Laser Pulse Generation and Ultrafast Pump-Probe Experiments

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Laser Pulse Generation and Ultrafast Pump-Probe
Experiments
By Brian Alberding
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Goals

• Basic Laser Principles
• Techniques for generating pulses
• Pulse Lengthening
• Pulse Shortening
• Ultrafast Experiments
• Transient Absorption Spectroscopy

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L.A.S.E.R
Light Amplification by Stimulated Emission of
Radiation
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Basic Laser

• Light Sources
• Gain medium
• Mirrors

R. Trebino
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Laser Cavity
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Gain Medium
Einstein Coefficients
E2
AN2 rate of Spontaneous emission
E1
E2
BN2I rate of Stimulated emission
E1
E h?
E2
BN1I rate of Stimulated absorption
E1
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To achieve lasing

• Stimulated emission must occur at a maximum (Gain
  gt Loss)
• Loss
• Stimulated Absorption
• Scattering, Reflections
• Energy level structure must allow for Population
  Inversion

E2
E1
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Obtaining Population Inversion
2-level system
3-level system
4-level system
Population Inversion is obtained for ?N lt 0 (?N
N1 N2)
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Summary Basic Laser

• Source light
• Reflective Mirrors (cavity)
• Gain Media
• Energy Level Structure
• Population Inversion
• Pumping Rate Upper laser State Lifetime
• Upper laser State Lifetime gt Cavity Buildup time

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Types of Lasers
Solid-state lasers have lasing material
distributed in a solid matrix (such as ruby or
neodymiumyttrium-aluminum garnet "YAG"). Flash
lamps are the most common power source. The
NdYAG laser emits infrared light at 1.064 nm.
Semiconductor lasers, sometimes called diode
lasers, are pn junctions. Current is the pump
source. Applications laser printers or CD
players. Dye lasers use complex organic dyes,
such as rhodamine 6G, in liquid solution or
suspension as lasing media. They are tunable over
a broad range of wavelengths. Gas lasers are
pumped by current. Helium-Neon lases in the
visible and IR. Argon lases in the visible and
UV. CO2 lasers emit light in the far-infrared
(10.6 mm), and are used for cutting hard
materials. Excimer lasers (from the terms
excited and dimers) use reactive gases, such as
chlorine and fluorine, mixed with inert gases
such as argon, krypton, or xenon. When
electrically stimulated, a pseudo molecule
(dimer) is produced. Excimers lase in the UV.
R. Trebino
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Quality of laser beams
Uncertainty Principle ?t ?? 1/4p
Irradiance vs. time
Spectrum
Long pulse
time
frequency
Short pulse
time
frequency
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Generating Pulses

• Q-switching
• Mode-Locking
• Passive
• Active
• Pulse Shortening
• Group Velocity Dispersion
• Pulse Lengthening - Chirp

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Q-Switching

• Alternate presence of oscillating laser beam
  within the cavity

• Methods
• Rotating mirror
• Saturable Absorber
• Electro-optic shutter
• Pockels Cell
• Kerr Cell
• Nanosecond timescales

R. Trebino
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Mode-Locking

• Technique
• Shutter between mirror and gain medium
• Shutter open All modes gain at same time
• Types
• Active
• Passive

R. Trebino
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Mode-Locking Methods

• Active Mechanical Shutters
• Acousto-Optic Switches (low gain lasers)
• Synchronous Pumping
• Passive
• Colliding Pulse
• Additive Pulse
• Kerr Lens

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Pulse Lengthening and Shortening
Group Velocity Dispersion The velocity of
different frequencies of light is
different within a medium.
Pulse Lengthening
Ultrashort Pulse
Any Medium
Chirped Pulse
Pulse Shortening
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Pump-Probe Experiment
R. Trebino
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White-Light Generation
n(?) n0(?) n2(?)I(?)
R. Trebino
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Types of Experiments

• Transient Absorption
• Fluorescence Upconversion
• Time Resolved IR
• Transient Coherent Raman and Anti-Stokes Raman
• Transient photo-electron spectroscopy

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Transient Absorption Model System

• Vibrational Relaxation (VR), Intersystem Crossing
  (ISC), and Internal Conversion (IC)
• Aspects of VR
• Pump wavelength dependence
• Density of states
• Probe wavelength dependence
• Franck-Condon Factors
• Full-spectrum, Kinetic trace
• Needed Information
• Steady State absorption and emission
• geometry
• Electron configuration

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James McCusker (MSU) Transition Metal Complexes

• Cr(acac)3 Oh, d3 complex
• Ligand field and charge transfer states

Ligand Field Emission
MLCT
Ground State 4A2
Photoluminescence Intensity (au)
Molar Absorptivity (M-1cm-1 x 103)
Excited States 2E, 4T2 2LMCT, 4LMCT
Ligand Field Abs
Wavelength (nm)
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Cr(acac)3
Ligand Field Transient Absorption 100 fs
excitation at 625 nm
Kinetic Data
Full Spectrum Data
480 nm probe t 1.09 0.06 ps
Red is single wavelength data at ?t 5 ps Blue
is nanosecond data at 90 K Long Lived 2E state
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Cr(acac)3
Ligand Field Transient Absorption 100 fs
excitation at 625 nm
Characteristic of Vibrational Relaxation
Pump Wavelength Dependence
C1 initial Abs amplitude a0 Long time offset
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Cr(acac)3
Jablonski Diagram
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FeII polypyridyl complexes

• Time scale of ?S ? 0 transitions
• Fe(tren(6-R-py)3)2
• d6 complex, Oh geometry
• R H Low Spin, 1A1 ground state
• R CH3 High Spin, 5T2 ground state

tren(py) tris(2-pyridylmethyliminoethyl)amine
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Fe(tren(6-R-py)3)2 Complexes Steady State
Absorption
R H
R CH3 similar to Fe(tren(6-H-py)3)2
ground state
Calculated Difference Middle Top (
) Nanosecond Data (dotted line)
Provides template for 5T2 excited state in low
spin complex
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Fe(tren(6-H-py)3)2
100 fs excitation at 400 nm
LMCT excitation fs timescale decay Bleach at long
times
R CH3 (5T2) No Abs at 620 nm R H (1A1)
Abs at 620 nm
620 nm Probe t1 80 20 fs, t2 8 3 ps
ps timescale decay is Vibrational Relaxation
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Fe(tren(6-H-py)3)2
100 fs excitation at 400 nm
5T2 state is populated in 700 fs Other excited
states decay faster than time resolution Vibration
al Relaxation occurs on ps timescale
?T 700 fs (black line) ?T 6 ps (blue
line) Calculated difference of R CH3/R H (red
line)
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Dynamics in Transition Metal Complexes

• Relative Rates of VR, ISC, and IC can vary
  depending on the system
• kISC gt kVR
• Fast spin forbidden transitions
• ?S 1, ?S 2 Spin Orbit Coupling

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Other Work and Applications

• Transition Metal Complexes
• Ligand Field States contribute to
  photosubstitution and photoisomerization
  processes
• Electron transfer processes and photovoltaics
• Dr. Bern Kohler DNA photodamage, skin cancer

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References

• Stimulated Emission http//hyperphysics.phy-astr
  .gsu.edu/hbase/mod5.html
• Laser Cavity http//micro.magnet.fsu.edu/primer/
  java/lasers/heliumneonlaser/index.html
• Silvfast, Laser Fundamentals, 2nd ed., Cambridge
  University Press, pg. 439-467
• J. Am. Chem. Soc., 2005, 127, 6857-6865.
• J. Am. Chem. Soc., 2000, 122, 4092-4097.
• Coordination Chemistry Reviews, 250 (2006),
  1783-1791
• Nature, 436, 25, 2006, 1141-1144.
• Rick Trebino, Georgia Tech University,
  http//www.physics.gatech.edu/gcuo/lectures/index.
  html, Optics 1 Lasers, Ultrafast Optics
  Introduction, Ultrafast Optics Pulse
  Generation, Ultrafast Optics Ultrafast
  Spectroscopy

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A dyes energy levels

• Dyes are big molecules, and they have complex
  energy level structure.

S2 2nd excited electronic state
Lowest vibrational and rotational level of this
electronic manifold
Energy
S1 1st excited electronic state
Excited vibrational and rotational level
Laser Transition
Pump Transition
Dyes can lase into any (or all!) of the
vibrational/rotational levels of the S0 state,
and so can lase very broadband.
S0 Ground electronic state
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Saturable Absorber
Intensity
Round trips (k)
Notice that the weak pulses are suppressed, and
the strong pulse shortens and is amplified.
After many round trips, even a slightly saturable
absorber can yield a very short pulse.
R. Trebino
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Absorption spectra following oxidation and
reduction
Oxidation
Reduction
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Jablonski Diagram Fe(tren(6-H-py)3)2