Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Non-linear Optical Properties of Group IV (Si, Ge) Semiconductor Nanocrystals

		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.

		163.　Kenji Imakita, Masahiko Ito, Ryo Naruiwa, Minoru Fujii, and Shinji Hayashi, Ultrafast third order nonlinear optical response of donor and acceptor codoped and compensated silicon quantum dots　 Applied Physics Letters, Vol. 101(4), 041112, pp. 1-3 (2012).
		Nonlinear optical responses of phosphorus and boron codoped and compensated silicon quantum dots (Si-QDs) embedded in borophosphosilicate glass were studied by a z-scan and an optical Kerr gate methods under femtosecond excitation at 780 nm. The nonlinear refractive index (n2) and the two photon absorption coefficients (β) of compensated Si-QDs were found to be enhanced several times compared to those of intrinsic Si-QDs. The response time was shorter than our time resolution of 100 fs at room temperature.

		161.　Kenji Imakita, Masahiko Ito, Ryo Naruiwa, Minoru Fujii, and Shinji Hayashi, Enhancement of ultrafast nonlinear optical response of silicon nanocrystals by boron-doping　 Optics Letters, Vol. 37, No. 11, pp. 1877-1879 (2012).
		Nonlinear optical responses of boron (B)-doped silicon nanocrystals (Si-ncs) embedded in borosilicate glass were studied by z-scan and optical Kerr gate methods under femtosecond excitation at 780 nm. The nonlinear refractive index (n2) and the two photon absorption coefficients (β) of B-doped Si-ncs were found to be 3 times enhanced, compared to those of intrinsic Si-ncs. The response time was faster than 100 fs even at 5 K. The origin of the large nonlinear optical response was discussed, based on the experimental data of n2, electron spin resonance spectra, and linear absorption spectra.

		138.　Masahiko Ito, Kenji Imakita, Minoru Fujii, and Shinji Hayashi, Nonlinear Optical Properties of Phosphorus Doped Silicon Nanocrystals/Nanoclusters Journal of Physics D: Applied Physics, Vol.43, 505101 pp. 1-5 (2010).
		Comprehensive studies have been performed on the nonlinear optical responses of silica films containing phosphorus (P)-doped Si nanoclusters and/or nanocrystals. In P-doped Si nanocrystals, enhancements of the nonlinear refractive indices (n2) and two photon absorption coefficients (β) relative to those of intrinsic Si nanocrystals were observed. The analysis of electron spin resonance and absorption spectra revealed that P donors are responsible for the large enhancements of n2 and β.

		136.　Masahiko Ito, Minoru Fujii, Kenji Imakita, and Shinji Hayashi Nonlinear Optical Properties of Silicon Nanoclusters/Nanocrystals Doped SiO₂ Films - Annealing Temperature Dependence Journal of Applied Physics, Vol. 108, 063512 pp. 1-5 (2010).
		Comprehensive studies on nonlinear refractive indices (n2) of SiO2 films containing Si nanocrystals and/or nanoclusters (SiO2:Si-ncs) are performed. The comparison of the nonlinear refractive indices with the electron spin resonance signals reveals that defect states play a major role in the large n2 when the annealing temperature is low, i.e., when Si nanoclusters exist in films. On the other hand, when Si nanocrystals are grown by high-temperature annealing, the contribution of defect states becomes small and that of the quantized electronic states of Si nanocrystals becomes large. The present results demonstrate that both the defect states and the quantized electronic states should be taken into account to explain the origin of large n2 of SiO2:Si-ncs and to optimize the structure to maximize n2.

		121. Kenji Imakita, Masahiko Ito, Minoru Fujii, and Shinji Hayashi, "Nonlinear Optical Properties of Phosphorous-doped Si Nanocrystals Embedded in Phosphosilicate Glass Thin Films,” Optics Express, Vol. 17, No. 9 , pp. 7368-7376 (2009).
		Nonlinear optical properties of phosphorus (P) -doped silicon (Si) nanocrystals are studied by z-scan technique in femtosecond regime at around 1.6 eV. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β ) of Si-ncs are significantly enhanced by P-doping. The enhancement of n2 is accompanied by the increase of the linear absorption in the same energy region, suggesting that impurity-related energy states are responsible for the enhancement of the nonlinear optical response.

		120. Kenji Imakita, Masahiko Ito, Minoru Fujii, and Shinji Hayashi, "Nonlinear Optical Properties of Si Nanocrystals Embedded in SiO₂ Prepared by a Cosputtering Method,” Journal of Applied Physics, Vol. 105, 093531, pp. 1-5 (2009).
		Nonlinear optical properties of Si nanocrystals (Si-ncs) doped SiO2 prepared by a cosputtering method were studied by z-scan technique in a femtosecond regime at around 1.6 eV. The nonlinear refractive index (n2) and nonlinear absorption coefficient (β) were strongly enhanced compared to those of bulk Si and found to be about ∼ 2×10−13 cm2/W and ∼ 0.8 cm/GW, respectively. In the photon energy region from 1.48 to 1.65 eV, the n2 and β spectra followed the absorption spectra and no enhancement was observed in the band-edge photoluminescence region. In the diameter range of 2.7–5.4 nm, the size dependence of n2 coincided well with that calculated by a pseudopotential approach, suggesting that the discrete energy states of Si-ncs are responsible for the observed enhanced optical nonlinearity.