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  • ETH Zurich Ultrafast Laser Physics

    Ursula Keller / Lukas Gallmann

    ETH Zurich, Physics Department, Switzerland www.ulp.ethz.ch

    Chapter 10: Ultrafast Measurements

    Ultrafast Laser Physics!

  • Ultrafast laser physics (ULP)

    1as!

    Time

    1am!

    Length

    1 picosecond = 1 ps = 10–12 s 1 femtosecond = 1 fs = 10–15 s 1 attosecond = 1 as = 10–18 s

  • Measurement with !s time resolution

    Harold E. Edgerton, MIT" 1903-1990"

    !

    Flash photography: ! Flash lights driven by electronics !

    triggered flash lights " #µs time resolution "

    (already available 1935)" limited by flash duration " („light pulse duration“)!

  • The problem

    •$ Straightforward: Measure slow event with fast event

    •$ However, all detectors are time-integrating on these time scales

    •$ Solution: Map dynamics/ time axis to static observable!

    E. Muybridge: Animal Locomotion (1887)

  • The solution

    •$ The classical pump-probe approach:

    •$ Map time to translation in space: •$ Therefore •$ 1 nm resolution in yields 7 as resolution in •$ Delay is equivalent to real time if duration of probe pulse is

    negligible and process is perfectly reproducible •$ This idea can be generalized to other mappings of time to

    time-independent quantities

    ! = 2"x c

    # "x S(!x)" S(# )

    !x !

  • Pump-Probe Measurement

    2 !z = c!t!

    !t

    !z =1 µm " !t # 2 $ 3.3 fs

  • Differential Transmission Spectroscopy

    Laser beam splitter chopper !c

    "t

    probe

    pump

    device under test

    photo detector (PD)

    !!c

    JPD noise of probe

    signal signal = [T("t, Ipump) – T(Ipump = 0)] Iprobe

    Transmission of device under test with pump on pump off

    Ultrafast Pump-Probe Techniques

    • Why a chopper?

    • Why not the chopper in the probe pulse?

    • Why do you use a lock-in amplifier?

  • Differential Transmission Spectroscopy

    Ultrafast measurements need some kind of nonlinearities in the measurement system (i.e. intensity dependent transmission)

    Laser beam splitter chopper !c

    "t

    probe

    pump

    device under test

    photo detector (PD)

    !!c

    JPD noise of probe

    signal signal = [T("t, Ipump) – T(Ipump = 0)] Iprobe

    Transmission of device under test with pump on pump off

    Ultrafast Pump-probe Techniques

  • Different arrangements

    •$Noncollinear degenerate pump-probe measurements

    •$Collinear degenerate pump-probe measurements

  • Noncollinear degenerate pump-probe

    PUMP

    PROBE

    SAMPLE Polarisation

    k1

    k2

    "Langsamer" Detektor (zeitlich gemittelt)

    !t

    LINSECHOPPER

    Noncollinear: pump and probe beam not collinear good for signal-to-noise because pump power is not on detector

    Degenerate: pump and probe pulse have the same central wavelength

  • Collinear degenerate pump-probe

    PUMP

    PROBE SAMPLE

    Polarisation

    !t

    LINSE

    CHOPPER bei f

    PUMP PBS

    PROBE

    PBS

    1

    LOCK-IN VERSTÄRKER bei f1

    What is the reason for the PBS (polarizing beam splitters) in the set-up?

  • Collinear degenerate pump-probe

    PUMP

    PROBE SAMPLE

    Polarisation

    !t

    LINSE

    CHOPPER bei f

    PUMP PBS

    PROBE

    PBS

    1

    LOCK-IN VERSTÄRKER bei f1

    Potential problem?

    PUMP

    PROBE

    SAMPLE

    Polarisation

    !t

    LINSE

    CHOPPER bei f

    PUMP

    STRAHL- TEILER

    PROBE

    1

    LOCK-IN VERSTÄRKER bei f œ f1

    CHOPPER bei f2

    2

    Detector can be saturated by strong pump beam.

    -

  • Degenerate four-wave mixing

    PUMP

    PROBE SAMPLE

    k

    k2

    1

    E

    E 1

    2

    "Langsamer" Detektor

    Polarisation => Beugungsgitter

    Why is this set-up a degenerate four-wave mixing experiment?

  • Degenerate four-wave mixing

    PUMP

    PROBE SAMPLE

    k

    k2

    1

    E

    E 1

    2

    "Langsamer" Detektor

    Polarisation => Beugungsgitter

    Parallel polarization creates a transient diffraction grating inside the sample.

    This grating exists as long as there is a coherent excitation (i.e. within the dephasing time)

    Review articles: K.-H. Pantke und J. M. Hvam, "Nonlinear quantum beat spectroscopy in semiconductors," Int. J. of Modern Physics B, 8, 73-120, 1994 E. O. Göbel, "Ultrafast Spectroscopy of Semiconductors," Festkörperprobleme, Advances in Solid State Physics, 30, S. 269-294, 1990 J. Shah, "Ultrafast Spectroscopy of Semiconductors," Springer-Verlag

  • Degenerate four-wave mixing

    PUMP

    PROBE SAMPLE

    k

    k2

    1

    E

    E 1

    2

    "Langsamer" Detektor

    Polarisation => Beugungsgitter

    PUMP

    PROBE

    SAMPLE

    Polarisation

    k1

    k2

    "Langsamer" Detektor

    !t

    LINSE

    k22 - k1

  • Optical Gating

    SIGNAL

    PROBE

    NICHTLINEARER KRISTALL

    k1

    k2

    LANGSAMER DETEKTOR

    !t LINSE

    Application: time resolved femtosecond luminescence measurement

    T. C. Damen and J. Shah, "Femtosecond luminescence spectroscopy with 60 fs compressed pulses," Applied Phys. Lett. 52, 1291, 1988

    J. Shah, "Ultrafast Luminescence Spectroscopy using sum frequency generation," IEEE JQE, 24, 276-288, 1988

  • Optical Gating: Time-of-flight imaging

    Application of optical gating for “time-of-flight” imaging

    M. R. Hee, J. A. Izatt, J. M. Jacobson, J. G. Fujimoto, "Femtosecond transillumination optical coherence tomography," Optics Lett., vol. 18, pp. 950-952, 1993

    Biologisches Gewebe

    Abtasten

    Starke Streuung durchgelassener

    Laserpuls

    Laserpuls

    Lichtanteil mit wenig Streuung => Abbildung

    SIGNAL

    PROBE

    NICHTLINEARER KRISTALL

    k1

    k2

    LANGSAMER DETEKTOR

    !t LINSE

  • Optical coherence tomography (OCT)

    How does this work?

    Science, 254, 1178, 1991!

  • Optical coherence tomography (OCT)

    Michelson Interferometer Interference only within coherence length

    Science, 254, 1178, 1991!

  • Prof. J. G. Fujimoto, MIT, USA"

    30 fs pulse duration -> 10 µm axial resoluton "

    10 fs pulse duration -> 3 µm axial resolution "

    Optical coherence tomography (OCT)

  • Time resolved four-wave-mixing

    PUMP

    PROBE

    SAMPLE

    Polarisation

    k1

    k2

    "Langsamer" Detektor

    !t

    LINSE

    k22 - k1

    How do you do time resolved four-wave mixing?

  • Time resolved four-wave-mixing

    2

    PUMP

    PROBE #1

    SAMPLE Polarisation

    k1

    k2 !t

    LINSE

    k2 - k1 NICHTLINEARER KRISTALL

    !t LINSE

    PROBE #2

    1

    2

    LANGSAMER DETEKTOR

  • Photoconductive Switching

    V

    Laserpuls

    Output

    Photoconductive switch or Auston switch: D. H. Auston, "Picosecond optoelectronic switching and gating in silicon," Appl. Phys. Lett., vol. 26, pp. 101-103, 1975 D. H. Auston, P. Lavallard, N. Sol, D. Kaplan, "An amorphous silicon photodetector for picosecond pulses," Appl. Phys. Lett., vol. 36, pp. 66-68, 1980

  • Photoconductive Switching

    OUT

    +V I (t)

    Z 0Z 0

    I (t + !)

    Photoconductive sampling gate D. H. Auston, A. M. Johnson, P. R. Smith, J. C. Bean, "Picosecond optoelectronic detection, sampling, and correlation measurements in amorphous semiconductors" Appl. Phys. Lett., vol. 37, pp. 371, 1980

  • High-order harmonic generation in gases

    Classical electron-trajectories ! Multiple trajectories with same recombination energy but different excursion time exist

    P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993) Harmonic order

    Lo g(

    S tre

    ng th

    )

    Plateau Cutoff

    Spectrum of harmonics

    ti

    tr

    !1 ! 2 Short

    trajectory Long

    trajectory

    HHG

    +

  • Laser-based HHG: “a success story”

    Challenges/Problems of laser based HHG: Low pulse repetition rates and low pulse energy! limits signal-to-noise ("5 orders of magnitude reduction)

    HHG and attosecond science

    Laser-based HHG

    Intense ultrafast Ti:sapphire CPA (!800 nm, > 300"J) pulse repetition rate: "1 kHz (moving towards 10 kHz) pulse energy center: up to 100 eV pulse energy of attosecond pulses: < nJ pulse duration: "100 as

    Femtosecond domain: nJ pulses at 100 MHz 100 mW averag

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