A polymer is a large molecule, or macromolecule, composed of many repeated subunits. Due to their broad range of properties, both synthetic and natural polymers play essential and ubiquitous roles in everyday life. Polymers range from familiar synthetic plastics such as polystyrene to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many small molecules, known as monomers, their large molecular mass relative to small molecule compounds produces unique physical properties, including toughness, a tendency to form glasses and semicrystalline structures rather than crystals. The terms polymer and resin are synonymous with plastic; the term "polymer" derives from the Greek word πολύς and μέρος, refers to a molecule whose structure is composed of multiple repeating units, from which originates a characteristic of high relative molecular mass and attendant properties. The units composing polymers derive or conceptually, from molecules of low relative molecular mass.
The term was coined in 1833 by Jöns Jacob Berzelius, though with a definition distinct from the modern IUPAC definition. The modern concept of polymers as covalently bonded macromolecular structures was proposed in 1920 by Hermann Staudinger, who spent the next decade finding experimental evidence for this hypothesis. Polymers are studied in the fields of biophysics and macromolecular science, polymer science. Products arising from the linkage of repeating units by covalent chemical bonds have been the primary focus of polymer science. Polyisoprene of latex rubber is an example of a natural/biological polymer, the polystyrene of styrofoam is an example of a synthetic polymer. In biological contexts all biological macromolecules—i.e. Proteins, nucleic acids, polysaccharides—are purely polymeric, or are composed in large part of polymeric components—e.g. Isoprenylated/lipid-modified glycoproteins, where small lipidic molecules and oligosaccharide modifications occur on the polyamide backbone of the protein.
The simplest theoretical models for polymers are ideal chains. Polymers are of two types: occurring and synthetic or man made. Natural polymeric materials such as hemp, amber, wool and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellulose, the main constituent of wood and paper; the list of synthetic polymers in order of worldwide demand, includes polyethylene, polystyrene, polyvinyl chloride, synthetic rubber, phenol formaldehyde resin, nylon, polyacrylonitrile, PVB, many more. More than 330 million tons of these polymers are made every year. Most the continuously linked backbone of a polymer used for the preparation of plastics consists of carbon atoms. A simple example is polyethylene. Many other structures do exist. Oxygen is commonly present in polymer backbones, such as those of polyethylene glycol, DNA. Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain or network. During the polymerization process, some chemical groups may be lost from each monomer.
This happens in the polymerization of PET polyester. The monomers are terephthalic acid and ethylene glycol but the repeating unit is —OC—C6H4—COO—CH2—CH2—O—, which corresponds to the combination of the two monomers with the loss of two water molecules; the distinct piece of each monomer, incorporated into the polymer is known as a repeat unit or monomer residue. Laboratory synthetic methods are divided into two categories, step-growth polymerization and chain-growth polymerization; the essential difference between the two is that in chain growth polymerization, monomers are added to the chain one at a time only, such as in polyethylene, whereas in step-growth polymerization chains of monomers may combine with one another directly, such as in polyester. Newer methods, such as plasma polymerization do not fit neatly into either category. Synthetic polymerization reactions may be carried out without a catalyst. Laboratory synthesis of biopolymers of proteins, is an area of intensive research. There are three main classes of biopolymers: polysaccharides and polynucleotides.
In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA to RNA and subsequently translate that information to synthesize the specified protein from amino acids; the protein may be modified further following translation in order to provide appropriate structure and functioning. There are other biopolymers such as rubber, suberin and lignin. Occurring polymers such as cotton and rubber were familiar materials for years before synthetic polymers such as polyethene and perspex appeared on the market. Many commercially important polymers are synthesized by chemical modification of occurring polymers. Prominent examples inclu
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics describes the behaviour of visible and infrared light; because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays and radio waves exhibit similar properties. Most optical phenomena can be accounted for using the classical electromagnetic description of light. Complete electromagnetic descriptions of light are, however difficult to apply in practice. Practical optics is done using simplified models; the most common of these, geometric optics, treats light as a collection of rays that travel in straight lines and bend when they pass through or reflect from surfaces. Physical optics is a more comprehensive model of light, which includes wave effects such as diffraction and interference that cannot be accounted for in geometric optics; the ray-based model of light was developed first, followed by the wave model of light.
Progress in electromagnetic theory in the 19th century led to the discovery that light waves were in fact electromagnetic radiation. Some phenomena depend on the fact that light has both particle-like properties. Explanation of these effects requires quantum mechanics; when considering light's particle-like properties, the light is modelled as a collection of particles called "photons". Quantum optics deals with the application of quantum mechanics to optical systems. Optical science is relevant to and studied in many related disciplines including astronomy, various engineering fields and medicine. Practical applications of optics are found in a variety of technologies and everyday objects, including mirrors, telescopes, microscopes and fibre optics. Optics began with the development of lenses by Mesopotamians; the earliest known lenses, made from polished crystal quartz, date from as early as 700 BC for Assyrian lenses such as the Layard/Nimrud lens. The ancient Romans and Greeks filled glass spheres with water to make lenses.
These practical developments were followed by the development of theories of light and vision by ancient Greek and Indian philosophers, the development of geometrical optics in the Greco-Roman world. The word optics comes from the ancient Greek word ὀπτική, meaning "appearance, look". Greek philosophy on optics broke down into two opposing theories on how vision worked, the "intromission theory" and the "emission theory"; the intro-mission approach saw vision as coming from objects casting off copies of themselves that were captured by the eye. With many propagators including Democritus, Epicurus and their followers, this theory seems to have some contact with modern theories of what vision is, but it remained only speculation lacking any experimental foundation. Plato first articulated the emission theory, the idea that visual perception is accomplished by rays emitted by the eyes, he commented on the parity reversal of mirrors in Timaeus. Some hundred years Euclid wrote a treatise entitled Optics where he linked vision to geometry, creating geometrical optics.
He based his work on Plato's emission theory wherein he described the mathematical rules of perspective and described the effects of refraction qualitatively, although he questioned that a beam of light from the eye could instantaneously light up the stars every time someone blinked. Ptolemy, in his treatise Optics, held an extramission-intromission theory of vision: the rays from the eye formed a cone, the vertex being within the eye, the base defining the visual field; the rays were sensitive, conveyed information back to the observer's intellect about the distance and orientation of surfaces. He summarised much of Euclid and went on to describe a way to measure the angle of refraction, though he failed to notice the empirical relationship between it and the angle of incidence. During the Middle Ages, Greek ideas about optics were resurrected and extended by writers in the Muslim world. One of the earliest of these was Al-Kindi who wrote on the merits of Aristotelian and Euclidean ideas of optics, favouring the emission theory since it could better quantify optical phenomena.
In 984, the Persian mathematician Ibn Sahl wrote the treatise "On burning mirrors and lenses" describing a law of refraction equivalent to Snell's law. He used this law to compute optimum shapes for curved mirrors. In the early 11th century, Alhazen wrote the Book of Optics in which he explored reflection and refraction and proposed a new system for explaining vision and light based on observation and experiment, he rejected the "emission theory" of Ptolemaic optics with its rays being emitted by the eye, instead put forward the idea that light reflected in all directions in straight lines from all points of the objects being viewed and entered the eye, although he was unable to explain how the eye captured the rays. Alhazen's work was ignored in the Arabic world but it was anonymously translated into Latin around 1200 A. D. and further summarised and expanded on by the Polish monk Witelo making it a standard text on optics in Europe for the next 400 years. In the 13th century in medieval Europe, English bishop Robert Grosseteste wrote on a wide range of scientific topics, discussed light from four different perspectives: an epistemology of light, a metaphysics or cosmogony of light, an etiology or physics of light, a theology of light, basing it on the works Aristotle and Platonism.
Grosseteste's most famous disciple, Roger Bacon, wrote w
Jonty Hurwitz is an artist and entrepreneur. Hurwitz creates anamorphic sculptures, he is recognised for the smallest human form created using nano technology. Jonty Hurwitz was born in Johannesburg, South Africa, to Selwin, a hotelier and entrepreneur and Marcia Berger, a drama lecturer and teacher. Jonty spent his early life living in small hotels in rural towns in South Africa while his father built up his business. Jonty studied Electrical Engineering at the University of the Witwatersrand in Johannesburg from 1989 to 1993, his major was Signal Processing. He joined the University of Cape Town Remote Sensing Group as a full-time researcher under Professor Michael Inggs, publishing a paper on radar pattern recognition. Following his research post, Hurwitz traveled for a long period of time in India studying Yoga and wood carving. In a 2015, documentary by CNN International on Hurwitz's artwork, BBC Radio 2 art critic Estelle Lovatt commented on Hurwitz's work: "If Leonardo Da Vinci were alive today, he would have been doing what Jonty is doing.
He would have been using algorithms. No one else works like him today, his art is the mix between the emotional and the intelligent, that's what gives it that spark." Hurwitz's work focuses on the aesthetics of art in the context of human perception. His early body of sculpture was discovered by Estelle Lovatt during 2011 in an article for Art of England Magazine: "Thinning the divide gap between art and science, Hurwitz is cognisant of the two being holistically co-joined in the same way as we are comfortably split between our spiritual and operational self". Hurwitz began producing sculptures in 2008. In 2009, his first sculpture'Yoda and the Anamorph' won the People's Choice Bentliff Prize of the Maidstone Museum and Art Gallery. In 2009 he won the Noble Sculpture Prize and was commissioned to install his first large scale work in the Italian village Colletta di Castelbianco. In 2010, he was selected as a finalist for the 4th International Arte Laguna Prize in Italy. In January 2013, Hurwitz's anamorphic work was described by the art blogger Christopher Jobson.
In a short documentary about Hurwitz's "Generation Pi" philosophy by Vera Productions it is estimated that the sculpture received 20 million views online in the space of a few weeks. In early 2013 Hurwitz was introduced to the Savoy Hotel by London art agent Sally Vaughan. Hurwitz was commissioned to be Artist in Residence at the hotel and produce a sculpture of the hotel's iconic Mascot Kaspar the Cat. Hurwitz lived for several months in the hotel producing the sculpture. In the same year, Hurwitz was nominated for the Threadneedle Prize and exhibited a collection at the Institute of Contemporary Arts in London. By late 2013, in a special edition of Art of England on portraiture, Hurwitz was cited as the No. 1 portrait artist in the UK. In January 2014 Hurwitz was voted No. 46 in the top 100 artists of 2013 by the American art site, Empty Kingdom. In the same month, Hurwitz's anamorphic work was blogged as "The best of 2013" by the American Art and Culture magazine, Juxtapoz. In 2013 Hurwitz's work was curated by Science Gallery International for a touring group show entitled ‘Illusion’ curated by Trinity College Dublin.
The show presents a collection of installation artworks from around the world that affect human perception. The exhibition led to a 2014/2015 tour in the USA; the show moved on to Kuala Lumpur, Malaysia in 2015 and Leipzig, Germany in 2016. In late 2014, he released a series of "nano sculptures" under the title of ″Trust″; this series of works captured the attentions of both the scientific and art community, being cited by among others, Scientific American, Popular Science and Phys.org. In 2015, Hurwitz was elected a member of the Royal British Society of Sculptors. In 2016 the Royal Photographic Society selected a Scanning Electron Microscope photograph by Hurwitz and Stefan Diller as one of the top 100'Royal Society International Images for Science'. Hurwitz has produced a body of work using both catoptric anamorphosis. In an interview with Christopher Jobson, Hurwitz explains his anamorphic inspiration as follows: “I have always been torn between art and physics. In a moment of self-doubt in 2008, I wandered into the National Portrait Gallery and stumbled across a strange anamorphic piece by William Scrots, a portrait of Edward VI from 1546.
Followed shortly down the aisle by The Ambassadors by Hans Holbein from 1533. My life changed forever. I rushed home and within hours was devouring the works of M. C. Escher, Da Vinci and many more. In a breath I had found "brothers" in a smallish group of artists spanning 500 years with the same dilemma as me. Within two months I was deep in production of my first work. My art rests on the shoulders of giants, I am grateful to them." Anamorphosis as a form of art has a long history. A page in Leonardo Da Vinci's note book shows two strangely elongated sketches of a child's head and an eye; these distorted and hesitant drawings, the first known anamorphoses, from around 1485". In the mid-18th Century anamorphosis was used by Jacobite artists to secretly depict images of Bonnie Prince Charlie in the wake of brutal English censorship. Hurwitz is a pioneer in creating catoptric sculpture; until the creation of his first work Rejuvenation, anamorphic sculptures have not been known to have existed in art history.
In his online talks, Hurwitz explains that this is a function of processing power and that whilst painting is possible in a mirror, three dimensional anamorphosis could only have
Nonlinear optics is the branch of optics that describes the behaviour of light in nonlinear media, that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is observed only at high light intensities such as those provided by lasers. Above the Schwinger limit, the vacuum itself is expected to become nonlinear. In nonlinear optics, the superposition principle no longer holds. Nonlinear optics remained unexplored until the discovery in 1961 of second-harmonic generation by Peter Franken et al. at University of Michigan, shortly after the construction of the first laser by Theodore Harold Maiman. However, some nonlinear effects were discovered before the development of the laser; the theoretical basis for many nonlinear processes were first described in Bloembergen's monograph "Nonlinear Optics". Nonlinear optics explains nonlinear response of properties such as frequency, phase or path of incident light; these nonlinear interactions give rise to a host of optical phenomena: Second-harmonic generation, or frequency doubling, generation of light with a doubled frequency, two photons are destroyed, creating a single photon at two times the frequency.
Third-harmonic generation, generation of light with a tripled frequency, three photons are destroyed, creating a single photon at three times the frequency. High-harmonic generation, generation of light with frequencies much greater than the original. Sum-frequency generation, generation of light with a frequency, the sum of two other frequencies. Difference-frequency generation, generation of light with a frequency, the difference between two other frequencies. Optical parametric amplification, amplification of a signal input in the presence of a higher-frequency pump wave, at the same time generating an idler wave. Optical parametric oscillation, generation of a signal and idler wave using a parametric amplifier in a resonator. Optical parametric generation, like parametric oscillation but without a resonator, using a high gain instead. Half-harmonic generation, the special case of OPO or OPG when the signal and idler degenerate in one single frequency, Spontaneous parametric down-conversion, the amplification of the vacuum fluctuations in the low-gain regime.
Optical rectification, generation of quasi-static electric fields. Nonlinear light-matter interaction with free electrons and plasmas. Optical Kerr effect, intensity-dependent refractive index. Self-focusing, an effect due to the optical Kerr effect caused by the spatial variation in the intensity creating a spatial variation in the refractive index. Kerr-lens modelocking, the use of self-focusing as a mechanism to mode-lock laser. Self-phase modulation, an effect due to the optical Kerr effect caused by the temporal variation in the intensity creating a temporal variation in the refractive index. Optical solitons, an equilibrium solution for either an optical pulse or spatial mode that does not change during propagation due to a balance between dispersion and the Kerr effect. Cross-phase modulation, where one wavelength of light can affect the phase of another wavelength of light through the optical Kerr effect. Four-wave mixing, can arise from other nonlinearities. Cross-polarized wave generation, a χ effect in which a wave with polarization vector perpendicular to the input one is generated.
Modulational instability. Raman amplification Optical phase conjugation. Stimulated Brillouin scattering, interaction of photons with acoustic phonons Multi-photon absorption, simultaneous absorption of two or more photons, transferring the energy to a single electron. Multiple photoionisation, near-simultaneous removal of many bound electrons by one photon. Chaos in optical systems. In these processes, the medium has a linear response to the light, but the properties of the medium are affected by other causes: Pockels effect, the refractive index is affected by a static electric field. Acousto-optics, the refractive index is affected by acoustic waves. Raman scattering, interaction of photons with optical phonons. Nonlinear effects fall into two qualitatively different categories and non-parametric effects. A parametric non-linearity is an interaction in which the quantum state of the nonlinear material is not changed by the interaction with the optical field; as a consequence of this, the process is "instantaneous".
Energy and momentum are conserved in the optical field, making phase matching important and polarization-dependent. Parametric and "instantaneous" nonlinear optical phenomena, in which the optical fields are not too large, can be described by a Taylor series expansion of the dielectric polarization density P at time t in terms of the electrical field E: P = ε 0 ( χ E +
A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry; the process begins by coating a substrate with a light-sensitive organic material. A patterned mask is applied to the surface to block light, so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is applied to the surface. In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened by light, the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed. A positive photoresist is a type of photoresist in which the portion of the photoresist, exposed to light becomes soluble to the photoresist developer.
The unexposed portion of the photoresist remains insoluble to the photoresist developer. A negative photoresist is a type of photoresist in which the portion of the photoresist, exposed to light becomes insoluble to the photoresist developer; the unexposed portion of the photoresist is dissolved by the photoresist developer. Note: This table is based on generalizations which are accepted in the Microelectromechanical systems fabrication industry. Based on the chemical structure of photoresists, they can be classified into three types: Photopolymeric, photocrosslinking photoresist. Photopolymeric photoresist is a type of photoresist allyl monomer, which could generate free radical when exposed to light initiates the photopolymerization of monomer to produce a polymer. Photopolymeric photoresists are used for negative photoresist, e.g. methyl methacrylate. Photodecomposing photoresist is a type of photoresist that generates hydrophilic products under light. Photodecomposing photoresists are used for positive photoresist.
A typical example is e.g. diazonaphthaquinone. Photocrosslinking photoresist is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are used for negative photoresist. Off-Stoichiometry Thiol-Enes polymersFor Self-assembled monolayer SAM photoresist, first a SAM is formed on the substrate by self-assembly; this surface covered by SAM is irradiated through a mask, similar to other photoresist, which generates a photo-patterned sample in the irradiated areas. And developer is used to remove the designed part. In lithography, decreasing the wavelength of light source is the most efficient way to achieve higher resolution. Photoresists are most used at wavelengths in the ultraviolet spectrum or shorter. For example, diazonaphthoquinone absorbs from 300 nm to 450 nm; the absorption bands can be assigned to π-π * transitions in the DNQ molecule. In the deep ultraviolet spectrum, the π-π* electronic transition in benzene or carbon double-bond chromophores appears at around 200 nm.
Due to the appearance of more possible absorption transitions involving larger energy differences, the absorption tends to increase with shorter wavelength, or larger photon energy. Photons with energies exceeding the ionization potential of the photoresist can release electrons which are capable of additional exposure of the photoresist. From about 5 eV to about 20 eV, photoionization of outer "valence band" electrons is the main absorption mechanism. Above 20 eV, inner electron ionization and Auger transitions become more important. Photon absorption begins to decrease as the X-ray region is approached, as fewer Auger transitions between deep atomic levels are allowed for the higher photon energy; the absorbed energy can drive further reactions and dissipates as heat. This is associated with the contamination from the photoresist. Photoresists can be exposed by electron beams, producing the same results as exposure by light; the main difference is that while photons are absorbed, depositing all their energy at once, electrons deposit their energy and scatter within the photoresist during this process.
As with high-energy wavelengths, many transitions are excited by electron beams, heating and outgassing are still a concern. The dissociation energy for a C-C bond is 3.6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed; the resulting scission breaks the original polymer into segments of lower molecular weight, which are more dissolved in a solvent, or else releases other chemical species which catalyze further scission reactions. It is not common to select photoresists for electron-beam exposure. Electron beam lithography relies on resists dedicated to electron-beam exposure. Physical and optical properties of photoresists influence their selection for different processes. Resolution is the ability to differ the neighboring
Photolithography called optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a photosensitive chemical photoresist on the substrate. A series of chemical treatments either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist. In complex integrated circuits, a CMOS wafer may go through the photolithographic cycle as many as 50 times. Photolithography shares some fundamental principles with photography in that the pattern in the photresist etching is created by exposing it to light, either directly or with a projected image using a photomask; this procedure is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than with lithographic printing; this method can create small patterns, down to a few tens of nanometers in size.
It provides precise control of the shape and size of the objects it creates and can create patterns over an entire surface cost-effectively. Its main disadvantages are that it requires a flat substrate to start with, it is not effective at creating shapes that are not flat, it can require clean operating conditions. Photolithography is the standard method of printed circuit microprocessor fabrication; the root words photo and graphy all have Greek origins, with the meanings'light','stone' and'writing' respectively. As suggested by the name compounded from them, photolithography is a printing method in which light plays an essential role. In the 1820s, Nicephore Niepce invented a photographic process that used Bitumen of Judea, a natural asphalt, as the first photoresist. A thin coating of the bitumen on a sheet of metal, glass or stone became less soluble where it was exposed to light; the light-sensitivity of bitumen was poor and long exposures were required, but despite the introduction of more sensitive alternatives, its low cost and superb resistance to strong acids prolonged its commercial life into the early 20th century.
In 1940, Oskar Süß created a positive photoresist by using diazonaphthoquinone, which worked in the opposite manner: the coating was insoluble and was rendered soluble where it was exposed to light. In 1954, Louis Plambeck Jr. developed the Dycryl polymeric letterpress plate, which made the platemaking process faster. In 1952, the U. S. military assigned Jay W. Lathrop and James R. Nall at the National Bureau of Standards with the task of finding a way to reduce the size of electronic circuits in order to better fit the necessary circuitry in the limited space available inside a proximity fuze. Inspired by the application of photoresist, a photosensitive liquid used to mark the boundaries of rivet holes in metal aircraft wings, Nall determined that a similar process can be used to protect the germanium in the transistors and pattern the surface with light. During development and Nall were successful in creating a 2D miniaturized hybrid integrated circuit with transistors using this technique.
In 1958, during the IRE Professional Group on Electron Devices conference in Washington, D. C. they presented the first paper to describe the fabrication of transistors using photographic techniques and adopted the term “photolithography” to describe the process, marking the first published use of the term to describe semiconductor device patterning. Despite the fact that photolithography of electronic components concerns etching metal duplicates, rather than etching stone to produce a "master" as in conventional lithographic printing and Nall chose the term “photolithography” over “photoetching” because the former sounded “high tech.” A year after the conference and Nall’s patent on photolithography was formally approved on June 9, 1959. Photolithography would contribute to the development of the first semiconductor ICs as well as the first microchips. A single iteration of photolithography combines several steps in sequence. Modern cleanrooms use robotic wafer track systems to coordinate the process.
The procedure described here omits some advanced treatments, such as thinning agents or edge-bead removal. If organic or inorganic contaminations are present on the wafer surface, they are removed by wet chemical treatment, e.g. the RCA clean procedure based on solutions containing hydrogen peroxide. Other solutions made with trichloroethylene, acetone or methanol can be used to clean; the wafer is heated to a temperature sufficient to drive off any moisture that may be present on the wafer surface, 150 °C for ten minutes is sufficient. Wafers that have been in storage must be chemically cleaned to remove contamination. A liquid or gaseous "adhesion promoter", such as Bisamine, is applied to promote adhesion of the photoresist to the wafer; the surface layer of silicon dioxide on the wafer reacts with HMDS to form tri-methylated silicon-dioxide, a water repellent layer not unlike the layer of wax on a car's paint. This water repellent layer prevents the aqueous developer from penetrating between the photoresist layer and the wafer's
A photonic crystal is a periodic optical nanostructure that affects the motion of photons in much the same way that ionic lattices affect electrons in solids. Photonic crystals occur in nature in the form of structural coloration and animal reflectors, and, in different forms, promise to be useful in a range of applications. In 1887 the English physicist Lord Rayleigh experimented with periodic multi-layer dielectric stacks, showing they had a photonic band-gap in one dimension. Research interest grew with work in 1987 by Eli Yablonovitch and Sajeev John on periodic optical structures with more than one dimension—now called photonic crystals. Photonic crystals can be fabricated for two, or three dimensions. One-dimensional photonic crystals can be stuck together. Two-dimensional ones can be made by drilling holes in a suitable substrate. Fabrication methods for three-dimensional ones include drilling under different angles, stacking multiple 2-D layers on top of each other, direct laser writing, or, for example, instigating self-assembly of spheres in a matrix and dissolving the spheres.
Photonic crystals can, in principle, find uses. Existing applications include thin-film optics with coatings for lenses. Two-dimensional photonic-crystal fibers are used in nonlinear devices and to guide exotic wavelengths. Three-dimensional crystals may one day be used in optical computers. Three-dimensional photonic crystals could lead to more efficient photovoltaic cells as a source of power for electronics, thus cutting down the need for an electrical input for power. Photonic crystals are composed of periodic dielectric, metallo-dielectric—or superconductor microstructures or nanostructures that affect electromagnetic wave propagation in the same way that the periodic potential in a semiconductor crystal affects electron motion by defining allowed and forbidden electronic energy bands. Photonic crystals contain repeating regions of high and low dielectric constant. Photons either propagate depending on their wavelength. Wavelengths that propagate are called modes, groups of allowed modes form bands.
Disallowed bands of wavelengths are called photonic band gaps. This gives rise to distinct optical phenomena, such as inhibition of spontaneous emission, high-reflecting omni-directional mirrors, low-loss-waveguiding. Intuitively, the bandgap of photonic crystals can be understood to arise from the destructive interference of multiple reflections of light propagating in the crystal at the interfaces of the high- and low- dielectric constant regions, akin to the bandgaps of electrons in solids; the periodicity of the photonic crystal structure must be around half the wavelength of the electromagnetic waves to be diffracted. This is ~350 nm to ~650 nm for photonic crystals that operate in the visible part of the spectrum—or less, depending on average index of refraction; the repeating regions of high and low dielectric constant must, therefore, be fabricated at this scale, difficult. Photonic crystals have been studied in one form or another since 1887, but no one used the term photonic crystal until over 100 years later—after Eli Yablonovitch and Sajeev John published two milestone papers on photonic crystals in 1987.
The early history is well documented in the form of a story when it was identified as one of the landmark developments in physics by the American Physical Society. Before 1987, one-dimensional photonic crystals in the form of periodic multi-layer dielectric stacks were studied extensively. Lord Rayleigh started their study in 1887, by showing that such systems have a one-dimensional photonic band-gap, a spectral range of large reflectivity, known as a stop-band. Today, such structures are used in a diverse range of applications—from reflective coatings to enhancing LED efficiency to reflective mirrors in certain laser cavities. A detailed theoretical study of one-dimensional optical structures was performed by Vladimir P. Bykov, the first to investigate the effect of a photonic band-gap on the spontaneous emission from atoms and molecules embedded within the photonic structure. Bykov speculated as to what could happen if two- or three-dimensional periodic optical structures were used; the concept of three-dimensional photonic crystals was discussed by Ohtaka in 1979, who developed a formalism for the calculation of the photonic band structure.
However, these ideas did not take off until after the publication of two milestone papers in 1987 by Yablonovitch and John. Both these papers concerned high-dimensional periodic optical structures. Yablonovitch's main goal was to engineer photonic density of states to control the spontaneous emission of materials embedded in the photonic crystal. John's idea was to use photonic crystals to affect control of light. After 1987, the number of research papers concerning photonic crystals began to grow exponentially. However, due to the difficulty of fabricating these structures at optical scales, early studies were either theoretical or in the microwave regime, where photonic crystals can be built on the more accessible centimetre scale. By 1991, Yablonovitch had demonstrated the first three-dimensional photonic band-gap in the microwave regime; the structure that Yablonovitch was able