# Photonic Crystal-Based Sensors for Detecting Alcohol Concentration
## 1 Introduction  
Photonic crystals are promising materials for applications in optoelectronics and photonics as well as in the field of optical sensing. Photonic crystals are dielectric or metalodielectric materials whose periodic spatial variation of dielectric functions are achieved using advanced nano-structuring techniques and leads to formation of spectral photonic band gaps or stop gaps [1]. Photonic crystal sensors exploit the sensitivity of photonic crystal dispersion bands to the modification of their refractive index and periodicity modulation by gases or fluids. Optical sensing can be realized in the simplest form by detecting alteration of the photonic crystal reflectivity or transmission spectra due to the infiltration of photonic crystal voids by various materials [2]. Sensors based on optical measurements have generally proven to be faster, safer, and easier to implement than those employing electrical measurements [3]. In addition, it has recently been demonstrated that this three-dimensionally porous material showed perfectly optical sensing properties [4]. The electrochemical etching and photolithography were typical methods used for fabrication of three-dimensionally porous materials [5, 6]. However, the typical methods are relatively complicated, requiring a controlled etching process in a hydrofluoric acid solution or expensive photolithography procedures. In contrast, the fabrication of three-dimensionally porous materials from assembly of colloidal particles is process simple, cost effectiveness and extensibility to large-scale production. The process of this technique mainly involved the three steps. First, colloidal microspheres are organized onto the substrate to form the template by the self-assembly method. Second, particular material is infiltrated in the interstice of the microspheres. Third, the template is removed. Currently, the ethanol concentration is mostly determined by the alcohol meter in the market. This is mainly based on the refractive index change of the wines to calculate the concentration. The alcohol meter is only suitable for measuring the ethanol concentration in the distilled liquors but is notsuitable for brewed alcoholics. Surface Plasmon Resonance (SPR) [7] is another powerful inspection apparatus which can measure the ethanol concentration precisely, but the cost of the system is high and the operation procedure is complicated. Many of the critical system parameters of the SPR system, such as the incident angle, optical glass, and gold layer thickness, have to be controlled precisely to realize the high sensing performance [8]. In 2007, [PERSON] _et al._[9] found that the iridescent scales of the _Morphosulkowskyi_ butterfly produce a different optical response when exposed to different vapors. This optical response dramatically outperforms that of existing nano-engineered photonic sensors. To achieve a proper vapor response in nano-fabricated photonic structures, the combined effect of the structure surface properties and the detected vapor properties (surface tension, molar volume, and molecular shape and size of the vapors) must be considered. To develop an effective and simple mean for detection of the ethanol concentration, we consider using the iridescent photonic sensors because the iridescent scales of the \(M\). butterfly produce a different optical response when it was exposed to different chemical solvents. Owing to the inspiration from the nano-photonic sensors of the butterfly wings, we would attempt to prepare a sensor that could quantitatively identify the ethanol concentration of difference ethanol concentration solutions. Compared with other methods for ethanol concentration measuring, the advantage of the nano-engineered photonic sensor is low cost, non-polluting and highly sensitive. Although the sensitivity of the SPR approach (typically \(10^{-5}\) nm\({}^{-1}\)) is around 100 times higher than the present photonic crystal system, the SPR setup is quite complicated [8].