Broadband Polarization Research Articles

Compact and broadband on-chip silicon polarization beam splitter based on tilted subwavelength grating

Efficient polarization management is a key issue for on-chip silicon photonic integrated circuits. Here, we propose a compact and broadband on-chip silicon polarization beam splitter (PBS) using tilted subwavelength grating (SWG) structure. The key polarization separation region consists of an input tapered SWG, a titled SWG, and two output tapered SWGs at both output ports. The tilted SWG has a strong polarization dependence for the mode coupling, and we have introduced different grating duty cycles, positional offset, and a new, to the best of our knowledge, output mode transition structure to enhance the device performance. Based on these techniques, the key coupling length of the proposed PBS is only 5 µm (total length, 12 µm), together with polarization extinction ratio (ER) 21.2 dB (19.7 dB) and insertion loss 0.21 dB (0.39 dB) for the TE (TM) mode. Moreover, the device working bandwidth can be extended to 311 nm (154 nm) by keeping E R > 15 d B ( E R > 18 d B ), satisfying the on-chip broadband applications. We believe the proposed device could become a good candidate to boost the on-chip polarization management toward compact and broadband directions.

Near-infrared broadband polarization beam splitter with an Au nanocube array

A near-infrared broadband polarization beam splitter (PBS) is proposed and numerically simulated, which is realized by a directional coupler assisted with an Au nanocube array. The fundamental transverse electric (TE) mode can be coupled to the output of the upper waveguide by exciting the surface plasmon polaritons (SPPs), while the fundamental transverse magnetic (TM) mode is output directly from the lower waveguide. The length of the coupling region of the PBS is only 1 μm. It has a working bandwidth of 210 nm in the range of 1420 ∼ 1630 nm, covering three bands: S-, C- and L-bands. The simulation shows that the extinction ratio of TE and TM polarization is 20.31 dB and 14.15 dB, respectively.

Low-profile and wide-band RCS-reduction antenna array based on the metasurface polarization converter.

Antenna elements with a low profile and high front-to-back (FB) ratio mean that no additional reflective cavity is required when forming the array, which greatly helps to simplify and lighten the entire array system. In this paper, we enhance the FB ratio of the antenna to 35 dB while maintaining an ultra-low profile of 0.05 λ0 by attaching the broadband polarization conversion metasurfaces (PCMs) as the parasitic patches to the surface of the radiating patch. Meanwhile, the array formed by the proposed antenna is arranged in a checkerboard form, which makes it have a lower radar cross section (RCS) in the X- and Ku- bands. Even with PCMs loaded, the antenna element maintains a small size of 0.58 λ0 × 0.58 λ0, which ensures the proposed array can achieve the ± 45° beam scanning, making it suitable for the phased array. For verification, we propose a low-sidelobe array composed of the proposed antenna elements, which exhibits a low profile, high FB ratio, and broadband RCS reduction through simulation and measurement.

Silicon Photonic Wires for Broadband Polarization Entanglement at Telecommunication Wavelengths

Compact and scalable sources of broadband polarization entanglement at telecommunication wavelengths will pave the way for multiuser long-distance quantum communication at enhanced data rates, but progress toward this goal has been hindered due to large birefringence in conventional silicon-on-insulator nanowaveguides. The authors theoretically demonstrate, via dispersion engineering, the successful generation of polarization-entangled photon pairs over a broad range of wavelengths. This work also provides a strategy to avoid entanglement degradation due to polarization-mode dispersion. The proposed devices will be useful building blocks for large-scale quantum communication networks.

Broadband Polarization Rotator and Splitter Based on 70 nm-Etched Waveguides on SOI Platform

The standard silicon photonic platforms provide three-step silicon etching, i.e., 220 nm for full etching, 70 nm for shallow etching, and 130 nm (or 150 nm) for slab etching. Previously reported mode-evolution-based polarization rotators and splitters (PSRs) usually employ 130 nm-etched slab waveguides for adiabatic TM0-to-TE1 conversion, however, they are not compatible with the platforms adopting 150 nm-etching techniques. In this paper, we demonstrate a broadband PSR based on 70 nm-etched slab waveguides, which is compatible with all the platforms. The PSR consists of a bi-level taper and an inverse-tapered coupler. The length of the polarization rotator shrinks from hundreds to only thirty microns by employing the 70 nm-etched slab waveguides, while a high efficiency of >95% is achieved, covering an ultra-wide bandwidth from 1250 nm to 1650 nm. The proposed PSR shows superior performance over S, C, and L bands. Low cross-talk of <−20 dB and a loss of <1.5 dB are experimentally confirmed over a wavelength range of 75 nm.

Broadband and low-loss TM-pass polarizer using tilted subwavelength structures

Photonic systems built on the Silicon-on-Insulator platform exhibit a strong birefringence, and must thus be operated with a single polarization for most applications. Hence, on-chip polarizers that can effectively suppress an undesired polarization state are key components for these systems. Polarizers that extinguish TE polarized light while letting TM polarized light pass with low losses are particularly challenging to design for the standard 220 nm Silicon-on-Insulator platform, because the modal confinement is stronger for TE polarization than for TM polarzation. Here, we propose and design a broadband, low loss and high extinction ratio TM-pass polarizer by engineering a Bragg grating that reflects the fundamental TE mode into the first order TE mode using a subwavelength metamaterial which at the same time allows the TM mode to pass. Our device achieves an extinction ratio in excess of 20 dB, insertion losses below 0.5 dB and back-reflections of the fundamental TE mode of the order of -20 dB in a bandwidth of 150 nm as demonstrated with full 3D-FDTD simulations.

Design of Compact and Broadband Polarization Beam Splitters Based on Surface Plasmonic Resonance in Photonic Crystal Fibers.

In this work, a polarization beam splitter (PBS) based on surface plasmonic resonance is proposed and realized in a designed photonic crystal fiber (PCF). The PCF consists of two kinds of air holes with different diameters. Two solid silica cores near the center of the PCF are established by removing the cladding air holes. A gold film is plated at the external surface of the central air hole of the PCF to excite the surface plasmonic resonance. In order to minimize the length and improve the operation bandwidth of the PBS, the influences of the transversal structural parameters of the PCF are investigated in the context of both X and Y polarization beams. It was found that a 123.6-m-long PBS with an operation bandwidth of 314 nm could be realized after the global optimization of five structural parameters. The proposed PBS may have potential applications in micro-/nano-optical systems for sensing and communications.

Synthesis of two quasi-optical polarizers for the multi-frequency high-power millimeter wave system.

Millimeter wave systems based on powerful gyrotron can deliver megawatt microwave power, which is an important auxiliary heating method for magnetic confinement fusion devices. Reflective gratings are normally used as quasi-optical polarizers for polarization control of the wave in such systems. Based on the coordinate transformation method, we developed a numerical code to study the broadband polarization strategy. By considering the synthesis of two polarizers, we designed a pair of polarizers in the W band. Calculation results indicated that almost arbitrary polarization can be achieved in a wide frequency range. To verify the design, we set up a low-power test platform and experiments were carried out. The performance of the polarizers was tested from 80 to 105GHz with a step of 5GHz. The test results agreed well with the numerical results, indicating that the design is reasonable. With the development of multi-frequency gyrotrons, the study in this paper can be used as a reference for the design of broadband polarizers of multi-frequency millimeter wave systems.

Tri-functional metamaterials integrated with vanadium dioxide in terahertz regions

In this paper, we theoretically and numerically demonstrate a vanadium dioxide (VO2)-based metamaterial device with triple functionalities in the terahertz region. By altering the phase transition characteristic of VO2, broadband perfect absorption and asymmetric transmission (AT) can be switchable obtained in a wideband frequency regime. When VO2 operates in the conducting state, the designed metamaterial acts like a polarization-dependent broadband perfect absorber. The absorbance is above 95% for x-polarized lights in the frequency range from 5.2 to 7.1 THz, while the absorbance is as low as 20% when the polarization is along the y-axis. When VO2 serves in the insulating state, the designed metamaterial device exhibits an AT effect with efficiency ∼0.72 in terahertz regions. Moreover, broadband polarization conversion associated with the asymmetric transmission can be achieved in the terahertz region. The electric field distributions combined with the current vectors are introduced to explore the physical mechanism. Besides, the performance of proposed metamaterial can be adjusted by altering the structural parameters. This designed metamaterial may have potential applications in exploring optoelectronic devices, such as optical switching, optical diode and thermal emitters in THz frequencies.

Broadband absorption and polarization conversion switchable terahertz metamaterial device based on vanadium dioxide

A broadband absorption and polarization conversion switchable terahertz metamaterial device can be obtained by introducing vanadium dioxide with the metal to insulate phase transition. As the vanadium dioxide is in the metallic state, the device is a broadband absorber with over 90% absorption from 1.03 THz to 2.62 THz, and the broadband absorption performance is not sensitive to both the oblique incident angle and polarization. When vanadium dioxide is in insulate state, the device performs as a polarization converter with two functions of linear to cross polarization and linear to circular polarization conversion. The polarization conversion efficiency of linear to cross polarization conversion is above 90% from 1.81 THz to 2.39 THz, while the ellipticity of linear to circular polarization conversion is over 0.9 in the ranges of 0.75–1.69 THz, 2.48–2.59 THz and 2.81–2.90 THz, respectively. The proposed bifunctional device provides the potential in THz devices with tunable and diverse functionalities.

Fabrication tolerant and broadband polarization splitter-rotator based on adiabatic mode evolution on thin-film lithium niobate

A polarization splitter-rotator device can facilitate on-chip polarization-division multiplexing to enhance the transmission data rate. Here, we propose and experimentally demonstrate a polarization splitter-rotator based on adiabatic mode evolution on the thin-film lithium niobate platform. The measured results for a fabricated device show low insertion losses of <-0.5 dB and large extinction ratios of >20 dB over the 110-nm band. Large fabrication tolerance is also demonstrated with extinction ratios of >15 dB in the wavelength range of 1465-1630 nm for a waveguide width variation of 80 nm.

Single/Dual/Triple Broadband Metasurface Based Polarisation Converter with High Angular Stability for Terahertz Applications.

This paper presents design and characterisation of a new compact metasurface based linear polarisation converter for terahertz applications. The metasurface unit cell with periodicity of 0.292 consists of an asymmetrically oriented planar double semicircular goblet-shaped resonators. It is printed on a polydimethylsiloxane (PDMS) dielectric substrate backed by a gold layer that acts as a ground plane. This metasurface structure exhibits a broadband cross-polarisation conversion in the frequency range of 0.72–0.99 THz with a polarisation conversion ratio (PCR) > 95% and angular stability > for both TE and TM modes. However, the PCR for the single band is >99% at resonant frequencies of 0.755 and 0.94 THz, while the optimised design shows 100% PCR over a BW of 95 GHz. Furthermore, slight modification and optimisation of the broadband design results in quad-ring and slotted DSGRs that produce dual and triple broadband polarisation conversion, respectively. The quad-ring DSGR performs polarisation conversion for frequency range of 0.70–1.08 and 1.61–1.76 THz while the slotted DSGR shows the triple broadband cross-conversion for frequency range of 0.67–0.85, 1.04–1.11, and 1.62–1.76 THz with PCR > 95%. This design is simple, easy to modify to implement single and multi broadband polarisation conversion with high PCR at terahertz regime. In addition to that, it is easy to fabricate and integrate with other components like multiple-input multiple-output terahertz antennas for mutual coupling reduction.

Broadband ultra-compact polarization splitter-rotator using diagonally overlapped bi-layer architecture.

A broadband and ultra-compact polarization splitter-rotator based on diagonally overlapped bi-layer architecture and an asymmetrical directional coupler is proposed on a silicon-on-insulator platform. By leveraging the structure over supermode theory, a 1-dB bandwidth of 220 nm, extinction ratio (ER) of <19dB, and cross talk (XT) of <-15.85dB within the span of 1400-1700 nm and coupling length of 4.62 µm are achieved. In addition, TM0-TE0 conversion loss of ∼0.19dB, ER of 35.88 dB, and XT of -30.46dB can be obtained at 1550 nm. The fabrication tolerances are also analyzed, indicating that the insertion losses remain below 1 dB over 1460-1620 nm in terms of width errors and layer-to-layer misalignments within ±10nm. The results show that the proposed device is very suitable to utilize between fibers and for polarization diversity of on-chip systems for broadband operation as well as ultra-compact integration.

Switchable bi-functional metamaterial based on vanadium dioxide for broadband absorption and broadband polarization in terahertz band

A switchable bi-functional optical metamaterial for broadband absorption and broadband cross-polarization conversion is proposed in the terahertz band based on the phase transition of vanadium dioxide (VO2). When VO2 is in the metal phase, the designed metamaterial has the function of a broadband absorber. This absorber consists of a VO2 top structure, a polyimide layer, and VO2 film. Calculated results show that the absorptance of the absorber is higher than 90% in the 3.33–5.62 THz range. Because of its symmetry, the absorber is insensitive to polarization and maintains high absorptance in incident angles from 0° to 60°. When VO2 is in the insulator phase, the designed metamaterial has the function of a broadband cross-polarization converter. This polarization converter comprises two gap gold square rings, a polyimide layer, and a gold substrate. The calculated results show that more than 90% cross-polarization conversion can be achieved at the vertical incidence of x and y polarization in the 2.54–4.55 THz range. Nearly 100% cross-polarization conversion can be achieved at 2.62, 3.29, and 4.33 THz. The designed metamaterial has a wide potential application prospect in fields including stealth technology, solar electromagnetism, and radar communication.

A single-layered metasurface inspired broadband polarization reconfigurable printed monopole antenna for hybrid wireless applications

A single-layered metasurface-loaded high-gain & broadband polarization reconfigurable λo/4 printed monopole antenna is investigated in this article. The proposed antenna configuration comprises of Y-shaped radiating monopole over partial ground plane with the extended twin parasitic conducting strips (PCS), is loaded with a single-layered metasurface (MS) reflector. To attain circular polarization (CP) characteristics, a BAR 50-02 V PIN Diode from Infineon is used to short the partial ground plane and one of the twin parasitic conducting strips (PCS-L). By utilizing the grid-slotted sub patches on the rectangular reflecting surfaces of MS layer with a volumetric dimension of 2λo × 1.65λo × 0.02λo, is placed just below Y-shaped monopole radiator (PYMA) at a height of 0.33λo, in which broadened impedance (IBW) and 3-dB axial ratio bandwidth (ARBW) with the enhanced CP antenna gain are achieved. Finally, proposed prototype of 1.33λo × 0.9λo × 0.02λ o, where λo = 5 GHz is fabricated and measured. It offers an measured IBW of 48.45 % (3.57–5.85 GHz), ARBW of 25.96 % (4.19–5.44 GHz) and the CP antenna gain of >8.35 dBic with antenna efficiency of >75 % in the desired operating bands. From the above results, the performance of single-layered metasurface-inspired polarization reconfigurable antenna confirms its suitability for hybrid wireless applications and also, it can be extended towards the scope of designing efficient RF energy harvesting system.

Optimal design for a broadband Stokes polarimeter of liquid crystal variable retarders.

Liquid crystal variable retarders (LCVRs) are the core component for rapid and high-precision broadband polarization detection. Additionally, the ability to suppress noise greatly affects the results of polarization measurements. In this work, a solving optimal design approach is proposed for building a high-performance broadband Stokes polarimeter based on LCVRs, which greatly reduces the influences of data fluctuation from liquid crystals and dispersion on the experimental results. This method relies on evaluation criteria of the condition number (CN) to build a gradual optimization that includes the following three steps: fixing the fast axis angles, meeting the requirements of a wideband, and ensuring a minimum CN. Additionally, with the method of increasing the measurement analysis vector, we ensure the whole band in the low CN and offer a solution to the problem of the difficulty in optimizing the LCVRs caused by the large change of retardance at 490-700 nm. Finally, the rapid and high-precision Stokes measurement of 490-700 nm wavelengths is achieved. We test the performance of the polarimeter after optimization in our simulation and experiment, which shows that the total RMS error is less than 0.032 and the single point error is small. This work not only reduces the influence of LCVR error on the experimental results but also makes it possible to apply LCVRs to 490-700 nm detection.

Wide Bandgap Perovskite Photovoltaic Cells for Stray Light Recycling in a System Emitting Broadband Polarized Light

AbstractPerovskite based photovoltaic (PV) cells are unique in combining low open‐circuit voltage losses with a broad bandgap tunability. This makes them an ideal PV cell to recycle photons back into electrical power in a variety of illumination systems or light emitting devices. Here, advantage of these features is taken and wide bandgap (WBG) perovskite PV cells are incorporated in devices suitable for display illumination and demonstrate a high yield in stray light recycling back into electricity with up to a 37.5% power conversion efficiency. The specific device considered is a modified half‐cylinder photonic plate designed to emit diffused broadband polarized light using a nonabsorbing reflective polarizer based on a random dielectric layer distribution. It is experimentally demonstrated that light recycling using appropriately tuned WBG perovskite PV cells becomes very efficient when implemented in systems where the light is emitted from narrowband sources, even if the emission spans a broad wavelength range.

Anisotropic coding metasurfaces and their active manipulation based on vanadium dioxide for multifunctional applications in the terahertz region.

Various kinds of metasurfaces have been proposed because they can be tailored to achieve the desired modulations on electromagnetic wave that do not occur in nature. Compared to conventional metamaterials, coding metasurfaces integrated with information science theory possess numerous distinctive advantages - simple design, time-saving and compatibility with digital devices. Here we propose terahertz multifunctional anisotropic reflective metasurfaces with a metal-insulator-metal cavity structure whose top constructional layer consists of a pair of gold arc-rings and a gold cut-wire located between them. Two different functions of narrow-band absorption and broadband polarization conversion are realized based on different coding matrices using the binary codes '0' and '1'. Furthermore, we integrate a specific coding metasurface with vanadium dioxide (VO2) to realize a temperature-controlled active metasurface. Through the temperature change, dynamic functionalities switching between a narrow-band polarization converter with a polarization conversion ratio over 94% and an efficient low-pass filter are achieved under the phase transition of VO2, and the active metasurface is polarization independent. The proposed coding metasurfaces are verified numerically and experimentally, and have promising applications in terahertz modulation and functional devices.

VO2-assisted multifunctional metamaterial for polarization conversion and asymmetric transmission.

A VO2-assisted temperature-controlled multifunctional metamaterial polarization converter with large asymmetric transmission (AT) is proposed by introducing a gold-VO2 grating. The converter can be switched between reflection mode and transmission mode by controlling the phase transition. When VO2 is in the metallic state, the converter works in reflection mode, converting the incident forward/backward linearly/circularly polarized waves into the cross-polarized waves, and the broadband polarization conversion rates (PCRs) can reach 90% with relative bandwidth of up to 91.1% and 87.5%, respectively; when VO2 is in the insulating state, the converter shows giant AT effect for circularly polarized waves at 0.64 THz and 1.28 THz. The multifunctional polarization converter holds great potential in the fields of communication and imaging, which provides a new way to design optical devices such as polarizers, isolators.

(Keynote) Van Der Waals Active Metasurfaces and Heterostructures for Phase Modulation and Polarization Conversion

A grand challenge for nanophotonics is the realization of tunable metasurfaces enabling active control of the key constitutive properties of light – amplitude, phase, wavevector and polarization. Active metasurfaces that enable dynamic modulation of reflection amplitude, phase and polarization have been recently explored using several active materials and modulation phenomena, including carrier index in plasmonic ENZ structures, reorientation of liquid crystal molecules, electrooptic effects in quantum well heterostructures and index change in phase change materials. The rapid advances in understanding of exciton resonances in layered van der Waals materials has now stimulated thinking about active metasurfaces that exploit excitonic modulation phenomena to enable ‘van der Waals active metasurfaces’. As one example, I will describe recent advances in electrically reconfigurable polarization conversion across the telecommunication wavelength range in van der Waals layered materials, integrated in a Fabry-Pérot cavity. The large electrical tunability of the excitonic birefringence in tri-layer black phosphorus enables spectrally broadband polarization conversion over nearly half the Poincaré sphere. We observe both linear to circular and cross-polarization conversion with voltage, demonstrating dynamic access to polarization diversity. As a second example, we discuss the observed large gate tunability of the complex refractive index in monolayer MoSe2 by Fermi level modulation near the A and B excitonic resonances for temperatures between 4 K to 150 K. By tuning the charge density, we observe both temperature and carrier dependent epsilon-near-zero response in the permittivity and transition from metallic to dielectric near the A exciton energy, and directly observe active phase modulation in monolayer MoSe2 gated heterostructures, whose voltage dependence is consistent with our complex index measurements. These results have broad implications for the use of monolayer transition metal dichalcogenides in active metasurfaces, and I also will give a general outlook for the wide range of possibilities for active van der Waals metasurfaces.