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Femtosecond lasers. István Robel Department of Physics and Radiation Laboratory University of Notre Dame June 22, 2005. Outline. Basics of lasers Generation and properties of ultrashort pulses Nonlinear effects: second harmonic generation white light generation

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femtosecond lasers

Femtosecond lasers

István Robel

Department of Physics and Radiation Laboratory

University of Notre Dame

June 22, 2005

slide2

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
slide3

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
slide4

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
slide5

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
slide6

Amplification of light

Population Inversion

“Negative temperature”

Energy

Molecules

I0

I >I0

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

slide7

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

slide8

Basic components of a laser

I0

I1

I3

I2

Laser medium in excited state

Mirror,

R = 100%

Mirror,

R < 100%

Ioutput

  • General characteristics of laser radiation:
  • Coherent (typical coherence length 1m)
  • Monochromatic (Dl/l=10-6)
  • Directional (mrad beam divergence )
  • Polarized
slide9

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
slide10

Ultrashort laser pulses

Irradiance vs. time

Spectrum

Long pulse

time

frequency

Short pulse

frequency

time

Heisenberg uncertainty principle:

Dt*Dn≥1

e.g. for a 150fs pulse:

Dn=7THz (e.g. n=600THz @ l=500nm)

Dl=6nm wavelength spread @ l=500nm

slide11

Frequency modes of the laser cavity

Frequency modes of the laser cavity

due to the spatial confinement:

e.g. for a 1m long cavity:

Dn=1.5GHz

DE=0.6meV

Dl=0.001A

slide13

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. B65, 277-294 (1997)

slide14

Group velocity dispersion: Chirp

In a medium different frequencies propagate with different velocities

slide15

Pulse compression

  • Spatial separation of different frequencies
  • Longer optical path for the frequencies that are “ahead”
  • Recombination of different frequencies in a short pulse
slide16

Amplification of short laser pulses

Energy levels

pump

Output

Laser oscillator

Amplifier medium

R=100%

R<100%

  • Difficulties:
  • beam only passes once through amplifier medium
  • Output intensity is changing in every roundtrip and intensity is lower than in cavity
slide17

Pockels cell and cavity dumping

V

Polarizer

R=100%

R=100%

Pockels cell

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 doesn’t change.

If V = Vp, the pulse polarization switches to its orthogonal state.

slide18

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
slide19

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

slide20

Nonlinear Optics

Nonlinear polarization:

P=e(c1E+c2E2+...)

Phase matching condition ensures conservation of momentum:

For a photon:

Second harmonic!

Higher frequencies occur due to the non-linear response of the material at high intensities

slide21

Self phase modulation and white light continuum

Intensity, au

Wavelength, nm

A wide range of frequencies is generated with a short, intense pulse

775 nm, 150 fs pulse in sapphire crystal

slide22

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

l=1.55mm

Pumped with

Cw diode laser

l=1mm

P=150mW

Pulse

compressor

Second

Harmonic

Generation

Pulse

Stretcher

First Level

Ti:Sapphire

Regenerative

amplifier

Pockels cell

with

HV supply

and delay timer

Pulse

compressor

Second and

Third

harmonic

Output

Nd:YAG pump laser

Second Level

slide23

Transient absorption spectroscopy

Unexcited medium

Excited medium

Unexcited medium absorbs heavily at wavelengths corresponding to transitions from ground state.

Excited medium absorbs weakly at wavelengths corresponding to transitions from ground state.

  • 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
slide24

Experimental Setup: Pump-Probe configuration

Chopper

Sample

Cell

Filter Wheel

CLARK

-MXR

CPA-2010

Pump

Probe

Optical Delay Rail

Ocean Optics

S2000 CCD Detector

(7fs -1.6 ns)

Ultrafast Systems

Frequency Doubler

775 nm, 1 kHz

1 mJ/pulse

To PC

  • 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
slide26

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
slide27

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 User’s Manual, Clark-MXR Inc, (2001)

W. Demtröder, Laser spectroscopy, Springer, 1998

Ultrashort Laser Pulse Phenomena