Reconfigurable VO2 metasurfaces with hybrid electro-optical control: manipulating THz radiation with 0.3 W/cm2 light.

Dynamicallyprogrammable metasurfaces capable of manipulating terahertz (THz) wavefronts in various manners depending on external controls are highly desired for next-generation wireless communication systems and new tools for THz diagnostics. Such metasurfaces may utilize the insulator-to-metal transition in V O 2, which can be induced both electrically and optically. Optical control is especially convenient for individual addressing to each meta-atom, but it is hampered by the high optical switching threshold of V O 2. We experimentally realize V O 2-based THz metasurfaces with hybrid electro-optical control when the metasurface is brought close to the transition point by an almost-threshold current, and then is easily switched by unfocused continuous-wave light. We were able to control the metasurface THz transmission by 0.4W/c m 2 near-IR light, while purely optical switching required tightly focused light with an intensity of >3×105 W/c m 2. After correcting for the fact that a tightly focused spot dissipates heat easier, we estimate that the optical switching threshold reduction due to the electric current alone is ∼2 orders of magnitude. Finally, coating the metasurface with Au nanoparticles further reduced the threshold by 30% due to plasmonic effects.

Photothermal Effect and Phase Transition in VO2 Enhanced by Plasmonic Particles.

Phase change metasurfaces based on VO2, which are pre-heated with electric current and optically addressed by projected structured light hologram, are considered to become a new paradigm in programmed THz/middle IR flat optics. Macroscopic quasi-homogeneous arrays of Au nanoparticles show large near IR absorption and a significant photothermal effect capable of boosting a light-triggered switching of VO2 and are to be carefully examined. We propose a new approach to simultaneously probe the altered temperature and electric conductivity of a hybrid Au particle-VO2 film composite by monitoring a phase shift and attenuating a surface acoustic wave in a YX128° cut LiNbO3 substrate. The method shows a temperature resolution of 0.1 °C comparable with the best existing techniques for studying nanoobjects and surfaces. The laser-induced photothermal effects were characterized in a macroscopic array of Au nanostars (AuNSts) with different surface coverage. In a monolayer of 10 nm Au, coupled plasmonic nanoparticles were deposited on the LiNbO3 substrate. An optically triggered insulator-metal transition assisted by photothermal effect in AuNSts/VO2/TiO2/LiNbO3 composites was studied at varied light power. We believe that the proposed SAW-based method is of significant importance for the characterization and optimization of radiation absorbing or/and electrically heated elements of metasurfaces and other devices for lab-on-chip and optical communication/processor technology.

The effect of size, charge state and composition on the binding of propene to yttrium-doped gold clusters.

The catalytic activity of metal clusters can be easily tuned by their size, charge state, or the introduction of dopant atoms. Here, the dopant-, charge- and size-dependent propene adsorption on gold (Au n +) and yttrium doped gold (Au n-1Y+) clusters (n = 4-20) was investigated using combined gas-phase reaction studies and density functional theory computations. The increased charge transfer between the cluster and propene in the cationic clusters considerably enhances the propene binding on both pure and yttrium-doped species, compared to their neutral cluster counterparts, while yttrium-doping lowers the propene binding strength in a size-dependent way compared to the pure gold clusters. Chemical bonding and energy decomposition analysis indicate that there is no covalent bond between the cluster and propene. The preferred propene binding site on a cluster is indicated by the large lobes of its LUMO, together with the low coordination number of the adsorption site. In small yttrium-doped gold clusters propene can not only bind to the electron-deficient yttrium atom, but also to the partially positively-charged gold atoms. Therefore, by controlling the charge of the clusters, as well as by introducing yttrium dopants, the propene binding strength can be tuned, opening the route for new catalytic applications.

Controlling the Adsorption of Carbon Monoxide on Platinum Clusters by Dopant-Induced Electronic Structure Modification.

A major drawback of state-of-the-art proton exchange membrane fuel cells is the CO poisoning of platinum catalysts. It is known that CO poisoning is reduced if platinum alloys are used, but the underlying mechanism therefore is still under debate. We study the influence of dopant atoms on the CO adsorption on small platinum clusters using mass spectrometry experiments and density functional calculations. A significant reduction in the reactivity for Nb- and Mo-doped clusters is attributed to electron transfer from those highly coordinated dopants to the Pt atoms and the concomitant lower CO binding energies. On the other hand Sn and Ag dopants have a lower Pt coordination and have a limited effect on the CO adsorption. Analysis of the density of states demonstrates a correlation of dopant-induced changes in the electronic structure with the enhanced tolerance to CO poisoning.

Optical absorption spectra of palladium doped gold cluster cations.

Photoabsorption spectra of gas phase Au(n)(+) and Au(n-1)Pd(+) (13 ≤ n ≤ 20) clusters were measured using mass spectrometric recording of wavelength dependent Xe messenger atom photodetachment in the 1.9-3.4 eV photon energy range. Pure cationic gold clusters consisting of 15, 17, and 20 atoms have a higher integrated optical absorption cross section than the neighboring sizes. It is shown that the total optical absorption cross section increases with size and that palladium doping strongly reduces this cross section for all investigated sizes and in particular for n = 14-17 and 20. The largest reduction of optical absorption upon Pd doping is observed for n = 15.

Thermal radiation of laser heated niobium clusters Nb(+)(N), 8 ⩽ N ⩽ 22.

The thermal radiation from small, laser heated, positively charged niobium clusters has been measured. The emitted power was determined by the quenching effect on the metastable decay, employing two different experimental protocols. The radiative power decreases slightly with cluster size and shows no strong size-to-size variations. The magnitude is 40-50 keV/s at the timescale of several microseconds, which is the measured crossover time from evaporative to radiative cooling.