Bio-inspired photonics or bio-inspired optical materials are the application of biomimicry (the use of natural models, systems, and elements for human innovations [1] ) to the field of photonics (the science and application of light generation, detection, and manipulation [2] ). This differs slightly from biophotonics which is the study and manipulation of light to observe its interactions with biology. [3] One area that inspiration may be drawn from is structural color, which allows color to appear as a result of the detailed material structure. [4] Other inspiration can be drawn from both static and dynamic camouflage in animals like the chameleon [5] or some cephalopods. [6] Scientists have also been looking to recreate the ability to absorb light using molecules from various plants and microorganisms. [7] Pulling from these heavily evolved constructs allows engineers to improve and optimize existing photonic technologies, whilst also solving existing problems within this field.
One of the earliest encounters with biological photonics was as early as the 6th century B.C.E (before common era). The Greek philosopher Anaximander, widely regarded as the first scientist, had a student named Anaximenes, who had the first documented mention of bioluminescence. [8] [9] He described seeing a glow in the water when striking it with an oar. [10] Similarly, Aristotle also experienced the same phenomena, which he documented in works like Meteorologica [11] and De Coloribus. [12] He mentions seeing "things which are neither fire nor forms of fire seem to produce light by nature. [12] "
Although it was experienced that early, there was still no explanation to why it was occurring. It was not until the early microscopes, utilized by Robert Hooke in the mid-1600s, [13] that allowed humans to observe nature in greater detail. Hooke himself published what he had seen in the text Micrographia in 1665. [14] Here he describes various biological structures such as the feathers of colorful birds, wing and eyes of flies, and pearlescent scales of silverfish. This ability to look at the microstructures of nature, gave scientists information on the mechanisms behind the interactions between biology and light. The Theorie of Imperfection, published by the Russian biophysicist Zhuralev and American biochemist Seliger, is the first working hypothesis about the ultra-weak emission of photons by biological systems. [15] Further developments in microscopy, like scanning electron microscopy (SEM), [16] only increased this and would allow scientists to mimic these observed structures.
In addition, the concept of biomimicry was spurred by many scientists, including Leonardo da Vinci. He spent a great deal of time studying the anatomy of birds and their flight capabilities. As demonstrated by various sketches and notes he left behind, he even attempted to create a "flying machine". [17] Although unsuccessful, it was one of the earliest examples of biomimicry.
Molecular biomimetics involves the design of optical materials based on specific molecules and/or macromolecules to induce coloration. [18] Molecularly Imprinted Polymers (MIPs) are specifically aimed at sensing macromolecules. [19] They can also form them into specific structures that change color. [20] Pigment-inspired materials aiming for specific molecular light absorption have been developed as for example melanin-inspired films prepared by polymerization of melanin precursors such as dopamine and 5,6-dihydroxyindole to provoke color saturation. [21] [22] [23] Polydopamine is a synthetic polymer with color properties similar to melanin. [24] It can also act to enhance the vibrancy and stability of structural colors. [20] Materials based on the multi-layer stacking of guanine molecular crystals found in living organisms (e.g. fish [25] and chameleons [26] ) have been proposed as potential reflective coatings and solar reflectors. Protein-based optical materials, for instance self-assembling reflectin proteins found in cephalopods [27] [28] and silk, [29] have incited interest in artificial materials for camouflage systems, [30] electronic paper (e-paper) [31] and biomedical applications. [32] Non-protein biological macromolecules such as DNA have also been utilized for bio-inspired optics. [33] The most abundant biopolymer on earth, cellulose, has been also utilized as a principal component for bio-optics. [34] [18] Modification of wood or other cellulose sources can mitigate scattering and absorption of light leading to optically interesting materials such as transparent wood and paper. [35] [36] Pressure and solvent polarity affect the color of a manufactured cellulose membrane, to the point of detection by the naked eye. Cellulose can also be used as nanofibrils or nanocrystals after treatments. One such treatment involves a nitrating agent to form nitrocellulose. [20] Cellulose nanocrystals can polarize light. [37]
Structural color is a type of coloration that arises from the interaction of light with nano-sized structures. [38] This interaction is possible because these photonic structures are of the same size as the wavelength of light. Through a mechanism of constructive and destructive interference, certain colors get amplified, while others diminish.
Photonic structures are abundant in nature, existing in a wide range of organisms. Different organisms use different structures, each with a different morphology designed to obtain the desired effect. Examples of this are the photonic crystal underlying the bright colors in peacock feathers [39] or the tree-like structures responsible for the bright blue in some Morpho butterflies. [40]
An example of bio-inspired photonics using structures is the so-called moth eye. Moths have a structure of ordered cylinders in their eyes that do not produce color, but instead reduce reflectivity. [41] This concept has led to creation of antireflective coatings. [42]
A combination of chemical structure and how it interacts with visible light creates color within organisms' nature. [4] The creation of specific biological photonics requires identifying the chemical components of the structure, the optical response created by the physics and the structure's function. [20] The complex structures created by nature can range from simple, quasi-ordered structures to hierarchical complex formations. [4]
Nature sometimes manipulates the nanostructure, such as its crystal lattice parameters in order to create its patterns and colors. [20] [43] [44] [45] [46] The Barbule (the individual strands of a feather that hold its color) of the peacock is made of an outer layer of keratin and an inner layer containing an array of melanin rods connected by keratin with holes separating them. When the melanin rods are parallel to the lattice arrangement of the structure of the keratin outer layer it creates the brown color. The rest of the colors of the feather are created by changing the spacing of the melanin layers. [20] [47] [48] [49]
Aperiodic Photonic Structures do not have a unit cell and are capable of creating band gaps without the requirement of a high index of refraction difference. Also known as quasi-ordered crystal structure creates blue and green coloring. [20]
These are twisted multilayers where fibers are aligned in the same direction and each layer they are slightly rotated. [50] [51] This structure allows nature to reflect polarized light and creates an intense value due to Bragg reflection. [4] [20] [51]
As a form of application, biophotonics are used in order to indicate antibacterial and self-healing properties. Since the existence of silver nanoparticles prevent bacterial adhesion (there is already bacteria existing in the hydrogel) it causes hydrogel degradation and color fading. This allows for the engineered hydrogel to display with color its integrity after self-healing. [4] [52]
Nanoarchitectures contribute to the iridescence of butterflies and beetles. Multilayers are common, typically in a 1-D or 3-D structure, 2-D structures are more rare. [53] Disorder and irregularity in the structure are “intentional” and adapted to the habitat. The structure has been successfully recreated and can be used as a coating. [54] It is also used in some applications where stable, vibrant color is required. It is flexible enough that it can be designed to have a pattern. [55]
When observing fireflies (Photuris sp.) using SEM, it was observed that their light emitting cuticle had a specific 2D periodic structure. It is structured following a “factory roof” like pattern with scales oriented at a tilted slope and a sharp edge on the protruding side of the scales. [56] [57] When modeling a similar structure using a photoresist layer on light emitting diodes (LEDs), it resulted in a 68% power increase and 55% increase in light extraction efficiency (LEE). This technology reduces the amount of energy consumed to produce the same amount of light. [58]
Responsive materials are materials or devices that can respond to external stimuli as they occur. A little bit of time is taken to adjust to the new surroundings, but the idea remains consistent with what is seen in nature. The most commonly used examples are the chameleon or octopus, as their responsive skin allows them to change the color or even the texture of their skin. [59] The mechanisms behind these tactics are called chromatophores, which are pigment-filled sacs that uses muscles and nerves to change the animal's external appearance. These chromatophores are activated by neuronal activity, so an animal can change its color just by thinking about it. [60] The animal uses another mechanism to be able to know what color or shape to take; a photo-sensitive cell within their skin called opsin is able to detect light (and possibly color). The animal can use these opsins to their advantage to quickly assess their surroundings, before turning on their chromatophores to accurately camouflage to their circumstances.
A lot of creatures have camouflage incorporated into their bodies — take the fish in the figure on the right for example. In this hypothetical, the animal can appear in two different ways depending on their surroundings: in the middle of the ocean away from all solid objects, it can appear near-translucent; near the sea floor where potential predators will only sense it from above, it can turn darker to naturally blend in with the rocky bottom. Many fish, such as the marine hatchetfish, use a combination of camouflage techniques to achieve these appearances. [61] Silvering, a common tactic, utilizes highly reflective scales to reflect the surrounding light effectively enough to make the scales appear invisible from the side. Counterillumination, a tactic used more by deep-sea dwellers, uses a luminous organ located in the bottom of the body to emit light in order to appear brighter from underneath. At this angle, the light emitted is at an intensity meant to replicate the sunlight as it appears on the surface of the water. Thus, from below the creature is essentially invisible to many predators.
Within the luminous organ is a laminar structure of photocytes and nerve branches, with relatively small gap junctions between them. [62] It is thought that the vast interconnectivity and the layered structure of these neuro-photocyte units is what allows a deep-sea fish to rapidly respond to a situation with spontaneous luminescence. Because all of the nerves are directly connected to the spinal cord (and by extension, the brain), researchers believe that electronic signals can trigger these photocytes to react. [63] With this line of thinking, scientists are working to develop technology using this type of neuro-photocyte unit.
These biologically inspired materials can be applied in many different circumstances. [64] This technology can be used to camouflage objects, create a device that can mold its shape yet still retain its desired properties, or even help people in relation to biomedical applications. A coating of this technology can help incorporate a foreign body into a living ecosystem, i.e. a human body. The technology of this device allows a person's antibodies to detect the new object as a non-threat, thus permitting easier acceptance of manmade tools into the body, such as a cardiac pacing device to the chest.
Biomimetics or biomimicry is the emulation of the models, systems, and elements of nature for the purpose of solving complex human problems. The terms "biomimetics" and "biomimicry" are derived from Ancient Greek: βίος (bios), life, and μίμησις (mīmēsis), imitation, from μιμεῖσθαι (mīmeisthai), to imitate, from μῖμος (mimos), actor. A closely related field is bionics.
A photonic crystal is an optical nanostructure in which the refractive index changes periodically. This affects the propagation of light in the same way that the structure of natural crystals gives rise to X-ray diffraction and that the atomic lattices of semiconductors affect their conductivity of electrons. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, as artificially produced, promise to be useful in a range of applications.
Iridescence is the phenomenon of certain surfaces that appear to gradually change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstructures or thin films. Examples of iridescence include soap bubbles, feathers, butterfly wings and seashell nacre, and minerals such as opal. Pearlescence is a related effect where some or most of the reflected light is white. The term pearlescent is used to describe certain paint finishes, usually in the automotive industry, which actually produce iridescent effects.
A metamaterial is any material engineered to have a property that is rarely observed in naturally occurring materials. They are made from assemblies of multiple elements fashioned from composite materials such as metals and plastics. These materials are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Metamaterials derive their properties not from the properties of the base materials, but from their newly designed structures. Their precise shape, geometry, size, orientation and arrangement gives them their smart properties capable of manipulating electromagnetic waves: by blocking, absorbing, enhancing, or bending waves, to achieve benefits that go beyond what is possible with conventional materials.
Optical computing or photonic computing uses light waves produced by lasers or incoherent sources for data processing, data storage or data communication for computing. For decades, photons have shown promise to enable a higher bandwidth than the electrons used in conventional computers.
A distributed Bragg reflector (DBR) is a reflector used in waveguides, such as optical fibers. It is a structure formed from multiple layers of alternating materials with different refractive index, or by periodic variation of some characteristic of a dielectric waveguide, resulting in periodic variation in the effective refractive index in the guide. Each layer boundary causes a partial reflection and refraction of an optical wave. For waves whose vacuum wavelength is close to four times the optical thickness of the layers, the interaction between these beams generates constructive interference, and the layers act as a high-quality reflector. The range of wavelengths that are reflected is called the photonic stopband. Within this range of wavelengths, light is "forbidden" to propagate in the structure.
Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons.
Harry Albert Atwater, Jr. is an American physicist and materials scientist and is the Otis Booth Leadership Chair of the division of engineering and applied science at the California Institute of Technology. Currently he is the Howard Hughes Professor of Applied Physics and Materials Science and the director for the Liquid Sunlight Alliance (LiSA), a Department of Energy Hub program for solar fuels. Atwater's scientific effort focuses on nanophotonic light-matter interactions and solar energy conversion. His current research in energy centers on high efficiency photovoltaics, carbon capture and removal, and photoelectrochemical processes for generation of solar fuels. His research has resulted in world records for solar photovoltaic conversion and photoelectrochemical water splitting. His work also spans fundamental nanophotonic phenomena, in plasmonics and 2D materials, and also applications including active metasurfaces and optical propulsion.
Reflectins are a family of intrinsically disordered proteins evolved by a certain number of cephalopods including Euprymna scolopes and Doryteuthis opalescens to produce iridescent camouflage and signaling. The recently identified protein family is enriched in aromatic and sulfur-containing amino acids, and is utilized by certain cephalopods to refract incident light in their environment. The reflectin protein is responsible for dynamic pigmentation and iridescence in organisms. This process is “dynamic” due to its reversible properties, allowing reflectin to change an organism's appearance in response to external factors such as needing to camouflage or send warning signals.
A colloidal crystal is an ordered array of colloidal particles and fine grained materials analogous to a standard crystal whose repeating subunits are atoms or molecules. A natural example of this phenomenon can be found in the gem opal, where spheres of silica assume a close-packed locally periodic structure under moderate compression. Bulk properties of a colloidal crystal depend on composition, particle size, packing arrangement, and degree of regularity. Applications include photonics, materials processing, and the study of self-assembly and phase transitions.
Multiphoton lithography of polymer templates has been known for years by the photonic crystal community. Similar to standard photolithography techniques, structuring is accomplished by illuminating negative-tone or positive-tone photoresists via light of a well-defined wavelength. A critical difference is, however, the avoidance of photomasks. Instead, two-photon absorption is utilized to induce a dramatic change in the solubility of the resist for appropriate developers.
A nanolaser is a laser that has nanoscale dimensions and it refers to a micro-/nano- device which can emit light with light or electric excitation of nanowires or other nanomaterials that serve as resonators. A standard feature of nanolasers includes their light confinement on a scale approaching or suppressing the diffraction limit of light. These tiny lasers can be modulated quickly and, combined with their small footprint, this makes them ideal candidates for on-chip optical computing.
A photonic metamaterial (PM), also known as an optical metamaterial, is a type of electromagnetic metamaterial, that interacts with light, covering terahertz (THz), infrared (IR) or visible wavelengths. The materials employ a periodic, cellular structure.
A metamaterial absorber is a type of metamaterial intended to efficiently absorb electromagnetic radiation such as light. Furthermore, metamaterials are an advance in materials science. Hence, those metamaterials that are designed to be absorbers offer benefits over conventional absorbers such as further miniaturization, wider adaptability, and increased effectiveness. Intended applications for the metamaterial absorber include emitters, photodetectors, sensors, spatial light modulators, infrared camouflage, wireless communication, and use in solar photovoltaics and thermophotovoltaics.
Structural coloration in animals, and a few plants, is the production of colour by microscopically structured surfaces fine enough to interfere with visible light instead of pigments, although some structural coloration occurs in combination with pigments. For example, peacock tail feathers are pigmented brown, but their microscopic structure makes them also reflect blue, turquoise, and green light, and they are often iridescent.
Photonic molecules are a form of matter in which photons bind together to form "molecules". They were first predicted in 2007. Photonic molecules are formed when individual (massless) photons "interact with each other so strongly that they act as though they have mass". In an alternative definition, photons confined to two or more coupled optical cavities also reproduce the physics of interacting atomic energy levels, and have been termed as photonic molecules.
Plasmonics or nanoplasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices, and finds applications in sensing, microscopy, optical communications, and bio-photonics.
An electromagnetic metasurface refers to a kind of artificial sheet material with sub-wavelength thickness. Metasurfaces can be either structured or unstructured with subwavelength-scaled patterns in the horizontal dimensions.
Silvia Vignolini is an Italian physicist who is Director of research at the Max Planck Institute of Colloids and Interfaces and Professor of Chemistry and Bio-materials in the Yusuf Hamied Department of Chemistry at the University of Cambridge. Her research investigates natural photonics structures, the self-assembly of cellulose and light propagation through complex structures. She was awarded the KINGFA young investigator award by the American Chemical Society and the Gibson-Fawcett Award in 2018.
In photonics, a meta-waveguide is a physical structures that guides electromagnetic waves with engineered functional subwavelength structures. Meta-waveguides are the result of combining the fields of metamaterials and metasurfaces into integrated optics. The design of the subwavelength architecture allows exotic waveguiding phenomena to be explored.