Generation Of Optical Vortices Research Articles

Broadband and high-efficiency accelerating beam generation by dielectric catenary metasurfaces

Abstract Self-accelerating beams show considerable captivating phenomena and applications owing to their transverse acceleration, diffraction-free and self-healing properties in free space. Metasurfaces consisting of dielectric or metallic subwavelength structures attract enormous attention to acquire self-accelerating beams, owing to their extraordinary capabilities in the arbitrary control of electromagnetic waves. However, because the self-accelerating beam generator possesses a large phase gradient, traditional discrete metasurfaces suffer from insufficient phase sampling, leading to a low efficiency and narrow spectral band. To overcome this limitation, a versatile platform of catenary-inspired dielectric metasurfaces is proposed to endow arbitrary continuous wavefronts. A high diffraction efficiency approaching 100% is obtained in a wide spectral range from 9 to 13 μm. As a proof-of-concept demonstration, the broadband, high-efficiency and high-quality self-accelerating beam generation is experimentally verified in the infrared band. Furthermore, the chiral response of the proposed metasurfaces enables the spin-controlled beam acceleration. Considering these superior performances, this design methodology may find wide applications in particle manipulation, high-resolution imaging, optical vortex generation, and so forth.

Positive-negative tunable liquid crystal lenses based on a microstructured transmission line

In this work, a novel technique to create positive-negative tunable liquid crystal lenses is proposed and experimentally demonstrated. This structure is based on two main elements, a transmission line acting as a voltage divider and concentric electrodes that distribute the voltage homogeneously across the active area. This proposal avoids all disadvantages of previous techniques, involving much simpler fabrication process (a single lithographic step) and voltage control (one or two sources). In addition, low voltage signals are required. Lenses with switchable positive and negative focal lengths and a simple, low voltage control are demonstrated. Moreover, by using this technique other optical devices could be engineered, e.g. axicons, Powell lenses, cylindrical lenses, Fresnel lenses, beam steerers, optical vortex generators, etc. For this reason, the proposed technique could open new venues of research in optical phase modulation based on liquid crystal materials.

Bi-channel near- and far-field optical vortex generator based on a single plasmonic metasurface

With the recent development of the metasurface, generating an optical vortex in optical far or near fields is realized in various ways. However, to generate vortices in both the near and far fields simultaneously is still a challenge, although it has great potential in the future compact and versatile photonic system. Here, a bi-channel optical vortex generator in both the near and far fields is proposed and demonstrated within a single metasurface, where the surface plasmon vortex and the far-field optical vortex can be simultaneously generated under circularly polarized light. The ability of generating vortices with arbitrary topological charges is experimentally demonstrated, which agrees well with simulations. This approach provides great freedom to integrate different vortex generators in a single device and offers new opportunities for integrated optical communications, trapping, and other related fields.

Application of a binary curved fork grating for the generation and detection of optical vortices outside the focal plane

In this paper, we consider using a binary curved fork grating not only for the simultaneous formation of several Bessel vortex beams but also for detecting the presence of angular harmonics (optical vortices) in an analyzed beam outside the focal plane. In this case, we use the effect of a longitudinal shift of focus (or correlation peak) when the lens is supplemented with an axicon. The combination of a lens and an axicon provides, in the focal plane, the generation of narrow light rings whose radii do not depend on the vortex phase component. Thus, the detection of an optical vortex cannot be carried out in the focal plane, but only at a certain distance from it, which provides additional security for information transmission. In addition, the use of these gratings makes it possible to simultaneously manipulate microparticles in various planes, and they can be used for fabricating complex three-dimensional metastructures as well as the single-shot pulse laser processing of transparent materials or polymers.

Electrically-tunable positioning of topological defects in liquid crystals

Topological defects are a consequence of broken symmetry in ordered systems and are important for understanding a wide variety of phenomena in physics. In liquid crystals (LCs), defects exist as points of discontinuous order in the vector field that describes the average orientation of the molecules in space and are crucial for explaining the fundamental behaviour and properties of these mesophases. Recently, LC defects have also been explored from the perspective of technological applications including self-assembly of nanomaterials, optical-vortex generation and in tunable plasmonic metamaterials. Here, we demonstrate the fabrication and stabilisation of electrically-tunable defects in an LC device using two-photon polymerisation and explore the dynamic behaviour of defects when confined by polymer structures laser-written in topologically discontinuous states. We anticipate that our defect fabrication technique will enable the realisation of tunable, 3D, reconfigurable LC templates towards nanoparticle self-assembly, tunable metamaterials and next-generation spatial light modulators for light-shaping.

Generation and measurement of high-order optical vortices by using the cross phase.

We investigate a method for the generation and measurement of high-order optical vortices (OVs) by using the cross phase (CP), which is applied to implement interconversion between Laguerre-Gauss (LG) beams and Hermite-Gaussian beams in the far-field. Experimentally, we generate LG beams, which are a kind of typical OVs, with 20 radial nodes, and measure OVs with topological charges up to 200 via the CP. On this basis, we discuss the relationship between intensity distributions and the waist radius of initial light beams. This work provides an alternative method to generate and measure high-order OVs, which is useful in the fields of optical micro-manipulation, high-dimensional quantum entanglement, and remote sensing of the angular rotation of structured objects.

Off-axis spiral phase mirrors for generating high-intensity optical vortices.

In this work, we present a novel, to the best of our knowledge, and practical method for generating optical vortices in high-power laser systems. Off-axis spiral phase mirrors are used at oblique angles of incidence in the beam path after amplification and compression, allowing for the generation of high-power optical vortices in almost any laser system. An off-axis configuration is possible via modification of the azimuthal gradient of the spiral phase helix and is demonstrated with a simple model using a discrete spiral staircase. This work presents the design, fabrication, and implementation of off-axis spiral phase mirrors in both low- and high-power laser systems.

Polarization insensitive all-dielectric metasurfaces for the ultraviolet domain

In recent years, metasurfaces have provided a tempting path to replace conventional optical components where an abrupt phase change is imposed on an incident wave using a periodic array of unit cells. Till date, highly efficient dielectric metasurfaces have been demonstrated in infrared and visible domains. However, due to the lower bandgap of typical dielectric materials, such metasurfaces present strong absorption in the ultraviolet (UV) domain, and thus, hamper their realization at shorter wavelengths. In this paper, we utilize a large bandgap dielectric material, niobium pentoxide (Nb2O5), to construct an ultra-thin and compact transmission-type metasurface that manipulates the phase of an incident wave using an array of Nb2O5 nano-cylinder. By the virtue of numerical optimization, complete 2π phase coverage along with the high transmission efficiency (around 88.5%) is achieved at 355nm. Such efficient control over the phase of the incident wave enabled us to realize the polarisation insensitive self-accelerating parabolic, reciprocal, and logarithmic Airy beams (ABs) generating metasurfaces with the efficiency of 70%, 72% and 77%, respectively. In addition to this, we also demonstrate auto focusing Airy optical vortex (AFAOV) generators where the metasurfaces are designed to combine the phase profiles of an abruptly focusing Airy (AFA) beam and that of spiral phase plate (SPP). The AFAOV is generated with efficiency of 70% (for l = 3) and 72% (for l = 5).

Taming the swirl of self-structured liquid crystal q-plates

Spontaneously formed liquid crystal topological defects under external fields offer a nature-assisted route to the creation of geometric phase optical vortex generators (q-plates). Here we report on the consequences of the unavoidable swirled transverse spatial distribution of the optical axis of such optical elements on the beam shaping and we propose a swirl-compensation scheme based on the arithmetic of geometric phase optical elements.

Smart optically induced nonlinear photonic crystals for frequency conversion and control

We extend the functionality of nonlinear photonic crystals by fabricating a structure combining a few individual optical transformations in second harmonic generation. In particular, we employed all-optical spontaneous polarization reversal in ferroelectrics to create nonlinear holograms with curved fork-like spatial modulation of nonlinearity for the generation of second harmonic optical vortices. The curved fork-like structure represents a combination of a standard vortex harmonic generator (fork structure) and an axicon, thereby allowing us to realize the second harmonic “perfect vortices” whose diameter remains constant independently of their topological charge.

Phase engineering with all-dielectric metasurfaces for focused-optical-vortex (FOV) beams with high cross-polarization efficiency

Metasurfaces, the two-dimensional (2D) metamaterials, facilitate the implementation of abrupt phase discontinuities using an array of ultrathin and subwavelength features. These metasurfaces are considered as one of the propitious candidates for realization and development of miniaturized, surface-confined, and flat optical devices. This is because of their unprecedented capabilities to engineer the wavefronts of electromagnetic waves in reflection or transmission mode. The transmission-type metasurfaces are indispensable as the majority of optical devices operate in transmission mode. Along with other innovative applications, previous research has shown that Optical-Vortex (OV) generators based on transmission-type plasmonic metasurfaces overcome the limitations imposed by conventional OV generators. However, significant ohmic losses and the strong dispersion hampered the performance and their integration with state-of-the-art technologies. Therefore, a high contrast all-dielectric metasurface provides a compact and versatile platform to realize the OV generation. The design of this type of metasurfaces relies on the concept of Pancharatnam-Berry (PB) phase aiming to achieve a complete 2π phase control of a spin-inverted transmitted wave. Here, in this paper, we present an ultrathin, highly efficient, all-dielectric metasurface comprising nano-structured silicon on a quartz substrate. With the help of a parameter-sweep optimization, a nanoscale spatial resolution is achieved with a cross-polarized transmission efficiency as high as 95.6% at an operational wavelength of 1.55 µm. Significantly high cross-polarized transmission efficiency has been achieved due to the excitation of electric quadrupole resonances with a very high magnitude. The highly efficient control over the phase has enabled a riveting optical phenomenon. Specifically, the phase profiles of two distinct optical devices, a lens and Spiral-Phase-Plate (SPP), can be merged together, thus producing a highly Focused-Optical-Vortex (FOV) with a maximum focusing efficiency of 75.3%.

Theoretical Prediction of Umbilics Creation in Nematic Liquid Crystals with Positive Dielectric Anisotropy.

Optically assisted electrical generation of umbilic defects, arising in homeotropically aligned nematic liquid crystal cells and known as topological templates for the generation of optical vortices, are reported in nematic liquid crystals with positive dielectric anisotropy in detail. It is shown that nematic liquid crystals with positive dielectric anisotropy can serve as a stable and efficient medium for the optical vortex generation from both linearly and circularly polarized input Gaussian beams. Hybrid cells made from a thin layer of nematic liquid crystal confined between a photoresponsive slab of iron-doped lithium niobate and a glass plate coated with an active material, i.e., indium tin oxide, were studied. Exposure to a laser beam locally induces a photovoltaic field in the iron-doped lithium niobate substrate, which can penetrate into the liquid crystal film and induce realignment of molecules. The photovoltaic field drives charge carrier accumulation at the interface of indium tin oxide with the liquid crystal, which effectively modifies the shape and symmetry of the electric field. The photovoltaic field has a continuous radial distribution in the transverse xy-plane, weakening with increasing distance from the light irradiation center, where the electric field is normal to the cell plane. Umbilics are created as a result of the liquid crystal tendency to realign parallel to the electric field. Numerical studies of the transmitted intensity profiles in between linear polarizers reveal optical vortex pattern (of four and eight brushes) characteristics for the umbilical defects. The application of crossed circular polarizers results in annular-shaped intensity patterns as a result of spin-to-orbital angular momentum conversions, which give rise to the optical vortices.

Comparative simulation of spiral zone plates with a relief from silica glass and silver

Comparative simulation of optical vortices generation by spiral zone plates with a relief from silver and silica glass using a frequency depended finite difference time domain method. The right circularly polarized Gaussian pulse was used as incident light. An analysis of the Umov-Poynting vector, amplitude and phase distributions in the focal plane shows was the presence of complex optical vortices with an inverse energy flow in its’ centre. Influence of the amplitude spiral zone plate relief height on energy backflow is studied. Comparison of simulation results for two types of spiral zone plate confirms the possibility of using the amplitude analogue instead of the phase zone plate since it is simpler to manufacture.

Laser-Inscribed Stress-Induced Birefringence of Sapphire.

Birefringence of is demonstrated inside cross-sectional regions of 100 m, inscribed by axially stretched Bessel-beam-like fs-laser pulses along the c-axis inside sapphire. A high birefringence and retardance of at mid-visible spectral range (green) can be achieved using stretched beams with axial extension of 30–40 m. Chosen conditions of laser-writing ensure that there are no formations of self-organized nano-gratings. This method can be adopted for creation of polarization optical elements and fabrication of spatially varying birefringent patterns for optical vortex generation.

Reconfigurable Periodic Liquid Crystal Defect Array via Modulation of Electric Field

AbstractTopological defects in liquid crystal (LC) phases have been considered critical from the areas in topology and self‐assembly, as well as in applications such as optical vortex generation, particle manipulation system, and template for material micropatterning. An approach for generating and modulating various patterns of LC defects is presented in a single cell by varying the electrode configurations. Periodic LC defect arrays including −1 topological defect in the nematic phase can be achieved by simply adjusting crossed electrodes. Specifically, the fourfold symmetric −1 defect pattern is used as a vortex beam generator and a particle trapping agent with control either of the frequency of the applied electric field or the temperature. The approach suggested here would be beneficial to extend the use of LC patterns in lithographic tools and optoelectronic devices.

Generation of optical vortices using a uniaxially aligned azo-dye-doped liquid crystal cell and space-variant polarization projection system.

A scheme is presented for an optical vortex (OV) generator comprising a uniaxially aligned azo-dye-doped liquid crystal (ADDLC) and a space-variant polarization projection (SVPP) system. The SVPP system consisting of an electro-optic modulator and a micro-electromechanical system projects a time-averaged SVP field equal to a vector beam onto the ADDLC and fabricates a three-dimensional twisted anisotropic structure, which has spatial phase modulation properties from plane to helical shape. The generation of OVs with odd- and even-numbered topological charge is experimentally demonstrated. As a flexible and broadband spatial light modulator, the proposed scheme should be applicable to the research and development of OV applications.

Wavelength-adaptable effective q-plates with passively tunable retardance

Wave retarders having spatially varying optical axes orientations, called q-plates are extremely efficient devices for converting spin to orbital angular momentum of light and for the generation of optical vortices. Most often, these plates are designed for a specific wavelength and have a homogeneous constant retardance. The present work provides a polarimetric approach for overcoming both these limitations. We theoretically propose and experimentally demonstrate q-plates with tunable retardance, employing a combination of only standard q-plates and waveplates. A clear prescription is provided for realizing wavelength indepedent q-plates for a desired retardance, with a potential for ultrafast switching. Apart from the potential commercial value of the proposed devices, our results may find applications in quantum communication protocols, astronomical coronography, angular momentum sorting and in schemes that leverage optical vortices and spin to orbital angular momentum conversion.

Generation of optical vortices by a pair of optical fiber orthogonal–dislocated gratings

In this paper, we present an effective method to generate optical vortices (OVs) by a pair of optical fiber orthogonal-dislocated gratings (ODGs). A pair of optical fiber ODGs can be achieved by using the UV-side-exposure method. We analyze that the fundamental mode can be selectively coupled to OVs with topological charge of 1 or −1 through tuning the distance between the centers of the two gratings. Even when the distance between the centers of the two gratings is not accurate or the written direction of the pair of gratings is not orthogonal, OVs can also be generated. Finally, the purity of OV generated by ODGs is calculated.

Reconfigurable generation of optical vortices based on forward stimulated intermodal Brillouin scattering in subwavelength-hole photonic waveguides

An all-optically reconfigurable generation of optical vortices would be highly beneficial to the implementation of next-generation optical communication and advanced information processing. The previously demonstrated approaches based on the parametric nonlinear optical processes, however, have exhibited limited conversion efficiency due to the group velocity mismatch and nonlinear phase shifts, and require the cumbersome preparation of either the optical element or initial seed beam having a non-zero topological charge. Here, we propose and analyze a novel scheme for highly efficient all-optical generation and control of optical vortices based on the dynamic acoustic vortex grating created by forward stimulated intermodal Brillouin scattering in a subwavelength-hole photonic waveguide. The dual-frequency pump beams in two different hybrid optical modes drive an acoustic vortex mode, which transforms a signal in the fundamental optical mode into an optical vortex mode. This scheme not only eliminates the need for the initial preparation of an angular-momentum-carrying medium or an optical vortex seed but also guarantees high modal purity and nearly 100% conversion efficiency assisted by the energy-momentum conservation. We also investigate the feasibility and practicability of the subwavelength-hole waveguides by examining the intermodal conversion efficiency and robustness of guidance of the optical vortices, taking into account the impact of the Kerr-type nonlinear effects on the intermodal Brillouin interactions based on our rigorous full-vectorial analytical theory.

Controlled generation of vortex and vortex dipole from a Gaussian pumped optical parametric oscillator.

We report on direct generation of optical vortices from a continuous-wave (cw), Gaussian beam pumped doubly resonating optical parametric oscillator (DRO). Using a 30-mm long MgO doped periodically poled lithium tantalate (MgO:sPPLT) crystal based DRO, pumped in the green by a frequency-doubled Yb-fiber laser in Gaussian spatial profile we have generated signal and idler beams in vortex mode of order, l = 1, tunable across 970-1178 nm. Controlling the overlap between the Gaussian pump beam with the fundamental cavity mode of the resonant signal and idler beams of the DRO through the tilt of the pump beam and/or the cavity mirror in transverse plane, we have generated both signal and idler beams in vortex and vortex dipole spatial profiles. Using the theoretical formalism for the vortex beam generation through the superposition of two Gaussian beams we have numerically calculated the spatial profile of the generated beam in close agreement with our experiment results. The generic experimental scheme can be used to generate optical vortex across the electromagnetic spectrum and in all time scales (cw to ultrafast) using suitable OPO.