Post on 19-Jun-2020
ultra short laser pulses
creation and application
atom physics seminar – Moritz Zaiß – 26.06.07
atom physics seminar
ultra short laser pulses
• what? - why? - how?
• creation and optimisation
• typical experimental setup
• properties of existing pulsed lasers
• applications
atom physics seminar – Moritz Zaiß – 26.06.07
overview
What are ultra short laser pulses? ultra short laser pulses
pulses are called ultra short if they only consist of a few wave cycles
-0.8
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exp(-x**2)*sin(3*x)
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exp(-x**2)*sin(10*x)E
tt
One wavelength of 790nm corresponds to 2.6 fs => few cycle pulses mean short pulses
simple gaussian „gedanken“ pulses
atom physics seminar – Moritz Zaiß – 26.06.07
ultra short laser pulses
• high time resolution
to resolve eg. vibration modes in H2 molecules
• high energy densities
for plasma physics, electron motion controlling,material procession
Why do you want to create ultra short laser pulses?
atom physics seminar – Moritz Zaiß – 26.06.07
ultra short laser pulses
fourier transformation
How do you create them?
We need a spectrum with a certain bandwidth to have a fine time resolution.And we need a method to superpose the modesin a way, that a short pulse comes out.
atom physics seminar – Moritz Zaiß – 26.06.07
E(ω)
ω - ω0
creation of laser pulsesultra short laser pulses
• short pulse needs a high bandwidthestimation via uncertainty relation
exact value for gaussian pulses:
atom physics seminar – Moritz Zaiß – 26.06.07
Δω Δt ≥ 0.441
creation of laser pulsesultra short laser pulses
In wavelength this means:
we need a laser medium which amplifies wavelengths from about 740nm to 840 nm
Titan:Saphir laser: 670 to 1070 nm with maximum at 790 nm.
atom physics seminar – Moritz Zaiß – 26.06.07
creation of laser pulsesultra short laser pulses
but in the cavity of a laser there are only some wavelengths allowed→ standing waves → fabry-pérot interferometer
difference betweentwo adjacent modes
atom physics seminar – Moritz Zaiß – 26.06.07
laser gainbandwidth
frequency
inte
nsity
condition forstanding waves
frequency
inte
nsity mode structure in cavity
creation of laser pulsesultra short laser pulses
atom physics seminar – Moritz Zaiß – 26.06.07
laser gainbandwidth
frequency
inte
nsity
frequency
inte
nsity
laser output spectrum
Question: What happens if the spectrum gets discrete?
~10^9 Hz
creation of laser pulsesultra short laser pulses
surging
• independent phase of the modes=>continuous wave lasers
• locked phase of the different modes=> train of pulses
atom physics seminar – Moritz Zaiß – 26.06.07
mode lockingultra short laser pulses
active mode locking:• acousto-optic modulator with frequency f• amplitude modulation through diffractionmodulated function ~ cos(ωt)*cos(ft)exites frequencies ω -f and ω +f
addition theorem:
atom physics seminar – Moritz Zaiß – 26.06.07
cos(ωt) cos(ωt)*cos(ft)
ω -f ; ω +fω
if f = Δ ω => mode locking
mode lockingultra short laser pulses
active mode locking:
considered in time domain its shutting and opening a weak gate
the time between two pulses is given by the resonator length τ = 2L/c
atom physics seminar – Moritz Zaiß – 26.06.07
gate
mode lockingultra short laser pulses
passive mode locking:• refraction depends on intensity - Kerr effect• gaussian power distribution⇒ the refractive index experienced by the beam is greater in the centre than at the edge. ⇒ the Kerr medium works like a lens for high intensity light.
atom physics seminar – Moritz Zaiß – 26.06.07
pulse behaviour an optimisationultra short laser pulses
• dispersion of the pulses in a medium
• compensation of the dispersive effects
• pulse amplifying and optimal compression
atom physics seminar – Moritz Zaiß – 26.06.07
dispersionultra short laser pulses
k1: inverse group velocityk2: group dispersion:different wavelengths havedifferent speed => spreading of the envelope
The non-linear dispersion has the following effects
this is called upchirp
atom physics seminar – Moritz Zaiß – 26.06.07
but no change in the ω spectrum
fourier limit
ultra short laser pulses
fourier transformation
σ 1/σ
this theoretical limit is called fourier limitand the aim is to reach this limit through compressing methods
ω0
atom physics seminar – Moritz Zaiß – 26.06.07
compensation: prism compression
ultra short laser pulses
refraction of light depends on wavelength
Through the distance I you can adjust the compensation:If l becomes larger, the red beam travels a longer and longerdistance through the prism where its velocity is smaller
atom physics seminar – Moritz Zaiß – 26.06.07
compensation: grating compressionultra short laser pulses
diffraction of light depends on wavelength
atom physics seminar – Moritz Zaiß – 26.06.07
ultra short laser pulses
avoiding high peak powers in the amplifierthrough stretching
or dispersive medium (glass)
pumped Ti:S cristal
atom physics seminar – Moritz Zaiß – 26.06.07
chirped pulse amplification
a real pulse laser
ultra short laser pulses
Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser)
prismcompressor
amplifier withpockels cell
oscillatorpulse stretcher
photo diode
photo diode
pockels cell
Nd:YAG pump laser
Nd:YAG pump laser
oscillator
atom physics seminar – Moritz Zaiß – 26.06.07
a real pulse laser
ultra short laser pulses
Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser)
prismcompressor
Amplifier withpockels cell
oscillatorpulse stretcher
photo diode
photo diode
pockels cell
Nd:YAG pump laser
Nd:YAG pump laser
oscillator
atom physics seminar – Moritz Zaiß – 26.06.07
neodym doted yttrium aluminium granat solid state laser
a real pulse laser
ultra short laser pulses
Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser)
prismcompressor
amplifier withpockels cell
oscillatorpulse stretcher
photo diode
photo diode
pockels cell
Nd:YAG pump laser
Nd:YAG pump laser
oscillator
atom physics seminar – Moritz Zaiß – 26.06.07
a real pulse laser
ultra short laser pulses
Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser)
prismcompressor
amplifier withpockels cell
oscillatorpulse stretcher
photo diode
photo diode
pockels cell
Nd:YAG pump laser
Nd:YAG pump laser
oscillator
atom physics seminar – Moritz Zaiß – 26.06.07
fourier limit
ultra short laser pulses
fourier transformation
σ 1/σ
this theoretical limit is called fourier limitand the aim is to reach this limit through compressing methods
ω0
atom physics seminar – Moritz Zaiß – 26.06.07
ultra short laser pulseslaser pulses in a non-linear medium – Kerr effect
atom physics seminar – Moritz Zaiß – 26.06.07
ultra short laser pulses
⇒ frequency widening⇒ possibility to compress the pulse even more (smaller fourier limit)
laser pulses in a non-linear medium – kerr effect
pulse shape phase frequency varation
atom physics seminar – Moritz Zaiß – 26.06.07
change of the pulse in the non linear medium
increasing the bandwith
ultra short laser pulses
• n2 is small so you need long non-linear dispersive media• high intensity => use of inert gas , solid medium would be destroyed
fibre tube argon or neon gas cell
atom physics seminar – Moritz Zaiß – 26.06.07
prismcompressor
properties of todays pulsed lasersultra short laser pulses
25 fs5-10fs
~650 as
400 µJ/pulse100 µJ/pulse
~1 µJ/pulse
Ti:S λcentral: 795nmcompressed
X–ray or XUV
durationpulse energylaser type
only about 2 cycles
with fs-laserpulses there are intensitiesof about 10 W/cm possible.15 2
solar constant: 0.1366 W/cm 2
atom physics seminar – Moritz Zaiß – 26.06.07
⇒ intensity of sunlight shining on the area of austria bundled to one cm 2
These are values of 5 years old dissertations so the current state of the art in energies/intensities are orders of magnitudes higher.
ultra short laser pulses
How are ultra fast processes measured with ultra short pulses?
example:
oscillations of deuterium molecules
atom physics seminar – Moritz Zaiß – 26.06.07
applications of modern pulsed lasers
vibration of D2
ultra short laser pulses
deuterium molecule in the ground state
D2
atom physics seminar – Moritz Zaiß – 26.06.07
potential a.u.
nucleus distance a.u
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D2
Vibration of D2
ultra short laser pulsesatom physics seminar – Moritz Zaiß – 26.06.07
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potential a.u.
nucleus distance a.u
multi photon ionisation
P ~ In
Vibration of D2
ultra short laser pulsesatom physics seminar – Moritz Zaiß – 26.06.07
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„Lochfraß“
potential a.u.
nucleus distance a.u
no longer an eigenstate
Vibration of D2
ultra short laser pulsesatom physics seminar – Moritz Zaiß – 26.06.07
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superposition of almost onlythe first and second stateosscillation with Δω
potential a.u.
nucleus distance a.u
Vibration of D2
ultra short laser pulsesatom physics seminar – Moritz Zaiß – 26.06.07
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low probability of ionisation
high probability of ionisation⇒ ionisation rate depends on time between the first and the second pulse
potential a.u.
nucleus distance a.u
Vibration of D2
ultra short laser pulses
delay time / fs
11.1 fs oscillations
We get this graph to assign the frequency.
D events x 104
2+
atom physics seminar – Moritz Zaiß – 26.06.07
Vibration of D2
ultra short laser pulsesatom physics seminar – Moritz Zaiß – 26.06.07
ωΔω
A(ω)fourier transformation of the measured data
11.1 fs oscillation agrees excellent with the theoretical value 11.14 fs
This time dependency was first time measured with7 fs ultra short laser pulses in time domain
ultra short laser pulses
laser pulse
ion detector
helm
holtz
indu
ctor
s
electron detector
applications of modern puls lasers
reaction microscope
D2 gas
atom physics seminar – Moritz Zaiß – 26.06.07
further applications of modern pulsed lasers
ultra short laser pulses
• meteorologic applications
• laser spectroscopy
• coherent control of electrons in atoms
• fine metal processing
• dental treatments
• fusion
atom physics seminar – Moritz Zaiß – 26.06.07
ultra short laser pulsesapplications of modern puls lasers
http://www.aist.go.jp/aist_e/latest_research/2006/20060210/20060210.html
three dimensional images in the airvisualization of "real 3D images" using laser pulses
atom physics seminar – Moritz Zaiß – 26.06.07
gas discharge through high intensity
ultra short laser pulsesapplications of modern puls lasers
http://www.aist.go.jp/aist_e/latest_research/2006/20060210/20060210.html
summaryultra short laser pulses
• medium with band spectrum• mode locking• stretching• amplifying• increase the bandwidth• compressing
⇒ ultra short laser pulses
creation application
• high time resolution for measurementsof fast systems e.g. atomic systems
• high intensitiy and high precisione.g. material processing
⇒ excellent tool for future physics
Thank You for Your attention!
atom physics seminar – Moritz Zaiß – 26.06.07
sourcesultra short laser pulses
CPA: http://www.icuil.org/article.php?articlesID=103D – Images : http://www.aist.go.jp/aist_e/latest_research/2006/20060210/20060210.htmlapplications: http://www.weltderphysik.de/de/1511.phpFusion: http://www.llnl.gov/str/Petawatt.htmlPlots: MuPad, gnuplot
1. K. Zrost, “Wechselwirkung von Atomen und kleinen Molekülen mit intensiven, ultrakurzen
2. U. Morgner et al., “Erzeugung und Anwendung ultrakurzer Lichtpulse,”Physikalische
3. Th. Brabec and F. Krausz, “Intense few-cycle laser fields: Frontiers of nonlinearoptics,”Rev. Mod. Phys. 72 (2000) 545
4. H. Hentschel et al., “Attosecond metrology,” Nature 414 (2001) 509,
atom physics seminar – Moritz Zaiß – 26.06.07
Vibration of D2
ultra short laser pulses
Now the R-wavepacket of the D2 oscillates with a certain frequency.We can measure this frequency if we shoot another pulse which ionizes the D2.If we vary the time between the pulses and measure the rate of D2 we will find,that the events oscillate, too.
D D+ +
2e-
D D+ +
2e-
ionisation is more probable ionisation is less probable
atom physics seminar – Moritz Zaiß – 26.06.07
ultra short laser pulsesapplications of modern puls lasers
atom physics seminar – Moritz Zaiß – 26.06.07