Full vectorial feature of second-harmonic generation with full P...
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Full vectorial feature of second-harmonic generation
with full Poincaré beams
Li Zhang (张 莉)
1,2
,FeiLin(林 飞)
2
, Xiaodong Qiu (邱晓东)
2
, and Lixiang Chen (陈理想)
2,
*
1
School of Physics and Optoelectronic Engineering, Foshan University, Foshan 528000, China
2
Department of Physics, Collaborative Innovation Center for Optoelectronic Semiconductors and Efficient Devices,
and Jiujiang Research Institute, Xiamen University, Xiamen 361005, China
*Corresponding author: chenlx@xmu.edu.cn
Received April 18, 2019; accepted June 6, 2019; posted online August 2, 2019
We demonstrate the full vectorial feature of second-harmonic generation (SHG), i.e., from infrared full Poincaré
beams to visible full Poincaré beams, based on two cascading type I phase-matching beta barium borate crystals
of orthogonal optical axes. We visualize the structured features of the vectorial SHG wave by using Stokes
polarimetry and show the interesting doubling effect of the polarization topological index, i.e., a low-order full
Poincaré beam is converted to a high-order one. However, the polarization singularities of both C points and L
lines are found to keep invariant during the SHG process. Our scheme could offer a deeper understanding on the
interaction of vectorial light fields with media and can be generalized to other nonlinear optical effects.
OCIS codes: 190.2620, 260.6042.
doi: 10.3788/COL201917.091901.
Second harmonic generation (SHG), as the first nonlin-
ear optical phenomenon observed in the experiment, can
be traced back to the seminal work about frequency
converting from 694.3 to 347.2 nm i n crystalline quartz
in 1961
[1]
. In the following year, Kleinman presented
the electromagnetic theory of nonlinear dielectric polari-
zation to explain the SHG phenomenon
[2]
. In 1968,
Bloembergen et al. discovered the SHG effect on the
semiconductor–air and metal–air interfaces
[3]
.As
polarization-dependent SHG signals could provide use-
ful information of the structural properties, SHG has
become a standard spectroscopic tool to characterize
materials
[4]
. Later, inspired by this attractive feature,
Freund developed a kind of biological SHG imaging,
i.e., SHG microscopy, to investigate the pola rity of
collagen fibers in rat tail tendons
[5]
. Since then, owing
to no energy deposition to its interacted matters and
localized excitation, SHG microscopy h as been utilized
for clinical imaging purposes to substantially reduce
photobleaching and phototoxicity relative to fluores-
cence methods
[6]
. Nowadays, it has become a mature im-
aging technology and is widely used in biology and
medicine fields
[7]
. Except for these applications in spec-
troscopy, the SHG process was also widely used for a
high-power single-frequency fiber laser source
[8]
,manipu-
lation and generation of optical vortices
[9]
,andoptical
image processing
[10]
.Wenotethatalloftheaboveworks
are almost focusing on the interaction of scalar light
fields with nonlinear m edia.
Recently, vector beams, possessing the spatially
inhomogeneous polarization states in the cross section
of light, have attracted tremendous attention in optical
communication
[11]
, optical micromanipulation
[12]
,high-
numerical-aperture focusing
[13]
, and polarization pattern
entanglement
[14]
. Their polarization features, such as
singularities
[15]
and the Pancharatnam– Berry phase
[16]
,
have been widely explored. Some researchers have also
investigated the SHG process of vector beams, including
the vector Gaussian beam
[17]
, the vortex beams with ra-
dial or azimuthal polarization
[18,19]
, and the cylindrical
vector beam
[20]
. Besides, the singularities contribution
[21]
and the conservation of topological charge
[22]
during
the SHG process were investigated. Recently, we also
visualized the hidden topological structures of a full
Poincaré (FP) beam via type II SHG in the potassium
titanyl phosphate (KTP) crystals
[23]
. However, we note
that, due to the type II phase-matching condition, the
vectorial feature of the fundamental light with spatially
inhomogeneous polarization cannot be well maintained
after the SHG process; namely, the generated SHG
beam becomes merely a scalar light field with a uniform
polarization state.
We also note that the polarization state of light usually
plays an irreplaceable role in optical manipulation
[24]
,
optical communication
[25]
, and lithography
[26]
. In this re-
gard, how to maintain the vectorial nature of light fields
during the process of SHG is meaningful and desirable
from both the theoretical and applied points of view.
Here, we present an experiment to realize SHG from
infrared vector fields to visible vector fields, based on two
cascading type I phase-matching beta barium borate
(BBO) crystals, whose fast axes are just configured to
be perpendicular to each other. It is noted that such a
two-crystal geometry was actua lly not new
[27,28]
, which
can be traced back to the seminal work of constructing
a bright source of polarization-entangled photon pairs
via parametric down-conversion
[29]
. Here, we further
explored such a configuration for an experimental
COL 17(9), 091901(2019) CHINESE OPTICS LETTERS September 2019
1671-7694/2019/091901(5) 091901-1 © 2019 Chinese Optics Letters
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