Ultrashort Phase Locked Laser Pulses for Asymmetric Electric Field Studies of Molecular Dynamics

Transcription

1 Ultrashort Phase Locked Laser Pulses for Asymmetric Electric Field Studies of Molecular Dynamics Kelsie Betsch University of Virginia Departmentt of Physics AMO/Fourth Year Seminar April 13, 2009

2 Overarching Goal Application of asymmetric electric fields to molecular dynamics studies Two color fields Ultrashort Carrier- -Envelope (CE) phase stabilized pulses Synthesis of CEP stabilized pulses

3 A Bit about Laser Pulses E 0 (t)=exp(-(t/τ) 2 ) envelope E(t) = E 0 (t)cos(ωt+φ) 0.0 Carrier envelope phase carrier λ = 800 nm τ = 30 fs φ = Time (fsec)

4 1.0 E(t) = E 0 (t)cos(ωt+φ) E 0 (t)=exp(-(t/τ) 2 ); λ=800nm; φ= τ = 4 fs -0.5 τ = 6 fs Time (fsec) Time (fsec) Time (fsec) τ = 8 fs

5 Electric Field Asymmetryy vs Pulse Duration Forward Peak α = - 1 Backward Peak 1.0 τ = 4 fs Forward Peak Backward Peak Time (fsec)

6 Electric Field Asymmetry vs Pulse Duration Forward Peak α = - 1 Backward Peak τ = 2 fs Forward Peak 0.0 alpha Backward Peak Time (fsec) Pulse Duration (fs) 35

7 Electric Field Asymmetry vs Pulse Duration Forward Peak α = - 1 Backward Peak τ = 30 fs Forward Peak alpha Time (fsec) Backwards Peak Pulse Duration (fs) 35

8 E(t) = E 0 (t)cos(ωt+φ) E 0 (t)=exp(-(t/τ) 2 ); λ=800nm; τ=5fs φ φ = φ = π/ Time (fsec) Time (fsec) φ = π 0.0 α= Time (fsec)

9 E(t) = E E 0 (t)=exp(-(t/τ) 0 (t)cos(ωt+φ) 2 ); λ=800nm; τ=30fs 1.0 φ = φ = π Time (fsec) Time (fsec) α=0.002

10 Ultrashort laser pulses can look different Pulse Duration φ (Carrier Envelope Phase) Why would atoms or molecules care?

11 High Harmonic Generation

12 HHG: Three-Step Model 1. Ionization 2. Acceleration hυ hυ E max =I p +3.17U p 3. Recombination

13 T. Brabec and F. Krausz, Rev. Mod. Phys. 72, 545 (2000). Baltuska et al, IEEE J. Sel. Top. Quant. Electron. 9, 972 (2003).

14 Two-Color Asymmetric Fields for N 2 Ionization N 2+ tunnel ionization yield is sensitive to the electric field strength Highly non-linear field dependence 2 0 V(z) +Fz V(z) (a.u.) z-coordina ate (a.u.) 2 4 6

15 E(t) = E 0 (t)[cos(ωt)+ γcos(2ωt+ φ)] E 0 (t)=exp(-(t/τ) 2 ); λ=800nm; τ=30fs; γ= =1 2 φ = 0 2 φ = π Time (fsec) Time (fsec)

16 vacuum chamber TOF spectrometer +V χ (2) λ/2 Calcite plate 800nm Time (fsec) Time (fsec)

17 Phase Dependence of N φ=π N2 2+ Yield (a.u.) γ=0.15 α= Time (fsec) Relative Delay (blue periods) φ= γ=0.15; φ=π/2; α= Time (fsec) Time (fsec)

18 Asymmetric Molecular Dissociation Asymmetric dissociation Attributed to enhanced ionization at R c 2% field asymmetry g1 g2 from Dan Pinkham's dissertation

19 2 0 V(z) z-coordinate (a.u.) V(z) +Fz (a.u.) z-coordinate (a.u.) V(z) +Fz (a.u.) z-coordinate (a.u.)

20 CEP in D 2+ Dissociation D 2 D + +D Control forward/backward ejection of D+ by controlling φ shows optical control of electron localization Kling et al, Science 312, 246 (2006).

21 Towards Carrier-Envelope Phase Stabilized Pulses Origin of phase stabilization Laser Reconstruction process

22 Time Domain Frequency Comb Theory Ideal case: Pulse circulating in cavity of length L with carrier frequency ω c Output is a sequence of nearly identical pulses separated by the round trip time T=2L/v g where is the mean group velocity But v g v p => carrier/envelope phase shift φ per pulse Frequency Domain v g v p Spectrum is a comb of laser modes spaced by ω r centered at ω c Continuous shift => offset frequency ω 0 = φ/t from being exact harmonics of the repetition frequency ω n =nω r +ω 0

23 Frequency Comb Theory ω r easy to measure ω 0 need more than one optical octave: ω n =nω r +ω 0 Red side, mode number n ω n = nω r + ω 0 Blue side, mode number 2n ω 2n = 2nω r + ω 0 Beat them together: 2ω n - ω 2n = 2(nω r + ω 0 )- 2nω r + ω 0 = ω 0 Stabilizing ω r and ω 0 will stabilize f frequency comb

24 Light from laser oscillator Detects ω 0

25 Stabilizing ω 0 and ω r n=n(e) control ω 0 control ω r

26 Frequency Comb Block Diagram Diagnostic Equipment: Spectrum Analyzer, Oscilloscope, Frequency Counter Electronics to interface with 30 fs laser amplifier Offset locking electronics Photodiode APD** Pump laser AOM Oscillator PCF* Nonlinear interferometer Main base plate Photodiode Sound wave creates grating and diffracts an amount of the light *Photonic crystal fiber for spectral broadening **Avalanche photodiode

27 FC8004 Optical Frequency Synthesizer (MenloSy ystems GmbH) Photodiode APD Pump Laser Verdi V6 Oscillator Nonlinear interferometer

28 Conventional Laser Schematic Chirped Pulse Amplification ~5 ps 1 khz 2 W Grating Compressor ~30 fs 1 khz 1 W Evolution pump laser Ti:Sapphire laser amplifier PC ~5 ps 1 khz Millennia pump laser KM Labs laser oscillator ~20 fs 80 MHz 300 mw Grating Expander

29 CE Phase-Locked Laser Schematic Grating Compressor Fiber compressor ~30 fs ~20 fs 200 MHz 500 mw CE locked Evolution pump laser Ti:Sapphire laser amplifier PC Colinear f-to-2f Inter- ferometer 1 khz 1 W CE locked ~5 fs 1 khz 500 mw Verdi pump laser CEP locked laser oscillator Nonlinear f-to-2f interferometer Grating Expander Locking electronics CE locked Locking electronics

30 Laser Rebuild Former Setup Grating Expander Grating Compressor 30 fs amplifier KM Labs Oscillator Millennia Pump Pockels cell / seed beam separation optics Evolution Pump

31 Laser Rebuild New Setup Evolution pump Grating Compressor 30 fs amplifier Femtolasers oscillator Verdi pump Grating Expander f to 2f interferometer

32 Current output Oscillator is phase locked ~1 W, 800 nm, 1kHz, ~30 fs pulse output (?) Amplifier is not yet phase locked Working on coline ear f-to-2f interferometer Duration? Pulse shape?

33 Plans for the Future Extend two-color studies probe dissociation processes of O 2, CO 2 HOMO structure

34 e - e - e - e - R e R c t= 25 fs t

35 Plans for the Future Extend two-color studies probe dissociation processes of O 2, CO 2 HOMO structure CEP to study same proc cesses Greater time resolution Which step(s) is(are) phase-dependent Way to determine CE phase

36 e - e - e - e - R e R c t= 25 fs t

37 e - e - e - e - R e R c t= 25 fs t

38 e - e - e - e - R e R c t= 25 fs t

39 Acknowledgments Dr. Robert Jones Dr. Dan Pinkham Mary Kutteruf Dr. Brett Sickmiller