Tomáš Čižmár

Exploiting multimode waveguides for pure fibre-based imaging

There has been an immense drive in modern microscopy towards miniaturization and fibre-based technology. This has been necessitated by the need to access hostile or difficult environments in situ and in vivo. Strategies to date have included the use of specialist fibres and miniaturized scanning systems accompanied by ingenious microfabricated lenses. Here we present a novel approach for this field by utilizing disordered light within a standard multimode optical fibre for lensless microscopy and optical mode conversion. We demonstrate the modalities of bright- and dark-field imaging and scanning fluorescence microscopy at acquisition rates that allow observation of dynamic processes such as Brownian motion of mesoscopic particles. Furthermore, we show how such control can realize a new form of mode converter and generate various types of advanced light fields such as propagation-invariant beams and optical vortices. These may be useful for future fibre-based implementations of super-resolution or light-sheet microscopy.

Light-sheet microscopy using an Airy beam

Light-sheet microscopy facilitates rapid, high-contrast, volumetric imaging with minimal sample exposure. However, the rapid divergence of a traditional Gaussian light sheet restricts the field of view (FOV) that provides innate subcellular resolution. We show that the Airy beam innately yields high contrast and resolution up to a tenfold larger FOV. In contrast to the Bessel beam, which also provides an increased FOV, the Airy beams characteristic asymmetric excitation pattern results in all fluorescence contributing positively to the contrast, enabling a step change for light-sheet microscopy.

Optical conveyor belt for delivery of submicron objects

We demonstrate an optical conveyor belt that provides trapping and subsequent precise delivery of several submicron particles over a distance of hundreds of micrometers. This tool is based on a standing wave (SW) created from two counter-propagating nondiffracting beams where the phase of one of the beams can be changed. Therefore, the whole structure of SW nodes and antinodes moves delivering confined micro-objects to specific regions in space. Based on the theoretical calculations, we confirm experimentally that certain sizes of polystyrene particles jump more easily between neighboring axial traps and the influence of the SW is much weaker for certain sizes of trapped object. Moreover, the measured ratios of longitudinal and lateral optical trap stiffnesses are generally an order of magnitude higher compared to the classical single beam optical trap.

Shaping the light transmission through a multimode optical fibre: complex transformation analysis and applications in biophotonics

We present a powerful approach towards full understanding of laser light propagation through multimode optical fibres and control of the light at the fibre output. Transmission of light within a multimode fibre introduces randomization of laser beam amplitude, phase and polarization. We discuss the importance of each of these factors and introduce an experimental geometry allowing full analysis of the light transmission through the multimode fibre and subsequent beam-shaping using a single spatial light modulator. We show that using this approach one can generate an arbitrary output optical field within the accessible field of view and range of spatial frequencies given by fibre core diameter and numerical aperture, respectively, that contains over 80% of the total available power. We also show that this technology has applications in biophotonics. As an example, we demonstrate the manipulation of colloidal microparticles.

Multiple optical trapping and binding: new routes to self - assembly

The impact of optical forces in the physical and biological sciences now enables the manipulation of objects ranging in size from a cell down to a single atom. The mechanical effects of optical fields have profound and far-reaching consequences, and attention is increasingly focused upon the opportunities for the non-contact assembly of particles into specific geometries. The present overview focuses on the two aspects of multi-particle trapping and optical binding. These can broadly be grouped as methods based on light-mediated inter-particle interactions, offering potential for the organization of large numbers of micro- or nano-particles using optical forces alone.

Tunable Bessel light modes: engineering the axial propagation

Due to their immunity to diffraction, Bessel light modes potentially offer advantages in various applications. However, they do exhibit significant intensity variations along their axial propagation length which hampers their applicability. In this paper we present a technique to generate Bessel beams with a tunable axial intensity within the accessible range of spatial frequencies. The beam may be engineered to have a constant intensity along its propagation length. Finally, we demonstrate how one can form a Bessel beam with a varying propagation constant along its axial extent which results in a tunable scaling of its lateral cross-section.

Static optical sorting in a laser interference field

We present a unique technique for optical sorting of heterogeneous suspensions of microparticles, which does not require the flow of the immersion medium. The method employs the size-dependent response of suspended dielectric particles to the optical field of three intersecting beams that form a fringelike interference pattern. We experimentally demonstrate sorting of a polydisperse suspension of polystyrene beads of diameters 1, 2, and 5.2μm and living yeast cells.

Experimental and theoretical determination of optical binding forces

We present an experimental and theoretical study of long distance optical binding effects acting upon micro-particles placed in a standing wave optical field. In particular we present for the first time quantitatively the binding forces between individual particles for varying inter-particle separations, polarizations and incident angles of the binding beam. Our quantitative experimental data and numerical simulations show that these effects are essentially enhanced due to the presence of a reflective surface in a sample chamber. They also reveal conditions to form stable optically bound clusters of two and three particles in this geometry. We also show that the inter-particle separation in the formed clusters can be controlled by altering the angle of the beam incident upon the sample plane. This demonstrates new perspectives for the generation and control of optically bound soft matter and may be useful to understand various inter-particle effects in the presence of reflective surfaces.

A dual beam photonic crystal fiber trap for microscopic particles

The dual beam counterpropagating optical trap has found increased use in studies such as optical stretching, optical binding, Raman spectroscopy, and the trapping of high index particles. In this letter we demonstrate the use of photonic crystal fiber to realize a long range dual beam trap that may support multiple wavelengths simultaneously. We develop a dual wavelength conveyor belt for trapped particles and realize the first ever dual beam white light (supercontinuum) trap. This low coherence light trap permits long range longitudinal optical binding of microparticles in the trap with no deleterious interference effects.

Fibre based cellular transfection

Optically assisted transfection is emerging as a powerful and versatile method for the delivery of foreign therapeutic agents to cells at will. In particular the use of ultrashort pulse lasers has proved an important route to transiently permeating the cell membrane through a multiphoton process. Though optical transfection has been gaining wider usage to date, all incarnations of this technique have employed free space light beams. In this paper we demonstrate the first system to use fibre delivery for the optical transfection of cells. We engineer a standard optical fibre to generate an axicon tip with an enhanced intensity of the remote output field that delivers ultrashort (~ 800 fs) pulses without requiring the fibre to be placed in very close proximity to the cell sample. A theoretical model is also developed in order to predict the light propagation from axicon tipped and bare fibres, in both air and water environments. The model proves to be in good agreement with the experimental findings and can be used to establish the optimum fibre parameters for successful cellular transfection. We readily obtain efficiencies of up to 57 % which are comparable with free space transfection. This advance paves the way for optical transfection of tissue samples and endoscopic embodiments of this technique.