Bio-Inspired Photonic Structures: Prototypes, Fabrications and Devices

Like the ability of electron regulation of electronic semiconductors, the photonic analogs usually considered as photonic structure materials are regarded as essential for light manipulation [1, 2]. With particular designs of photonic structures, they are expected to achieve different far-field and near-field optical features and thus lead to a perspective in all-optical circuit [3]. Though humankind has entered the nano-scale realm several decades ago, it is still a hard task for engineers to explore novel optical functional devices due to the limited experiences and originalities on artificial photonic structures design and the desired optical features. Additionally, it is also great challenges to fabricate photonic structures, owing to their sub-optical-wavelength to sub-micron featured sizes, especially in a high dimensional way by nano-fabrication technologies today.


Introduction
Like the ability of electron regulation of electronic semiconductors, the photonic analogs usually considered as photonic structure materials are regarded as essential for light manipulation [1,2]. With particular designs of photonic structures, they are expected to achieve different far-field and near-field optical features and thus lead to a perspective in all-optical circuit [3]. Though humankind has entered the nano-scale realm several decades ago, it is still a hard task for engineers to explore novel optical functional devices due to the limited experiences and originalities on artificial photonic structures design and the desired optical features. Additionally, it is also great challenges to fabricate photonic structures, owing to their sub-optical-wavelength to sub-micron featured sizes, especially in a high dimensional way by nano-fabrication technologies today.
By contrast, nature are found to develop photonic structures millions of years before our initial attempts. Diversified photonic structures, most of which are sophisticated and hierarchic, are revealed in beetles, butterflies, sea animals and even plants in recent surveys [4][5][6][7][8][9][10]. The exhibited optical features are regarded to have particular biological functions such as signal communications, conspecific recognition, and camouflage, which are optimized under selection pressure. Naturally, the occurring photonic structures provide us ideal 'blueprints' on design and stimulate similar optical functional devices. Various fabrication methods of bio-inspired photonic structures are explored [11][12][13][14][15][16]. By chemical methods (e.g. Sol-Gel, colloidal crystallization, chemical systhesis), nanoimprint lithography and nanocasting, physical layer deposition (PLD), atomic layer deposition (ALD), and etc., bio-inspired photonic structures, their reverse counterparts, and applications are achieving greater success than ever before. This Chapter will review the typical bio-inspired photonic structures and focus on the biomimetic fabrications, the corresponding optical functions and the prototypes of optical devices. It is organized as follows: four subsections are introduced in Section 2, in which each describes one catagory of bio-inspired optical functional devices. In every subsection, the nature prototype is introduced first, then followed by biomimetic fabrication methods and optical features of artificial analogs, finally closed on the bio-inspired optical devices. A brief perspective is given in Section 3.

Prototypes
In arthropodal animals such as butterflies, nipple arrays with typical spacing of optical wavelength or subwavelength are commonly found on surfaces of their compound eyes, which are believed helpful to the light-harvest efficiency of the biological visual system [17]. With optical impedance matching to the ambience, the light transmission are enhanced.
Another analogous examples are the transparent wings of some lepidoptera insects like hawkmoths [18] and cicada [19] (Fig.1 (a)). The tapered pillars lead to gradual changes of refraction index in view of effective medium theory ( Fig.1 (b)) and therefore play a key role in minimizing the reflection over broadband and large viewangles. In order to physically explain the anti-reflection origin, the Fresnel equations are given as follows.
where r s and r p refer to reflection coefficients of s polarised light (the electric field of the light perpendicular to the incident plane) and p polarised light (the electric field in the incident light plane), n 1 and n 2 are refraction indices of neighboring mediums, respectively. The relationship between incident angle i 1 and refraction angle i 2 is given by Snell's Law n 1 sini 1 =n 2 sini 2 . From the equations, e.g., for normal incidence, r s and r p are suppressed to near zero if n 1 and n 2 have very close values [20,21].

Bio-inspired fabrications
Inspired by nature, many efforts are made in exploring techniques for fabricating the anti-reflection nanostructures and a variety of methods, e.g., conformal- Oblique angle deposition (OAD) is usually employed to fabricate anisotropic film which is originated from the oblique growth of contained nanorods with a tilted angle to the substrate surface normal. The so-called CEFR method, which rotates the substrates at a high speed under OAD, leads to the straight growth of a dense array columns to substrate surface rather than helical structures which are formed under low rotation rate. That is, CEFR method is suitable for conformal replication of the photonic structure with a curved surface even under thick film deposition. With compound eyes of the fruit fly as bio-templates, it is reported artificial replica is successfully fabricated and hence similar optical features are inherited, respectively [22].
Many lithography techniques are applied for fabricating antireflection structures, among which colloidal lithography is a much simpler approach [23]. With colloidal crystals as masks, the silicon substrate is etched by reaction ion etching (RIE). During the fabrication duration, the colloidal spheres are etched by RIE gradually firstly, leading to a reduced transverse cross section of the spheres and thus an increasing exposure of the substrate. Attributed to the features of RIE, the etching rates of the apex and the junction parts of the spheres are not uniform, resulting in the etching morphology modification from frustum to cone arrays on the substrate finally [24].
Electron cyclotron resonance (ECR) plasma etching technique is employed by researchers to fabricate antireflection structures with much higher aspect ratio surfaces [25][26][27]. With the selected gas-mixture consisting of SiH 4 , CH 4 , Ar, and H 2 , one step and self-masked dry etching are realized for fabricating high density nanotip arrays on a 6-inch silicon wafer. The fabrication progress is illustrated in the schematic diagram of Fig. 2(a). In brief, SiC clusters, which size and density can be tuned via process temperature, gas pressure, and composition, are formed on surfaces of the silicon substrate due to the reaction of SiH 4 and CH 4 plasma. Ar and H 2 are responsible for the dry etching process. The SiC clusters then act as nanomasks or nanocaps to protect the underlying substrate from etching, thus forming an aperiodic array of silicon nanotips with their lengths varying from ∼1000 nm to ∼ 16 μm finally ( Fig. 2(b) and (c)). Even superior to natural prototypes, the biomimetic antireflection structures exhibit striking omnidirectional low reflection shown in Fig. 2(d) over a broad range of wavelengths from ultraviolet to terahertz region, irrespective of polarization.
Avoiding time-consuming and complicated mask fabrication, scientists also attempt to directly use cicada wings or insect eyes as bio-templates [19,28,29]. For example, the nipple arrays on wing surfaces are stamped under certain pressure on glass-phase PMMA, which is at higher degree than its glass-transition temperature, supported by silicon wafer. A release process makes the polymer reverse nanostructures of the bio-templates. With the patterned PMMA as a mask or a mold, inverse or similar structures of the cicada wing are achieved by RIE or thermodeposition. A schematic diagram of the NIL, the fabricated structures, and the natural templates for comparison are illustrated in Fig. 3(a), Fig. 3(b), and Fig. 3(c), respectively. It is also worthy to note that an extra advantage of using bio-templates in NIL process is the notable low-surface-tension, which is vital for the release process, due to the wax layer commonly found on surfaces of plants and insects.
Taking advantages of accurate thickness control and three-dimensional (3D) fabrication of ALD, conformal replica is accomplished after ALD growth and sintering the hybrid structures with fly eyes as bio-templates, achieving similar anti-reflective features in the artificial analog finally [29].

Potential applications
Due to the high reflectance of silicon solar cells (more than 30%) induced by the high index contrast of silicon and air according to the Fresnel equations, scientists are already aware of the vital roles of high-quality antireflection coatings at early ages of solar cell fabrications [30]. Inspired by antireflection structures found in moth eyes, nanodomes or similar architectures are reproduced on surfaces of solar cells, leading to a dramatic light absorption increase and therefore a superior efficiency improvements than that of quarter-wavelength antireflection coating [31][32][33], as shown in Fig. 4(a)-(d). The recent theoretical investigations even report a high light trapping close to the Yablonovitch limit in the silicon solar cell by optimizing a double-sided antireflection structure design ( Fig. 4(e) and (f)) [34].
The biomimetic antireflection structures can also play a key role in light extraction of light-emitting devices (LEDs) [35][36][37]. Because of the total internal reflection and the waveguiding modes in the glass substrate, only about 20% amount of the generated light can irradiate from the LEDs. By fabricating silica cone arrays on the surfaces of the ITO glass substrate to modulate the above two bottleneck factors, the light luminance efficiency of white LEDs is significantly improved by a factor of 1.4 in the normal direction and even larger enhancement for large viewing angles.
Another fascinating application of the inspired antireflection structures is the use in the micro Sun sensor for Mars rovers [38]. On the basis of the recorded image by an active pixel sensor, the location coordinates of the rover can be calculated. However, the ghost image originating from the multiple internal reflection of the optical system leads to severe limitation of the accuracy. By fabricating dense nanotip arrays on the surfaces of the sensor, the internal reflection is minimized to be nearly 3 orders of magnitude lower than that of no treatments, resulting in a more reliable three-axis attitude information.

Prototypes
Besides the well known coloration change strategy via migrations and volumes change of pigment granules such as chameleons, nature develops a second approach which is known as structural coloration change (SCC). By varying photonic structure characterizations, incident light angle, or the refraction index contrast of the color-produced optical system via the environmental stimuli, reversible coloration changes, which are basically passive, are revealed in fishes, beetles, and birds [39][40][41][42][43][44][45][46]. Most structural basis of SCC are attributed to the one-dimensional (1D) reflectors. For example, the damselfish Chrysiptera cyanea can change its color from blue to ultraviolet rapidly under stressful conditions, which is triggered by the simultaneous change in the spacing of adjoining reflecting plates made of guanine in the iridophore cell [47][48][49]. In insect world, the coloration change of longhorn beetles Tmesisternus isabellae from golden to red is revealed to originate from the swollen multilayer after water absorption [50] (c)). Another origin of structural coloration change is the tilted angle variability of the nanoplates with respect to incident light, which is found in tropic fish neon tetra Paracheirodon innesi [51][52][53] (Fig. 5(d), (e), and (f)). Physically, the underlying mechanism of the coloration change in the mentioned 1D biological photonic structures can be understood according to the given formula where d is the layer thickness, n is refraction index, and θ is angle of refraction. The subscripts represent the layer index. The angle of refraction at different layers θ 1 and θ 2 can be obtained from Snell's law n 1 sinθ 1 =n 1 sinθ 1 =sinθ 0 , where θ 0 is the incident angle from air. From the equation, it is easy to elucidate that either n, d, or θ 0 is related to the optical path in the multilayer and thus leads to shifting interference peaks at different wavelengths.
In biological system, high-dimensional photonic structures responsible for the coloration change are also discovered, though they are rare. An intriguing example is hercules beetles Dynastes hercules which can alter their appearance from khaki-green to black providing the ambience changes from dry to a high humidity level [54][55][56] (Fig. 5(g) and (h)). The 3D photonic crystal structures (nanoporous structures) are filled with water instead of air voids in dry status, rendering different refraction index contrast and thus the variation of Bragg scattering ( Fig. 5(i), here only 2D cross section is illustrated). However, the underlying physics of coloration change induced by high-dimensional photonic structures is no other than that of their 1D counterparts.

Bio-inspired fabrications and applications
Due to the obvious appearance changes which are easy to be picked up with the naked eye, color-tunable devices are explored to identify the status changes by temperature, vapor, solvent, humidity in ambience, the applied mechanical force, electric field, magnetic field and etc. Besides the sensors, some novel writing system ('paper and ink') are developed. The key idea is to modify period (d), refraction index (n), viewangle (θ) or their combinations of photonic structures, just like nature shows us.

d variation
Because of relatively simple control by the environmental stimuli and large spectral variation which can be recognized by the naked eye, approaches on the modulation of photonic structure period are always of scientist interests in obtaining tunable color applications.
'M-Ink' is a mixture of colloidal nanocrystal clusters (CNCs), solvent and photocurable resin ( Fig. 6(a)). With the superparamagnetic Fe 3 O 4 nanocrystals encapsulated by silica shell, 'M-Ink' can response to external magnetic fields. The role of the resin is to provide repulsive force which balances the attractive force of the CNCs. Without external magnetic fields, CNCs are randomly dispersed (infinite period) in liquid resin. The exhibited coloration is consistent with the magnetite, to be brown. After applying magnetic fields, the CNCs are assembled to form chain-like structures along the magnetic field lines ( Fig. 6(b)). The additional magnetic force, the intrinsic force among the CNCs, and the repulsive force by resin establish dynamic balance with variation of the external magnetic fields, tuning the distance between the neighboring CNC (finite period). The switchable period then determines the color of the light diffracted from the CNC chain, leading to a full color show (Fig. 6(c)). The final step is to fix the desired coloration. After exposure to ultraviolet (UV) light at different exerted magnetic fields locations, the chain-like CNCs can be frozen in the solidified resin instantaneously, remaining the periods of the chains undistorted and accomplishing high-resolution color pattern fabrication (Fig. 6(d)) [57].
With similar principle, the electric field-driven tunable color sensor is also realized by highly charged polystyrene (PS) colloids which form non-close-packed face-centered cubic (fcc) lattice [58]. Tuning the period along [111] direction by the balance of the exerted electrostatic force and the repulsive force, the exhibited coloration changes as a result of the applied electric field. The so-called 'P-Ink' is an electroactive material which consists of inverse opal inside polyferrocenylsilane (PFS) derivatives matrix. Such ink fabrication includes 3 primary steps: An opal film made of silica spheres is deposited onto glass substrate first by self-assembly; UV light is exposed to the sample in order to solidify the matrix and then form a stable PFS/silica composite; With diluted HF, inverse opal structure are realized in the elastomeric polymer matrix. By applying tunable voltage, macroscopic swelling and shrinking of the polymer matrix and microscopic Bravais lattice change responsible for the reverse coloration occur [59]. Stimulated by electrical forces, quite a few switchable coloration devices or sensors based on other materials or circumstances can be found elsewhere [60][61][62].
Many pressure-based photonic and even laser devices are reported [63][64][65][66]. Using monodispersed PS spheres to form cubic close packing (ccp) structures which are embedded in polydimethylsiloxane (PDMS) matrix, reverse colors are observed simply by stretching and releasing the rubber sheet. Upon mechanical stress, the lattice is elongated along the applied force direction, while the interplanar spacing in the perpendicular direction (i.e. distance between the (111) planes) decreases because of the nearly invariance volume of the rubber. The compressed distance leads to a blue-shift, e.g., from red to green [63]. Such opal rubber is believed to have practical applications such as a color indicator, tension meter or elongation strain sensor. The inverse opal structures (filled by air voids) in elastomer network are also synthesized [64,67] (Fig. 7(a)). The porous elastomeric photonic crystals (EPCs) show highly reversible optical response to compressive force, e.g., 60 nm spectral blue-shift under a compressive pressure of ∼ 15 kPa in the structures having 350-nm void size. Although the coloration change can be attributed to the Bravais lattice deformability, like the mechanism mentioned before. However, it is especially noteworthy that porous EPCs remain nearly undeformed in orthogonal directions when an external pressure is exerted in one direction, which can be ascribed to the high filling factor of air voids. The air voids enable the distortion of the cross-sectional void spaces from roughly circular to elliptical shape and a reduction of the air volume fraction under pressure. The elastic deformation feature of such structures helps to reduce the redistribution of stress along lateral directions when compressed by a patterned surface, leading to novel biometric applications such as the fingerprint recognition devices, as shown in Fig. 7(b). Additionally, air voids of porous EPCs provide a platform to incorporate with other functional materials for us to explore new applications. For instance, filling PbS quantum dots in the air voids, the photoluminescence (PL) emission, which leads to many potential applications in the near-infrared region, can be modified by overlapping with the forbidden bandgap of the inverse opal structures (Fig. 7(c)).
The chemical solvents are also used to be as stimuli. An interesting example is invention of new type 'photonic paper/ink' system [68, 69]. With novel soft materials consisting of closely packed PS and polymer elastomer (colloids and PDMS), the exhibited coloration can be altered reversely by immersing the materials into silicone liquid ('writing process') and an evaporation process ('erasing process'). The spacing between the (111) planes is adjusted by the strength of interaction between PDMS matrix and the contained silicone oligomers with different molecule weight in the liquid. With different solvents ('ink'), the swelling and shrinkage of the matrix show a featured reversible shift of Bragg diffraction peak. A multilayer based on alternating Teflon-like layer and Au nanoparticle/Teflon-like layer composite layer operates not in visible but optical telecommunication wavelength range [70]. When the structure is exposed to different organic solvent vapor (e.g. acetone, ethanol, methanol, water, chloroform, and etc.), the molecules enter the metal/polymer composite inside the holes and microvoids in its structure, resulting in the swelling up, e.g., for acetone vapors at a molar fraction of 0.25, with an relative increase of 12.5% in thickness to an equilibrium state while leaving the Teflon-like layer unchanged due to its inert chemical features. The measured reflectance show a large variation of 0.2 μm, which is advantageous to the detection of the organic/inorganic vapors.
Other external stimuli such as UV light, heat, or chemical reaction are applied to trigger and fabricate coloration sensitive materials as well by reversely controlling the spacing of the responsible photonic structures. Detailed information can be found in some references and recent reviews [14,15,61,[71][72][73][74].

n variation
Besides d, the refraction index n is another attribute of photonic structures. Different approaches to alter refraction index, in which infiltration is most regular, are revealed in order to achieve novel visual applications. Inspired by beetles D. hercules, 3D nanoporous structures are fabricated by so-called dip-coating deposition, achieving humidity sensing [75]. The 3D architecture, which is inverse opal structure, is reported to have its reflective peak red-shifting 14 nm when the relative humidity changes from 25% to 98%. It is worth noting that after treatments by O 2 plasma, the 3D photonic structures exhibit large bandgap shift of 137 nm and dramatic coloration change from bluish green in dry state to red in fully wet state. The modified hydrophilic feature play an important role in water collection in air voids at different level of humidity, leading to larger variation of refraction index contrast. The other clew for the remarkable coloration change can be ascribed to the high filling factor of air voids of inverse opal structures. The high value of ∼ 76% results in a wider modulation of refraction index contrast by the amount of solvent absorption. Tuning hydrophilic features by chemical groups, the inverse opal film can even be used as 'Watermark-Ink' system [76].
In the studies, the inside porous structures are functionalized with chemical group (R1, R2, R3, or R4) through vaporizing an alkylchlorosilane. The chemical functionalities of the structures then are erased and the surface is reactivated by O 2 plasma exposure, leaving a patterned functionalized region where is masked by a PDMS polymer. With iterations of such kind of functionalization and reactivation, the opal structures are locally patterned by different chemical groups, leading to differentiate hydrophilic feature ( Fig. 8(a) and (b)). When immersing the film into specific fluids, regulated infiltration by the wettability occurs spatially in the film and thus surveys different optical responses (i.e. different patterns, Fig.  8(c) and (d)). The fabricated structures are expected to have applications in encryption as well as colorimeter.
Besides infiltration, phase-transition materials can also induce refraction index variation and switchable coloration, providing the transition conditions are satisfied. The best-known phase-transition material maybe is liquid crystals (LCs). Above the phase-transition temperature of 34 • , LCs change their nematic phase which is anisotropic to isotropic phase, leading to a significant change of refraction index and coloration, e.g., in inverse opal structures. By mixing the active material into LCs, sensitive reflection and polarization triggered by UV light are reported by a series of subsequent researches [77][78][79]. Besides LCs, various sensitive materials, including Ag 2 Se, WO 3 and ferroelectric ceramics, are also found functionally in incorporating with diversified photonic structures to obtain tunable structural coloration materials and thus fabricate thermo-or electro-sensors [80][81][82][83].

θ variation
Iridescence is a characteristic of structural coloration, which is determined by band dispersions of photonic structures. With various incident angle or observation direction, the perceived coloration is changing. Hence, it is feasible to explore novel tunable color devices by tuning orientations of the photonic structures [84][85][86]. Just like the fish neon tetra P. innesi, magnetochromatic microspheres alter their appearance by tilting the inner photonic structures with respect to the incidence (Fig. 9). As mentioned before, with superparamagnetic Fe 3 O 4 particles coated by silica in PEGDA emulsions, the applied magnetic field guides to form photonic chains, in which the interpaticle distances depend on the balance of the attractive and repulsive force. Polymerized by UV light, microspheres are solidified and the inner periodic photonic chains, which give rise to structural coloration, are fixed permanently. Dispersing the microspheres into liquids, various colors are observed in top view due to the random orientations of the photonic chains in the microspheres. By applying external magnetic fields, the microspheres tend to rotate in order to keep the magnetic chains inside in the direction of the magnetic vector, leading to a homogenous coloration of green exhibited. The feature of the switchable coloration 'on' and 'off' status by the external stimuli is believed to have applications in color display, signage, bio-and chemical detection, and magnetic field sensing.

Prototypes
Structural coloration results from the interaction of light and photonic structures with featured size of visible wavelengths. It is even more widespread than pigmentary coloration in animal world. Some literatures have well reviewed the field comprehensively. In the Chapter, we do not plan to pay attention to the overall structural coloration but only focus on a specific subject 'structural color mixing' [87][88][89][90][91][92]. In tiger beetles Cicindela oregona (Fig. 10(a)-(c)) [87,88], the honeycomb-like pits are found on surfaces of the elytra. Under microscope, the brown morph actually includes blue-green patches in the red background, while the black one consists of magenta patterns surrounded by dull green. The microscopic colors are the results of light interference by the multilayer structures in the cuticle. However, due to the colored patches (40-80 μm across) are too small to be resolved by the unaided eye (the resolution d is determined by the Rayleigh criterion d=l×0.61λ/D, where l is distance of distinct vision, λ the wavelength and D the average pupil diameter of humankind), the perceived coloration is a mixture of the discrete colors, leading to a totally different exhibition from that equipped with microscope. From the investigations of photonic structures of beetles Chlorophila obscuripennis ( Fig. 10(d)-(f)), we revealed the color is a juxtaposition in a smaller region (∼ 10×10 μm 2 ) by green on the ridges and cyan in centers of the pits. Furthermore, the pits on the elytra surface give rise to diffused light reflected over a wide large of angles, leading to inconspicuous coloration shown [91]. In butterflies Papilio palinurus ( Fig. 10(g)-(i)), the scanning electronic micrographs show surfaces in the scales comprise a similar two-dimensional (2D) pits (4-6 μm in diameter and 3 μm at the greatest depth). The flat regions between and in pits appear yellow, and the inclined region contributes to blue color. Because of the limitation of human eyes' resolution, the butterfly displays a mixture color of green, i.e. yellow plus blue turns to be green. Due to sufficient depth of the pits, light which normally incidents on the inclined side can experience dual reflection and is back-reflected. The retro-reflection is found to play a key role in the polarization conversion of incident light, which is crucial in some novel optical applications. Thanks to symmetric feature of the pits, no macroscopic polarization effects can be observed [90]. On the cover scale surfaces of butterflies Suneve coronata, however, the natural occurring triangular grooves array not symmetric but in 1D way with a period of ∼ 2 μm, giving rise to polarization conversion of the normal incident light in a specific direction. Intriguingly, it is revealed the coloration macroscopically remains unconscious change under different polarized illumination. The reason is the curly scales, which induce polarization conversion between the neighboring rows of scales but in the orthogonal direction. The nanostructures having different magnitude in size therefore cancel out the macroscopic polarization effects ( Fig. 12(a)-(d)) [89].

Bio-inspired fabrications and applications
Structural coloration may be especially crucial for future color and related industry because of the non-fading feature (if the photonic structures are undeformed) [93,94] and environmental friendliness. Naturally, structural coloration mixing inherits the advantages. Mimicking the nature, mixed structural color and its application can be obtained. For example, PS colloids with a diameter of 5 μm are assembled on a gold-coated silicon substrate. A 2.5-μm-thick layer of platinum or gold is then deposited to fill the interspaces of the colloids by electrochemical approach, creating a negative replica. After removal of the PS colloids by ultrasonic waves and a sputtering thin carbon film, a multilayer of quarter-wave titania and alumina films is grown by ALD ( Fig. 11(a)), inheriting the morphology of hexagonally arranged pits of the negative replica ( Fig. 11(b)). The sample exhibits similar color mixing with that of butterflies P. blumei (Fig. 11(c) and (d)). Moreover, by modifying the surface morphology of the imitation (Fig.  11(e)), even visual information, e.g., a picture, can be encoded into the photonic structures which display a striking appearance change from pale blue in the specular direction to red in retro-reflection ( Fig. 11(f) and (g)). The bio-inspired work is expected to find applications in security labelling field or color industry such as painting and coating [95].
Inspired by color mixing researches, some novel applications based on polarization conversion are designed [89]. For instance, by etching periodic triangular-like grooves on surfaces of a flat multilayer, the film displays a coloration of green, which is actually a mixed color from yellow in the flat region and blue in the grooves in normal incidence, as shown in Fig. 12(e). Because of the broken symmetry of the 1D structures, the blue color can be suppressed by using a polarizer on light path, leading to the exhibited coloration change to yellow (i.e. the hue in flat regions). That is, the grooved areas turn the light polarization π/2 and thus are shadowed by the inserted polarizer. Following the ideas, an encoded pattern which comprises grooved areas can be distinguished with the background consisting of perpendicular grooves via the luminosity level under different polarized light incidence ( Fig. 12(f)). The two imaged examples inspired by the grooved structures of the butterfly show us great prospective in fields such as anti-fake and encryption.

Other bio-inspired fabrications and applications
Besides the main categories, some other bio-inspired photonic applications are also reported [96][97][98][99][100]. For examples, the natural scales of Morpho sulkowskyi are extremely sensitive to the different environmental vapors, which lead to dramatically improved responses as units of potential sensor applications compared with that of current devices [101]. Using black scales of the butterfly Papilio paris and Thaumantis diores as templates, hierarchically periodic microstructure titania replica was systhesized by chemical procedures. The high surface area inherited from the natural templates is great advantages to the light harvesting efficiency and dye sorption when the replica is used as photoanode in dye-sensitized solar cell [100]. With Morpho as bio-templates, alumina replicas show the potential applications in waveguides and beam-splitters under thin film coating by ALD and sintering [97]. Under thick film coating, the hybrid structure is found not to inherit the 'Christmas-tree' structures but develop a 'Pyramid-like' structures which have potential applications in light trapping [99].
With the complex photonic structures (multilayer plus 2D amorphous structures) of beetles Trigonophorus rothschildi varians as 'blueprints', the artificial counterpart which is fabricated by FIB etching holes through a 5×SiO/SiGe multilayer structure show similar optical features like non-specularity and only slightly angle-dependent reflectance [98]. Further bio-inspired work is undergoing which is stimulated by a recent revealed 3D architecture [102], aiming at the ultra-negative angular dispersion of diffraction and potential novel dispersive optical elements.

Perspectives
In the Chapter, several important kinds of bio-inspired photonic applications are reviewed, including antireflection devices, color-tunable sensors, structural color mixing applications and etc. The nature nourishes scientists the functional optical applications either the blueprints of photonic architecture or directly the bio-templates. Due to the higher index of inorganic materials used, the mimicking photonic structures even show better optical performances as well as enhanced mechanical properties of high temperature tolerance, stability and infrangibility. The biomimetic applications are anticipated to help our life better in the near future. However, complicated photonic structures (e.g. those of high-dimensional, hierarchic, amorphous features in nature) still remains hardly reproduced or, if they are fabricated successfully, the efforts involved are so great using the traditional fabrication ways that optical devices can not commercially explored. Thorough physical mechanism understanding as well as better fabrication approach explorations may help to simply the structure fabrications, achieve similar optical functions and realize commercial applications. In addition, adding substances such as functional chemical groups, fluorescence particles, metal, or other active materials, the mimicking photonic structures allow the properties of interest to be augmented, which may open a new window of novel optical device exploration.
Although the photonic biomimicry is in its infancy, we believe that the bio-inspired optical device would surely have profound impacts on our modern society.