**1. Introduction**

256 Photonic Crystals – Innovative Systems, Lasers and Waveguides

Zlatanovic, S., Mirkarimi, L. W., Sigalas, M. M., Bynum, M. A., Chow, E., Robotti, K. M.,

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Burr, G. W., Esener, S., & Grot, A. (2009). Photonic crystal microcavity sensor for ultracompact monitoring of reaction kinetics and protein concentration. *Sensors and Actuators, B: Chemical*, Vol. 141, No. 1, (August 2009), pp. 13-19, ISSN 0925-

> In the last couple of decades we have been witnessing an enormous technological advancement in the field of micro-technology to the extent that nowadays we talk about nanotechnology. Faster computers, LCD based mobiles, nanoparticles for UV absorption in suntan lotions are just few of many examples where nanotechnology plays a fundamental role. The merit of this is mainly in the advance of the fabrication methods. Present techniques such as Focused Ion Beam (FIB) lithography guarantee a resolution of less than 10 nanometers which is about five times more precise than ten years before. Also Photonic Crystals (PhCs), among the others, take advantage from this extremely high resolution level allowing a downscale that permits the realization of structures which in principle can work at vey high energy. Historically PhCs were known as Bragg mirrors and only in 1987 (Yablonovitch, 1987; Sajeev, 1987) with the works of Yablonovitch and Sajeev the term Photonic Crystals was introduced. Nowadays, besides their natural application as filters in particular under full band gap conditions, PhCs see a number of applications: optical fibers (Birks et al., 1997; Zhao et al., 2010), vertical cavity surface emitting lasers (Yokouchi et al., 2003), high reflection coatings, temperature sensors (Song et al., 2006), high efficiency solar cells (Bermel et al., 2007), electric field detectors (Song & Proietti Zaccaria, 2007), non-linear analysis (Malvezzi et al., 2002; Malvezzi et al., 2003), just to name a few. Many are the techniques for the fabrication of PhCs, for example by means of focused-ion beam (Cabrini et al., 2005), two-photon fabrication (Deubel et al., 2004), laser-interference (Proietti Zaccaria et al., 2008a) or waver-fusion techniques (Takahashi et al., 2006). Here we shall focus on the role that PhCs can play for another exciting discipline known as *Plasmonics*. It refers to the capability of some devices of sustaining an *optical surface* mode, namely an electromagnetic wave travelling at the interface between two different materials such as a dielectric and a metal. Such a wave originates from the coupling of incident photons on the interface with

<sup>\*</sup> Anisha Gopalakrishnan1, Gobind Das1, Francesco Gentile1**,**2, Ali Haddadpour3, Andrea Toma1, Francesco De Angelis1, Carlo Liberale1, Federico Mecarini1, Luca Razzari1, Andrea Giugni1, Roman Krahne1 and Enzo Di Fabrizio1,2

*<sup>1</sup>Nanobiotech Facility, Italian Institute of Technology, Genova ,Italy* 

*<sup>2</sup>BIONEM lab., Departement of Clinical and Experimental Medicine, Magna* 

*Graecia University, viale Europa, Catanzaro, Italy* 

*<sup>3</sup>Department of Electrical and Computer Engineering, University of Tabriz, Iran* 

Photonic Crystals for Plasmonics: From Fundamentals to Superhydrophobic Devices 259

The chosen dielectric PhC is made of circular columns of high refractive index material, namely silicon (n=3.6) surrounded by air (n=1). The radius of the columns is r=300nm and lattice period P=1m. As expected, only TM polarization shows zero transmission regions, in particular three band gaps below 1m-1 are shown in Fig. 2.1. On the other hand, TE does not sustain any band gap. These results remain true even increasing the columns dielectric value or changing the columns radius. No full band gap is then found for this kind of structure.

Fig. 2.1. TM band structure for a 2D square silicon columns photonic crystal. Three band

The behavior of 2D metallic photonic crystals is fundamentally different from what expected by 2D dielectric photonic crystals (Zhao et al., 2009; Sakoda et al., 2001; Ito & Sakoda, 2001). In fact, the use of metallic dispersive materials strongly modifies the light behavior in periodic structures, both for TM and TE polarizations. In particular, TM polarization shows a cut-off frequency c and no modes are found below it. This implies the existence of a TM gap below c. From a physical point of view this is related to the existence of free electrons in metallic materials. Similarly, TE polarization shows a behavior which is absent in dielectric 2D photonic crystals. In fact, in specific range of frequencies, metallic photonic crystals show TE polaritonic band gap close to the plasma frequency. Physically it is associated to the creation of surface plasmon polaritons on the metallic columns of the crystal. These peculiarities of metallic PhCs have the merit of increasing the chance of a full band gap also for square crystals. Hence, maintaining the same geometrical configuration as in the previous section, namely a square structure of columns in air, we have numerically analyzed the band gap

formation when metallic columns are considered instead of dielectric ones.

 

**2.2.1 Full band gap in metallic 2D photonic crystals: Drude vs. Lorentz model** 

We start by considering the Drude model (Rakic´ et al., 1998) to describe the metallic parts of

() 1 *<sup>p</sup> <sup>f</sup>*

 

2 0 2

*i* 

0

(2.1)

**2.1.1 Full band gap in dielectric 2D photonic crystals** 

gaps are shown.

the crystal:

**2.2 Metallic photonic crystals**

the existing free electrons. This kind of mode is known as Surface Plasmon Polariton (SPP). Photonic crystals, as a translational modulation of the refractive index, have already been playing a very crucial role in plasmonics. In fact, they can provide the missing wave vector for the coupling between photons and free electrons of a metal layer (Raether, 1988). Here we have chosen to face three situations relating PhCs and Plasmonics:


The first topic concerns the fundamentals of SPP and PhCs. No particular application will be suggested, but mostly we will focus on the theory behind the generation of SPP inside a metallic photonic crystal. The next two topics are, on the other hand, strictly related to applications. We will concentrate our attention on Raman spectroscopy, as a very important tool for the investigation of the optical properties of many kinds of samples, such as semiconductors or proteins. In particular, metallic ordered (periodic) structures will be used either as artificial SERS substrates or as combiners of SERS and super-hydrophobic effect for few molecules detection. This description will offer a general overview of the important functions that PhCs can hold in Plasmonics and how we could start thinking more intensively of PhCs realized with metallic materials.
