Laser Pulse Generation and
Ultrafast Pump-Probe Experiments
By Brian Alberding
Goals
• Basic Laser Principles
• Techniques for generating pulses
– Pulse Lengthening
– Pulse Shortening
• Ultrafast Experiments
– Transient Absorption Spectroscopy
L.A.S.E.R
Light Amplification by Stimulated Emission of Radiation
Basic Laser
• Light Sources
• Gain medium
• Mirrors
I
I0
R = 100%
I3
I1
Laser medium
I2
R < 100%
R. Trebino
Laser Cavity
Gain Medium
Einstein Coefficients
E2
AN2 = rate of Spontaneous emission
E1
E2
BN2I = rate of Stimulated emission
E1
E = hν
E2
BN1I = rate of Stimulated absorption
E1
To achieve lasing:
• Stimulated emission must occur at a
maximum (Gain > Loss)
– Loss:
• Stimulated Absorption
• Scattering, Reflections
• Energy level structure must allow for
Population Inversion
E2
E1
Obtaining Population Inversion
2-level system
2
3-level system
3
2
Fast decay
Laser
Pump
Transition
Laser
Transition
N1
1
N2
1
d N
 2 BI N  AN  AN
dt
N 
N
1  I / I sat
4-level system
3
Fast decay
2
d N
  BIN  BI N  AN  AN
dt
N  N
Pump
Transition
Laser
Transition
1
0
Fast decay

1  I / I sat
1  I / I sat
Population Inversion is obtained for ΔN < 0 (ΔN = N1 – N2)
d N
 BIN  BI N  AN
dt
N   N
I / I sat
1  I / I sat
I sat 


Summary – Basic Laser
• Source light
• Reflective Mirrors (cavity)
• Gain Media
– Energy Level Structure
– Population Inversion
3
2
Fast decay
Pump
Transition
Laser
Transition
1
0
Fast decay
• Pumping Rate ≥ Upper laser State Lifetime
• Upper laser State Lifetime > Cavity Buildup time
Types of Lasers
Solid-state lasers have lasing material distributed in a solid matrix (such
as ruby or neodymium:yttrium-aluminum garnet "YAG"). Flash lamps are
the most common power source. The Nd:YAG 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 farinfrared (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
Quality of laser beams
Uncertainty Principle:
Δt Δν ≥ 1/4π
Irradiance vs. time
Spectrum
Long pulse
time
frequency
time
frequency
Short pulse
Generating Pulses
• Q-switching
• Mode-Locking
– Passive
– Active
• Pulse Shortening
– Group Velocity Dispersion
• Pulse Lengthening - Chirp
Q-Switching
• Alternate presence of oscillating laser
beam within the cavity
•Methods
Output intensity
-Electro-optic shutter
•Pockels Cell
•Kerr Cell
Cavity Gain
-Saturable Absorber
Cavity Loss
-Rotating mirror
100%
0%
Time
•Nanosecond timescales
R. Trebino
Mode-Locking
• Technique
– Shutter between
mirror and gain
medium
– Shutter open: All
modes gain at
same time
• Types
– Active
– Passive
R. Trebino
Mode-Locking Methods
• Active – Mechanical Shutters
• Passive
– Colliding Pulse
– Additive Pulse
– Kerr Lens
Shortest Pulse Duration (fs)
– Acousto-Optic Switches (low gain lasers)
– Synchronous Pumping
1000
Active mode
locking
Passive mode locking
Colliding pulse
mode locking
100
Intra-cavity pulse
Ti-Sapphire compression
10
'65
'70
'75
'80 '85
Year
'90
'95
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
Pulse Shortening:
The longer wavelengths
traverse more glass.
Chirped Pulse
Pump-Probe Experiment
The excite pulse changes the sample
absorption seen by the probe pulse.
Excite
pulse
Delay
Lens
Change in probe
pulse energy
Probe
pulse
Slow
detector
Sample
Delay
R. Trebino
White-Light Generation
n(ν) = n0(ν) + n2(ν)I(ν)
Generally, small-scale self-focusing occurs,
causing the beam to breakup into
filaments.
R. Trebino
Types of Experiments
•
•
•
•
Transient Absorption
Fluorescence Upconversion
Time Resolved IR
Transient Coherent Raman and AntiStokes Raman
• Transient photo-electron spectroscopy
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
James McCusker (MSU):
Transition Metal Complexes
• Cr(acac)3: ~Oh, d3 complex
MLCT
Ligand Field
Emission
Ligand Field Abs
Wavelength (nm)
Photoluminescence Intensity (au)
Molar Absorptivity (M-1cm-1 x 103)
– Ligand field and charge transfer states
Ground State: 4A2
Excited States:
2E, 4T
2
2LMCT, 4LMCT
Cr(acac)3
Ligand Field Transient Absorption
100 fs excitation at 625 nm
Kinetic Data
Full Spectrum Data
480 nm probe
Red is single wavelength data at Δt = 5 ps
τ = 1.09 ± 0.06 ps
Blue is nanosecond data at 90 K
Long Lived = 2E state
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
Cr(acac)3
Jablonski Diagram
FeII polypyridyl
complexes
• Time scale of ΔS ≠ 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
[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
)
[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
τ1 = 80 ± 20 fs, τ2 = 8 ± 3 ps
ps timescale decay is
Vibrational Relaxation
[Fe(tren(6-H-py)3)]2+
~100 fs excitation at 400 nm
5T
2
state is populated in 700 fs
Other excited states decay
faster than time resolution
Vibrational Relaxation occurs
on ps timescale
ΔT = 700 fs (black line)
ΔT = 6 ps (blue line)
Calculated difference of R = CH3/R = H (red line)
Dynamics in Transition Metal
Complexes
• Relative Rates of VR, ISC, and IC can
vary depending on the system
– kISC > kVR
• Fast spin forbidden transitions
– ΔS = 1, ΔS = 2; Spin Orbit Coupling
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
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”
A dye’s energy levels
•Dyes are big molecules, and they have
complex energy level structure.
S1: 1st excited
electronic state
Energy
S2: 2nd excited
electronic state
Pump Transition
S0: Ground
electronic state
Lowest vibrational and
rotational level of this
electronic “manifold”
Excited vibrational and
rotational level
Laser Transition
Dyes can lase into any (or
all!) of the vibrational/
rotational levels of the S0
state, and so can lase very
broadband.
Intensity
Saturable Absorber
Short time (fs)
k=1
k=2
k=3
k=7
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
Absorption spectra following
oxidation and reduction
Oxidation
Reduction
Jablonski Diagram [Fe(tren(6-H-py)3)]2+