Question

A semiconductor laser operates mainly on the principle of

• A. Spontaneous emission
• B. Stimulated emission
• C. Black body radiation
• D. Photoelectric effect

Hint:

- LED = Spontaneous Emission (Incoherent light) - LASER = Stimulated Emission (Coherent light) In stimulated emission, one incoming photon goes in, and two identical, in-phase photons come out.

Answer 1

The Correct Option is B

Concept: The word LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. All lasers, including gas, solid-state, and semiconductor injection lasers, require a quantum mechanical process where an incoming photon triggers an excited electron to drop to a lower energy state, releasing an identical twin photon.

Step 1: Physics of light emission in semiconductors.
In a semiconductor laser (typically utilizing a forward-biased \(p\)-\(n\) junction made from direct bandgap materials like GaAs), electrons are injected into the conduction band and holes into the valence band. This creates a non-equilibrium state known as population inversion, where a higher density of electrons resides in the upper energy state compared to the lower energy state. There are two main ways these electrons can recombine with holes:
• Spontaneous Emission: An electron drops down randomly on its own schedule, emitting a photon with arbitrary phase and direction. This is the operating principle of a standard Light Emitting Diode (LED).
• Stimulated Emission: An existing photon whose energy exactly matches the bandgap energy (\(E_g = h\nu\)) passes close to an excited electron. This photon interacts field-wise with the electron, inducing it to transition down immediately. Crucially, the newly emitted photon shares the exact same frequency, phase, polarization, and directional vector as the stimulating photon.

Step 2: Optical feedback and amplification.
As these identical photons bounce back and forth between the polished parallel cleavage planes of the semiconductor crystal (acting as a Fabry-Pérot resonant cavity), they trigger a cascade of additional stimulated emissions. This amplification yields a coherent, monochromatic, and highly directional laser beam.

Step 3: Verifying the options.
The dominant process driving laser amplification is stimulated emission, which corresponds directly to option (B).