ultrafast laser physics - eth zürich - homepage | eth zürich · pdf...
Post on 23-Mar-2018
225 Views
Preview:
TRANSCRIPT
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"xc
# "xS(!x)" S(# )
!x !
Differential Transmission Spectroscopy
Laserbeam splitter chopper
!c"t
probe
pump
device under test
photo detector (PD)
!!c
JPD noise of probe
signalsignal = [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)
Laserbeam splitter chopper
!c"t
probe
pump
device under test
photo detector (PD)
!!c
JPD noise of probe
signalsignal = [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
SAMPLEPolarisation
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
PROBESAMPLE
Polarisation
!t
LINSE
CHOPPER bei f
PUMPPBS
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
PROBESAMPLE
Polarisation
!t
LINSE
CHOPPER bei f
PUMPPBS
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
PROBESAMPLE
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
PROBESAMPLE
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
PROBESAMPLE
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
!tLINSE
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
!tLINSE
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
SAMPLEPolarisation
k1
k2!t
LINSE
k2 - k1 NICHTLINEARER KRISTALL
!tLINSE
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
+VI (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
Log(
Stre
ngth
)
Plateau Cutoff
Spectrum of harmonics
ti
tr
!1 ! 2Short
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 average power
Attosecond domain: nJ pulses at 1 kHz 1 !W average power
Streaking techniques instead of pump-probe attosecond resolved measurements strongly signal-to-noise limited use phase sensitive techniques instead:
energy streaking: mapping time to energy angular streaking: mapping time to angular momentum
attosecond pulse synchronized with strong infrared field strong infrared field can be used for streaking
[1] R. Kienberger et al., Science, 207, 1144 (2002) [2] R. Kienberger et al., Nature, 427, 817 (2006) [3] E. Goulielmakis et al., Science, 305, 1267 (2004)
energy streaking: mapping time to energy (linear polarized streaking field)
Attosecond streak camera •$ Most versatile and most successful technique to date:
Attosecond streak camera Hentschel et al., Nature 414, 509 (2001)
1.$ The attosecond pulse and an intense, short infrared pulse are overlapped in/on a medium being studied – they can be delayed with respect to each other
2.$ The attosecond pulse ionizes the medium
3.$ The vector potential of the infrared pulse shifts the resulting electron spectrum in energy as a function of the relative delay
Measured at ETH, 2012
Streaking techniques instead of pump-probe attosecond resolved measurements strongly signal-to-noise limited use phase sensitive techniques instead:
energy streaking: mapping time to energy angular streaking: mapping time to angular momentum
attosecond pulse synchronized with strong infrared field strong infrared field can be used for streaking
angular streaking: mapping time to angular momentum (circular polarized)
5 fs
time measurement =
angle measurement
no as pulses!
P. Eckle, A. Pfeiffer, C. Cirelli, A. Staudte, R. Dörner, H.-G. Muller, M. Büttiker, U. Keller, Science 322, 1525, 2008
Delay in tunnel ionization
30!
A. S. Landsman, M. Weger, J. Maurer, R. Boge, A. Ludwig, S. Heuser, C. Cirelli, L. Gallmann, U. Keller Optica 322, 1525 (2008)
How long does it take for an electron to traverse the tunneling barrier in tunnel-ionization of helium?
top related