Selected terms

Absorption spectroscopy

Measuring the absorption of radiation or the absorption spectrum of a sample is a widely used spectroscopy technique.Light of a specific wavelength interacts with a sample and the amount of radiation absorbed is measured, giving information about the sample such as concentration, layer thickness etc.

ATR crystals

are used for ATR spectroscopy  (ATR= attenuated total reflection). This is a sampling technique used in conjunction with infrared spectroscopy which enables opaque samples to be examined directly in the solid, liquid or gas state without further preparation e.g. coatings, polymer foils and liquid samples, e.g. solvent mixtures. The method was first presented by Harrick in 1960 and Fahrenfort in 1961. Measuring the intensity of the reflected light allows conclusions to be drawn about the sample. The core element of this method is the internal reflection element (IRE) which uses a property of total internal reflection resulting in an evanescent wave. The IRE is usually a special ATR crystal, which allows for multiple refractions.

Beam splitter

Beam splitter are optical components that separate a single light beam into two partial beams or different wavelengths, whereby a certain part is reflected and another part is transmitted. This can be achieved by suitable coatings (half mirror, dichroic beam splitter, polka dot structures), or by using prevented total internal reflection (beam splitter cube). There are polarizing beam splitters (pole cubes) that either transmit or reflect differently polarized light.

Birefringence

In optics, birefringence is the property of non-cubic (optically anisotropic) materials to split a light beam into two mutually polarized sub-beams. The cause of this effect lies in the different refractive indices (no and ne) depending on the propagation direction and polarization of the light. A prominent example of such a material is calcite (limestone, also double spar), in which the double refraction was discovered by Erasmus Bartholin in 1669. Isotropic materials can also become birefringent as a result of external influences such as mechanical stress (deformation or stress birefringence), electrical fields (electrical birefringence, Kerr effect) or magnetic fields (magnetic birefringence, Cotton-Mouton effect, generally see magneto-optics).

Bridgman-Stockbarger

(named after Percy Williams Bridgman and Donald C. Stockbarger) is a technique used for the production of large single crystals, like fluorides, silicon or gallium arsenide. A crucible filled with raw material powder is inserted into a furnace with different temperature zones. The raw material melts in the hotter part of the furnace. Crystallisation begins when the molten raw material is transferred into a cooler section of the oven. The seed crystals which are oriented along the axis of the crucible grow the fastest. To accelerate this process we use specially shaped crucibles.

Centring Error

The centring error is a measure for the deviation of the optical axis of a lens from its geometrical axis. Often the axis of the lateral surface is regarded as the reference axis of a lens because the so-called optical axis is only a virtual concept. If the centres of curvature of the lens faces are on the reference axis then the lens is centred. The permitted centring error is given in angle minutes. See also DIN ISO 10110-6.

Cherenkov Counter

or Cerenkov detectors (after Pavel A. Cherenkov born 1904 in Voronez).The maximum velocity attainable is the speed of light in a vacuum (c = 300 000 km/s). When travelling through a material such as glass, the speed of light is less (v = c/index of refraction). A very fast particle passing though a material at a velocity greater than that at which light can travel through that material emits light. This process is quite similar to the production of a sonic boom by airplanes exceeding the speed of sound. The light radiation is emitted in a cone whose angle is directly related to the particle’s velocity and the refraction index of the material. Generally, the Cherenkov counter consists of a radiator (the medium in which the light is generated, e.g. gases, NaF2 or aerogel), optical elements to collect and direct the light and photo detectors. Cherenkov counters allow the discrimination between lighter and heavier particles and the determination of their energy. They are used in nuclear-, high-energy and astroparticle physics.

Coating

Coating in optics is used to create optical layers, usually multiple layers, to adjust the optical properties (reflection, transmission, absorption) of a surface. Their function is based on the interference effect and/or the intrinsic propert.

Filters

selectively transmit light having certain properties (wavelengths, polarization, or (often incidentally) by angle of incidence) while blocking the remainder. There are adjustable filters (e.g. for adjustable effects), edge filters (Long Wave Pass or Short Wave Pass which transmit longer and shorter wavelengths respectively while blocking or absorbing the remaining wavelengths), polarizing filters (made from anisotropic foils or with dielectric reflecting surfaces) or interference filters (based on the optical interference of optical coatings).

Ion selective Electrodes

also known as a specific ion electrodes (SIE), are transducers (or sensors) that convert the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter. The sensing part of the electrode is usually made as an ion-specific membrane. A distinction is made between homogeneous and heterogeneous solid membranes. Homogeneous solid membranes are crystal plates (e.g. LaF3 for the detection of fluoride ions) or homogeneous mouldings (e.g. AgCl for the detection of chloride ions and Ag_2S for silver or sulphide ions). There are also liquid membranes (organic solvents), polymer – gel membranes and glass membranes.

IR Range

Electro-magnetic waves in the range between visible light and longer microwaves are called infrared radiation (IR for short). They cover the range between 780 nm and 1 mm, and include Terahertz radiation. Like visible light, humans can perceive Infrared waves, but they are felt as heat. IR is often further sub-divided into NIR (near infrared: 750-3000 nm), MIR (mid infrared: 3-50 µm) and FIR (far infrared: 50-1000 µm, which includes the Terahertz range of 100-1000 µm).

Lenses

are optical components with two refractive faces, of which at least one is convex or concave. The most important property of a lens or array of lenses is its imaging properties. The most crucial aspect of a lens is its focal length, i.e. the measure of how strongly the system converges (focuses) or diverges (defocuses) light. The simplest lenses are spherical lenses: their two surfaces are parts of the surfaces of spheres, with the lens axis ideally perpendicular to both surfaces. Each of the surfaces can be convex (bulging outwards from the lens) or concave (depressed into the lens). As such, the surfaces can be assigned radii of curvature. A distinction is made between converging lenses (two convex faces or one convex and one planar face; ideally, a beam of light running parallel to the optical axis is collected in one point, the focus) or diverging lenses (two concave faces or one concave and one planar face; a beam of light after passing through the lens appears to be emanating from a particular point on the axis in front of the lens.

Nacken-Kyropoulos

(named after Richard Nacken and Spyro Kyropoulos) is a technique for the production of single crystals, like alkali halides or sapphires.

A cooled rod is inserted into the melt for the crystal to grow on. At the beginning a polycrystalline region is formed at the rod but by reduction of the diameter of the grown material during pulling, single crystalline growth is afforded. Nowadays single crystal seeds are used.

Objectives

are optical elements that gather light from the object being observed and focus it to produce a real image. They are used in microscopes, telescopes, cameras, binoculars and slide projectors and many other optical instruments.

Orientation

Crystal orientation is the classification of the inclination of the crystal surface in relation to its crystal lattice. Crystal orientation is usually denoted by the Miller indices, e.g. (111) or (110) (and directions parallel or perpendicular to it). The Miller index can only be used for single crystals, i.e. not just a solid but a homogenous uniformly oriented solid. Orientated crystals are used in the semiconductor industry or in fundamental physics. Specific crystal orientation can be achieved by cleavage (see material properties) or by directional cutting.

Prisms

are optical elements that refract light or break it up into its constituent spectral colours by applying the principles of total internal reflection and wavelength dependant refraction. The traditional geometrical shape is that of a triangular prism with a triangular base and rectangular sides, although not all prisms are this shape, depending on their application (e.g. pentaprism). 

Scintillation detectors

are instruments which measure the energy and intensity of ionizing radiation. Scintillation detectors (also called scintillation counters) are one of the oldest methods for the detection of radioactive or X-ray radiation. The detector houses a scintillator in such a way as to shield it from external light sources (and humidity, e.g. for hygroscopic sodium iodide). The scintillator is coupled to an electronic light sensor such as a photomultiplier tube (PMT) or a photodiode. When excited by ionizing radiation the scintillator reemits some of the absorbed energy in the form of light. This very weak flash of light is absorbed by the PMT which reemits it in the form of electrons via the photoelectric effect. This results in an electrical pulse which can be analysed.  Operational areas include nuclear medicine (Positron Emission Tomography: PET), particle physics, electron microscopes and X-ray scanners in security scanners.

Spectroscopy

Spectroscopy is a collective term for methods that examine the energy spectrum of a sample by breaking down radiation according to its energy. Spectroscopes were used for the visual observation of optical spectra, recording devices are called spectrometers. The latter also work in other areas of the electromagnetic spectrum and with particles such as electrons or ions. The sample can be excited with one type of radiation and then another emission of the sample can be examined.

The investigation of the light emission and absorption of molecules and atoms using grating and prism spectrometers are the oldest spectroscopic methods. They are therefore also referred to as classical spectroscopy. Molecular spectroscopy examines the interaction of molecules with electromagnetic fields. This enables the characterization of molecular properties such as bond lengths and strengths as well as the identification of the atomic components. The observed molecular spectra differ from the atomic spectra by many more, mostly overlapping lines (“bands”). The reason for this is that the molecules can absorb or emit energy not only through electron transfers, but also through vibrations of the atoms against each other and rotations of the molecule around one of its axes.

Stress induced birefringence

Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials. Isotropic solids do not exhibit birefringence unless under mechanical stress, either due to stress applied externally or as part of the manufacturing process. Mechanical stress affects the optical properties of certain materials which is undesirable in many applications (e.g. lenses for polarizing optics).  The ability to understand and quantify residual stress and its effect on optical components is therefore essential in controlling the parameters of crystal growth and optimising quality.

Surface Roughness

Roughness is a term used in surface physics and describes the unevenness of a surface. There are different calculation methods for the quantitative characterization of the roughness considering each of the different peculiarities of the surface. The surface roughness can be influenced by polishing, grinding, lapping, etching or evaporation. There are three measures of roughness using µm as unit: the mean roughness R_a (average distance between a measurement point on the surface to the centreline), the so called root-mean-squared roughness (rms = root mean square average of the profile height deviations from the mean line of the surface) and the averaged roughness height (10-point average roughness) R_max (see e.g. DIN ISO 4287)

Transmission spectroscopy

One of the most commonly used methods in spectroscopy is to measure the transmittance or transmittance spectrum of a sample. For this purpose, a sample is irradiated with light of specific wavelengths and the proportion of the measuring radiation that has penetrated the sample is measured. There are characteristic interactions between light and sample and information about the concentration, layer thickness, bond states or specific molecule groups is obtained.

UHV-Viewports

are hermetically sealed optical components which are typically used for visual or broad band energy transmission into and out of UHV (ultrahigh vacuum) systems. There are various specifications regarding leak rates, transmission performance and coatings – some coatings can be removed by heating the UHV-Viewport to a specified temperature without the need to open the vacuum chamber.

UV Range

Ultraviolet or UV radiation, (also called ultraviolet light or UV light), is not visible to the human eye. The term ultraviolet (i.e. „beyond the violet“) originates from the fact that the UV spectrum, which covers the wavelength range from 15 nm to 400 nm, begins with waves somewhat shorter than those that humans are just able to perceive as the colour blue-violet.

VIS Range

Only light in the wavelength range from ~ 400 nm (violet) to 800 nm (red) is visible to the human eye. This is called the visible wavelength range (VIS=visible).

VUV Range

The higher energies of the ultraviolet spectrum are called vacuum UV (VUV), because under normal environmental conditions they are absorbed by air.  Any experiments involving these waves must therefore be carried out in a vacuum.There are no clearly defined boundaries between the VUV range and short-wave UV radiation and X-rays. Historically, the VUV range is defined as the wavelength range from 100 to 200 nm.

Wavefront distortion

Where a homogenous light beam consisting of many light waves passes through an inhomogeneous material with a varying refractive index n (density variations in the material) or of non-uniform thickness, the individual light waves will be distorted proportionally to the distance travelled through the material, resulting in a deterioration of the optical image. This can be measured by analyzing the interference patterns in a two-arm or phase-shifting interferometer, where one “arm” contains the material to be measured and the other a reference sample. 

Wavelength

The range of electro-magnetic waves covers a wide spectrum of wavelengths from a trillionth of a meter in length up to waves which are some 1000 meters long (you can visualize the wavelength as the distance between two neighbouring wave crests or wave troughs).

These waves (or energy radiations) are generated by the movement of atomic particles and their electrical charges (which in turn bring magnetic forces into play). In a vacuum, electro-magnetic waves travel at the speed of light (30000000 m/s). The relationship between frequency and wavelength is determined by the following formula: Wavelength =speed of light / frequency

Windows

are optical components that only allow the passage of waves within a specific range.  They come in a range of shapes and specifications (e.g. parallelism, surface quality, dimensional tolerances), depending on the type of application.

X-ray-monochromators

By its nature, electromagnetic radiation is made up of different wavelengths (polychromatic – poly= many) unless manipulated. Using a monochromator (Greek: mono = one + chroma = colour) the desired wavelengths can be isolated by absorption or reflection of the unwanted wavelengths.  X-ray monochromators are devices made of crystal (often curved for precollimation and focusing) which can select a defined wavelength of radiation. It operates through the diffraction process according to Bragg's law. The radiation is reflected not only on the crystalline surface but also on the crystal lattice as the X-rays penetrate the crystal.  A beam reflected by the crystal surface covers a shorter distance than a beam reflected from within the crystal.  This optical path difference causes destructive interference between most the waves.  Only those wavelengths which at the specified angle satisfy the Bragg conditions experience constructive interference.