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Start of funding 01.01.2015

Broadband grating couplers for lightwave nanoelectronic circuitry

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Electronics systems are currently limited to the multi-gigahertz range by fundamental obstacles in their architecture, which are predicted to lead, in the medium to long range, to substantial deviations from Moore’s law. Within the collaboration between the Physics Department of the Ludwig-Maximilians-Universität (LMU) in Garching and the Lawrence Berkeley National Laboratories‘ Molecular Foundary (MF) in Berkeley, we plan to introduce a paradigm shift in the design of nanoelectronic devices, where tailored light fields will allow active control on electron currents in nanocircuits. As a key technology in ultrafast plasmonic nanocircuitry the proposed project aims at producing and investigating novel broadband grating couplers. In design studies, all grating couplers will be theoretically modeled and hereafter produced using state-of-the-art nanofabrication technologies at the ML facilities. Consecutively, at LMU, the grating couplers are investigated by means of attosecond-resolved laser spectroscopy.

Final report:
With decreasing feature size, the architecture of conventional, silicone-based electronic circuitry will reach fundamental physical limits, which confine the maximum achievable clock rates. A possible alternative, which moves gradually into the focus of fundamental and applied research, is described by the term of lightwave nanoelectronic circuitry, which would in principle support significant higher clock rates. Here, the transport of information is not realized by the transport of charges but by surface plasmon polaritons, which can exist as collective interaction of valence electrons at a metal-dielectric interface. However, the success of such a paradigm shift depends sensitively on the availability of suitable components for the generation, transport and processing of the respective signals in a plasmon-based circuitry. Within this project, we investigated the possibility of manufacturing broadband grating couplers for the in- and outcoupling of surface plasmon polaritons, where special attention is drawn on the usability of ultrafast switching rates.

During the first part of the project, samples were manufactured in the Molecular Foundry (MF) at the Lawrence Berkeley National Laboratory (LBNL). Here, gold nanowires were used as a basis on which an even surface was prepared as cross section via focused ion beam milling (FIB). To support a dissipation-free plasmon propagation, the inherent surface roughness was reduced in consecutive polishing steps, where small dose rates were used during the FIB process. The last step comprised the definition of the grating structure on the prepared surface, where grating constant, depth and width of grating grooves and its lateral dimensions were varied. To achieve signal transport at high clock rates, the grating geometry must support the in- and outcoupling of ultrashort pulses: for the application of few-cycle laser pulses, centered in the near-infrared regime, the supported spectral width should range from 500 nm to more than 1100 nm. Besides fan-shaped gratings, where the supported spectral range varies spatially across the grating geometry, chirped gratings with increasing and decreasing grating constant could be realized.

The second part included the characterization of different grating geometries in numerical simulations. Here, we employed the finite-difference time domain (FDTD) method, which was realized by a commercially available software package. The results demonstrated that components fabricated from gold can be used in plasmonic circuitry centered in the near-infrared spectral regime and allows fast propagation velocities. While the attenuation for gold is larger as compared to different materials like silver, gold offers big advantages regarding the chemical stability and in-situ integration of more complex geometries, which can be grown using established techniques. Beyond this, the simulations validated that the designed grating geometries are capable to support a large spectral range and thereby allow the in- and outcoupling of ultrashort few-cycle femtosecond pulses.

In the final step, the incoupled plasmonic excitations should be investigated by experimental means. Therefore, the attosecond streaking technique was considered, which allows sufficient temporal resolution to achieve sub-cycle characterization of plasmons. Despite the vast recent achievements, the application of the attosecond streaking technique on nanostructured targets is still in its infancy, which is why within the funding period of the project this last step remained elusive.