PHYS 17200 Lecture Notes - Lecture 15: Stimulated Emission, Spontaneous Emission, Photon
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6 Apr 2015
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PHYS 17200 - Modern Mechanics - Lecture 15_Energy Quantization
Photons
● discrete “packets” of electromagnetic energy and momentum that travel at the speed of
light
● E_photon = h*v_light = h*c/ lambda_light
○ E_photon = Energy of a photon
○ h = Planck’s constant
○ v_light = speed of photon
○ c = speed of light
○ lambda_light = wavelength of photon
Hydrogen Atom
● Internal Energy States
○ There is a wave-particle character to the explanation of the discrete internal
energy states of atoms as well.
● Quantum Model of the Interaction between atom’s charged particles and It’s
Environment (The Electromagnetic Field)
○ Absorption:
■ photon is absorbed
■ electron jumps to higher energy level
○ Spontaneous emission:
■ photon is emitted
■ electron jumps to lower energy level
○ Stimulated emission:
■ external photon causes electron jump to lower energy level
■ a photon is emitted
■ the original photon is not absorbed!
How does a laser work?
● Lasers contain atomic “oscillators” that emit identical photons, with the same energy,
phase, direction….
● Ordinary emission is “spontaneous.”
● Laser emission is “stimulated.”
● The excited atom could spontaneously decay and emit a photon, at some random time.
● A photon of energy equal to the excited state can “stimulate” the atomic oscillator to emit
its photon in phase with the resonant “driving” photon.
● If there are many atoms in the same excited state, there can be a cascade or “chain
reaction” that greatly multiplies the number of stimulated photons….
● However, ground state absorption can quench this effect. Must instead have a
“population inversion”, not the exponential “thermal” distribution of populated states
mentioned earlier.