consisting of bismuth (Bi)-doped silica thin films and different kinds of spacer, i.e., Si, silica, Si-rich silica, layers are grown and the luminescence properties are studied. When samples were annealed at a low temperature, Bi-related near infrared active centers (BRACs) were formed at interfaces between Bi-doped silica and Si-rich silica (or silicon) due to the reduction of Bi3þ to BRACs by silicon. On the other hand, films annealed at a high temperature showed similar emission behaviors to bulk glasses. The results demonstrated here establish a new strategy for the control of BRACs and building peculiar Bi activated film structures.

		159.　Sa chu rong gui, Kenji Imakita, Minoru Fujii, Zhenhua Bai, Shinji Hayashi, Luminescence properties of Bi-doped oxidized porous silicon thin films　 Optical Materials, Vol. 34, pp. 1161-1164 (2012).
		Luminescence properties of Bi-doped oxidized porous silicon (OPS) thin films were studied. It was found that this material shows two broad luminescence bands centered at 845 nm with the FWHM of 120 nm and at 1410 nm with that of 220 nm under 488 nm excitation. A detailed analysis of the 3D plot of PL intensities versus excitation and emission wavelengths revealed that these luminescence bands arise from at least two different kinds of Bi luminescence centers. The broad luminescence covering the whole telecommunication window (1.2–1.6 μm) suggests that Bi-doped OPS thin films can be a candidate material for a broadband waveguide-type optical amplifier at optical telecommunication wavelengths.

		154.　Yuji Miwa, Hong-Tao Sun, Kenji Imakita, Minoru Fujii, Yu Teng, Jianrong Qiu, Yoshio Sakka, and Shinji Hayashi, Sensitized broadband near-infrared luminescence from bismuth-doped silicon-rich silica films　 Optics Letters, Vol. 36, No. 21, pp. 4221-4223, November 1 (2011).
		Developing Si compatible optical sources has attracted a great deal of attention owing to the potential for forming inexpensive, monolithic Si-based integrated devices. In this Letter, we show that ultra broadband near-IR (NIR) luminescence in the optical telecommunication window of silica optical fibers was obtained for Bi-doped silicon-rich silica films prepared by a co-sputtering method. Without excess Si, i.e., Bi-doped pure silica films, no luminescence was observed in the NIR range. A broad Bi-related NIR photoluminescence appears when excess Si was doped in the Bi-doped silica. The luminescence properties depended strongly on the amount of excess Si and the annealing temperature. Photoluminescence results suggest that excess Si acts as an agent to activate Bi NIR luminescence centers and also as an energy donor to transfer excitation energy to the centers. It is believed that this peculiar structure might find some important applications in Si photonics.

		132.　Hong-Tao Sun, Fumiaki Shimaoka, Yuji Miwa, Jian Ruan, Minoru Fujii, Jianrong Qiu, and Shinji Hayashi, Sensitized superbroadband near-IR emission in bismuth glass/Si nanocrystal superlattices　 Optics Letters, Vol. 35, Issue 13, pp. 2215-2217 (2010) .
		We show that sensitized superbroadband near-IR (NIR) emission in bismuth glass/Si nanocrystal superlattices can be realized. Photoluminescence is enhanced by 1 order of magnitude in this structure. We observed that the excitation wavelength dependence of the NIR emission does not show any distinct structure corresponding to the direct transition of bismuth IR-active centers. Our results suggest that the enhanced emission might result from the energy transfer from Si nanocrystals to IR-active bismuth. This structure may find broad applications for broadband amplifiers and broadly tunable laser sources.