laser

History

Although the theoretical possibility of stimulated emission was substantiated by Einstein back in 1917, the first device to use this principle for microwaves—the maser—was created by Charles Townes and his group in 1954. Then in 1960, Theodore Maiman built the first laser that emitted visible light, using a ruby crystal.

How it works

First, the laser's active medium (e.g., a crystal or gas) is pumped with energy from an external source. Atoms go into an excited state, and then, 'shedding' excess energy, emit photons. These initial photons stimulate other excited atoms to emit exactly identical photons—thus an avalanche of identical light particles is born. Two parallel mirrors repeatedly pass light through the medium, amplifying it until part of it exits through one of the mirrors, forming a directed beam.

History

Although the theoretical possibility of stimulated emission was substantiated by Einstein back in 1917, the first device to use this principle for microwaves—the maser—was created by Charles Townes and his group in 1954. Then in 1960, Theodore Maiman built the first laser that emitted visible light, using a ruby crystal.

How it works

First, the laser's active medium (e.g., a crystal or gas) is pumped with energy from an external source. Atoms go into an excited state, and then, 'shedding' excess energy, emit photons. These initial photons stimulate other excited atoms to emit exactly identical photons—thus an avalanche of identical light particles is born. Two parallel mirrors repeatedly pass light through the medium, amplifying it until part of it exits through one of the mirrors, forming a directed beam.

History

In 1917, Einstein theoretically predicted stimulated emission. In 1954, Charles Townes, James Gordon, and Herbert Zeiger created the first maser, operating on ammonia. Six years later, Theodore Maiman demonstrated the first optical laser using ruby. Soon Ali Javan designed a helium-neon laser (1961), and Robert Hall the first semiconductor laser (1962). These discoveries transformed science, telecommunications, medicine, and manufacturing.

How it works

The active medium (for example, neodymium in a YAG crystal or a helium-neon mixture) is excited by external pumping, transferring atoms to a metastable energy level. When the upper-level population exceeds the lower-level population (population inversion), a spontaneously emitted photon stimulates emission from neighboring excited atoms, generating a cascade of coherent photons. Inside the optical resonator (e.g., a Fabry–Pérot etalon), light is repeatedly reflected by mirrors and amplified on each pass. Part of the radiation exits through a semi-transparent mirror. Photons, being bosons, accumulate in a single quantum state, ensuring identical parameters of the laser beam. In semiconductor lasers, feedback is achieved through Bragg diffraction on periodic structures (distributed feedback) with the condition mλ = 2nΛ.

Open problems & fun facts

Development of compact and cheap X-ray lasers pumped by incoherent sources.; Increasing the efficiency and power of semiconductor lasers while reducing size.; Generation of ultrashort pulses (attosecond range) with controlled phase and envelope shape.; Achieving conditions for laser-driven thermonuclear fusion with uniform target compression.

💡 Red-beam laser pointers use a miniature diode laser based on gallium-aluminum arsenide, which efficiently emits light with a current of just a few milliamps.