Nanophotonic sensor technology

e446a30d-597d-4571-8525-471335c2a8e4

Closed

EPSRC
EPSRC
EPSRC
EPSRC
EPSRC

No funds listed.

Sept. 30, 2017

March 31, 2021

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.