Refractometry with Ultralow Detection Limit Using Anisotropic Whispering-Gallery-Mode Resonators

Abstract

The intrinsic sensitivity of whispering-gallery-mode resonators aimed at measuring refractive index can be extremely high, although their practical performance is compromised by temperature fluctuations that masquerade as refractive-index changes. We present a triple-mode approach that delivers simultaneous and independent sensing of temperature and refractive-index changes in the same resonator. The frequency difference between two orthogonally polarized modes is used to sense temperature which is then actively stabilized to ∼1 μK over 15 minutes. We then detect a frequency difference between two modes of different wavelengths to obtain a refractive-index measurement that is free of temperature fluctuations. This triple-mode technique delivers a state-of-the-art detection limit of 8 × 10 −9 refractive-index units, despite the resonator size being 100 times larger than that typically used for sensitive refractometric sensing.

Detection of Single Nanoparticles Using the Dissipative Interaction in a High-QMicrocavity

“…Optical microcavities featuring high-Q factors and small mode volumes, such as Fabry-Perot cavities [3], photonic crystals [4,5], microspheres [6][7][8][9], microrings [10][11][12][13][14], microtoroids [15][16][17], microbubbles [18][19][20], and microtubes [21,22] have been widely investigated in sensing applications. In general, the microcavity sensing depends mainly on reactive (i.e., dispersive) interactions, resulting in a resonance wavelength shift [23][24][25][26] or mode splitting [26,27], which essentially responds to the real part of the polarizability of the targets. Via reactive sensing, a single virus [28,29] and a single nanoparticle [30][31][32][33] have been detected experimentally.…”

Measuring the Charge of a Single Dielectric Nanoparticle Using a High-QOptical Microresonator

“…Over the past few years, optical whispering-gallerymode (WGM) microresonators including microspheres [1][2][3], microrings [4][5][6][7][8], microtoroids [9][10][11][12], microbubbles [13][14][15], and microtubes [16,17] have become valuable tools in sensing applications due to the significantly enhanced light-matter interaction provided by their ultrahigh-Q factors and small mode volumes [18,19]. So far, by monitoring either the cavity-resonant wavelength shift (mode shift) [20][21][22][23][24][25][26] or mode splitting [27][28][29][30], single nanoparticle binding events have been resolved. The former sensing scheme has a large detection range in particle size, while the latter is immune to various noises such as environmental temperature drift.…”

Self‐Assembled Nanostructured Photonic‐Plasmonic Metasurfaces for High‐Resolution Optical Thermometry

“…The reported responsivities reaching up to 200 pm per °C indicate a higher resolution than other implementations, such as localized surface plasmon resonators, surface plasmon polariton resonators, Bragg gratings, or Tamm structures . Yet, although record resolutions down to the 10 −9 °C range have been obtained with microresonators, the reported responsivities imply that achieving even a 10 −3 °C resolution requires a picometer precision in the determination of the resonance wavelength. Reaching such precision requires specific devices, such as highly monochromatic lasers as light source or bulky optoelectronic devices for signal processing.…”

“…In this context, monitoring the optical response of temperature‐sensitive integrated photonic elements, such as microresonators or microinterferometers has arisen as an appealing solution. With such elements, temperature monitoring requires tracking the wavelength variation of narrow, high‐quality‐factor resonances (spectral width at the picometer scale).…”

“…The on‐chip implementation of such structures required using lithography techniques or building periodic multilayers thicker than the wavelength of light. Such fabrication routes are also needed for the implementation of microresonators, microinterferometers, or Tamm plasmonic structures, respectively. Here, the metasurface structure is compatible with a lithography‐free two‐layer fabrication process, affordable either by physical deposition or solution‐processing.…”