Tunable Visibly Transparent Optics Derived from Porous Silicon

Abstract

Visibly transparent porous silicon dioxide (PSiO 2 ) and PSiO 2 /titanium dioxide (TiO 2 ) optical elements were fabricated by thermal oxidation, or a combination of thermal oxidation and atomic layer deposition infilling, of an electrochemically etched porous silicon (PSi) structure containing an electrochemically defined porosity profile. The thermally oxidized PSiO 2 structures are transparent at visible wavelengths and can be designed to have refractive indices ranging from 1.1 to 1.4. The refractive index can be increased above 2.0 through TiO 2 infilling of the pores. Applying this oxidation and TiO 2 infilling methodology enabled tuning of a distributed Bragg reflector (DBR) formed from PSi across the visible spectrum. At the maximum filling, the DBR exhibited a transmission of 2% at 620 nm. Simulations match well with measured spectra. In addition to forming DBR filters, phase-shaping gradient refractive index (GRIN) elements were formed. As a demonstration, a 4 mm diameter radial GRIN PSiO 2 element with a parabolic, lens-like phase profile with a calculated focal length of 1.48 m was formed. The calculated focal length was reduced to 0.80 m upon the addition of TiO 2 . All the structures showed broad transparency in the visible and were stable to the materials conversion process.

Resonant Mode Engineering of Photonic Crystal Sensors Clad with Ultralow Refractive Index Porous Silicon Dioxide

“…Once on quartz, the patterned PSi thin film is gently rinsed with hexanes and left to dry in air. After drying, the patterned PSi thin film is thermally oxidized by exposure to a dry oxygen atmosphere, first at 500 °C to stabilize the microstructure and then at 925 °C to fully convert the PSi into PSiO 2 . The patterned PSiO 2 film with ultralow refractive index, as determined by thin film characterization of a planar PSiO 2 film on quartz ( Figure ), serves as a platform for sputter deposition of TiO 2 .…”

Porous Si-SiO2 based UV Microcavities

“…The effects of dry oxidation on BRs have been well studied within the VIS-NIR range 31,32,45 , where it is reported a refractive index decrease due to SiO 2 growing inside the Si matrix. Other authors reported a mixture of OPS and titanium dioxide (TiO 2 ) to form transparent BRs in the VIS range where the oxidation process eliminates optical losses across the VIS range 23 . Several studies on BRs have been made in the UV range using different Si substrates and applying two oxidation processes [20][21][22] .…”

“…After oxidation, the stopband shifted to 440 nm. In the second stage, OPS BRs were infiltrated with TiO 2 , and the stopband red-shifted to 492 nm; at maximum infilling, with TiO 2 , the stopband had a transmission of 2% (at 620 nm) 23 . M. Ghulinyan and C. J. Oton reported MCs centered in the infrared region where the PS is almost transparent; absorption losses play a much less important role than other loss mechanisms such as light scattering by the pores and the interfaces between layers 12,24 .…”

Metallothermic Reduction of Silica Nanoparticles to Porous Silicon for Drug Delivery Using New and Existing Reductants

“…In the last two decades,p orous Si has been extensivelye xplored as an interesting optical material, [1][2][3][4][5][6] drug delivery vehicle, [7][8][9][10][11] sensor, [12][13][14][15][16][17][18] gas storage medium, [19,20] anode material for Li-ion batteries, [21][22][23][24] and in other energy conversion systems. [25][26][27][28] The utility of porous Si is highly dependento ni ts surfacea rea, crystallinity,m orphology,a nd pore volume, which are often dictatedb yt he synthetic methods used for its preparation.…”