248 research outputs found
Stimulated Raman adiabatic passage into continuum
We propose a technique which produces nearly complete ionization of the
population of a discrete state coupled to a continuum by a two-photon
transition via a lossy intermediate state whose lifetime is much shorter than
the interaction duration. We show that using counterintuitively ordered pulses,
as in stimulated Raman adiabatic passage (STIRAP), wherein the pulse coupling
the intermediate state to the continuum precedes and partly overlaps the pulse
coupling the initial and intermediate states, greatly increases the ionization
signal and strongly reduces the population loss due to spontaneous emission
through the lossy state. For strong spontaneous emission from that state,
however, the ionization is never complete because the dark state required for
STIRAP does not exist. We demonstrate that this drawback can be eliminated
almost completely by creating a laser-induced continuum structure (LICS) by
embedding a third discrete state into the continuum with a third control laser.
This LICS introduces some coherence into the continuum, which enables a
STIRAP-like population transfer into the continuum. A highly accurate analytic
description is developed and numerical results are presented for Gaussian pulse
shapes
Broadband sum frequency generation via chirped quasi-phase-matching
An efficient broadband sum frequency generation (SFG) technique using the two
collinear optical parametric processes \omega 3=\omega 1+\omega 2 and \omega
4=\omega 1+\omega 3 is proposed. The technique uses chirped quasi-phase-matched
gratings, which, in the undepleted pump approximation, make SFG analogous to
adiabatic population transfer in three-state systems with crossing energies in
quantum physics. If the local modulation period %for aperiodically poled
quasi-phase-matching first makes the phase match occur for \omega 3 and then
for \omega 4 SFG processes then the energy is converted adiabatically to the
\omega 4 field. Efficient SFG of the \omega 4 field is also possible by the
opposite direction of the local modulation sweep; then transient SFG of the
\omega 3 field is strongly reduced. Most of these features remain valid in the
nonlinear regime of depleted pump
- …