Physics
Laser Operation
Quick fact
The first working laser was built in 1960 by Theodore Maiman using a synthetic ruby crystal, and it produced a pulse lasting only a few hundred microseconds.
Why this is interesting
You've seen laser pointers and maybe even had your eyes lasered—but how does a small device produce such an intensely focused, single-color beam of light?
Read the full explanation
Understanding Laser Operation
Imagine a crowd of people on a dance floor. Most are standing still (ground state), but some have been handed glow sticks and are excited (excited state). In a laser, we need more people with glow sticks than without—this is called population inversion. If someone with a glow stick happens to bump into another excited person, that second person also lights up and they both release their glow sticks in perfect sync. That is stimulated emission: one photon triggers another identical photon. Now put mirrors at both ends of the dance floor: the light bounces back and forth, triggering more and more photons. One mirror is only partially reflective, so a tiny bit of light escapes each bounce—that's the laser beam. The result is a concentrated flood of identical, synchronized light waves.
A deeper explanation
Laser operation hinges on three core principles. First, a medium (gas, crystal, or semiconductor) is pumped with energy (electrical or optical) to elevate electrons into higher energy levels. These electrons quickly drop to a metastable state that holds them longer than usual. With sufficient pumping, more electrons reside in the metastable state than in the ground state—population inversion. Second, when a photon of the exact energy difference between the metastable and ground state encounters an excited electron, it triggers stimulated emission: the electron drops down and emits a second photon identical in phase, frequency, direction, and polarization to the incident photon. Third, the optical cavity (two mirrors) reflects these photons back and forth through the medium, amplifying the light through repeated stimulated emission. The cavity ensures that only photons traveling exactly perpendicular to the mirrors sustain oscillation, producing a highly directional beam. One mirror is partially transmitting, allowing a small fraction of the light to exit as the laser output. This coherent, monochromatic, and low-divergence beam is the hallmark of laser operation.