Azimuthally Polarized Beam Research Articles

Vector beam generation based on the nanometer-scale rectangular holes

A kind of plasmonic vector beam generator is proposed with the help of a compact nano-hole structure. The structure is composed of the homogeneous rectangular holes which are etched in a silver film and arranged in an Archimedes' spiral. Under the circularly polarized light illumination, each rectangular hole is identified as a quarter-wave plate, and the vector beams including the radial and azimuthal polarization beams can form through properly orientating and spirally arranging these holes. The numerical simulations according to the finite-difference time-domain method provide the parameters of the optimized structure of the vector beam generator. The theoretical analysis reveals the formation principle of the vector beams. The practical measurement for the diffraction of the proposed structure verifies the vector beam generation.

Highly Efficient Vectorial Fiber Laser With Switchable Output

We reported an efficient ytterbium-doped vectorial fiber laser modulated by a low loss q-plate, which can manipulate the intra-cavity polarization evolution in a flexible and compact way. The vectorial fiber laser can deliver watt-level radially and azimuthally polarized laser around $1~\mu \text{m}$ from the same laser cavity with slope efficiency greater than 70.6%, and the polarization extinction ratio exceeding 97%. The polarization behavior of the radial and azimuthal polarization beams can be switched by simply controlling the intra-cavity polarization element.

A subwavelength spot and a three-dimensional optical trap formed by a single planar element with azimuthal light

The generation of subwavelength spots smaller than the Abbe diffraction limit has attracted great interest due to the various applications in many fields, such as high-density optical data storage and particle manipulation. Planar optics that can miniaturize conventional refractive optics have become increasingly attractive. In this work, we first formed a subwavelength bright spot and a three-dimensional optical trap under the illumination of an azimuthally polarized (AP) beam by only a single planar element, a spiral zone plate (SZP). Initially, the SZP was proposed as a computer-generated hologram to generate optical phase singularities. However, the SZP in this work was used to focus and modulate the incident AP beam with a vortex phase simultaneously. Therefore, no additional vortex phase modulating element was introduced in our method. The SZP has an ultra-long focal length of 250λ for a numerical aperture (NA) of 0.95 and an incident wavelength of 632.8 nm. The generated spot is purely transversely polarized with a lateral full width at half maximum (FWHM) of 0.43λ beyond the diffraction limit of 0.54λ. The generated focal field formed a stable optical trap for a Rayleigh dielectric particle in three dimensions.

Fano resonance with high local field enhancement under azimuthally polarized excitation

Being an enabling technology for applications such as ultrasensitive biosensing and surface enhanced spectroscopy, enormous research interests have been focused on further boosting the local field enhancement at Fano resonance. Here, we demonstrate a plasmonic Fano resonance resulting from the interference between a narrow magnetic dipole mode and a broad electric dipole mode in a split-ring resonator (SRR) coupled to a nanoarc structure. Strikingly, when subjected to an azimuthally polarized beam (APB) excitation, the intensity enhancement becomes more than 60 times larger than that for a linearly polarized beam (LPB). We attribute this intensity enhancement to the improved conversion efficiency between the excitation and magnetic dipole mode along with improved near-field coupling. The APB excited Fano structure is further used as a nanoruler and beam misalignment sensor, due to the high sensitivity of intensity enhancement and scattering spectra to structure irregularities and excitation beam misalignment. Interestingly, we find that, regardless of the presence of structural translations, the proposed structure still maintains over 60 times better intensity enhancement under APB excitation compared to LPB excitation. Moreover, even if the APB excitation is somewhat misaligned, our Fano structure still manages to give a larger intensity enhancement than its counterpart excited by LPB.

Biomimetic surface structuring using cylindrical vector femtosecond laser beams

We report on a new, single-step and scalable method to fabricate highly ordered, multi-directional and complex surface structures that mimic the unique morphological features of certain species found in nature. Biomimetic surface structuring was realized by exploiting the unique and versatile angular profile and the electric field symmetry of cylindrical vector (CV) femtosecond (fs) laser beams. It is shown that, highly controllable, periodic structures exhibiting sizes at nano-, micro- and dual- micro/nano scales can be directly written on Ni upon line and large area scanning with radial and azimuthal polarization beams. Depending on the irradiation conditions, new complex multi-directional nanostructures, inspired by the Shark’s skin morphology, as well as superhydrophobic dual-scale structures mimicking the Lotus’ leaf water repellent properties can be attained. It is concluded that the versatility and features variations of structures formed is by far superior to those obtained via laser processing with linearly polarized beams. More important, by exploiting the capabilities offered by fs CV fields, the present technique can be further extended to fabricate even more complex and unconventional structures. We believe that our approach provides a new concept in laser materials processing, which can be further exploited for expanding the breadth and novelty of applications.

Broadband active tuning of unidirectional scattering from nanoantenna using combined radially and azimuthally polarized beams.

We propose an approach to actively tune the scattering pattern of a Mie-type spherical antenna. The scheme is based on separate control over the induced electric dipole and induced magnetic dipole using two coherent focused beams of radial polarization and azimuthal polarization, respectively. By carefully tuning the amplitude and phase relation of the two beams, a broadband unidirectional scattering can be achieved, even at the antenna's resonant wavelength where the antenna scatters efficiently. By moving the focus of one beam, a drastic switch of the unidirectional scattering can be observed. Such a scheme enables the design of ultra-compact optical switches and directional couplers based on nanoantennas.

Experimental realization of focal field engineering of the azimuthally polarized beams modulated by multi-azimuthal masks

This paper experimentally investigates the controlling of a focal field of azimuthally polarized (AP) Gaussian beams with multi-azimuthal masks. Based on the angular diffraction theory, an explicit model is established to describe the intensity and polarization distributions of the focal fields. It reveals that the focal fields of the modulated AP beams are closely related to the mask structure. By designing the parameters of the mask structure, we could consciously manage the intensity and polarization distributions of the focal fields of an AP Gaussian beam. Our experimental results also demonstrate the feasibility of controlling focal field characteristics by engineering the mask structure, and suggest an effective approach for manipulating the focal field of vector beams.

Peculiarities of second harmonic generation by paraxial beams with radial / azimuthal polarization in type II nonlinear crystal

In this work we investigate the light patterns generated by paraxial radial and azimuthal polarization beams in type-II nonlinear crystal. We show that in paraxial case the second harmonic intensity pattern generated by the radial / azimuthal polarization beams can be expressed similarly to the Hermite-Gaussian HG11 mode. In addition, numerical simulations were carried out taking into account diffraction, walk-off, and pump depletion. The numerical simulations have shown that even with pump depletion the resulting second harmonic beam consists of 4 maxima. Also, experimental results are presented, which confirm theoretical predictions. Interference patterns indicate phase shifts of π between neighbouring maxima.

Generation of azimuthally polarized beams in fast axial flow CO2 laser with hybrid circular subwavelength grating mirror

A hybrid circular subwavelength grating mirror is proposed and fabricated as a rear mirror in a fast axial flow CO2 laser system to generate azimuthally polarized beams (APBs). This grating mirror, with particular gold-covered ridges and nanopillar-stuffed grooves, performs wideband TE wave reflectivity and high polarization selectivity. It shows that the polarization selectivity mechanism lies in the gold ridge's high reflectivity to the TE wave and the lower TM wave reflectivity, which are the result of the mode leaking into substrate through the dielectric-like nanopillar layer. Finally, a high-quality 550 W APB is obtained in subsequent experiments, which provides potential applications in drilling and welding.

Breaking the diffraction-limited resolution barrier in fiber-optical two-photon fluorescence endoscopy by an azimuthally-polarized beam

Although fiber-optical two-photon endoscopy has been recognized as a potential high-resolution diagnostic and therapeutic procedure in vivo, its resolution is limited by the optical diffraction nature to a few micrometers due to the low numerical aperture of an endoscopic objective. On the other hand, stimulated emission depletion (STED) achieved by a circularly-polarized vortex beam has been used to break the diffraction-limited resolution barrier in a bulky microscope. It has been a challenge to apply the STED principle to a fiber-optical two-photon endoscope as a circular polarization state cannot be maintained due to the birefringence of a fiber. Here, we demonstrate the first fiber-optical STED two-photon endoscope using an azimuthally-polarized beam directly generated from a double-clad fiber. As such, the diffraction-limited resolution barrier of fiber-optical two-photon endoscopy can be broken by a factor of three. Our new accomplishment has paved a robust way for high-resolution in vivo biomedical studies.

Plasmonic petal-shaped beam for microscopic phase-sensitive SPR biosensor with ultrahigh sensitivity

Differential phase measurement between radially polarized (RP) and azimuthally polarized (AP) beams is an important technique in microscopic surface plasmon resonance (SPR) biosensors as reported in our earlier works [Opt. Lett.37, 2091 (2012); Appl. Phys. Lett.102, 011114 (2013)]. However, such a technique suffers complex beam splitting, detection, and data processing procedures for RP and AP beams which may lower the accuracy of phase measurement. In this Letter, a novel plasmonic petal-shaped vector beam is proposed instead of RP and AP beams, greatly simplifying the sensor system and enabling single measurement in differential interferometry. Moreover, an improved ultrahigh sensitivity on the order of 10(-7) refractive index units (RIUs) is experimentally verified in the proposed system.

Effect of polarization purity of cylindrical vector beam on tightly focused spot

The tightly focused spots of cylindrical vectors (CVs) are dependent on polarization composition. We experimentally demonstrate the effect of polarization purity (PP) of the CV beam on the tightly focused spot quantitatively, which should be strictly controlled for the effective applications of the CV beam. The focal spots measured by a knife-edge scanning method showed that the azimuthally polarized (AP) component increases the transverse field and the size of the focal spots, while the radially polarized component results in a nonzero intensity distribution at the center of the focus even in a high PP AP beam.

Imaging using cylindrical vector beams in a high-numerical-aperture microscopy system

Imaging of object structures using cylindrical vector beams in an aplanatic solid immersion lens (SIL) microscope is investigated. Based on a complete optical model of an aplanatic SIL microscope, images of some object structures using radial polarization, azimuthal polarization, and azimuthal vortex beams are simulated. Some interesting imaging effects of these object structures are observed. For example, counterintuitively, it is found that, compared to linear and circular polarizations, radial polarization requires a larger pinhole to acquire a good image and resolution. Similarly, it is shown that an azimuthal vortex beam provides good images for a variety of object structures and pinhole sizes. Theories and explanations are provided to justify the observed effects. The presented results play an important role in high-numerical-aperture optical imaging.

Vector beam synthesis by means of astigmatic conversion

We propose a new scheme based on a modified Mach–Zehnder interferometer for generating vector beams. The beams are formed with the help of a special diffractive optical element, which performs the astigmatic transformation of an input Hermite–Gaussian beam into two complexly conjugated Laguerre–Gaussian beams. The scheme allows us to obtain radially and azimuthally polarized beams and beams with a more complex polarization structure. A theoretical estimation for the transformation of beams both of radial and azimuthal polarization has been fulfilled.

Investigation on partially coherent vector beams and their propagation and focusing properties

The propagation and focusing properties of partially coherent vector beams including radially polarized and azimuthally polarized (AP) beams are theoretically and experimentally investigated. The beam profile of a partially coherent radially or AP beam can be shaped by adjusting the initial spatial coherence length. The dark hollow, flat-topped, and Gaussian beam spots can be obtained, which will be useful in trapping particles. The experimental observations are consistent with the theoretical results.

Effect of spatial coherence on propagation, tight focusing, and radiation forces of an azimuthally polarized beam

We report experimental generation of an azimuthally polarized (AP) beam with variable spatial coherence. The effect of spatial coherence on the propagation properties of an AP beam is studied both numerically and experimentally, and our experimental results agree well with the theoretical predictions. The dependence of the intensity distribution of an AP beam focused by a high numerical aperture objective lens on its initial spatial coherence is illustrated numerically, and it is found that we can shape the beam profile of a tightly focused AP beam by varying its initial spatial coherence. Furthermore, the radiation forces on Rayleigh particles induced by a tightly focused AP beam are studied, and we find that the tightly focused AP beam can be used to trap a Rayleigh particle whose refractive index is larger or smaller than that of the ambient by varying its initial spatial coherence. Our results will be useful for particle trapping and material thermal processing.

Analytical expression for the diffraction field of an axicon using the ray-tracing and interference method

We present a simple analytical expression for calculating the vector field distribution behind an axicon using the ray tracing and interference method. This expression can be used for analyzing vector beam propagation throughout the whole space behind the axicon, with any base angle. Using the present analytical expression, we have studied the propagation characteristics of the four kinds of polarizations, linear polarization, left-handed circular polarization (i.e., the radial polarization), right-handed circular polarization, and azimuthal polarization, incident on the axicon. We found that the longitudinally-polarized component has to be considered for a large base angle and high refractive index of axicons, apart from the azimuthally-polarized beam of incidence.

Spatially-variable retardation plate for efficient generation of radially- and azimuthally-polarized beams

We investigate conversion of a linearly-polarized Gaussian beam to a radially- or an azimuthally-polarized doughnut (0, 1) ∗ Laguerre–Gaussian (LG) beams, performed with a spatially-variable retardation (SVR) plate. The SVR plate is composed of eight sectors of a λ/2 retardation plate, each one with different orientation of the to crystal’s slow axis. The analysis reveal that nearly-pure radially- or azimuthally-polarized LG(01) ∗ beam with M 2 = 2.2 can be obtained, while the transformation efficiency is 89.6%. In the experiments, performed with Nd:YAG laser, we transformed a Gaussian beam with M 2 = 1.3 to a radially- and azimuthally-polarized (0, 1) ∗ Laguerre–Gaussian beams with M 2 = 2.5. We carefully characterized the polarization state of the obtained radially- and azimuthally-polarized beams, measuring Stokes parameters. The polarization purity of the obtained beams, calculated from the measured data, was as high as 96%.