Design of all-optical, hot-electron current-direction-switching device based on geometrical asymmetry

Plasmonic nanostar photocathodes for optically-controlled directional currents

Plasmonic nanocathodes offer unique opportunities for optically driving, switching, and steering femtosecond photocurrents in nanoelectronic devices and pulsed electron sources. However, angular photocurrent distributions in nanoplasmonic systems remain poorly understood and are therefore difficult to anticipate and control. Here, we provide a direct momentum-space characterization of multiphoton photoemission from plasmonic gold nanostars and demonstrate all-optical control over these currents. Versatile angular control is achieved by selectively exciting different tips on single nanostars via laser frequency or linear polarization, thereby rotating the tip-aligned directional photoemission as observed with angle-resolved 2D velocity mapping and 3D reconstruction. Classical plasmonic field simulations combined with quantum photoemission theory elucidate the role of surface-mediated nonlinear excitation for plasmonic field enhancements highly concentrated at the sharp tips (Rtip = 3.4 nm). We thus establish a simple mechanism for femtosecond spatiotemporal current control in designer nanosystems.

Plasmonic metaresonances: harnessing nonlocal effects for prospective biomedical applications

Metal nanoparticles (MNPs) possess optical concentration capabilities that can amplify and localize electromagnetic fields into nanometer length scales. The near-fields of MNPs can be used to tailor optical response of luminescent semiconductor quantum dots (QDs), resulting in fascinating optical phenomena. Plasmonic metaresonances (PMRs) form a class of such optical events gaining increasing popularity due to their promising prospects in sensing and switching applications. Unlike the basic excitonic and plasmonic resonances in MNP-QD nanohybrids, PMRs occur in the space/time domain. A nanohybrid experiences PMR when system parameters such as QD dipole moment, MNP-QD centre separation or submerging medium permittivity reach critical values, resulting in the plasmonically induced time delay of the effective Rabi frequency experienced by the QD asymptotically tending to infinity. Theoretical analyses of PMRs available in the literature utilize the local response approximation (LRA) which does not account for the nonlocal effects of the MNP, and neglect the MNP dependence of the QD decay and dephasing rates which hinder their applicability to QDs in the close vicinity of small MNPs. Here, we address these limitations using an approach based on the generalized nonlocal optical response (GNOR) theory which has proven to yield successful theoretical explanations of experimentally observed plasmonic phenomena. Our results indicate that, omission of the MNP nonlocal response and the associated decay/dephasing rate modifications of the QD tend to raise implications such as significant over-estimation of the QD dipole moment required to achieve PMR, under-estimation of the critical centre separation and prediction of significantly lower near-PMR QD absorption rates, in comparison to the improved GNOR based predictions. In light of our observations, we finally suggest two prospective applications of PMR based nanoswitches, namely, aptamer based in vitro cancer screening and thermoresponsive polymer based temperature sensing. To demonstrate the latter application, we develop and utilize a proof of concept (two dimensional) skin tumor model homogeneously populated by MNP-QD nanohybrids. Our simulations reveal a novel near-PMR physical phenomenon observable under perpendicular illumination, which we like to call the margin pattern reversal, where the spatial absorption pattern reverses when the near-PMR QDs switch from the bright to dark state.

Controlling resonance energy transfer in nanostructure emitters by positioning near a mirror

The ability to control light-matter interactions in quantum objects opens up many avenues for new applications. We look at this issue within a fully quantized framework using a fundamental theory to describe mirror-assisted resonance energy transfer (RET) in nanostructures. The process of RET communicates electronic excitation between suitably disposed donor and acceptor particles in close proximity, activated by the initial excitation of the donor. Here, we demonstrate that the energy transfer rate can be significantly controlled by careful positioning of the RET emitters near a mirror. The results deliver equations that elicit new insights into the associated modification of virtual photon behavior, based on the quantum nature of light. In particular, our results indicate that energy transfer efficiency in nanostructures can be explicitly expedited or suppressed by a suitably positioned neighboring mirror, depending on the relative spacing and the dimensionality of the nanostructure. Interestingly, the resonance energy transfer between emitters is observed to "switch off" abruptly under suitable conditions of the RET system. This allows one to quantitatively control RET systems in a new way.

Obvious modulation of rectifying performance by conjugation breaking of the bridging fragment in donor–bridge–acceptor molecular diodes

Large modulation of rectifying performance is obtained by breaking conjugation of the bridging fragment in donor–bridge–acceptor molecular diodes.

Theoretical analysis of hot electron injection from metallic nanotubes into a semiconductor interface

Hot electron injection from a metallic nanotube under optical illumination into a semiconductor layer in contact with its inner boundary.