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Generation of ultrashort light pulses
Adriana Pálffy-Buß
Max-Planck-Institut für Kernphysik, Heidelberg
Visualising fast processes
1877 - galloping horse ms resolution
Shutter controls exposure/illumination time
Short time scales – ultrashort light pulses
Time scales
Age of the universe14 Billion years1 second Shortest pulse
ever generated - 67 as
Human/biological time scale
geological/astronomicaltime scale
CamerasLaser pulses
Nuclear time scale
Electronic timescale
Molecular timescale
Historical development
1960, T. MaimanFirst laser – short pulses
1991, W. Sibbett Kerr-lense mode locking: few fs
1961 Q-Switching – ns pulses
1964 Mode locking: ps to fs pulses
„few-cycle pulse“
Limitation - 1 cycle @ 800 nm – 2.7 fs
Attosecond pulses
Require higher frequencies – XUV, X-ray!
1987 – 1988 – ''High harmonic generation'' - HHG
fs laser pulse
atomic mediumnoble gas
as pulse
Experiment/Detector
Intensity > 10¹ W/cm²⁴
M. Ferray, A. L’Huillier et al. J. Phys. B 21, L31 (1988)
Mechanism
Semiclassical: „3-Step-Model“
P. Corkum, Phys. Rev. Lett. 71, 1994 (1993)
Phase relation between the optical fs pulse and the as pulse!
Why high harmonics?
Classical trajectories starting close to phase 0
Laser phase
Ele
ctro
n po
sitio
n
Lase
r fie
ld
„short“ and „long“ trajectories with the same energy
„Cutoff“-trajectory with maximal energy
Why high harmonics?
Classical trajectories starting close to phase 0
Laser phase
Ele
ctro
n po
sitio
n
Lase
r fie
ld
Next field max/min are starting point for further trajectories
In symmetrical atoms and fields this leads to identical recollision energies!
Superposition at detector:
only odd multiples of laser frequency „High harmonics“
HHG
„high harmonic generation“
Energy
Inte
nsity
M. Ferray, A. L’Huillier et al. J. Phys. B 21, L31 (1988)
„cutoff“-Energy
@Plateau, Fourier limit
perturbative regime Not yet HHG!
Scattering of the wave packet – 10 ³ geometrical conversion factor in 2D⁻
Realization
Many atoms instead of one - „Phase Matching“
→ index of refraction
Generation of a single as pulse
ionizes!
3,3 fs, 720 nm
Conversion factor 10⁻⁶
80 eV, < 100 as
E. Goulielmakis et al., Science 320, 1614 (2008)
Pulse duration record 67 as K. Zhao et al., Opt. Lett. 37, 3891 (2012)
Photon energy record 1.6 keV T. Popmintchev et al., Science 336, 1287 (2012)
Requires same transit time for optical and HHG pulse!
Application example
streaking
„Streaking“ - visualizing a fs pulse
Ionization on the as time scale
E. Goulielmakis, Science 305, 1267 (2004)
Summary
sub-fs pulses, as short as 67 as
10 eV – 1.6 keV
Semiclassical trajectories„3-Step“-Model
Generation of ultrashort laser pulses for observation of fast electron dynamics
HHG- High Harmonic Generation
Energy
Inte
nsi
ty
Exciting applications
Attosecond pulses, Agostini & DiMauro, Rep. Prog. Phys. 67, 813 (2004) Attosecond physics, Krausz & Ivanov, Rev. Mod. Phys. 81, 163 (2009) HHG, Kohler et al., Adv. At. Mol. Opt. Phys. 61, 159 (2012)
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