Coherent optical interactions in graphene multilayers

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

Coherent control of light interaction with graphene

We report the experimental observation of all-optical modulation of light in a graphene film. The graphene film is scanned across a standing wave formed by two counter-propagating laser beams in a Sagnac interferometer. Through a coherent absorption process the on-axis transmission is modulated with close to 80% efficiency. Furthermore we observe modulation of the scattered energy by mapping the off-axis scattered optical signal: scattering is minimized at a node of the standing wave pattern and maximized at an antinode. The results highlight the possibility to switch and modulate any given optical interaction with deeply sub-wavelength films.Comment: 4 pages, 4 figure

Photoluminescence Mapping over Laser Pulse Fluence and Repetition Rate as a Fingerprint of Charge and Defect Dynamics in Perovskites

Defects in metal halide perovskites (MHP) are photosensitive, making the observer effect unavoidable when laser spectroscopy methods are applied. Photoluminescence (PL) bleaching and enhancement under light soaking and recovery in dark are examples of the transient phenomena that are consequent to the creation and healing of defects. Depending on the initial sample composition, environment, and other factors, the defect nature and evolution can strongly vary, making spectroscopic data analysis prone to misinterpretations. Herein, the use of an automatically acquired dependence of PL quantum yield (PLQY) on the laser pulse repetition rate and pulse fluence as a unique fingerprint of both charge carrier dynamics and defect evolution is demonstrated. A simple visual comparison of such fingerprints allows for assessment of similarities and differences between MHP samples. The study illustrates this by examining methylammonium lead triiodide (MAPbI3) films with altered stoichiometry that just after preparation showed very pronounced defect dynamics at time scale from milliseconds to seconds, clearly distorting the PLQY fingerprint. Upon weeks of storage, the sample fingerprints evolve toward the standard stoichiometric MAPbI3 in terms of both charge carrier dynamics and defect stability. Automatic PLQY mapping can be used as a universal method for assessment of perovskite sample quality

Geometries for the coherent control of four-wave mixing in graphene multilayers OPEN

Deeply sub-wavelength two-dimensional films may exhibit extraordinarily strong nonlinear effects. Here we show that 2D films exhibit the remarkable property of a phase-controllable nonlinearity, i.e., the amplitude of the nonlinear polarisation wave in the medium can be controlled via the pump beam phase and determines whether a probe beam will "feel" or not the nonlinearity. This is in stark contrast to bulk nonlinearities where propagation in the medium averages out any such phase dependence. We perform a series of experiments in multilayer graphene that highlight some of the consequences of the optical nonlinearity phase-dependence, such as the coherent control of nonlinearly diffracted beams, single-pump-beam induced phase-conjugation and the demonstration of a nonlinear mirror characterised by negative reflection. The observed phase sensitivity is not specific to graphene but rather is solely a result of the dimensionality and is therefore expected in all 2D materials. Two-dimensional optical media with deeply sub-wavelength or mono-atomic thickness such as photonic metamaterials, plasmonic, heterostructure layered materials, and structured or few-layer graphene provide the advantage of being able to control (enhance, suppress or modulate) optical interactions. Their functionality in the linear regime is well established in terms of cloaking 1 , ultrafast modulators 2 and optical magnetis

Geometries for the coherent control of four-wave mixing in graphene multilayers

Deeply sub-wavelength two-dimensional films may exhibit extraordinarily strong nonlinear effects. Here we show that 2D films exhibit the remarkable property of a phase-controllable nonlinearity, i.e., the amplitude of the nonlinear polarisation wave in the medium can be controlled via the pump beam phase and determines whether a probe beam will "feel" or not the nonlinearity. This is in stark contrast to bulk nonlinearities where propagation in the medium averages out any such phase dependence. We perform a series of experiments in multilayer graphene that highlight some of the consequences of the optical nonlinearity phase-dependence, such as the coherent control of nonlinearly diffracted beams, single-pump-beam induced phase-conjugation and the demonstration of a nonlinear mirror characterised by negative reflection. The observed phase sensitivity is not specific to graphene but rather is solely a result of the dimensionality and is therefore expected in all 2D materials

Policy Paper 01/2018

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Climate Change Impacts, Adaptation and Policy Preferences: A Snapshot of Farmers’ Perceptions in India

Not AvailableClimate change is now considered as a major issue by the researchers, governments, civil society organizations, farmers and general public. It is a focal point of discussions at community to global levels. With respect to agriculture, there is a need to evolve a strategy containing technologies, policies, investments and interventions that help communities and governments prepare for the adverse impacts of climate change in terms of adaptation on one hand and those measures that will help mitigation by slowing down the emission of GHGs on the other. It is desirable that any such strategy considers farmers’ beliefs, perceptions, responses, policy preferences, etc. Since Indian agriculture is diverse and climate change is complex, the nature of problem and the farmers’ responses will be diverse and heterogeneous. Understanding what farmers think with respect to climate change is also essential for mainstreaming adaptation into development planning which is considered as key to enhancing resilience. Thus, there is a need for formulation of policies that are regionally differentiated and acceptable to farmers. Therefore, an effort was made to understand farmers’ perceptions as part of the ICAR-led scheme ‘National Initiative (now Innovations) on Climate Resilient Agriculture’ (NICRA) launched during 2011.Not Availabl

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