Coherent optical interactions in graphene multilayers
Rao, Shraddha M. S.
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The work discussed in this thesis deals with the generation, control and modulation of optical interactions in two-dimensional materials, specifically in unpatterned, subwavelength graphene multilayers, using the process of Coherent Perfect Absorption (CPA). It aims to address the problem of inefficient light-matter coupling at the nanoscale by studying new geometries for enabling total absorption in 50% absorbing graphene films. Total optical absorption is demonstrated and a 80% modulation of the absorption and scattering is achieved by controlling the relative phases of the interacting optical beams. Degenerate four-wave mixing (DFWM) in graphene multilayers leads to the generation of optical phase conjugation and negative refraction. These nonlinear responses are generated with a conversion efficiency of 5 x 10-5, and using the CPA arrangement their amplitudes are modulated with a modulation contrast of 100%. It is shown that the two-dimensionality of graphene gives rise to a ‘phase-dependent’ nonlinearity, which differs significantly from that in bulk materials. The optical nonlinearity in graphene is seen to be controlled by the relative phases of the interacting optical fields in a manner such that the nonlinear polarisation itself can be switched on or off. The phase-dependent nonlinearity of the two-dimensional medium is then explored in three alternative geometries. The first one uses only two input beams, and a light-with-light modulation of the nonlinear signals is observed with a contrast of 90%. The second geometry involves a single beam interacting with the sample, wherein, nonlinear signals are generated in a self-pumping mode, due to reflection from a mirror placed very close to the graphene sample. The last configuration also uses a mirror in order to require only a single light beam and leads to the observation of a ‘negative reflection’ signal. Finally, a nonlinear imaging technique ‘phase-contrast imaging’ is performed using a traditional DFWM configuration with three input optical fields. A phase-object applied on one of the pump beams is transformed into an intensity object in the resulting negative refraction. A few basic phase objects are imaged on the negatively refracted beam and are reported in this work, offering a possible application for the advantages offered by two-dimensional optical nonlinearities.