Emerging Directions in Nanophotonics: Opportunities and Challenges

Nanophotonics deals with optical interactions and dynamics on nanoscale1. Some new directions being pursued in our lab are Cd –free quantum dots such as ternary core-shell CuInS₂/ZnS and Silicon Quantum dots, which also exhibit two-photon excitation suitable for two-photon in vitro and in vivo bioimaging. We are also developing plasmonic semiconductors such as self-doped copper chalcogenide (Cu_2−xSe and Cu_2−xS) nanoparticles. Here the localized surface plasmon resonance (LSPR) is derived from free carriers in nonstoichiometric binary or multicomponent semiconductor nanoparticles. The talk will also discuss our work on multiphoton harvesting and photon transforming nanostructures (Optical nanotransformers) that shift photons from one spectral range to another for many emerging biomedical and photovoltaic technologies, which require light of a specific wavelength range, not readily deliverable to the site where it is needed. We have introduced new multistep energy transfer pathways for greatly enhancing both upconversion and down conversion (Quantum cutting) processes.

A new direction for nanophotonics is for Chiral Photonics in which the main objective of our research program is rational design of new chiral polymers and polymer-based nanocomposites with extremely large linear, nonlinear and magneto-optical activity via establishing structure-property relations. In chiral polyfluorene thin films, we demonstrated extremely large chiral nonlinearity. Another new direction pursued is magnetic field control of light in in a chiral polymer nanocomposite to achieve large magneto-optic coefficient, which can enable sensing of extremely weak magnetic field due to brain waves. Another direction is Spin photonics involving interaction of Structured Light with a Chiral Plasmonic Metasurface. Our modeling reveals an approach for giant enhancement and broad spectral tunability of the chiro-optic response of a plasmonic nanohelix metasurface by exploiting interaction with complex light, endowed with both spin and orbital angular momentum.

In bio-nanophotonics, a new approach involves nonlinear nanocrystals such as ZnO with which we can use four wave mixing, sum frequency generation and second harmonic generation to convert a deep tissue penetrating Near IR light at the targeted biological site to a desired shorter wavelength light suitable for bio imaging or activation of a therapy. Yet another direction for imaging, sensing and light controlled therapy is development of nanostructures that can respond in the spectral regions called NIR window II (1,000 nm to 1,300 nm) and NIR window III (1,600 nm to 1,870 nm) which provide deeper penetration through biological tissues. A new area of our bio-nanophotonics program is brain research and the emerging field of Neurophotonics, where we apply field responsive materials for functional mapping of the brain using optical and photoacoustic imaging, as well as magneto-optics to sense region selectively the magnetic field due to brain waves. We have also demonstrated remote and noninvasive actuation of optogenetic stimulation of brain activity. Our vision is to utilize Neurophotonics to quantify the cognitive states, thus paving the pathway to a quantified human paradigm.