Lasers

Spontaneous Emission  Atomic electrons can be excited by external radiation. EM wavesEM waves Electrons in a fieldElectrons in a field  Atoms emit light when the electron returns to the ground state. Random release time Transition occurs spontaneously

Stimulated Emission  Some atoms can be induced to emit light when they are excited. Triggering photonTriggering photon  The triggering photon is not absorbed. Continues after interacting  The emitted photon is coherent with the triggering photon. Same phase Same direction Same polarization

Avalanche  Many atoms can be pumped to an excited state at once. Few begin dischargeFew begin discharge  The light strikes other atoms and a coherent avalanche of light forms. Semisilvered mirror amplifies the beamSemisilvered mirror amplifies the beam  Light Amplification from Stimulated Emission of Radiation = LASER

Population Inversion  All lasers require a sustained population in the excited state. Population inversion  A helium-neon laser maintains a continuous inversion. Electrically pump helium Collisions excite neon Initial discharge is lasing  Further discharge to ground state is spontaneous. Electrical pumping E H2 E H1 collisions 1.96 eV Helium E N1 E N2 E N4 E N3 Neon eV eV

Laser Types  Dye lasers Organic molecules in solution: rhodamine Tunable frequencies  Semiconductor lasers Diode lasers Small low-power next  Solid-state lasers Solid matrix: ruby, yag lasersSolid matrix: ruby, yag lasers  Gas lasers Single or mixes gases: HeNe, CO2Single or mixes gases: HeNe, CO2  Excimer lasers From excited and dimers:From excited and dimers: Reactive gases Cl, FReactive gases Cl, F Molecule forms and lases on breakupMolecule forms and lases on breakup