Quantum Physics

28.12 Lasers and Holography 923is usually a net absorption of energy, because when the system is in thermal equilibrium,there are many more atoms in the ground state than in excited states.However, if the situation can be inverted so that there are more atoms in an excitedstate than in the ground state, a net emission of photons can result. Such acondition is called population inversion. This is the fundamental principle involvedin the operation of a laser, an acronym for light amplification by stimulatedemission of r adiation. The amplification corresponds to a buildup of photons inthe system as the result of a chain reaction of events. The following three conditionsmust be satisfied in order to achieve laser action:1. The system must be in a state of population inversion (that is, more atoms inan excited state than in the ground state).2. The excited state of the system must be a metastable state, which means its lifetimemust be long compared with the otherwise usually short lifetimes of excitedstates. When that is the case, stimulated emission will occur before spontaneousemission.3. The emitted photons must be confined within the system long enough to allowthem to stimulate further emission from other excited atoms. This is achievedby the use of reflecting mirrors at the ends of the system. One end is totally reflecting,and the other is slightly transparent to allow the laser beam to escape.One device that exhibits stimulated emission of radiation is the helium–neongas laser. Figure 28.20 is an energy-level diagram for the neon atom in this system.The mixture of helium and neon is confined to a glass tube sealed at the ends bymirrors. A high voltage applied to the tube causes electrons to sweep through it,colliding with the atoms of the gas and raising them into excited states. Neonatoms are excited to state E 3 * through this process and also as a result of collisionswith excited helium atoms. When a neon atom makes a transition to state E 2 , itstimulates emission by neighboring excited atoms. This results in the productionof coherent light at a wavelength of 632.8 nm. Figure 28.21 summarizes the stepsin the production of a laser beam.ENERGYE 3 *E 2Metastable stateλ = 632.8 nmOutputenergyInputenergyE 1Figure 28.20 Energy-level diagramfor the neon atom in a helium–neonlaser. The atom emits 632.8-nm photonsthrough stimulated emission inthe transition E * 3 : E 2 . This is thesource of coherent light in the laser.hfSpontaneous emission—random directionsLaseroutputMirror oneStimulating waveon axisEnergy input(a)Mirror two(b)Courtesy of HRL Laboratories LLC, Malibu, CAFigure 28.21 (a) Steps in theproduction of a laser beam. The tubecontains atoms, which represent theactive medium. An external source ofenergy (optical, electrical, etc.) isneeded to “pump” the atoms toexcited energy states. The parallelend mirrors provide the feedback ofthe stimulating wave. (b) Photographof the first ruby laser, showing theflash lamp surrounding the ruby rod.