Nanodot couplers provide efficient near-field energy transfer

Nanometer-scale photonic integrated circuits are one way to create the optical devices required by future systems.¹ These integrated circuits consist of nanometer-scale dots. An optical near field, the thin film of light on the surface of a nanometric material, is used as the signal carrier. Since an optical near field is free from the diffraction of light due to its size-dependent localization and resonance features, nanophotonics enables the fabrication, operation, and integration of nanometric devices.

Driving such a device requires an external, conventional, diffraction-limited photonic mechanism with a far/near-field converter. A potential converter is a plasmon waveguide. A plasmon is a quasiparticle made of electron plasma and a photon. The waveguide employs a metallized silicon wedge structure to convert far-field light into an optical near field via a one-dimensional plasmon mode.² Compared with a metallic core waveguide, or wire, a chain of closely spaced metallic nanoparticles should reduce energy loss³ because of the lower metal content and plasmon resonance in the metallic nanoparticles. Energy transfer in the nanodot coupler relies on near-field coupling between the plasmon-polariton modes of neighboring particles.

We compared the spatial distribution of the optical near-field intensity in a linear nanodot coupler and a metallic core waveguide. To create the nanodot coupler, carbon hemispheres were deposited and coated with a 100-nm-thick gold film: see Figure 1(a). The same process was applied to carbon wire to make the metallic core waveguide: see Figure 1(c). In the nanodot coupler we found plasmon-polariton transfer as long as 4μm. Furthermore, the propagation loss was 10 times lower than that of the metallic core waveguide: see Figure 1(b), (d) and (g). Similar energy transfer was observed in a zigzag-shaped nanodot coupler with low energy loss at the corners: see Figure 1(f) and (h).⁴ Such high flexibility in the arrangement of nanoparticles is an outstanding advantage of optical far/near-field conversion for driving nanophotonic devices.

We fabricated nanodot couplers at all scales with a self-assembling method that builds nanodot chains by controlling desorption with an optical near field.^5,6 A nanodot chain of metallic nanoparticles was created using radio frequency sputtering under illumination on a glass substrate with a simple groove 100nm wide and 30nm deep. During deposition of the metal, linearly polarized light illuminating the groove (E₉₀) excites a strong optical near field at the edge of the groove—see Figure 2(b)—which induces desorption of the deposited metallic nanoparticles.⁵ A metallic dot has strong optical absorption due to plasmon resonance. This can induce desorption of a deposited metallic nanodot when it reaches the resonant diameter.⁵ As the deposition of metallic dots proceeds, growth is governed by a trade-off between deposition and desorption. This plus the photon energy of the incident light determines dot size. Consequently, the metallic nanoparticles should align along the groove.

Illumination with 2.33 and 2.62eV light during aluminum deposition caused the formation of chains of 99.6nm-diameter nanodots with 27.9nm separation, and chains of 84.2nm-diameter nanodots with 48.6nm separation, respectively: see Figure 2(d) and (e). This occurred along a 100-μm-long groove in a highly size- and position-controlled manner. The dot size decreased in proportion to the increase in photon energy: 99.6nm × (2.33/2.62) = 88.5 ∼ 84.2nm. This indicates that photon energy determines size and that the size-controlled dot-chain formation occurs due to photodesorption of the deposited metallic nanoparticles. The ratios of the center-center distance (d) and radius (a) of the nanodots—d/a = 2.56(2.33eV) and 3.15 (2.62eV)—are similar to the optimum value (2.4–3.0) for the efficient transmission of optical energy along a nanodot coupler as calculated using Mie's theory.⁷ Our results thus imply that d was set at the optimum distance for efficient energy transfer of the optical near field, since a field of that strength can induce desorption of the deposited metallic nanoparticles and result in position-controlled dot-chain formation.

In summary: Our deposition method is based on a photodesorption reaction. As such, illumination at multiple photon energies using a simple lithographically patterned substrate could enable the simultaneous fabrication of size- and position-controlled nanoscale structures with different particle sizes as well as with other metals or semiconductors. This combination of features has the potential to dramatically increase the production throughput of the nanoscale structures that future systems will require.