On the Influence of Spatial Dispersion on the Performance of Graphene-Based Plasmonic Devices

We investigate the effect of spatial dispersion phenomenon on the performance of graphene-based plasmonic devices at THz. For this purpose, two different components, namely a phase shifter and a low-pass filter, are taken from the literature, implemented in different graphene-based host waveguides, and analyzed as a function of the surrounding media. In the analysis, graphene conductivity is modeled first using the Kubo formalism and then employing a full-$k_\rho$ model which accurately takes into account spatial dispersion. Our study demonstrates that spatial dispersion up-shifts the frequency response of the devices, limits their maximum tunable range, and degrades their frequency response. Importantly, the influence of this phenomenon significantly increases with higher permittivity values of the surrounding media, which is related to the large impact of spatial dispersion in very slow waves. These results confirm the necessity of accurately assessing non-local effects in the development of practical plasmonic THz devices.Comment: 5 pages, 18 figures, 2 table

Nonreciprocal Wavefront Engineering with Time-Modulated Gradient Metasurfaces

We propose a paradigm to realize nonreciprocal wavefront engineering using time-modulated gradient metasurfaces. The essential building block of these surfaces is a subwavelength unit cell whose reflection coefficient oscillates at low frequency. We demonstrate theoretically and experimentally that such modulation permits tailoring the phase and amplitude of any desired nonlinear harmonic and determines the behavior of all other emerging fields. By appropriately adjusting the phase delay applied to the modulation of each unit cell, we realize time-modulated gradient metasurfaces that provide efficient conversion between two desired frequencies and enable nonreciprocity by (i) imposing drastically different phase gradients during the up/down conversion processes and (ii) exploiting the interplay between the generation of certain nonlinear surface and propagative waves. To demonstrate the performance and broad reach of the proposed platform, we design and analyze metasurfaces able to implement various functionalities, including beam steering and focusing, while exhibiting strong and angle-insensitive nonreciprocal responses. Our findings open an alternative direction in the field of gradient metasurfaces, in which wavefront control and magnetic-free nonreciprocity are locally merged to manipulate the scattered fields

A Rat Model of Gambling Behavior and its Extinction: Effects of Win Probability on Choice in a Concurrent-Chains Procedure

Two experiments examined the effects of varying the probability of “wins” within a rat model of gambling. On a concurrent-chains procedure, rats could choose between a “work” lever on which a fixed 20 responses produced a food pellet or a “gamble” lever, where on some trials (“wins”) only one response was required for reinforcement while on other trials 40 responses were required. Despite the fact that the work lever was always associated with the higher overall reinforcement rate, rats frequently chose to respond on the gamble lever. The frequency with which rats chose the gamble lever varied as a function of win probability. Extinction of the gamble choice (i.e., gamble-lever choices no longer resulted in wins) resulted in consistent choice of the work lever. The behavioral baselines reported in the present study may prove useful for investigators interested in employing a rat model of gambling