atom physics seminar ultra short laser pulses

Transcription

1 atom physics seminar ultra short laser pulses creation and application

2 ultra short laser pulses overview what? - why? - how? creation and optimisation typical experimental setup properties of existing pulsed lasers applications

3 What are ultra short laser pulses? pulses are called ultra short if they only consist of a few wave cycles One wavelength of 790nm corresponds to 2.6 fs => few cycle pulses mean short pulses simple gaussian gedanken pulses E exp(-x**2)*sin(10*x) exp(-x**2)*sin(3*x) t t

4 Why do you want to create 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

5 How do you create them? fourier transformation E(ω) We need a spectrum with a certain bandwidth to have a fine time resolution. And we need a method to superpose the modes in a way, that a short pulse comes out. ω - ω0

6 ultra short laser pulses creation of laser pulses short pulse needs a high bandwidth estimation via uncertainty relation exact value for gaussian pulses: Δω Δt 0.441

7 creation of 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.

8 creation of laser pulses intensity laser gain bandwidth frequency but in the cavity of a laser there are only some wavelengths allowed standing waves fabry-pérot interferometer condition for standing waves difference between two adjacent modes

9 creation of laser pulses intensity laser gain bandwidth frequency intensity mode structure in cavity laser output spectrum ~10^9 Hz frequency frequency Question: What happens if the spectrum gets discrete?

10 creation of laser pulses independent phase of the modes =>continuous wave lasers locked phase of the different modes => train of pulses surging

11 mode locking active mode locking: acousto-optic modulator with frequency f amplitude modulation through diffraction modulated function ~ cos(ωt)*cos(ft) exites frequencies ω -f and ω +f addition theorem: cos(ωt) ω cos(ωt)*cos(ft) ω -f ; ω +f if f = Δ ω => mode locking

12 mode locking active mode locking: considered in time domain its shutting and opening a weak gate gate the time between two pulses is given by the resonator length τ = 2L/c

13 mode locking 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.

14 pulse behaviour an optimisation dispersion of the pulses in a medium compensation of the dispersive effects pulse amplifying and optimal compression

15 dispersion The non-linear dispersion has the following effects k1: inverse group velocity k2: group dispersion:different wavelengths have different speed => spreading of the envelope this is called upchirp but no change in the ω spectrum

16 fourier limit fourier transformation σ 1/σ ω0 this theoretical limit is called fourier limit and the aim is to reach this limit through compressing methods

17 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 longer distance through the prism where its velocity is smaller

18 compensation: grating compression diffraction of light depends on wavelength

19 chirped pulse amplification or dispersive medium (glass) avoiding high peak powers in the amplifier through stretching pumped Ti:S cristal

20 a real pulse laser Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser) Nd:YAG pump laser prism compressor photo diode amplifier with pockels cell pockels cell oscillator oscillator photo diode pulse stretcher Nd:YAG pump laser

21 a real pulse laser Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser) neodym doted yttrium aluminium granat solid state laser Nd:YAG pump laser prism compressor photo diode Amplifier with pockels cell pockels cell oscillator oscillator photo diode pulse stretcher Nd:YAG pump laser

22 a real pulse laser Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser) Nd:YAG pump laser prism compressor photo diode amplifier with pockels cell pockels cell oscillator oscillator photo diode pulse stretcher Nd:YAG pump laser

23 a real pulse laser Nd:YAG laser ( Neodym doted Yttrium-Aluminium-Granat solid-state laser) Nd:YAG pump laser prism compressor photo diode amplifier with pockels cell pockels cell oscillator oscillator photo diode pulse stretcher Nd:YAG pump laser

24 fourier limit fourier transformation σ 1/σ ω0 this theoretical limit is called fourier limit and the aim is to reach this limit through compressing methods

25 laser pulses in a non-linear medium Kerr effect

26 laser pulses in a non-linear medium kerr effect pulse shape phase frequency varation change of the pulse in the non linear medium frequency widening possibility to compress the pulse even more (smaller fourier limit)

27 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 prism compressor

28 ultra short laser pulses properties of todays pulsed lasers laser type Ti:S λcentral: 795nm compressed pulse energy 400 µj/pulse 100 µj/pulse duration 25 fs 5-10fs X ray or XUV ~1 µj/pulse ~650 as with fs-laserpulses there are intensities 15 2 of about 10 W/cm possible. solar constant: W/cm 2 only about 2 cycles 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.

29 applications of modern pulsed lasers How are ultra fast processes measured with ultra short pulses? example: oscillations of deuterium molecules

30 vibration of D2 D potential a.u deuterium molecule in the ground state D nucleus distance a.u

31 Vibration of D2 potential a.u. 0 D2 D multi photon ionisation -8 P ~ I n nucleus distance a.u

32 Vibration of D2 potential a.u. 0 D2 D Lochfraß -8 no longer an eigenstate nucleus distance a.u

33 Vibration of D2 potential a.u. 0 D2 D superposition of almost only the first and second state osscillation with Δω nucleus distance a.u

34 Vibration of D2 potential a.u. 0 D2 D high probability of ionisation ionisation rate depends on time low probability between of ionisation the first and the second pulse nucleus distance a.u

35 Vibration of D2 We get this graph to assign the frequency. D + events x fs oscillations delay time / fs

36 Vibration of D2 fourier transformation of the measured data A(ω) Δω ω 11.1 fs oscillation agrees excellent with the theoretical value fs This time dependency was first time measured with 7 fs ultra short laser pulses in time domain

37 applications of modern puls lasers reaction microscope ion detector D2 gas laser pulse helmholtz inductors electron detector

38 further applications of modern pulsed lasers meteorologic applications laser spectroscopy coherent control of electrons in atoms fine metal processing dental treatments fusion

39 applications of modern puls lasers three dimensional images in the air visualization of "real 3D images" using laser pulses gas discharge through high intensity

40 applications of modern puls lasers

41 summary creation application medium with band spectrum mode locking stretching amplifying increase the bandwidth compressing ultra short laser pulses high time resolution for measurements of fast systems e.g. atomic systems high intensitiy and high precision e.g. material processing excellent tool for future physics Thank You for Your attention!

42 sources 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 nonlinear optics, Rev. Mod. Phys. 72 (2000) H. Hentschel et al., Attosecond metrology, Nature 414 (2001) 509, CPA: 3D Images : applications: Fusion: Plots: MuPad, gnuplot

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44 ultra short laser pulses 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 - ionisation is more probable + D D+ 2e - ionisation is less probable

45 applications of modern puls lasers