CHEM 524

CHEM 524 - Course Outline (Sect. 3)

II.D. Laser light sources (see general Laser Handout, p.2, old but useful summary)

1. General aspects

Unique Properties: coherence, directionality, spectral purity
Stimulated emission – mechanism to amplify one transition

Temperature inverted levels—non-equilibrium population -- ni>nj, Ei>Ej
--4-level system works best,
--Cavity create standing wave—cycle light wave between mirrors,
--stimulate emission from the gain medium,
--Narrow frequency, direction, polarization

c, Properties – Types

timing: cw – continuous wave -- Pulsed – down to fs (10^-15 s)
tune: single or multiple lines -- broad band (100’s - 1000’s cm^-1)
modes: transverse (cross-section intensity distribution), TM_oo—ideal Gaussian

--longitudinal (standing wave - each mirror at a node) -- source of ultimate resolution

power: -- cw (mW to a few W) – biggest – CO₂ welders

--pulsed -- can be many MW but short times, can saturate oscillator, deplete inversion

--add amplifier to get highest powers (Star-Wars, etc)

2. Types available

a. Gas ion -- lines or narrow bands, figure L-3

Atomic -- low efficiency -- cw stable, rare gas ions, most in visible

--power is expensive/sensitive, optics

--can be intracavity doubled if high power (Fred design)

Ar 514, 488 + weak uv
Kr red + 350
HeNe 633 low power
HeCd 441
Cu blue pulse

Molecular --higher power, pulsed – 100-500 mJ/pulse

Excimer --rare gas and halogen, excited state dimer has no bound ground state,

Discharge capacitor through high pressure transverse to lasing direction

-- pulsed: high rep. rate and high power, fast deplete nj

-- Beam quality poor-- ok for transverse dye-- can be improved w/special optics

XeCl 308 , good for dye pump
XeF 351
ArF 193 , good for photochem + VUV source
KrCl 222
KrF 249
F₂ 157 , good VUV (photochem)
N₂ -- "un-laser" -- super radiance 337.1 nm, 3-5 ns pulses -- can be good dye pump, low power fluorescence

b.Solid state -- note 1^st laser was "ruby" -- red, pulsed, inefficient, (2^nd HeNe – also inefficient.)

• Nd+3 YAG dominate -- work horse of pulsed laser field – pump other device

-- 1.06 m -- high power, good efficiency

-- Normally -- flashlamp pumped-- Xe discharge/-µs -- need Q-switch to control pulse (8-12 ns)

--traditional pulsed modest rep rate (few Hz) -- now available at MHz and even cw (lower power)

-- can be diode laser pumped -- beam quality high

-- power 100’s mJ/pulse amplifier get more,

IR oscillator or -- non linear crystals another– high efficiency

-- double (532), triple (355), quadruple (266)

• Other ions and materials available, typically R.E. ion & near IR

c. IR lasers -- variously tunable

• CO₂ (and variants--CO, NO2, HCl, HBr, H2O, HF) --asymmetric stretch to bend (overtone) or sym. stretch,

--molecular vib -- rot transition (9.6-10.6 µ -- centers of bands)

--multiple line (coarse tune 2 cm^-1/ line), high pressure more continuous discharge and collision excite

high power cw or pulsed -- 4 level, fast relax, one of most efficient lasers

• Diode: vis to IR, low power , tune each over narrow band

multilayer chip (crystal), reverse bias diode – size ~1 mm cavity, beam ~f/1

Ga (In) As -- vis and near IR, moderate power (100’s mW), --fiber optic communication

Pb (Sn) Te -- near to mid IR (3-30 m) power ~1 mW (cw)– high resolution IR absorption spectroscopy, remote sense

Modes – each very narrow, separated by few cm^-1, hop between

oscillate on (5-10) at a time, add monochromator for single mode

--change composition for other regions

--each crystal tune ~100 cm^-1 by temperature (T)

--each mode tune ~2 cm^-1 by current (I) until hop

• F-center -- excite with laser, operate like dye, tune w/grating– limited (~100 cm-1)

– change x tal for bigger shift, F-center: M⁺X^- xtal e^- trap, needs to be cooled

near IR, cw

d. Tunable visible lasers

• Dye laser -- excite with pump laser (YAG double/triple or excimer) or flash lamp --or cw (Ar+)

-- pseudo four-level (fast relax vibration in ground. state.),

-- Timing: mimic time character of pump

--Tune (with grating/prism/etalon) over fluorescence band – smooth, width depend on vibronic envelope

--Big shifts -- change dye (400-700 possible, near IR very unstable)

--Relatively high efficiency (~10% of pump power)

-- transverse or longitudinal pump -- power depends on pump/very high

need amplifier stage avoid saturation,

-- Major resource for spectroscopy, resolution can be high

--Can be operated at very high resolution with accessory tuning --Designs: jet (cw), ring (traveling wave ). etalon tune modes, transverse + amplifier

• Ti: Sapphire -- solid state -- dye-like laser -- red tunability into near IR

very high efficiency and power capability

Non-linear Devices –transform – one frequency in, different ones out, but depend on high power, index match-- figure L-17

IR:

Optical parametric oscillator: w_p = w_i + w_s --LiNbO₃ typical at YAG (1-4 m)
Difference crystal: w₃ = w₁ - w₂ -- tune w₃ output by tune w₂ vs. w₁ figure L-16

UV/vis:

Sum or Doubler setup, results, : w₃ = w₁ + w₂ or w₀ = 2w_i KDP, KD^*P, BBO (uv)
Tripler (gas): w₀ = 3w_i --Typical polarizable rare gas, Xe
Raman shift-gas, setup,, multiple frequency shifts, Results, again, again,: w₀ = w_i ± nw_vib --often use H₂ w_vib ~4000 cm^-1