Photonic Crystals
Application of photonic crystals (PhCs) is a new direction in gas sensor design. PhCs can consist of periodic arrangements of dielectric materials with different refractive indexes, which can be divided into one-dimensional PhC, two-dimensional PhC and th
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Photonic Crystals
6.1
Photonic Crystals in Gas Sensors
The application of photonic crystals (PhCs) is a new direction in gas sensor design (Lambrecht et al. 2007; Sünner et al. 2008; Srivastava et al. 2011; Zhao et al. 2011). The first concept of PhCs was proposed by Yablonovitch (1987) and John (1987) in 1987. Essentially, PhCs contain regularly repeating internal regions of high and low dielectric constant. In particular, as shown in Fig. 6.1, PhCs can consist of periodic arrangements of dielectric materials with different refractive indexes, which can be divided into 1D PhCs, 2D PhCs, and 3D PhCs according to the structures. Compared with 1D PhCs (e.g., gratings) and 3D PhCs (e.g., opals), 2D PhCs are easier to fabricate and cover a wider research value. In general, there are two kinds of 2D PhCs: one is the air-hole type PhC and the other is the pillar-column type PhC. The fabrication of 2D air-hole type PhCs with triangular lattice structure is simple, and it has the largest PhC bandgap (Jamois et al. 2002). In particular, these structures can consist of a slab of material (such as silicon) which can be patterned using techniques borrowed from the semiconductor industry. Such chips offer the potential to combine photonic processing with electronic processing on a single chip. Therefore, 2D PhCs attract widespread interest and are the most frequently used structures (Jamois et al. 2002; Beiu and Beiu 2008; García-Rupérez et al. 2010). Imprinting in polymers is one of the approaches widely used for fabrication of 2D PhC sensors (Boersma et al. 2011). Photonic crystal fiber (see below) is another approach to design such 2D structures. The fabrication of these fibers using microstructured polymeric materials (MPOF) was also reported by Van Eijkelenborg et al. (2003). For 3D PhCs, various techniques have been used including photolithography and etching techniques similar to those used for integrated circuits. Some of these techniques are already commercially available. To circumvent nanotechnological methods with their complex machinery, alternative approaches have been followed to grow PhCs as self-assembled structures from colloidal crystals. An example of a PhC prepared using this approach is shown in Fig. 6.2. Mass-scale 3D PhC films and fibers can now be produced using a shear-assembly technique which stacks 200–300-nm colloidal polymer spheres into perfect films of fcc lattice. Research has shown that PhCs possess unique properties such as a photonic bandgap (PBG) (Cheng et al. 2009), photonic localization (Maloshtan and Kilin 2007), slow light devices based on PhCs (Adachi et al. 2010), and so on (Zhao et al. 2011). It was established that the propagation of photons in PhCs is similar to the propagation of electrons in conducting crystals. In analogy with the electronic structure, the PhC presents a periodic potential to a photon propagating through it, resulting in the photonic band. In other words, the incident light whose wavelength lies within the PBG cannot propagate through the PhC region, and the t
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