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Letter Vol. 48,No. 9 /1 May 2023 / Optics Letters 2357
Electrically controlled dual-mode polarization beam
splitter using a nematic liquid crystal
Vaibhav Sharma AND Aloka Sinha∗
Department of Physics, Indian Institute of Technology, New Delhi, India
*aloka@physics.iitd.ac.in
Received 13 January 2023; revised 22 March 2023; accepted 22 March 2023; posted 23 March 2023; published 25 April 2023
Polarization handling using an external source is highly
desirable in applied optics and photonics to increase the
degree of freedom of an optical system. Here we report an
electrically controlled polarization beam splitter (PBS) by
sandwiching the nematic liquid crystal (LC) between two
equilateral prisms. The presented LC-PBS is operated in
two different modes: non-splitting mode and polarization
splitting mode. The externally applied voltage can switch
the mode of the PBS, which makes the device active and
flexible. The proposed electrically controlled PBS exhibits
features such as bistability with highly stable modes, large
splitting angle, wider operating range, and ease of fabrica-
tion with lower cost. © 2023 Optica Publishing Group
https://doi.org/10.1364/OL.484857
The polarization beam splitter (PBS) plays an essential role in
many fields for various applications, including optical mod-
ulation [1], polarization-based interferometry [2], logic gate
operations [3], medical imaging [4], data storage [5], and quan-
tum computing [6]. A PBS is used to split the light into two
different directions with different polarization states [often ver-
tical (S) and horizontal (P) polarization states]. In conventional
PBS [7], the birefringence material is used to separate the two
polarization states either by refraction or reflection. There are
some limitations to this type of PBS. Refraction-based PBS
requires longer interaction (e.g., Wollaston prism) length to get
an observable separation between the two polarized beams [7].
Reflection-type PBS based on Brewster’s law (e.g., MacNeille
cube polarizer) requires multiple layers so that the Spolarized
light can be easily reflected. Other than this, the reflection-based
polarization splitting of the beam can also be achieved by using
the birefringence prisms (e.g., Glan–Taylor prism). In this tech-
nique, two air-spaced birefringence prisms are oriented with
both optical axes parallel to the plane of reflection and parallel
to the entry and exit sides. The polarization splitting is achieved
at the prism’s air interface by total internal reflection (TIR) [7].
The PBSs referred to above are passive, i.e., their polarization-
splitting characteristics are fixed once they are manufactured.
The externally controlled features are required in PBS to make
them active and flexible for various applications such as quan-
tum computing [6] and externally controlled logic operations
[3]. A liquid crystal (LC) can be an alternative material in
this device because it can change its optical properties under
the externally applied electric or optical field. In recent years,
LCs have been extensively used to fabricate different optical
devices, including beam steerer [8], polarization selector [9],
mode size converter [10], and random laser [11]. The combina-
tion of prism and LC geometry is widely used to design several
optical devices, including beam deflectors [12,13], polariza-
tion beam steerers [14], retarders [15], LC-projection systems
[16], quantum-mechanical barrier tunneling compounds [17],
and gratings [18,19]. The LC-beam deflector or steerer [12–14]
can split the polarization, but the splitting angle is very small
(∼1°), and because of that, they cannot be used effectively in
beam splitting applications. Also, the fabrication process of such
a beam splitter is complex and requires a photolithography pro-
cess with multiple steps. The LC-filled prism systems [15,16]
are used for polarization splitting, but like a conventional beam
splitter, the device has only one mode (polarization splitting
mode). The device does not exhibit a non-splitting mode and
hence it has only one degree of freedom. Also due to its large
dimensions, the maximum transmittance of S- and P-polarized
light is 20–25% only [15]. The higher losses reduce the extinc-
tion ratio of the two orthogonal polarizations, which is the key
parameter of a PBS. Other techniques such as LC phase gating
[18] based polarization splitting require well-defined multiple
hybrid alignments of the LCs by elaborate and complicated
photolithographic techniques, and the bulk polymer-dispersed
LC gratings [19] require the polarization holographic technique
which inevitably involves a difficulty and complexity in precisely
controlling the anisotropic phase separation of LC droplets and a
polymer matrix. In addition to the above-mentioned techniques,
beam splitting is also achieved by using a directional coupler
[20–22], photonic crystal fiber [23,24], and metamaterials [25],
but these are primarily for silicon photonics applications in
integrated optics platforms. Recently, an LC-based optically
controlled PBS was developed by Jau et al. [26]. They filled
the air space between the two prisms with azobenzene-LC and
showed the change in the polarization properties of light by
altering the phase of the LC using an external optical source.
The device required two optical laser sources of very high power
(1–2 W/cm2) to excite the LC molecules. Also, the non-splitting
mode (NSM) was not stable in this PBS; after a few hours,
the NSM mode of the device changed itself because of the
cis–trans isomerization of the azo-LC molecules. In this Letter,
we propose an electrically controlled and dual-mode PBS by
0146-9592/23/092357-04 Journal ©2023 Optica Publishing Group
