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Smart Optical Metamaterials: A route towards electro-tuneable fast-reversible self-assembly of nanoparticles at controlled electrochemical interfaces

Periodic Reporting for period 1 - S-OMMs (Smart Optical Metamaterials: A route towards electro-tuneable fast-reversible self-assembly of nanoparticles at controlled electrochemical interfaces)

Reporting period: 2018-07-16 to 2020-07-15

Numerous smart optical applications of the future belong to a new generation of artificial materials comprising nanoscale building-blocks, capable of exhibiting extraordinary optical responses. Recent progress in nanotechnology enabled developing such optical metamaterials (OMMs) economically via voltage-controlled self-assembly of metallic nanoparticles (NPs) at electrochemical interfaces. For instance, a dense layer of metallic NPs of nanometer thickness strongly reflects incident light like a ‘mirror’, whereas a sparse layer enacts a ‘window’ by allowing light to pass through. This unique class of OMMs provides a platform for realizing switchable mirror/window, absorber/mirror devices (via NP assembly/disassembly on metallic substrate) etc. Besides tuneable optics, dense OMMs could also generate abundance of tuneable electric-field ‘hot-spots’ for ultrasensitive detection of trace amount of molecules. This project aimed at developing ‘smart’ electrotuneable OMMs for novel applications in materialising programmable mirrors, tuneable optical-filters and -cavities, and molecule-detectors.

Smart electro-tuneable optical devices are particularly important for society as these could play a vital role in minimizing the global energy needs by either harvesting solar radiation or by tuning optical devices dynamically with much lower voltage variation or by making the devices more efficient and long lasting. These OMMs as an ultrasensitive detector of biological and chemical molecules can sense any threat to food, health and security.

The objectives were to explore new architectures of electrodes for NP assembly to deliver application-specific, narrow or broadband, reflectance and/or transmittance spectra, develop new schemes for efficient voltage control over NP assembly/disassembly to achieve fast switching between different arrangements of NPs for fast alteration of the system’s optical response.
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.
This project delivered on designing a few specific ‘smart’ electro-tuneable optical devices based on custom-made electrochemical interfaces. It laid the foundation for designing tunable optical devices economically, based on OMMs via controlled self-assembly of metallic NPs at patterned electrodes. This new class of electro-switchable mirror-windows can selectively allow or block solar light depending on weather conditions. Tuneable optical-filters and -cavities, where tuning of optical properties can be achieved by sub-volt voltage variation, will be particularly useful in low-power integrated photonic devices. OMMs with dense plasmonic NP arrays act as bed of electric field ‘hot-spots’ could also revolutionise sensing of trace, tiny amount of hazardous molecules.

LEDs are used almost everywhere in day-to-day life — starting from TVs, traffic lights, bill boards, general & automotive lighting to sanitizations. With NP meta-grids LEDs are expected to become more energy-efficient and long lasting. With promising benefits like reduction in global energy needs of LED lighting, this work was covered by many press releases internationally.

All-in-all, electro-switchable mirror-windows will help towards making modern buildings and agricultural green-houses energy positive, contributing to the fight against climate change. Fine optical-filters tuneable by sub-volt variation will be crucial in new optical communication systems. OMMs with electrotunable NP-arrays will revolutionise ultrasensitive detection of trace amounts of hazardous molecules in our fight against pollutants, epidemics, illegal substances, and chemical terrorism.
Schematics of electrochemical optical metamaterials