spectroscopy 2: electronic transitions chapter 14
Post on 19-Dec-2015
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• Light Amplification by Stimulated Emission of Radiation
• Requirements for laser action
• Laser-active medium (e.g., gas, dye, crystal, etc)
• Metastable excited state (i.e., fairly long-lived)
• Population inversion (i.e., more in excited state)
• Cavity (for positive feedback or gain)
LasersLasers
Fig 14.28 Transitions involved in one kind of three-level laser
100
51
49
Many ground statemolecules must be
excited
Fig 14.29 Transitions involved in a four-level laser
100
1
0
Only one ground statemolecule must be
excited for populationinversion!!
Fig 14.30 Schematic of steps leading to laser action
Active laser medium
Pumping creates population inversion
Each photon emittedstimulates anotheratom to emit a photon
coherent radiation
Laser medium confinedto a cavity
Fig 14.30 Principle of Q-switching
Active medium ispumped while cavity
is nonresonant
Resonance is suddenlyrestored resulting in agiant pulse of photons
Fig 14.32 The Pockels cell(When cell is “off” cavity isresonant)
(a) When “on”, plane-polarizedray is circularly polarized
(b) Upon reflection from endmirror, it re-enters Pockelscell
(c) Ray emerges for cell plane-polarized by 90o
The power density of a 1 mW laser pointer whenfocused to a spot of around 2 um(which isn't difficult with a simple convex lens)is around... 250,000,000 W/m2 !
Table 17.4 Characteristics of laser radiation
• High power – enormous number of photons/time
• Monchromatic – essentially one wavelength
• Collimated beam – parallel rays
• Coherent – all em waves in phase
• Polarized –electric field oscillates in one plane
Types of Practical LasersTypes of Practical Lasers
(a) Solid-state lasers
e.g., Ruby, Nd-YAG, diode
(b) Gas lasers
e.g., He-Ne, Ar-ion, CO2, N2
(c) Chemical and exiplex (eximer) lasers
e.g., HCl, HF, XeCl, KrF
(d) Dye lasers
e.g., Rhodamine 6G, coumarin
Transitions involved in a ruby laser
Laser medium:
Al2O3 doped with Cr3+ ions
Output: cw at ~ 20kW
Disadvantage: >50% of
population must be pumped
to 2E metastable state
10-7 s
3 ms
103 W
/m2
Transitions involved in a Nd-YAG laser
Laser medium:
YAG doped with Nd3+ ions
Output: ~ 10 TW in sub-ns
pulses
Advantage: Only one ion in
population must be pumped
to 4F metastable state
0.23 ms
65 W/m
2