High Quality Factor Metasurfaces for Real-Time Ocean Observation

Changing climate conditions are driving fundamental shifts to our marine and freshwater ecosystems. Phytoplankton are microscopic organisms responsible for half of global photosynthetic carbon fixation, but certain phytoplankton taxa can produce powerful, low molecular weight biotoxins that contaminate drinking water sources, harm wildlife, are detrimental to human health, and pose economic threat to coastal communities. Understanding how environmental drivers impact plankton nutrient cycling and toxin production is key to advancing climate resilience but remains an outstanding challenge. Dominant methods of studying these marine toxins and their genes are based on liquid chromatography with tandem mass spectrometry (LC/MS-MS) and polymerase chain reaction (PCR), but these techniques are costly, require sophisticated infrastructure, and lack remote, autonomous, real-time detection capabilities central to understanding coupled hydrographic and climate dynamics.<br/>In this talk, I will describe the development of a nanophotonic sensor for real-time aquatic biotoxin detection using surface-functionalized high quality factor (high-Q) silicon metasurfaces. I will discuss the design of our sensor, which is formed of sub-wavelength silicon nanobars, of dimensions 500 nm x 600 nm x 160 nm, periodically modulated to enable free space coupling to guided mode resonances. We demonstrate that these guided mode resonances generate a strong local field enhancement through high quality factors exceeding 1,000 both in simulation and in fabricated devices. We show that increased field penetration into the surrounding medium and long resonant lifetimes together enable sensitive and quantitative readout of the local dielectric environment, where small perturbations generate strong spectral shifts to the resonant mode. We demonstrate that by using free space coupling into the guided mode resonances, the optical responses of individual resonators can be spatially resolved and simultaneously read out as a change in resonance wavelength, or as a change to the scattering intensity on a 2D CCD array using a home-built infrared optical microscope. We apply this platform to the detection of two aquatic biotoxins: domoic acid, a neurotoxin produced by the marine diatoms, <i>Pseudo-nitzschia spp.</i>, responsible for human and wildlife mortalities, and microcystin, a liver toxin produced by cyanobacteria that poses a major threat to drinking and agricultural water supplies. We show that by employing an antibody binding assay with tailored surface functionalization, our high-Q metasurfaces can exhibit strong shifts in resonance wavelength of ~5 nm, offering a straightforward approach to detection with high antibody binding specificity. Finally, I will discuss the integration of our high-Q metasurfaces with the Environmental Sample Processor, an autonomous robotic water sampler developed at the Monterey Bay Aquarium Research Institute (MBARI), that offers a pathway for bringing the optical bench to the sea through<i> in situ</i> sample acquisition, processing, and analysis.