Iterative phase-retrieval method for generating stereo array of polarization-controlled focal spots

Generation of the stable propagation Bessel beam and the axial multifoci beam with pure phase elements

A recently proposed method is upgraded to convert two amplitude phase modulation systems (APMSs) to pure phase elements (PPEs), for generating the stable propagation Bessel beam and the axial multifoci beam, respectively. Phase functions of the PPEs are presented analytically. Numerical simulations by the complete Rayleigh-Sommerfeld method demonstrate that the converted PPE has implemented the same optical functionalities as the corresponding APMS, in either the longitudinal or the transverse direction. Compared with the traditional APMS, the converted PPE possesses many advantages such as fabrication process simplification, system complexity reduction, production cost conservation, alignment error avoidance, and experimental precision enhancement. These inherent advantages position the PPE as an ideal choice and driving force behind further advancements in optical system technology.

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.

Fast calculation of tightly focused random electromagnetic beams: controlling the focal field by spatial coherence

Focusing of a vectorial (electromagnetic) optical beam through a high numerical aperture can be investigated by means of the Richards-Wolf diffraction integral. However, such an integral extends from two-dimensional to four-dimensional, greatly increasing the computation time and therefore limiting the applicability, when light with decreased spatial coherence is considered. Here, we advance an effective protocol for the fast calculation of the statistical properties of a tightly focused field produced by a random electromagnetic beam with arbitrary state of spatial coherence and polarization. The novel method relies on a vectorial pseudo-mode representation and a fast algorithm of the wave-vector space Fourier transform. The procedure is demonstrated for several types of radially (fully) polarized but spatially partially coherent Schell-model beams. The simulations show that the computation time for obtaining the focal spectral density distribution with 512 × 512 spatial points for a low coherence beam is less than 100 seconds, while with the conventional quadruple Richards-Wolf integral more than 100 hours is required. The results further indicate that spatial coherence can be viewed as an effective degree of freedom to govern both the transverse and longitudinal components of a tightly focused field with potential applications in reverse shaping of focal fields and optical trapping control.

Projection Printing of Ultrathin Structures with Nanoscale Thickness Control

Spatial control of photon energy has been a central part of many light-based manufacturing processes. We report a direct projection printing method for ultrathin structures with nanoscale thickness control by using a patterned evanescent field. The evanescent field is induced by total internal reflection at the interface between the substrate and a prepolymer solution, and it is patterned by a phase-only spatial light modulator. The ultrathin structure is printed on a high-refractive-index glass substrate through photopolymerization. An iterative algorithm is used to calculate the phase pattern for generating arbitrary holography images and making the image plane to coincide with the interface. The thickness of the pattern is limited by the penetration depth of the evanescent field. Experiment results demonstrated that polymer structures as thin as 200 nm can be patterned without significant process optimization. Such fine control in thickness could transform many techniques such as light-based 3D printing and laser direct-write manufacturing.

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.

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.

Nanoscale 3D printing of hydrogels for cellular tissue engineering

Hydrogel scaffolds that mimic the native extracellular matrix (ECM) environment is a crucial part of tissue engineering. It has been demonstrated that cell behaviors can be affected by not only the hydrogel's physical and chemical properties, but also its three dimensional (3D) geometrical structures. In order to study the influence of 3D geometrical cues on cell behaviors as well as the maturation and function of engineered tissues, it is imperative to develop 3D fabrication techniques to create micro and nanoscale hydrogel constructs. Among existing techniques that can effectively pattern hydrogels, two-photon polymerization (2PP)-based femtosecond laser 3D printing technology allows one to produce hydrogel structures with 100 nm resolution. This article reviews the basics of this technique as well as some of its applications in tissue engineering.

3D point scanning super-resolution microscopy via polarization modulation

We report a new approach to achieving super-resolution in point-scanning microscopy through polarization modulation for the first time, to the best of our knowledge. By modulating linearly polarized incident light, the emission extent of fluorescent dyes changes periodically, adding sparsity in each recording, which contributes to the super-resolution reconstruction. To recover the super-resolution result, a sparse penalty-based deconvolution method is implemented onto the polarization-modulated dataset subsequently. By simply inserting a vortex half-wave retarder into a typical confocal microscope, we obtain the super-resolution experimental results of both nuclear pore complex proteins and tubulins in vero cells, which evidence a sub-diffraction resolution of λ/5. In addition, three-dimensional (3D) super-resolution on spatial distributed single molecules is simulated, where the significant resolution improvement in both lateral and axial directions further confirms its capacity in 3D imaging applications. Considering no constraint on fluorescence dyes and easy implementation in a point-scanning microscope, we envision that the polarization-modulated confocal microscope would be a helpful alternative in biological imaging.

Resolution criteria in double-slit microscopic imaging experiments

Double-slit imaging is widely used for verifying the resolution of high-resolution and super-resolution microscopies. However, due to the fabrication limits, the slit width is generally non-negligible, which can affect the claimed resolution. In this paper we theoretically calculate the electromagnetic field distribution inside and near the metallic double slit using waveguide mode expansion method, and acquire the far-field image by vectorial Fourier optics. We find that the slit width has minimal influence when the illuminating light is polarized parallel to the slits. In this case, the claimed resolution should be based on the center-to-center distance of the double-slit.

Generation of a controllable multifocal array from a modulated azimuthally polarized beam

In this Letter, the focal spot areas of an azimuthally polarized beam modulated with a vortex-0-2π-phase plate or a π-phase-step plate are numerically found to be smaller than a radially polarized beam for three pupil functions with uniform, Gaussian, and Bessel-Gauss profiles. Several types of multizone phase plates are theoretically designed and numerically simulated for generating tight multifocal arrays from the azimuthally polarized beams for what we believe is the first time. The positions and polarization states of the multifocal arrays can be controlled simply by varying the pattern of the multizone plates. The produced multifocal array with controllable position and polarization is beneficial to parallel optical recording and parallel optical imaging.