Annular optical trapping of metallic nanoparticles using the azimuthally polarized beam.

The unique physical and chemical properties make metallic nanoparticles promising for broad applications in many fields. Exploring the dynamics of metallic nanoparticles in optical traps is crucial for exploiting optical tweezers to advance the applications of metallic particles. In this paper, we present a detailed study of the annular optical trapping of gold nanoparticles with azimuthal polarization. Theoretical analysis based on the T-matrix method shows that the gold nanoparticles experience optical forces pointing to the equilibrium position along the radial direction, while there is no force along the azimuthal direction at this equilibrium position. Therefore, a tightly focused azimuthally polarized beam captures gold nanoparticles in an annular region. Experimental measurements of the motion trajectory of the confined gold nanoparticles reveal a donut profile consistent with the theoretical predictions. Our work reported in this paper is expected to deepen our understanding of the interactions between metallic nanoparticles and light and promote the application of metallic nanoparticles.

Zero-order free holographic optical tweezers.

Holographic optical tweezers (HOTs) use spatial light modulators (SLM) to modulate light beams, thereby enabling the dynamic control of optical trap arrays with complex intensity and phase distributions. This has provided exciting new opportunities for cell sorting, microstructure machining, and studying single molecules. However, the pixelated structure of the SLM will inevitably bring up the unmodulated zero-order diffraction possessing an unacceptably large fraction of the incident light beam power. This is harmful to optical trapping because of the bright, highly localized nature of the errant beam. In this paper and to address this issue, we construct a cost-effective, zero-order free HOTs apparatus, thanks to a homemade asymmetric triangle reflector and a digital lens. As there is no zero-order diffraction, the instrument performs excellently in generating complex light fields and manipulating particles.

Optical trapping of gold microparticles using linearly polarized, centrally obstructed Gaussian beams.

Metallic microparticles larger than the illumination wavelength are commonly considered poor optical trapping candidates due to their high extinction coefficient. This paper presents a numerical and experimental study on the three-dimensional (3D) trapping of gold microparticles using a centrally obstructed Gaussian beam based on the T-matrix method. The range of particle size for stable optical trapping is determined. For the trapping numerical aperture of 1.32 and illumination wavelength of 1.064 µm, numerical analysis proves that 3D trapping of gold microparticles with a radius bigger than 1.0 µm can be readily achieved. By imprinting a digital lens to the spatial light modulator, we slightly defocus the centrally obstructed Gaussian beam to shift the trapping location to the focal plane for clear observation. Experimental results demonstrate stable trapping of gold microparticles with a radius greater than 1.4 µm at high-power illumination, agreeing well with the theoretical predictions. The presented work should be of interest to the community applying metallic microparticles to relevant research.

Simultaneous optical trapping and imaging in the axial plane: a review of current progress.

Optical trapping has become a powerful tool in numerous fields such as biology, physics, chemistry, etc. In conventional optical trapping systems, trapping and imaging share the same objective lens, confining the region of observation to the focal plane. For the capture of optical trapping processes occurring in other planes, especially the axial plane (the one containing the z-axis), many methods have been proposed to achieve this goal. Here, we review the methods of acquiring the axial-plane information from which axial plane trapping is observed and discuss their advantages and limitations. To overcome the limitations existing in these methods, we developed an optical tweezers system that allows for simultaneous optical trapping and imaging in the axial plane. The versatility and usefulness of the system in axial-plane trapping and imaging are demonstrated by investigating its trapping performance with various optical fields, including Bessel, Airy, and snake-like beams. The potential applications of the reported technique are suggested to several research fields, including optical pulling, longitudinal optical binding, tomographic phase microscopy (TPM), and super-resolution microscopy.

Generation of a double-ring perfect optical vortex by the Fourier transform of azimuthally polarized Bessel beams.

The perfect optical vortex (POV), the ring size being independent of its topological charge, has found potential applications in optical tweezers and optical communications. In this Letter, we report a new kind of POV, termed as double-ring POV (DR-POV), whose diameters of the two rings are independent of topological charge. We theoretically demonstrate that such a vortex is the Fourier transform of an azimuthally polarized Bessel beam. Experimental results agree well with theoretical prediction. We further investigate the vortex nature of the DR-POV through an interferometric method, showing that the two rings of the vortex have the same topological charge value (magnitude and sign). The specular properties of the DR-POV may find application in optical tweezers, such as trapping and rotating of low-refractive-index particles in the dark region between the two rings.

Multi-color structured illumination microscopy for live cell imaging based on the enhanced image recombination transform algorithm.

Structured illumination microscopy (SIM) has attracted considerable interest in super-resolution, live-cell imaging because of its low light dose and high imaging speed. Obtaining a high-quality reconstruction image in SIM depends on the precise determination of the parameters of the fringe illumination pattern. The image recombination transform (IRT) algorithm is superior to other algorithms in obtaining the precise initial phase without any approximation, which is promising to provide a considerable solution to address the difficulty of initial phase estimation at low-modulation-depth conditions. However, the IRT algorithm only considers a phase shift of π∕2, which limits its applications in general scenarios. In this letter, we present a general form of IRT algorithm suitable for arbitrary phase shifts, providing a powerful tool for parameter estimation in low signal-to-noise cases. To demonstrate the effectiveness of the enhanced IRT algorithm, we constructed a multicolor, structured illumination microscope and studied at super-resolution, the cargo traffic in HRPE cells, and monitored the movement of mitochondrial structures and microtubules in COS-7 cells. The custom SIM system using the enhanced IRT algorithm allows multicolor capability and a low excitation intensity fluorescence imaging less than 1 W/cm2. High-quality super-resolution images are obtained, which demonstrates the utility of this approach in imaging in the life sciences.