Efficient Polarization Beam Splitter Based on All-Dielectric Metasurface in Visible Region
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In this paper, we present an all-dielectric gradient metasurface, composed of periodic arrangement of differently sized cross-shaped silicon nanoblocks resting on the fused silica substrate, to realize the function of polarization split in visible region. The cross-shaped silicon block arrays can induce two opposite transmission phase gradients along the x-direction for the linear x-polarization and y-polarization. By properly designing, the metasurface can separate the linearly polarized light into x- and y-polarized ones, which propagate at the same angle along the left and right sides of the normal incidence in the x-z plane. Particularly, when a beam with the polarization angle of 45.0° is incident on the proposed device, the x- and y-polarized transmitted ones possess nearly equal intensity within the wavelength range from 579 to 584 nm. We expect the proposed polarization beam splitter can play an important role for future free-space optical devices.
KeywordsPhase shift Metasurface Polarization beam splitters Refraction Visible region
Distance between neighboring units along the x-direction
Phase difference between neighboring units along the x-direction
Total transmission intensity
- Ix-pol .
Intensity of x-polarized transmitted beam
Intensity of y-polarized transmitted beam
Refractive index of the incident medium
Refractive index of the transmitted medium
Angle of incidence
Angle of anomalous refraction
Incident wavelength in vacuum
In recent years, metasurfaces, two-dimensional subwavelength structures composed of nanoantennas in an array configuration, have obtained enormous attentions. Metasurface can manipulate the incident light on a subwavelength scale because its ultrathin structured thickness introduces abrupt changes of the incident beam parameters. For example, the phase [1, 2, 3, 4, 5], amplitude [6, 7, 8, 9], and polarization [10, 11, 12, 13] of the incident beams can be manipulated by adjusting the shape, size, and orientation of the subwavelength nanoantennas. In comparison with the conventional bulky materials, the metasufrace devices are easier to be fabricated and their ultrathin thickness in the optical path can greatly suppress transmission losses. Based on the above exciting advantages, metasurfaces have been used in many applications, such as polarization converter [11, 12, 13], full-color printing , holography , flat lenses , optical vortex generation [4, 17], and spectrum splitting [18, 19, 20, 21].
Metallic nanostructures were utilized to constitute metasurfaces with beam deflection originally [1, 22, 23]. The required 2π phase coverage can generally be achieved based on two methods. The one is generating two independent resonances, each of which introduces a phase shift of π. The other is to spatially rotate the polarization-dependent subwavelength resonators from 0° to 180°. However, the absorption losses of metallic metasurfaces limit the efficiency in transmission mode. All-dielectric metasurfaces have recently been proposed to substitute the metallic ones due to their low absorption losses [24, 25, 26, 27, 28]. To date, three different approaches have been demonstrated to realize the 2π phase shift in the all-dielectric metasurfaces, geometric phase , Mie resonance [2, 4, 7], and Fabry–Pérot resonance [3, 28]. The first method is similar to the above second way of metallic metasurface; it works for circularly polarized light. The second mechanism covers the full 2π phase range based on spectrally overlapping magnetic and electric resonances; the metasurface designed based on this way is also known as Huygens metasurface. The third method, just as the one utilized in this paper, uses high aspect ratio nanoantennas to obtain the desired phase control. The antennas can be considered as truncated waveguides in this case, and transmission phase is manipulated by the effective refractive index of the fundamental mode in differently sized dielectric antennas. Silicon is generally applied in all-dielectric metasurface devices [2, 3, 4] for its high refractive index, low loss, and mature process manufacturing. As for some other low refractive index materials, such as silica (SiO2), silicon nitride (Si3N4), and titanium dioxide (TiO2), their losses may be ignored, but the higher aspect ratios make the fabrication very challenging.
Polarization beam splitter, a device which can separate an optical beam into two orthogonally polarized components propagating along different paths, is an important component in optical systems. Polarization beam splitters reported in the literatures are designed primarily based on the following structures, including subwavelength structures [29, 30, 31], hybrid plasmonic couplers , gratings , multimode interference (MMI) structures , and asymmetrical directional couplers [35, 36]. Farahani and Mosallaei  proposed an infrared reflectarray metasurface to reradiate incoming light into two orthogonally polarized reflective beams. Guo et al.  designed a polarization splitter based on silicon metasurfaces at the specific wavelength of 1500 nm. In this work, we propose a simple and large-angle deflected polarization beam splitter based on dielectric metasurface, which is constructed by different cross-shaped silicon resonator arrays atop of the silica substrate. When x- or y-polarized light is normally incident, the polarization direction of the transmitted light is the same as that of the incident light. At a wavelength of 583 nm, the deflected angle is 46.78° and the deflection efficiency is 63.7% under x-polarized incidence, while the deflection efficiency is 66.4% and the deflected angle is − 46.78° for y-polarized one. Furthermore, the proposed device is capable of separating the linearly polarized light into x- and y-polarized ones. Especially, when the polarization of the incident light is at an angle of 45° to the x-axis, two orthogonally polarized transmitted beams possess approximately equal intensities within the wavelength region from 579 to 584 nm.
Results and Discussions
Optical parameters at different wavelengths
In summary, we design a polarization beam splitter based on the all-dielectric metasurface in visible region. The metasurface is composed of cross-shaped silicon nanoblock arrays placed on top of silica dielectric substrate. When the incident light is polarized at the angle 45° relative to x-direction, identical intensities of the x- and y-polarized output signals are 0.336 at the operating wavelength 583 nm, which accounts for 46.3% of the total transmission intensity. Moreover, the proposed device exhibits equal-power polarization beam splitting performance for 45° polarized incidence within the wavelength region from 579 to 584 nm. We expect the polarization beam splitter can be further applied in the future all-optical integrated devices.
National Key R&D Program of China (2016YFA0301300); National Natural Science Foundation of China (NSFC) (Nos.61671090 and 61875021); BUPT Excellent Ph.D. Students Foundation (CX2018114); Fund of State Key Laboratory of Information Photonics and Optical Communications (IPOC20172204); Guangxi Key Laboratory of Wireless Wideband Communication and Signal Processing, Natural Science Foundation of Beijing (2192036).
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.
JL and CL carried out the simulation and analysis. YL, TW, and LY supervised the writing of the manuscript. YW created the figures. HY and ZY supervised the whole work. All the authors have read and approved the final manuscript.
The authors declare that they have no competing interests.
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