In optics, an ultrashort pulse of light is an electromagnetic pulse whose time duration is of the order of a picosecond (10⁻¹² second) or less. Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators. They are commonly referred to as ultrafast events. Amplification of ultrashort pulses almost always requires the technique of chirped pulse amplification, in order to avoid damage to the gain medium of the amplifier.

They are characterized by a high peak intensity (or more correctly, irradiance) that usually leads to nonlinear interactions in various materials, including air. These processes are studied in the field of nonlinear optics.

In the specialized literature, "ultrashort" refers to the femtosecond (fs) and picosecond (ps) range, although such pulses no longer hold the record for the shortest pulses artificially generated. Indeed, x-ray pulses with durations on the attosecond time scale have been reported.

The 1999 Nobel Prize in Chemistry was awarded to Ahmed H. Zewail, for the use of ultrashort pulses to observe chemical reactions at the timescales on which they occur, opening up the field of femtochemistry.

The aspect ratio of a geometric shape is the ratio of its sizes in different dimensions. For example, the aspect ratio of a rectangle is the ratio of its longer side to its shorter side – the ratio of width to height, when the rectangle is oriented as a "landscape".

The aspect ratio is most often expressed as two integer numbers separated by a colon (x:y), less commonly as a simple or decimal fraction. The values x and y do not represent actual widths and heights but, rather, the proportion between width and height. As an

 example, 8:5, 16:10, 1.6:1, 8⁄5 and 1.6 are all ways of representing the same aspect ratio.

In objects of more than two dimensions, such as hyperrectangles, the aspect ratio can still be defined as the ratio of the longest side to the shortest side.

Surface roughness, often shortened to roughness, is a component of surface texture. It is quantified by the deviations in the direction of the normal vector of a real surface from its ideal form. If these deviations are large, the surface is rough; if they are small, the surface is smooth. In surface metrology, roughness is typically considered to be the high-frequency, short-wavelength component of a measured surface. However, in practice it is often necessary to know both the amplitude and frequency to ensure that a surface is fit for a purpose.

Roughness plays an important role in determining how a real object will interact with its environment. In tribology, rough surfaces usually wear more quickly and have higher friction coefficients than smooth surfaces. Roughness is often a good predictor of the performance of a mechanical component, since irregularities on the surface may form nucleation sites for cracks or corrosion. On the other hand, roughness may promote adhesion.

Laser engraving is the practice of using lasers to engrave an object. Laser marking, on the other hand, is a broader category of methods to leave marks on an object, which also includes color change due to chemical/molecular alteration, charring, foaming, melting, ablation, and more. The technique does not involve the use of inks, nor does it involve tool bits which contact the engraving surface and wear out, giving it an advantage over alternative engraving or marking technologies where inks or bit heads have to be replaced regularly.

The impact of laser marking has been more pronounced for specially designed "laserable" materials and also for some paints. These include laser-sensitive polymers and novel metal alloys.

The term laser marking is also used as a generic term covering a broad spectrum of surfacing techniques including printing, hot-branding and laser bonding. The machines for laser engraving and laser marking are the same, so that the two terms are sometimes confused by those without knowledge or experience in the practice.

An optical waveguide is a physical structure that guides electromagnetic waves in the optical spectrum. Common types of optical waveguides include optical fiber and transparent dielectric waveguides made of plastic and glass.

Optical waveguides are used as components in integrated optical circuits or as the transmission medium in local and long haul optical communication systems.

Optical waveguides can be classified according to their geometry (planar, strip, or fiber waveguides), mode structure (single-mode, multi-mode), refractive index distribution (step or gradient index) and material (glass, polymer, semiconductor).

Multiphoton lithography (also known as direct laser lithography or direct laser writing) of polymer templates has been known for years^[timeframe?] 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. The fundamental difference is, however, the avoidance of reticles. Instead, two-photon absorption is utilized to induce a dramatic change in the solubility of the resist for appropriate developers.

Hence, multiphoton lithography is a technique for creating small features in a photosensitive material, without the use of complex optical systems or photomasks. This method relies on a multi-photon absorption process in a material that is transparent at the wavelength of the laser used for creating the pattern. By scanning and properly modulating the laser, a chemical change (usually polymerization) occurs at the focal spot of the laser and can be controlled to create an arbitrary three-dimensional periodic or non-periodic pattern. This method has been used for rapid prototyping of structures with fine features.

Polydimethylsiloxane (PDMS), also known as dimethylpolysiloxane or dimethicone, belongs to a group of polymeric organosilicon compounds that are commonly referred to as silicones.PDMS is the most widely used silicon-based organic polymer, as its versatility and properties lead to many applications.

It is particularly known for its unusual rheological (or flow) properties. PDMS is optically clear and, in general, inert, non-toxic, and non-flammable. It is one of several types of silicone oil (polymerized siloxane). Its applications range from contact lenses and medical devices to elastomers; it is also present in shampoos (as it makes hair shiny and slippery), food (antifoaming agent), caulking, lubricants and heat-resistant tiles.

A microplate or microtiter plate (Microtiter is a registered trademark in the United States therefore should not be used generically, attribution must be given), microwell plate, multiwell, is a flat plate with multiple "wells" used as small test tubes. The microplate has become a standard tool in analytical research and clinical diagnostic testing laboratories. A very common usage is in the enzyme-linked immunosorbent assay (ELISA), the basis of most modern medical diagnostic testing in humans and animals.

A microplate typically has 6, 12, 24, 48, 96, 384 or 1536 sample wells arranged in a 2:3 rectangular matrix. Some microplates have been manufactured with 3456 or 9600 wells, and an "array tape" product has been developed that provides a continuous strip of microplates embossed on a flexible plastic tape.

Tempered or toughened glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the interior into tension. Such stresses cause the glass, when broken, to shatter into small granular chunks instead of splintering into jagged shards as ordinary annealed glass does. The granular chunks are less likely to cause injury.

Tempered glass is used for its safety and strength in a variety of applications, including passenger vehicle windows, shower doors, aquariums, architectural glass doors and tables, refrigerator trays, mobile phone screen protectors, bulletproof glass components, diving masks, and plates and cookware.

• 3.3Applications
  • 3.3.1Windows
  • 3.3.2As substrate for semiconducting circuits
  • 3.3.3In lasers
  • 3.3.4In endoprostheses

A foil is a very thin sheet of leaf, usually made by hammering or rolling. Foils are most easily made with malleable leaves, such as aluminium, copper, tin, and gold. Foils usually bend under their own weight and can be torn easily. For example, aluminium foil is usually about 1/1000 inch (0.03 mm), whereas gold (more malleable than aluminium) can be made into foil only a few atoms thick, called gold leaf. Extremely thin foil is called metal leaf. Leaf tears very easily and must be picked up with special brushes.

Foil is commonly used in household applications. It is also useful in survival situations, in the form of a "space blanket", where the reflective surface reduces the degree of hypothermia caused by thermal radiation.

Engineering plastics are more robust and are used to make products such as vehicle parts, building and construction materials, and some machine parts. In some cases they are polymer blends formed by mixing different plastics together (ABS, HIPS etc). Engineering plastics can replace metals in vehicles, reducing their weight, with a 10% reduction improving fuel efficiency by 6-8%. Roughly 50% of the volume of modern cars is made of plastic but this only accounts for 12-17% of the vehicle weight.

High-performance plastics usually expensive, with their use limited to specialised applications which make use of their superior properties.

• Aramids: a class of heat-resistant and strong synthetic fibers used in aerospace and military applications, includes Kevlar and Nomex, and Twaron.
• Polyetheretherketone (PEEK): strong, chemical- and heat-resistant thermoplastic; its biocompatibility allows for use in medical implant applications and aerospace moldings. It is one of the most expensive commercial polymers.
• Polyetherimide (PEI) (Ultem): a high-temperature, chemically stable polymer that does not crystallize
• Polyimide: a high-temperature plastic used in materials such as Kapton tape
• Polysulfone: high-temperature melt-processable resin used in membranes, filtration media, water heater dip tubes and other high-temperature applications
• Polytetrafluoroethylene (PTFE), or Teflon: heat-resistant, low-friction coatings used in non-stick surfaces for frying pans, plumber's tape and water slides
• Polyamide-imide (PAI): High-performance engineering plastic extensively used in high performance gears, switches, transmission and other automotive components, and aerospace parts

An interposer is an electrical interface routing between one socket or connection to another. The purpose of an interposer is to spread a connection to a wider pitch or to reroute a connection to a different connection.

Interposer comes from the Latin word "interpōnere", meaning "to put between". They are often used in BGA packages, multi-chip modules and High Bandwidth Memory.

A common example of an interposer is an integrated circuit die to BGA, such as in the Pentium II. This is done through various substrates, both rigid and flexible, most commonly FR4 for rigid, and polyimide for flexible. Silicon and glass are also evaluated as an integration method.Interposer stacks are also a widely accepted, cost-effective alternative to 3D ICs. There are already several products with interposer technology in the market, notably the AMD Fiji/Fury GPU,and the Xilinx Virtex-7 FPGA. In 2016, CEA Leti demonstrated their second generation 3D-NoC technology which combines small dies ("chiplets"), fabricated at the FDSOI 28 nm node, on a 65 nm CMOS interposer.

A probe card is an interface between an electronic test system and a semiconductor wafer. Typically the probe card is mechanically docked to a prober and electrically connected to a tester. Its purpose is to provide an electrical path between the test system and the circuits on the wafer, thereby permitting the testing and validation of the circuits at the wafer level, usually before they are diced and packaged. It consists, normally, of a printed circuit board (PCB) and some form of contact elements, usually metallic, but possibly of other materials as well.

A semiconductor manufacturer will typically require a new probe card for each new device wafer and for device shrinks (when the manufacturer reduces the size of the device while keeping its functionality) because the probe card is effectively a custom connector that takes the universal pattern of a given tester and translates the signals to connect to electrical pads on the wafer. For testing of DRAM and FLASH memory devices these pads are typically made of aluminum and are 40–90 um per side. Other devices may have flat pads, or raised bumps or pillars made of copper, copper alloys or many types of solders such as lead-tin, tin-silver and others.

Fan-out wafer-level packaging (also known as wafer-level fan-out packaging, fan-out WLP, FOWL packaging, FO-WLP, FOWLP, etc.) is an integrated circuit packaging technology, and an enhancement of standard wafer-level packaging (WLP) solutions.

In conventional technologies, a wafer is diced first, and then individual dies are packaged; package size is usually considerably larger than the die size. By contrast, in standard WLP flows integrated circuits are packaged while still part of the wafer, and the wafer (with outer layers of packaging already attached) is diced afterwards; the resulting package is practically of the same size as the die itself. However, the advantage of having a small package comes with a downside of limiting the number of external contacts that can be accommodated in the limited package footprint; this may become a significant limitation when complex semiconductor devices requiring a large number of contacts are considered.

Fan-out WLP was developed to relax that limitation. It provides a smaller package footprint along with improved thermal and electrical performance compared to conventional packages, and allows having higher number of contacts without increasing the die size.

In contrast to standard WLP flows, in fan-out WLP the wafer is diced first. But then the dies are very precisely re-positioned on a carrier wafer or panel, with space for fan-out kept around each die. The carrier is then reconstituted by molding, followed by making redistribution layer atop the entire molded area (both atop the chip and atop the adjacent fan-out area), and then forming solder balls on top.

Applications : vortex coronagraph , optical tweezers spectral efficiency, orbital angular momentum based multiplexing, Quantum computing, cryptography, Stimulated Emission Depletion (STED) Microscopy, polariton fluids, quantum vortices

Optical tweezers (originally called single-beam gradient force trap) are scientific instruments that use a highly focused laser beam to hold and move microscopic and sub-microscopic objects like atoms, nanoparticles and droplets, in a manner similar to tweezers. If the object is held in air or vacuum without additional support, it can be called optical levitation.

The laser light provides an attractive or repulsive force (typically on the order of piconewtons), depending on the relative refractive index between particle and surrounding medium. Levitation is possible if the force of the light counters the force of gravity. The trapped particles are usually micron-sized, or smaller. Dielectric and absorbing particles can be trapped, too.

Optical tweezers are used in biology and medicine (for example to grab and hold a single bacterium or cell like a sperm cell, blood cell or DNA), nanoengineering and nanochemistry (to study and build materials from single molecules), quantum optics and quantum optomechanics (to study the interaction of single particles with light). The development of optical tweezing by Arthur Ashkin was lauded with the 2018 Nobel Prize in Physics.

A laser diode (LD, also injection laser diode or ILD, or diode laser) is a semiconductor device similar to a light-emitting diode in which a diode pumped directly with electrical current can create lasing conditions at the diode's junction

Driven by voltage, the doped p–n-transition allows for recombination of an electron with a hole. Due to the drop of the electron from a higher energy level to a lower one, radiation, in the form of an emitted photon is generated. This is spontaneous emission. Stimulated emission can be produced when the process is continued and further generates light with the same phase, coherence and wavelength.

The choice of the semiconductor material determines the wavelength of the emitted beam, which in today's laser diodes range from infra-red to the UV spectrum. Laser diodes are the most common type of lasers produced, with a wide range of uses that include fiber optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray disc reading/recording, laser printing, laser scanning and light beam illumination. With the use of a phosphor like that found on white LEDs, Laser diodes can be used for general illumination.

An axicon is a specialized type of lens which has a conical surface. An axicon transforms a laser beam into a ring shaped distribution. They can be convex or concave and be made of any optical material. The combination with other axicons or lenses allows a wide variety of beam patterns to be generated. It can be used to turn a Gaussian beam into a non-diffractive Bessel-like beam. Axicons were first proposed in 1954 by John McLeod.

Axicons are used in atomic traps and for generating plasma in wakefield accelerators. They are used in eye surgery in cases where a ring-shaped spot is useful.

The Axicon is usually characterized by the ratio of the diameter of the ring to the distance from the lens tip to image plane d/l.

A depolarizer or depolariser is an optical device used to scramble the polarization of light. An ideal depolarizer would output randomly polarized light whatever its input, but all practical depolarizers produce pseudo-random output polarization.

Optical systems are often sensitive to the polarization of light reaching them (for example grating-based spectrometers). Unwanted polarization of the input to such a system may cause errors in the system's output.

The active laser medium (also called gain medium or lasing medium) is the source of optical gain within a laser. The gain results from the stimulated emission of photons through electronic or molecular transitions to a lower energy state from a higher energy state previously populated by a pump source.

Examples of active laser media include:

• Certain crystals, typically doped with rare-earth ions (e.g. neodymium, ytterbium, or erbium) or transition metal ions (titanium or chromium); most often yttrium aluminium garnet (Y₃Al₅O₁₂), yttrium orthovanadate (YVO₄), or sapphire (Al₂O₃);^[1] and not often Caesium cadmium bromide (CsCdBr₃)
• Glasses, e.g. silicate or phosphate glasses, doped with laser-active ions;
• Gases, e.g. mixtures of helium and neon (HeNe), nitrogen, argon, carbon monoxide, carbon dioxide, or metal vapors;
• Semiconductors, e.g. gallium arsenide (GaAs), indium gallium arsenide (InGaAs), or gallium nitride (GaN).
• Liquids, in the form of dye solutions as used in dye lasers