New architectures of electrodes (as substrate for NP assembly) in the form of optical meta-trenches, meta-fences and meta-cavities (see figure below) were explored for targeted assembly of NPs to realize voltage-controlled blocking/passing of incident light. Based on switchable assembly (disassembly) of gold NPs on thick flat silver electrode, an electrotuneable nanoplasmonic absorber (mirror) was first demonstrated. Taking two of those parallel electrodes, but much thinner (10–20 nm that allows transmission), a fast switchable optical device involving NPs in electrolytic solution filled in a micro-optical-cavity (formed between those electrodes) was designed. In such ‘Fabry–Perot interferometer’ cavity, sub-volt polarization of electrodes changes the density of NP arrays assembled on the electrodes that alters the inter-NP gap in the array, and thereby the optical response of the cavity. This meta-cavity allowed fast tuning of narrowband transmission spectrum to realize a tuneable optical filter.
A new architecture of NPs’ stacked assembly on charged columnar electrodes allowed obtaining broadband reflectivity (or ‘shielding’ of incident rays) with growing number of NP layers, controlled via voltage variation. These meta-fences based on ZnO columnar electrodes (nanowires) was designed and the best structures were specified for NPs to quickly populate/depopulate the interface of the charged columnar electrodes.
Control over inter-NP gap in the assembly of NPs allowed tuning of reflectivity profile of the ensemble, where generated electric field ‘hot-spots’ facilitated sensing based on reflectivity. These electrotuneable platforms were demonstrated as new ultrasensitive detectors based on amplified surface enhanced Raman spectroscopy (SERS), but detected through much simpler reflectivity measurements.
A detailed theoretical framework was developed for accurate description of these optical devices involving voltage-controlled self-assembly of NPs at custom-made electrochemical interfaces. The theoretical framework—developed for calculating optical responses (viz. reflection, transmission and absorption) of layers of plasmonic NPs and for estimating inter-NP gap in a monolayer—was corroborated with rigorous computer simulations on commercial tools and systematically tested against proof-of-the-concept experiments. All experiments were navigated by theory and their data were treated by it. Such concerted approach allowed designing nanoplasmonic systems based on self-assembling 2D NP-arrays for reflectivity-based sensing, thermo-responsive optical switching, and an electrotuneable platform for amplified SERS.
A stunning finding of a potentially huge economic impact was made, as a ‘side product’ of this project. Related not with electrovariability, but with the properties of designed NP arrays, it has been discovered and explored theoretically, and tested by simulations. These are nanophotonically-modified LED-devices based on embedding a dense array of plasmonic NPs between the LED chip and epoxy casing. This provides the effect of extraordinary transmission that allows to extract almost 100% of light from LEDs, in contrast to a standard LED that loses some 20% of light to internal reflection and which leads to overheating and degradation of LEDs. With the optimized NP array, called ‘meta-grid’, the LEDs will get energy-efficient and long-lasting.
The main findings of this project were disseminated through ten high impact peer-reviewed journal publications, many international press releases, one editorial as a guest editor in a special issue of an Institute of Physics journal, six conference publications/talks, several invited talks, two patent applications submitted to Imperial Innovations. It also brought in four new external academic collaborations, one new industry collaboration, and more funding for further exploration.
