How Do Lasers Work?

In 1917, Albert Einstein published a paper predicting that light could trigger atoms to release energy as identical copies of the triggering photon. He called it stimulated emission. For 43 years, it was a theoretical curiosity. Then, on May 16, 1960, Theodore Maiman aimed a xenon flash lamp at a synthetic ruby crystal the size of a fingertip — and produced the world's first laser beam.

Today, lasers etch circuit boards, perform eye surgery, play your Blu-Ray discs, and carry internet traffic across oceans. Every one of them runs on the same quantum-mechanical trick Einstein described a century ago.

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How Light Interacts with Atoms

To understand lasers, you need to understand the three ways a photon can interact with an atom.

Absorption

An atom in its lowest energy state — the ground state — can absorb a photon if the photon's energy exactly matches the gap between the ground state and a higher energy level. The atom jumps to the higher level; the photon vanishes. This is how sunglasses work: the dye molecules absorb specific frequencies of light and convert them to heat.

$E_\text{photon} = hf = E_2 - E_1$

Spontaneous Emission

An atom in an excited state is unstable. After a characteristic lifetime (nanoseconds to milliseconds), it drops back down spontaneously, releasing a photon in a random direction with a random phase. This is how an ordinary light bulb works — atoms are thermally excited and decay randomly in all directions, producing incoherent light.

Stimulated Emission — Einstein's Prediction

Einstein's 1917 insight was a third possibility. If a photon with exactly the right energy passes an already-excited atom, it can stimulate the atom to emit a second photon that is identical to the first: same wavelength, same direction, same phase.

This is the key to lasers. One photon becomes two. Two become four. The beam grows exponentially — but only if enough atoms remain in the excited state to sustain it.

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Energy Levels: Ground State, Excited States, and Metastable States

Real atoms have many energy levels. For lasing, three types matter:

Ground state (E₀): The lowest energy level. Atoms stay here indefinitely unless pumped.

Excited / pump level: A short-lived state reached by absorbing pump energy. Lifetime is typically nanoseconds — atoms fall out almost immediately via non-radiative decay (releasing heat, not light).

Metastable state: The crucial level. An unusually long lifetime — microseconds to milliseconds — because quantum-mechanical selection rules partially forbid rapid decay. Atoms accumulate in the metastable state instead of falling through immediately. This is where stimulated emission happens.

The metastable state is the laser's energy reservoir. Without it, atoms would decay too quickly to build the population needed for lasing.

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Population Inversion: The Key to Laser Action

Under thermal equilibrium, most atoms occupy the ground state. A photon passing through such a material is more likely to be absorbed than to trigger stimulated emission — the medium attenuates the beam.

Population inversion is the condition where more atoms occupy the upper laser level than the lower level:

$N_2 > N_1$

When this condition is met, stimulated emission dominates over absorption. A photon passing through the gain medium triggers more emissions than it suffers absorptions — the beam is amplified. The gain coefficient is:

$g = \sigma (N_2 - N_1)$

where $\sigma$ is the stimulated emission cross-section (a quantum-mechanical property of the transition) and $g > 0$ means net amplification.

Achieving population inversion requires pumping — supplying energy faster than the metastable state decays. This is why a two-level system cannot lase: any pump that excites atoms also stimulates them back down at equal rates. Three or four levels are needed to break this symmetry.

Try it: Set pump power to 0% — atoms stay in the ground state, no inversion. Increase pump power above ~40% and watch the metastable population build. At high pump power, the "Population inversion: YES" badge lights green and the cavity photon count climbs.

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The Optical Cavity: Mirrors That Build a Beam

A gain medium alone is an amplifier, not a laser. To turn amplification into a sustained directed beam, you need an optical cavity: two mirrors facing each other, with the gain medium between them.

Photons born by spontaneous emission travel in random directions. Most escape the sides of the gain medium and are lost. But photons travelling along the cavity axis bounce back and forth, passing through the gain medium repeatedly. Each pass amplifies the beam through stimulated emission.

One mirror is fully reflective (100%). The other — the output coupler — transmits a small fraction (typically 1–5%), allowing the usable beam to escape. The threshold condition for sustained lasing is:

$G^2 \cdot R_1 \cdot R_2 \cdot (1 - L)^2 \geq 1$

where $G$ is the single-pass gain, $R_1, R_2$ are mirror reflectivities, and $L$ is the single-pass (per-pass) loss fraction. Because every photon in the output beam was born from stimulated emission, all photons share identical phase, direction, and wavelength — making laser light coherent and monochromatic.

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Four-Level vs Three-Level Laser Systems

Not all lasers achieve population inversion the same way.

Three-level systems (e.g. Ruby): Atoms pump from ground → pump band → metastable, then lase back to ground. Because lasing terminates at the ground state, maintaining inversion requires keeping more than half of all atoms excited — demanding very intense pumping. Ruby lasers typically operate in short pulses.

Four-level systems (e.g. He-Ne, Nd:YAG): Atoms pump from ground → pump level → metastable, lase to a lower laser level (not ground), then rapidly decay back to ground. Because the lower laser level quickly empties, population inversion is much easier to maintain. Four-level lasers operate continuously (CW — continuous wave).

Property	Three-level (Ruby)	Four-level (He-Ne, Nd:YAG)
Lower laser level	Ground state	Intermediate level
Pump threshold	Very high	Low
Operation mode	Typically pulsed	Continuous wave possible
Example wavelength	694 nm	632.8 nm / 1064 nm

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The History of the Laser

1917 — Einstein predicts stimulated emission. In his paper on the quantum theory of radiation, Einstein derives the A and B coefficients governing spontaneous and stimulated emission. This is the theoretical foundation of every laser.

1954 — The maser. Charles Townes and James Gordon at Columbia University build the first maser (Microwave Amplification by Stimulated Emission of Radiation) — the microwave precursor to the laser. Townes shares the 1964 Nobel Prize in Physics for this work.

1958 — Schawlow and Townes propose the optical maser. Townes and Arthur Schawlow publish a paper describing how to extend the maser principle to infrared and visible light using an optical cavity with mirrors — describing the laser in theory.

1960 — Maiman builds the first laser. Theodore Maiman at Hughes Research Laboratories fires a xenon flash lamp around a synthetic ruby rod and detects coherent red light at 694.3 nm. He submits the paper to Physical Review Letters, which rejects it; it is published in Nature instead.

1961 — He-Ne laser. Ali Javan, William Bennett, and Donald Herriott at Bell Labs build the first gas laser and the first continuously operating laser, emitting at 1152 nm (near-infrared). Visible red emission at 632.8 nm followed in 1962.

1962 — Semiconductor laser diode demonstrated simultaneously by three independent groups at GE, IBM, and MIT Lincoln Laboratory — the ancestor of every laser pointer, CD player, and fibre-optic transmitter today.

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Common Laser Types and Their Wavelengths

Laser Type	Medium	Wavelength	Colour	Common Use
He-Ne	Helium-neon gas	632.8 nm	Red	Lab instruments, barcode scanners
Ruby	Cr³⁺ in Al₂O₃	694.3 nm	Deep red	Holography, tattoo removal
Nd:YAG	Nd³⁺ in YAG crystal	1064 nm	Near-infrared	Cutting, welding, range-finding
CO₂	CO₂ gas	10,600 nm	Far-infrared	Industrial cutting, surgery
GaN diode	GaN semiconductor	405–450 nm	Violet-blue	Blu-Ray, lithography
Er:fibre	Er³⁺ in silica fibre	1550 nm	Near-infrared	Fibre-optic communications

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Worked Examples

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Applications of Lasers

Medicine: Excimer lasers (193 nm) reshape corneas in LASIK surgery. Nd:YAG lasers treat retinal tears. Pulsed dye lasers remove tattoos and birthmarks by shattering pigment granules with nanosecond pulses.

Communications: Infrared laser diodes at 1550 nm carry internet traffic through fibre-optic cables. A single fibre carries terabits per second using wavelength-division multiplexing — many lasers at slightly different wavelengths sharing one fibre.

Manufacturing: CO₂ and Nd:YAG lasers cut sheet metal and drill circuit boards. Femtosecond-pulse lasers machine glass and ceramics without heat damage — the pulse is too short for heat to diffuse.

Science: Laser cooling uses photon momentum to slow atoms to microkelvin temperatures, enabling Bose-Einstein condensates and the world's most accurate atomic clocks. LIGO's laser interferometers detect gravitational wave distortions smaller than one-thousandth the diameter of a proton.

Consumer electronics: Semiconductor laser diodes read Blu-Ray discs (405 nm), power laser projectors, transmit signals in optical mice, and serve as the ranging sensors in phone and car lidar systems.

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Frequently Asked Questions

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Quick Reference

Quantity	Formula	Notes
Photon energy	$E = hf = hc/\lambda$	Use $hc = 1240$ eV·nm
Population inversion	$N_2 > N_1$	Upper laser level > lower level
Gain coefficient	$g = \sigma(N_2 - N_1)$	σ in cm², N in cm⁻³
Threshold condition	$G^2 R_1 R_2 (1-L)^2 \geq 1$	Round-trip gain ≥ round-trip loss
Einstein B coefficient	$B_{21}$	Rate coefficient for stimulated emission
Einstein A coefficient	$A_{21}$	Rate coefficient for spontaneous emission