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Topic: Stimulated emission

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	Stimulated emission - Wikipedia, the free encyclopedia
		Stimulated emission is really a quantum mechanical phenomenon but it can be understood in terms of a classical field and a quantum mechanical atom.
		Stimulated emission can be modelled mathematically by considering an atom which may be in two electronic energy states, the ground state (1) and the excited state (2), with energies E
		The critical detail of stimulated emission is that the emitted photon is identical to the stimulating photon in that it has the same frequency, phase and polarisation (thus the two photons are totally coherent).
	en.wikipedia.org /wiki/Stimulated_emission (718 words)

	Coherence in Stimulated Emission
		The purpose of this paper is to investigate the identity or near identity of the stimulated emission with the stimulating radiation.
		Almost all of the analysis of stimulated emission in the physical science literature assumes that the stimulated emission is identical to the stimulating radiation in terms of direction and frequency.
		On the other hand, time reversal of stimulated emission when the momentum transfer is opposite to the case of absorption involves two coherent photon impinging upon an atom and resulting in the atom moving to an excited state and one of the photons continuing in the direction it was moving before the event.
	www.sjsu.edu /faculty/watkins/stimem.htm (2731 words)

	Olympus FluoView Resource Center: Introduction to Lasers
		The first stimulated emission was achieved in the microwave region of the spectrum, but now lasers are available that emit ultraviolet and infrared light, and progress is being made toward producing lasers for the x-ray spectral region.
		The primary problem in achieving stimulated laser emission is that, under normal conditions of thermodynamic equilibrium, the population, or number of atoms or molecules at each energy level, is not favorable to stimulated emission.
		Stimulated emission results from the fact that more atoms are available in the upper excited (metastable) state than in the lower state where absorption of light would most likely occur.
	www.olympusfluoview.com /theory/laserintro.html (6678 words)

	Laser - Wikipedia, the free encyclopedia
		A LASER (Light Amplification by Stimulated Emission of Radiation) is an optical source that emits photons in a coherent beam.
		The gain medium is a material of controlled purity, size, and shape, which uses a quantum mechanical effect called stimulated emission (discovered by Einstein while researching the photoelectric effect) to amplify the beam.
		The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization.
	en.wikipedia.org /wiki/Laser (3901 words)

	MSN Encarta - Laser
		They are then “stimulated” by external photons to emit the stored energy in the form of photons, a process known as stimulated emission.
		Light amplification is achieved as the photons move back and forth between two parallel mirrors, triggering further stimulated emissions.
		Stimulated emission, the underlying process for laser action, was first described theoretically by Albert Einstein in 1917.
	uk.encarta.msn.com /encyclopedia_761578658/laser.html (474 words)

	Stimulated Emission Pumping (Site not responding. Last check: 2007-11-06)
		From this excited state, the atom either decays spontaneously back to the ground state, or the transition to the ground state is stimulated by the interaction with a second laser beam.
		The stimulation of the emission predetermines frequency and direction of the photon.
		However, due to the large number of photons in the beam, it is impossible to distinguish the few additional photons emerging from stimulated emission from the background.
	www.mpq.mpg.de /qdynamics/projects/pistol/stimulated.html (137 words)

	Howzit Work? Laser Light (Site not responding. Last check: 2007-11-06)
		Amplification by stimulated emission, or oscillation, are ways of extracting more of this energy.
		Stimulated Emission: Inside a laser, there is a high concentration of excited atoms.
		This collision stimulates the emission of a second photon with the same energy as the first.
	fathom.org /Stephen/hdtw/laser.html (482 words)

	The Helium-Neon Laser
		The new photon produced in stimulated emission has the same wavelength and polarization, and is emitted in the same direction, as the stimulating photon.
		It is sometimes useful for purposes of analogy to think of the stimulated emission process as a "cloning" process for photons, as depicted in Fig.
		The stimulated emission process should be contrasted with spontaneous emission processes that, because they are not caused by any preceding event, produce photons that are emitted isotropically, with random polarization, and over a broader range of wavelengths.
	cat.middlebury.edu /~PHManual/heliumneon.html (2196 words)

	Quantum properties of light
		The treatment here is limited to a review of the characteristics of absorption, emission, and stimulated emission which are essential to an understanding of lasers and their applications.
		The stimulated emission of light is the crucial quantum process necessary for the operation of a laser.
		A population inversion cannot be achieved with just two levels because the probabability for absorption and for spontaneous emission is exactly the same, as shown by Einstein and expressed in the Einstein A and B coefficients.
	hyperphysics.phy-astr.gsu.edu /hbase/optmod/qualig.html#c2 (838 words)

	MSN Encarta - Laser
		An excited atom can then be “stimulated” by a photon of exactly the same color (or, equivalently, the same wavelength) as the photon this atom is about to emit spontaneously.
		If the photon approaches closely enough, the photon can stimulate the excited atom to immediately emit light that has the same wavelength and is in step with the photon that interacted with it.
		This stimulated emission is the key to laser operation.
	encarta.msn.com /encyclopedia_761578658/Laser.html (894 words)

	Lasers
		A LASER (Light Amplification by Stimulated Emission of Radiation) is an electromagnetic oscillator which combines light amplification with feedback.
		In the process of stimulated emission, a photon of energy hf perturbs an excited atom or molecule and causes it to relax to a lower level, emitting a photon of the same frequency, phase, and polarization as the perturbing photon.
		Stimulated emission is the basis for photon amplification and it is the fundamental mechanism underlying all laser action.
	electron9.phys.utk.edu /optics421/modules/m7/lasers.htm (1911 words)

	Pulsed_OH_maser_emission
		In this case, the broadband pulsar spectrum exhibits excess line emission at 1720 MHz as the pulsar's photons stimulate the creation of additional photons in an intervening OH cloud.
		It has long been assumed that stimulated emission plays an important role in astrophysical OH line radiation because it can be shown that the emitting clouds are not in local thermodynamic equilibrium.
		Since this stimulated emission is driven by the pulsar pulse, it varies on a few millisecond timescale, which is orders of magnitude shorter than the quickest maser variations previously detected.
	www.atnf.csiro.au /news/newsletter/feb05/page1.html (1315 words)

	He-Ne Laser Theory
		Stimulated emission occurs when an photon passes in close proximity to an atom excited to the energy of that photon (E = hf).
		Obviously, each time an excited atom is stimulated to emit a photon, the population of excited atoms decreases by one.
		Once we achieve a population inversion and experience stimulated emissions, there must be some mechanism to increase the gain of the laser output.
	www.phy.davidson.edu /StuHome/jimn/He-Ne/Pages/Theory.htm (1246 words)

	Olympus Microscopy Resource Center: Physics of Light and Color - Stimulated Emission in a Laser Cavity: Interactive ...
		The amplification of light by stimulated emission is a fundamental concept in the basic understanding of laser action.
		Figure 1(a) shows the beginning of stimulated emission, which is amplified in Figure 1(b) through Figure 1(g) as the light is reflected from the mirrors positioned at the cavity ends.
		Finally, at the equilibrium state (Figure 1(h)), the cavity is saturated with stimulated emission.
	www.olympusmicro.com /primer/java/lasers/heliumneonlaser/index.html (1075 words)

	Olympus Microscopy Resource Center: Physics of Light and Color - Spontaneous and Stimulated Processes: Interactive Java ...
		In addition, transitions from one energy level to another must be possible in order for light emission to occur, and these transitions include both spontaneous and stimulated emission.
		Once an electron resides in the upper level, it can either spontaneously emit a light wave and drop to the lower level (spontaneous emission) or it can be stimulated by a light wave to emit a second wave and return to the lower level (stimulated emission).
		The emitted light could easily stimulate emission from another exited atom, but so few are available that the emission more likely will first encounter an atom in the ground state, and will be absorbed instead.
	www.olympusmicro.com /primer/java/lasers/electroncycle (912 words)

	First observations of stimulated electromagnetic emission at Arecibo (Site not responding. Last check: 2007-11-06)
		Many systematic spectral features of the emissions bear a close resemblance to those observed in ionospheric modification experiments in the auroral zone and scale in accordance with detailed theory.
		This proves that these sideband emissions are not dependent on specific geophysical conditions but are due to fundamental interaction processes in the ionospheric plasma.
		We also report the discovery of unique, short-lived HF sideband emissions that are less systematic than those observed previously and accompanied by a quenching of the HF enhanced plasma and ion lines in the Arecibo 430 MHz incoherent scatter radar spectra.
	www.agu.org /pubs/crossref/1989/89GL00576.shtml (147 words)

	Laser Intro (Site not responding. Last check: 2007-11-06)
		This photon is of the right frequency to stimulate emission from another excited state particle, which emits another photon which can stimulate another excited state particle, and so on.
		In addition to stimulated emission processes there are also stimulated absorption processes in which a ground state particle absorbs a photon matching the energy gap and jumps to the excited state.
		Since the probabilities for stimulated absorption and emission processes are equal (relative to population of the ground and excited states -- Einstein's famous result), it is clearly detrimental to the laser to have any particles in the ground state.
	www.unc.edu /~dtmoore/laser_intro.html (427 words)

	ATNF Science Highlights 2004
		Although maser emission from OH clouds in the ISM has long been studied, these are the first measurements that directly demonstrate the stimulated amplification of a signal propagating through the interstellar medium: the maser amplification is clearly modified as the pulsar cycles on and off.
		Since this stimulated emission is driven by the pulsar pulse, it varies on a timescale of a few milliseconds, orders of magnitude shorter than the quickest maser variations previously detected.
		The Parkes search for OH absorption and stimulated emission in the spectrum of pulsars yielded many interesting results in the direction of the pulsar B1641-45.
	www.atnf.csiro.au /research/highlights/2004/weisberg/weisberg.html (1098 words)

	Spontaneous and stimulated emission (from light) -- Britannica Concise Encyclopedia - The online encyclopedia you ... (Site not responding. Last check: 2007-11-06)
		When the rate of absorption was equated to the rate of emission (i.e., to the sum of spontaneous and stimulated emission), an equation of the same form as Planck's law was obtained.
		Accordingly, it was much easier to develop microwave amplification by stimulated emission of radiation (maser) than the corresponding laser effect with radiation in or near the visible spectrum.
		The existence of stimulated emission can be fitted into a classical wave theory, like many other concepts that originated in relation to theories of photons.
	www.britannica.com /ebc/article-69325 (1210 words)

	eMedicine - Lasers, General Principles and Physics : Article by Steve Lee, MD (Site not responding. Last check: 2007-11-06)
		The theoretic principles behind the laser were developed as early as 1917, when Einstein laid the groundwork for stimulated emission in his treatise "On the Quantum Theory of Radiation." In 1955, Gordon produced the maser, the microwave predecessor of the laser.
		Emissions are produced when the pump is modulated to create discrete laser pulses, which usually are broad and randomly shaped.
		This population inversion is accomplished by using a mechanical opaque shutter or by inserting a high-speed, electrically sensitive, polarizable optical shutter known as a Pockel cell between the 2 mirrors of the laser.
	www.emedicine.com /ent/topic40.htm (4053 words)

	Plasma Laser Stars
		Cooperative emission of photons from an inverted population, usually defined as a device in which more stimulated emission is occurring than spontaneous emission.
		The effect of the cavity is not only to maintain a large enough electromagnetic field strength to stimulate emission from excited ions but also to maintain feedback and thus coherence of the output beam.
		Photons that produce the quantum transition by spontaneous emission have a much better chance of being amplified because they are more numerous than stellar photons at that particular wavelength that those from the nearby flbody, and so the emission line laser radiation from quasars is mostly from amplified spontaneous emission rather than amplified photospheric emission.
	laserstars.org /glossary/laser.html (1039 words)

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