01-12-2012, 04:59 PM
LASER
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LASER ACTION
Laser action is based on amplification of EM waves by means of forced or induced atoms or molecules.
A laser radiation uses three fundamental phenomena when EM waves interacts with the matter namely
Spontaneous absorption
Let us consider two energy level having energy E1 & E2 resp.
The atom will remain in ground state unless some external stimulant is applied to it.
When an EM wave i.e photon of particular freq fall on it , there is finite probability that atom will jump form energy state E1 to E2.
Spontaneous emission
Consider an atom in higher state (E2).
It can decay to lower energy level by emitting photon.
Emitted photon have energy hv=E2-E1.
Life time of excited state is 10-9sec.
There are metastable state i.e. transition from this state is not allowed acc to selection rule.
There life time is 10-3 sec.
Atom in this state can’t jump to lower state at there own.
When an photon of suitable freq arrive it make the atom in metastable unstable.
The emitted photon is in coherence with incident photon.
Stimulated vs Spontaneous Emission
Stimulated emission requires the presence of a photon. An “incoming” photon stimulates a molecule in an excited state to decay to the ground state by emitting a photon. The stimulated photons travel in the same direction as the incoming photon.
Spontaneous emission does not require the presence of a photon. Instead a molecule in the excited state can relax to the ground state by spontaneously emitting a photon. Spontaneously emitted photons are emitted in all directions.
Population Inversion
The process of producing a population inversion is called pumping.
Examples:
→by lamps of appropriate intensity
→by electrical discharge
Optical Resonator
It is an set up used to obtain amplification of stimulated photons, by oscillating them back & forth between two extreme limits. Consist of:
Two plane or concave mirrors placed co-axially.
One mirror is reflecting & other is partially reflecting.
FOUR LEVEL LASER:
STEP 1- PUMPING: atoms are excited to higher energy level by providing energy from ext. source.
STEP 2- POPULATION INVERSION:
atom via radiation less decay, decays to metastable state and hence population inversion take place.
STEP 3- LASER ACTION: atom from metastable state decays to lower state by stimulated emission and hence laser action take place.
STEP 4- BACK TO GROUND STATE:
atom from excited state decays to lower state by spontaneous emission.