ultra short laser pulses - max planck society · ultra short laser pulses atom physics seminar –...

ultra short laser pulses

creation and application

atom physics seminar – Moritz Zaiß – 26.06.07

atom physics seminar


• what? - why? - how?

• creation and optimisation

• typical experimental setup

• properties of existing pulsed lasers

• applications


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

-0.6

-0.4

-0.2

0

0.2

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0.6

0.8

1

-20 -15 -10 -5 0 5 10 15 20

exp(-x**2)*sin(3*x)

-1

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0

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1

-15 -10 -5 0 5 10 15

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



• 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?



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.


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:


Δω Δt ≥ 0.441


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.



but in the cavity of a laser there are only some wavelengths allowed→ standing waves → fabry-pérot interferometer

difference betweentwo adjacent modes


laser gainbandwidth

frequency

inte

nsity

condition forstanding waves

frequency

inte

nsity mode structure in cavity



laser gainbandwidth

frequency

inte

nsity

frequency

inte

nsity

laser output spectrum

Question: What happens if the spectrum gets discrete?

~10^9 Hz


surging

• independent phase of the modes=>continuous wave lasers

• locked phase of the different modes=> train of pulses


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:


cos(ωt) cos(ωt)*cos(ft)

ω -f ; ω +fω

if f = Δ ω => mode locking


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


gate


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.


pulse behaviour an optimisationultra short laser pulses

• dispersion of the pulses in a medium

• compensation of the dispersive effects

• pulse amplifying and optimal compression


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


but no change in the ω spectrum

fourier limit



σ 1/σ

this theoretical limit is called fourier limitand the aim is to reach this limit through compressing methods

ω0


compensation: prism compression


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


compensation: grating compressionultra short laser pulses

diffraction of light depends on wavelength



avoiding high peak powers in the amplifierthrough stretching

or dispersive medium (glass)

pumped Ti:S cristal


chirped pulse amplification

a real pulse laser


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


a real pulse laser



prismcompressor

Amplifier withpockels cell


photo diode

photo diode

pockels cell

Nd:YAG pump laser

Nd:YAG pump laser

oscillator


neodym doted yttrium aluminium granat solid state laser

a real pulse laser



prismcompressor

amplifier withpockels cell


photo diode

photo diode

pockels cell

Nd:YAG pump laser

Nd:YAG pump laser

oscillator


fourier limit



σ 1/σ

this theoretical limit is called fourier limitand the aim is to reach this limit through compressing methods

ω0


ultra short laser pulseslaser pulses in a non-linear medium – Kerr effect



⇒ 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


change of the pulse in the non linear medium

increasing the bandwith


• 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


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


⇒ 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.


How are ultra fast processes measured with ultra short pulses?

example:

oscillations of deuterium molecules


applications of modern pulsed lasers

vibration of D2


deuterium molecule in the ground state

D2


potential a.u.

nucleus distance a.u

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0 0.5 1 1.5 2

D2

Vibration of D2

ultra short laser pulsesatom physics seminar – Moritz Zaiß – 26.06.07

-8

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D2D2+

potential a.u.


multi photon ionisation

P ~ In

Vibration of D2


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D2D2+

„Lochfraß“

potential a.u.


no longer an eigenstate

Vibration of D2


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D2D2+

superposition of almost onlythe first and second stateosscillation with Δω

potential a.u.


Vibration of D2


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D2D2+

low probability of ionisation

high probability of ionisation⇒ ionisation rate depends on time between the first and the second pulse

potential a.u.


Vibration of D2


delay time / fs

11.1 fs oscillations

We get this graph to assign the frequency.

D events x 104

2+


Vibration of D2


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


laser pulse

ion detector

helm

holtz

indu

ctor

s

electron detector

applications of modern puls lasers

reaction microscope

D2 gas


further applications of modern pulsed lasers


• meteorologic applications

• laser spectroscopy

• coherent control of electrons in atoms

• fine metal processing

• dental treatments

• fusion


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


gas discharge through high intensity


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!


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,


Vibration of D2


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


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