Physic Labs

Frontier physics

Lasers and light–matter interaction

Watch a resonance model between an optical field and a two-level system: vary field strength and noise to see the energy exchange. Relate observations to the resonance condition ℏω=E2−E1\hbar\omega=E_2-E_1 and stimulated emission.

Advanced

⚠ Real lasers can permanently damage the retina even at low power — never look into the beam or its reflections.

Equipment

  • Field–two-level-atom interaction model (panel 1)
  • «Parameter» slider (field strength) and «Noise» slider (decoherence)
  • Time-evolution plot with the «Run time» checkbox

Procedure

  1. Set the resonant condition

    In panel 1, set «Noise» to 0 and sweep «Parameter» from low to high. Notice the two-level system's response peaks only when the field frequency matches the level spacing — a stand-in for ℏω=E2−E1\hbar\omega=E_2-E_1.

  2. Follow the energy exchange in time

    In panel 2, keep «Run time» checked, set «Physical parameter» mid-range, and watch the population oscillate between levels — the signature of Rabi oscillations. Raise the parameter to see the exchange rate grow with the field amplitude.

  3. Add noise to break coherence

    Raise «Noise» gradually while keeping the field strength fixed. Watch the population oscillation die out: decoherence and noise scramble the phase, destroying resonance — why real lasers need a resonant cavity and a steady population inversion.

  4. Predict the laser threshold

    Find the parameter value where the response shifts from weak to clearly amplified, then explain: once inversion is large enough, stimulated emission beats absorption and light is amplified. Change the initial conditions and predict the new threshold before checking.

Simulation

Experiment history

Einstein introduced stimulated emission in 1917 when he rederived Planck's law statistically: an incoming photon can induce an excited atom to emit a second, phase-matched photon. The idea lay dormant three decades until Charles Townes, Nikolai Basov, and Alexander Prokhorov independently proposed microwave masers (1954); Townes and Arthur Schawlow then extended the concept to light in their landmark 1958 paper. Theodore Maiman operated the first laser — a pulsed ruby device — at Hughes Research Laboratory in 1960. Within a decade the laser went from «a solution looking for a problem» to a quantitative instrument: high-resolution spectroscopy, atom trapping, optical communications, and surgery. The 1964 Nobel Prize honored Townes, Basov, and Prokhorov; later prizes (1981, 1997, 2005, 2018) rewarded ever finer control of light–matter interaction.

Related physicists

Niels BohrRichard Feynman

Related library topics

Lasers and light–matter interaction