A technique for recording, and later reconstructing, the amplitude and phase distributions of a coherent wave disturbance. Invented by Dennis Gabor in 1948, the process was originally envisioned as a possible method for improving the resolution of electron microscopes. While this original application has not proved feasible, the technique is widely used as a method for optical image formation, and in addition has been successfully used with acoustical and radio waves. This article discusses holography with electromagnetic waves in the optical and microwave regions of the electromagnetic spectrum, and its potential use with x-rays. For holography with sound waves. See also: Acoustical holography
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Article
Laser
A device that uses the principle of amplification of electromagnetic waves by stimulated emission of radiation, generating a coherent beam of light in the infrared, visible, or ultraviolet portions of the electromagnetic spectrum. The term "laser" is an acronym for "light amplification by stimulated emission of radiation," or a light amplifier. Light amplification is achieved in the so-called gain medium, which is a solid, liquid, or gaseous material that possesses suitable properties such that when the material is externally excited, the probability of stimulated radiation emission in it exceeds the probability of radiation absorption. The vast majority of laser devices (Fig. 1) are amplifiers made into oscillators by feeding appropriately phased output back into the light input. In practice, the unmodified word “laser” has come to mean a device that actually is an oscillator, while the modifier “amplifier” as in “laser amplifier” (technically redundant) or in “optical amplifier” is generally used when referring to the gain medium operating without feedback. See also: Amplifier; Light; Maser; Oscillator; Radiation
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Laser spectroscopy
Spectroscopy with laser light or, more generally, studies of the interaction between laser radiation and matter. Lasers have led to a revolution of classical spectroscopy, because laser light can far surpass the light from other sources in brightness, spectral purity, and directionality; and if required, laser light can be produced in extremely intense and short pulses, far enhancing the possibility of observing nonlinear phenomena. The use of lasers can greatly increase the resolution and sensitivity of conventional spectroscopic techniques, such as absorption spectroscopy, fluorescence spectroscopy, or Raman spectroscopy. Moreover, interesting phenomena have become observable in the resonant interaction of intense coherent laser light with matter. Some of these effects have become the basis for powerful spectroscopic methods, which offer unprecedented spectral resolution, or which permit the investigation of properties of matter that could not be observed previously. Laser spectroscopy has become a wide and diverse field, with applications in numerous areas of physics, chemistry, and biology.