Presentation is loading. Please wait.

Presentation is loading. Please wait.

Femtosecond lasers István Robel

Similar presentations

Presentation on theme: "Femtosecond lasers István Robel"— Presentation transcript:

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

2 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

3 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

4 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

5 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

6 Amplification of light
I >I0 Light amplification by stimulated emission occurs when passing through gain medium Energy Population Inversion Molecules “Negative temperature” 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

7 Four-level laser The four-level system is the ideal laser system. fast
Molecules accumulate in this level, leading to an inversion with respect to this level. slow Laser transition fast

8 Basic components of a laser
Mirror, R = 100% R < 100% I0 I1 I2 I3 Laser medium in excited state Ioutput General characteristics of laser radiation: Coherent (typical coherence length 1m) Monochromatic (Dl/l=10-6) Directional (mrad beam divergence ) Polarized

9 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

10 Ultrashort laser pulses
Heisenberg uncertainty principle: Dt*Dn≥1 e.g. for a 150fs pulse: Dn=7THz (e.g. l=500nm) Dl=6nm wavelength l=500nm Long pulse Short pulse Irradiance vs. time Spectrum time frequency

11 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

12 Generation of short pulses by mode-locking

13 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, (1997)

14 Group velocity dispersion: Chirp
In a medium different frequencies propagate with different velocities

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

16 Amplification of short laser pulses
pump Output Laser oscillator Amplifier medium R=100% R<100% Energy levels Difficulties: beam only passes once through amplifier medium Output intensity is changing in every roundtrip and intensity is lower than in cavity

17 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 doesn’t change. V Pockels cell Polarizer R=100% If V = Vp, the pulse polarization switches to its orthogonal state.

18 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

19 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

20 Nonlinear Optics Higher frequencies occur due to the non-linear response of the material at high intensities Nonlinear polarization: P=e(c1E+c2E2+...) For a photon: Second harmonic! Phase matching condition ensures conservation of momentum:

21 Self phase modulation and white light continuum
Wavelength, nm Intensity, au A wide range of frequencies is generated with a short, intense pulse 775 nm, 150 fs pulse in sapphire crystal

22 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 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

23 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

24 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 To PC Optical Delay Rail Frequency Doubler Ocean Optics S2000 CCD Detector Sample Cell Filter Wheel Chopper CLARK -MXR CPA-2010 775 nm, 1 kHz 1 mJ/pulse (7fs -1.6 ns) Probe Pump Ultrafast Systems

25 Femtosecond Transient Absorption Spectroscopy at NDRL

26 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

27 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

Download ppt "Femtosecond lasers István Robel"

Similar presentations

Ads by Google