Incident Circular Polarization Research Articles

Enhanced circular dichroism of crossed nanorods with nanowire

Circular dichroism (CD) is a signal that characterizes the optical properties of chiral structures. Structures with intense CD signals have valuable applications in molecular chemistry, pharmaceuticals, and biosensing. This work proposes the use of a nanowire to increase the CD signal of crossed nanorods. The separation of resonant wavelengths of transmission under left-handed and right-handed circular polarization incidence (LCP and RCP) increases because the electric interaction between the upper nanorod and nanowire under LCP incidence is different from that under RCP incidence. The increased separation of resonant wavelengths, in turn, enhances the CD signals. In addition, two new CD modes appear, and these modes can be tuned by changing the structural parameters of the proposed structure. The present results will guide the design of plasmonic chiral nanostructures for enhancing the CD signal.

Broadband and high-efficiency transmissive-type nondispersive polarization conversion meta-device

Metamaterials with tailored electromagnetic properties hold great promise for the manipulation of electromagnetic waves. Here, we propose an intriguing off-resonance meta-device to control the polarization states and dispersive features of the electromagnetic waves simultaneously. The proposed meta-device is composed of stacks of bilayer gear-like metallic patterns, which exhibit significant anisotropy due to the geometric asymmetry. The results show that the considered meta-device operates in a transmission mode and can achieve broadband and nondispersive cross-polarization transmission with a high polarization conversion ratio nearly 100% for both of the normal incident linear and circular polarization waves. Moreover, the broadband and efficient polarization conversion behavior of the meta-device can be maintained regardless of the incident angle for the circular polarization waves. These fascinating performances enable the meta-device to act as an excellent half-wave plate being greatly beneficial for polarization manipulation.

Full telecomband covered half-wave meta-reflectarray for efficient circular polarization conversion

We report a full telecomband covered half-wave meta-reflectarray for efficient circular polarization conversion at near-infrared frequencies, which is composed of regular Z-shaped antenna array above a conductive ground layer separated with a dielectric spacer. The calculations of polarization conversion spectra demonstrate the ultra-broadband operation at 690 to 2010 nm continuously covering the whole telecom windows, with more than 90% conversion efficiency in full bandwidth. The relative bandwidth of as high as 98% is far higher than those reported previously never exceeding 50%. Sufficiently high conversion efficiencies of beyond 90% can be steadily performed for either left- or right-handed circular polarization incidence. The underlying mechanism is interpreted through the cross-coupling of multiple intrinsic resonances of meta-atoms. The effects of geometry and oblique incidence on the polarization conversion are discussed, showing the acceptable technical tolerance and wide operating angle. The proposed half-wave meta-reflectarray shows promising application in the field of on-chip integrated spin-based logic devices and optical communications.

A Novel THz Half-Wave Polarization Converter for Cross-Polarization Conversions of Both Linear and Circular Polarizations and Polarization Conversion Ratio Regulating by Graphene

Polarization converter is firstly proposed with the functions of simultaneously converting a linear polarization (LP) and a circular polarization (CP) to its corresponding cross polarizations, and polarization conversion ratio (PCR) of the converter with graphene strips is also controllable in this paper. A reflection-type converter is designed based on a metasurface in the terahertz region. It consists of periodic unit cells with strip-loaded half elliptical rings, a dielectric spacer layer, and a gold layer. The converter with PCR regulating also includes graphene strips on the gold layer. Both the LP and CP incidences can be converted into their cross-polarized ones with good PCR performances in a 2-THz band (2.1–2.9 THz, 32%). The graphene strips on the gold layer that disturbs the gold layer currents can regulate the PCR, typically from 2% to 98% for LP illuminations and from 60% to 98% for CP illuminations by adjusting the chemical potential ${\mu} _{{\text{c}}}$ of the graphene. Also, the physical mechanisms of the polarization conversions were investigated.

Spoof surface plasmon polaritons excitation and wavefront control by Pancharatnam–Berry phase manipulating metasurface

Realizing fine control of surface plasmon polaritons (SPPs) and spoof surface plasmon polaritons (SSPPs) is highly desired in many integrated photonic and microwave applications, but the flexibility to control the wavefront of SPPs and SSPPs still need addressing. In this paper, a Pancharatnam–Berry (PB) phase manipulating metasurface (PMM) was designed to achieve SSPPs excitation and wavefront control. Under circular polarization (CP) incidence, simply by designing the rotation angle of the unit cells the reflection phase spatial distribution can be manipulated. By means of different phase profiles on the 2D unit cells array, the SSPPs can be excited with various wavefront shapes, without the need of special excitation structure pattern. Meanwhile, a plasmonic metal is also designed to support SSPPs with both TE and TM polarizations, which can efficiently guide out the energies from the input CP waves. As a proof of concept, a PB PMM composed of N-shape metallic structure was designed. Through designing the rotation of the unit cells, two typical phase profiles were designed to excite SSPPs in arbitrary slant direction or focusing. This scheme could be used to achieve SSPPs excitation with many other wavefront shapes, and would also enable promising applications in other spectra.

Polarization-controlled asymmetric excitation of surface plasmons

Free-space light can be coupled into propagating surface waves at a metal–dielectric interface, known as surface plasmons (SPs). This process has traditionally faced challenges in preserving the incident polarization information and controlling the directionality of the excited SPs. The recently reported polarization-controlled asymmetric excitation of SPs in metasurfaces has attracted much attention for its promise in developing innovative plasmonic devices. However, the unit elements in these works were purposely designed in certain orthogonal polarizations, i.e., linear or circular polarizations, resulting in limited two-level polarization controllability. Here, we introduce a coupled-mode theory to overcome this limit. We demonstrated theoretically and experimentally that, by utilizing the coupling effect between a pair of split-ring-shaped slit resonators, exotic asymmetric excitation of SPs can be obtained under the x-, y-, left-handed circular, and right-handed circular polarization incidences, while the polarization information of the incident light can be preserved in the excited SPs. The versatility of the presented design scheme would offer opportunities for polarization sensing and polarization-controlled plasmonic devices.

Asymptotic theory of circular polarization memory.

We establish a quantitative theory of circular polarization memory, which is the unexpected persistence of the incident circular polarization state in a strongly scattering medium. Using an asymptotic analysis of the three-dimensional vector radiative transfer equation (VRTE) in the limit of strong scattering, we find that circular polarization memory must occur in a boundary layer near the portion of the boundary on which polarized light is incident. The boundary layer solution satisfies a one-dimensional conservative scattering VRTE. Through a spectral analysis of this boundary layer problem, we introduce the dominant mode, which is the slowest-decaying mode in the boundary layer. To observe circular polarization memory for a particular set of optical parameters, we find that this dominant mode must pass three tests: (1)this dominant mode is given by the largest, discrete eigenvalue of a reduced problem that corresponds to Fourier mode k=0 in the azimuthal angle, and depends only on Stokes parameters U and V; (2)the polarization state of this dominant mode is largely circular polarized so that |V|≫|U|; and (3)the circular polarization of this dominant mode is maintained for all directions so that V is sign-definite. By applying these three tests to numerical calculations for monodisperse distributions of Mie scatterers, we determine the values of the size and relative refractive index when circular polarization memory occurs. In addition, we identify a reduced, scalar-like problem that provides an accurate approximation for the dominant mode when circular polarization memory occurs.

Laguerre-Gaussian quasi-modal q-plates from nanostructured glasses

A quasi-modal version of the recently introduced Laguerre-Gaussian modal q-plates [Rafayelyan and Brasselet, Opt. Lett. 42, 1966–1969 (2017)] is proposed and implemented using femtosecond direct laser writing of space-variant nanogratings in the bulk of silica glass. The corresponding design consists of linear azimuthal modulation of the optical axis orientation and polynomial radial modulation of the retardance profile. Experimental demonstration is made for Laguerre-Gaussian modes with azimuthal indices l =(1, 2, 3) and radial index p = 0. Such quasi-modal q-plates overcome previous limitations regarding the robustness of modality against the handedness of the incident circular polarization state.

Polarization‐controlled surface plasmon holography

AbstractThe ability of generating arbitrary surface plasmon (SP) profiles in a controllable manner is of particular interest in designing plasmonic imaging, lithography and forcing devices. During the past decades, holography has gained enormous interest and achievements in free‐space three‐dimensional displays. Here, by applying a two‐dimensional version of holography, we experimentally demonstrate a generic method to control the SP profiles. Through controlling the orientation angles of two separated slits under circular polarization incidence, the amplitude and phase of the excited SPs can be freely manipulated, which allows direct generation of the desired SP profiles. A series of controllable SP holography schemes are theoretically and experimentally demonstrated, where the holographic SP profiles with high imaging quality can be dynamically modulated by varying the circular polarization handedness or orientation angle of linear polarization. The universality and simplicity of the proposed design strategies would offer promising opportunities for practical plasmonic applications. image

Transverse spin and transverse momentum in scattering of plane waves.

We study the near field to the far field evolution of spin angular momentum (SAM) density and the Poynting vector of the scattered waves from spherical scatterers. The results show that at the near field, the SAM density and the Poynting vector are dominated by their transverse components. While the former (transverse SAM) is independent of the helicity of the incident circular polarization state, the latter (transverse Poynting vector) depends upon the polarization state. It is further demonstrated that the interference of the transverse electric and transverse magnetic scattering modes enhances both the magnitudes and the spatial extent of the transverse SAM and the transverse momentum components.

Centrally Symmetric Focusing of Surface Plasmon Polaritons with a Rectangular Holes Arrayed Plasmonic Lens

Nano rectangular holes etched in a gold thin film are optimized and arrayed to form a plasmonic lens. We demonstrate both theoretically and numerically that the proposed plasmonic lens can focus surface plasmon polaritons to a centrally symmetric field with spatially homogeneous arbitrary polarization incidence, including linear, elliptical, and circular polarization incidence. Centrally symmetric field distributions with highly confined spots whose size beyond the diffraction limit are achieved. Particularly, the intensity distribution of the focused field is insensitive to the polarization direction of linearly polarized incident light and the inclination angle of elliptically polarized incident light. Moreover, the intensity of the focal spot can be modulated by the ellipticity angle of elliptically polarized incident light. The intensity is maximized for right-handed circular polarization incidence, and is twice that for linear polarization incidence, while that is null for left-handed circular polarization incidence.

Focusing surface plasmon polaritons and detecting Stokes parameters utilizing nanoslits distributed plasmonic lenses.

A method to detect the full Stokes parameters utilizing a double-ring and Archimedes-curves distributed nanoslits plasmonic lenses is proposed. We demonstrate theoretically and numerically that both of these two plasmonic lenses can focus surface plasmon polaritons to centrally symmetric fields with subwavelength-sized focal spots under linear, elliptical, and circular polarization incidence. The intensity at the focal spots is modulated by the polarization state of incident light. Utilizing this intensity polarization sensitivity, the full Stokes parameters of incident light are detected by recoding only four intensities at the focal spots of these two plasmonic lenses.

Fast-response and high-efficiency optical switch based on dual-frequency liquid crystal polarization grating

A dual-frequency liquid crystal polarization grating is fabricated by photoalignment and demonstrated as an optical switch. A high diffraction efficiency up to 95% is obtained for a single first order with circular incident polarization. Via merely alternating the frequency of applied electric field, the switch On and Off time reach 350 μs and 550 μs, respectively. This work supplies a new design for fast-response and high-efficiency optical switch with the merits of easy fabrication and low power consumption.

Polarization-independent longitudinal multi-focusing metalens.

A novel multi-focusing metalens in the longitudinal direction has been proposed and investigated based on the equal optical path principle, which is independent on the incident polarizations and can be suitable for both of the linear and circular polarization incidences simultaneously. Here, three novel designing principles: partitioned mode, radial alternating mode and angular alternating mode, have been proposed firstly for constructing different types of the longitudinal multi-focusing metalenses. The performances of the designed metalenses based on the different designed methods have also been analyzed and investigated in detail, and the intensity ratio of the focusing spots can be tuned easily by modulating the numbers of the relative type of nanoantennas, which is significant for the micro-manipulating optics and the multi-imaging technology in the integrated optics.

Graphene circular polarization analyzer based on spiral metal triangle antennas arrays.

In this paper we propose a circular polarization analyzer based on spiral metal triangle antenna arrays deposited on graphene. Via the dipole antenna resonances, plasmons are excited on graphene surface and the wavefront can be tailed by arranging metal antennas into linetype, circular or spiral arrays. Especially, for spiral antenna arrays, the geometric phase effect can be cancelled by or superposed on the chirality carried within circular polarization incidence, producing spatially separated solid dot or donut shape fields at the center. Such a phenomenon enables the graphene based spiral metal triangle antennas arrays to achieve functionality as a circular polarization analyzer. Extinction ratio over 550 can be achieved and the working wavelength can be tuned by adjusting graphene Fermi level dynamically. The proposed analyzer may find applications in analyzing chiral molecules using different circularly polarized waves.

Adjustable wideband reflective converter based on cut-wire metasurface

We present the design, analysis, and measurement of a broadband reflective converter using a cut-wire (CW) metasurface. Based on the characteristics of LC resonances, the proposed reflective converter can rotate a linearly polarized (LP) wave into its cross-polarized wave at three resonance frequencies, or convert the LP wave to a circularly polarized (CP) wave at two other resonance frequencies. Furthermore, the broad-band properties of the polarization conversion can be sustained when the incident wave is a CP wave. The polarization states can be adjusted easily by changing the length and width of the CW. The measured results show that a polarization conversion ratio (PCR) over 85% can be achieved from 6.16 GHz to 16.56 GHz for both LP and CP incident waves. The origin of the polarization conversion is interpreted by the theory of microwave antennas, with equivalent impedance and electromagnetic (EM) field distributions. With its simple geometry and multiple broad frequency bands, the proposed converter has potential applications in the area of selective polarization control.

Ultra-thin optical vortex phase plate based on the L-shaped nanoantenna for both linear and circular polarized incidences

Based on the L-shaped gold nanoantennas, the ultra-thin optical vortex phase plates (VPPs) have been designed to generate the optical vortex beams with different topological charges, which are independent of the incident polarization states and suitable for both X/Y linear and circular polarization incidences simultaneously. The phase and amplitude of transmitted cross-polarization light can be simultaneously manipulated by changing two degrees of freedom (the length and the width) in the L-shaped nanoantenna unit. Evolution properties of the generated vortex beam are demonstrated and analyzed. The different interactions of angular momentums between light and the VPP in the different incident polarization states have also been investigated fully. The designed VPP shows a superior broadband characteristics in near-infrared wavelength ranging from 750nm to 1200nm, which enable a potential implication for integrated optics and vortex optics.

Tunable graphene-coated spiral dielectric lens as a circular polarization analyzer.

We propose a tunable circular polarization analyzer based on a graphene-coated spiral dielectric lens. Spatially separated solid dot shape (or donut shape) field can be achieved if the geometric shape of analyzer and incident circular polarization possess the opposite (or same) chirality. Moreover, distinct from the narrow working bandwidth of a traditional circular polarization analyzer, the focusing and defocusing effects in the analyzer are independent of the chemical potential of graphene, and depend only on the dielectric permittivities and the grating occupation ratio. Combined with the strong tunability of graphene plasmons, the operation wavelength of analyzer can be tuned by adjusting the graphene chemical potential without degrading the performance. The proposed analyzer could be used in applications in chemistry or biology, such as analyzing the physiological properties of chiral molecules based on circular polarization.

A novel Raman optical activity instrument operating in the deep ultraviolet spectral region

Raman optical activity (ROA) has been exclusively observed in the visible (VIS) and near-infrared (NIR) spectral regions to date. During the last few years, we have designed, constructed and tested the first ROA instrument, operating in the deep-ultraviolet (DUV) spectral region employing 244-nm excitation. This novel DUV ROA instrument is based on a backscattering geometry and incident circular polarization modulation (ICP); it makes use of a fast DUV imaging lens-based spectrograph and specially designed DUV grade polarization optics. The performance of this instrument has been evaluated by analysing measured non-resonant DUV ROA spectra of non-absorbing enantiomeric liquid samples and by comparing these with corresponding ROA spectra recorded in the visible spectral region. Copyright © 2015 John Wiley & Sons, Ltd.

Optical beam interactions with a periodic array of Fresnel zone plates

The interactions of first-order elegant Laguerre–Gaussian beams (ELG) with a two-dimensional periodic array are analysed theoretically and numerically. The structure consists of a periodic composition of two-zone Fresnel plates engraved in a silver film. The beam field is composed of periodic sequences of beams of circular or polar polarization incidence upon the structure. The beam axes coincide with the symmetry axes of every fourth Fresnel zone plate placed periodically along two orthogonal coordinates of a horizontal plane of the structure. It is shown that the beam-structure interaction results in substantial cross-polarization coupling, higher-order mode excitation, strong focussing and the extraordinary transmission of the optical field. An interpretation of the results is given per an analogy to the beam-structure interactions observed at planar, homogeneous and isotropic dielectric interfaces and layers.