Themes > Science > Physics > Optics >

Themes > Science > Physics > Optics >

Themes > Science > Physics > Optics > Laser Tutorial > Creating a Population Inversion Finding substances in which a population inversion can be set up is central to the develpment of new kinds of laser. The first material used was synthetic ruby. Ruby is crystalline alumina (Al2O3) in which a small fraction of the Al3+ ions have been replaced by chromium ions, Cr3+. It is the chromium ions that give rise to the characteristic pink or red colour of ruby and it is in these ions that a population inversion is set up in a ruby laser. In a ruby laser, a rod of ruby is irradiated with the intense flash of light from xenonfilled flashtubes. Light in the green and blue regions of the spectrum is absorbed by chromium ions, raising the energy of electrons of the ions from the ground state level to the broad F bands of levels. Electrons in the F bands rapidly undergo non-radiative transitions to the two metastable E levels. A non-radiative transition does not result in

the emission of light; the energy released in the transition is dissipated as heat in the ruby crystal. The metastable levels are unusual in that they have a relatively long lifetime of about 4 milliseconds (4 x 10-3 s), the major decay process being a transition from the lower level to the ground state. This long lifetime allows a high proportion (more than a half) of the chromium ions to build up in the metastable levels so that a population inversion is set up between these levels and the ground state level. 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. This light can then interact with other chromium ions that are in the metastable levels causing them to emit light of the same wavelength by stimulated

emission. As each stimulating photon leads to the emission of two photons, the intensity of the light emitted will build up quickly. This cascade process in which photons emitted from excited chromium ions cause stimulated emission from other excited ions is indicated below: The ruby laser is often referred to as an example of a three-level system. More than three energy levels are actually involved but they can be put into three categories.These are; the lower level form which pumping takes place, the F levels into which the chromium ions are pumped, and the metastable levels from which stimulated emission occurs. Other types of laser operate on a four level system and , in general, the mechanism of amplification differs for different lasing materials. However, in all cases, it is necessary to set up a population inversion so that stimulated emission occurs more often than absorption. themes/sciences/physics/optics/ LaserTutorial/Creating/Creating.h Figure 3.2 Indirect and Direct gap semiconductors In equilibrium, the charge carriers occupy their lowest energy states, electrons at the bottom of the conduction band, and holes at the top of the valence band. In silicon these states do not have the same momentum. Therefore if a recombination is

to result in the emission of a photon, which has little momentum, a quantum of lattice vibration (a phonon) must also be created to carry away the excess momentum. This is known as an indirect process and such semiconductors are known as INDIRECT GAP semiconductors. The two particle process is not favoured, and recombinations in indirect gap semiconductors usually occur by thermal or collisional processes. The silicon chips in a computer do not glow, they just get warm. GaAs however is a DIRECT GAP semiconductor. The minima of electron and hole energies occur at the same momentum. Thus recombination can result in a photon alone. LED action was first observed in GaAs in 1952. The light is emitted in a narrow range of wavelengths which is determined by the size of the band gap of the semiconductor. The red, green and yellow LEDs available today are made using semiconductor compounds with different band gaps, for example GaAs, GaAlAs, AlInGaP, GaAsP. The LED must satisfy two more conditions before it is possible to make a laser diode. Firstly stimulated emission must be able to dominate over absorbtion and spontaneous emission, and secondly there must be an optical cavity. Fortunately both

are relatively simple to achieve. The population inversion required for stimulated emission to dominate is realised by increasing the current through the diode. This increases the density of electron-hole pairs in the junction region, and thus creates an inversion. The optical cavity is made by cleaving the crystal along two parallel crystal planes, perpendicular to the junction plane. This creates optically flat and parallel surfaces. They do not need to be coated to act as laser mirrors as the high refractive index of GaAs gives the GaAs-air interface a reflectivity of 35% which is found to give sufficient feedback to sustain laser action. 4-Level Laser Scheme

n n k l m n excitation n m

slow radiative decay k l fast(ish) l m m

inversion fast to maintain population Helium-Neon Laser Energy Level Scheme Ne 2S 1 Resonant Collisional Energy Transfer 3S

632.8 nm He 2P + 1S He ionised by electron impact in

discharge Collisions CO2 Laser Energy Level Scheme N2 N2 vibrational states excited by collision but cannot emit as no dipole moment CO2 v=2

002 10-6 s v=1 10-4 s 001 10.6 100

010 Collisions and radiative decay

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