Label-free DNA biosensing by topological light confinement

Large-area and transparent all-dielectric metasurfaces sustaining photonic bound states in the continuum (BICs) provide a set of fundamental advantages for ultrasensitive biosensing. BICs bridge the gap of large effective mode volume with large experimental quality factor. Relying on the transduction mechanism of reactive sensing principle, herein, we first numerically study the potential of subwavelength confinement driven by topological decoupling from free space radiation for BIC-based biosensing. Then, we experimentally combine this capability with minimal and low-cost optical setup, applying the devised quasi-BIC resonator for PNA/DNA selective biosensing with real-time monitoring of the binding event. A sensitivity of 20 molecules per micron squared is achieved, i.e. ≃0.01 pg. Further enhancement can easily be envisaged, pointing out the possibility of single-molecule regime. This work aims at a precise and ultrasensitive approach for developing low-cost point-of-care tools suitable for routine disease prescreening analyses in laboratory, also adaptable to industrial production control.

Optimized Virtual Optical Waveguides Enhance Light Throughput in Scattering Media

Ultrasonically sculpted gradient-index optical waveguides make it possible to non-invasively steer and confine light inside scattering media. This confinement capability has applications in tissue and brain imaging, where virtual optical waveguides can be used on their own or cascaded with physical optical elements. The level of light confinement strongly depends on ultrasound parameters such as modulation pattern, frequency, and amplitude, as well as the material parameters of the scattering medium such as the refractive index, scattering coefficient, and phase function. We provide a characterization of these dependencies for a radially symmetric virtual optical waveguide. To this end, we develop a physically-accurate simulator, and use it to quantify how different ultrasound and material parameters affect light confinement. We explain our observations through a qualitative analysis of the behavior of multiply scattered light. We use the results of this analysis to demonstrate that, by properly designing ultrasound parameters, we can achieve a fourfold improvement in light confinement compared to previous virtual optical waveguide designs. We additionally show that virtual optical waveguides can achieve up to 50% light throughput enhancement compared to an ideal external lens, in a medium that mimics the scattering properties of human bladder, and at an optical thickness of one transport mean free path. Lastly, we show experimental results that corroborate the simulation predictions. In particular, we demonstrate for the first time that virtual optical waveguides effectively recycle scattered light in turbid media, and can achieve a 15% light throughput enhancement at five transport mean free paths.

Infrared Polaritonic Biosensors Based on Two-Dimensional Materials

In recent years, polaritons in two-dimensional (2D) materials have gained intensive research interests and significant progress due to their extraordinary properties of light-confinement, tunable carrier concentrations by gating and low loss absorption that leads to long polariton lifetimes. With additional advantages of biocompatibility, label-free, chemical identification of biomolecules through their vibrational fingerprints, graphene and related 2D materials can be adapted as excellent platforms for future polaritonic biosensor applications. Extreme spatial light confinement in 2D materials based polaritons supports atto-molar concentration or single molecule detection. In this article, we will review the state-of-the-art infrared polaritonic-based biosensors. We first discuss the concept of polaritons, then the biosensing properties of polaritons on various 2D materials, then lastly the impending applications and future opportunities of infrared polaritonic biosensors for medical and healthcare applications.