Dynamical analysis of the effect of elliptically polarized
laser pulses on molecular alignment and orientation
Jingsong Liu (刘劲松), Qiyuan Cheng (程起元), Daguang Yue (岳大光),
Xucong Zhou (周旭聪), and Qingtian Meng (孟庆田)*
School of Physics and Electronics, Shandong Normal University, Jinan 250358, China
*Corresponding author: qtmeng@sdnu.edu.cn
Received June 26, 2018; accepted August 27, 2018; posted online Septembe r 20, 2018
In this Letter, we study the molecular alignment and orientation driven by two elliptically polarized laser pulses.
It is shown that the field-free molecular alignment can be achieved in a three-dimensional (3D) case, while the
field-free molecular orientation is only along the x and y directions, and that the field-free alignment and ori-
entation along different axes are related to the populations of the rotational states. It is demonstrated that
changing the elliptic parameter is efficient for controlling both in-pulse and post-pulse molecular alignment
and orientation. The delay time also has an influence on the field-free molecular alignment and orientation.
OCIS codes: 320.5390, 020.1670.
doi: 10.3788/COL201816.103201.
In recent decades, with the development of laser technol-
ogy, dynamical analysis of laser-controlled molecular
alignment and orientation has attracted much attention
due to its significance in chemical reactions, high-order
harmonic generation, multiphoton ionization, surface
processing, and so on
[1–10]
. For a linear polar molecule,
molecular alignment means that the molecular axis is
along the pulse polarization direction, while the molecu lar
orientation is based on the alignment and points to a spe-
cific direction.
Earlier methods to align and orient molecules include
using a dc electric field through the permanent dipole
interaction
[11,12]
. Later, researchers found that using the
dc electric field combined with a non-resonant laser pulse
can obtain a higher degree of alignment or orientation
[13]
.
However, the strong dc field can easily induce Stark effect
in real applications, so it cannot be widely used and re-
placed by various laser pulses. Up to now, there are many
kinds of laser techniques to achieve alignment and orien-
tation both in experiment and theory, such as the two-
color and multi-color laser pulse
[14–17]
, the single-cycle or
few-cycle terahertz (THz) laser pulse and the combination
of different pulses
[18–24]
. The related studies showed
that the laser intensity, pulse duration, pulse shape,
and pulse delay, as well as the temperature of the system,
have a significant effect on the molecular alignment and
orientation. Based on the different pulse durations, two
different molecular alignment and orientation dynamic
regimes were found, i.e., adiabatic and non-adiabatic
regimes of alignment and orientation
[25–27]
. For the forme r
case, in which the pulse duration is long compared to the
rotational period of the molecule, a pendular state is cre-
ated, and the molecular ensemble evolves along it. The lat-
ter case, also named field-free molecular alignment and
orientation, shows an opposite situation, which can be
viewed as a rapid “kick ” to drive the molecular axis
toward the laser field direction, and after the pulse, the
alignment and orientation can be repeatedly emerged.
Therefore, using laser techniques to control the field-free
molecular alignment and orientation in a non-adiabatic
regime is more valuable for real applications.
So far, dynamical analysis of alignment and orientation
driven by linearly polarized laser pulses for linear mole-
cules has been widely studied both experimentally and
theoretically. By contrast, the field-free molecular align-
ment and orientation of linear molecules induced by ellip-
tically polarized laser pulses are relatively rarely discussed.
In recent years, some of the alignment and ori entation
induced by elliptically polarized laser pulses have been re-
ported. For example, Daems et al.
[28]
and Hertz et al.
[29]
have used el liptically polarized laser pulses to achieve
three-dimensional (3D) field-free molecular alignment
and shown a different quantum process. Maan et al.
[30]
have achieved field-free molecular orientation by delayed
elliptically polarized laser pulses and analyzed the effect of
pulse parameters on the rotational excitation and orienta-
tion dynamics; however, they only selected one direction
to discuss. Due to the different properties from linearly po-
larized laser pulses, molecular alignment and orientation
by elliptically polarized laser pulses have become an inter-
esting topic.
In this Letter, we u se the density matrix method to
study the molecular alignment and orientation driven
by two elliptically polarized laser pulses. We describe
the full Hamiltonian of the system and the numerical cal-
culation method in detail. We compare the alignment
and orientation results steered by the elliptically polar-
ized laser pulse, and analyze the population s of the rota-
tional states after the laser pulse. Then, the effects of
the delay time and elliptic parameters on alignment
and orientation are discussed. F inally, the conclusions
are drawn.
COL 16(10), 103201(2018) CHINESE OPTICS LETTERS October 10, 2018
1671-7694/2018/103201(6) 103201-1 © 2018 Chinese Optics Letters
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