Plasmonic waveguides based on synthetic nanomembranes

Structuring metal dielectrics with nanometer-scale precision enables electromagnetic-field concentration and control at the subwavelength level. Use of surface-plasmon polaritons (SPPs), evanescent waves propagating at a conductor-dielectric interface, has ushered in the field of plasmonics.^1,2 Plasmonic waveguides and circuits will be critical to a new generation of devices with the compact dimensions of integrated electronics and the speed of photonics.³ Existing applications of plasmonics include ultrasensitive chemical and biological sensors,⁴ plasmon nanoguides and circuits,⁵ and photovoltaic cells.⁶ Photonics is vastly extended by plasmonic nanostructures, including subwavelength plasmonic crystals⁷ and plasmonic metamaterials.⁸

The primary limitations of plasmonic materials are heavy absorption losses, large frequency dispersions, and a limited choice of materials.⁹ Longer propagation paths and larger design freedom could be ensured through use of synthetic free-standing nanomembranes, quasi-2D structures with extremely large aspect ratios.¹⁰ Such structures are intrinsically symmetric in an electromagnetic sense, ensure much lower losses, and therefore support long-range SPP propagation^11–13 (see Figure 1). In contrast to conventional SPP guides with bulk substrates, here SPPs propagate along a quasi-2D guide. A nanomembrane is less than 100nm thick, with lateral dimensions often larger than 1cm. In spite of their enormous aspect ratios, nanomembranes are quite robust and even allow manipulation with free hands.^14,15 They are probably the only element from the nanotechnology toolbox that can be seen with an unaided eye and manipulated without special equipment.

Figure 1. Long-range surface-plasmon-polariton (SPP) propagation on a self-supported nanomembrane. ε₁, ε₂: Dielectric functions. a.u.: Arbitrary units.

In our research, we choose to produce plasmonic nanomembrane guides through multifunctionalization.¹⁶ A biological nanomembrane without functionalization would be a simple inanimate object. Synthetic-nanomembrane nanofunctionalization vastly expands their applicability. Primary approaches to membrane multifunctionalization include using nanofillers, lamination (multilayering), additive and subtractive patterning, and surface sculpting (see Figure 2).

Figure 2. Nanomembrane multifunctionalization. (a) Nanofillers, (b) lamination, (c) additive (top) and subtractive (bottom) patterning, and (d) surface sculpting.

One may incorporate nanofillers into nanomembranes. Examples include plasmonic metal-nanoparticle arrays,¹⁷ which themselves may serve as plasmonic guides, but also other components such as carbon nanotubes, which also mechanically reinforce nanomembranes. Another approach is to laminate a larger number of strata, thereby obtaining multilayer plasmonic crystals.¹⁸ Methods for this approach include the layer-by-layer technique,¹⁹ Langmuir-Blodgett deposition, and dip-and-drop coating.¹⁶ Patterning by rendering protrusions or apertures can be effected, for example, by directed energy or particle beams.²⁰ This enables fabrication of complex structures, such as fishnet-type negative-refractive-index metamaterials. Nanomembrane waveguides can also be sculpted to form various 3D shapes^21,22 (see Figure 3). Obviously, two or more of these may be combined.

Figure 3. Artificial ion channels (red cylinders) built into a three-layer nanomembrane.

One of our research directions is lamination of nanomembranes with layers of transparent conductive oxides.²³ These materials are seen as a viable low-loss alternative to metals in plasmonics.⁹ We used indium oxide, indium-doped tin oxide, and aluminum-doped zinc oxide nanoparticles synthesized from nonaqueous solution and deposited them using dip-or-drop coating.²³

Another research direction is aimed at plasmon-waveguide chemical-sensor selectivity enhancement. The idea is to laminate the plasmon guide with another nanomembrane, incorporating artificial nanopores.²⁴ Use of synthetic gated-ion channels enables selective and switchable transport of analytes to the guide surface²⁵ and opens a pathway toward biomimetic enhancement of plasmonic structures.

In combination, artificial free-standing nanomembranes and plasmonics have opened up new research directions, further expanded by multifunctionalization. Nanomembranes can be used to fabricate stretchable and foldable plasmonic waveguides, circuits, and devices that are transferable to various substrates, including those that are curvilinear. They can be patterned and stacked to form 3D photonic and plasmonic crystals. They may be dynamically tuned through stretching and folding. They can be fabricated in various shapes, such as nanoribbons, and their properties can be engineered through multifunctionalization. There are many applications of nanomembrane guides—such as chemical and biological sensors—and photonic circuitry, such as active devices and photodetectors. Some functionalization approaches include those that are biomimetic, which facilitates the introduction of bionics into plasmonics. Available material choice even includes those that may be incompatible with biology. Possible limits are far away and are the subject of ongoing research.