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Topic: Population inversion

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	Olympus Microscopy Resource Center: Physics of Light and Color - Introduction to Lasers
		A population inversion can be produced through two basic mechanisms, either by creating an excess of atoms or molecules in a higher energy state, or by reducing the population of a lower energy state.
		Because each atom's residence time in the metastable state is relatively long, the population tends to increase and leads to a population inversion between the metastable state and the lower ground state (which is continuously being depopulated to the highest level).
		Because the population inversion is not created between the ground state and the upper level, the number of atoms or molecules that must be elevated is dramatically reduced in this model.
	www.olympusmicro.com /primer/lightandcolor/lasersintro.html (6884 words)

	Lasers (E-I-1)
		Population inversions in gases—Inversions in gases are generally produced by applying a voltage across a gas discharge tube that consists of a long, narrow glass or ceramic tube serving to confine the gain medium, with electrodes installed at each end of the tube.
		Population inversions in crystalline solids and glasses—As in the case of liquids, when energy levels in solids are excited, typically by irradiating those solids with light, the levels tend to decay much more rapidly via collisions with their surrounding neighbors rather than by radiating their energy in the form of light.
		Population inversions in semiconductors—Inversions in semiconductors are produced when joining a p-doped semiconductor material with an n-doped semiconductor material in a similar way to that of producing a transistor to create a pn junction.
	cord.org /step_online/st1-5/st15ei1.htm (3291 words)

	Magneto Thermodynamics, Part 2
		For instance, a population of molecules that have magnetic properties, when under the influence of a magnetic field, may be constrained to movement in a single direction, and yet in the absence of a magnetic field, this same population may have nearly unlimited directions of movement.
		In a population of Hydrogen atoms at room temperature, the vast majority of electrons are in the orbit N = 1, and the effective temperature of the orbital population is very close to zero.
		In thermodynamics terms, this is known as a population inversion, and under the rules of classical thermodynamics, was considered a physical impossibility...
	www.geocities.com /electrogravitics/mt2.html (1728 words)

	Physics News Update Number 100
		The recent GRO mapping of the burst population found an isotropic distribution, thus challenging an earlier supposition that gamma bursts came primarily from pulsars, which were believed to reside mostly in the galactic plane.
		Population inversion is the condition in which a majority of atoms in a laser's active medium is maintained in excited states.
		In a paper to be published in the journal Optics Communications, Jin-Yue Gao of Jilin University in China reports the use of microwave radiation to prevent ground-state atoms in a sodium vapor from absorbing photons produced in the vapor, thereby obviating the need for population inversion.
	www.aip.org /pnu/1992/physnews.100.htm (486 words)

	CHAPTER-9
		To understand the concept of a population inversion, recall from Chapter 3 that Boltzmann statistics tell us that there are more spins in the lower spin state than the upper one of a two spin state system.
		Population inversion is the interchange of the populations of these two spin states so that there are more spins in the upper state then the lower one.
		The population distribution between the four levels is such that the lowest state has the greatest population and the highest the lowest population.
	www.cis.rit.edu /htbooks/nmr/chap-9/chap-9.htm (1220 words)

	MaserPatentApplication_Specs
		In order to sustain the population inversion, it is necessary to further resonate the inverted population, more specifically, to elevate the Cooper pairs’ binding energy by more than two energy levels without the pair’s energy ever falling to the ground bound state.
		Population inversion in the Bose regime is characterized by a spontaneous bundling of free phonons into bundles such as a bundle of two superposed free phonons 28.
		Consider a population inversion into an eighth excited state in which Cooper pairs are bound by a high-amplitude vibration represented by a bundle of 256 superposed phonons. The partially reflective means are calibrated to allow only the bundles of 256 superposed free phonons to break through.
	borisvolfson.com /Maser_Method.html (2877 words)

	Introduction to Lasers
		In order to achieve population inversion in a three-level laser, most of the atoms in the ground state must be pumped up to the upper laser level.
		As a result, the populations of the ground state and the metastable states become inverted and lasing takes place if a cavity is provided.
		Population inversion is achieved somewhat differently in the cw helium-neon (He-Ne) gas laser.
	kottan-labs.bgsu.edu /teaching/workshop2001/chapter4a.htm (3310 words)

	physics - Population inversion
		In physics, specifically statistical mechanics, the concept of population inversion is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a laser.
		To understand the concept of a population inversion, it is necessary to understand some thermodynamics and the way that light interacts with matter.
		As described above, a population inversion is required for laser operation, but cannot be achieved in our theoretical group of atoms with two energy-levels when they are in thermal equilibrium.
	www.physicsdaily.com /physics/Population_inversion (2075 words)

	Quantum properties of light
		The achievement of a significant population inversion in atomic or molecular energy states is a precondition for laser action.
		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.
		The case of the helium-neon laser illustrates one of the ways of achieving the necessary population inversion.
	hyperphysics.phy-astr.gsu.edu /hbase/optmod/qualig.html (838 words)

	He-Ne Laser Theory
		Therefore, the key to the laser's action is population inversion: there must be more atoms in the desired excited state than in the state to which the electons transition.
		Due to selection rules for electron transitions, this state is metastable, which is not only handy, but necessary for a population inversion to occur (click here to see an energy-level diagram of what's going on).
		Once we achieve a population inversion and experience stimulated emissions, there must be some mechanism to increase the gain of the laser output.
	webphysics.davidson.edu /alumni/jimn/He-Ne/pages/Theory.htm (1246 words)

	Powering a Laser: Pumping and Population Inversion
		The power must maintain the necessary population inversion to keep the laser process going, and that implies a pumping mechanism to elevate electrons to that metastable state.
		The use of helium to "pump" electrons into a metastable state of neon in the helium-neon laser is an example of such a mechanism.
		This requires a proper combination of population inversion and lifetime of the excited state.
	hyperphysics.phy-astr.gsu.edu /HBASE/optmod/laspow.html (277 words)

	Spectroscopic Applications (Experimental)
		For this reason, a major thrust over the last few years has been applications of frequency swept laser pulses, which in the adiabatic limit produce population inversion or state-to-state transfer even if the sweep is not perfectly linear or if the intensity is not uniform.
		We also demonstrated that frequency swept pulses could quantitatively transfer population from intermediate to final states, which can be useful for techniques such as stimulated emission pumping.
		This population can easily be detected by measuring the total fluorescence or scattering emitted by the sample, or by measuring the total absorption.
	www.princeton.edu /~wwarren/Specexp (1192 words)

	Population inversion - Wikipedia, the free encyclopedia
		In physics, specifically statistical mechanics, a population inversion occurs when a system (such as a group of atoms or molecules) exists in state with more members in an excited state than in lower energy states.
		The concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a laser.
		Because at least half the population of atoms must be excited from the ground state to obtain a population inversion, the laser medium must be very strongly pumped.
	en.wikipedia.org /wiki/Population_inversion (2133 words)

	Olympus Microscopy Resource Center: Physics of Light and Color - Diode Lasers: Interactive Java Tutorial
		This situation is achieved under the conditions of a population inversion in the active medium, a process whereby the electron population of an upper energy level is induced to grow larger than that of a lower level.
		At the junction between p-type and n-type semiconductors, a population inversion can be created among the energy levels of electrons and holes if sufficient electrical energy is applied to the junction.
		With the dominance of stimulated emission that results from a population inversion, the only remaining requirement for laser action is to confine emission so that it is directed primarily along the junction boundaries, and undergoes amplification or gain before being emitted from the semiconductor material.
	www.olympusmicro.com /primer/java/lasers/diodelasers (3136 words)

	OPTICAL AMPLIFICATION WITHOUT POPULATION INVERSION (Site not responding. Last check: 2007-10-12)
		Optical amplification is a first step in the laser process in which a single photon, emitted by an excited atom, stimulates other atoms to emit photons having identical attributes, resulting in a buildup of light intensity.
		Optical amplification normally requires a population inversion whereby a majority of atoms must be excited into a higher energy state; otherwise the ground state atoms, which readily absorb photons, would sabotage this process.
		This technique may one day be exploited in a working laser, particularly in an x-ray or gamma- ray laser, where population inversion is difficult to achieve because excited states are so short-lived.
	newton.ex.ac.uk /aip/glimpse.txt/physnews.121.1.html (221 words)

	Laser tutorial 5 (WSRNet) (Site not responding. Last check: 2007-10-12)
		Finding substances in which a population inversion can be set up is central to the develpment of new kinds of laser.
		This population inversion is the condition required for stimulated emission to overcome absorption and so give rise to the amplification of light.
		In an assembly of chromium ions in which a population inversion has been set up, some will decay spontaneously to the ground state level emitting red light of wavelength 694.3 nm in the process.
	members.aol.com /WSRNet/tut/t5.htm (480 words)

	Encyclopedia of Laser Physics and Technology - population inversion, laser gain (Site not responding. Last check: 2007-10-12)
		Note that population inversion is a state which deviates from thermal equilibrium: in thermal equilibrium, the population of the lower level is always higher, and net gain can never occur.
		In most solid state lasers, the concept of population inversion is not very useful, because the upper and lower energy levels actually consist of large numbers of slightly different energy levels.
		This shows that population inversion in the sense of >50% inversion is not required to obtain optical amplification: for long wavelengths, where the emission is typically much stronger than the absorption, gain is achieved even for rather low excitation levels.
	www.rp-photonics.com /population_inversion.html (426 words)

	Lasers - Population Inversion (Site not responding. Last check: 2007-10-12)
		A quantitative measurement of population inversion applies only to the two levels which are directly involved in the transition.
		This results in a macroscopic electric polarization vector which causes the populations to oscillate between the ground and excited state with a frequency which is proportional to the electric field intensity of the the pump.
		If the pulse is turned off after the population has performed one half of a Bloch oscillation then they would all be left in the excited state, hence a population inversion has been created.
	laserstars.org /glossary/inversion.html (349 words)

	Warren - Lasers
		For example, we used frequency swept laser pulses to generate narrowband population inversion in Na vapor (effectively a three-level system) with broadband laser pulses.
		We have experimentally demonstrated inversion by frequency sweeps when many energy levels are resonant, for example because the pulse includes a variety of rotational lines.
		Explicitly, the population in state J"=N will be transferred into the excited state if the frequency separation between the two vibronic transitions involving this state (R(N)-P(N)) is larger than the pulse bandwidth.
	www.princeton.edu /~wwarren/laser.shtml (1983 words)

	Inverse - Wikipedia, the free encyclopedia
		Inversion (algebra), a pair of adjacent out-of-order elements of a permutation (viewed as a list).
		Population inversion, in statistical mechanics, when a system exists in state with more members in an excited state than in lower energy states
		Sexual inversion, in biology, the switching from one sex to the opposite among some animal species.
	en.wikipedia.org /wiki/Inversion (290 words)

	Population
		By electrical or optical pumping, carriers can be excited to higher states and the population of the states can be changed.
		For lasing, the upper radiative transition states must have a large population of electrons than the ground state i.e.
		This also requires the lifetimes of each state to be correct to allow population inversion.
	www.sp.phy.cam.ac.uk /~SiGe/Population.html (126 words)

	Theory
		Each time an excited atom is stimulated to emit a photon, the population of excited atoms decreases by one.
		The key to the laser's action is population inversion, or maintaining more Ne atoms in the desired 3S excited state than in the 2P state.
		This sets up a population inversion: higher population of atoms in the 3s than in the 2p.
	www.phy.davidson.edu /StuHome/joesten/IntLab/HeNeLaser/theory.htm (1007 words)

	Short-pulse Population Inversion and Transmittance (Site not responding. Last check: 2007-10-12)
		The maximal population transfer in a two-level system induced by an ultrashort, linearly polarized laser pulse and the optimal pulse shape for maximal population transfer depend critically on the pulse duration and phase.
		In the case of temporally symmetric pulses, half-cycle pulses are optimal and are capable of inverting the population completely.
		For antisymmetric pulses, bimodal pulses are optimal but cannot invert the population completely.
	www.chemistry.bnl.gov /GPMD/old-pages/MUCKAB1.HTML (87 words)

	Laser Intro (Site not responding. Last check: 2007-10-12)
		Thus in a laser we strive to create a "population inversion" where most or all of the particles are in the excited state.
		This is achieved by adding energy to the laser medium (usually from an electrical discharge or an optical source such as another laser or a flashlamp); this process is called pumping.
		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)

	JYI Volume Three Features: Discovery of the First and Only Laser in Outer Space
		In a population inversion, a higher energy level is more populated than another energy level that is lower in energy.
		If there are only two levels in the cycle, a population inversion can never be formed because, as Einstein proved, the probability of absorption is equal to the probability of stimulated emission for a given transition.
		The roundabout answer to this question is that a population of molecules will not be in thermodynamic equilibrium if the molecules have not had a chance to thermalize with the surrounding environment.
	www.jyi.org /volumes/volume3/issue3/features/peterson.html (2414 words)

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