Lasers: The Story of Stimulated Emission and Population Inversion
Understand the quantum physics behind lasers - from atomic transitions to the clever trick of metastable states that makes laser action possible.
Why Laser Physics is Crucial for JEE
LASER (Light Amplification by Stimulated Emission of Radiation) represents one of the most important applications of quantum mechanics. In JEE, laser concepts appear in:
- Direct concept-based questions (JEE Main)
- Energy level diagram problems (JEE Advanced)
- Numerical problems on population inversion
- Comparison of different laser types
Absorption: The Starting Point
🔬 What Happens:
When an atom in ground state ($E_1$) absorbs a photon of energy exactly equal to $E_2 - E_1$, it jumps to excited state ($E_2$).
Photon + Atom($E_1$) → Atom($E_2$)
Energy Level Diagram:
🎯 Key Point:
Photon Energy Must Match: $h\nu = E_2 - E_1$
The incident photon's energy must exactly match the energy difference between levels for absorption to occur.
Spontaneous Emission: Random Light
🔬 What Happens:
An atom in excited state ($E_2$) spontaneously decays to ground state ($E_1$) after an average lifetime, emitting a photon.
Atom($E_2$) → Atom($E_1$) + Photon
Characteristics:
- Random direction
- Random phase
- Incoherent light
- Different frequencies
Energy Level Diagram:
Stimulated Emission: The Laser Magic
🔬 What Happens:
An incident photon stimulates an excited atom to decay, emitting a second photon that is identical to the first.
Photon + Atom($E_2$) → 2 Photons + Atom($E_1$)
Key Features:
- Same direction as incident photon
- Same phase (coherent)
- Same frequency
- Same polarization
Energy Level Diagram:
🎯 The Amplification Process:
1 photon in → 2 photons out → 4 photons out → 8 photons out...
This chain reaction creates the intense, coherent light we call laser.
Population Inversion: The Laser Requirement
🎯 The Problem:
Under normal conditions, most atoms are in ground state. Absorption dominates over stimulated emission.
More atoms in E₁ → More absorption than emission
💡 The Solution - Population Inversion:
We need more atoms in excited state than ground state:
N₂ > N₁ (Population Inversion)
This makes stimulated emission dominate over absorption.
Metastable State: Making Inversion Possible
🔬 What is a Metastable State?
A special excited state with much longer lifetime (10⁻³ s) compared to normal excited states (10⁻⁸ s).
Why it's crucial:
- Atoms stay excited longer
- Allows buildup of excited atoms
- Makes population inversion achievable
- Enables stimulated emission to dominate
Three-Level Laser System:
🎯 Ruby Laser Example:
Step 1: Optical pumping excites Cr³⁺ ions to E₃
Step 2: Fast non-radiative decay to metastable E₂
Step 3: Atoms accumulate in E₂ (population inversion)
Step 4: Stimulated emission from E₂ to E₁ produces laser light
🚀 JEE Problem-Solving Strategies
Key Concepts to Remember:
- Stimulated emission requires population inversion
- Population inversion requires metastable state
- Laser light is coherent, monochromatic, directional
- Normal light is incoherent, polychromatic, multidirectional
Exam Tips:
- Draw energy level diagrams for visual understanding
- Remember lifetime values: normal vs metastable states
- Practice numericals on photon energy calculations
- Compare different laser types (Ruby, He-Ne, Semiconductor)
Advanced Laser Physics Available
Includes four-level systems, laser cavity modes, Q-switching, and JEE Advanced level problems
📝 Quick Self-Test
Try these JEE-level problems to test your understanding:
1. Why is population inversion necessary for laser action?
2. Compare the properties of light from spontaneous vs stimulated emission.
3. Calculate the wavelength of photon emitted when an electron jumps from energy level -1.5 eV to -3.4 eV.
Ready to Master Modern Physics?
Get complete access to laser physics, photoelectric effect, and nuclear physics for JEE