Alù and Engheta (Phys. Rev. Lett. 104, 213902, 2010) proposed chip-scale optical interconnects built as wireless links between impedance-matched nanoantenna pairs, each antenna fed by a plasmonic transmission line and loaded in its nanoscale gap by a lumped dielectric nanodisk (an 'optical nanocircuit' element), and showed theoretically that such links can strongly outperform plasmonic waveguide interconnects over micrometre separations. They left open the systematic characterization needed to turn this concept into a practical interconnect technology: the operation bandwidth and its trade-offs with the matching/loading design, strategies for multiplexing and separating many simultaneously active free-propagating signals in a dense on-chip environment, the trade-off between the reduced field confinement of the wireless link and propagation loss, robustness to polarization misalignment, multipath and fabrication tolerances, and integration of the nanocircuit loads with other photonic and electronic circuitry. Later work (2012-2023) has experimentally realized individual links, demonstrated broadband and wavelength-multiplexed link designs, and integrated such antennas with silicon photonics, but a quantitative, experimentally validated system-level theory of multi-channel on-chip optical wireless networks - including crosstalk control among coexisting transmitter-receiver pairs and a general bandwidth-matching trade-off theory - remains open.

Background

Modern chip-scale data processing relies on optical interconnects, which today are implemented as wired waveguides. At the nanoscale, metal-based ('plasmonic') waveguides - structures guiding light as coupled electromagnetic charge-density waves (surface plasmons) confined below the diffraction limit - suffer strong ohmic absorption, so guided signals decay exponentially over micrometre distances. Alù and Engheta (2010) proposed an alternative modeled on radio practice: a wireless, broadcast-style link between two nanoantennas. An optical nanoantenna is a resonant metallic (or dielectric) nanostructure, typically tens to a couple of hundred nanometres in size, that converts localized (guided or near-field) optical energy into freely propagating radiation and vice versa. A central tool of their proposal is 'optical nanocircuits' (metatronics): the idea, developed in earlier work by the same group, that suitably shaped nanoparticles behave at optical frequencies as lumped circuit elements - e.g. a subwavelength dielectric nanodisk filling the gap between the dipole arms acts as a lumped capacitive load, with gap capacitance Cload≈επa2/gC_{\mathrm{load}}\approx\varepsilon\pi a^{2}/g for arm radius aa and gap thickness gg. Because a plasmonic nanostripline feeding the antenna has a high, dispersive characteristic impedance (hundreds of ohms) while an unloaded nanodipole has a sharply peaked input resistance at its open-circuit resonance, a dielectric nanodisk of permittivity ε=5ε0\varepsilon=5\varepsilon_{0} filling the 9 nm gap of a 120 nm long silver nanodipole shifts the antenna input impedance so that its real part matches the stripline and its reactance vanishes at f0=415f_{0}=415 THz - the optical analogue of antenna-feed impedance matching. With matched transmit and receive antennas, the received power then follows the free-space Friis law (scaling as 1/d21/d^{2} with separation dd), which over a few micrometres beats the exponential decay of a plasmonic stripline whose simulated attenuation constant is α=0.184 μm−1\alpha=0.184~\mu\mathrm{m}^{-1}. The authors also noted the costs of the wireless paradigm: weaker field confinement than a plasmonic guide, radiation loss, mixing between signals of different transmitter-receiver pairs, and unquantified sensitivity to polarization misalignment, multipath interference and fabrication nonidealities. Follow-up literature has grown into the subfield of on-chip optical wireless interconnects, also called Optical Wireless Networks-on-Chip (OWNoC), reviewed in Abadal, Han and Jornet's survey of chip-scale wave propagation and channel modeling from millimetre-wave to optics, and in reviews of optical nanoantennas such as Biagioni, Huang and Hecht. Relevant vocabulary from that literature: wavelength-division multiplexing (WDM, carrying independent data streams on separate wavelengths), orbital angular momentum (OAM) multiplexing (encoding channels in the helical phase structure of beams), full-duplex links (simultaneous transmission and reception), and scanning near-field optical microscopy (SNOM, imaging fields with a subwavelength probe).

Problem Statement

For optical wireless interconnects implemented as impedance-matched nanoantenna pairs of the type introduced by Alù and Engheta (2010) - each antenna fed by a plasmonic transmission line and loaded in its nanoscale gap by a lumped optical nanocircuit element - establish the quantitative system-level characterization required for them to serve as a practical on-chip interconnect technology. The single overarching objective is to determine the achievable performance envelope of matched-nanoantenna optical wireless links and the design rules that reach it; the jointly required results are: (1) the operation bandwidth such links can support, its limits, and its trade-offs against the impedance-matching/loading design; (2) systematic strategies for multiplexing and separating multiple simultaneously active free-propagating signals in a dense on-chip environment, including crosstalk between different transmitter-receiver pairs; (3) the trade-off between the reduced field confinement of the wireless link, relative to plasmonic waveguides, and propagation loss, together with its implications for applications requiring near-field enhancement; (4) robustness of the matched link to realistic nonidealities - polarization misalignment, multipath interference, and material/geometry fabrication tolerances beyond idealized examples; and (5) practical integration of the lumped nanocircuit loads (e.g., dielectric nanodisks of prescribed permittivity) or functionally equivalent matching structures, their on-chip placement, and coexistence with other photonic and electronic circuitry. The characterization is sought for the canonical impedance-matched nanoantenna link of Alù and Engheta (2010) and, wherever a result concerns the simultaneous operation of several links, for a representative multi-link on-chip arrangement; the conditions enumerated within a result hold jointly: result (4), for example, requires robustness to polarization misalignment, multipath interference, and material/geometry fabrication tolerances together, with the multipath condition evaluated in the representative multi-link arrangement.

References

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