## 1 Introduction  
Raman spectroscopy is a powerful vibrational spectroscopy technique that has many advantages. It is an analytical technique with high specificity for the identification of various chemical compounds [1; 2; 3]. In addition, it is a non-invasive approach that allows measurements to be performed without any sample preparation, and the measurement time is generally very short. However, the Raman signal is very low and difficult to detect. Statistically, for every 1-10 million photons bombarding a sample, only one will result in Raman scattering [4; 5]. For this reason, Raman spectroscopy is often coupled to substrates integrating metal plasmonic nanostructures that reproduce an SERS (surface enhanced Raman scattering) effect [6; 7; 8]. The molecules, which need to be analysed,are deposited on these substrates and are excited by an electromagnetic (EM) field, which is enhanced by the proximal metal nanostructures. The amplified EM field interacts with molecules and produces Raman scattering which is again amplified by the metal nanostructures [9; 10; 11]. In greater detail, the incident EM waves excite plasmon resonances within the metal nanostructures. Due to resonance effects, the local electric field is enhanced in the proximity of metal nanoparticles. The molecules at the metal surface then experience an enhanced local EM wave, and produce higher Raman signals than those molecules far from the metal surface. Moreover, since Raman signals are a type of EM wave, the same route through plasmon resonances will further enhance the outgoing Raman field. In the end, plasmon resonances are responsible for the double amplification Raman signals of molecules close to metal nanostructures, thus producing a surface-enhanced Raman effect. A detailed review of plasmon-enhanced spectroscopies can be found in [12]. The Raman signal enhancement strongly depends on the size and shape of the metal particles [13; 14; 15], and different methods to fabricate them exist in the literature [16; 17; 18]. The combination of a top down method, that is, electron beam lithography, with a bottom up technique, that is, chemical electroless deposition, allows a SERS substrate of a self-similar chain of silver nanospheres to be obtained [19]. Plasmonic nanoholes can be made with a template-stripping technique that involves the fabrication of a Si template obtained with electron beam lithography [20]. The Ag-decorated nanotip array, obtained with Electron Beam Induced Deposition and Silver Electroless Deposition, is another efficient 3D plasmonic device, that can be used to obtain a SERS effect [21]. The prototypes described above, although very efficient, are difficult, expensive and time-consuming to fabricate. Other techniques allow the fabrication of nanoplasmonic structures using faster and inexpensive processes. In this regard, metallic colloids [22; 23; 24], metallic films [25; 26], sensors fabricated by nanosphere lithography [27], and plasmonic systems exploiting nanoparticle self-assembly properties [28; 29] have been developed. However, some of the processes to achieve these kinds of low-cost SERS substrates lack reliability in regard to obtaining large sensing surfaces [30; 31]. These disadvantages in contrast to the benefits of Raman spectroscopy, which allows information to be obtained easily and without any particular sample preparation [32; 33]. Consequently, research involving alternative ways to maintain the SERS effect [34], is of crucial importance. Some structures existing in nature show, on the nanoscale, a conformation called photonic crystal. Photonic crystal, a periodic structure in an optical medium, creates unusual optical dispersion properties, which cause the iridescent colours in some butterfly wings and beetle armours and other structures present in nature [35; 36]. These structures, if metallized, can be assimilated to the SERS devices and used as optical sensors [37; 38; 39]. Here, we focus on the peacock feather, opal and diatoms, because they are easy to find in nature.