Paper published in Nanoscale

Our work on trapping 2D materials has been published as M. G. Donato, E. Messina, A. Foti, T. Smart, P. H. Jones, M. A. Iatì, R. Saija, P. G. Gucciardi & O. M. Maragò.  'Optical trapping and optical force positioning of two dimensional materials', Nanoscale 10 1245-1255 (2018).

From the abstract: In recent years, considerable effort has been devoted to the synthesis and characterization of two-dimensional materials. Liquid phase exfoliation (LPE) represents a simple, large-scale method to exfoliate layered materials down to mono- and few-layer flakes. In this context, the contactless trapping, characterization, and manipulation of individual nanosheets hold perspectives for increased accuracy in flake metrology and the assembly of novel functional materials. Here, we use optical forces for high-resolution structural characterization and precise mechanical positioning of nanosheets of hexagonal boron nitride, molybdenum disulfide, and tungsten disulfide obtained by LPE. Weakly optically absorbing nanosheets of boron nitride are trapped in optical tweezers. The analysis of the thermal fluctuations allows a direct measurement of optical forces and the mean flake size in a liquid environment. Measured optical trapping constants are compared with T-matrix light scattering calculations to show a quadratic size scaling for small size, as expected for a bidimensional system. In contrast, strongly absorbing nanosheets of molybdenum disulfide and tungsten disulfide are not stably trapped due to the dominance of radiation pressure over the optical trapping force. Thus, optical forces are used to pattern a substrate by selectively depositing nanosheets in short times (minutes) and without any preparation of the surface. This study will be useful for improving ink-jet printing and for a better engineering of optoelectronic devices based on two-dimensional materials.

Paper published in Optics Communications

Our work on the focal volume structure of spatially inhomogeneous polarization beams focused by a novel 'fractal' axicon has been published as Zhirong Liu, Kelin Huang, Xun Wang & P. H. Jones 'Tight focusing of radially polarized beams modulated by a fractal conical lens' Opt. Commun. 402 231-237 (2017).

From the abstract: A novel high numerical aperture (NA) focusing system with a fractal conical lens (FCL) is proposed, and tight focusing of radially polarized beams through the proposed optical system is investigated theoretically and numerically. The influence of several relevant factors, including the FCL’s stage $S$, objective lens’ NA, and truncation parameter ${\beta }_0$, on the targeted beam’s focusing characteristics in the focal region is discussed in detail. It is found that, when a FCL with S≥0$\mathrm{}$ is employed, position of the major focal point would shift from the geometric focal point, and the focused intensity distributions cannot maintain symmetrical about the focus any more, although they present different profiles for various truncation parameters ${\beta }_0$. When $S\ge 2$, multiple focal points can be generated, i.e., a single major focus and a series of subsidiary foci surrounding it along the optical axis, which form a focal region. These unique focusing characteristics with a FCL are remarkably different from that of without a FCL. The fascinating findings here may be taken advantage of when using radially polarized beams in exploiting new-type optical tweezers and making use of a FCL.

Paper published in Nano Letters

Our theoretical study of optical binding effects between nonspherical particles has been published as S. H. Simpson, P. Zemánek, O. M. Maragò, P. H. Jones & S. Hanna.  'Optical binding of nanowires', Nano Letters 17 3485-3492 (2017).

 From the abstract:  Multiple scattering of light induces structured interactions, or optical binding forces, between collections of small particles. This has been extensively studied in the case of microspheres. However, binding forces are strongly shape dependent: here, we turn our attention to dielectric nanowires.  Using a novel numerical model we uncover rich behavior. The extreme geometry of the nanowires produces a sequence of stationary and dynamic states. In linearly polarized light, thermally stable ladder-like structures emerge. Lower symmetry, sagittate arrangements can also arise, whose configurational asymmetry unbalances the optical forces leading to nonconservative, translational motion. Finally, the addition of circular polarization drives a variety of coordinated rotational states whose dynamics expose fundamental properties of optical spin. These results suggest that optical binding can provide an increased level of control over the positions and motions of nanoparticles, opening new possibilities for driven self-organization and heralding a new field of self-assembling optically driven micromachines.

Paper published in Journal of the Acoustical Society of America

Our work on acoustic and optical trapping of microbubbles has been published as G. Memoli, C. R. Fury, P. N. Gélat, K. O. Baxter & P. H. Jones.  'Acoustic force measurements on polymer-coated microbubbles in a microfluidic device', Journal of the Acoustical Society of America 141 3346 (2017).

From the abstract:  This work presents an acoustofluidic device for manipulating coated microbubbles, designed for the simultaneous use of optical and acoustical tweezers. A comprehensive characterization of the acoustic pressure in the device is presented, obtained by the synergic use of different techniques in the range of acoustic frequencies where visual observations showed aggregation of microbubbles. In absence of bubbles, the combined use of laser vibrometry and finite element modelling supported a non-invasive measurement of the acoustic pressure and an  enhanced understanding of the system resonances. Calibrated holographic optical tweezers were then used for a direct measurement of the acoustic forces acting on an isolated microbubble at low driving pressures and to confirm the spatial distribution of the acoustic field. This allowed quantitative pressure measurements by particle tracking using polystyrene beads and an evaluation of the related uncertainties. The extension of the tracking technique to polymer-coated microbubbles allowed acoustic force measurements at higher pressures, highlighting four peaks in the acoustic response of the device. Results and methodologies are relevant to acoustofluidic applications requiring a precise characterization of the acoustic field and, in general, to biomedical applications with microbubbles or deformable particles.

Paper published in IEEE Photonics Journal

Our work on a novel optical trapping system using fractal optics and spatially inhomogneous polarisation of light has been published as Zhirong Liu and P. H. Jones, 'Fractal conical lens optical tweezers', IEEE Photonics Journal 9 6500111 (2017)

From the abstract: We propose a novel optical tweezers composed of an annular beam with alternate radially and azimuthally polarized rings modulated by a fractal conical lens (FCL) and demonstrate its optical forces on Rayleigh dielectric particles both analytically and numerically. Owing to the optical system's particular focusing properties, which could generate a dark-centered or peak-centered intensity distribution in the focal region when selecting an appropriate truncation parameter in front of the focusing lens, the proposed FCL optical tweezers could selectively trap and manipulate dielectric mesoscopic particles with low- or high-refractive indices by appropriately adjusting the radius of the pupil or the beam. Finally, the stability conditions for effective trapping and manipulation Rayleigh particles are analyzed.

Paper published in JOSA A

Our paper on the optical trapping forces of a poarization-structured beam focused by a fractal (Devil's) lens has been published as Zhirong Liu and P. H. Jones, 'Radiation forces acting upon a Rayleigh particle by highly focused alternate radially- and azimuthally-polarized beams modulated by a Devil's Lens', Journal of the Optical Society of America A 33 2501-2508 (2016).

From the abstract:  We propose and demonstrate a novel high numerical aperture (NA) focusing system composed of an annular beam with alternate radially and azimuthally polarized rings, focused by a devil’s lens (DL), and further investigate its radiation forces acting upon a Rayleigh particle both analytically and numerically. Strongly focused cylindrical vector beams produce either dark-centered or peak-centered intensity distributions depending on the state of polarization, whereas the DL produces a series of foci along the propagation direction. We exploit these focusing properties and show that by selecting an appropriate truncation parameter in front of the focusing lens, the proposed optical focusing system can selectively trap and manipulate dielectric micro-particles with low or high refractive indices by simply adjusting the radius of the pupil or the beam. Finally, the stability conditions for effectively trapping and manipulating Rayleigh particles are analyzed. The results obtained in this work are of interest in possible applications in optical confinement and manipulation, sorting micro-particles, and making use of a DL.

Paper published in Optics Letters

Our paper on optical binding in two-dimensions has been published as Xiang Han, Hui Luo, Guangzong Xiao and P. H. Jones.  'Optically bound colloidal lattices in evanescent optical fields', Optics Letters 41 4935 (2016).

From the abstract: In this Letter, we demonstrate the formation of a stable two-dimensional lattice of colloidal particles in the interference pattern formed by four evanescent optical fields at a dielectric interface. The microspheres are observed to form a two-dimensional square lattice with lattice vectors inclined relative to the beam propagation directions. We use digital video microscopy and particle tracking to measure the Brownian motion of particles bound in the lattice, and use this to characterize fluctuations in the local ordering of particles using the bond orientational order parameter, the probability distribution of which is shown to be a chi-squared distribution. An explanation for the form of this distribution is presented in terms of fluctuations of the modes of a ring of particles connected by springs.