The aim of this project is to study the optical forces induced when a beam of light falls on a patterned, metamaterial surface. The ultimate objective will be to understand how the forces depend on the metamaterial structure and materials properties, with a view to using this knowledge to design a new class of sensors and actuators. Computational electromagnetics combined with machine learning will be used to achieve this goal.
The radiation pressure of light was first postulated by Kepler in 1619, to explain why comet tails point away from the sun. In fact, Maxwell’s electromagnetic theory suggests that light may interact with matter in more complex ways, and this ultimately gave rise to the idea of optical tweezers, for which Arthur Ashkin was awarded a share of the 2018 Nobel prize for Physics. In optical tweezers, a highly focused laser beam exerts pico-Newton-sized forces on micron-sized colloidal particles. In general, the particle (of refractive index greater than the surrounding medium) is attracted towards the region of highest intensity in the laser beam: the so-called intensity gradient force. However, by breaking the symmetry of either the particle or the beam, the direction of the optical force can be controlled and optical torques can also be induced. This gives rise to a novel range of applications including optical microtools, optical spanners, optical crank-shafts, optical Archimedes screws, and so forth.
One interesting way to fabricate asymmetric structures on a micron-scale, without using colloidal particles, is through surface patterning e.g. by depositing a patterned gold film using lithography. This will be the subject of the current project. If the patterning is on a scale comparable to or less than the wavelength of light, the surface is generally referred to as a metamaterial. As indicated above, optical forces occur in many different guises, including lateral forces arising from shape asymmetry, polarisation dependant optical torques and spin-dependant lateral forces. Recent work has demonstrated the possibility of macroscopic forces arising from this latter influence , leading to potential applications in light-operated actuators. Furthermore, if the substrate is a thin film or membrane, any deformation or change in its optical properties can lead to a dramatic change in optical interaction. This effect may be exploited for a range of sensor applications.
This will be a computer-based project using a range of computational electromagnetic techniques, previously developed for optical tweezers, to calculate optical forces acting on metamaterial surfaces. Part of the aim of the project will be to optimise the metamaterial structure for particular applications such as sensors and actuators. Machine learning techniques will be used for this aspect of the project, in particular taking advantage of the open-source TensorFlow package.
How to Apply Please make an online application for this project at http://www.bris.ac.uk/pg-howtoapply. Please select Physics PhD on the Programme Choice page. You will be prompted to enter details of this specific project in the ‘Research Details’ section of the form.
Anticipated start date: September 2019
Candidate RequirementsA first degree in physics or a related subject, normally at a level equivalent to at least UK upper second-class honours, or a relevant postgraduate master's qualification.
See international equivalent qualifications on the International Office website.
Funding UK/EU: UK and EU students who meet the eligibility requirements will be considered for an EPSRC DTP studentship. Funding will cover UK/EU tuition fees, maintenance at the UKRI Doctoral Stipend rate (£14,777 per annum, 2018/19 rate) and a training support fee of £1,000 per annum for a period up to 3.5 years.
Eligibility includes, but is not limited to, being a UK or EU national who was resident in the UK for 3 years prior to the start of the project.
Funding overseas: Overseas students are also welcome to apply for a limited number of School of Physics studentships. These will be fully funded studentships to outstanding overseas candidates.
Self-funded: We welcome all-year-round applications from self-funded students and students seeking their own funding from external sources.
 H. Magallanes and E. Brasselet, Nature Photonics, 12, 461-464, 2018