Polarization Conversion Process Research Articles

Separating radial and azimuthal polarizations of circular Airy vortex beam via uniaxial crystal.

Since Ciattoni A. et al. found that a particular circularly polarized beam propagating along the optical axis in a uniaxial crystal can generate a vortex with a reversed circular polarization, numerous studies of spin-orbit coupling in this polarization conversion process have been carried out. In this paper, from another perspective rather than the circular polarization conversion, for the first time we find that radial- and azimuthal-polarization components will be separated and finally focus on two separated focus points when circular Airy vortex beams propagate in a uniaxial crystal. Both the separation of the radial- and azimuthal-polarization components in positive and negative uniaxial crystals are investigated, and the physical mechanism of this phenomenon is explained in details. Moreover, the influences of the crystal length and birefringence on the separation of the radial- and azimuthal-polarization components are also discussed. Our results could offer deeper understanding of the propagation of light beam in uniaxial crystal and facilitate the flexible applications of circular Airy vortex beams.

Multifunctional acoustic holography based on compact acoustic geometric-phase meta-array

Optical geometric-phase metasurfaces provide a robust and efficient means for light wave control by simply manipulating the spatial orientations of the in-plane anisotropic meta-atoms, where polarization conversion plays a vital role. However, the concept of acoustic geometric-phase modulation for acoustic field control remains unexplored because airborne acoustic waves lack a similar optical polarization conversion process. In this work, a new type of acoustic meta-atom with deep-subwavelength feature size is theoretically investigated and further applied to acoustic field engineering based on the emerged concept of acoustic geometric phase. Herein, tunable acoustic geometric-phase modulation of designated order is obtained via the near-field coupled orbital angular momentum transfer process, and the topological charge-multiplexed acoustic geometric phase endows our meta-arrays with multiple functionalities. Our work extends the capacity of the acoustic geometric-phase meta-arrays in high-quality acoustic field reconstruction and offers new possibilities in multifunctional acoustic meta-holograms.

Mitigating Vortex Splitting by Controlling the Wavefront Isophase Line Curvature of Vector Autofocusing Airy Vortex Beams in Free Space

The match between the orbital angular momentum (OAM) transmission and receipt is a prerequisite for vortex beams as a carrier of the wireless optical communication system in free space. However, the vortex splitting induced by atmospheric turbulence results in an offset in the average OAM measured after propagating over the free space optical communication link. Therefore, how to reduce the vortex splitting effect to improve the stability of orbital angular momentum propagation is studied in this paper. First, a new parameter (m) is introduced to modulate the radial distribution of the polarization state of the autofocusing Airy vortex beams (AAVBs). Second, to control the optical field distribution in focal plane, the wavefront isophase line curvature of the AAVBs is reconstructed by using the Pancharatnam-Berry phase generated in the polarization conversion process. Finally, by comparing the vortex splitting rates of the AAVBs with different wavefrontisophase line curvatures under different turbulence environments, it can be found that the vortex splitting in free space can be effectively mitigated by choosing an appropriate wavefront isophase line curvature. In addition, the mitigation effect is more obvious in the atmosphere with stronger turbulence. This study provides a new feasible method to control the phase structure of vortex beams and promises potential applications in the OAM-based free space optical communication system.

Intensity-tunable terahertz bandpass filters based on liquid crystal integrated metamaterials.

In this paper, we demonstrate an intensity-tunable THz bandpass filter by introducing liquid crystal (LC) integrated with asymmetric frequency selective surface (FSS) and subwavelength metal gratings. Here, the tunable THz filter is derived from the inner polarization state conversion in composited devices, and the incident linear polarization can be converted into 90° orthogonal components. By controlling the LC orientation under the applied electric field with the metamaterial electrodes, the polarization conversion process can be actively modulated; thus, the polarization-dependent and tunable THz bandpass filter is achieved. Based on the multilayer design and the inner Fabry-Perot-like resonance mechanism, the LC-integrated metamaterials filter presents better filtering performance than the single FSS filter, and the Q-value is improved from 7.7 to 13.8 at the working frequency. Our simulated work paves the way for the design of new and efficient THz filters.

Second harmonic imaging of plasmonic Pancharatnam-Berry phase metasurfaces coupled to monolayers of WS2

Abstract The nonlinear processes of frequency conversion such as second harmonic generation (SHG) usually obey certain selection rules, resulting from the preservation of different kinds of physical quantities, e.g. the angular momentum. For the SHG created by a monolayer of transition-metal dichalcogenides (TMDCs) such as WS2, the valley-exciton locked selection rule predicts an SHG signal in the cross-polarization state. By combining plasmonic nanostructures with a monolayer of TMDC, a hybrid metasurface is realized, which affects this nonlinear process because of an additional polarization conversion process. Here, we observe that the plasmonic metasurface modifies the light-matter interaction with the TMDC, resulting in an SHG signal that is co-polarized with respect to the incident field, which is usually forbidden for the monolayers of TMDC. We fabricate such hybrid metasurfaces by placing plasmonic nanorods on top of a monolayer WS2 and study the valley-exciton locked SHG emission from such system for different parameters, such as wavelength and polarization. Furthermore, we show the potential of the hybrid metasurface for tailoring nonlinear processes by adding additional phase information to the SHG signal using the Pancharatnam-Berry phase effect. This allows direct tailoring of the SHG emission to the far-field.

Monthly variation on the propagation and evolution of internal solitary waves in the northern South China Sea

In this paper, internal solitary waves (ISWs) in the northern South China Sea (NSCS) are investigated with respect to their propagation and evolution processes in different months. To achieve that, environmental parameters associated with ISW are first calculated using climatological datasets derived from the Simple Ocean Data Assimilation 3.3.1 (SODA3.3.1) reanalysis. The environmental parameters are significantly distinct from month to month, especially the quadratic nonlinearity coefficient α. It is negative in the deep basin and turns positive on the shallow continental shelf. A clear dividing line that α equals to zero can be found along the shelf from northeast to southwest from January to February and from October to December. The dividing line is interrupted starting from March and is mostly constrained on the northeast shelf, and from May to September only limited areas can positive α be found. Further simulations based on the variable extended Korteweg-de Vires (veKdV) model indicate that the polarity conversion process of ISWs therefore exhibits significant seasonality. It can be concluded from the simulation results that near the 200-m isobath on the continental shelf the elevation waves are more likely to appear from November to March owing to the positive values of α. The large-scale background current exhibits little effect on the deformation and polarity conversion of ISW in the NSCS although it prominently affects the environmental parameters around the shelf currents from Luzon strait.

Controlling phase of arbitrary polarizations using both the geometric phase and the propagation phase

Subwavelength spatial control of phase using metasurfaces is attractive for practical applications such as flat lens, vector beam generator, ultrathin wave-plate and high-resolution holography. All-dielectric metasurfaces provide two phase modulation mechanisms, which are propagation phase dependent on the dimension of meta-atoms and geometric phase dependent on the orientation of meta-atoms. Recently, it was proposed that by combining geometric phase and propagation phase, arbitrary and independent phase profiles can be imposed on a pair of orthogonal polarizations. However, a succinct expression of phase change introduced during an arbitrary polarization conversion process has not been presented. Here, we derive the geometric phase for the general case of nonorthogonal polarization conversions. Through the numerical simulations of a metasurface orbital angular momentum generator under both circular and elliptical polarization illuminations, we verify our theoretical analyses. This work presents the mechanism of controlling polarization and phase of arbitrary polarizations using both geometric phase and propagation phase, which exhibits the potential to enrich the capacity of polarization multiplexed metasurfaces.

Estimating depth of polarity conversion of shoaling internal solitary waves in the northeastern South China Sea

Internal Solitary Wave (ISW) normally converts its polarity from depression wave to elevation wave during shoaling process. This phenomenon has been verified by model studies and field observations. However, through the comparison between theoretical studies and observations, we find that Extended Kortweg-de Vries (EKdV) theory, often used by numerical simulations, cannot well predict the polarity conversion area of shoaling ISWs. Here we trace seismic reflectors within the water column, captured by exploration seismic data, and systematically analyze the polarity conversion area of the shoaling ISWs in the northeastern South China Sea (SCS). The results show that the ISWs in the northeastern SCS generally begin to convert their polarities from depression to elevation waves at the seafloor depth of about 200m, and turn into elevation waves at the seafloor depth of about 100m. The ISWs between depression and elevation waves are newly defined as transition waves, which are distributed between the seafloor depth of 130m and 220m; while elevation waves are distributed between 80m and 120m. We compared the observation with the theoretical critical conversion depth estimation by the two-layer model based on EKDV equation. The result suggests that the depth of the upper layer in the two-layer model can be selected as the depth of the maximum buoyancy frequency not the base of the mixing layer. Such selection can get better estimation of critical conversion depth of shoaling ISWs. Our analysis also suggests that the polarity conversion process is amplitude-dependent. The ratio of amplitude to depth for elevation waves is more than two times larger than that of the transition waves. This implies that the seafloor plays a more important role in shallow water than in deep water in controlling the amplitudes of ISWs during shoaling process.

Selective Conversion from p-Type to n-Type of Printed Bottom-Gate Carbon Nanotube Thin-Film Transistors and Application in Complementary Metal-Oxide-Semiconductor Inverters.

The fabrication of printed high-performance and environmentally stable n-type single-walled carbon nanotube (SWCNT) transistors and their integration into complementary (i.e., complementary metal-oxide-semiconductor, CMOS) circuits are widely recognized as key to achieving the full potential of carbon nanotube electronics. Here, we report a simple, efficient, and robust method to convert the polarity of SWCNT thin-film transistors (TFTs) using cheap and readily available ethanolamine as an electron doping agent. Printed p-type bottom-gate SWCNT TFTs can be selectively converted into n-type by deposition of ethanolamine inks on the transistor active region via aerosol jet printing. Resulted n-type TFTs show excellent electrical properties with an on/off ratio of 106, effective mobility up to 30 cm2 V-1 s-1, small hysteresis, and small subthreshold swing (90-140 mV dec-1), which are superior compared to the original p-type SWCNT devices. The n-type SWCNT TFTs also show good stability in air, and any deterioration of performance due to shelf storage can be fully recovered by a short low-temperature annealing. The easy polarity conversion process allows construction of CMOS circuitry. As an example, CMOS inverters were fabricated using printed p-type and n-type TFTs and exhibited a large noise margin (50 and 103% of 1/2 Vdd = 1 V) and a voltage gain as high as 30 (at Vdd = 1 V). Additionally, the CMOS inverters show full rail-to-rail output voltage swing and low power dissipation (0.1 μW at Vdd = 1 V). The new method paves the way to construct fully functional complex CMOS circuitry by printed TFTs.

Polarization-sensitive color in butterfly scales: polarization conversion from ridges with reflecting elements.

Polarization-sensitive color originates from polarization-dependent reflection or transmission, exhibiting abundant light information, including intensity, spectral distribution, and polarization. A wide range of butterflies are physiologically sensitive to polarized light, but the origins of polarized signal have not been fully understood. Here we systematically investigate the colorful scales of six species of butterfly to reveal the physical origins of polarization-sensitive color. Microscopic optical images under crossed polarizers exhibit their polarization-sensitive characteristic, and micro-structural characterizations clarify their structural commonality. In the case of the structural scales that have deep ridges, the polarization-sensitive color related with scale azimuth is remarkable. Periodic ridges lead to the anisotropic effective refractive indices in the parallel and perpendicular grating orientations, which achieves form-birefringence, resulting in the phase difference of two different component polarized lights. Simulated results show that ridge structures with reflecting elements reflect and rotate the incident p-polarized light into s-polarized light. The dimensional parameters and shapes of grating greatly affect the polarization conversion process, and the triangular deep grating extends the outstanding polarization conversion effect from the sub-wavelength period to the period comparable to visible light wavelength. The parameters of ridge structures in butterfly scales have been optimized to fulfill the polarization-dependent reflection for secret communication. The structural and physical origin of polarization conversion provides a more comprehensive perspective on the creation of polarization-sensitive color in butterfly wing scales. These findings show great potential in anti-counterfeiting technology and advanced optical material design.

The Behaviors of Internal Solitary Waves near the Continental Shelf of South China Sea Inferred from Satellite Images

The behaviors of internal solitary waves (ISWs) near the continental shelf of SCS are analyzed. We interpret manifestations of four kinds of ISWs in MODIS images: mode-1 depression wave, bolus, mode-1 elevation wave, and mode-2 convex wave. The geographical distributions of these ISWs are investigated. Bolus and mode-1 elevation wave, the two evolution stages of polarity conversion process of depression ISW, only exists around 22˚N, which is in agreement with the sign of mode-1 nonlinear coefficient α1. The mode-2 convex waves generally follow the southern part of mode-1 depression ISW, where the steep terrain favors their generation. The information of a mode-1 depression ISW, such as amplitude and propagation speed, is also derived from the MODIS image.

Simulation study on SAR imaging of internal solitary wave polarity conversion process

This paper presents a Synthetic Aperture Radar (SAR) imaging simulation method to observe internal solitary waves (ISWs). The method is based on Lynett and Liu's horizontal fully two-dimensional ISW propagation model and a SAR ocean surface imaging model [Lynett, P.J. and Liu, P.L., 2002, A two-dimensional depth-integrated model for internal wave propagation over variable bathymetry. Wave Motion, 36, pp. 221–240]. We simulate ISWs propagating in a variable depth ocean and the resulting internal wave polarity conversion process. Analysis of the simulated SAR images shows the process of ISWs of depression changing to ISWs of elevation. Our conclusion is that the radar signature of ISWs during this process are seriously weakened but do not disappear during the polarity conversion. Comparing our simulation results with a local ERS-2 satellite image, this simulation study also validates the effects of ocean topography on the propagation of internal waves by showing ISW fission and isobath-parallelized propagation direction.

Dependence of the gain factor of the reflective polarizer on the configuration of optical sheets

The correlation between the optical performance of the reflective polarizer, which is a key optical component for the brightness enhancement of the liquid crystal display (LCD), and the configuration of optical sheets was investigated in a direct‐lit CCFL (cold‐cathode fluorescent lamp) backlight. The optical gain of the reflective polarizer, the polarization state of the light emitted from each film, and the loss factor for the polarization conversion process occurring in the lower part of the backlight were determined using a phenomenological approach for the polarization recycling process. The present study suggests that the correlation between the optical performance of the brightness enhancement films and the backlight configuration should be carefully considered in the optimization of the backlight structure.

Nonlinear dynamics in laser polarization conversion by stimulated scattering in nematic liquid crystal films.

We present a study of the nonlinear dynamics of a coherent polarization conversion process in nematic liquid crystal films. This effect is mediated by two-beam coupling between the incident polarized laser and its orthogonally polarized (lower-frequency) noise component scattered by the director axis fluctuations. A complete model for the director dynamics derived from the basic equations is presented. The existence of complex, time-, and intensity-dependent dynamics of the director motion such as oscillations and various bifurcations including period doubling is revealed.

Propagation of polarized high-energy photons in crystals

The density matrix is obtained for short-wavelength photons passing through a medium. For sufficiently large thicknesses of a target, this matrix is expressed by the forward scattering amplitude of a photon. For multi-GeV photons, scattering via virtual e +e − pairs (VP) is the most important process. Its amplitude in a crystal is calculated. The results obtained are used to find the optimal thicknesses, orientations and types of crystals for the linear-to-circular polarization conversion process.

Polarization conversion from diffraction gratings.

If a surface-modulated diffraction grating is arranged so that the scattering vector does not lie in the plane of incidence of the illuminating radiation, then polarization conversion can occur. This conversion can be strongly enhanced if a surface mode, such as a surface-plasmon-polariton or guided mode, is excited at the grating surface. In this paper we develop a mathematical description of the polarization conversion process, using a method that transforms the grating surface into a planar system. The predicted polarization conversion is compared very favorably with data for surface-mode-enhanced conversion.