Question

LASER action needs

• A. High temperature
• B. Semiconductor
• C. High pressure
• D. Number inversion

Hint:

The key to LASER action is to have more atoms "ready to emit" (in the excited state) than atoms "ready to absorb" (in the ground state). This special state is called population inversion.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
LASER stands for Light Amplification by Stimulated Emission of Radiation. The core principle behind a laser is creating a condition where stimulated emission is more probable than absorption or spontaneous emission.
Step 2: Detailed Explanation:
For light amplification to occur, there must be more atoms or molecules in a higher energy (excited) state than in a lower energy (ground or intermediate) state. This non-equilibrium condition is known as population inversion or number inversion.
When population inversion is achieved, a photon passing through the medium is more likely to trigger a stimulated emission (creating an identical photon) than to be absorbed. This leads to a chain reaction and amplification of light.
Let's analyze the other options:
(A) High temperature: Generally leads to thermal equilibrium, where lower energy states are more populated, preventing population inversion.
(B) Semiconductor: While semiconductors are used as the gain medium in diode lasers, they are not a requirement for all types of lasers (e.g., gas lasers, crystal lasers).
(C) High pressure: Not a general requirement. In gas lasers, pressure is a critical parameter but is not always high. High pressure can sometimes hinder laser action due to increased atomic collisions.
Step 3: Final Answer:
Population inversion (or number inversion) is the fundamental and necessary condition for laser action. Therefore, option (D) is correct.