Bio‐inspired optical structures for enhancing luminescence

Abstract Luminescence is an essential signal for many plants, insects, and marine organisms to attract the opposite sex, avoid predators, and so on. Most luminescent living organisms have ingenious optical structures which can help them get high luminescent performances. These remarkable and efficient structures have been formed by natural selection from long‐time evolution. Researchers keenly observed the enhanced luminescence phenomena and studied how these phenomena happen in order to learn the characteristics of bio‐photonics. In this review, we summarize the optical structures for enhancing luminescence and their applications. The structures are classified according to their different functions. We focus on how researchers use these biological inspirations to enhance different luminescence processes, such as chemiluminescence (CL), photoluminescence (PL), and electroluminescence (EL). It lays a foundation for further research on the applications of luminescence enhancement. Furthermore, we give examples of luminescence enhancement by bio‐inspired structures in information encryption, biochemical detection, and light sources. These examples show that it is possible to use bio‐inspired optical structures to solve complex problems in optical applications. Our work will provide guidance for research on biomimetic optics, micro‐ and nano‐optical structures, and enhanced luminescence.


INTRODUCTION
Luminescent materials are widely applied in the fields of sensors, information anti-counterfeiting, light-emitting diodes (LEDs), and laser light sources. Due to the limitation of the material properties, the luminescent performance of a device needs to be improved to satisfy the strict requirements of practical applications. For example, fluorescent anticounterfeiting applications require high security, multiple enemies via the processes of bioluminescence, fluorescence, or phosphorescence. They evolved many unique optical structures, which produce wonderful effects of enhancing light extraction, making luminescence directional, and so on. [17][18][19][20] Researchers found inspiration from them and exploited these optical phenomena to develop photonic devices. [21][22][23][24][25] With the development of nanomaterials and nanotechnology, many processing techniques were used to fabricate different bioinspired optical structures. [26][27][28] The manufacturing methods of structured luminescent devices include depositing luminescent nanomaterials on biological templates as well as using imprinting or photolithography to introduce optical structures to the luminescent materials. [29][30][31] Furthermore, calculations and simulation results of bio-inspired optical structures help people to understand the fundamentals of these optical structures and guide the design and fabrication of luminescent devices. The smart solutions in organisms to address optical challenges provide plenty of motivations for developing novel artificial photonic materials and luminescent devices, such as color filters, artificial eyes, and electronic skin. [32][33][34] Here, we focus on enhancing luminescence by bio-inspired optical structures and their application to luminescent devices ( Figure 1). First, we classify the organisms with actively or passively optical emitting signals as well as their micro-and nano-optical structures. Second, we report bio-inspired optical structures to enhance different types of luminescence processes and their applications in anti-counterfeiting, biochemical detection, and light sources. Finally, we summarize advantages, disadvantages, and improvement methods of bio-inspired optical structures for enhancing luminescence.

 OPTICAL STRUCTURES IN ORGANISMS FOR ENHANCING OPTICAL SIGNALS
Luminescence exists in a series of organisms, such as marine organisms, plants, and insects. [35][36][37][38] Analysis and interpretation of these fluorescent signals can provide information about their physiological state, how species communicate with each other, and the presence of specific chemicals. Many interesting interactions occur between optical structures and luminescence produced by pigments. With the development of microscopic technology and the improved accuracy of observation, researchers have revealed the optical structures existing on surfaces of organisms and studied them in detail. [39,40]

. Enhanced fluorescence emission
In 2005, researchers discovered an internal light-filtering effect in the flowers of Mirabilis jalapa. There are two pigments (betaxanthin and betacyanin) in flowers for generating fluorescence, and the emission of betaxanthin is absorbed by betacyanins. This special mechanism creates a showy pattern with contrasting fluorescent on the petals to enhance visibility to pollinators. [41] This example demonstrates the important role of the enhanced fluorescence signal for the organism. Fluorescent pigments are also present in different photonic structures, which strongly control the intensity and direction of the emitted light. The swallowtail (Papilio) butterfly has pigment-infused two-dimensional (2D) photonic crystal (PhC) structures on its wing scales from colored regions. This distributed Bragg reflector (DBR) structure inhibits the in-plane emission and enhances its out-of-plane emission, leading to directional fluorescence. [42] The same phenomenon of directed fluorescence enhancement occurs in male butterflies Troïdes magellanus (Papilionidae) (Figure 2A) because the fluorophores in the hindwings are confined in the three-dimensional (3D) structures consisting of periodicallydistributed ridges with triangular cross sections. [43] The 3D structure serves as a diffraction grating that reflects light and couples fluorescence to a structure acting as an optical waveguide, leading to a directional emission. On the back of some beetles, natural pigments exist in the photonic structures of the elytra scales. For example, the male Hoplia coerulea beetle has a porous photonic structure that is embedded with fluorophores. This structure consists of thin pure cuticle layers with periodically arranged and mixed air-cuticle porous layers ( Figure 2B). [44] The fluorophores in the structures show, depending on the position of the photonic bandgap, enhanced or suppressed fluorescence emission because resonant structures can modify the density of optical states (DOS), which is related to electromagnetic modes of propagation, the decay rate of fluorescence, and the lifetime of the excited state. [45]

. Enhanced light extraction
Some organisms can generate bioluminescence through a chemical reaction in their bodies. [46][47][48] However, the refractive index mismatch at interfaces always causes reflection and loss of light. Surprisingly, organisms have evolved special optical structures which can extract light from the highrefractive-index interior to the air. For example, fireflies can efficiently output light for intimidating enemies or attracting a mate. Bay and co-workers studied multi-dimensional optical structures existing in the luminous area of the firefly's body ( Figure 2C). [49] The edge structures from jagged and misfitted scales lead to enhanced light extraction by diffuse transmission. The reduction of the refractive index in the emission region also contributes to the efficient extraction of the emission. The nipple array structures on the moth-eye surface form anti-reflection structures which reduce the glare of the eyes and avoid detection by predators. [50] Oh and co-workers reported that the nanopillar array structures with a ∼0.5 fill factor, that is, the relative areal fraction of sub-wavelength nanostructures, achieved a gradual decrease in reflectivity with increasing height ( Figure 2D). The structures can be used to increase light transmittance and improve the sensitivity of moth eyes to sunlight. [51] The same phenomenon has been found on the wings of butterflies. For example, the randomly-distributed height of pillar structures with a mean height of 500 nm and a variance of 100 nm on the wings of glasswing butterflies leads to low broadband reflection below 2.2% for normal incidence and large view angles. [52] Fabricating the anti-reflection structures can effectively extract emission light, resulting in the enhanced luminescence performance of light-emitting devices.

 BIO-INSPIRED OPTICAL STRUCTURES FOR HIGHLY EFFICIENT LUMINESCENCE
As we saw in the previous section, in order to adapt to changes in the environment and ensure survival, organisms have already created some fantastic optical structures. Therefore, studying the underlying mechanism in the structures for enhancing luminescence plays an essential role in enabling practical applications of luminescent materials. Based on the source of excitation, luminescence can be classified into different types, such as chemiluminescence (CL), photoluminescence (PL), and electroluminescence (EL). Luminescent materials that are used in CL, PL, and EL processes mainly include rare-earth luminescent materials, quantum dot luminescent materials, and organic luminescent materials. In addition to selecting luminescent materials with matching energy levels, efficient carrier transport, and high luminous efficiency, it is also necessary to match the size of the optical structure to the emission wavelength to achieve the highest luminescence enhancement. Researchers made many efforts to use bio-inspired optical structures to enhance different luminescence types, which are described in detail in the following subsections.

. Enhanced CL
During chemical reactions, excited substances emit a type of light radiation called CL. CL can be used to determine reactants and catalysts in chemical reactions and has been developed for biochemical analysis and detection. [53] It is produced on three conditions: i) the reaction must provide sufficient excitation energy, ii) the chemical energy can be accepted by the substance and generate an excited state, and iii) the excited state molecule must have high quantum efficiency to release photons. However, due to the low fluorescence quantum yield and weak luminescence intensity, it is difficult for CL systems to achieve the requirements of high-sensitivity sensors for biochemical analysis and detection. Bio-inspired optical structures offer inspiration to improve the quantum yield and luminescence intensity of CL. Shi and co-workers proposed a strategy to enhance CL using PhC structures ( Figure 3A). [54] The 3D PhC structure suppresses the emission at the central wavelength of the stopband and enhances the emission at the edge of the stopband. The PhC structure with matched photonic stopband makes the CL achieve a 44.9-times enhancement of emission intensity and the highest decay rate. Meanwhile, the advantages of this large-area PhC film are the decreased consumption of excitons and the weak quenching of fluorescence. In addition, researchers became interested in the CL system in fireflies. Chen and coworkers analyzed the optical structures in the reflective layer of firefly lanterns and fabricated an artificial film with high reflectivity for enhancing CL ( Figure 3B). [55] They combined a chemical solution and randomly distributed hollow silicon particles between two glasses. This photonic glass exhibited high reflectivity over a broad wavelength region, facilitating CL extraction. The hollow particles with high specific surface enhanced transfer of the interface and increased the rate of reaction, leading to a 55.3-times improved CL intensity. This bio-inspired investigation provided a new avenue to design the structures for enhancing CL, which will be beneficial in sensors, imaging, and light sources.

. Enhanced PL
PL is the process of a fluorophore undergoing a radiative transition from an excited state back to its ground state and generating photons. The different photoluminescent materials have different positions of the localized energy levels in the forbidden band leading to distinct radiative transitions. PL materials include long afterglow materials, up-conversion materials, aggregation-induced emission (AIE) materials, and so on. [56][57][58][59][60][61][62] The environments also affect the spontaneous emission of luminescent materials in addition to the intrinsic electronic properties of the material. For example, nanoscale resonant cavities can enhance the spontaneous emission rate of luminescent materials based on the Purcell effect. The enhancement or suppression of spontaneous emission can be realized by changing the density of states of the electromagnetic field through optical structures. To improve the quantum efficiency and luminous intensity of PL, it is necessary to combine luminescent materials with optical structures. Upconversion nanoparticles (UCNPs), which can convert near-infrared light to visible light, become possible through two-photon or multi-photon processes. To improve the luminescence intensity and energy conversion efficiency, Yin and co-workers presented a significantly increased upconversion luminescence by combining the band edge effect of 3D opal PhC and the surface plasmon effect of gold nanorods. The Reproduced with permission. [54] Copyright 2013, Wiley-VCH. (B) (i) The composition of the bioinspired CL light-emitted device. (ii) The CL intensity versus time for a control sample and experimental samples with different photonic structure thicknesses. Reproduced with permission. [55] Copyright 2015, Springer Nature. metal nanoparticles distributed on the luminescent films absorb the matched incident light and generate a localized surface plasmon resonance (LSPR) which transfers the energy to the material to control the luminescent properties. Numerical simulations of the electromagnetic field of the plasmonic gold nanorods showed a size-dependent field enhancement. At the same time, it was observed that small UCNPs lead to luminescent centers within the interaction range of the gold nanorods. When the average size of the UCNPs is 5 nm, the length of the gold nanorod is 115 nm, and the photonic stop band (PSB) of an opal PhC structure is at 980 nm, the upconversion luminescence of UCNPs had been enhanced by more than 10 3 folds, while the detectable excitation threshold had been reduced by three orders of magnitude to 0.37 mW mm −2 . [63] Mao and co-workers incorporated UCNPs into a 2D PhC film by electrostatic interactions on the surface. The discontinuous dielectric PhC structure film forms a strongly enhanced local electric field region, which resulted in approximately 130-and 350-fold enhancements for green and red luminescence, respectively ( Figure 4A). [64] The 2D PhC film can be applied to large-area devices. Gao and co-workers employed butterfly wings with PhC structures as a biological template to improve the luminescence of UCNPs. They achieved enhanced luminescence by tuning the bandgaps of PhC structures. The results showed that the increased local DOS led to the enhancement of decay rates. Overall, the hybrid PhC structures are suitable for producing biophotonic devices with tunable upconversion emission. [65] The PhC structure can also be used for long afterglow materials to enhance the emission intensity and prolong the glowing time. Shi and co-workers assembled PhC on a substrate and covered them with a mixture of SrAl 2 O 4 :Eu and polydimethylsiloxane (PDMS) ( Figure 4B). [66] Due to the bandgap of PhC structures, the luminescence intensity and the emission direction are improved. The reflected light is reabsorbed and stimulates the luminescence. With an optimized lattice constant of the PhC structure, a two-times afterglow intensity and a 1.7-times afterglow time of SrAl 2 O 4 :Eu without any dopants were achieved. Enhancing the luminescence of afterglow materials with special light storage and release properties is of great interest for displays, lighting, and security signals.
Perovskite materials with high fluorescence quantum yield and tunable wavelength are widely applied in light-emitting devices such as LEDs and lasers. Therefore, the enhancement of luminescence becomes essential for the applications of perovskite emitters. Suppression and redistribution of spontaneous emission caused by PhC structures occur in grating and opal patterned perovskite devices. Wang and co-workers introduced a subwavelength grating metasurface structure to enhance the PL of perovskite films by nanoimprinting technique. [67] The simulation results of light transport The intensity spectra of emission from the perovskite opal film with the increasing pump power. Reproduced with permission. [68] Copyright 2018, Wiley-VCH. spectroscopy indicated the existence of high-quality cavity modes. Steady-state PL spectra showed that the metasurface structure significantly enhances the PL intensity by a factor of eight because the optimization of the metasurface leads to great dielectric resonance. Zhou and co-workers used the opal template to prepare 3D perovskite PhC ( Figure 4C). [68] Amplified spontaneous emission of the 3D perovskite PhC appeared at a low-power threshold under lasing excitation because of the strong coherent scattering and a high-intensity resonant field of the PhC structure. Furthermore, the PhC structures also can control the propagation of emission light and improve the in-coupling and out-coupling of radiation by matching resonant modes of PhC and both the excitation and the emission of the materials. [69] The perovskite nanocrystals on the top of PhC structures achieved a sixteen times enhancement of the PL intensity and an increased spontaneous emission rate.

. Enhanced light extraction for highly efficient EL
In the EL process, photons are generated due to the injection of an applied current into the layer containing the active material. Various semiconductor materials with high PL quantum efficiency, such as GaN, organic molecule, perovskite, have been employed to realize EL processes. However, the external quantum efficiency (EQE) of EL is still low due to the limitations originating from non-radiative recombination and light trapping from defects. [70,71] Cao and co-workers introduced additives into a precursor solution to prepare perovskite thin films with submicrometre platelets ( Figure 5A). [72] These randomly distributed submicrometre structures enhanced the extraction of all-direction light without spectrum shift and angular dependence. Furthermore, the amino acids passivate the defects in the perovskite surface, leading to reducing the non-radiative recombination. This LED achieved 20.7% EQE and 12% energy conversion efficiency. Ye and co-workers combined a perovskite active layer with an improved light outcoupling structure ( Figure 5B). [73] This photonic structure enhanced the extraction of light from trapped modes and reduced emission losses caused by the reabsorption in the perovskite layer, which maintained a low temperature in the device after prolonged operation. Finally, a long-lived red LED with an EQE of 21.2% was realized.
To achieve LEDs with directional emission and polarization characteristics, challenges concerning the characteristics of traditional Lambertian light sources need to be solved. Fu and co-workers used a grating as a DBR to suppress air modes and extract waveguide modes for shaping beams ( Figure 5C). [74] The patterned samples only had strong diffraction peaks from the transverse electric (TE) waveguide mode, which corresponded to high directivity with a small divergence angle. The LEDs with highly directional beam shapes have great potential in solid-state lighting, holographic displays, and stereoscopic displays.

. Information encryption technology
Information encryption technology is a protection strategy used to ensure the safe and effective transmission during an exchange of information. It is necessary to strengthen the security level of information and provide sufficient characters and encoding procedures in the communication industry. [75][76][77] Many researchers exploited unique optical effects produced by photonic structures to enhance the luminescent signal or increase the number of information channels for highly-secure information encryption. Meng and coworkers designed a hydrophilic-modified UCNPs integrated bilayer inverse opal PhC film. [78] The schematic diagram in Figure 6A shows that the synergistic effect of the photonic bandgap of double layers realized optical amplification and enhanced the fluorescence signal. Furthermore, the researchers demonstrated the application of a triple anticounterfeiting technology based on the bilayer photonic structure and UCNPs. Under the irradiation of NIR lasing, the device exhibited a green luminescence pattern that cannot be seen under natural light. It ensured the anti-counterfeiting ability and improved the reliability and security of information. Zhou and co-workers demonstrated angle-dependent anti-counterfeiting applications based on synergistic upconversion enhancement caused by multiple physical effects ( Figure 6B). [79] They introduced flexible opal PhC employing a nanoimprinting technique. The printing pattern combined with UCNPs provided good optical contrast allowing highsensitivity detection. Because of the LSPR and two-photon effect, angle-dependent bright and clear quick response code patterns were seen under infrared laser excitation at different angles. This device can be applied to manufacturing flexible and large-area security products. Physical unclonable function (PUF) provides another form of anti-counterfeiting with advanced security. Wan and co-workers replicated the micro-and nano-structures on the surfaces of natural plants through a simple, green, and environmentally friendly fabrication process. [80] The bio-inspired PDMS films exhibited strong optical scattering effects under coherent illumination. This speckle response was used as a unidirectional and unique encoding mechanism. Compared with traditional digital cryptography, it was fundamentally secure because of the unreproducible structures from nature. The feasibility of using bio-inspired PUFs as cryptographic primitives in information encryptions was experimentally validated, which showed the possible practical applications in verification and anti-counterfeiting.

. Highly sensitive sensors
Sensors need to detect extremely low concentrations of analytes in challenging physical and chemical backgrounds, which leads to expensive detection systems and complex multi-step detection processes to achieve the required high sensitivity. To address these challenges, researchers enhanced the luminous signals by utilizing bio-inspired optical structures. [81][82][83][84] For example, a common method for heavy metal ion detection is based on a fluorescent film sensor with immobilized sensitive molecules. The advantages are washability and reversibility of detection. The PhC structure with a photonic bandgap can be introduced to enhance the fluorescence signal for increased sensitivity. [85,86] Zhang and co-workers constructed a fluorescence-enhancing film sensor with a rapidly diffusible macroporous silica inverse opal structure ( Figure 7A). [87] The detection mechanism is that the reaction of a Rhodamine 6G derivative with Bi 3+ ions emits an enhanced fluorescent signal. The intensity of fluorescence increased with increasing concentration. Moreover, the concentration of Bi 3+ ions within a certain range can be quantitatively analyzed by a calibration curve. By adding different ions to the sensor, the stronger emission intensity resulted in a great selectivity for Bi 3+ ions in the co-detection of multiple ions. To further improve the detection limit of metal ions, Yan's group demonstrated multiple heterostructure PhC (MHPhC) films composed of continuous colloidal crystals with different diameters ( Figure 7B). [88] The wide bandgap of PhC structures enabled simultaneous matching of excitation and emission wavelengths, which enhanced the emission intensity of different dyes. To achieve successful multi-recognition in large-scale samples, sensors with diverse structures provide a possibility for sufficient information acquisition and signal enhancement. Huang and co-workers designed PhC detection chips for multianalyte sensing with different micro-and nano-structures by utilizing self-assembled particles on hydrophilic patterns. [89] Figure 7C shows the 3D morphology of the PhC in the array. The selective fluorescence enhancement caused by the periodic structure and observation angle lets the PhC distinguish the subtle differences in the response of different ions. The photonic bandgap was blue-shifted from the center to the edge region of the PhC pixel. Experiments showed that the highly integrated and printable multianalyte chip with PhC structures achieved efficient detection of metal ions and was successfully applied to groundwater samples. Drug and biomolecule detection is widely used in early diagnosis and medical treatment. [90][91][92][93] However, detecting analytes from highly diluted solutions is still a challenge. Hou and co-workers designed a PhC chip with hydrophilic and hydrophobic micropatterns inspired by desert beetles ( Figure 8A). [94] The analyte molecules concentrated on the PhC dots due to the wettability difference of hydrophilic and hydrophobic micropatterns and produced strong emission by the fluorescence enhancement effect of the PhC structures. In addition, the PhC chip realized ultrasensitive detection of cocaine molecules after modification with DNA aptamers and reduced the limit of cocaine molecule detection. Furthermore, multi-component detection also has a strong potential for applications and needs to be improved. Qin et al. reported a rainbow structural-color chip that enhanced the fluorescence emission over a broad wavelength range to detect different signals in multisaccharide chemical analysis ( Figure 8B). Different concentrations of twelve saccharides and their mixtures were successfully distinguished by the structured chip. [95] Zhang and co-workers designed an AIE-doped polyionic liquid-based PhC sphere detector for differential detection of multiple analytes ( Figure 9A). [96] One single photonic sphere included the ionic liquid units, which can interact with different analytes for efficient detections. This detector can be responsive to twenty natural amino acids with enhanced selectivity and excellent scalability. The single PhC sphere strategy showed the application potential for precise identification of complex multi-analyte detection. Zhao's group developed a bioinspired PhC barcode for multiplexed and sensitive nucleic acid detection ( Figure 9B). [97] Different probes can be immobilized on the surface of PhC spheres with different structural colors to realize multiplex detection. The experiment proved that detecting Human Papilloma Virus nucleic acid by PhC barcodes had ultra-high sensitivity, high accuracy, and specificity. In summary, these sensors with bio-inspired structures demonstrated the ability to detect susceptible and multiple substances in biological detections.

. Highly bright light sources
LEDs have developed for many years, and they are now the most commonly used light source. However, the low EQE still needs to be optimized. The EQE is defined as the ratio of photons emitted to the injected electrons. Periodic gratings, microlens arrays, and highly-order nanorod arrays were used to reduce the optical losses to improve the EQE of LEDs. [98] The main optical losses in LED devices include the waveguided modes loss, interface loss between the air and substrate, and plasmon polariton loss. When people face problems about how to reduce the emission losses of light sources, solutions can be found by observing light management phenomena in nature. [99][100][101][102] Researchers found many inspirations for enhancing the light extraction of LEDs in the optical structure of living organisms. Kim and co-workers reproduced a layered structure composed of longitudinal nanostructures with a period of 250 nm and asymmetric microstructures with a width of 10 µm using PDMS by imitating the optical structures in firefly lanterns ( Figure 10A). [103] This bio-inspired organic LED (OLED) exhibited enhanced light extraction efficiency by 61% and side-enhanced Lambertian emission, thus improving the wide-angle illumination. It is also demonstrated that light extraction efficiency can be increased by introducing external components. Kwon and co-workers investigated rough-surface microlens arrays with radius of curvature of ∼28 µm based on moth-eye surface structures. They fabricated multifunctional optical films with polyvinyl alcohol materials with good physical flexibility and superior conformity ( Figure 10B). [104] These special films had excellent transparency due to the anti-reflection effect provided by the moth-eye nanostructures. Adding this film to LED arrays can exhibit enhanced power efficiency by 4% and reduce power consumption, which is used in flexible and large-area LED arrays. Liang and co-workers adopted the nanopatterned imprinting to fabricate the high-transmittance and flexible fluoropolymer (FFP) films with nanoconical structures for enhancing light extraction efficiency ( Figure 10C). [105] The TE and transverse magneticmodes of deep ultraviolet (DUV) LEDs can be significantly improved with this approach by 20.5% and 21.8%, respectively. This strategy is cost-effective from the packaging point of view because it could be scalably fabricated and covered on conventional light-emitting devices. Park and co-workers found inspiration from the petal surface of tulips and prepared hierarchical surface structures composed of a wavy surface with a radius of curvature of 50 µm and surface relief grating with a 500 nm period. [106] The structures on the films had a special diffraction pattern which helped to improve the outcoupling efficiency of the light-emitting source.
Lasers have the characteristics of monochromaticity, directionality, and coherence. They are widely used as light sources in illumination, display, and integrated photonic devices. Optical structures are used to enhance the interaction between photons and materials, leading to improved performance of lasers. For example, PhC structures can serve as microcavities.
The strong optical confinements of PhC microcavities can significantly reduce the size of the device and improve the quality factor of the cavity, which is suitable for high-density and functional integrated optical circuits. Jia and co-workers fabricated a MAPbI 3 laser using a one-dimensional (1D) grating with a 403 nm period as a second-order distributed feedback structure. [107] This continuous wave DFB laser covered with a 50 nm thick gold layer and a 15 nm thick Al 2 O 3 interlayer can operate for 25 ns with the threshold of 5 kW cm −2 under the excitation of InGaN diode laser at 160 K, which provides an idea for the development of electrically pumped perovskite lasers. To further control the modes of lasers, Chen and coworkers embedded the perovskite thin film into the 2D PhC resonator with a 450 nm period, which formed a single-mode PhC band-edge laser with a threshold of 68.5 ± 3.0 µJ cm −2 ( Figure 11A). [108] They realized an arrayed laser display device with this 2D PhC structured laser. Subsequently, Schünemann et al. fabricated perovskite films with an inverse opal structure using 3D PhC as templates. [109] A single-mode distributed feedback laser with stable emission was fabricated by solution process, which did not require expensive and complicated processes. In addition, researchers found that some optical structures in living organisms such as the quasi-period PhC structures on the wings of butterfly and the random nanostructures on the wings of cicada can also act as resonators for lasing. [110][111][112] Wang and co-workers used butterfly wings with a quasi-periodic structure as a resonant cavity. They added zinc oxide particles as a gain material and obtained the Fabry-Pérot (F-P) mode laser emission at 61 µJ pumping energy ( Figure 11B). [113] Due to the great flexibility of butterfly wings, F I G U R E   (A) (i) Surface topographies of a firefly lantern template and a bio-inspired organic OLED. The inset shows the out-coupling and high transmission effect that happened in a firefly lantern. (ii) Maximum EQE of an OLED device with different structures at four inclined angles. (iii) Photographs of emission from OLEDs with flat (upper) and structured (lower) surfaces. Reproduced with permission. [103] Copyright 2016, American Chemical Society. (B) (i) Schematic of fabricating the optical film with moth-eye-inspired structures. (ii) Digital photographs of flexible LED array displays covered with large-area antireflective diffuser films. Reproduced with permission. [104] Copyright 2016, American Chemical Society. (C) (i) Top-view and (ii) cross-sectional SEM images of the patterned template for nanoimprinting. (iii) The atomic force microscope image and (iv) digital photograph of nanopatterned template. (v) Transmittance of the template with and without the FFP film. The inset plot exhibits the enhancement of effective transmittance. (vi) The photographs of the fabricated FFP film and DUV-LED with FFP film. Reproduced with permission. [105] Copyright 2019, American Chemical Society. the output of the laser can be maintained under bending. This work proved that natural biological structures can provide effective optical feedback. Li and co-workers reproduced the micromastoid structure from the lotus leaf surface by soft lithography (Figure 11C). [114] These randomly distributed structures scattered the coherent light and increased the travel time and path of light for enhancing the amplification of lasing. The optical cavity in the shape of an equilateral triangle which consisted of three mastoid structures showed the laser emission. Furthermore, the bio-inspired laser has tunable emission spectrum by moving the position of excitation or bending the flexible substrate.

 CONCLUSION AND PROSPECT
Light signals in the natural environment are crucial to the evolution and selection of organisms. The biological systems continuously optimize complex tissues and structures to adapt to the changes in the environment. Based on the experiences of luminescence enhancement in biological optical systems, researchers designed bio-inspired optical structures and fabricated optical devices for information encryption, biochemical detection, and light-emitting sources. Although many bio-inspired strategies for enhancing luminescence have been developed in recent years, some organisms and Insets are the IR camera pictures of the patterned film below (left) and above (right) the lasing threshold pump power. Reproduced with permission. [108] Copyright 2016, American Chemical Society. (B) (i) The SEM image of wings with the ZnO NPs which formed an F-P laser. (ii) The intensity spectra of bio-inspired F-P laser with increasing pump energy. Inset shows the plot of the lasing threshold test. Reproduced with permission. [113] Copyright 2015, Springer Nature. (C) (i) SEM images of the surface structure of bio-inspired PDMS template. Inset is an SEM image of a single papilla hill structure. (ii) The intensities of random laser in flat (green), convex (blue), and concave (red) states. Reproduced with permission. [114] Copyright 2020, American Chemical Society.
their employed optical phenomena have not yet been clearly explained. Further studies on the properties and functions of optical systems in organisms will be exploited to get optimized bio-inspired optical morphologies and structures for enhanced luminescence. We hope that the following approaches will address these issues: For complex applications, the functions and performance of optical structures need to be reconsidered due to different requirements in technical applications. Apart from studying the simple structures and their mechanisms, combining two or more structures also achieve an improved performance of device. [115,116] In-depth study of bio-inspired structures can be executed through nanoscale characterization and computer simulation techniques.
It is also crucial to enhance the resolution of the bioinspired structures by improving the manufacturing processes. Utilizing high-precision metasurface structures makes it possible to achieve luminescence enhancement in nonlinear optics. [117,118] For example, the second and third harmonics are helpful in the research of biological imaging and detection, and they show tremendous application potential.
To solve the need for flexible and wearable devices in the fields of display, lighting, and detection, researchers should continue to develop flexible and large-area light-emitting devices, such as flexible displays, wearable biosensors, and smart electronic devices. [119][120][121][122][123][124][125] Exploring continuous fabrication technologies is a sensible path to achieving comprehensive coverage of complex optical structures of large-area devices. [126][127][128] The processes also should develop in the direction of simple operation, low costs, and sustainable resources. Reasonable design of materials and structures can avoid the influences of substrates, electrodes, etc., so that the device maintains excellent luminescence characteristics and stability under large strain and geometric deformation. With the development of advanced materials, structures, and processing technologies, enhanced luminescence devices with bio-inspired optical structures will have bright prospects in information encryption, medical diagnosis, and energy-saving lighting.

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflicts of interest.