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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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Presentation on theme: "Femtosecond lasers István Robel Department of Physics and Radiation Laboratory University of Notre Dame June 22, 2005."— Presentation transcript:

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

2 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 Outline

3 AbsorptionSpontaneous emission 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 Spontaneous emission

4 Two types of particles in nature: bosons and fermions Bosons Examples: photons, He 4 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 Bosons and fermions

5 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 Stimulated emission

6 Energy Population Inversion Molecules Negative temperature Light amplification by stimulated emission occurs when passing through gain medium I0I0 I >I 0 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 Amplification of light

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

8 Mirror, R = 100% Mirror, R < 100% I0I0 I1I1 I2I2 I3I3 Laser medium in excited state I output General characteristics of laser radiation: Coherent (typical coherence length 1m) Monochromatic ( / =10 -6 ) Directional (mrad beam divergence ) Polarized Basic components of a laser

9 Shortest event ever measured (indirectly): decay of tau-lepton 0.4x s Period of nuclear vibrations: 0.1x s Shortest event ever created: 250 attosecond ( s) x-ray pulse (2004) Bohr orbit period in hydrogen atom: 150 attoseconds Single oscillation of 600nm light: 2 fs ( s) 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 -9 s) Period of mid-range sound vibrations: ms Time scales in nature

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

11 Frequency modes of the laser cavity due to the spatial confinement: e.g. for a 1m long cavity: =1.5GHz E=0.6 eV =0.001A Frequency modes of the laser cavity

12 Generation of short pulses by mode-locking

13 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) Mode-locking by non-linear polarization rotation

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

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

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

17 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 = V p, the pulse polarization switches to its orthogonal state. V Pockels cell Polarizer R=100% Pockels cell and cavity dumping

18 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 Regenerative amplifier

19 Oscillator StretcherAmplifierCompressor Pulse is stretched first to avoid high intensity artifacts in the amplifier Amplified pulse is compressed to obtain the short pulse duration Chirped Pulse Amplification

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

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

22 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 =1.55 m Pumped with Cw diode laser =1 m P=150mW Pulse compressor Second Harmonic Generation Pulse Stretcher First Level Nd:YAG pump laser Ti:Sapphire Regenerative amplifier Pockels cell with HV supply and delay timer Pulse compressor Second and Third harmonic Second Level Output The Clark CPA-2010 Laser System

23 Unexcited mediumExcited 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 Transient absorption spectroscopy

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

25 Femtosecond Transient Absorption Spectroscopy at NDRL

26 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 Applications of pulsed lasers

27 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 Resources and References

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