Ultrafast Laser Physics!

Transcription

1 Ultrafast Laser Physics! Ursula Keller / Lukas Gallmann ETH Zurich, Physics Department, Switzerland Chapter 10: Ultrafast Measurements Ultrafast Laser Physics ETH Zurich

2 Ultrafast laser physics (ULP) Time 1as! 1 picosecond = 1 ps = s 1 femtosecond = 1 fs = s 1 attosecond = 1 as = s Length 1am!

3 Measurement with!s time resolution Harold E. Edgerton, MIT" "! Flash photography:! Flash lights driven by electronics! triggered flash lights " #µs time resolution " (already available 1935)" limited by flash duration " ( light pulse duration )!

4 $ Straightforward: Measure slow event with fast event The problem $ However, all detectors are time-integrating on these time scales E. Muybridge: Animal Locomotion (1887) $ Solution: Map dynamics/ time axis to static observable!

5 The solution $ The classical pump-probe approach: $ Map time to translation in space:! = 2"x c $ Therefore # "x S(!x) " S(# )!x! $ 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

6 Pump-Probe Measurement!t 2!z = c!t!!z = 1 µm "!t # 2 $ 3.3 fs

7 Ultrafast Pump-Probe Techniques Differential Transmission Spectroscopy Laser beam splitter probe pump chopper! c "t J PD device under test noise of probe photo detector (PD) signal = [T("t, I pump ) T(I pump = 0)] I probe Transmission of device under test with Why a chopper? pump on pump off Why not the chopper in the probe pulse? Why do you use a lock-in amplifier?! c signal!

8 Ultrafast Pump-probe Techniques Differential Transmission Spectroscopy Laser beam splitter probe pump chopper! c "t J PD device under test noise of probe photo detector (PD) signal = [T("t, I pump ) T(I pump = 0)] I probe Transmission of device under test with pump on pump off! c signal! Ultrafast measurements need some kind of nonlinearities in the measurement system (i.e. intensity dependent transmission)

9 Different arrangements $ Noncollinear degenerate pump-probe measurements $ Collinear degenerate pump-probe measurements

10 Noncollinear degenerate pump-probe PUMP PROBE CHOPPER LINSE "Langsamer" Detektor (zeitlich gemittelt) k 1 k 2 Polarisation!t SAMPLE 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

11 Collinear degenerate pump-probe PUMP CHOPPER bei f 1 PBS Polarisation PROBE PUMP LOCK-IN VERSTÄRKER bei f 1 PROBE!t LINSE SAMPLE PBS What is the reason for the PBS (polarizing beam splitters) in the set-up?

12 Collinear degenerate pump-probe CHOPPER bei f1 PUMP PBS Polarisation PROBE PUMP LOCK-IN VERSTÄRKER bei f 1 PROBE!t LINSE SAMPLE PBS PUMP CHOPPER bei f 1 STRAHL- TEILER Polarisation PROBE PUMP LOCK-IN VERSTÄRKER be i f œ - f 1 2!t CHOPPER bei f 2 LINSE SAMPLE PROBE Potential problem? Detector can be saturated by strong pump beam.

13 Degenerate four-wave mixing PUMP E 1 "Langsamer" Detektor k 1 PROBE E 2 SAMPLE k 2 Polarisation => Beugungsgitter Why is this set-up a degenerate four-wave mixing experiment?

14 Degenerate four-wave mixing PUMP E 1 "Langsamer" Detektor k 1 PROBE E 2 SAMPLE k 2 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, , 1994 E. O. Göbel, "Ultrafast Spectroscopy of Semiconductors," Festkörperprobleme, Advances in Solid State Physics, 30, S , 1990 J. Shah, "Ultrafast Spectroscopy of Semiconductors," Springer-Verlag

15 Degenerate four-wave mixing PUMP E 1 "Langsamer" Detektor k 1 E 2 SAMPLE k 2 PROBE Polarisation => Beugungsgitter PUMP LINSE SAMPLE k 1 PROBE Polarisation!t k 2 2k 2 - k 1 "Langsamer" Detektor

16 Optical Gating SIGNAL NICHTLINEARER KRISTALL k 1 PROBE!t LINSE k 2 LANGSAMER DETEKTOR 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, , 1988

17 Optical Gating: Time-of-flight imaging Laserpuls Starke Streuung durchgelassener Laserpuls Abtasten Biologisches Gewebe Lichtanteil mit wenig Streuung => Abbildung SIGNAL NICHTLINEARER KRISTALL k 1 PROBE 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 , 1993!t LINSE k 2 LANGSAMER DETEKTOR

18 Optical coherence tomography (OCT) Science, 254, 1178, 1991! How does this work?

19 Optical coherence tomography (OCT) Michelson Interferometer Interference only within coherence length Science, 254, 1178, 1991!

20 Optical coherence tomography (OCT) 30 fs pulse duration -> 10 µm axial resoluton " 10 fs pulse duration -> 3 µm axial resolution " Prof. J. G. Fujimoto, MIT, USA"

21 Time resolved four-wave-mixing PUMP LINSE SAMPLE k 1 PROBE Polarisation!t k 2 2k 2 - k 1 "Langsamer" Detektor How do you do time resolved four-wave mixing?

22 Time resolved four-wave-mixing Polarisation PUMP LINSE SAMPLE k 1 PROBE #1!t 1 k 2 2k 2 - k 1 NICHTLINEARER KRISTALL PROBE #2!t 2 LINSE LANGSAMER DETEKTOR

23 Photoconductive Switching Laserpuls Output V Photoconductive switch or Auston switch: D. H. Auston, "Picosecond optoelectronic switching and gating in silicon," Appl. Phys. Lett., vol. 26, pp , 1975 D. H. Auston, P. Lavallard, N. Sol, D. Kaplan, "An amorphous silicon photodetector for picosecond pulses," Appl. Phys. Lett., vol. 36, pp , 1980

24 Photoconductive Switching I (t) +V Z 0 Z 0 I (t +!) OUT 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

25 High-order harmonic generation in gases Spectrum of harmonics Log(Strength) Plateau Cutoff Harmonic order P. B. Corkum, Phys. Rev. Lett. 71, 1994 (1993) Classical electron-trajectories! Multiple trajectories with same recombination energy but different excursion time exist HHG t r + t i! 1! 2 Short trajectory Long trajectory

26 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 Laser-based HHG: a success story Femtosecond domain: nj pulses at 100 MHz 100 mw average power Attosecond domain: nj pulses at 1 khz 1!W average power Challenges/Problems of laser based HHG: Low pulse repetition rates and low pulse energy! limits signal-to-noise ("5 orders of magnitude reduction)

27 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 energy streaking: mapping time to energy (linear polarized streaking field) [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)

28 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

29 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) time measurement = angle measurement 5 fs no as pulses! P. Eckle, A. Pfeiffer, C. Cirelli, A. Staudte, R. Dörner, H.-G. Muller, M. Büttiker, U. Keller, Science 322, 1525, 2008

30 Delay in tunnel ionization How long does it take for an electron to traverse the tunneling barrier in tunnel-ionization of helium? 30! A. S. Landsman, M. Weger, J. Maurer, R. Boge, A. Ludwig, S. Heuser, C. Cirelli, L. Gallmann, U. Keller Optica 322, 1525 (2008)

31 and much more.