This is a picture of the Nd-YAG laser that Derek and Jim used in their multi-photon absorption and raman scattering lab.
The Nd-YAG laser is an optically pumped solid-state laser that can produce very high-power emissions. This is a result of its lasing medium operating as a four-level system.
The lasing medium is the colorless, isotropic crystal Y2Al5O12 (Yttrium-Aluminum Garnet - YAG). When used in a laser, about 1% of the Yttrium is replaced by Neodymium. The energy levels of the Nd3+ ion are responsible for the fluorescent properties, i.e., active particles in the amplification process.
Population inversion results from shining light on this crystal. If the light is intense enough, atoms within the crystal that absorb this light transition from ground state into the absorption bands This is often done with a flash lamp - often a quartz tube filled with a noble gas through which high energy stored in a capacitor is discharged, emitting in the blue and ultra-violet (see diagram below).
Atoms transition efficiently from their broad absorption bands (shown as the E3 level in the above diagram) to the upper energy (laser) levels. The radiative decays to the ground-state from these bands have long life-times, on the order of micro-seconds, as compared to the fast transitions to the upper energy levels (on the order of nano-seconds). Approximately 99% of the ions that are excited to the absorption band transfer to the upper energy levels. These levels are characterized by a relatively long lifetime, on the order of milli-seconds. Due to this long life-time, they de-excite almost solely due to spontaneous emission.
The Nd-YAG laser used in our labs uses a cylindrical crystal. The crystal forms the laser cavity and has reflective ends - one coated so that it is 100% reflective, and the other is either sufficiently reflective, or is coated to allow only part of the amplified light to pass - enough feed-back so that oscillation may occur. The following diagram may help visualize this apparatus.
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Davis, Christopher C. Lasers and Electro-Optics. New York: Cambridge University Press, 1996.
Eastham, Derek. Atomic Physics of Lasers. Philadelphia: Taylor and Francis, 1986.