Fundamental Processes of Radiation
1. Spontaneous Emission
- Occurs when an excited atom (energy state E2) returns spontaneously to a lower state (E1) without external stimulation.
- Emits a photon of energy:
Characteristics:
- Random in direction and phase.
- Basis of ordinary light sources (e.g., light bulbs, LEDs).
2. Induced (Stimulated) Emission
- Discovered by Einstein (1917).
- Occurs when an excited atom at E2 is struck by a photon of frequency matching the energy difference (E2−E1)
- The atom emits a second photon identical to the first (same frequency, phase, polarization, and direction).
- This principle forms the basis of laser action (Light Amplification by Stimulated Emission of Radiation).
Energy Level Populations
- Normal Population:
- Most atoms are in the ground state (E1).
- Spontaneous emission dominates, so no amplification.
- Population Inversion:
- Condition where more atoms occupy excited state E2 than ground state E1.
- Achieved using pumping methods (optical, electrical, or chemical).
- Essential for laser operation because it ensures that stimulated emission > absorption.
Main Components of a Laser
- Active Medium:
- Material where amplification occurs.
- Can be gas (He-Ne, CO₂), solid (ruby, Nd:YAG), liquid (dye lasers), or semiconductor.
- Contains active centers (atoms, ions, or molecules) that undergo excitation.
- Pump Source:
- Provides external energy to achieve population inversion.
- Methods: optical pumping (flashlamp), electrical discharge, chemical reactions.
- Optical Resonator (Cavity):
- Formed by two mirrors:
- Fully reflective mirror.
- Partially transmitting mirror (output coupler).
- Photons bounce back and forth → stimulate further emissions → coherent amplification.
- Output Coupler:
- The partially transparent mirror that lets out the coherent laser beam.