Lasers are distinguished from other light sources by their coherence. Spatial (or transverse) coherence is typically expressed through the output being a narrow beam, which is diffraction-limited. Laser beams can be

 ...

A laser consists of a gain medium, a mechanism to energize it, and something to provide optical feedback. The gain medium is a material with properties that allow it to amplifylight by way of stimulated emission. Light of a specific wavelength that passes

 ...

Electrons and how they interact with electromagnetic fields are important in our understanding of chemistry and physics.
Stimulated emission
In the

 ...

Foundations
In 1917, Albert Einstein established the theoretical foundations for the laser and the maserin

 ...

Gas lasers
Following the invention of the HeNe gas laser, many other gas discharges have been found to amplify light coherently. Gas lasers using many different gases have been built and used for many purposes. The

 ...

When lasers were invented in 1960, they were called "a solution looking for a problem". Since then, they have become ubiquitous, finding utility in thousands of highly varied applications

 ...

A laser can be classified as operating in either continuous or pulsed mode, depending on whether the power output is essentially continuous over time or whether its output takes

 ...