Incident Polarization State Research Articles

Differentiation of suspended particles by polarized light scattering at 120°.

Probing suspended particles in seawater, such as microalgae, microplastics and silts, is very important for environmental monitoring and ecological research. We propose a method based on polarized light scattering to differentiate different suspended particles massively and rapidly. The optical path follows a similar design of a commonly used marine instrument, BB9, which records backscattering of non-polarized light at 120°. In addition, polarization elements are added to the incident and scattering path for taking polarization measurements. Experiments with polystyrene microspheres, porous polystyrene microspheres, silicon dioxide microspheres, and different marine microalgae show that by carefully choosing the incident polarization state and analyzing the polarization features of the scattered light at 120°, these particles can be effectively differentiated. Simulations based on the Mie scattering theory and discrete dipole approximation (DDA) have also been conducted for particles of different sizes, shapes and refractive indices, which help to understand the relationship between the polarization features and the physical properties of the particles. The laboratory system may serve as a prove-of-concept prototype of new instrumentations for applications on board or even with submersibles.

Linear-cavity cylindrical vector lasers based on all-fiber mode converters

We propose three mode converters based on coupling of the fundamental mode and the first group higher-order modes in fiber tapers, angled fiber facets, and transversely dislocated fibers, respectively. The insertion loss and coupling efficiency of the fiber tapers are 0.541 dB and 10.6% respectively, which are much better than those of the angled fiber facets (2.4 dB and 2.9%) or the transversely dislocated fibers (1.2 dB and 5.6%). Cylindrical vector beams are delivered from an erbium-doped fiber laser after separately incorporating the mode converters into the cavity. The output spectra of three lasers are located at 1548.7 nm with the same 3-dB bandwidth of 0.03 nm, and spatial distributions are switchable between radially and azimuthally polarized states by adjusting the incident polarization state of the fundamental mode. The linear-cavity all-fiber lasers are capable of generating flexible and cost-effective cylindrical vector beams, which are quite attractive for fiber-based sensing and communications.

Dynamic Janus Metasurfaces in the Visible Spectral Region.

Janus monolayers have long been captivated as a popular notion for breaking in-plane and out-of-plane structural symmetry. Originated from chemistry and materials science, the concept of Janus functions have been recently extended to ultrathin metasurfaces by arranging meta-atoms asymmetrically with respect to the propagation or polarization direction of the incident light. However, such metasurfaces are intrinsically static and the information they carry can be straightforwardly decrypted by scanning the incident light directions and polarization states once the devices are fabricated. In this Letter, we present a dynamic Janus metasurface scheme in the visible spectral region. In each super unit cell, three plasmonic pixels are categorized into two sets. One set contains a magnesium nanorod and a gold nanorod that are orthogonally oriented with respect to each other, working as counter pixels. The other set only contains a magnesium nanorod. The effective pixels on the Janus metasurface can be reversibly regulated by hydrogenation/dehydrogenation of the magnesium nanorods. Such dynamic controllability at visible frequencies allows for flat optical elements with novel functionalities including beam steering, bifocal lensing, holographic encryption, and dual optical function switching.

Visible frequency plasmonic perfect absorber made of a thin metal layer containing cylindrical grooves

In this communication, a visible frequency plasmonic perfect absorber (PPA) made of a thin metal layer (gold and aluminum) with an array of cylindrical grooves is demonstrated and studied numerically using the finite difference time domain solver. A novel design concept is used to achieve perfect absorption at tunable wavelengths between 400 and 1000 nm. The PPA unit cell is made of a cylindrical groove of appropriate depth, and the size of the groove is typically set to half the array period. By simply changing the array period, the perfect absorption (100%) spectral position is set to a desired wavelength. The PPA is observed to be very effective for wide incident angles and polarization states of input radiation. The study reports an attractive alternative way of designing all-metal-based PPA that may have potentials for various plasmonic and photonic applications.

Diffraction efficiency performance of random anti-reflecting subwavelength surface structures on prefabricated fused silica binary gratings.

Random anti-reflecting subwavelength surface structures have been reported to enhance transmission of optical windows and lenses. Specifically, for fused silica substrates, 99.9% specular transmission has been verified by various groups. Diffractive optical elements, such as gratings, also experience net Fresnel losses on both their planar and structured surfaces. We investigated the performance of prefabricated 50% duty-cycle, binary, fused silica linear gratings, with a period of 1.6μm, before and after application of random anti-reflecting subwavelength surface structures, in order to reduce their initial Fresnel reflectivity. We compared the diffraction order directions and their efficiencies at three test wavelengths: 594, 612, and 633nm, for both TE(s) and TM(p) incident light polarization states, under three different mountings: normal, first Bragg, and second Bragg incidence. We report transmission enhancement of the sum of all propagating grating orders for all cases tested by factors between 2% and 10%, with reduction of the respective reflected orders by similar ratios. Transmission enhancement of the -2 diffraction order at Bragg incidence suggests that the random etch has different rates between the raised and lowered linear grating topography.

Analyzing characteristics of spiral vector beams generated by mixing of orthogonal LP11 modes in a few-mode optical fiber.

We have demonstrated a method for the generation of spiral vector beams using a few-mode optical fiber. The beams are generated due to the combination of orthogonal linearly polarized modes with a tilted polarization vector in the few-mode optical fiber. The excitation of these modes is controlled by the incident state of polarization and coupling conditions at the input end of the fiber. The beams with clockwise and counterclockwise nature of spiral polarization are generated, and switching between them is demonstrated. The obtained results emphasize the strong dependency of polarization of generated spiral vector beams and other orthogonal linearly polarized modes on the input state of polarization. The simulated results of mode mixing are verified experimentally with good agreement.

Hybridization-induced broadband terahertz wave absorption with graphene metasurfaces

Electromagnetic (EM) wave absorption plays a vital role in photonics. While metasurfaces are proposed to absorb EM waves efficiently, most of them exhibit limited bandwidth and fixed functionalities. Here, we propose a broadband and tunable terahertz (THz) absorber based on a graphene-based metasurface, which is constructed by a single layer of closely patterned graphene concentric double rings and a metallic mirror separated by an ultrathin SiO2 layer. Plasmonic hybridization between two graphene rings significantly enlarges the absorption bandwidth, which can be further tuned by gating the graphene. Moreover, the specific design also makes our device insensitive to the incident angle and polarization state of impinging EM waves. Our results may inspire certain wave-modulation-related applications, such as THz imaging, smart absorber, tunable sensor, etc.

Ge2 Sb2 Te5 -Based Tunable Perfect Absorber Cavity with Phase Singularity at Visible Frequencies.

The metal-dielectric stacks-based asymmetric Fabry-Perot (F-P) cavity systems have recently attracted much interest from the scientific community for realizing perfect absorption over the spectral bands from visible to infrared since they possess a lithography-free design that is cost-effective and scalable. This study experimentally demonstrates an asymmetric F-P cavity system for achieving tunable wide angle perfect absorption and phase singularity. The proposed system shows tunable multiband perfect absorption in the visible spectral region by incorporating an ultrathin layer of phase change material such as Ge2 Sb2 Te5 (GST) in the stack. The system shows multi-narrowband perfect absorption with a maximum of 99.8% at a specific incident angle and polarization state when the GST is in amorphous phase; however, the absorption bands blueshift and broaden after switching to the crystalline phase. More importantly, the proposed scheme shows tunable phase singularity at the reflection-less point. The obtained tunable perfect absorption and abrupt phase change are solely due to the presence of a highly absorbing ultrathin layer of GST in the stack. Experimental results are validated using an analytical simulation model based on a transfer matrix method. The proposed scheme could find potential applications in active photonic devices such as phase-sensitive biosensors and absorption filters.

Measurement of Giant Spin Splitting of Reflected Gaussian Beams

We investigate theoretically and experimentally the beam shifts of a Gaussian beam reflected by an air-glass interface at Brewster incidence. For a linearly polarized incident beam, the reflected beam will undergo a spatial spin splitting within the plane of incidence. This spin splitting is up to 83 μm when the incident beam waist is 90 μm. Additionally, the reflected beam will be deflected along the direction perpendicular to the incident plane, i.e., undergo an angular shift. The maximum angular shift has been theoretically predicted and experimentally confirmed. Both the spatial spin splitting and angular shift can be controlled by the incident polarization state. These controllable giant beam shifts have potential applications in the design of spin-photonic devices and the optical sensing.

Two-dimensional arbitrary nano-manipulation on a plasmonic metasurface.

In this Letter, we report on a plasmonic nano-ellipse metasurface with the purpose of trapping and two-dimensional (2D) arbitrary transport of nanoparticles by means of rotating the polarization of an excitation beam. The locations of hot spots within a metasurface are polarization dependent, thus making it possible to turn on/off the adjacent hot spots and then convey the trapped target by rotating the incident polarization state. For the case of a metasurface with a unit cell of perpendicularly orientated nano-ellipses, the hot spots with higher intensities are located at both apexes of the nano-ellipse whose major axis is parallel to the direction of polarization. When the polarization gradually rotates to its counterpart direction, the trapped particle may move around the ellipse and transfer to the most adjacent ellipse, due to the unbalanced trap potentials around the nano-ellipse. Clockwise and counterclockwise rotation would guide the particle in a different direction, which makes it possible to convey the particle arbitrarily within the plasmonic metasurface by setting a time sequence of polarization rotation. As confirmed by the three-dimensional finite-difference time-domain analysis, our design offers a novel scheme of 2D arbitrary transport with nanometer accuracy, which could be used in many on-chip optofluidic applications.

Invited Article: An active terahertz polarization converter employing vanadium dioxide and a metal wire grating in total internal reflection geometry

Active broadband terahertz (THz) polarization manipulation devices are challenging to realize, but also of great demand in broadband terahertz systems. Vanadium dioxide (VO2) shows a promising phase transition for active control of THz waves and provides broadband polarization characteristics when integrated within grating-type structures. We creatively combine a VO2-based grating structure with a total internal reflection (TIR) geometry providing a novel interaction mechanism between the electromagnetic waves and the device, to realize a powerful active broadband THz polarization-controlling device. The device is based on a Si-substrate coated with a VO2 layer and a metal grating structure on top, attached to a prism for generating the TIR condition on the Si-VO2-grating interface. The grating is connected to electrodes for electrically switching the VO2 between its insulating and conducting phases. By properly selecting the incident angle of the THz waves, the grating direction, and the incident polarization state, we first achieved a broadband intensity modulator under a fused silica prism with an average modulation depth of 99.75% in the 0.2-1.1 THz region. Additionally, we realized an active ultra-broadband quarter-wave converter under a Si prism that can be switched between a 45° linear rotator and a quarter wave converter in the 0.8-1.5 THz region. This is the first demonstration of an active quarter-wave converter with ultra-broad bandwidth performance. Our work shows a highly flexible and multifunctional polarization-controlling device for broadband THz applications.

Fully Polarimetric Bistatic Radar Calibration With Modified Dihedral Objects

We introduce the design of dihedral reflectors that are used to calibrate an instrumented, dual-linearly polarized, wide-bistatic compact radar range operating at 160 GHz. The polarization scattering matrix of this bistatic calibration object may be tailored to rotate the incident linear polarization state to a slant polarization state in a fashion that is similar to the 90° dihedral that is used for calibrating monostatic polarimeters. The bistatic calibration object that we describe here is capable of introducing this polarization rotation while simultaneously steering the mainlobe of its specular reflection to the receiver. As we demonstrate, this characteristic of the mainlobe-steered dihedral (MSD) object can be achieved over a wide range of bistatic angles. The MSD object possesses a variety of desirable qualities for the calibration of bistatic polarimeters, but they are particularly useful for those systems that do not possess the capability to rotate their feed arrangement (e.g., millimeter-/ submillimeter-wave waveguide-based systems). Through both computational and experimental results, we demonstrate the capability of the MSD object to calibrate and characterize the accuracy (better than 0.5 dB) and polarization purity (~50 dB) of a 160 GHz dual-linearly polarized, bistatic compact radar range at 15°, 45°, and 75° bistatic angles.

Tunable-Focusing of Surface Plasmon Polaritons With a Slightly Irregular Line of Nanofootprints

Tunable surface plasmon polariton launchers are highly demanded in various applications of nanophotonics for achieving controllable plasmonic signals coupling and multiplexing. Here, a tunable-focusing device with simplified structure is suggested. It consists of an array of nanofootprints. This device allows us polarization-controlled tunable plasmonic directing and focusing on the two-dimensional metallic surface. The simulation and experimental results indicate that the focal position of the excited plasmon field can be flexibly tuned between two distinct positions just by manipulating the incident polarization state. Hence, this nanofootprints structure can serve effectively as controllable plasmonic routers and demultiplexers. It also has promising application prospects in many other fields of optics.

Generalized method for probing ideal initial polarization states in multibeam Lloyd's mirror interference lithography of 2D scale gratings

Delicate control of the initial polarization states of sub-beams incident to the interferometer unit in multibeam Lloyd's mirror interference lithography is of great significance for the fabrication of high-precision two-dimensional (2D) scale gratings. In this work, the optimization of the initial polarization states of incident sub-beams is formulated as a constrained multiobjective optimization problem. A generalized method that allows the incident sub-beams to initially have the general elliptical polarization states in the optimization is proposed based on the multiobjective genetic algorithm. It is demonstrated that the method is capable of delivering ideal initial polarization states with which the cross-interference between the two once-reflected sub-beams on the grating substrate of the interferometer unit can be completely eliminated from the resultant 2D interference fringes. The feasibility of the proposed method is verified through the comparison between the simulated 2D interference fringes and the fabricated 2D scale gratings under the achieved ideal initial polarization states.

Controllable symmetric and asymmetric spin splitting of Laguerre-Gaussian beams assisted by surface plasmon resonance.

The spin splitting of light beams carrying orbital angular momentum (OAM) is investigated theoretically in attenuated internal reflection in the Kretschmann configuration. The excitation of the surface plasmon resonance (SPR) can significantly enhance the OAM-induced Imbert-Fedorov (IF) shift and the OAM-dependent spin splitting. The cooperation effect of these two shifts will result in an asymmetric spin splitting, which varies with the incident angle and polarization state. Specially, at the SPR angle, the OAM-induced IF shift vanishes, and the OAM-dependent spin splitting will cause a symmetric spin splitting. However, when the incident beam is horizontally polarized, the OAM-induced IF shift predominates. Thus the two spin components of the reflected will not be split; instead, they will be shifted together. This flexible control of the optical spin splitting can find applications in quantum information and precision metrology.

Enhanced Electromagnetic Energy Harvesting with Subwavelength Chiral Structures

Efficient collection of energy from electromagnetic radiation is important to emerging technologies for wireless power transfer, but is challenging in dynamic environments, because performance generally depends on the light's polarization direction. The authors show that engineering the chirality of a subwavelength structure can enable uniform collection of energy from all orientations of an incident polarization state. Such structures also enjoy enhanced performance when the chirality of the incident field matches that of the structure. This approach could enable miniaturization and more robust wireless powering of moving devices.

Grating-coupled surface plasmon resonance on bulk stainless steel.

Grating-coupled surface plasmon resonance (SPR) is demonstrated with one-dimensional gratings fabricated on the surface of bulk stainless steel using imprinting combined with electrochemical etching. The extent of light coupling and the wavelengths of SPR peaks were characterized with respect to the incident angle and polarization states of the light. When the plane of incidence was orthogonal to the grating grooves, only TM polarization was absorbed at two different wavelengths. In the plane of incidence parallel to the grooves, a single resonance peak was observed only when the incident light was TE-polarized. The dependence of SPR wavelengths on the incident angle was in good agreement with theoretical consideration.

Universal phase-only spatial light modulators.

Universal phase-only spatial light modulators (UP-SLMs) are proposed and demonstrated by simulation and experiment. UP-SLMs, which consist of unitarily coupled electrically controllable wave plates, convert the optically anisotropic properties of the paired two wave plates into an effective single isotropic phase plate and can be realized with electrically tunable birefringence materials such as uniaxial liquid crystals (LCs). The universal approaches are applicable to any uniaxial anisotropic materials based cells, any LC cell operation modes and any incident light polarization states. Further the UP-SLMs are experimentally demonstrated even with commercial LCD panels having low geometrical rotational symmetry. These UP-SLMs will play significant roles in wave optics, communications, information displays, digital holography, quantum optics and quantum information technology by harnessing their unique capacity of modulating the only phase of linearly, circularly, elliptically, azimuthally, radially and even randomly polarized light.

Giant spin splitting induced by orbital angular momentum in an epsilon-near-zero metamaterial slab.

An orbital angular momentum (OAM)-induced spin splitting is theoretically predicted when a higher-order Laguerre-Gaussian beam is transmitted through a metamaterial slab. The upper bound of this spin splitting is found to be |ℓ|w0/(|ℓ|+1)1/2, where ℓ and w0 are the incident OAM and beam waist, respectively. By optimizing the structure parameter of the metamaterial, as well as the incident angle, the OAM-induced spin splitting can reach more than 0.99 of the upper bound in the cases of both the horizontal and vertical incident polarization states, and the transmitted light fields turn out to be full Poincaré beams. These findings provide a deeper insight into the spin-orbit interaction, and, thereby, facilitate the development of spin-based applications.

Influence of Incident Polarization State on Performance of Real-time Transverse Force Sensing using Stokes Parameters in Fiber Bragg Grating

Stokes parameters can be advantageously used to obtain real-time transverse force measurements with uniform FBGs. In this paper, we demonstrate here that the state of polarization (SOP) of incident light can be used to improve the performance of the sensor. The model and the simulations are presented. A 3-paddle fiber polarization controller was used to adjust the azimuth angle and ellipse angle of the light. The experimental results show the great influences of incident SOP on the sensitivity and linear range of the system, which is well agreed with the theoretical prediction. The highest sensitivity of 0.9475/kgf was obtained with good linearity.