An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves. Optical cavities are a major component of lasers, surrounding the gain medium and providing feedback of the laser light, they are used in optical parametric oscillators and some interferometers. Light confined in the cavity reflects multiple times producing standing waves for certain resonance frequencies; the standing wave patterns produced. Different resonator types are distinguished by the focal lengths of the two mirrors and the distance between them; the geometry must be chosen. Resonator types are designed to meet other criteria such as minimum beam waist or having no focal point inside the cavity. Optical cavities are designed to have a large Q factor. Therefore, the frequency line width of the beam is small indeed compared to the frequency of the laser. Light confined in a resonator will reflect multiple times from the mirrors, due to the effects of interference, only certain patterns and frequencies of radiation will be sustained by the resonator, with the others being suppressed by destructive interference.
In general, radiation patterns which are reproduced on every round-trip of the light through the resonator are the most stable, these are the eigenmodes, known as the modes, of the resonator. Resonator modes can be divided into two types: longitudinal modes, which differ in frequency from each other; the basic, or fundamental transverse mode of a resonator is a Gaussian beam. The most common types of optical cavities consist of two facing spherical mirrors; the simplest of these is the plane-parallel or Fabry–Pérot cavity, consisting of two opposing flat mirrors. While simple, this arrangement is used in large-scale lasers due to the difficulty of alignment. However, this problem is much reduced for short cavities with a small mirror separation distance. Plane-parallel resonators are therefore used in microchip and microcavity lasers and semiconductor lasers. In these cases, rather than using separate mirrors, a reflective optical coating may be directly applied to the laser medium itself; the plane-parallel resonator is the basis of the Fabry–Pérot interferometer.
For a resonator with two mirrors with radii of curvature R1 and R2, there are a number of common cavity configurations. If the two radii are equal to half the cavity length, a concentric or spherical resonator results; this type of cavity produces a diffraction-limited beam waist in the centre of the cavity, with large beam diameters at the mirrors, filling the whole mirror aperture. Similar to this is the hemispherical cavity, with one plane mirror and one mirror of radius equal to the cavity length. A common and important design is the confocal resonator, with mirrors of equal radii to the cavity length; this design produces the smallest possible beam diameter at the cavity mirrors for a given cavity length, is used in lasers where the purity of the transverse mode pattern is important. A concave-convex cavity has one convex mirror with a negative radius of curvature; this design produces no intracavity focus of the beam, is thus useful in high-power lasers where the intensity of the intracavity light might be damaging to the intracavity medium if brought to a focus.
A transparent dielectric sphere, such as a liquid droplet forms an interesting optical cavity. In 1986 Richard K. Chang et al. demonstrated lasing using ethanol microdroplets doped with rhodamine 6G dye. This type of optical cavity exhibits optical resonances when the size of the sphere or the optical wavelength or the refractive index is varied; the resonance is known as morphology-dependent resonance. Only certain ranges of values for R1, R2, L produce stable resonators in which periodic refocussing of the intracavity beam is produced. If the cavity is unstable, the beam size will grow without limit growing larger than the size of the cavity mirrors and being lost. By using methods such as ray transfer matrix analysis, it is possible to calculate a stability criterion: 0 ⩽ ⩽ 1. Values which satisfy the inequality correspond to stable resonators; the stability can be shown graphically by defining a stability parameter, g for each mirror: g 1 = 1 − L R 1, g 2 = 1 − L R 2,and plotting g1 against g2 as shown.
Sputter deposition is a physical vapor deposition method of thin film deposition by sputtering. This involves ejecting material from a "target", a source onto a "substrate" such as a silicon wafer. Resputtering is re-emission of the deposited material during the deposition process by ion or atom bombardment. Sputtered atoms ejected from the target have a wide energy distribution up to tens of eV; the sputtered ions can ballistically fly from the target in straight lines and impact energetically on the substrates or vacuum chamber. Alternatively, at higher gas pressures, the ions collide with the gas atoms that act as a moderator and move diffusively, reaching the substrates or vacuum chamber wall and condensing after undergoing a random walk; the entire range from high-energy ballistic impact to low-energy thermalized motion is accessible by changing the background gas pressure. The sputtering gas is an inert gas such as argon. For efficient momentum transfer, the atomic weight of the sputtering gas should be close to the atomic weight of the target, so for sputtering light elements neon is preferable, while for heavy elements krypton or xenon are used.
Reactive gases can be used to sputter compounds. The compound can be formed on the target surface, in-flight or on the substrate depending on the process parameters; the availability of many parameters that control sputter deposition make it a complex process, but allow experts a large degree of control over the growth and microstructure of the film. One of the earliest widespread commercial applications of sputter deposition, still one of its most important applications, is in the production of computer hard disks. Sputtering is used extensively in the semiconductor industry to deposit thin films of various materials in integrated circuit processing. Thin antireflection coatings on glass for optical applications are deposited by sputtering; because of the low substrate temperatures used, sputtering is an ideal method to deposit contact metals for thin-film transistors. Another familiar application of sputtering is low-emissivity coatings on glass, used in double-pane window assemblies; the coating is a multilayer containing silver and metal oxides such as zinc oxide, tin oxide, or titanium dioxide.
A large industry has developed around tool bit coating using sputtered nitrides, such as titanium nitride, creating the familiar gold colored hard coat. Sputtering is used as the process to deposit the metal layer during the fabrication of CDs and DVDs. Hard disk surfaces use other sputtered materials. Sputtering is one of the main processes of manufacturing optical waveguides and is another way for making efficient photovoltaic solar cells. Sputter coating in scanning electron microscopy is a sputter deposition process to cover a specimen with a thin layer of conducting material a metal, such as a gold/palladium alloy. A conductive coating is needed to prevent charging of a specimen with an electron beam in conventional SEM mode. While metal coatings are useful for increasing signal to noise ratio, they are of inferior quality when X-ray spectroscopy is employed. For this reason when using X-ray spectroscopy a carbon coating is preferred. An important advantage of sputter deposition is that materials with high melting points are sputtered while evaporation of these materials in a resistance evaporator or Knudsen cell is problematic or impossible.
Sputter deposited. The difference is due to different elements spreading differently because of their different mass but this difference is constant. Sputtered films have a better adhesion on the substrate than evaporated films. A target contains a large amount of material and is maintenance free making the technique suited for ultrahigh vacuum applications. Sputtering sources are compatible with reactive gases such as oxygen. Sputtering can be performed top-down. Advanced processes such as epitaxial growth are possible; some disadvantages of the sputtering process are that the process is more difficult to combine with a lift-off for structuring the film. This is because characteristic of sputtering, makes a full shadow impossible. Thus, one cannot restrict where the atoms go, which can lead to contamination problems. Active control for layer-by-layer growth is difficult compared to pulsed laser deposition and inert sputtering gases are built into the growing film as impurities. Pulsed laser deposition is a variant of the sputtering deposition technique in which a laser beam is used for sputtering.
Role of the sputtered and resputtered ions and the background gas is investigated during the pulsed laser deposition process. Sputtering sources employ magnetrons that utilize strong electric and magnetic fields to confine charged plasma particles close to the surface of the sputter target. In a magnetic field, electrons follow helical paths around magnetic field lines, undergoing more ionizing collisions with gaseous neutrals near the target surface than would otherwise occur; the sputter gas is an inert gas such as argon. The extra argon ions created as a result of these collisions lead to a higher deposition rate; the plasma can be sustained at a l
Zinc sulfide is an inorganic compound with the chemical formula of ZnS. This is the main form of zinc found in nature, where it occurs as the mineral sphalerite. Although this mineral is black because of various impurities, the pure material is white, it is used as a pigment. In its dense synthetic form, zinc sulfide can be transparent, it is used as a window for visible optics and infrared optics. ZnS exists in two main crystalline forms, this dualism is a salient example of polymorphism. In each form, the coordination geometry at Zn and S is tetrahedral; the more stable cubic form is known as zinc blende or sphalerite. The hexagonal form is known as the mineral wurtzite, although it can be produced synthetically; the transition from the sphalerite form to the wurtzite form occurs at around 1020 °C. A tetragonal form is known as the rare mineral called polhemusite, with the formula S. Zinc sulfide, with addition of few ppm of suitable activator, exhibits strong phosphorescence, is used in many applications, from cathode ray tubes through X-ray screens to glow in the dark products.
When silver is used as activator, the resulting color is bright blue, with maximum at 450 nanometers. Using manganese yields an orange-red color at around 590 nanometers. Copper gives long-time glow, it has the familiar greenish glow-in-the-dark. Copper-doped zinc sulfide is used in electroluminescent panels, it exhibits phosphorescence due to impurities on illumination with blue or ultraviolet light. Zinc sulfide is used as an infrared optical material, transmitting from visible wavelengths to just over 12 micrometers, it can be shaped into a lens. It is made as microcrystalline sheets by the synthesis from hydrogen sulfide gas and zinc vapour, this is sold as FLIR-grade, where the zinc sulfide is in a milky-yellow, opaque form; this material when hot isostatically pressed can be converted to a water-clear form known as Cleartran. Early commercial forms were marketed as Irtran-2 but this designation is now obsolete. Zinc sulfide is a common pigment, sometimes called sachtolith; when combined with barium sulfate, zinc sulfide forms lithopone.
Fine ZnS powder is an efficient photocatalyst, which produces hydrogen gas from water upon illumination. Sulfur vacancies can be introduced in ZnS during its synthesis. Both sphalerite and wurtzite are intrinsic, wide-bandgap semiconductors; these are prototypical II-VI semiconductors, they adopt structures related to many of the other semiconductors, such as gallium arsenide. The cubic form of ZnS has a band gap of about 3.54 electron volts at 300 kelvins, but the hexagonal form has a band gap of about 3.91 electron volts. ZnS can be doped as either a p-type semiconductor; the phosphorescence of ZnS was first reported by the French chemist Théodore Sidot in 1866. His findings were presented by A. E. Becquerel, renowned for the research on luminescence. ZnS was used by Ernest Rutherford and others in the early years of nuclear physics as a scintillation detector, because it emits light upon excitation by x-rays or electron beam, making it useful for X-ray screens and cathode ray tubes; this property made zinc sulfide useful in the dials of radium watches.
Zinc sulfide is produced from waste materials from other applications. Typical sources include smelter and pickle liquors, it is a by-product of the synthesis of ammonia from methane where zinc oxide is used to scavenge hydrogen sulfide impurities in the natural gas: ZnO + H2S → ZnS + H2O It is produced by igniting a mixture of zinc and sulfur. Since zinc sulfide is insoluble in water, it can be produced in a precipitation reaction. Solutions containing Zn2+ salts form a precipitate ZnS in the presence of sulfide ions. Zn2+ + S2− → ZnSThis reaction is the basis of a gravimetric analysis for zinc. Zinc and Sulfur at The Periodic Table of Videos Composition of CRT phosphors University of Reading, Infrared Multilayer Laboratory optical data melting point
A mirror is an object that reflects light in such a way that, for incident light in some range of wavelengths, the reflected light preserves many or most of the detailed physical characteristics of the original light, called specular reflection. This is different from other light-reflecting objects that do not preserve much of the original wave signal other than color and diffuse reflected light, such as flat-white paint; the most familiar type of mirror is the plane mirror. Curved mirrors are used, to produce magnified or diminished images or focus light or distort the reflected image. Mirrors are used for personal grooming or admiring oneself, for viewing the area behind and on the sides on motor vehicles while driving, for decoration, architecture. Mirrors are used in scientific apparatus such as telescopes and lasers and industrial machinery. Most mirrors are designed for visible light. There are many types of glass mirrors, each representing a different manufacturing process and reflection type.
An aluminium glass mirror is made of a float glass manufactured using vacuum coating, i.e. aluminium powder is evaporated onto the exposed surface of the glass in a vacuum chamber and coated with two or more layers of waterproof protective paint. A low aluminium glass mirror is manufactured by coating silver and two layers of protective paint on the back surface of glass. A low aluminium glass mirror is clear, light transmissive and reflects accurate natural colors; this type of glass is used for framing presentations and exhibitions in which a precise color representation of the artwork is essential or when the background color of the frame is predominantly white. A safety glass mirror is made by adhering a special protective film to the back surface of a silver glass mirror, which prevents injuries in case the mirror is broken; this kind of mirror is used for furniture, glass walls, commercial shelves, or public areas. A silkscreen printed glass mirror is produced using inorganic color ink that prints patterns through a special screen onto glass.
Various colors and glass shapes are available. Such a glass mirror is durable and more moisture resistant than ordinary printed glass and can serve for over 20 years; this type of glass is used for decorative purposes. A silver glass mirror is an ordinary mirror, coated on its back surface with silver, which produces images by reflection; this kind of glass mirror is produced by coating a silver, copper film and two or more layers of waterproof paint on the back surface of float glass, which resists acid and moisture. A silver glass mirror provides clear and actual images, is quite durable, is used for furniture and other decorative purposes. Decorative glass mirrors are handcrafted. A variety of shades and glass thickness are available. A beam of light reflects off a mirror at an angle of reflection equal to its angle of incidence; that is, if the beam of light is shining on a mirror's surface, at a θ ° angle vertically it reflects from the point of incidence at a θ ° angle, vertically in the opposite direction.
This law mathematically follows from the interference of a plane wave on a flat boundary. In a plane mirror, a parallel beam of light changes its direction as a whole, while still remaining parallel. In a concave mirror, parallel beams of light become a convergent beam, whose rays intersect in the focus of the mirror. Known as converging mirror In a convex mirror, parallel beams become divergent, with the rays appearing to diverge from a common point of intersection "behind" the mirror. Spherical concave and convex mirrors do not focus parallel rays to a single point due to spherical aberration. However, the ideal of focusing to a point is a used approximation. Parabolic reflectors resolve this. Parabolic reflectors are not suitable for imaging nearby objects because the light rays are not parallel. Objects viewed in a mirror will appear not vertically inverted. However, a mirror does not "swap" left and right any more than it swaps top and bottom. A mirror reverses the forward/backward axis. To be precise, it reverses the object in the direction perpendicular to the mirror surface.
Because left and right are defined relative to front-back and top-bottom, the "flipping" of front and back results in the perception of a left-right reversal in the image. Looking at an image of oneself with the front-back axis flipped results in the perception of an image with its left-right axis flipped; when reflected in the mirror, your right hand remains directly opposite your real right hand, but it is perceived as the left hand of your image. When a person looks into a mirror, the image is front-back reversed, an effect similar to the holl
A dichroic filter, thin-film filter, or interference filter is a accurate color filter used to selectively pass light of a small range of colors while reflecting other colors. By comparison, dichroic mirrors and dichroic reflectors tend to be characterized by the colors of light that they reflect, rather than the colors they pass. Dichroic filters can filter light from a white light source to produce light, perceived by humans to be saturated in color. Although costly, such filters are popular in theatrical applications. Dichroic reflectors are used behind a light source to reflect visible light forward while allowing the invisible infrared light to pass out of the rear of the fixture, resulting in a beam of light, cooler; such an arrangement allows a given light to increase its forward intensity while allowing the heat generated by the backward-facing part of the fixture to escape. Many quartz halogen lamps have an integrated dichroic reflector for this purpose, being designed for use in slide projectors to avoid melting the slides, but now used for interior home and commercial lighting.
This improves whiteness by removing excess red. For these applications non cool beam lamps must be used. Dichroic filters use the principle of thin-film interference, produce colors in the same way as oil films on water; when light strikes an oil film at an angle, some of the light is reflected from the top surface of the oil, some is reflected from the bottom surface where it is in contact with the water. Because the light reflecting from the bottom travels a longer path, some light wavelengths are reinforced by this delay, while others tend to be canceled, producing the colors seen. In a dichroic mirror or filter, instead of using an oil film to produce the interference, alternating layers of optical coatings with different refractive indices are built up upon a glass substrate; the interfaces between the layers of different refractive index produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are added by vacuum deposition.
By controlling the thickness and number of the layers, the frequency of the passband of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation and so do not become nearly as hot as the equivalent conventional filter. Where white light is being deliberately separated into various color bands, the similar dichroic prism is used instead. For cameras, however it is now more common to have an absorption filter array to filter individual pixels on a single CCD array. Recessed or enclosed luminaires that are unsuitable for use with dichroic reflector lights can be identified by the IEC 60598 No Cool Beam symbol. In fluorescence microscopy, dichroic filters are used as beam splitters to direct illumination of an excitation frequency toward the sample and at an analyzer to reject that same excitation frequency but pass a particular emission frequency; some LCD projectors use dichroic filters instead of prisms to split the white light from the lamp into the three colours before passing it through the three LCD units.
Older DLP projectors transmit a white light source through a color wheel which uses dichroic filters to switch colors sent through the Digital micromirror device. Newer projectors may use LED light sources to directly emit the desired light wavelengths, they are used as laser harmonic separators. They separate the various harmonic components of frequency doubled laser systems by selective spectral reflection and transmission. Dichroic filters are used to create gobos for high-power lighting products. Pictures are made by overlapping up to four colored dichroic filters. Photographic enlarger color heads use dichroic filters to adjust the color balance in the print. Much better filtering characteristics than conventional filters Ability to fabricate a filter to pass any passband frequency and block a selected amount of the stopband frequencies Because light in the stopband is reflected rather than absorbed, there is much less heating of the dichroic filter than with conventional filters Much longer life than conventional filters.
Because the wavelength of light selected by the filter varies with the angle of incidence of the light, such jewelry has an iridescent effect, changing color as the earrings swing. Another interesting application of dichroic filters is spatial filtering. With a technique licensed from Infitec, Dolby Labs uses dichroic filters for screening 3D movies; the left lens of the Dolby 3D glasses transmits specific narrow bands of red and blue frequencies, while the right lens transmits a
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is thus the inverse of the spatial frequency. Wavelength is determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings and is a characteristic of both traveling waves and standing waves, as well as other spatial wave patterns. Wavelength is designated by the Greek letter lambda; the term wavelength is sometimes applied to modulated waves, to the sinusoidal envelopes of modulated waves or waves formed by interference of several sinusoids. Assuming a sinusoidal wave moving at a fixed wave speed, wavelength is inversely proportional to frequency of the wave: waves with higher frequencies have shorter wavelengths, lower frequencies have longer wavelengths. Wavelength depends on the medium. Examples of wave-like phenomena are sound waves, water waves and periodic electrical signals in a conductor.
A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric and the magnetic field vary. Water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary. Wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in sinusoidal waves over deep water a particle near the water's surface moves in a circle of the same diameter as the wave height, unrelated to wavelength; the range of wavelengths or frequencies for wave phenomena is called a spectrum. The name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any wave pattern can be described in terms of the independent propagation of sinusoidal components; the wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, where v is called the phase speed of the wave and f is the wave's frequency.
In a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the phase speed is the speed of light, about 3×108 m/s, thus the wavelength of a 100 MHz electromagnetic wave is about: 3×108 m/s divided by 108 Hz = 3 metres. The wavelength of visible light ranges from deep red 700 nm, to violet 400 nm. For sound waves in air, the speed of sound is 343 m/s; the wavelengths of sound frequencies audible to the human ear are thus between 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light. A standing wave is an undulatory motion. A sinusoidal standing wave includes stationary points of no motion, called nodes, the wavelength is twice the distance between nodes; the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed.
For example, for an electromagnetic wave, if the box has ideal metal walls, the condition for nodes at the walls results because the metal walls cannot support a tangential electric field, forcing the wave to have zero amplitude at the wall. The stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Wavelength and wave velocity are related just as for a traveling wave. For example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. Traveling sinusoidal waves are represented mathematically in terms of their velocity v, frequency f and wavelength λ as: y = A cos = A cos where y is the value of the wave at any position x and time t, A is the amplitude of the wave, they are commonly expressed in terms of wavenumber k and angular frequency ω as: y = A cos = A cos in which wavelength and wavenumber are related to velocity and frequency as: k = 2 π λ = 2 π f v = ω
Mirrored sunglasses are sunglasses with a reflective optical coating on the outside of the lenses to make them appear like small mirrors. The lenses give the wearer's vision a brown or grey tint; the mirror coating decreases the amount of light passing through the tinted lens by a further 10–60%, making it useful for conditions of sand, water and higher altitudes. Mirrored sunglasses are one-way mirrors; the color of the mirror coating is independent of the tint of the lenses. It is determined by the structure of the layer, their popularity with police officers in the United States has earned them the nickname "cop shades". The two most popular styles for these are dual lenses set in metal frames, "Wraparound". Wraparound sunglasses are quite popular in the world of extreme sports; the simplest version of a mirror coating is a single layer of a deposited thin film of a suitable metal prepared by ion beam deposition, sputter deposition or vapor deposition. However, this kind of coating is prone to scratching, degrades in a corrosive environment like salt water.
More modern reflective coatings have several alternating layers of specific thickness, made of dielectric materials and sometimes metals. The metal layer can be made from titanium, nickel or chromium, or from an alloy like Nichrome or Inconel, has thickness ranging from 0.5 to 9 nanometers. The dielectric layer comprises a suitable oxide, e.g. chromium oxide, silicon dioxide, or titanium dioxide. The manufacturing process is similar to making anti-reflective coating, mirror and antireflective coatings can be deposited in the same sequence of operations. Photochromic lens