Nanophotonic sensor technology

Lead Research Organisation: University of York
Department Name: Physics


The lateral resolution of the guided mode resonance is of order 2-5 micrometres, which is determined by the fact that the guided mode resonance is a grating resonance, i.e. the resonant mode samples multiple grating periods. By changing the nature of the resonance to a particle resonance, higher spatial resolution may be achieved. Examples for this approach are the Mie resonances of high-index dielectric nanoparticles, or the plasmonic resonances of metal nanoparticles , the latter having been used extensively in the localised surface plasmon resonance (LSPR) sensing and imaging (SPRI) techniques. While these Mie resonance techniques offer higher spatial resolution than the grating resonances, their spectral resolution is lower and restricted to Q-factors of order Q approximately equals 10-20, i.e. an order of magnitude lower than the Q-factors of Q approximately equals 200-300 typically achieved with our grating resonances. Therefore, both grating resonances and particle resonances intrinsically achieve a very similar combined spectral-spatial resolution.

We propose to explore a novel type of resonant structure based on the recently introduced metal-insulator-metal modes (MIM). MIM modes are particle resonances that afford similar Q-factors to guided mode resonances yet also very high spatial resolution. The origin of this remarkable combination is the exceptional slow light effect they exhibit (The PI made a number of pioneering contributions to slow light research in the late 2000's). These slow light effects manifest themselves as high-k localised modes. For example, these modes can achieve Q-factors as high as 100 and group indices as high as 30, thereby affording resonant modes that are extremely highly localised yet spectrally narrow.

The reason MIM modes have such low loss is that most of their field resides in the dielectric space between two metal plates, i.e. similar to a capacitor. This means that the mode is extremely well shielded from the outside world, hence not useful for sensing at all. In a pilot study, we have now established that the MIM mode can be sufficiently modified that most of its field extends outside the capacitor geometry while the high k and the high Q properties are maintained.

In a further investigation, we verified the sensitivity of the mode fand noted a sensitivity of approximately 600nm/RIU against bulk refractive index changes, which is remarkably high and indicates the part-plasmonic nature of this mode.

In conclusion, MIM modes allow us to combine the high spectral resolution available with guided mode resonances, the high spatial resolution available with particle resonances and the high sensitivity of surface plasmon modes in an exciting new sensing modality.


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509802/1 01/10/2016 31/03/2022
1947536 Studentship EP/N509802/1 01/10/2017 31/03/2021 George Duffett
Description We have developed an effective strategy for detecting the presence of biomedically relevant molecules such as proteins with plasmonic refractive index sensors. We confine light in metal-insulator-metal (MIM) nanoresonators. The MIM's two metal plates shield the resonant mode from the outside world. We found that the resonant mode overlaps with the outside world at the edges of the resonator. This overlap between the resonant mode and surrounding media makes the MIM very sensitive to refractive index changes close to its surface. This strategy is more suitable for detecting the presence of biomedically relevant molecules than maximising the bulk sensitivity, which does not necessarily guarantee that a sensor will detect small changes close to its surface.

We verified the MIM's high surface sensitivity by changing the refractive index of the material within 10nm of the MIM's surface and measuring the resonant mode's peak wavelength shift. We also measured the MIM's bulk sensitivity by flowing different concentrations of salt and sugar solutions over its surface. We flowed these solutions over the MIM by designing and assembling 3D printed microfluidic channels. This microfluidic assembly will allow us to detect biologically relevant molecules such as proteins binding to the MIM surface.

We developed two main methods of fabricating these sensors: a top-down approach of e-beam lithography and a bottom-up approach using commercially available nanoparticles. The sensors fabricated via the bottom-up nanoparticle approach are cheaper, easier, faster, more scalable and have a better optical response than sensors fabricated by e-beam lithography.

We have identified the origin of the MIM's high surface sensitivity though two different simulation techniques. Our simulations show that the high surface sensitivity is due to electric field hotspots at the corners of the resonator.
Exploitation Route The insights from the work funded through this award are highly relevant for the design of next generation optical biosensors. Other researchers can benefit from our study by optimizing their sensors for high surface sensitivity as opposed to high bulk sensitivity.

The the MIM sensor itself can be taken forward in an industry context. It has many use-cases such as detecting and measuring pollutants, monitoring food quality, early
disease detection and drug discovery. It could be developed as a product in any of these areas, owing to the scalability of the fabrication process we developed.
Sectors Agriculture, Food and Drink,Healthcare