Femtosecond lasers

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Femtosecond lasers
István Robel Department of Physics and Radiation
Laboratory University of Notre Dame June 22,
2005
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Outline

• Basics of lasers
• Generation and properties of ultrashort pulses
• Nonlinear effects
• second harmonic generation
• white light generation
• Amplification of short laser pulses
• Ultrafast laser spectroscopy

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Spontaneous emission
Absorption
Spontaneous emission
Ground state
Ground state

• Characteristics of spontaneous emission
• Random process
• Photons from different atoms are not coherent
• Random direction of emitted photon
• Random polarization of emitted photon

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Bosons and fermions
Two types of particles in nature bosons and
fermions

• Bosons
• Examples photons, He4 atoms, Cooper pairs
• A quantum state can be occupied by infinite many
  bosons
• Bose-Einstein condensation all bosons in a
  system will occupy the same quantum state
  (examples supeconductivity, superfluid He,
  laser)
• integer spin

• Fermions
• Examples are electrons, protons, neutrons,
  neutrinos, quarks
• Pauli exclusion principle every quantum state
  can be occupied by 1 fermion at most
• Half-integer spin

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Stimulated emission
Ground state

• The emitted photon is in the same quantum state
  as the incident photon
• same energy (or wavelength),
• same phase (coherent)
• same polarization
• same direction of propagation

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Amplification of light
I0
I gtI0
Light amplification by stimulated emission occurs
when passing through gain medium
Competing processes Absorption only possible
if an atom is not in the excited
state Spontaneous emission important if the
lifetime of the excited state is too short
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Four-level laser
fast
Molecules accumulate in this level, leading to an
inversion with respect to this level.
The four-level system is the ideal laser system.
slow
Laser transition
fast
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Basic components of a laser
Ioutput

• General characteristics of laser radiation
• Coherent (typical coherence length 1m)
• Monochromatic (Dl/l10-6)
• Directional (mrad beam divergence )
• Polarized

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Time scales in nature

• Shortest event ever measured (indirectly) decay
  of tau-lepton 0.4x10-24 s
• Period of nuclear vibrations 0.1x10-21s
• Shortest event ever created 250 attosecond
  (10-18s) x-ray pulse (2004)
• Bohr orbit period in hydrogen atom 150
  attoseconds
• Single oscillation of 600nm light 2 fs (10-15s)
• Vibrational modes of a molecule ps timescale
• Electron transfer in photosynthesis ps timescale
• Period of phonon vibrations in a solid ps
  timescale
• Mean time between atomic collisions in ambient
  air 0.1 ns (10-9s)
• Period of mid-range sound vibrations ms

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Ultrashort laser pulses
Heisenberg uncertainty principle DtDn1
e.g. for a 150fs pulse Dn7THz (e.g. n600THz
_at_ l500nm) Dl6nm wavelength spread _at_ l500nm
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Frequency modes of the laser cavity
Frequency modes of the laser cavity due to the
spatial confinement
e.g. for a 1m long cavity Dn1.5GHz DE0.6meV Dl
0.001A
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Generation of short pulses by mode-locking
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Mode-locking by non-linear polarization rotation

• The polarization of very high intensity pulses
  is rotated when passing through a nonlinear
  medium
• Using a polarizer low energy pulses can be
  filtered out, only the high energy mode-locked
  pulse gets amplified

Nelson et al Appl. Phys. B 65, 277-294 (1997)
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Group velocity dispersion Chirp
In a medium different frequencies propagate with
different velocities
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Pulse compression

• Spatial separation of different frequencies
• Longer optical path for the frequencies that are
  ahead
• Recombination of different frequencies in a
  short pulse

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Amplification of short laser pulses
pump
Output
Laser oscillator
Amplifier medium
R100
Rlt100

• Difficulties
• beam only passes once through amplifier medium
• Output intensity is changing in every roundtrip
  and intensity is lower than in cavity

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Pockels cell and cavity dumping
The Pockels cell is a material that rotates the
polarization of light if a voltage is applied on
it
If V 0, the pulse polarization doesnt change.
If V Vp, the pulse polarization switches to its
orthogonal state.
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Regenerative amplifier
M mirror TFP thin film polarizer FR Faraday
rotator PC Pockels cell

• Amplification of the seed pulse
• Seed pulse has to be injected when gain is
  maximal
• Has to be ejected when pulse height and
  stability is maximal

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Chirped Pulse Amplification

• Pulse is stretched first to avoid high intensity
  artifacts in the amplifier
• Amplified pulse is compressed to obtain the
  short pulse duration

Oscillator
Stretcher
Amplifier
Compressor
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Nonlinear Optics
Higher frequencies occur due to the non-linear
response of the material at high intensities
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Self phase modulation and white light continuum
A wide range of frequencies is generated with a
short, intense pulse
775 nm, 150 fs pulse in sapphire crystal
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The Clark CPA-2010 Laser System
Parameters Wavelength of fundamental 775
nm Pulse duration 150 fs Pulse energy 1mJ Power
per pulse 7 GW Repetition rate 1KHz Wavelength
of second harmonic 387 nm Pulse duration 150
fs Pulse energy 0.25mJ
Er doped fiber oscillator 25KHz l1.55mm Pumped
with Cw diode laser l1mm P150mW
Pulse compressor
Second Harmonic Generation
Pulse Stretcher
First Level
TiSapphire Regenerative amplifier
Pockels cell with HV supply and delay timer
Pulse compressor
Second and Third harmonic
Output
NdYAG pump laser
Second Level
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Transient absorption spectroscopy

• Varying the delay between excitation pulse and
  probe pulse results time-dependent measurement of
  phenomenon
• Time resolution is limited by the length of the
  excitation pulse

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Experimental Setup Pump-Probe configuration

• Sample is excited by short laser pulse (pump)
• Differential absorbance of the sample is
  measured by a delayed second pulse (probe)
• Time dependence is measured by changing the
  delay of the probe pulse

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Femtosecond Transient Absorption Spectroscopy at
NDRL
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Applications of pulsed lasers

• Time dependent measurements of
• Thermalization of hot electron in a metal or
  semiconductor
• Electron-phonon heat transfer
• Decay of surface plasmon oscillations
• Quantum beats
• Electron transfer processes
• Exciton lifetime in semiconductors
• Charge carrier relaxation in semiconductors
• Electron- and energy transfer in molecules
• Photoinduced mutations in DNA

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Resources and References
R. Trebino, Frequency-resolved Optical Gating
The Measurement of Ultrashort Laser Pulses, Book
News Inc., (2002) R. Trebino, Lectures in Optics
(Georgia Tech Lecture Notes) K. Ekvall, Time
Resolved Laser Spectroscopy, Ph.D. Thesis, RIT
Stockholm, (2000) B. B. Laud, Lasers and
Non-Linear Optics, Wiley, (1991) CPA 2010 Users
Manual, Clark-MXR Inc, (2001) W. Demtröder,
Laser spectroscopy, Springer, 1998 Ultrashort
Laser Pulse Phenomena