1. Lasers

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

3. Some atoms can be induced to emit light when they are excited. Triggering 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 Stimulated Emission

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

5. 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. Population Inversion collisions EN4 EH2 1.96 eV EN3 20.61 eV Electrical pumping 18.70 eV EN2 EN1 EH1 Helium Neon

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