Frontier physics
Lasers and light–matter interaction
A laser relies on stimulated emission in a population-inverted medium; resonance governs energy exchange between electromagnetic fields and matter.
A laser relies on stimulated emission in a population-inverted medium; resonance governs energy exchange between electromagnetic fields and matter.
Definition: Core idea
Two levels E₁<E₂ resonate with photons hν=E₂−E₁. An incident photon can be absorbed or trigger emission of a photon with matching frequency, phase, direction, and polarization. Laser action requires gain above losses, commonly achieved by pumping a population inversion and using a feedback cavity.
Model and interpretation
Two levels E₁<E₂ resonate with photons hν=E₂−E₁. An incident photon can be absorbed or trigger emission of a photon with matching frequency, phase, direction, and polarization. Laser action requires gain above losses, commonly achieved by pumping a population inversion and using a feedback cavity.
Example: Quantitative example
A laser transition has ΔE=2.0 eV. Estimate its resonant frequency and wavelength.
Solution
ν=ΔE/h≈4.84×10¹⁴ Hz and λ=c/ν≈620 nm (orange-red light).
Quick check
In a pumped two-level medium, gain is proportional to the population inversion ; without inversion, absorption exceeds stimulated emission. A resonant photon induces a transition that emits a second photon with matching frequency, phase, and direction, while the cavity selects strongly fed-back modes. Optical Bloch equations couple the field to material polarization; when the Rabi frequency is not small compared with linewidth, nonlinear responses such as Rabi oscillations and saturation matter.
A semiclassical model treats the electromagnetic field classically but keeps quantized material levels; the dipole approximation is valid when wavelength greatly exceeds atomic size. In the weak-field limit, linear response gives frequency-dependent polarization; near resonance, absorption and dispersion change sharply. Strong fields require density-matrix dynamics to describe saturation, spontaneous emission, and mode competition.
Finite linewidth limits coherence and sets the gain bandwidth; cavity losses must be smaller than round-trip gain for self-sustained laser oscillation. Short laser pulses have broad spectra due to the time–frequency relation, enabling femtosecond pulses. Strong-field interactions also produce AC Stark shifts and multiphoton processes.
What enables gain on a two-level laser transition?
Which statement best describes “Lasers and light–matter interaction”?
References
- Peter W. Milonni, Joseph H. Eberly (2010). Laser Physics