超快光学第24章阿秒脉冲解析

Why try to make attosecond pulses?
Bohr-orbit time in hydrogen: 152 attoseconds
Molecular vibrations can also be very fast. H2 vibrational oscillation period: ~ 7 fs
% E (w) III(wT / 2 ) F (w
-4
-2
0
2
4
wT/2
A series of modes, and the shorter the pulse the broader the spectrum.
Generating short pulses = Mode-locking
time
frequency
So for a sub-fs pulse, Dn > 1015 Hz. This is a lot!
Imagine a half-cycle pulse. Its electric field looks like this:
Electric field
Single-cycle pulses
Because this pulse field has nonzero area, it has an w = 0 component—which has infinite wavelength! And this wavelength diffracts away immediately:
0 Time
Locking vs. not locking the பைடு நூலகம்hases of the laser modes (frequencies)
Intensity vs. time Random phases Light bulb
Time
Intensity vs. time Locked phases Ultrashort pulse!
Attosecond pulses are short-wavelength pulses.
A single-cycle red pulse:
A single-cycle 800-nm pulse has a period of 2.7 fs.
t (and z)
To achieve a period of 1 fs requires a wavelength of 300 nm.
Sources: The physics of attosecond light pulses, Pierre Agostini and Louis F DiMauro Rep. Prog. Phys. 67 (2004) 813. Chang Hee NAM and Kyung Taec Kim 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
Compressing a single-cycle red pulse:
If you compress a single-cycle pulse of one wavelength, you necessarily reduce its wavelength!
The Fourier transform of an infinite train of pulses
Sub-fs X-ray pump
Sub-fs X-ray probe
Bandwidth
The uncertainty principle requires that Dt Dn > 1.
Irradiance vs. time
Long pulse
Spectrum
time
frequency
Short pulse
An infinite train of identical pulses can be written: E(t) t
E(t) = III(t/T) * f(t)
-3T
-2T
-T
0
T
2T
3T
where f(t) represents a single pulse and T is the time between pulses. The Convolution Theorem states that the Fourier Transform of a convolution is the product of the Fourier Transforms. So: F {f(t)} F {E(t)}
zR w2 / 0
Electric field
Such a pulse immediately evolves into a single-cycle pulse:
0
Time
The shortest possible pulse of a given wavelength is one cycle long.
Single-cycle and attosecond light pulses
Bandwidth, bandwidth, bandwidth… UV wavelength… Possible routes:
Raman scattering High-harmonic generation
Measurement of attosecond pulses
X-ray Excite-Probe Spectroscopy
With sub-fs x-ray pulses, we could trace inner shell relaxation X-Ray Pump / X-Ray Probe Spectroscopy processes.
t =0
t >0
Time
Methods for generating many equally spaced modes with 1015 Hz bandwidth