With the availability of high-resolution miniature spatial light modulators (SLMs) new methods in optical microscopy have become feasible. The SLMs discussed in this review consist of miniature liquid crystal displays with micron-sized pixels that can modulate the phase and/or amplitude of an optical wavefront. In microscopy they can be used to control and shape the sample illumination, or they can act as spatial Fourier filters in the imaging path. Some of these applications are reviewed in this article. One of them, called spiral phase contrast, generates isotropic edge enhancement of thin phase samples or spiral-shaped interference fringes for thicker phase samples, which can be used to reconstruct the phase topography from a single on-axis interferogram. If SLMs are used for both illumination control and spatial Fourier filtering, this combination for instance allows for the generalization of the Zernike phase contrast principle. The new SLM-based approach improves the effective resolution and avoids some shortcomings and artifacts of the traditional method. The main advantage of SLMs in microscopy is their flexibility, as one can realize various operation modes in the same setup, without the need for changing any hardware components, simply by electronically switching the phase pattern displayed on the SLMs.

What spatial light modulators can do for optical microscopy

We study theoretically and experimentally the spatial intensity distribution of the zero-order Bessel beam formed by the axicon which possess a rounded tip. Such a tip generates a refracted beam that interferes with the quasi-Bessel beam created behind the axicon. In turn an undesired intensity modulation occurs that significantly disturbs the unique properties of the quasi-Bessel beam--namely the constant shape of the lateral intensity distribution and the slow variation of the on-axis beam intensity along the beam propagation. We show how the spatial filtration of the beam in the Fourier plane improves this spatial beam distribution and removes the undesired modulation. We use an efficient numerical method based on Hankel transformations to simulate the propagation of the beam behind the axicon and filter. We experimentally measure the intensity distribution of the beam in many lateral planes and subsequently reconstruct the spatial intensity distribution of the beam. Computed and measured beam distributions are compared and the obtained agreement is very good.

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High quality quasi-Bessel beam generated by round-tip axicon

We present the first theoretical and experimental study of dielectric sub-micron particle behaviour in an optical field generated by interference of co-propagating non-diffracting beams of different propagation constants. In such a field, there are periodic oscillations of the on-axial intensity maxima (self- imaging) that are frequently mentioned as useful for optical trapping. We show thatinthreedimensionsthisistrueonlyforverysmallparticlesandtheincreasing number of interfering beams does not enable confinement of substantially bigger particles under the studied conditions. Experimentally, we succeeded in optical confinement of beads radii from 100nm up to 300nm but only with the help of fluid flow against the beams propagation. We observed self-organization of the particles into the periodic 1D array with the interparticle distance equal to 7.68 µm. We observed how a bead jump from one trap to the neighbouring- occupied trap caused a domino effect propagating with constant velocity over the subsequent occupied traps. Phase shift in one beam induced controlled bi-directional shift of the whole structure over a maximal distance of 250 µm in two co-propagating Bessel beams.

https://iopscience.iop.org/article/10.1088/1367-2630/8/3/043/pdf

Sub-micron particle organization by self-imaging of non-diffracting beams

We present a simple method using an axicon and spatial light modulator to create multiple parallel Bessel beams and precisely control their individual positions in three dimensions. This technique is tested as an alternative to classical holographic beam shaping commonly used now in optical tweezers. Various applications of precise control of multiple Bessel beams are demonstrated within a single microscope giving rise to new methods for three-dimensional positional control of trapped particles or active sorting of micro-objects as well as “focus-free” photoporation of living cells. Overall this concept is termed a ‘biophotonics workstation’ where users may readily trap, sort and porate material using Bessel light modes in a microscope.

/pdf/generation-of-multiple-bessel-beams-for-a-biophotonics-javio08u9h.pdf

Generation of multiple Bessel beams for a biophotonics workstation

We present the theoretical and experimental study of nondiffracting Bessel beams as a device for optical manipulation and confinement of nanoparticles. We express analytically the optical forces acting on a nanoparticle placed into a single and two counter-propagating non-paraxial nondiffracting beams created behind the axicon. Nanoparticle behavior in these configurations is predicted by computer simulations. Finally we demonstrate experimentally how standing waves created from two independent counter-propagating nondiffraction beams confines polystyrene beads of radii 100 nm, and organizes them into a one-dimensional chain 1 mm long. Phase shift in one beam causes the motion of the whole structure of the standing wave together with any confined objects over its extent.

An optical nanotrap array movable over a milimetre range

In this letter, we report multiple-channel longitudinal optical guiding of microparticles using an array of Bessel beams generated from a composite hologram fabricated by ultraviolet lithography. The optical guiding efficiency of each Bessel beam in the optical array is investigated experimentally. The rod-like core of each Bessel beam, with its nondiffracting and self-reconstruction property, has been shown to offer strong capability for optical guiding along the propagation axis possibly even in multiple-microfluidic channels massively.

Microfabricated-composite-hologram-enabled multiple channel longitudinal optical guiding of microparticles in nondiffracting core of a Bessel beam array

The authors present experimental results of self-referenced spiral interference realized by a hollow spiral phase plate fabricated by electron beam lithography. Comparing with conventional interference systems, the proposed phase element provides a simple and robust approach to obtaining spiral interference fringes in a self-referenced interferometric way. Experimental implementation confirms that the element can be employed as an external tool kit for simple modification of existing optical microscopes to interferometers.

S. H. Tao

Papers

Microfabricated-composite-hologram-enabled multiple channel longitudinal optical guiding of microparticles in nondiffracting core of a Bessel beam array

Hollow spiral phase plate for self-referenced spiral interferometry