Exploiting multimode waveguides for pure fibre-based imaging

We acknowledge support from the UK Engineering and Physical Science Research CouncilThere has been an immense drive in modern microscopy towards miniaturisation and fibre based technology. This has been necessitated by the need to access hostile or diffcult environments in-situ and in-vivo. Strategies to date have included the use of specialist fibres and miniaturised scanning systems accompanied by ingenious microfabricated lenses. We present a novel approach for this field by utilising disordered light within a standard multimode optical fibre for lensless microscopy and optical mode conversion. We demonstrate the modalities of bright-field and dark-field imaging and scanning fluorescence microscopy at acquisition rates allowing observation of dynamic processes such as Brownian motion of mesoscopic particles. Furthermore, we show how such control can realise 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.Publisher PDFPeer reviewe

Optical manipulation : advances for biophotonics in the 21st century

We thank the UK Engineering and Physical Sciences Research Council for funding (Grant Nos. EP/P030017/1 and EP/R004854/1).Significance: Optical trapping is a technique capable of applying minute forces that has been applied to studies spanning single molecules up to microorganisms. AIM: The goal of this perspective is to highlight some of the main advances in the last decade in this field that are pertinent for a biomedical audience. Approach: First, the direct determination of forces in optical tweezers and the combination of optical and acoustic traps, which allows studies across different length scales, are discussed. Then, a review of the progress made in the direct trapping of both single-molecules, and even single-viruses, and single cells with optical forces is outlined. Lastly, future directions for this methodology in biophotonics are discussed. Results: In the 21st century, optical manipulation has expanded its unique capabilities, enabling not only a more detailed study of single molecules and single cells but also of more complex living systems, giving us further insights into important biological activities. Conclusions: Optical forces have played a large role in the biomedical landscape leading to exceptional new biological breakthroughs. The continuous advances in the world of optical trapping will certainly lead to further exploitation, including exciting in-vivo experiments.Publisher PDFPeer reviewe

Selective and optimal illumination of nano-photonic structures using optical eigenmodes

Using optical eigenmodes defined by the interaction between the electromagnetic fields and photonic structures it is possible to determine the optimal illumination of these structures with respect to a specific measurable quantity. One such quantity considered here is the electric field intensity in the hotspot regions of an array of nano-antennas. This paper presents two possible methods, both based on optical eigenmodes, to determine the optimal and most efficient illumination that couples to a single hotspot on top of a single nano-antenna taken from an array of nano-antennas. The two methods are compared in terms of cross-talk and overall coupling efficiency.Comment: Paper presented at the TaCoNa-Photonics meeting October 2011, Bad Honnef, German

Is there an optimal basis to maximise optical information transfer?

We establish the concept of the density of the optical degrees of freedom that may be applied to any photonics based system. As a key example of this versatile approach we explore information transfer using optical communication. We demonstrate both experimentally, theoretically and numerically that the use of a basis set with fields containing optical vortices does not increase the telecommunication capacity of an optical system.Publisher PDFPeer reviewe

Wavefront correction enables vibrational imaging of bacteria with multimode fibre probes

Raman spectroscopy is a valuable tool for non-invasive and label-free identification of sample chemical composition. Recently a few miniaturized optical probes emerged driven by the need to address areas of difficult access, such as in endoscopy. However, imaging modality is still out of reach for most of them. Separately, recent advances in wavefront shaping enabled different microscopies to be applied in various complex media including multimode fibers. Here we present the first and thinnest to date Raman fiber imaging probe based on wavefront shaping through a single multimode fiber without use of any additional optics. We image agglomerates of bacteria and pharmaceuticals to demonstrate the capability of our method. This work paves the way towards compact and flexible Raman endoscopy. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.Publisher PD

Raman imaging through a single multimode fibre

UK Engineering and Physical Sciences Research Council (EPSRC) (EP/J01771/X); European Union project FAMOS (FP7 ICT no. 317744); PreDiCT-TB consortium (IMI 115337); European Union’s Horizon 2020 Marie Sklodowska-Curie Actions (MSCA) (707084).Vibrational spectroscopy is a widespread, powerful method of recording the spectra of constituent molecules within a sample in a label-free manner. As an example, Raman spectroscopy has major applications in materials science, biomedical analysis and clinical studies. The need to access deep tissues and organs in vivo has triggered major advances in fibre Raman probes that are compatible with endoscopic settings. However, imaging in confined geometries still remains out of reach for the current state of art fibre Raman systems without compromising the compactness and flexibility. Here we demonstrate Raman spectroscopic imaging via complex correction in single multimode fibre without using any additional optics and filters in the probe design. Our approach retains the information content typical to traditional fibre bundle imaging, yet within an ultra-thin footprint of diameter 125 µm which is the thinnest Raman imaging probe realised to date. We are able to acquire Raman images, including for bacteria samples, with fields of view exceeding 200 µm in diameter.Publisher PDFPeer reviewe

Wide-field 3D optical imaging using temporal focusing for holographically trapped microparticles

Funding. Engineering and Physical Sciences Research Council (EPSRC) (EP/J01771X/1, EP/M000869/1).A contemporary challenge across the natural sciences is the simultaneous optical imaging or stimulation of small numbers of cells or colloidal particles organised into arbitrary geometries. We demonstrate the use of temporal focusing with holographic optical tweezers in order to achieve depth-resolved two-photon imaging of trapped objects arranged in arbitrary three dimensional geometries using a single objective. Trapping allows independent position control of multiple objects by holographic beam shaping. Temporal focusing of ultrashort pulses providesawide-field two-photon depth-selective activation of fluorescent samples. We demonstrate wide-field depth-resolved illumination of both trapped fluorescent beads and trapped HL60 cells in suspension with full 3D positioning control. These approaches are compatible with implementation through scattering media and can be beneficial for emergent studies in colloidal science and particularly optogenetics, offering targeted photoactivation over a wide area with µm depth control precision.PostprintPeer reviewe

Optical forces and torques on eccentric nanoscale core–shell particles

Funding: Q.S. and A.D.G. acknowledge support of the Australian Research Council Centre of Excellence for Nanoscale BioPhotonics (CNBP; Grant No. CE140100003), K.D. acknowledges the UK Engineering and Physical Sciences Research Council (Grant EP/P030017/1), Q.S. acknowledges the support of an Australian Research Council Discovery Early Career Researcher Award (Grant No. DE150100169), and A.D.G. acknowledges the support of an Australian Research Council Future Fellowship (Grant No. FT160100357). This research was undertaken with the assistance of resources from the National Computational Infrastructure (NCI Australia), an NCRIS enabled capability supported by the Australian Government (Grant No. LE160100051).The optical trapping and manipulation of small particles is an important tool for probing fluid properties at the microscale. In particular, microrheology exploits the manipulation and rotation of micron-scale particles to probe local viscosity, especially where these properties may be perturbed as a function of their local environment, for example in the vicinity of cells. To this end, birefringent particles are useful as they can be readily controlled using optically induced forces and torques, and thereby used to probe their local environment. However, the magnitude of optical torques that can be induced in birefringent particles is small, and a function of the particle diameter, meaning that rotational flow cannot readily be probed on length scales much small than the micron level. Here we show modeling that demonstrates that eccentric spherical core–shell nanoparticles can be used to generate considerable optical torques. The eccentricity is a result of the displacement of the center of the core from the shell. Our results show that, for particles ranging from 90 to 180 nm in diameter, we may achieve rotation rates exceeding 800 Hz. This fills a missing size gap in the rotation of microparticles with optical forces. The diameter of particle we may rotate is almost an order of magnitude smaller than the smallest birefringent particles that have been successfully rotated to date. The rotation of eccentric core–shell nanoparticles therefore makes an important contribution to biophotonics and creates new opportunities for rheology in nanoscale environments.PostprintPeer reviewe

Generation of Bessel-like beams with reduced sidelobes for enhanced light-sheet microscopy

The authors acknowledge financial support from the Ministry of Human Resource Development, New Delhi through the SPARC project. KD acknowledges support from the Australian Research Council and the European Union’s Horizon 2020 research and innovation programme under the H2020 FETOPEN project “Dynamic”.Bessel beams have found important applications due to their propagation invariant nature. However, the presence of sidelobes has proven a hindrance in key imaging and biophotonics applications. We describe the design and generation of sidelobe-suppressed Bessel-like beams (SSBB) that provide enhanced contrast for light-sheet imaging. The sidelobe suppression is achieved by the interference of two Bessel beams with slightly different wavevectors. Axicon phase functions for each Bessel beam are combined into a single phase function using the random multiplexing technique. This phase function is realised using a spatial light modulator to generate a SSBB. The generated beam at 633 nm has a 1/e2 radius of 44 µm and a propagation invariant distance of 39 mm which is more than four times that of the Rayleigh range of a Gaussian beam with the same 1/e2 radius. Within this distance, the overall peak intensity of the sidelobes of the SSBB is less than 10% that of the main lobe peak intensity. In addition, through numerical simulation for the recovery of spatial frequencies, we show that the SSBB improves image contrast compared to a Bessel beam for light-sheet imaging. We also show that the designed phase function can be realised using a meta-optical element.Publisher PDFPeer reviewe