Youngdae Koh

Photonic polymer replicas from distributed Bragg reflectors structured porous silicon.

Well defined 1-dimentional (1-D) photonic crystals of polymer replicas have been successfully obtained. DBR porous silicon containing nanometer-scale pores are prepared by an anodic electrochemical etch of p(++)-type silicon wafer. The resulting DBR porous silicon film removed from the substrate by applying an electropolishing current has been thermally oxidized in the furnace at 400 degrees C for 3 h. Oxidized DBR PSi/polystyrene composite films are prepared by casting of polymer solution onto a free-standing porous silicon photonic crystal layer. Flexible photonic polymer replicas have been prepared after the removal of oxidized DBR PSi matrix in HF/H2O mixture solution. Polymer replicas exhibit a sharp resonance in the reflectivity spectrum. Optical characteristics of photonic polymer replica indicate that the surface of polymer film has a negative structure of DBR PSi. This replica is stable in aqueous solutions for several days without any degradation.

1-D photonic crystals of free-standing DBR PSi for sensing and drug delivery applications

Free-standing multilayer distributed Bragg reflectors (DBR) porous silicon dielectric mirrors, prepared by electrochemical etching of crystalline silicon using square wave currents are treated with polystyrene to produce flexible, stable composite materials in which the porous silicon matrix is covered with caffeine-impregnated polystyrene. Optically encoded DBR PSi/polystyrene composite films retain the optical reflectivity. Optical characteristics of DBR PSi/polystyrene composite films are stable and robust for 2 hrs in a pH

Smart-particles-containing Multiple Photonic Band Gaps Based on Rugate-structured Porous Silicon

Photonic crystals (PCs) containing multi-encoded rugate porous silicon (PSi) were prepared by applying a composite waveform, a sum of three computer-generated pseudo-sinusoidal current waveforms. The PCs displayed three sharp photonic band gaps in the optical reflectivity spectrum. Three types of rugate PSi (H-, HO-, and R-terminated rugate PSi) were used in this study. Free-standing rugate PSi films were obtained from the silicon substrate by applying an electropolishing current and were then made into particles by using an ultrasono-method in an ethanol solution. Sensing experiments using these particles with organic solvents such as methanol and hexane were performed. Condensation of the organic vapors in the pores increased the refractive index of all the particles and resulted in a red shift in the photonic peaks. The specificity of adsorption or capillary condensation at the surfaces of the smart particles depended dramatically on the surface chemistry.

FABRICATION AND CHARACTERIZATION OF PRISMATIC BAND FILTER GRADIENT RUGATE POROUS SILICON

Band filter gradient (BFG) rugate porous silicon (PSi) was generated by an electrochemical etching of silicon wafer through an asymmetric electrode configuration in aqueous ethanolic HF solution. BFG rugate PSi prepared from anisotropic etching conditions displayed the reflection band whose reflection maximum varied spatially across the PSi. BFG rugate PSi displayed a prismatic reflection due to the disproportion of current from the position of cathode across the silicon wafer. In this work, we have developed a method to create planar gradient refractive index in Si substrate. Reflection peak of BFG rugate PSi was shifted to shorter wavelength and its pore sizes and film thickness were decreased across the silicon wafer.

Fabrication of Optically Encoded Images on Porous Silicon

Optical images on the porous silicon exhibiting Febry-Perot fringe pattern have been prepared by using an electrochemical etching of p-type silicon wafer (boron-doped, orientation, resistivity ) and beam projector. The images remained in the substrate displayed an optical images correlating to the optical pattern and could be useful for optical data storage. A decrease in the effective optical thickness of the Febry-Perot layers was observed, indicative of a change in refractive index induced by exposing of porous silicon to the white light. This provides the ability to fabricate complex optical encoding in the surface of silicon.

Dual Photonic Transduction of Porous Silicon for Sensing Gases

Porous silicon exhibiting dual optical properties, both fringe (optical reflectivity) and photoluminescence had been developed and used as chemical sensors. Porous silicon samples were prepared by an electrochemical etch of p-type silicon wafer (boron-doped, orientation, resistivity ; ). Two different types of porous silicon, fresh porous silicon (Si-H terminated) and oxidized porous silicon (Si-OH terminated)by the thermal oxidation, were prepared. Then the samples were exposed to the vapor of various organics, such as methanol, acetone, hexane, and toluene. Both reflectivity and photoluminescence were simultaneously measured under the exposure of organic vapors for sensing VOC`s. These surface-modified samples showed unique respond in both reflectivity and photoluminescence with various organic vapors. While polar molecules exhibit greater quenching photoluminescence, molecules having higher vapor pressure show greater red shift for reflectivity.

DBR PSi/Polymer Composite Materials -Dual Photonic Characteristics

DBR (distributed Bragg reflectors) PSi (porous silicon) composite films displaying dual optical properties, both optical reflectivity and photoluminescence had been developed. DBR PSi samples were prepared by electrochemical etch of heavily doped silicon wafers (boron doped, polished on the face, resistivity of , Siltronix, Inc.). Free-standing DBR PSi films were treated with PMMA (polymethyl methacrylate) to produce flexible, stable composite materials in which the PSi matrix is covered with PMMA containing photoluminescent polysiloles. Optical characteristics of DBR PSi/polysilole-impregnated PMMA composite materials exhibit both their photonic reflectivity at 565 nm and photoluminescence at 510 nm, simultaneously. A possible application of this materials will be discussed.