Class 12 > Unit # 26: Atomic Physics > LASER

Talha's Physics Academy

Spontaneous Emission, Stimulated Emission, Population Inversion & Laser Action

Video Lecture: LASER & Emission Processes

Watch the complete lecture detailing laser principles, optical pumping, metastable states, and stimulated emission:

Watch directly on YouTube (https://youtu.be/c0zIcp7WbKQ)

LASER: Light Amplification by Stimulated Emission of Radiation

“When an atom or molecule retains its excess energy until stimulated or induced to emit the energy in the form of light, which is known as LASER.”

Lasers are designed to produce and amplify this stimulated form of light into intense and focused beams. Compared to conventional sources of ordinary light, laser light is highly intense, monochromatic (single wavelength), and emitted in a unidirectional, coherent beam limited by diffraction.

Core Principles and Mechanisms

1. Stimulated or Induced Absorption: When an atom in its ground state absorbs an incident photon of energy $\Delta E = h\nu$, it transitions into one of its allowed excited states. The lifetime of an atom in a typical excited state is approximately $10^{-8}\text{ seconds}$.
2. Spontaneous Emission: The process of photon emission by an excited atom without any external influence is called spontaneous emission. In an excited state, the lifetime is very short, so the probability that the atom in state $E_2$ will return to the lower state $E_1$ by spontaneously emitting radiation ($E = h\nu$) is very high. The emitted radiation is incoherent with respect to other photons.
3. Stimulated Emission: If an atom is in an excited state $E_2$, the action of external radiation with a frequency $\nu$ can force (induce or stimulate) a transition back to the ground state, releasing a secondary photon with the exact same energy. These two identical photons are exactly in phase and coherent. When a cascade of stimulated emissions occurs, light amplification takes place, producing a beam that is coherent, monochromatic, and parallel.
4. Population Inversion: Population inversion is the key to producing laser light: the process of achieving a greater population of electrons in a higher energy state compared to a lower energy state. Under normal thermal equilibrium conditions, the number of electrons in the lower energy state ($E_1$) is always greater than in the higher energy state ($E_2$). Population inversion cannot be achieved in a simple two-energy-level system.
5. Metastable States: Consider a system with three energy levels ($E_1 < E_2 < E_3$). The intermediate energy level $E_2$ is referred to as a metastable state, characterized by a much longer lifetime of about $10^{-3}\text{ seconds}$ (roughly 100,000 times longer than standard excited states).
6. Optical Pumping: When light energy equal to the energy difference between $E_3$ and $E_1$ ($E_3 - E_1$) is supplied, electrons in the lower ground state ($E_1$) gain sufficient energy and jump into the higher excited state ($E_3$). This process of supplying external energy is called optical pumping.
7. Laser Action: Because of the short lifetime ($10^{-8}\text{ s}$) of state $E_3$, only a small number of electrons accumulate there; they rapidly undergo a radiationless transition down to the metastable state $E_2$. In the metastable state $E_2$, electrons remain for a significantly longer period ($10^{-3}\text{ s}$), causing the population entering $E_2$ to exceed the population leaving it. As a result, a large number of electrons accumulate in the metastable state. Once population inversion is achieved, exposing state $E_2$ to a triggering beam of photons induces stimulated emission, producing intense laser light.

Fig: Energy level diagram illustrating optical pumping, population inversion, and laser action.

© 2026 Talha's Physics Academy. All rights reserved.