Stimulated Raman Adiabatic Passage into continuum

1
Stimulated Raman Adiabatic Passage into continuum
Andon Rangelov (Sofia University,
Bulgaria) Nikolay Vitanov (Sofia University,
Bulgaria) Ennio Arimondo (Pisa University,
Italy)
Control of Quantum Dynamics of Atoms, Molecules
and Ensembles by Light (TOK)
Engineering, Manipulation and Characterization of
Quantum States of Matter and Light (RTN)
Palermo, 2 June 2007
2
Outline
1. What is Stimulated Raman Adiabatic Passage
(STIRAP) ?

• Three-level atom - ? configuration
• Hamiltonian, dark state, counterintuitive scheme

2. What is Laser Induced Continuum Structure
(LICS) ?

• LICS configuration
• Fano autoionization configuration
• STIRAP via the continuum

3. How to optimize the ionization ?

• Direct ionization into continuum, Ionization via
  intermediate state REMPI, STIRAP
• STIRAP into continuum with LICS scheme
• Hamiltonian, pulse sequence, quasi-dark state

3
Three level atom- ? configuration
Hamiltonian In Rotating Wave Approximation (RWA)

Stokes
Pump
adiabatic basis (eigenvectors of H(t) )
4
STIRAP in ? configuration
Conditions Adiabatic evolution
Counterintuitive order of pulses
Result Highly efficient population transfer
5
Laser Induced Continuum Structure (LICS)
LICS
control laser
pump laser
Two discrete states , are coupled to
continuum states with pump and control
lasers. If control laser is strong, a structure
occurs in the otherwise flat continuum.
6
STIRAP via the continuum
Conditions for STIRAP via a continuum 1)
Adiabatic evolution 2) Counterintuitive order of
pulses 3) Two-photon detuning
For purely bound states -these conditions can be
easily fulfilled. When involving continuum
states -high laser intensities are required to
fulfill the condition of adiabaticity.
T.Peters, L.P.Yatsenko, and T.Halfmann, Phys.
Rev. Lett. 95, 103601 (2005)
7
How to optimize the ionization
We consider ionization of atom initially in
ground state
1) Direct ionization it requires a very strong
pulse with a short wave length.
2) Ionization via intermediate state REMPI
(resonantly-enhanced multiphoton ionization)
works, if small decay form the intermediate state
3) Ionization via intermediate (which decay with
large rate) naturally leads to counterintuitive
pulse ordering, as in STIRAP. It is not really
STIRAP, because of continuum, which does not
allow the formation of a dark state, which is a
coherent superposition of discrete states.
8
Can we use STIRAP with LICS to optimize
ionization?
STIRAP into continuum
STIRAP into continuum with LICS
Stokes
Pump
9
Hamiltonian, pulse sequence, quasi-dark state
STIRAP into continuum with LICS
Adiabatic elimination of the continuum states
Continuum
Stokes
Fano parameter
pump Rabi frequency
detuning between and
decay rate from
Pump
Stark shifts of states and
Ionization widths of states and
10
Hamiltonian, pulse sequence, quasi dark state
STIRAP into continuum with LICS
Hamiltonian matrix is non-Hermitian
Continuum
Stokes
Fano parameter
pump Rabi frequency
detuning between and
decay rate from
Pump
, could be incorporated in the
detunings
Ionization widths of states and
11
Hamiltonian, pulse sequence, quasi dark state
STIRAP into continuum with LICS
Hamiltonian matrix is non-Hermitian
Continuum
Stokes
Pulse ordering
Less population in Stokes before
pump Maximum ionization Stokes before control To
have LICS control close to the pump
This means that in the beginning of the
evolution
Pump
12
Schrödinger equation in adiabatic basis
gives the connection between adiabatic and
diabatic bases
where
adiabatic diagonal Hamiltonian
nonadiabatic coupling
If the time evolution is slow we can neglect the
nonadiabatic coupling
Then
where
dynamics is determined by the initial conditions
on the adiabatic states
Instead of the complicated adiabatic states we
make the approximation
13
Quasi-dark state of Hamiltonian
is assumed to be small parameter (
)
Also
If is stable ( ) and at two-photon
resonance ( ) , the quasi-dark state
turns into the well-known dark state written for
small values of angle
14
Initially only the quasi-dark state is populated.
In the adiabatic limit the populations are
where
numerical simulations for Gaussian pulse shapes
of Stokes,pump,control
Hamiltonian
15
An important difference between LICS-STIRAP and
STIRAP The control pulse plays a important role
in achieving a high ionization rate.
16
An important difference between LICS-STIRAP and
STIRAP The control pulse plays a important role
in achieving a high ionization rate.
17
An important difference between the scheme that
we proposed and STIRAP In STIRAP the arrival
and the departure of the pulses should be in the
proper time window, while in our technique it is
only important how the pulses arrive but not what
is the order of their departures
18
There is a laser intensity for which the
ionization rate reach it maximal value and after
that is saturated (regime of the adiabaticity)
19
Summary

• An interesting analytic prediction, for an
  optimal population transfer into continuum was
  presented

• The control pulse plays a very important role in
  achieving a high ionization rate

• The pulse ordering is important for how the
  pulses arrive but not what is the order of their
  departures

• Applications Rydberg atom ionization
  efficiency close to unity with negligible
  population into discrete states and efficient
  photoionization of a Bose-Einstein condensate.

A. Rangelov, N. Vitanov, E. Arimondo, submitted
to PRA