Lasing

Lasing is the process of light amplification by stimulated emission of radiation. It occurs when an active medium is energized, creating a population inversion where more particles are in excited states than in lower ones. In this condition, incoming photons can stimulate the emission of additional photons that are identical in frequency, phase, and direction, leading to amplification of light.

The mechanism relies on stimulated emission plus feedback from an optical cavity. Photons produced by spontaneous emission seed the process, but the cavity mirrors reflect light back and forth, selecting and reinforcing specific modes. When the optical gain of the medium exceeds the losses in the cavity, a threshold is reached and coherent light is emitted as a beam with high directionality and brightness.

Lasers typically exhibit high temporal and spatial coherence, a narrow spectral linewidth, and a well-defined beam. The wavelength is determined by the energy levels of the gain medium and the resonant modes of the cavity. Lasers can be categorized by the gain medium, including gas lasers, solid-state lasers, dye lasers, and semiconductor (diode) lasers, with common examples such as He–Ne, Nd:YAG, and diode lasers. Fiber lasers and laser diodes are notable derivatives.

Historically, the term laser stands for Light Amplification by Stimulated Emission of Radiation. The concept emerged in the 1950s–1960s, with the first working laser demonstrated in 1960 by Theodore Maiman using a ruby medium. Since then, lasers have found widespread applications in industry, medicine, communications, science, and everyday technology.

Safety considerations include eye and skin hazards from intense, focused light, necessitating appropriate shielding, interlocks, and protective eyewear in many applications.