Inhomogeneous Polarization States Research Articles

All-silicon metasurface-enabled non-coaxial superposition of vector vortex beams

Vector vortex beams (VVBs) possess great applications in fundamental science, from particle trapping to quantum communication. Recently, the complex system for evaluating VVBs has been simplified by implementing metasurfaces and demonstrating the evolutionary trends of its coaxial polarization states. Nevertheless, it is quite challenging to obtain the polarization singularities generated in the terahertz (THz) band when non-coaxial superposition is performed by two multiplexed VVBs. Herein, a minimalist all-silicon metasurface platform is proposed, capable of being implemented to evaluate the non-coaxial superposition states of two identical VVBs. The employment of spin-polarization multiplexing techniques gives higher encoding degrees of freedom within the orthogonal circular polarization channel, enabling the superposition behavior of inhomogeneous polarization states to be performed at predefined focal planes. A systematic analysis of the newly generated polarization singularities in the interaction region is performed by applying the full Stokes polarization vector. With the ultra-compact configuration, the experimental results obtained by the proposed strategy under THz wave illumination exhibit good agreement with the numerical simulations. Thus, such mechanisms offer potential applications for multiple particle capture based on non-coaxial superimposition of VVB and provide efficient meta-platform in data encryption, high-resolution imaging, and quantum communications.

Manipulation of polarization topology using a Fabry–Pérot fiber cavity with a higher-order mode optical nanofiber

Optical nanofiber cavity research has mainly focused on the fundamental mode. Here, a Fabry–Pérot fiber cavity with an optical nanofiber supporting the higher-order modes (TE01, TM01, HE21 o , and HE21 e ) is demonstrated. Using cavity spectroscopy, with mode imaging and analysis, we observed cavity resonances that exhibited complex, inhomogeneous states of polarization with topological features containing Stokes singularities such as C-points, Poincaré vortices, and L-lines. In situ tuning of the intracavity birefringence enabled the desired profile and polarization of the cavity mode to be obtained. We believe these findings open new research possibilities for cold atom manipulation and multimode cavity quantum electrodynamics using the evanescent fields of higher-order mode optical nanofibers.

Manipulation of Longitudinally Inhomogeneous Polarization States Empowered by All‐Silicon Metasurfaces

AbstractPolarization plays a key role in fundamental science, and manipulating the evolutionary trends of longitudinal polarization states can provide new implements for light–matter interaction. However, existing 3D optical devices can only manipulate the polarization conversion in a single transverse plane. Recently, methods to control polarization by cascading bulky optical systems are miniaturized using metasurfaces. Nevertheless, it is quite challenging to generate focused beams with vectorial properties for metasurfaces carrying inhomogeneous polarization profiles. Here, a single‐layer all‐silicon metasurface operating in the terahertz (THz) band is demonstrated to address all those aforementioned challenges in one go, and pencil‐like beams with inhomogeneous polarization profiles can be imparted along the propagation direction. The introduction of a latitudinal polarization control factor representing the initial phase difference enables near‐even manipulation of the evolutionary trend of the longitudinal polarization state according to the extended focal depth. By manipulating the topological charges within two orthogonal circularly polarized channels to accomplish coherent superposition, a focused vortex beam with vectorial properties can be generated in desired planes. Two functional demonstrations based on this approach are established experimentally, offering promising opportunities for structured light on meta‐optics.

Electro-Optic Modulation of Higher-Order Poincaré Beam Based on Nonlinear Optical Crystal

Vector beams (VBs) have spatially inhomogeneous polarization states distribution and have been widely used in many fields. In this paper, we proposed a method to modulate polarization states of higher-order Poincaré (HOP) beams and designed a system based on Mach-Zehnder interferometers, in which polarization state (include azimuth and ellipticity) of generated HOP beams were modulated by linear electro-optic (EO) effect of nonlinear optical crystals. Using this method, the polarization state of generated HOP beams could be controlled by voltage signal applied on EO crystals, which makes the process of the polarization state change with no optical element moving and mechanical vibrations. Besides, due to the flexibility of the voltage signal, the polarization state could be switched directly and immediately.

Real-Time Phase Conjugation of Vector Vortex Beams

Vector vortex beams have played a fundamental role in the better understanding of coherence and polarization. They are described by spatially inhomogeneous polarization states, which present a rich optical mode structure that has attracted much attention for applications in optical communications, imaging, spectroscopy and metrology. However, this complex mode structure can be quite detrimental when propagation effects such as turbulence and birefringence perturb the beam. Optical phase conjugation has been proposed as a method to recover an optical beam from perturbations. Here we demonstrate full phase conjugation of vector vortex beams using three-wave mixing. Our scheme exploits a fast non-linear process that can be conveniently controlled via the pump beam. Our results pave the way for sophisticated, practical applications of vector beams.

Effect of Sr2+ on phase structure and properties for 0.6(Na0.5Bi0.5)TiO3-0.4(Bi1-ySry)TiO3 relaxor ferroelectrics

0.6(Na0.5Bi0.5)TiO3-0.4(Bi1-ySry)TiO3 ceramics (abbreviated BNT-BSyT, y = 0.2, 0.3, 0.5, 0.7, and 0.9 mol) were fabricated using a traditional solid-state processing route, and the effects of Sr2+ on phase structure, di- and ferroelectric properties, and electrostrictive properties were systematically investigated. The values of γ for BNT-BSyT ceramics are greater than 1.8, revealing that these materials exhibit relaxor ferroelectric properties derived from the co-occupation of Bi3+, Na+, and Sr2+ in the A-site and inhomogeneous polarization states (i.e., coexistence of rhombohedral and tetragonal states). In addition, Td decreases with increasing Sr2+ content along with a stable and increased tetragonal phase. BNT-BS0.9T ceramic has a slightly large maximum polarization Pmax = 16.15 μC/cm2, a small remanent polarization Pr = 1.6 μC/cm2, and a relatively good energy storage property (density W1 = 0.220 J/cm3 and efficiency η = 72.13%), indicating that Sr2+ substitution helps to promote energy storage characteristics. Moreover, there is a superior electrostrictive coefficient (Q33 = 0.0385 m4/C2) for BNT-BS0.3T ceramic. Therefore, we propose that this lead-free electrostrictor is a good candidate for practical applications.

Nonlinear frequency conversion of vectorial optical fields with a Mach-Zehnder interferometer

Vectorial optical fields with inhomogeneous polarization states have attracted great interest for their unique properties in both quantum and classical physics. In this paper, we propose a method to realize such nonlinear frequency conversion of the vectorial optical fields, which is by using a Mach–Zehnder interferometer. The separated arms enable two nonlinear processes to simultaneously proceed in two vertical directions. In our experiment, nonlinear frequency conversion of fundamental frequency vectorial optical fields with topological charge l=3 is studied. To illustrate the flexibility of this method, Taiji and clover patterns with inhomogeneous polarization distribution are also generated at second harmonic wavelengths. This research proposes a more flexible method to realize nonlinear frequency conversion of the vectorial optical fields and may have potential applications in the generation of ultrafast vectorial optical fields.

Magnetoelectric response of quantum structures driven by optical vector beams

Key advances in the generation and shaping of spatially structured photonic fields in both the near and far fields render possible the control of the duration, the phase, and the polarization state of the field distributions. For instance, optical vortices having a structured phase are nowadays routinely generated and exploited for a range of applications. While the light-matter interaction with optical vortices have been well studied, the distinctive features of the interaction of quantum matter with vector beams, meaning fields with spatially inhomogeneous polarization states, have yet to be explored in full detail, which is done here. We analyze the response of atomic and low-dimensional quantum structures to irradiation with radially or azimuthally polarized cylindrical vector beams. Striking differences from vortex beams are found: Radially polarized vector beams drive radially breathing charge-density oscillations via electric-type quantum transitions. Azimuthally polarized vector beams do not affect the charge at all but trigger, via a magnetic vector potential, a dynamic Aharonov-Bohm effect, meaning a vector-potential-driven oscillating magnetic moment. In contrast to vortex beams, no unidirectional currents are generated. Atoms driven by a radially polarized vector beam exhibit angular-momentum-conserving quadrupole transitions tunable by a static magnetic field, while when excited with an azimuthally polarized beam, different final-state magnetic sublevels can be accessed.

Polarization mediated coherent and incoherent Bessel-moiré generation.

We demonstrate an efficient approach for the investigation of polarization states of a Bessel beam along the propagation direction. Furthermore, we propose a method to generate Bessel-moiré using a birefringent lens and Wollaston prism. The analysis showed that an experimentally generated incoherent moiré pattern is analogous to a theoretically simulated pattern. Moreover, we verified that inhomogeneous polarization states of a Bessel beam are still present in Bessel-moiré along its propagation direction. Our observation of Bessel-moiré due to mechanical vibration depicts the fact that incoherent moiré is stable, whereas the coherent moiré is sensitive to external vibration only along its propagation direction.

Designer vector beams maintaining a robust intensity profile on propagation through turbulence

We report experiments on propagation of scalar and vector optical beams through random phase screens mimicking turbulence and show that the intensity profile of the beam containing a C-point polarization singularity shows maximally robust behavior. This observation is explained in terms of the polarization and orbital angular momentum (OAM) diversity in the beam. The $l = 0$ and $l = 1$ OAM states whose vector combination leads to the C-point singularity are seen to produce complementary speckle intensity patterns with significant negative correlation on propagation through a random phase screen. This unique property of C-point singularity makes it superior to other inhomogeneous polarization states as demonstrated in our experiments. The results provide an important generic guideline for designing beams that can maintain optimally robust beam intensity profile on passing through random phase fluctuations and are expected to have a number of applications.

Creation of independently controllable multiple focal spots from segmented Pancharatnam-Berry phases

Recently, based on space-variant Pancharatnam-Berry (PB) phases, various flat devices allowing abrupt changes of beam parameters have been predicted and demonstrated to implement intriguing manipulation on spin states in three dimensions, including the efficient generation of vector beams, spin Hall effect of light and light-guiding confinement, and so on. Here, we report on the construction of independently controllable multiple focal spots with different inhomogeneous polarization states by utilizing segmented PB phases. Combining the phase shift approach with PB phases, we engineer fan-shaped segmented PB phases and encode them onto two spin components that compose a hybrid polarized vector beam in a modified common-path interferometer system. Experimental results demonstrate that the fan-shaped segmented PB phase enables the flexible manipulation of focal number, array structure and polarization state of each focal spot. Furthermore, we demonstrate that this fan-shaped approach enables to flexibly tailor the polarization state and the spin angular momentum distribution of a tightly focused field, which have potential applications in optical manipulation, tailored optical response and imaging etc.

On-demand tailored vector beams.

We introduce an effective optical system to produce optical beams with arbitrary, inhomogeneous polarization states. Using our system, we are capable of generating vector beams with discretionarily chosen transverse complex fields in a straightforward way. We generate several different instances of well-known vector beams and the less common spirally polarized vector beams, as well as a full Poincaré beam. We visually show the continual transition between azimuthally and radially polarized beams via a collection of spirally polarized beams. We experimentally determine the polarization states of the generated beams and quantitatively assess the performance of our system. We find that the measured polarization distributions accurately coincide with the intended input polarization distributions.

Collapse dynamics of a vector vortex optical field with inhomogeneous states of polarization

Based on a pair of coupled 2D nonlinear Schrödinger equations, the collapse dynamics of a vector field with hybrid states of polarization (SoP) in a Kerr medium is demonstrated. The critical power for an optical field to collapse is present, and the full vectorial numerical simulations provide detailed information about the evolution and partial collapse of the vector field in a Kerr medium. Our results reveal that the optical field prefers to collapse in linearly-polarization, as a result of the self-focusing effect difference in linearly, elliptically and circularly polarized components. The SoP in the field cross-section changes and propagates with a spiral trajectory when the vector beams are imposed with a vortex. The vectorial effect on the collapse of a vector optical field can prevail over the noise even though it reaches 10% amplitude of the optical field. The unique feature of these structured collapses of a vector optical field may lead to new phenomena in the interaction of light with matter.

Effect of a spiral phase on a vector optical field with hybrid polarization states

The propagation dynamics of a vector field with inhomogeneous states of polarization (SoP) imposed a vortex is studied using the angular spectrum method. The evolution of SoP in the cross section of the field during propagation is analyzed numerically by the Stokes polarization parameters. The results indicate that SoP in the field cross section rotate along the propagation axis during propagation due to the existence of a vortex. In addition, the interaction between the phase singularity and the polarization singularity leads to the creation or annihilation of the optical field in the central region. In particular, the distributions of the transverse energy flow and both spin and orbital optical angular momentum fluxes in the cross section of the vortex vector optical field depend sensitively on both the vortex and polarization topology charges.

4 × 20 Gbit/s mode division multiplexing over free space using vector modes and a q-plate mode (de)multiplexer.

Vector modes are spatial modes that have spatially inhomogeneous states of polarization, such as, radial and azimuthal polarization. In this work, the spatially inhomogeneous states of polarization of vector modes are used to increase the transmission data rate of free-space optical communication via mode division multiplexing. A mode (de)multiplexer for vector modes based on a liquid crystal q-plate is introduced. As a proof of principle, four vector modes each carrying a 20-Gbit/s quadrature phase shift keying signal (aggregate 80 Gbit/s) on a single wavelength channel (λ∼1550 nm) were transmitted ∼1 m over the lab table with <-16.4 dB mode crosstalk. Bit error rates for all vector modes were measured at the 7% forward error correction threshold with power penalties <3.41 dB.

Measuring the self-healing of the spatially inhomogeneous states of polarization of vector Bessel beams

We experimentally measured the self-healing of the spatially inhomogeneous states of polarization of vector Bessel beams. Radially and azimuthally polarized vector Bessel beams were experimentally generated via a digital version of Durnin's method, using a spatial light modulator in concert with a liquid crystal q-plate. As a proof of principle, their intensities and spatially inhomogeneous states of polarization were experimentally measured using Stokes polarimetry as they propagated through two disparate obstructions. It was found, similar to their intensities, that their spatially inhomogeneous states of polarization self-healed. The self-healing can be understood via geometric optics, i.e., the interference of the unobstructed conical rays in the shadow region of the obstruction, and may have applications in, for example, optical trapping.

Diffraction of convergent spherical waves with all possible polarization states using the Luneburg integral method

We present a complete electromagnetic study, which includes electric, magnetic, and Poynting vector fields of diffracted convergent spherical waves under all possible polarization states compatible with Maxwell's equations. Exit pupil boundary conditions for these polarizations were obtained by means of Hertz potentials. Using these boundary conditions, two-dimensional Luneburg diffraction integrals for the three components of electric and magnetic fields were formulated, and after some approximations, we showed that the complete electromagnetic description of the inhomogeneous polarization states of spherical waves is reduced to the knowledge of seven one-dimensional integrals. The consistency of the method was tested by comparison with other previously reported methods for linearly polarized (LP), TE, and TM polarizations, while the versatility of the method was showed with the study of nonstandard polarization states, for example, that resulting from the superposition of TE and TM dephased spherical waves, which shows a helicoidal behavior of the Poynting vector at the focalization region, or the inhomogeneous LP state that exhibits a ring structure for the Poynting vector at the focal plane.