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Spectroscopy 2:
Electronic Transitions
CHAPTER 14
Lasers
• 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)
Fig 14.28 Transitions involved in one kind of three-level laser
51
Many ground state
molecules must be
excited
100
49
Fig 14.29 Transitions involved in a four-level laser
1
Only one ground state
molecule must be
excited for population
inversion!!
0
100
Fig 14.30 Schematic of steps leading to laser action
Active laser medium
Laser medium confined
to a cavity
Pumping creates
population inversion
coherent radiation
Each photon emitted
stimulates another
atom to emit a photon
Fig 14.42 Summary of features needed for efficient laser
action
Fig 14.30 Principle of Q-switching
Active medium is
pumped while cavity
is nonresonant
Resonance is suddenly
restored resulting in a
giant pulse of photons
Fig 14.32 The Pockels cell
(When cell is “off” cavity is
resonant)
(a) When “on”, plane-polarized
ray is circularly polarized
(b) Upon reflection from end
mirror, it re-enters Pockels
cell
(c) Ray emerges for cell planepolarized by 90o
Intensity
Fig 14.33 Mode-locking for producing ultrashort pulses
Fig 14.34 Mode-locking for producing ultrashort pulses
Table 14.4 Characteristics of laser radiation
• High power – enormous number of photons/time
•
The power density of a 1 mW laser pointer when
focused 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 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
10-7 s
3 ms
Laser medium:
Al2O3 doped with Cr3+ ions
Output: cw at ~ 20kW
Disadvantage: >50% of
population must be pumped
to 2E metastable state
Transitions involved in a Nd-YAG laser
Laser medium:
0.23 ms
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
Fig 14.43 Transitions involved in a helium-neon laser
5 mol:1 mol
Fig 14.44 Transitions involved in a argon-ion laser
Blue-green
Fig 14.45 Transitions involved in a carbon dioxide laser
Fig 14.46 Molecular potential energy curves for an
exiplex laser
Population
is always
zero
Fig 14.47 Optical absorption spectrum of
Rhodamine 6G
Fig 14.48 Dye laser configuration