Research

Minoru FUJII （藤井　稔） : Research on Mesoscopic Materials

RESEARCH ACTIVITIES

Porous Silicon Based Polarizing Elements for Visible to Infrared Range

		160.　Shinsuke Shichi, Minoru Fujii, Tomoki Nishida, Hidehiro Yasuda, Kenji Imakita, and Shinji Hayashi, Three-dimensional structure of (110) porous silicon with in-plane optical birefriengence　 Journal of Applied Physics, Vol. 111, 084303, pp. 1-6 (2012).
		Electrochemical etching of a (110) oriented Si wafer results in a porous silicon (PSi) layer which exhibits a strong in-plane optical birefringence. We study the refractive index ellipsoid of (110) PSi by angle-resolved optical transmittance measurements and reveal that it is a biaxial crystal. The angle-resolved transmission electron microscope observations demonstrate that pores grow along the directions in between the <100> crystal axes and the etching current flow and these directions depend on the etching current density. The etching current density dependence of the pore direction indicates that the shape of the index ellipsoid can be controlled by the etching condition.

		108. Nobuyuki Ishikura, Minoru Fujii, Kohei Nishida, Shinji Hayashi, and Joachim Diener, "Dichroic Rugate Filters Based on Birefringent Porous Silicon,” Optics Express, Vol. 16, No. 20, pp. 15531-15539 (2008).
		Rugate filters made of anisotropically nanostructured birefringent silicon have been fabricated and studied by polarization-resolved transmission measurements. Electrochemical etching of a (110) oriented Si wafer results in porous silicon layers which exhibit a strong in-plane birefringence. We demonstrate that a sinusoidal refractive index variation of birefringent porous silicon combined with index-matching layers and apodization results in a dichroic rugate filter having a stop-band dependent on the polarization direction of the incident light without higher-order harmonics and sidelobes. We also demonstrate that the combination of different dichroic rugate filters allow us to realize filters with more complex properties in a single preparation step.

		P58. Nobuyuki Ishikura, Minoru Fujii, Kohei Nishida, Shinji Hayashi, Joachim Diener, Minoru Mizuhata, and Shigehito Deki, "Design and Fabrication of Extended-Bandwidth Rugate Filters Made of Porous Silicon,” ECS Transactions, Vol. 16, No. 3, pp. 55-59 (2008). (Proceedings of ECS 214th meeting, Honolulu, Hawai, October 12 - 17 (2008).
		We designed and fabricated porous silicon-based broadband rugate filters by combining up to seven rugate structures. A near-infrared stop-band filter having a reflection band width of 1926 nm with no higher-order harmonics and very small sidelobes were realized. The suppression of higher-order harmonics and the reduction of sidelobes also allowed us to produce a high quality pass-band filter having three high transmittance regions consisting of broad stop-bands.

		106. Nobuyuki Ishikura, Minoru Fujii, Kohei Nishida, Shinji Hayashi, Joachim Diener, Minoru Mizuhata, and Shigehito Deki, "Broadband Rugate Filters Based on Porous Silicon,” Optical Materials, Vol. 31, pp. 102-105 (2008).
		We designed and fabricated porous silicon-based broadband rugate filters by combining up to five rugate structures. A near-infrared stop-band filter having a reflection band width of 1356 nm with no higher-order harmonics and very small sidelobes were realized. The suppression of higher-order harmonics and the reduction of sidelobes also allowed us to produce a high quality pass-band filter exhibiting a high transmittance region sandwiched by two broad stop-bands.

		70. Nicolei, Kuenzner, Joachim Diener, Egon Gross, Dmitri Kovalev, Victor Yu. Timoshenko, and Minoru Fujii, "Form Birefringence of Anisotropically Nanostructured Silicon,” Physical Reviev B, Vol. 71, pp. 195304-1-8, May (2005).
		We present a detailed study of the anisotropic optical properties of mesoporous silicon layers prepared from substrates having different doping levels under various preparation conditions. We demonstrate that the morphology of the layers strongly depends on the preparation conditions. It correlates with measured optical anisotropy values and defines the directions of the optical axes. The experimental data are explained in the framework of an effective medium model which takes into account the different morphologies of the layers. Modifications of the optical anisotropy of the layers in a controlled manner by filling the pores with dielectric substances and by oxidation of the structure confirm that form birefringence is the origin of the optical anisotropy.

		61. Joachim Diener, Nicolei Künzner, Egon Gross, Dmitri Kovalev, and Minoru Fujii, "Planar Silicon-based Light Polarizers,” Optics Letters, Vol. 29, No. 2, pp. 195-197, January (2004).
		Silicon-based thin-film polarizers operating in the visible and near-infraed spectral range are fabricated by electrochemical etching of bulk silicon wafers. Anisotropically etched (110) porous silicon layers exhibit a strong in-plane anisotropy of the refractive index. Stackes of alternating layers with different mean refractive indices and thicknesses act as dichroic Bragg reflectors or microcavities, respectively. Both structures have two distinct reflection and transmission bands depending on the polarization of the incident linearly polarized light. Planar polarizers are realized through the combination, in one structure, of a dichroic reflector with either a second reflector or a microcavity with different spectral responses.

		50. J. Diener, N. Künzner, D. Kovalev, E. Gross, F. Koch, and M. Fujii, "Dichroic Behavior of Multilayer Structures Based on Anisotropically Nanostructured Silicon,” Journal of Applied Physics, Vol. 91, No. 10, pp. 6704-6709, May (2002).
		Multilayer structures of anisotropically nanostructured (birefringent) silicon have been fabricated and studied by polarization-resolved reflection and transmission measurements. We demonstrate that stacks of birefringent porous silicon layers with alternating refractive indices and thicknesses act as dichroic Bragg reflectors or dichroic microcavities with a transmission/reflection dependent on the polarization direction of the incident light. The possibility of separate fine tuning of two orthogonally polarized transmission/reflection bands and their spectral splitting is demonstrated.

		44. N. Kuenzner, D. Kovalev, J. Diener, E. Gross, V. Yu. Timoshenko, G. Polisski, F. Koch, and M. Fujii, "Giant Birefringence in Anisotropically Nanostructured Silicon,” Optics Letters, Vol. 26, No. 16, pp. 1265-1267, August (2001).
		We performed a study of the in-plane birefringence of anisotropically nanostructured Si layers, which exhibit a greater difference in the main value of the anisotropic refractive index than that of natural birefringent crystals. The anisotropy parameters were found to be strongly dependent on the typical size of the Si nanowires used to assemble the layers. This finding opens the possibility of an application of birefringent Si retarders to a wide spectral range for control of the polarization state of light.