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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.

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goals
Goals
  • Basic Laser Principles
  • Techniques for generating pulses
    • Pulse Lengthening
    • Pulse Shortening
  • Ultrafast Experiments
    • Transient Absorption Spectroscopy
l a s e r
L.A.S.E.R

Light Amplification by Stimulated Emission of Radiation

basic laser

I

I0

I1

I3

I2

Laser medium

R = 100%

R < 100%

Basic Laser
  • Light Sources
  • Gain medium
  • Mirrors

R. Trebino

gain medium
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
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

3

3

2

N2

Fast decay

Fast decay

2

2

Laser

Pump Transition

Laser Transition

Pump Transition

Laser Transition

N1

1

1

1

Fast decay

0

Obtaining Population Inversion

2-level system

3-level system

4-level system

Population Inversion is obtained for ΔN < 0 (ΔN = N1 – N2)

summary basic laser

3

Fast decay

2

Pump Transition

Laser Transition

1

Fast decay

0

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 > Cavity Buildup time
slide10

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 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

quality of laser beams
Quality of laser beams

Uncertainty Principle: Δt Δν ≥ 1/4π

Irradiance vs. time

Spectrum

Long pulse

time

frequency

Short pulse

time

frequency

generating pulses
Generating Pulses
  • Q-switching
  • Mode-Locking
    • Passive
    • Active
  • Pulse Shortening
    • Group Velocity Dispersion
  • Pulse Lengthening - Chirp
q switching

Output intensity

100%

Cavity Gain

Cavity Loss

0%

Time

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

mode locking
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 mode locking

1000

Passive mode locking

Shortest Pulse Duration (fs)

Colliding pulse mode locking

100

Intra-cavity pulse compression

10

Ti-Sapphire

'65

'70

'75

'80

'85

'90

'95

Year

Mode-Locking Methods
  • Active – Mechanical Shutters
    • Acousto-Optic Switches (low gain lasers)
    • Synchronous Pumping
  • Passive
    • Colliding Pulse
    • Additive Pulse
    • Kerr Lens
pulse lengthening and shortening

The longer wavelengths traverse more glass.

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:

pump probe experiment

The excite pulse changes the sample absorption seen by the probe pulse.

Excite pulse

Slow

detector

Sample

Probe pulse

Lens

Change in probe pulse energy

Delay

Delay

Pump-Probe Experiment

R. Trebino

white light generation

Generally, small-scale self-focusing occurs, causing the beam to breakup into filaments.

White-Light Generation

n(ν) = n0(ν) + n2(ν)I(ν)

R. Trebino

types of experiments
Types of Experiments
  • Transient Absorption
  • Fluorescence Upconversion
  • Time Resolved IR
  • Transient Coherent Raman and Anti-Stokes Raman
  • Transient photo-electron spectroscopy
transient absorption model system
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
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

Molar Absorptivity (M-1cm-1 x 103)

Photoluminescence Intensity (au)

Ligand Field Abs

Excited States:

2E, 4T2

2LMCT, 4LMCT

Wavelength (nm)

cr acac 3
Cr(acac)3

Ligand Field Transient Absorption

100 fs excitation at 625 nm

Kinetic Data

Full Spectrum Data

480 nm probe

τ = 1.09 ± 0.06 ps

Red is single wavelength data at Δt = 5 ps

Blue is nanosecond data at 90 K

Long Lived = 2E state

cr acac 323
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 324
Cr(acac)3

Jablonski Diagram

fe ii polypyridyl complexes
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
[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 excitedstate in low spin complex

fe tren 6 h py 3 2
[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 3 228
[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

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
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
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
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”
slide32

A dye’s 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

Pump Transition

Laser 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

saturable absorber

k = 3

Short time (fs)

k = 1

k = 2

k = 7

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