He-Ne laser and speckle
He-Ne laser and speckle
Gibbous moon
Gibbous moon
Water droplet
Water droplet
Electron tree
Fractal electron tree or Lichtenberg figure
Oscilloscope trace
Oscilloscope trace
Soap bubble colour fringes
Soap bubble colour fringes

About us

The Optical Theory Group is part of the Photonics Division within the Physics Department at Imperial College London. The group is lead by Dr. Matthew R. Foreman who currently holds a Royal Society University Research Fellowship.

Research

More information on some of our past and present projects can be found by visiting our Research pages.

Recent publications

J. Berk and M. R. Foreman, "Role of Multiple Scattering in Single Particle Perturbations in Absorbing Random Media" arXiv , 2105.02800 (2006).

Abstract : Speckle patterns produced by disordered scattering systems exhibit a sensitivity to addition of individual particles which can be used for sensing applications. Using a coupled dipole model we investigate how multiple scattering can enhance field perturbations arising in such random scattering based sensors. Three distinct families of multiple scattering paths are shown to contribute and the corresponding complex enhancement factors derived. Probability distributions of individual enhancement factors over the complex plane are characterised numerically within the context of surface plasmon polariton scattering in which absorption is shown to play an important role. We show that enhancements become more strongly dependent on individual scatterer properties when absorption losses are larger, however, amplitude enhancements ~10^2, comparable to low loss surface plasmons, are achievable through sensor optimisation. Approximate analytic expressions for the complex mean enhancements are also found, which agree well with simulations when loop contributions are negligible.

J. Berk and M. R. Foreman, "Multiple Scattering Enhanced Single Particle Sensing" arXiv , 2105.02798 (2006).

Abstract : Methods to increase the light scattered from small particles can help improve the sensitivity of many sensing techniques. Here, we investigate the role multiple scattering plays in perturbing the scattered signal when a particle is added to a random scattering environment. Three enhancement factors, parametrising the effect of different classes of multiple scattering trajectories on the field perturbation, are introduced and their mean amplitudes explored numerically in the context of surface plasmon polariton scattering. We demonstrate that there exists an optimum scatterer density at which the sensitivity enhancement is maximised, with factors on the order of 10^2 achievable. Dependence of the enhancement factors on scatterer properties are also studied.

F. Azeem, L. S. Trainor, P. A. Devane, D. S. Norman, A. Rueda, N. J. Lambert, M. Kumari, M. R. Foreman, H. G. L. Schwefel, "Dielectric perturbations: anomalous resonance frequency shifts in optical resonators" Opt. Lett. 46, 2477-2480 (2006).

Abstract : Small perturbations in the dielectric environment around a high quality whispering gallery mode resonator usually lead to a frequency shift of the resonator modes directly proportional to the polarizability of the perturbation. Here, we report experimental observations of strong frequency shifts that can be opposite and even exceed the contribution of the perturbations polarizability. The mode frequencies of a lithium niobate whispering gallery mode resonator are shifted using substrates of refractive indices ranging from 1.50 to 4.22. Both blue- and red-shifts are observed, as well as an increase in mode linewidth, when substrates are moved into the evanescent field of the whispering gallery mode. We compare the experimental results to a theoretical model by Foreman et al. and provide an additional intuitive explanation based on the Goos-Hänchen shift for the optical domain.