ABSTRACT
Graphene, despite its centrosymmetric structure, is predicted to have a substantial second order nonlinearity, arising from non-local effects. However, there is disagreement between several published theories and experimental data. Here we derive an expression for the second order conductivity of graphene in the non-local regime using perturbation theory, concentrating on the difference frequency mixing process, and compare our results with those already published. We find a second-order conductivity (σ(2) ≈ 10-17 AmV-2) that is approximately three orders of magnitude less than that estimated from recent experimental results. This indicates that nonlinear optical coupling to plasmons in graphene cannot be described perturbatively through the electronic nonlinearity, as previously thought. We also show that this discrepancy cannot be attributed to the bulk optical nonlinearity of the substrate. As a possible alternative, we present a simple theoretical model of how a non-linearity can arise from photothermal effects, which generates a field at least two orders of magnitude larger than that found from perturbation theory.
ABSTRACT
Recently, we demonstrated an all-optical coupling scheme for plasmons, which takes advantage of the intrinsic nonlinear optical response of graphene. Frequency mixing using free-space, visible light pulses generates surface plasmons in a planar graphene sample, where the phase matching condition can define both the wavevector and energy of surface waves and intraband transitions. Here, we also show that the plasmon generation process is strongly intensity-dependent, with resonance features washed out for absorbed pulse fluences greater than 0.1 J m-2 This implies a subtle interplay between the nonlinear generation process and sample heating. We discuss these effects in terms of a non-equilibrium charge distribution using a two-temperature model.This article is part of the themed issue 'New horizons for nanophotonics'.
ABSTRACT
BACKGROUND: The ingestion of foreign bodies is the second most frequent cause of aorto-esophageal fistulization after aneurysms of the thoracic aorta. CASE REPORT: A 28-year-old man died from catastrophic hematemesis 7 days after ingesting a bone splinter. The CT scan performed 3 days before the fatal event had demonstrated a fistulous formation originating in the aorta. DISCUSSION: Aorto-esophageal fistulae develop progressively from the esophageal perforation caused by the foreign body. Clinically, there is medical chest pain, followed by arterial hematemesis and finally terminal exsanguination. Diagnosis must be achieved during the free intervals in this triad of often rapidly succeeding signs.
