Photonic Bandgap Structures Novel Technological Platforms for Physical, Chemical and Biological Sensing

Photonic crystals (PhCs), able to manipulate light at a scale on the order of the wavelength or even shorter, have the potential for developing new technologies and devices for a wide range of applications. In particular, PhCs have been used to investigate, both theoretically and experimentally, photonic sensors because of their peculiar properties, such as the capability of enhancing field-matter interaction and control over the group velocity. In this chapter the basic physics of photonic crystals with specific reference to the origin of the photonic band gap and to their design for achieving specific dispersion properties, is reviewed.

Photonic crystals are periodic structures with spatial variation of the dielectric, magnetic and metallic properties in one, two and three dimensions. For the present we confine this discussion to mixed dielectric structures. The propagation of electromagnetic waves through these crystals may result in pass and stop bands, depending on the structure. The wave propagation is governed by Maxwell's Eqs. and these calculations are usually performed for infinite structures and subsequently modified for finite structures. There are different approaches to solving the wave propagation problem and these range from the plane wave expansion method (PWEM) [1], the transfer matrix method [2], the finite difference time domain method (FDTD) [3] and the finite element method [4] which are among the most popular. In this chapter only the plane wave expansion and the finite difference methods are discussed.

Photonic Crystals (PhCs) provide significant capabilities in terms of emission control, localization, guiding, dense integration, high-speed operation, and the ability to engineer the dispersion properties of a given material. As a result, they have led the way towards true miniaturization of nanophotonic circuits, and hold the key to achieving the long-sought goal of large-scale integrated photonic systems on a chip, including integrated photonic chemical/biological sensor devices. In this chapter, we present brief discussion and summary of major achievements in theory, applications and fabrication of photonic crystals. We present applications relying on engineering the confinement properties as well as the dispersive properties and present some of the fabrication techniques for both planar and three-dimensional photonic crystal structures.

Photonic Crystal Fibers (PCFs) have extended the range of capabilities in optical fibers, both by improving well-established properties and introducing new features. PCFs are optical fibers that employ a microstructured arrangement in a background material of different refractive index. The background material is often undoped silica and a low index region is typically provided by air voids running along the length of the fiber.The strong wavelength dependency of the effective refractive index and the inherently large design flexibility of the PCFs allow for a whole new range of novel properties. Such properties include endlessly single-moded fibers, extremely nonlinear fibers and fibers with anomalous dispersion in the visible wavelength region. Fabrication of PCF, like in conventional fiber fabrication, starts with a fiber preform. PCF preforms are formed by stacking a number of capillary silica tubes and rods to form the desired air/silica structure. This way of creating the preform allows a high level of design flexibility as both the core size and shape as well as the index profile throughout the cladding region can be controlled.When the desired preform has been constructed, it is drawn to a fiber in a conventional high-temperature drawing tower and hair-thin photonic crystal fibers are readily produced in kilometer lengths.

Light emitting organic materials, such as conjugated polymers and low-molar-mass compounds, exhibit appealing characteristics for the realization of active optical devices, including high luminescence efficiency, wide emission tunability, as well as simple and cheap processing. In particular, photonic crystals and their integration in planar and vertical microcavities based on organics provide an effective approach to control the light-matter interaction in solids, and to design and fabricate devices. Examples of applications which are here reviewed include distributed feedback lasers, nanopatterned organic light-emitting diodes and photonic crystal waveguides, and vertical architectures exploited in resonators, photodiodes, and microcavities showing strong exciton-photon coupling.

Both patterned periodic or random structures can exhibit large slow down factors associated with large field enhancements. These properties can be exploited to good effect for a variety of sensing functions. We describe here some basic mechanisms responsible for these effects and discuss their use. Some experimental results are reported using porous form-birefringent super-lattice structures which can be scaled in size for use from microwave to optical frequencies.

Circular resonators are key components for diverse applications, ranging from optical telecommunication systems through basic research involving highly confined fields and strong photon-atom interactions to biochemical and rotation sensing. The important properties of circular resonators are the Q factor, the free spectral range (FSR) and the modal volume, where the last two are determined by the resonator radius and the index contrast. The total-internal-reflection (TIR) mechanism employed in “conventional” resonators links these attributes and limits the ability to construct small footprint devices exhibiting large FSR and small modal volume on one hand, and high Q on the other. Recently, a new class of circular resonator, based on a single defect surrounded by radial Bragg reflectors, has been proposed and demonstrated. The radial Bragg confinement breaks the link between the modal volume and the Q, and paves the way for the realization of compact, high-Q resonators. These characteristics as well as the unique mode profile and polarization properties of these resonators and lasers make them attractive tools for nanometer scale semiconductor lasers, ultrasensitive detectors, as well as to study nonlinear optics, light tweezers and cavity QED.

The unique feature of photonic crystal fiber (PCF) both as a light guide and a gas/liquid transmission cell allows synergistic integration of optics and microfluidics. There is a growing interest in PCF as a platform for spectroscopy-based chemical and biological sensing. In this chapter, we focus on PCF for sensing and detection using surface-enhanced Raman scattering (SERS). We examine the state-of-theart, challenges and opportunities in the development of SERS-active PCF sensors.

In this Chapter, we provide a compact overview of possible applications of the “guided resonance” phenomenon in photonic crystal slabs to sensing scenarios. More specifically, starting from the outline of the basic aspects, we review a number of selected results from the recent topical literature dealing with applications to displacement, chemical, and biological sensing. Besides theoretical and physical issues, emphasis is also placed on experimental and technological aspects.

Photonic crystal sensors have recently drawn much attention due to their inherent compactness and high sensitivity. Different types of photonic crystal sensors rely on one main transduction mechanism: perturbations in environmental conditions alter index of refraction or periodicity of the structure which is transferred to a detectable change in optical power or spectrum. This transduction mechanism facilitates utilization of photonic crystal for sensor applications ranging from pressure and displacement measurements to detection of chemical composition change and binding events. In this chapter, we give a summary of different types of photonic crystal sensors such as waveguides, microcavities and self-assembled 3D structures and review their application to physical, chemical and biological sensing. We discuss advantages and limitations of the technology.

Remarkable developments and improvement have been observed in the field of optical fiberbased sensors in the last decade, for physical, chemical and biological applications. Many new sensors for specific analytes have been reported, novel sensing chemistries or transduction principles have been introduced, and applications in various fields have been realized. This boost has been pushed also by the mature fabrication technology of photonic crystal fibers whose peculiar properties well fit requirement for efficient sensing systems. Discussions on the main issues related to the use of photonic crystal fibers in biosensing applications is reported. Specific description of fiber functionalization for the detection of biologically relevant analytes is described highlighting deposition procedures of selective probes.