Dynamic three-dimensional multifocal spots in high numerical-aperture objectives

Uniform intensity chiral optical field by multifocal synthesis

Chiral optical beams that carry orbital angular momentum (OAM) have a broad range of applications such as optical tweezers, chiral microstructure fabrication, and optical communications. However, some chiral optical beams have inhomogeneous intensity distribution that limits the application in these fields. In this Letter, two different types of chiral optical fields with uniform intensity and arbitrary length were proposed based on the amplitude encoding method and multifocal synthesis. The intensity distribution of the chiral optical fields is determined by the distance between the focal points that can greatly extend the modulation length of the chiral optical field. Moreover, since each focal point contains modulable amplitude and phase, an arbitrary interception of the optical field can be realized by selectively retaining a part of the focal points. By partitioning the chiral optical field and assigning different topological charges, the OAM space-division multiplexing and independent tunability of the topological charges can be realized. In addition, the composite multi-petal vortex array formed by combining two different chiral optical fields can greatly enhance the information capacity of the optical communications and may have potential applications in fields such as particle manipulation.

Generation of multi-focus shaping with high uniformity based on an improved Gerchberg-Saxton algorithm

The Gerchberg-Saxton (GS) algorithm has been extensively employed in computational holography and beam shaping with the advantages of quick iteration speed and high energy utilization. However, the GS algorithm is prone to trapping into local optima and not reaching ideal outcomes, leading to poor shaping quality. In this paper, a method of random disturbance superposition (RDS) was proposed to feedback GS amplitude, which could stably and universally achieve over 95% high uniformity shaping of multiple beams without other complex operations. In light of this, this paper also covered how this technique affected energy utilization. It has been discovered that the introduction of perturbation could decrease the energy utilization. By analyzing the mechanism, a phase value replacement (PVR) method was proposed, which could effectively improve energy utilization without reducing uniformity. Finally, the simulation results were experimentally validated and met expectations very well. This method helps to accurately control the energy distribution of multiple beams and has a driving effect on laser precision processing technology.

Speckle suppression in arbitrary parallel holographic illumination by the spatial frequency regaining method

This Letter proposes a spatial frequency regaining method for parallel holographic illumination (SFR-PHI) to suppress speckle noise in phase-only computer-generated holography (CGH). Based on the accurate calculation of the beam bandwidth, this method uses the bandwidth-limited quadratic initial phase and weighted constraint iteration to generate the optimized phase hologram, which can provide the accurate spatial frequency of multiple illumination patterns. The results show that SFR-PHI performs superiorly in speckle suppression for generating dozens of illumination patterns in parallel and with arbitrary shapes and numbers. Compared with other speckle-suppression methods, it exhibits significant advantages in terms of accuracy and modulation efficiency.

Modulation of high-quality internal multifoci based on modified three-dimensional Fourier transform

We report an efficient method to generate arbitrary three-dimensional (3D) parallel multifoci inside a material. Taking into account the numerical aperture of the objective lens and the refractive index of the material, the Ewald cap was modified with a longer radius, then the whole 3D intensity distribution inside the material could be calculated using only a single Fourier transform (FT). By introducing the adaptive weight coefficient, the uniformity of the 3D multifoci improves from 81.3% to 98.9%. By adjusting the axial resolution of the Ewald cap, the uniformity of the axial multifoci improves from 85.9% to 99.7%. In the experiment, we have realized one-dimensional (1D), 2D, and 3D structures inside the fused silica, which are in excellent agreement with the simulation results. The experimental results of the "H-U-S-T" structure demonstrate that customized arbitrary intensity distribution inside the material can be realized.

Bifocal flat lens with different imaging characteristics for a dual-sensor imaging system

Wide field of view (FOV) images and magnified images can be taken simultaneously by dual-sensor imaging systems. Here, we propose an approach for creating a bifocal flat lens with different imaging characteristics of its two foci, which makes dual-sensor imaging systems more integrated and miniaturized. That is, two special parts of two different conventional ZP are extracted and then combine the two elements in a specific way. So that there are two foci with different characteristics along the optical axis, one is long focus with higher resolution, the other is short focus with long depth of focus (DOF). Under the proposed approach, a thin and light bifocal diffractive lens (BDL) with thickness of 0.6 μm is developed. The long and short focal lengths of the BDL are ~ 81 mm and ~ 27 mm, respectively, with a diameter of 6 mm. We experimentally demonstrate that the long focus of the BDL is capable of taking high-resolution magnified images, and its resolution is up to 21.90″. The short focus is able to take wide FOV with long DOF images, and two objects spread 2880 mm apart can be imaged clearly. The experiment results demonstrate that all of these metrics are better than those of a conventional refractive lens.

Controllable perfect optical vortex generated by complex amplitude encoding

We propose a new paradigm for generating the perfect optical vortex (POV) with a controlled structure and orbital angular momentum (OAM) distribution in the focal region of a tightly focused system. The superiority of the proposed technique is demonstrated with an experiment involving the dynamic manipulation of small particles. This technique for creating the POV could open new routes to optical manipulation based on OAM.

Realization of flexible and parallel laser direct writing by multifocal spot modulation

In this investigation, we propose a strip segmentation phase (SSP) method for a spatial light modulator (SLM) to generate independent multifocal spots when the beam passes through a high numerical aperture (NA) lens. With the SSP method, multifocal spots can be generated with each spot independently, flexibly and uniformly distributed. The performance of the SSP method is first validated with numerical simulation. Then, by applying the modulation method with SLM and importing the beams into an inverted fluorescence microscopy system with a high-NA lens, the spot distribution and their shapes can be observed by fluorescent image. The fluorescent image exhibits high uniformity and high consistency with the aforementioned numerical simulations. Finally, we dynamically load a series of phase maps on SLM to realize continuous and independent spot movement in a multifocal array. By laser direct writing on photoresist, a complex NWU-shape structure can be realized flexibly with multi-task fabrication capability. The SSP method can significantly improve the efficiency and flexibility of laser direct writing. It is also compatible with most recent techniques, e.g., multiphoton absorption, stimulated emission depletion and photo-induced depolymerization etc., to realize parallel super-resolution imaging and fabrications.

Dynamic control of magnetization spot arrays with three-dimensional orientations

We report a new paradigm for achieving magnetization spot arrays with controllable three-dimensional (3D) orientations. Toward this aim, we subtly design a tailored incident beam containing three parts and further demonstrate that the designed incident beam is phase-modulated radial polarization. Based on the raytracing model under tight focusing condition and the inverse Faraday effect on the magneto-optic (MO) film, the magnetization field components along the y-axis and z-axis directions are generated through the focus. In particular, we are able to garner orientation-tunable 3D magnetization under different numerical apertures of the focusing objectives by adjusting the ratios between the three parts of incident beam. Apart from a single magnetization spot, magnetization spot arrays capable of dynamically controlling 3D orientation in each spot can also be achieved by multi-zone plate (MZP) phase filter. Such a robust magnetization pattern is attributed to not only the constructive interferences of three orthogonal focal field components, but also the position translation of each magnetization spot resulting from shifting phase of the MZP phase filter. It is expected that the research outcomes can be beneficial to spintronics, magnetic encryption and multi-value MO parallelized storage.

Generation of controllable chiral optical fields by vector beams

Chirality is common in nature, describing not only the geometrical property of a three-dimensional object, but also an intrinsic feature of an optical field. Chiral optical fields are attracting increasing attention due to their potential applications in chiral light-matter interaction. Here we demonstrate a strategy to realize a controllable chiral optical field by tightly focusing two tailored vector beams in a 4π optical microscopic system. By modulating the wavefronts of the incident vector beams with appropriately designed phase masks, a chiral optical field with multiple spots carrying switchable handedness or controllable chirality can be generated. The location, the number and the handedness of such chiral spots can be arbitrarily adjusted depending on the actual application requirements. In addition to trapping and manipulating multiple particles, this controllable chiral optical field may find applications in enantioselective separation, chiral detection and chiral sensing at the nanoscale.

Chiral optical field generated by an annular subzone vortex phase plate

We proposed and experimentally demonstrated a novel method for generating a chiral beam with controllable intensity twist lobes and direction by using annular subzone (AS) vortex phase plates, which is composed of different ASs and different vortex phases. The phase distribution continuity between two adjacent ASs determines the intensity distribution of the light field. The rotated direction of the optical filed is determined by the topological charge sign. The number of intensity twist lobes is determined by the topological charge gradient between adjacent subzones. The experimental results show that this method is effective and practical, which offers broad potential applications in particle manipulation, chiral microstructure fabrication, and optical tweezers.

Non-iterative dartboard phase filter for achieving multifocal arrays by cylindrical vector beams

We proposed an analytically designed non-iterative dartboard phase filter (DPF) to achieve multifocal arrays by cylindrical vector beams. The DPF is composed of sectors, which is two-dimensionally divided in polar coordinates, along the radial and azimuthal directions. Meanwhile, a modulation factor was first proposed and introduced into the DPF to improve the intensity uniformity of the generated multifocal array. By the proposed DPF, the one-dimensional, two-dimensional and three-dimensional multifocal arrays are generated, which have intensity uniformities larger than 92.5%. The focal position and polarization of these generated multifocal arrays can be controlled, while the transverse sizes of each focal spot are subwavelength. The proposed DPF and the generated multifocal arrays have potential applications in the fields of polarization-multiplexed data storage, polarization-sensitive nanophotonic devices and parallel direct laser writing.

Dynamic control of transverse magnetization spot arrays

We propose a feasible strategy for firstly constructing diffraction-limited light-induced magnetization spot arrays capable of dynamically controlling transverse polarization orientation of each spot. To achieve this goal, we subtly design a tailored incident light comprised of two sorts of beams and sufficiently demonstrate tit's production through phase modulation of a radially polarized beam. Via tightly focusing counter-propagating composite illuminating beams in a 4π optical microscopic configuration, two orthogonally polarized focal fields with π/2 phase difference between them are formed, inducing a three-dimensional (3D) super-resolved transverse magnetization spot in the magnetic-optical (MO) film. Exploiting the ideal of the multi-zone plate (MZP) filter, we further achieve versatile magnetization spot arrays with controllable in-plane polarization direction in each spot. Such well-defined magnetization behavior is attributed to not merely the coherent interference of vectorial optical waves, but also non-overlapping superposition of localized focal fields. Our achievable outcomes pave the way for practical applications in spintronics and multi-value MO parallelized storage.