An ultra-small photonic crystal (PC) optical microcavity to detect small changes in concentration of chemical analytes in solutions is demonstrated. The high electric field concentration in small modal volumes of PC microcavities leads to high quality factor and efficient light-matter interaction with ultrasmall volumes of analyte and makes PC microcavities very sensitive to changes in refractive index of the surrounding medium. A linear PC microcavity was fabricated with four missing holes in a triangular lattice of air rods in a GaAs slab with an embedded quantum-dot active region on a GaAs substrate. The detection scheme depends upon the shifts of the sharp resonances supported by polymer-coated PC microcavities, due to change in refractive index, when in contact with a solution with a particular ionic concentration. For ClO₄- anions, the resonance peaks shift by 20nm over the concentration range 10^-3M to 10^-6M. A smaller shift of 5nm is observed for Ca²⁺ ions over the same range. The device is a demonstration of PC microcavities in ion sensing. The microcavity sensing area is smaller than in microsphere-based detection or surface plasmon resonance imaging techniques and the devices are ideal as optical sensors for "lab-on-a-chip" microfluidic applications with minuscule volumes of analyte.

The efficiency of standard incandescent light sources is limited by strong thermal emission in the infrared regime. It is possible that emission of light may be more efficient when the conventional tungsten filament is replaced by metallic photonic crystals that have large photonic band gaps in the infrared and can suppress the thermal emission of blackbody emitters. One approach toward fabricating photonic crystal structures with highly ordered periodic features on an optical length scale involves colloidal crystal templating to produce inverse opals. Metallic inverse opals were synthesized using chemical vapor deposition (CVD) and wet chemical methods capable of producing granules, thin films and monolithic pieces. Thin films were prepared by infiltrating silica opal films with tungsten hexacarbonyl in a CVD process, reducing tungsten in hydrogen and removing the silica template by HF etching. A range of soluble metal precursors, including tungsten(VI) chloride, tungsten(V) ethoxide and acetylated peroxotungstic acid, were infiltrated into self-assembled, colloidal crystal arrays comprised of monodisperse poly(methyl methacrylate) (PMMA) spheres. The infiltrated composites were processed under reducing conditions to produce metallic inverse replicas of the template. The influence of processing conditions on structural properties, including thickness of skeletal walls, window openings and solid filling fraction, was studied. A monolithic tungsten inverse opal with dimensions of 0.5 × 0.5 × 0.2 cm was resistively heated in an inert atmosphere and thermal emission was observed. The wet chemical methods provide a low cost alternative to expensive nanolithographic methods for the fabrication of three-dimensional periodic metallic structures.

A new class of colloidal crystals, whose structural color can be tuned by changing lattice constants, was fabricated. They were composed of polystyrene (PS) submicron particles embedded in a silicone elastomer. The particles were self-assembled into a cubic close packing (ccp) structure, and the ccp (111) planes were parallel to the substrate. These ccp (111) planes produced the structural color of the crystal film by the Bragg's diffraction of incident light. The center-to-center distance between the planes, d was tuned by two approaches. One of the approaches involves the tuning of structural color by increasing d. A composite film fabricated with 202-nm PS particles exhibited a green color; its color changed to red due to the swelling of PDMS elastomer in solvents. On the other hand, a composite film coated on an elastic rubber sheet was stretched horizontally. Therefore, the lattice distance of ccp (111) planes decreased and the wavelength of reflected light reduced as a function of sheet elongation. In contrast, the structural color of the elastic rubber sheet was changed by applying mechanical stress. By tuning the structural color, composite films can be used for optical sensing, thereby avoiding special detector equipment.

Photonic bandgap (PBG) structures have remarkable optical properties that can be exploited for biosensing applications. We describe the fabrication of 1-D PBG biosensors using porous silicon. The optical properties of porous silicon PBGs are sensitive to small changes of refractive index in the porous layers, which makes them a good sensing platform capable of detecting binding of the target molecules to the bioreceptors. The material nanostructure and device configuration that lead to optimum performance of the devices are investigated in detail by modeling the optical response. It is shown that porous silicon based PBG sensors are useful for detecting biological matter, from small molecules to larger proteins.

We study the interaction of silicon photonic crystal nanocavities with infiltrated colloidal PbS nanocrystals as a viable and efficient source for achieving indistinguishable and single photons. Nanocrystal-nanocavity coupling is predicted at near-infrared wavelengths, suggesting the possibility towards exciting silicon-based nanophotonic lasers, and novel efficient sources for fiber and silicon-based quantum information networks and systems. Two effective designs for nanocrystal-nanocavity coupling are illustrated that exhibit moderate to high cavity quality factors, and ultra-small modal volumes for spontaneous emission enhancements. It is shown that in principle our system can approach the observation of strong exciton-cavity coupling in a solid-state implementation at room temperature.

We review the studies conducted in our group concerning electromagnetic properties of metamaterials and photonic crystals with negative effective index of refraction. In particular, we demonstate the true left handed behavior of a 2D composite metamaterial, by analyzing the electric and magnetic response of the material components systematically. The negative refraction, subwavelength focusing, and flat lens phenomena using 2D dielectric photonic crystals are also presented.

Certain select structures in photonic crystals (PhCs) exhibit complete photonic band gap i.e. a frequency region where the photonic band gaps for both polarizations (i.e. transverse electric and transverse magnetic modes) exist and overlap. One of the most fundamental applications of the photonic band gap structures is the design of photonic crystal waveguides, which can be made by inserting linear defects in the photonic crystal structures. By setting closely two parallel 2D PhC waveguides, a directional waveguide coupler can be designed, which can be used to design a polarization splitter. In this paper we design a polarization splitter in a photonic crystal structure composed of two dimensional honeycomb pattern of dielectric rods in air. This photonic crystal structure exhibits a complete photonic band gap that extends from λ = 1.49 μm to λ = 1.61 μm, where lambda is the wavelength in free space, providing a large bandwidth of 120 nm. A polarization splitter can be made by designing a polarization selective coupler. The coupling lengths at various wavelengths for both polarizations have been calculated using the Finite Difference Time Domain method. It has been shown that the coupling length, for TE polarization is much smaller as compared to that for the TM polarization. This principle is used to design a polarization splitter of length 32 μm at λ = 1.55 μm. Further, the spectral response of the extinction ratios for both polarizations in the two waveguides at propagation distance of 32 μm has been studied.

A one-dimensional model is presented to explain the physics of solid core photonic crystal fibers. The model provides a clear way to demonstrate many of the interesting characteristics of these fibers: variation of cladding index with wavelength, endlessly single-mode operation, short wavelength index limit, long wavelength index limit, and variation of these properties with the air/silica fraction. The effective index is calculated for a laminar cladding consisting of periodic layers of alternating high and low index dielectrics. The waveguide model consists of the same periodic layers surrounding a high-index core through which most of the light propagates. The light is confined by total internal reflection. The model is shown to be an accurate analogue for a more complicated two-dimensional finned dielectric waveguide.

Microstructured fibers (MOFs) are among the most innovative developments in optical fiber technology in recent years. These fibers contain arrays of tiny air holes that run along their length and define the waveguiding properties. Optical confinement and guidance in MOFs can be obtained either through modified total internal reflection, or photonic bandgap effects; correspondingly, they are classified into index-guiding Holey Fibers (HFs) and Photonic Bandgap Fibers (PBGFs). MOFs offer great flexibility in terms of fiber design and, by virtue of the large refractive index contrast between glass/air and the possibility to make wavelength-scale features, offer a range of unique properties. In this paper we review the current status of air/silica MOF design and fabrication and discuss the attractions of this technology within the field of sensors, including prospects for further development. We focus on two primary areas, which we believe to be of particular significance. Firstly, we discuss the use of fibers offering large evanescent fields, or, alternatively, guidance in an air core, to provide long interaction lengths for detection of trace chemicals in gas or liquid samples; an improved fibre design is presented and prospects for practical implementation in sensor systems are also analysed. Secondly, we discuss the application of photonic bandgap fibre technology for obtaining fibres operating beyond silica's transparency window, and in particular in the 3μm wavelength region.

We report the calculation of the attenuation coefficient of a probing optical mode due to interaction with metallic nanoparticles randomly distributed in the air holes of a solid core photonic crystal fiber (PCF) for SERS-based sensing and detection. The approach employed is an approximation of the solid core PCF with conventional curricular fiber considering a similar total internal reflection mechanism for mode propagation and almost complete concentration of the mode flux in the core of PCF. Losses due to the absorption and radiative scattering of electromagnetic energy by nanoparticles are examined. The analysis demonstrates a critical dependence of the absorption losses approaching the resonant localized surface plasmon's excitation and very fast rise of radiative losses with the increase of nanoparticle size. The physics of proper integral Raman spectroscopy is also discussed.

Raman spectroscopy using a microstructured optical fiber is discussed, with focus on evanescent sensing. It is shown that the optimum fiber has a lattice pitch close to the airhole diameter.

We have recently fabricated continuous semiconducting micro and nanowires within the empty spaces of highly ordered microstructured (e.g., photonic crystal or holey) optical fibers (MOF's). These systems contain the highest aspect ratio semiconductor micro- and nanowires yet produced by any method: centimeters long and ~100 nm in diameter. These structures combine the flexible light guiding capabilities of an optical fiber with the electronic and optical functionalities of semiconductors and have many potential applications for in-fiber sensing, including in-fiber detection, modulation, and generation of light.

Although long-period gratings (LPGs) and photonic crystal fibers (PCFs) have emerged at the same time and been around for almost ten years, the fabrications of fiber components in PCFs have attracted great attention in recent years. Post processing of PCFs with a CO₂ laser is very powerful and versatile method for making miniature compact fiber-based devices including LPGs and phase-shifted LPGs. This article will review our research work on fabrication of those gratings in PCFs by use of focused beam from a CO₂ laser and point-by-point writing fashion. Either the mechanical stress relaxation technique or surface deformation method is employed in the design and fabrication of the gratings. The characterizations of the inscribed LPGs in PCFs at high temperature and high strain are also described. The potential applications of PCF-LPG devices for gas sensing have been discussed. Unlike the PCF-based gas sensor that detects the analytes by the interaction of light with gases through the absorption of the evanescent wave in the holes of fiber cladding, the PCF-LPG gas sensing works by the interrogating of the shifts of different resonance wavelength and strength of core-cladding mode coupling in the transmission spectrum. The advantages of the PCF-LPG sensing devices are: (1) high temperature insensitive and stability; (2) compactness when packaged; (3) practical use under hazardous conditions and in high temperature environment.

We describe the design and fabrication of an integrated optical disk resonator and demonstrate its ability to perform as an environmental sensor. The device consists of a 500 micron radius disk side-coupled to a straight bus waveguide, fabricated in silicon oxy-nitride (SiON). The guiding layer has a refractive index of 1.8 and is 350 nm thick. Since the devices require few processing steps and can be fabricated using the well-established techniques of plasma enhanced chemical vapor deposition and optical lithography, they are reasonably easy to produce. By monitoring the transmission of 1550 nm light through the resonators, we can measure changes in the refractive index at the surface. We determine a sensitivity of 1.0 x 10-5 to changes in the surface index with experiments using sucrose solutions of varying concentrations.

In this paper, we report the highly birefringent elliptical core photonic crystal fibers (EC-PCF). The scalar and vectorial effective index methods are used to obtain the average refractive index of 2D triangular photonic crystal cladding. The first order perturbation technique is used to obtain the birefringence properties of EC-PCF. Birefringence of the EC-PCF obtained, is in the order of 10^{- 2}, which is 10 times higher than that reported for conventional elliptical core fiber. It is shown narrowing the space between the air holes and enlarging the air-hole size in the cladding of PCF can achieve high birefringence. It is observed that for every design of PCF, an optimum hole-pitch exhibits for which EC-PCF shows high birefringence. Further, the effect of core area on birefringence is also studied and it has been revealed that high birefringence can be obtained for smaller core area of EC-PCF. Both Scalar effective index and Vector effective index method with the perturbation approach are applied to compare the high birefringence of EC-PCF.

Photonic crystals are the subject of intense study due to their capability to provide precise control of optical transmission characteristics through the choice of their periodic lattice parameters and the material with which they are fabricated. The sensitivity of photonic crystal band structure and defect transmission states to refractive index changes either at planar boundaries of a 2-D crystal or at individual lattice sites, make these structures potentially attractive for integrated point detection biosensor devices, given that scalable geometries and materials suitable for micro/nanofluidic addressing and co-integration with integrated source and detectors are achievable. Recent work in the literature has experimentally shown the efficacy of single defect induced intra-band gap optical transmission peak shifts for detection of refractive index changes occurring globally in either the superstrate or lattice void space. In this paper, we present results from the analysis of single and cluster defects in hole and rod geometry 2-D photonic crystal lattices to assess the efficacy of photonic crystal under selective addressing of cell clusters with micro/nano fluidic channels and under selective binding to activated crystal cell or rod surfaces by large biomolecules. Using MPB [MIT Photonic Bands] and Optiwave FDTD tools, we have analyzed the optical transmission properties of Si semiconductor photonic crystals as a function of defect cluster size and refractive index to identify design windows with viable fabrication dimensions, and measurable spectral shifts for ranges of induced refractive index change of interest. Modeling results are presented and efforts towards fabrication of prototype test structures described.