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.RLight Amplification by Stimulated Emission of Radiation

Basic Laser

• Light Sources • Gain medium• Mirrors

R = 100% R < 100%

I0 I1

I2I3 Laser medium

I

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Laser Cavity

Gain Medium

E1

E2

BN1I = rate of Stimulated absorption

Einstein Coefficients

E2

E1

E2

E1

BN2I = rate of Stimulated emission

AN2 = rate of Spontaneous emission

E = hν

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

satI

d NBIN BI N AN A N

dt

2

d NBI N AN A N

dt

Laser Transition

Pump Transition

Fast decay

Fast decay

1

2

3

0

2

1

N2

N1

Laser

Fast decay

Laser Transition

Pump Transition

1

23

2-level system 3-level system 4-level system

1 / sat

NN

I I

1 /

1 /sat

sat

I IN N

I I

d NBIN BI N A N

dt

/

1 /sat

sat

I IN N

I I

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

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

Laser Transition

Pump Transition

Fast decay

Fast decay

1

2

3

0

Types of LasersSolid-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.

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Quality of laser beams

Long pulse

Short pulse

Irradiance vs. time Spectrum

time

time

frequency

frequency

Uncertainty Principle: Δt Δν ≥ 1/4π

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

100%

0%Time

Cav

ity L

oss

Cav

ity G

ain

Output intensity•Methods

-Rotating mirror

-Saturable Absorber

-Electro-optic shutter

•Pockels Cell

•Kerr Cell

•Nanosecond timescalesR. Trebino

Mode-Locking• Technique

– Shutter between mirror and gain medium

– Shutter open: All modes gain at same time

• Types– Active– Passive

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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

'65 '70 '75 '80 '85 '90 '95

10

100

1000S

hort

est

Pul

se D

ura

tion

(fs)

Year

Active mode locking

Passive mode locking

Colliding pulse mode locking

Intra-cavity pulse compressionTi-Sapphire

Pulse Lengthening and Shortening

Group Velocity Dispersion – The velocity of different frequencies of light is different within a medium.

Ultrashort Pulse Any Medium Chirped Pulse

The longer wavelengths traverse more glass.

Pulse Lengthening:

Pulse Shortening:

Pump-Probe Experiment

Delay

Slow detector

Excite pulse Sample

LensProbe pulse

Cha

nge

in p

robe

pu

lse

ene

rgy

Delay

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

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White-Light Generation

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

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n(ν) = n0(ν) + n2(ν)I(ν)

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

• 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

– Ligand field and charge transfer states

Wavelength (nm)

Ph

oto

lum

ine

sc

en

ce

Inte

ns

ity (a

u)

Mo

lar

Ab

so

rpti

vit

y (

M-1c

m-1 x

10

3 )

Ground State: 4A2

Excited States:

2E, 4T2

2LMCT, 4LMCT

Ligand Field Abs

MLCTLigand Field Emission

Cr(acac)3

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

Ligand Field Transient Absorption

100 fs excitation at 625 nm

Kinetic Data Full Spectrum Data

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

620 nm Probe

τ1 = 80 ± 20 fs, τ2 = 8 ± 3 ps

LMCT excitation

fs timescale decay

Bleach at long times

R = CH3 (5T2): No Abs at 620 nm

R = H (1A1): Abs at 620 nm

ps timescale decay is Vibrational Relaxation

[Fe(tren(6-H-py)3)]2+

~100 fs excitation at 400 nm

ΔT = 700 fs (black line)

ΔT = 6 ps (blue line)

Calculated difference of R = CH3/R = H (red line)

5T2 state is populated in 700 fs

Other excited states decay faster than time resolution

Vibrational Relaxation occurs on ps timescale

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.

S0: Ground electronic state

S1: 1st excited electronic state

S2: 2nd excited electronic state

Ene

rgy

Laser Transition

Lowest vibrational and rotational level of this electronic “manifold”

Excited vibrational and rotational level

Dyes can lase into any (or all!) of the vibrational/rotational levels of the S0 state, and so can lase very broadband.

Pump Transition

Saturable Absorber

After many round trips, even a slightly saturable absorber can yield a very short pulse.

Short time (fs)

Inte

nsi

ty

Round trips (k)

k = 1

k = 7

Notice that the weak pulses are suppressed, and the strong pulse shortens and is amplified.

k = 2k = 3

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Absorption spectra following oxidation and reduction

Oxidation

Reduction

Jablonski Diagram [Fe(tren(6-H-py)3)]2+