We have designed and fabricated one dimensional photonic bandgap (PBG) structures from dielectric multilayers of porous silicon, with a periodic repetition of a unit cell consisting of 21 layers (95%) with the refractive index varying according to the envelope of a Gaussian function and another layer (5%) with a fixed refractive index. The structures can be designed to demonstrate the wavelength scalability within the visible as well as near infrared region. Three different structures have been stacked together to enhance the width of the PBG. The omnidirectional nature of the PBG was verified experimentally and theoretically up to 68° and 89° angles of incidence, respectively.

For enhancing the omnidirectional photonic bandgap ͑OPBG͒, we report the fabrication of two different configurations of one-dimensional, wavelength scalable dielectric multilayer structures of porous silicon, consisting of a unit cell formed by varying the refractive index of the multilayers according to the envelope of a Gaussian function. As compared to the already reported OPBG of 88 nm ͑in the complete angular range of 0°to 89°͒, an enhancement up to 204 nm ͑2.3 times͒ was observed on stacking, six different Gaussian structures ͑balanced mirror͒ with only 8 periods each. An unbalanced mirror structure, consisting of the six similar Gaussian structures as the balanced mirror, but having different sequence of periods, ͑configuration with 13, 6, 5, 5, 6, and 13 periods for each Gaussian, respectively͒ was seen to demonstrate the OPBG of 252 nm ͑enhanced by 2.86 times͒. The total optical thickness of both the structures was kept to be the same. The omnidirectional nature of the PBG was verified experimentally up to 68°and theoretically up to 89.9°angle of incidence.In the recent years, one-dimensional ͑1D͒ photonic bandgap structures such as dielectric multilayers with an omnidirectional bandgap in a specific wavelength region 1,2 have been studied extensively. Such structures have an advantage over simple metallic mirrors of being nondispersive and nonabsorbing in the visible and near infrared ͑NIR͒ range. 3,4 Alternating layers of dielectric materials with two different dielectric constants are the simplest possible photonic crystals 5 with many applications. [6][7][8][9][10][11][12] Due to the versatile nature of porous silicon, 13 it has been established as a promising material for photonic applications. 14 1D porous silicon photonic bandgap structures have already found many applications such as dielectric mirrors, 15 waveguides, 16 sensors 17 and many other devices. 18 Bruyant et al. reported omnidirectional mirrors, based on 1D photonic crystals from porous silicon, 19 in two kinds of structures: simple periodic and chirped periodic structure such that the thickness of the last bilayer was 3.5% longer than the first bilayer. Apart from that in 2005, presented a theoretical study to enlarge the omnidirectional bandgap ͑OPBG͒ of two multilayered structures ͑named balanced and unbalanced mirrors͒ by varying only the thicknesses of the periods along the depth of the structure. Using the transfer matrix method, Arriaga and Saldaña 21 theoretically demonstrated the presence of OPBGs in the silicon based mirrors, with a periodic repetition of a unit cell composed of multilayers with a Gaussian profile refractive index. Recently our group 22 designed and fabricated 1D photonic bandgap ͑PBG͒ structures from dielectric multilayers of porous silicon, with a periodic repetition of a unit cell consisting of 22 layers, where the refractive index of the 95% of the unit cell varied according to the envelope of a Gaussian function. Three different Gaussian structures ͑with 15 unit cells each͒ were stacked together to en...

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Morphological properties of thermochromic VO2—porous silicon based hybrids reveal the growth of well-crystalized nanometer-scale features of VO2 as compared with typical submicron granular structure obtained in thin films deposited on flat substrates. Structural characterization performed as a function of temperature via grazing incidence X-ray diffraction and micro-Raman demonstrate reversible semiconductor-metal transition of the hybrid, changing from a low-temperature monoclinic VO2(M) to a high-temperature tetragonal rutile VO2(R) crystalline structure, coupled with a decrease in phase transition temperature. Effective optical response studied in terms of red/blue shift of the reflectance spectra results in a wavelength-dependent optical switching with temperature. As compared to VO2 film over crystalline silicon substrate, the hybrid structure is found to demonstrate up to 3-fold increase in the change of reflectivity with temperature, an enlarged hysteresis loop and a wider operational window for its potential application as an optical temperature sensor. Such silicon based hybrids represent an exciting class of functional materials to display thermally triggered optical switching culminated by the characteristics of each of the constituent blocks as well as device compatibility with standard integrated circuit technology.

Macroporous silicon substrates, with square-shaped pores, have been used to crystallize hen egg white lysozyme by the sitting drop vapor diffusion method. The X-ray diffraction technique was used to determine the tetragonal structure of the crystals. Use of an asymmetric anodization procedure to produce pore size gradients in porous structure, ranging from 400 nm to 1 μm, resulted in the formation of sub-micrometer-sized protein crystals within the macroporous structure. The presence of the crystals was observed by field emission scanning electron microscopy and confirmed by Raman and infrared spectroscopy. The present work provides experimental evidence of sub-micrometer crystal growth from pore corners and rough sides of the pore walls, attributed to the reduction of the potential energy for nucleation, in accordance with the different mathematical models developed so far.