Stimulated Emission and Optical Gain
Stimulated emission lets one photon induce an excited atom to emit an identical photon, and a medium with population inversion turns this into net optical gain.
Definition
The process by which an incident photon stimulates an excited atom to emit a second, identical photon, and the resulting net amplification of light, or optical gain, achieved when more atoms occupy the upper level than the lower one.
Scope
This topic covers the microscopic basis of laser action. It includes the three radiative processes of absorption, spontaneous emission, and stimulated emission, related by the Einstein coefficients; the requirement of population inversion for net amplification and why it cannot occur in thermal equilibrium; the gain coefficient and its dependence on the inversion, cross section, and lineshape; saturation of the gain at high intensity; and the pumping schemes, notably three-level and four-level systems, used to create and maintain inversion. It establishes how a medium amplifies light coherently.
Core questions
- How do absorption, spontaneous, and stimulated emission relate to one another?
- Why is population inversion necessary for optical gain?
- What determines the magnitude of the gain coefficient?
- How do pumping schemes create and sustain population inversion?
Key concepts
- absorption
- spontaneous emission
- stimulated emission
- Einstein coefficients
- population inversion
- gain coefficient
- gain saturation
- three- and four-level pumping
Key theories
- Einstein A and B coefficients
- Einstein showed in 1917 that the rates of spontaneous emission, stimulated emission, and absorption are linked by fixed ratios; the existence of stimulated emission follows from requiring consistency with thermal radiation.
- Population inversion and gain saturation
- Net amplification requires more atoms in the upper than the lower level, achievable only by pumping; as the optical intensity grows it depletes the inversion, saturating the gain and stabilizing laser output.
Clinical relevance
Optical gain is the basis of every medical laser, and the same stimulated-emission physics, deliberately depleted, underlies stimulated-emission-depletion (STED) super-resolution microscopy used in biomedical research.
History
Einstein's 1917 paper on the quantum theory of radiation introduced stimulated emission and the coefficients that bear his name. The idea was turned into a device in the 1950s when Townes and, independently, Basov and Prokhorov realized amplification by stimulated emission of microwaves, work recognized by the 1964 Nobel Prize in Physics.
Key figures
- Albert Einstein
- Charles H. Townes
- Nikolay Basov
- Aleksandr Prokhorov
Related topics
Seminal works
- salehteich2019
- svelto2010
Frequently asked questions
- Why can't a normal material in equilibrium amplify light?
- In thermal equilibrium more atoms occupy lower energy levels than upper ones, so absorption outweighs stimulated emission and light is attenuated; amplification needs a non-equilibrium population inversion produced by pumping.
- What is special about a stimulated photon?
- The photon emitted by stimulated emission matches the stimulating photon in frequency, direction, phase, and polarization, which is why the amplified light is coherent.