Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Surface Plamson Enhanced Upconversion in Rare-earth Doped Materials

		(proceeding) 62. Taishi Nakano, Minoru Fujii, Kenji Imakita, Shinji Hayashi, Takaaki Tsuruoka, and Kensuke Akamatsu, "Enhancement of upconversion luminescence of Al₂O₃: Er³⁺, Yb³⁺ thin films by small assemblies of Au nanorods ,” Transactions of the Materials Research Society of Japan, Vol. 38, pp. 131-134 (2013). (Proceedings of IUMRS International Conference on Electronic Materials 2012, IUMRS-ICEM 2012, September 23(Sun)-28(Fri), 2012, Yokohama, Japan)
		Enhancement of upconversion luminescence by small assemblies of Au nanorods such as dimers, trimers, tetramers, etc, is studied by comparing upconversion images with electron microscope images. We show that small assemblies starting from dimers can enhance the upconversion luminescence, while isolated Au nanorods cannot. The analysis of the data reveals that the gaps between nanorods act as hot spots for the enhancement of upcnversion luminescence.

		179.　Toshiyuki Kodama, Minoru Fujii, Taishi Nakano, Kenji Imakita, Shinji Hayashi, "Enhancement of Upconversion Luminescence of Er and Yb Co-doped Y₂O₃ Nanoparticle by Ag Half-Shell", Optical Materials, Vol. 35, Issue. 12, pp. 2394–2399 (2013).
		Upconversion photoluminescence (PL) properties of single Y2O3 nanoparticles doped with Yb and Er (Y2O3:Yb,Er) with a Ag over-layer is studied. We traced the PL and light scattering images of individual nanoparticles by changing the thickness of a Ag over-layer. When the Ag thickness is relatively small and only the upper part of a nanoparticle is covered by Ag (Ag half-shell), the PL is strongly enhanced. On the other hand, when the Ag thickness is increased and a continuous Ag over layer is formed, the enhancement factor decreases. From the correlation between the enhancement factors of the upconversion PL and scattering intensities as well as the change of the PL lifetime, the mechanism of the PL enhancement is discussed.

		172.　Minoru Fujii, Taishi Nakano, Kenji Imakita, and Shinji Hayashi, Upconversion Luminescence of Er and Yb Co-Doped NaYF₄ Nanoparticles with Metal Shells The Journal of Physical Chemistry C, Vol. 117, pp. 1113-1120 (2013).
		Upconversion photoluminescence (PL) of a composite nanoparticle consisting of an Er and Yb codoped NaYF4 core and a Au shell is studied theoretically and experimentally. We first investigate the effects of a Au shell on the radiative and nonradiative emission rates of a dipole placed in a core, the absorption and scattering cross sections of a composite nanoparticle, and the electric field within a core at the excitation wavelength. We then synthesize the composite nanoparticle and study the PL properties. From the analyses of the PL data in combination with the data obtained by theoretical calculations, the mechanism of the enhancement and quenching of upconversion PL by the formation of a Au shell is studied.

		102. Takeho Aisaka, Minoru Fujii, and Shinji Hayashi, "Enhancement of Upconversion Luminescence of Er Doped Al2O3 Films by Ag Island Films,” Applied Physics Letters Vol. 92, No. 13, pp. 132105-1-3 (2008).
		Strong enhancement of upconversion photoluminescence (PL) of Er3+ in the visible range was achieved by placing it near rough Ag island films. In order to understand the mechanism of the enhancement, PL was excited by 978 and 488 nm light and the enhancement factors were obtained in a wide spectral range (520 nm to 1.6 μm). The emission and excitation wavelength dependence of the enhancement factors revealed that both the incident electric fields and the radiative decay rate of Er3+ are enhanced due to the excitation of surface plasmons supported by Ag islands. It was found that the upconversion PL is more sensitive to the enhancement of the incident electric field than the downconversion PL because of the multiphoton absorption process. The present result indicates that the upconversion PL obtains more benefit from the metal-enhanced fluorescence technique than the downconversion PL.

		P60. Toshihiro Nakamura, Minoru Fujii, Yoshiko Usui, T. Aisaka, and Shinji Hayashi, "Enhancement of Upconversion Luminescence from Er Doped Al2O3 Films by a Thin Metal Layer ,” ECS Transactions, Vol. 16, Issue 31, pp. 87-93 (2009). (Proceedings of ECS 214th meeting, Honolulu, Hawai, October 12 - 17 (2008))
		The effect of a metal layer on upconversion luminescence of Er doped Al2O3 thin films is studied. Three visible luminescence peaks, which are assigned to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions of Er3+, are observed by excitation with 978.5 nm light. When a Au or Ag layer is deposited on top of the samples, the upconversion luminescence intensity is strongly enhanced. Roughness at an Al2O3-metal interface is considered to play a significant role for the enhancement. From the comparison of the enhancement factors between the upconversion PL and downconversion PL excited by 488.0 nm light, the mechanism of the PL enhancement is discussed.