Strong Polarization Selectivity Research Articles

Meta‐Structured Silicon Nanophotonic Polarization Beam Splitter with an Optical Bandwidth of 415 nm

AbstractThe polarization beam splitter (PBS) is a pivotal element in the polarization management of free‐space optical instruments and systems. Photonic integrated circuits for sensing, imaging, communications, and quantum‐information processing also have needs for monolithically integrated PBSs with an ultra‐broad optical bandwidth. In this paper, a novel silicon nanophotonic PBS inspired by the crystalline Glan–Thompson prism but implemented with silicon subwavelength‐grating (SWG) metamaterials is presented. Due to the tailored artificial anisotropy of SWGs, the meta‐prism functions like a thin‐film reflector or a waveguide crossing for different polarizations. Thus, the incident light can be steered with strong polarization selectivity and negligible wavelength dependence. Unlike conventional PBS designs, the routing of polarized light is enabled by the wavelength‐independent total internal reflection in anisotropy‐engineered effective media, thereby breaking the bandwidth limit. The device footprint is as small as ≈15 × 7 µm2. Low insertion losses of 0.6–1.7 dB and high extinction ratios of 20–30 dB are experimentally achieved spanning a record broad bandwidth of over 415 nm, ranging from 1.26 to 1.675 µm wavelength. These results represent, to the best of their knowledge, the most broadband integrated PBS ever demonstrated to date.

Novel polarization‐dependent combiner for wide field‐of‐view glass‐like near‐eye displays

AbstractWe propose a glass‐like wide field of view near‐eye display with a polarization‐dependent combiner (PDC). The novel PDC consists of two polarization volume lenses with strong polarization selectivity. By inserting a PDC to the proposed polarization‐time multiplexing system, we demonstrate an overall horizontal FOV of 50° while maintaining a compact form factor.

Patterning Liquid-Crystal Alignment for Ultrathin Flat Optics.

Liquid-crystal (LC)-based ultrathin flat optical elements (FOEs) exhibit several attractive properties, such as a high degree of optical tunability, strong polarization selectivity, nearly 100% diffraction efficiency, and a simple fabrication process. Investigating the alignment patterning of LC-FOEs to diversify their performance has attracted broad interest in the optics field. In this mini-review, we start from the photoalignment (PA) process and then dive into device structures and performances. By generating and recording the desired polarization fields on the PA layer, the LC molecules will follow the recorded patterns and establish the phase profiles for different functionalities, such as gratings and lenses. Because of the polarization dependency, LC-FOEs have found useful applications in near-eye displays. Understanding the interactions between the PA mechanism and LC molecules helps to optimize the device performance for novel optical systems.

Polarization Volume Gratings for Near-Eye Displays and Novel Photonic Devices

Liquid crystal-based reflective polarization volume grating (PVG), also known as a linear Bragg–Berry phase optical element or a member of volume Bragg gratings (VBGs), is a functional planar structure with patterned orientation of optical axis. Due to the strong polarization selectivity, nearly 100% diffraction efficiency, large diffraction angle, and simple fabrication process, PVGs have found potential applications in novel photonic devices and emerging near-eye displays. In this review paper, we describe the operation principles, discuss the optical properties, present the fabrication methods, and provide promising applications of PVGs for near-eye displays and novel photonic devices.

Bifocal Fresnel Lens Based on the Polarization-Sensitive Metasurface

Thin structured surfaces allow flexible control over propagation of electromagnetic waves. Focusing and polarization state analysis are among functions, required for effective manipulation of radiation. Here a polarization sensitive Fresnel zone plate lens is proposed and experimentally demonstrated for GHz spectral range. Two spatially separated focal spots for orthogonal polarizations are obtained by designing metasurface pattern, made of overlapping tightly packed cross and rod shaped antennas with a strong polarization selectivity. Optimized subwavelength pattern allows multiplexing two different lenses with low polarization crosstalk on the same substrate and provides a control over focal spots of the lens only by changing of the polarization state of the incident wave. More than a wavelength separation between the focal spots was demonstrated for a broad spectral range, covering half a decade in frequency. The proposed concept could be straightforwardly extended for THz and visible spectra, where polarization-sensitive elements utilize localized plasmon resonance phenomenon.

Photothermal properties of core-capped gold nanoparticles

Photothermal effects associated with noble metal nanostructures have shown wide potential applications in photo-thermal cancer therapy, photo-thermal imaging, nanomedicine, etc. These applications benefit from the localized surface plasmon resonance (LSPR) effect of the nanoparticles. Due to the LSPR effect, the nanoparticles exhibit unique optical properties such as strong scattering and absorption in the band ranging from visible to near-infrared region. The absorption enables the plasmonic nanoparticle to be a thermal source to increase the temperature of itself and the localized surrounding environment. Among these particels, the anisotropic core-capped nanostructures distinguish themselves by their strong polarization selectivity. The absorptions are different when the incident light is polarized in the directions vertical (90) and parallel (0) to its symmetry axis, respectively. At 90, a large red-shift can be achieved and the absorption cross section is greatly enhanced. Moreover, their absorption peaks can be flexibly manipulated by slightly adjusting one of the geometrical parameters. However, the photothermal responses to these parameters are left blank. In this paper, photothermal effects of SiO2@Au core-capped nanoparticles are studied based on the numerical finite elemental analysis method (COMSOL software). The thermal response to each of the paramenters, including shell thickness, core diameter, core-shell ratio, and metal surface coverage is achieved. The calculation shows that the temperature of these core-capped nanoparticles can be adjusted efficiently in the near infrared band by easily rotating the polarization, i.e. slightly adjusting the geometric parameters. Especially in a range between 30 and 70, the temperature varying with the polarization follows almost a linear relationship. The comparisons with other popular structures including solid sphere, core-shell and nanorod are also made. The results indicate that at a similar size, the core-capped structure can offer a higher temperature than solid spheres and core-shell structures. To obtain the same temperature variation, the core-capped one has a smaller size than a nanorod. The comparisons demonstrate that the core-capped structure can be an alternative to a high-efficient nano heat source in the photothemal applications.

High-contrast wide-field evanescent wave illuminated subdiffraction imaging

In this Letter, we show how to obtain high-contrast wide-field evanescent wave illuminated subdiffraction imaging through controlling nanoscale light-matter interaction. The light coupling, propagation, and far-field imaging processes show strong polarization selectivity and film quality dependence, which is used to improve the image-contrast-to-noise ratio (CNR) and to enlarge the field of view (FOV). We demonstrate experimentally high CNR subdiffraction imaging with lateral resolution of 122nm and FOV of thousands of micrometers square.

Compact polarization beam splitter for silicon photonic integrated circuits with a 340-nm-thick silicon core layer.

A polarization beam splitter (PBS) is proposed and realized for silicon photonic integrated circuits with a 340-nm-thick silicon core layer by introducing an asymmetric directional coupler (ADC), which consists of a silicon-on-insulator (SOI) nanowire and a subwavelength grating (SWG) waveguide. The SWG is introduced to provide an optical waveguide which has much higher birefringence than a regular 340-nm-thick SOI nanowire, so that it is possible to make the phase-matching condition satisfied for TE polarization only in the present design when the waveguide dimensions are optimized. Meanwhile, there is a significant phase mismatching for TM polarization automatically. In this way, the present ADC enables strong polarization selectivity to realize a PBS that separates TE and TM polarizations to the cross and through ports, respectively. The realized PBS has a length of ∼2 μm for the coupling region. For the fabricated PBS, the extinction ratio (ER) is 15-30dB and the excess loss is 0.2-2.6dB for TE polarization while the ER is 20-27dB and the excess loss is 0.3-2.8dB for TM polarization when operating in the wavelength range of 1520-1580nm.

Plasmonic nano‐slits assisted polarization selective detour phase meta‐hologram

AbstractTraditional detour‐phase hologram is a powerful optical device for manipulating phase and amplitude of light, but it is usually not sensitive to the polarization of light. By introducing the light‐metasurface interaction mechanism to the traditional detour phase hologram, we design a novel plasmonic nano‐slits assisted polarization selective detour phase meta‐hologram, which has attractive advantages of polarization multiplexing ability, broadband response, and ultra‐compact size. The meta‐hologram relies on the dislocations of plasmonic slits to achieve arbitrary phase distributions, showing strong polarization selectivity to incident light due to the plasmonic response of deep‐subwavelength slits. To verify its polarization sensitive and broadband responses, we experimentally demonstrate two holographic patterns of an optical vortex and an Airy beam at p‐ and s‐polarized light with wavelengths of 532nm, 633nm and 780nm, respectively. Furthermore, we realize an application example of the meta‐hologram as a polarization multiplexed photonic device for multi‐channel optical angular momentum (OAM) generation and detection. Such meta‐holograms could find widespread applications in photonics, such as chip‐level beam shaping and high‐capacity OAM communication. image

Purified plasmonic lasing with strong polarization selectivity by reflection.

As miniaturized light sources of size beyond the optical diffraction limit, surface plasmon lasers are of particular interest for numerous exciting applications. Although convincing demonstrations of plasmonic lasing have been reported with the metal-insulator-semiconductor (MIS) hybrid design using semiconductor nanomaterials, it remains a challenge that conventional photonic lasing may be triggered and misinterpreted as plasmonic lasing. One way to address this issue is to cut off photonic modes in the waveguide by strictly restricting the semiconductor thickness. Here we propose a novel hybrid design, namely the dielectric-metal-insulator-semiconductor (DMIS) design that potentially solves the challenge. Taking advantage of strong polarization selectivity by reflection effect in favor of the plasmonic mode, whispering-gallery mode cavities based on the proposed DMIS design suppress possible photonic lasing modes and relieve the semiconductor thickness for purified plasmonic lasing. Using these cavities, we demonstrate room-temperature purified plasmon lasing with cadmium sulphide square nanobelts atop of a deposited multilayer film. Approaches for further improvement of the plamsonic lasing performance are discussed. Our design provides a reliable platform for developing better surface plasmon nanolasers using new semiconductor nanomaterials.

Light transmission through a circular metallic grating under broadband radial and azimuthal polarization illumination

We studied the characteristics of a circular metallic grating illuminated by broadband radial and azimuthal polarizations. We demonstrated that this scenario is the cylindrical analogue of a one-dimensional Cartesian grating illuminated by TM and TE polarizations. We measured the transmission spectra of this structure and observed strong polarization selectivity and, specifically, a resonance for radial polarization excitation, indicating a strong coupling to surface plasmons. The structure may be attractive for applications where pure radial polarization is needed, such as tight focusing, material processing, and particle trapping.

Design and fabrication of sub-wavelength metal wire-grid and its application to experimental study of polarimetric imaging

Metal wire-grid whose period is far less than the illuminating wavelength exhibits strong polarization selectivity within a large field of view over a wide wave length range. Thus it could be used as the polarization-controller in the polarimetric imaging system. For infrared polarimetric imaging system, an aluminum wire-grid on the CaF substrate with period of 200nm and depth of 100nm was designed based on effective medium theory. The calculation results demonstrate that the designed wire-grid has the extinction ratio larger than 35 dB within a field of view ±20° over both middle wave infrared (MWIR) (3—5 μm) and long wave infrared (LWIR) (8—12 μm). The aluminum wire-grid was fabricated by electron beam lithography, reactive ion etching and plasma stripping. A clear-cut polarimetric image was obtained when wire-grid was placed in front of a MWIR thermal infrared imager.

Spatially Graded Nanostructured Chiral Films as Tunable Circular Polarizers

AbstractIn this paper, we introduce a simple single‐step method for creating spatially graded helical nanostructured thin films. The films mimic some of the interesting polarization and coloration properties found in nature and enhance the application prospects for helically structured thin films. Our helical nanostructures, fabricated using a variant of the glancing angle deposition technique (GLAD), are spatially graded with thicknesses that vary by several microns across substrate lengths of several tens of centimeters. These thickness gradations are predicted by simulation and verified by scanning electron microscopy (SEM). The resultant films act as Bragg multilayers and can be employed as optical filters which not only preferentially transmit one handedness of circularly polarized light, but also allow for spatially determined frequency tunability. Through spectroscopic measurements, we demonstrate that when appropriate deposition conditions are chosen these nanostructures exhibit strong polarization selectivity, concurrent with excellent frequency tunability. The preferentially transmitted peak wavelength can be changed from approximately 620 to 690 nm by translating the film over a spatial distance of 30 mm. These graded nanostructures can be incorporated into photonic and sensing devices. The graded helical nanostructures may also be useful for providing a graded scaffolding to support liquid crystals.

Coupled-mode analysis of polarization-splitting directional coupler

The theory of a polarization-splitting directional coupler is developed with a coupled-mode approach for two interacting planar optical waveguides separated by a thin metal film. For TM guided modes and TE guided modes the coupled-mode equations governing the evolution of the modal amplitudes are obtained in an analytically closed form that provides a clear description of how the presence of a metal film brings about a strong polarization selectivity in the coupling between two optical waveguides. Representative numerical results show that, when silver or gold is used as the metal, polarization splitting with extinction ratios ∼ − 30 dB and throughput losses ∼0.2 dB is attainable in three typical wavelengths of 0.85, 1.30, and 1.55 μm.