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
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
A Bit about Laser Pulses E 0 (t)=exp(-(t/τ) 2 ) envelope 1.0 0.5 E(t) = E 0 (t)cos(ωt+φ) 0.0 Carrier envelope phase carrier -0.5-1.0-100 λ = 800 nm τ = 30 fs φ = 0-50 0 50 100 Time (fsec)
1.0 E(t) = E 0 (t)cos(ωt+φ) E 0 (t)=exp(-(t/τ) 2 ); λ=800nm; φ=0 1.0 0.5 0.5 0.0 0.0-0.5 τ = 4 fs -0.5 τ = 6 fs -1.0-20 -10 0 1.0 10 20 Time (fsec) -1.0-20 -10 0 10 20 Time (fsec) 0.5 0.0-0.5-1.0-20 -10 Time (fsec) τ = 8 fs 0 10 20
Electric Field Asymmetryy vs Pulse Duration Forward Peak α = - 1 Backward Peak 1.0 τ = 4 fs Forward Peak 0.5 0.0-0.5-1.0-20 Backward Peak -10 0 10 20 Time (fsec)
Electric Field Asymmetry vs Pulse Duration Forward Peak α = - 1 Backward Peak 1.0 0.5 τ = 2 fs Forward Peak 0.0 alpha 0.6 0.5 0.4 0.3 0.2 0.1-0.5 Backward Peak -1.0-20 -10 0 10 20 Time (fsec) 0 0 5 10 15 20 25 30 Pulse Duration (fs) 35
Electric Field Asymmetry vs Pulse Duration Forward Peak α = - 1 Backward Peak 1.0 0.5 τ = 30 fs Forward Peak 0.0 0.6-0.5 0.5 alpha 0.4 0.3 0.2 0.1-1.0-100 -50 0 50 100 Time (fsec) Backwards Peak 0 0 5 10 15 20 25 30 Pulse Duration (fs) 35
E(t) = E 0 (t)cos(ωt+φ) E 0 (t)=exp(-(t/τ) 2 ); λ=800nm; τ=5fs φ 1.0 0.5 φ = 0 1.0 0.5 φ = π/2 0.0 0.0-0.5-0.5 1.0-1.0-20 -10 0 10 20 Time (fsec) 0.5-1.0-20 -10 0 10 20 Time (fsec) φ = π 0.0 α=0.07-0.5-1.0-20 -10 0 10 20 Time (fsec)
E(t) = E E 0 (t)=exp(-(t/τ) 0 (t)cos(ωt+φ) 2 ); λ=800nm; τ=30fs 1.0 φ = 0 1.0 φ = π 0.5 0.5 0.0 0.0-0.5-0.5-1.0-100 -50 0 50 100 Time (fsec) -1.0-100 -50 0 50 100 Time (fsec) α=0.002
Ultrashort laser pulses can look different Pulse Duration φ (Carrier Envelope Phase) Why would atoms or molecules care?
High Harmonic Generation
HHG: Three-Step Model 1. Ionization 2. Acceleration hυ hυ E max =I p +3.17U p 3. Recombination
T. Brabec and F. Krausz, Rev. Mod. Phys. 72, 545 (2000). Baltuska et al, IEEE J. Sel. Top. Quant. Electron. 9, 972 (2003).
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.) -2-4 -6-6 -4-2 0 z-coordina ate (a.u.) 2 4 6
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 1 1 0 0-1 -1-2 -4-2 0 2 4 Time (fsec) -2-4 -2 0 2 4 Time (fsec)
vacuum chamber TOF spectrometer +V χ (2) λ/2 Calcite plate 800nm 1.5 1.5 1.0 1.0 0.5 0.5 0.0 0.0-0.5-0.5-1.0-1.0-1.5-4 -2 0 2 4 Time (fsec) -1.5-4 -2 0 2 4 Time (fsec)
Phase Dependence of N + 2 1.5 0.20 1.0 0.5 φ=π N2 2+ Yield (a.u.) 0.15 0.10 0.05 0.0-0.5-1.0-1.5 1.5 1.0 γ=0.15 α=0.35-4 -2 0 2 4 Time (fsec) 0.5 1.5 1.0 0.5 0.00 0 1 2 3 4 Relative Delay (blue periods) 5 0.0-0.5 0.0-1.0 φ=0-0.5-1.0-1.5 γ=0.15; φ=π/2; α=0.0009-4 -2 0 2 4 Time (fsec) -1.5-4 -2 0 2 4 Time (fsec)
Asymmetric Molecular Dissociation Asymmetric dissociation Attributed to enhanced ionization at R c 2% field asymmetry g1 g2 from Dan Pinkham's dissertation
2 0 V(z) -2-4 2-6 0-6 -4-2 0 2 4 6 z-coordinate (a.u.) V(z) +Fz (a.u.) -2-4 2-6 0-6 -4-2 0 2 4 6-2 z-coordinate (a.u.) V(z) +Fz (a.u.) -4-6 -10-5 0 5 10 z-coordinate (a.u.)
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).
Towards Carrier-Envelope Phase Stabilized Pulses Origin of phase stabilization Laser Reconstruction process
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 www.menlosystems.com 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
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 www.menlosystems.com Stabilizing ω r and ω 0 will stabilize f frequency comb
Light from laser oscillator Detects ω 0
Stabilizing ω 0 and ω r n=n(e) control ω 0 control ω r
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
FC8004 Optical Frequency Synthesizer (MenloSy ystems GmbH) Photodiode APD Pump Laser Verdi V6 Oscillator Nonlinear interferometer
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
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
Laser Rebuild Former Setup Grating Expander Grating Compressor 30 fs amplifier KM Labs Oscillator Millennia Pump Pockels cell / seed beam separation optics Evolution Pump
Laser Rebuild New Setup Evolution pump Grating Compressor 30 fs amplifier Femtolasers oscillator Verdi pump Grating Expander f to 2f interferometer
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?
Plans for the Future Extend two-color studies probe dissociation processes of O 2, CO 2 HOMO structure
e - e - e - e - R e R c t= 25 fs t
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
e - e - e - e - R e R c t= 25 fs t
e - e - e - e - R e R c t= 25 fs t
e - e - e - e - R e R c t= 25 fs t
Acknowledgments Dr. Robert Jones Dr. Dan Pinkham Mary Kutteruf Dr. Brett Sickmiller