Tuning the exponential sensitivity of a bound-state-in-continuum optical sensor

Abstract

In this work, we investigate the evanescent field sensing mechanism provided by an all-dielectric metasurface supporting bound states in the continuum (BICs). The metasurface is based on a transparent photonic crystal with subwavelength thickness. The BIC electromagnetic field is localized along the direction normal to the photonic crystal nanoscale-thin slab (PhCS) because of a topology-induced confinement, exponentially decaying in the material to detect. On the other hand, it is totally delocalized in the PhCS plane, which favors versatile and multiplexing sensing schemes. Liquids with different refractive indices, ranging from 1.33 to 1.45, are infiltrated in a microfluidic chamber bonded to the sensing dielectric metasurface. We observe an experimental exponential sensitivity leading to differential values as large as 226 nm/RIU with excellent FOM. This behavior is explained in terms of the physical superposition of the field with the material under investigation and supported by a thorough numerical analysis. The mechanism is then translated to the case of molecular adsorption where a suitable theoretical engineering of the optical structure points out potential sensitivities as large as 4000 nm/RIU.

High-Sensitive Refractive Index Sensing Enabled by Topological Charge Evolution

“…The theoretical Qs of GMRs are infinite at normal incident angle (k = 0), which are also known as the bound states in the continuum (BICs) -discrete states with infinite lifetime even out-going waves are allowed. A series of pioneering works had demonstrated BIC-based RI sensors [28] upon silicon-on-insulator (SOI) [9], [29]- [31] and silicon nitride platforms [6], [12], [26], [32], [33]. By exiting at small incident angle (k ≈ 0), the Qs about 1 ∼ 3 × 10 4 had been achieved and resulted in DL up to 3 ∼ 6 × 10…”

Dielectric Nanohole Array Metasurface For High-Resolution Near-Field Sensing and Imaging

“…Two-dimensional plasmonic nanohole arrays, for example, can achieve bulk sensitivities above 700 nm/RIU [23,26] and dielectric nanohole arrays operating at 1550 nm can be even better with a demonstrated sensitivity of S ~ 800 nm/RIU [27] and theoretically up to 4000 nm/RIU in the visible range [19], while 1-D arrays typically achieve between 100-300 nm/RIU [28]. While the bulk sensitivity is easy to measure, it is not the most relevant parameter for a surface-a nity sensor; instead, the surface sensitivity should be used, which describes the response of the sensor to refractive index changes at the very surface of the sensor [25,29] which is much more representative of surface-bound proteins or DNA.…”

“…Further improvements are limited by the high losses of metals, however. Dielectric metasurfaces have therefore been introduced that support resonances with much higher quality factors [17][18][19]. Two major aspects have so far been missed, however.…”

Observation of spin-polarized directive coupling of light at bound states in the continuum

“…Reciprocity implies that far-field excitation of an ideal (infinite Q-factor) BIC mode is not possible. However, real structures can have finite, arbitrarily large Q-factors close enough to the BIC point, and partial coupling to the input light makes its spectral signature to appear as a special Fano-shaped resonance in quasi-BIC regime, which can be exploited for several applications [17][18][19][20]. In addition, of a certain interest is also the polarization structure of the quasi-BIC radiation [21,22].…”