Lasers And Semiconductors

Question 1

Spontaneous emission

Answer 1

The emission of photons through the random de-excitation of excited atoms, without external trigger. (Light is non-directional, not in phase and randomly polarized)

Question 2

Stimulated emission

Answer 2

The process where an incident photon of appropriate energy triggers the emission of another photon from excited atoms. The emitted photon has the same energy, direction, phase and polarization as the incident photon.

Question 3

Population inversion

Answer 3

When there are more atoms in the upper energy level than in the lower energy level

Question 4

Metastable state

Answer 4

An excited stage which has a longer lifetime than other excited states. Usually an excited state has a lifetime of 10^-6 s, but a metastable state can have a lifetime of about 10^-3s

Question 5

Band theory

Answer 5

When one brings two identical atoms (with identical energy levels) close enough together the energy levels split. When a large number of atoms are brought near, the split levels are so close they make up a band.