Physics Letters B 739 (2014) 189–195
Contents lists available at ScienceDirect
Physics Letters B
www.elsevier.com/locate/physletb
Optimization of Schwinger pair production in colliding laser pulses
F. Hebenstreit
a,b,∗
, F. Fillion-Gourdeau
c
a
Albert Einstein Center for Fundamental Physics, Institute for Theoretical Physics, Bern University, 3012 Bern, Switzerland
b
Institut für Theoretische Physik, Universität Heidelberg, 69120 Heidelberg, Germany
c
Université du Québec, INRS–Énergie, Matériaux et Télécommunications, Varennes, Québec, J3X 1S2, Canada
a r t i c l e i n f o a b s t r a c t
Article history:
Received
30 September 2014
Received
in revised form 20 October 2014
Accepted
24 October 2014
Available
online 30 October 2014
Editor: A.
Ringwald
Keywords:
Dynamically
assisted Schwinger mechanism
Optimal
control theory
Multi-start
method
Recent studies of Schwinger pair production have demonstrated that the asymptotic particle spectrum is
extremely sensitive to the applied field profile. We extend the idea of the dynamically assisted Schwinger
effect from single pulse profiles to more realistic field configurations to be generated in an all-optical
experiment searching for pair creation. We use the quantum kinetic approach to study the particle
production and employ a multi-start method, combined with optimal control theory, to determine a set
of parameters for which the particle yield in the forward direction in momentum space is maximized.
We argue that this strategy can be used to enhance the signal of pair production on a given detector in
an experimental setup.
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/3.0/). Funded by SCOAP
3
.
1. Introduction
The creation of electron–positron pairs from external electric
fields (Schwinger effect) has been a long-standing prediction of
quantum electrodynamics (QED) [1–3]. The breakdown of the QED
vacuum has not been observed yet because of the required electric
field strength which is of the order of E
S
∼ 10
16
V /cm. However,
recent theoretical studies as well as technological advances have
raised the hope that an experimental observation might become
feasible in the near future [4,5].
In
recent years, investigations have demonstrated that the
electron–positron spectrum is extremely sensitive to the applied
electric field profile [6–14]. Most notably, it has been shown that
the particle production can be significantly enhanced by using op-
timized
or tailored field configurations. In this respect, the dynam-
ically
assisted Schwinger effect was proposed as a mechanism to
enhance non-perturbative particle production by orders of magni-
tude:
Superimposing a strong but low-frequency field with a weak
but high-frequency field partially lifts the exponential suppression
of the Schwinger effect due to dynamical pair creation [15–19]. On
the other hand, it has also been shown that quantum interference
can result in a drastic enhancement or decrease of the particle
yield: For a sequence of single pulses it was demonstrated that
an order-of-magnitude variation of the particle yield occurs upon
*
Corresponding author.
E-mail
addresses: hebenstreit@itp.unibe.ch (F. Hebenstreit),
francois.fillion@emt.inrs.ca (F. Fillion-Gourdeau).
changing the interpulse time-lag while keeping all other param-
eters
fixed [20–23]. Most studies, however, have been based on
very simple field configurations – mostly superpositions of single
electric pulses – which are certainly not realistic in the sense of
representing experimentally relevant fields.
A
realistic field configuration, however, which is likely to be
generated in an all-optical experiment searching for pair creation is
supposed to be more complicated. Theoretical models of such field
configurations, which are in fact non-trivial solutions of Maxwell
equations in vacuum, include standing-wave beam pulses or su-
perpositions
of e-dipole pulses [24–26]. Previous investigations of
the asymptotic particle number (but not its spectrum) in these
electromagnetic backgrounds have only been based on the locally-
constant
field approximation [27–29].
To
a first approximation, the electromagnetic field in the fo-
cal
spot of these field configurations can also be approximated by
a spatially homogeneous but time-dependent electric field which
points in a given direction. This is based on the fact that the spatial
scale for particle production, which is set by the Compton wave-
length,
is orders of magnitude smaller than typical scales of optical
lasers. The typical time profile of electric fields, which have been
employed in the study of the asymptotic particle spectrum, is then
given by an envelope with subcycle structure, including a carrier
phase and/or a chirp [6–10].
In
this publication, we investigate the possibility of generat-
ing
an optimized field configurations from a superposition of two
colliding laser pulses of different amplitude and frequency. Accord-
ingly,
we extend the idea of the dynamically assisted Schwinger
http://dx.doi.org/10.1016/j.physletb.2014.10.056
0370-2693/
© 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/). Funded by
SCOAP
3
.