Ultrafast photoassociation of ultracold rubidium atoms

1
Ultrafast photoassociation of ultracold rubidium
atoms

• David McCabe
• Alex Dicks, Duncan England, Melissa Friedman,
  Hugo Martay, Emiliya Dimova, Jovana Petrovic,
  Ian Walmsley
• Clarendon Laboratory
• University of Oxford
• CoCoChem 2008

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Outline
1

• Introduction
• Pump-decay photoassociation and ground-state
  detection
• Pump-probe photoassociation and excited-state
  detection
• Outlook and summary

3
Introduction
2

• Context
• Interesting physics emerges at cold temperatures
• Atomic cooling mastered to quantum degeneracy
  (BEC)
• Extension to ultracold molecules not trivial
• Motivation
• High-resolution molecular spectroscopy
• Low-energy molecular collisions
• Quantum information processing
• Molecular BEC
• Method
• Ultrafast photoassociation of ultracold Rb atoms
  in a magneto-optical trap

4
Approaches to cold molecule formation
3

• Direct molecular cooling techniques
• Supersonic expansion of a gas jet
• Buffer gas cooling
• Stark deceleration

Low internal energy configuration BUT Translationa
lly hot (100s of mK)
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Approaches to cold molecule formation
3

• Direct molecular cooling techniques
• Supersonic expansion of a gas jet
• Buffer gas cooling
• Stark deceleration
• Indirect molecular cooling techniques
• Feshbach resonance

Low internal energy configuration BUT Translationa
lly hot (100s of mK)
Translationally cold BUT High vibrational state
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Approaches to cold molecule formation
3

• Direct molecular cooling techniques
• Supersonic expansion of a gas jet
• Buffer gas cooling
• Stark deceleration
• Indirect molecular cooling techniques
• Feshbach resonance
• Photoassociation
• Continuous wave

Low internal energy configuration BUT Translationa
lly hot (100s of mK)
Translationally cold BUT High vibrational state
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Approaches to cold molecule formation
3

• Direct molecular cooling techniques
• Supersonic expansion of a gas jet
• Buffer gas cooling
• Stark deceleration
• Indirect molecular cooling techniques
• Feshbach resonance
• Photoassociation
• Continuous wave
• Ultrafast

Low internal energy configuration BUT Translationa
lly hot (100s of mK)
Translationally cold AND Low vibrational state ??
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Role of pulse shaping in ultrafast
photoassociation
4
Frank-Condon overlaps to excited state vs pump
detuning from atomic D1 asymptote

• Sharp spectral filtering required on edge of
  atomic D1 asymptote

D1
D1
Plot by Jordi Mur Petit, UCL
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Role of pulse shaping in ultrafast
photoassociation
4
Frank-Condon overlaps to excited state vs pump
detuning from atomic D1 asymptote

• Sharp spectral filtering required on edge of
  atomic D1 asymptote

D1
D1
Plot by Jordi Mur Petit, UCL

• Focussing of wavepacket via chirp of
  photoassociation pulse

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Outline
5

• Introduction
• Pump-decay photoassociation and ground-state
  detection
• Ground-state detection scheme
• Coherent dissociation of Rb2 observed!
• Pump-probe photoassociation and excited-state
  detection
• Outlook and summary

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Pump-decay (1)Molecular detection
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• Time-of-flight (TOF) mass spectrometry
• Narrowband tunable 9ns pulsed dye laser _at_ 602nm
• Resonantly-enhanced multi-photon ionization
  scheme
• Rb2 2 photon, resonant
• Rb 3 photon, off-resonant

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Pump-decay (2)Experiment
7

• Background molecules in absence of
  photoassociation (PA) pulse

Oxford Brown et. al. PRL 96 173002
(2006) Freiberg/Berlin Salzmann et. al. PRA 73
023414 (2006)
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Pump-decay (2)Experiment
7

• Background molecules in absence of
  photoassociation (PA) pulse
• Suppression of Rb2 observed - dependent on PA
  pulse chirp

Oxford Brown et. al. PRL 96 173002
(2006) Freiberg/Berlin Salzmann et. al. PRA 73
023414 (2006)
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Pump-decay (2)Experiment
7

• Background molecules in absence of
  photoassociation (PA) pulse
• Suppression of Rb2 observed - dependent on PA
  pulse chirp
• Larger chirp gives greater suppression ? Coherent
  quenching phenomenon

Oxford Brown et. al. PRL 96 173002
(2006) Freiberg/Berlin Salzmann et. al. PRA 73
023414 (2006)
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Outline
8

• Introduction
• Pump-decay photoassociation and ground-state
  detection
• Pump-probe photoassociation and excited-state
  detection
• Time-resolved study of excited state dynamics
• Search for wavepacket dynamics
• Outlook and summary

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Pump-probe (1)Experiment
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• Direct, time-resolved ionization from excited
  state
• Time-of-flight detection
• Molecular signal swamped by large Rb ion signal

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Pump-probe (2)Results - simulations
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• Optimizing ionization pulse wavelength ? and FWHM
  bandwidth ??
• Ionization signal strengths simulated via ground
  singlet and triplet channels
• Best visibility dynamics for ?500nm, ?? 10nm

Simulations by Hugo Martay
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Pump-probe (2)Results - simulations
10

• Optimizing ionization pulse wavelength ? and FWHM
  bandwidth ??
• Ionization signal strengths simulated via ground
  singlet and triplet channels
• Best visibility dynamics for ?500nm, ?? 10nm

Simulations by Hugo Martay
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Pump-probe (3)Results - experiment
11

• Step in pump-probe signal at zero delay
• Subsequent dynamics unclear due to poor
  signal-to-noise and slow acquisition

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Outline
12

• Introduction
• Pump-decay photoassociation and ground-state
  detection
• Pump-probe photoassociation and excited-state
  detection
• Outlook and summary

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Outlook
13

• Technical improvements to molecular detection
• Improved pulse-shaping
• Sculpting and focussing of wavepacket evolution
• Identification of appropriate dump pulse
• Bose-Einstein condensate in a 3D lattice
  (collaboration with Chris Foot)
• Improve photoassociation efficiency
• One molecule per lattice site for long lifetimes
• Combine magnetic and optical Feshbach techniques

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Summary
14

• Ultrafast photoassociation
• Route to low internal energies and ultracold
  translational temperatures
• Pump-decay photoassociation experiments
• Coherent quenching of molecules observed!
• Pump-probe photoassociation experiments
• Time-resolved dynamics studies ongoing
• Future Molecules in a BEC

Thank you!
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Pump-decay (2)Experiment
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Coherent control

• Goal Complete control of two-body collision
  process

• Approach Phase and amplitude shaping of E(t)
• Open loop
• Closed loop

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MOT lab
BEC lab