Aucun titre de diapositive

1
Attosecond Light Pulses
Bandwidth, bandwidth, bandwidth UV
wavelength Possible routes Raman
scattering High-harmonic generation Measuremen
t of attosecond pulses
Sources The physics of attosecond light pulses,
Pierre Agostini and Louis F DiMauro Rep. Prog.
Phys. 67 (2004) 813 Reinhard Kienberger1, M.
Hentschel1, M. Drescher1,2, G. Reider1, Ch.
Spielmann1, Ferenc Krausz1 1Institut für
Photonik, Technische Universität Wien,
AUSTRIA 2Fakultät für Physik, Universität
Bielefeld, GERMANY
2
Why try to make attosecond pulses?
Molecular vibrations can be very fast!
Characteristic time scale Vibrational
oscillation period 7 fs (H2)
3
Why try to make attosecond pulses?
Characteristic time scale Bohr-orbit time in
hydrogen 152 attoseconds
4
Tracing the Evolution of Microscopic Enviroment
of Atoms in Molecular Reactions
t0
tgt0
Wkin(Auger)
Wkin(photo)
Energy levels of a selected atom
X-ray fluorescence
chemical shift ?W
M. Schnürer et al., PRL 85, 3392 (2000)
5
X-ray Excite-Probe Spectroscopy
With sub-fs x-ray pulses, we could trace the
inner shell relaxation processes
6
Bandwidth
The uncertainty principle requires that Dt Dn gt
1 So for a sub-fs pulse, Dn gt 1015 Hz This is a
lot!
7
Attosecond pulses are short-wavelength pulses.
The shortest possible pulse of a given wavelength
is one cycle long. If you compress a
single-cycle pulse of one wavelength, you change
its wavelength!
A single-cycle red pulse
A single-cycle 800-nm pulse has a period of 2.7
fs. To achieve a period of 1 fs requires a
wave-length of 300 nm.
A compressed single-cycle pulse, which is now
violet
8
An Infinite Train of Pulses and its Fourier
Transform

• An infinite train of identical pulses can be
  written

E(t) III(t/T) f(t)
where f(t) represents a single pulse and T is the
time between pulses. Its Fourier transform
A series of modes, and the shorter the pulse the
broader the spectrum.
9
Mode-locked vs. non-mode-locked light
Mode-locked pulse train
A train of short pulses
Non-mode-locked pulse train
Random phase for each mode
A mess
10
Generating short pulses mode-locking

• Locking the phases of the laser modes yields an
  ultrashort pulse.

11
Locked modes
So to make an attosecond pulse, we must generate
a massively broad set of modes (1015 Hz broad)
and then lock them in phase
12
Methods for generating many equally spaced modes
with 1015 Hz bandwidth

• Cascaded Raman scattering
• High-harmonic generation

But do they lock the modes?????
13
Raman scattering and attosecond pulses
Input two frequencies nearly resonant with a
Raman resonance.
At high intensity, the process cascades many
times.
Etc.
Input pulses
Raman processes can cascade many times, yielding
a series of equally spaced modes!
S. E. Harris and A. V. Sokolov PRL 81, 2894
14
Cascaded Raman generation
A. V. Sokolov et al. PRL 85, 85 562
This can be done with nanosecond laser pulses!
15
Experimental demonstration of cascaded Raman
scattering
Detuning from 2-photon resonance
2994 cm-1
- 400MHz
100MHz
700MHz
75,000 cm-1 (2.3 x 1015 Hz) of bandwidth has been
created!
A. V. Sokolov et al. PRL 85, 562
16
Numerical simulation of Raman scattering in D2
Propagation distance
0
1.8
3.6
z (cm)
Harris and Sokolov. PRL 81, 2894
17
Is the light produced using cascaded Raman
scattering an attosecond pulse?
Unfortunately, no one has measured the spectral
phase of this light, so we dont know its pulse
length And if you havent measured it, you
havent made it!
18
High Harmonic Generation in a gas
X-ray spectrometer
800 nm lt 1ps
detector
1015W/cm2
grating
Laser dump
HHG produces equally spaced harmonics out to as
much as the 300th harmonic, potentially as short
as 10 attoseconds!
19
High-harmonic generation has all the features
needed for attosecond pulse generation.
XUV region. Equally spaced frequencies and lots
of em. Overall very broad bandwidth (gt 1015
Hz). Spatial coherence. Reasonable stability. The
physics seems to imply that the XUV pulse should
be really short
Maybe HHG just naturally produces attosecond
pulses without even trying
20
Are the high harmonics actually in phase?
NO!
Measuring the relative phase of adjacent
harmonics
The resulting intensity vs. time
plateau
cutoff
100 fs input pulse
P. Antoine et al., PRL 77, 1234
21
HHG by sub-10fs pulses
The highest harmonics (and perhaps the shortest
XUV pulses?) have been generated using the
shortest input pulses.
Chang et al. PRL 79, 2967 Spielmann et al.
Science 278, 661
Priori et al. PRA 61,063801
22
Few-cycle-driven XUV/HHG emission is also more
efficient.
Measured XUV spectral intensity from neon
Wavelength (nm)
Note the broader harmonics from the 7-fs pulse
due to the spectrally broader excitation pulse.
M. Schnürer et al., PRL 83, 722 (1999) Ch.
Spielmann et al., Science 278, 661 (1997) Z.
Chang et al., PRL 79, 2967 (1997)
23
Theory says many-fs input pulses yield many-fs
XUV pulses, but few-fs input pulses could yield
attosecond XUV pulses
I. P. Christov, M. M. Murnane, and H. C. Kapteyn,
PRL 78, 1251, (1997)
A 5-fs input pulse in theory yields an attosec
XUV pulse
24
Theory predicts that few-cycle-driven XUV/HHG
emission consists of attosecond pulses.
Simulation
Isolated sub-femtosecond XUV pulse!
3D code N. Milosevic T. Brabec, 2001 I.
Christov et al., PRL 78, 1251 (1997) C. Kann et
al., PRL 79, 2971 (1997)
25
As with any long-sought milestone, false claims
abound.
Observation of attosecond light localization in
HHG
N. A. Papadogiannis et al. PRL 83,4289
These guys basically performed an interferometric
autocorrelation measurement, but with HHG as the
nonlinear process instead of SHG.
26
A false claim
These authors saw peaks, more indicative of the
broad spectra than a short pulse. They also saw
unphysical asymmetries, without question due to
noise. Dont believe everything you read, even
if its in a peer-reviewed scientific journal!
Observation of attosecond light localization in
HHG N. A. Papadogiannis et al. PRL 83,4289
Harmonic autocorrelation
27
HHG and the absolute phase (carrier-offset phase)
HHG is very sensitive to the peak intensity,
which is higher for a 0-abs phase (cos) than for
90-abs phase (sin).
Threshold for HHG
E(t) E(t) cos(?Lt)
E(t) E(t) sin(?Lt)
Multi cycle pulse
0
0
tfs
- 10
10
- 10
10
Input pulse instantaneous intensity
Few cycle pulse
0
0
- 5
5
- 5
5
28
Theory predicts more intense XUV for a zero
absolute phase.
A. Apolonski et al. PRL 85, 740
Intensity of the XUV harmonic radiation
Laser electric field
29
The absolute phase varies!
The intensity profile has a round-trip time of
L/vg, while the carrier wave has a round-trip
time of L/vf. If these times differ, the absolute
phase (also called the carrier-envelope offset
phase) will vary from pulse to pulse.
Pulse field vs. time
Carrier-envelope phase control of femtosecond
mode-locked lasers and direct optical frequency
synthesis, D. J. Jones et al. Science 288, 635
30
Variable absolute phase in the frequency domain
fceo (Df/2p) frep
Carrier-envelope phase control of femtosecond
mode-locked lasers and direct optical frequency
synthesis, D. J. Jones et al. Science 288, 635
The frequency of absolute-phase variation is
related to d, the extra residual frequency.
31
Stabilizing the absolute phase
If a pulse has more than one octave, it has both
f and 2 f for some frequency, f. Interfering
them in a SHG crystal yields two contributions at
2 f that from the original beam and the SH of
f. Simply measuring the spectrum is performing
spectral interferometry yields a fringe phase
Stabilizing the SH spectrum near 2 f stabilizes
the abs phase.
Apolonski et al. PRL 85 740
32
How to measure an attosecond pulse?
XUV Autocorrelation
NLO effects2-photon absorption 2-photon
ionization
t
Problemslow XUV fluxsmall sabs
focusing
NL
Kobayashi et al., Opt. Lett. 23, 64 (1998)
33
Attosecond autocorrelation
Tzallas et al (2003)
Ions are detected vs. displacement of the two
half mirrors. Theres a significant background
the contrast is less than 3.
34
Attosecond autocorrelation results
Autocorrelation traces of an attosecond pulse
train produced by harmonics 715, generated in
xenon. The XUV bursts of the train have a
duration of 780 80 as
35
RABITT (Reconstruction of Attosecond Beating by
Interference of Two-photon Transition)
RABBITT takes advantage of the interference of
the even-harmonic sidebands created when the XUV
pulse interacts with the intense IR laser pulse.
36
RABITT set up
The two SiO2 plates control the delay with
attosecond resolution.
XUV pulse is in the center of the annular IR beam.
Paul et al (2001)
37
RABITT results for a 250-as pulse
38
Attosecond streak camera
Compared to an attosecond pulse, a near-IR pulse
is slowly varying. Its field can be a linear
ramp, yielding a streak camera.
E. Constant et al., PRA 56, 3870
39
Momentum transfer depends on instant of electron
release within the wave cycle
40
Mapping time to momentum
Momentum change along the EL vector
800-nm laser electric field
?p(t7)
?p(t6)
?p(t5)
t7
t1
t2
t3
t4
t5
t6
?p(t4)
instant of electron release
?p(t3)
?p(t2)
?pi
?p(t1)
Incident X-ray intensity
0
500 as
-500 as
Optical-field-driven streak camera
J. Itatani et al., Phys. Rev. Lett. 88, 173903
(2002) M. Kitzler et al., Phys. Rev. Lett. 88,
173904 (2002)
41
Energy shift of sub-fs electron wave-packet
As we vary the relative delay between the XUV
pulse and the 800-nm field, the direction of the
emitted electron packet will vary.
tD
dN/dW
42
800-nm pulse field used for measuring attosecond
XUV pulses
Measured
Light electric field, EL(t) (107 V/cm)
Calculated from spectrum
Time t (fs)
E. Goulielmakis et al., Science 305, 1267 (2004)
43
Attosecond streak camera trace
90
80
70
Photoelectron kinetic energy eV
60
50
2
0
4
8
10
14
18
20
6
12
16
22
Delay
Dt
fs
E. Goulielmakis et al., Science 305, 1267 (2004)
44
Full characterization of a sub-fs, 100-eV XUV
pulse
Field-free spectrum
td -T0/4
td T0/4
? 250 attoseconds!!
45
Attosecond pulses conclusions
This is just the beginning. In 1980, the
shortest pulses were picoseconds long.
Femtosecond pulses were only a dream. Now, not
much later, attosecond pulses are real. Who
knows how much further well go, what well do
with them, and what well learn?