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. 2023 Jun 26;23(13):5925.
doi: 10.3390/s23135925.

Spatial Multiplexing of Whispering Gallery Mode Sensors

Stephen Holler  1 , Matthew Speck  1
Affiliations

Spatial Multiplexing of Whispering Gallery Mode Sensors

Stephen Holler et al. Sensors (Basel). .

Abstract

Whispering gallery mode resonators have proven to be robust and sensitive platforms for the trace detection of chemical and/or biological analytes. Conventional approaches using serially addressed resonators face challenges in simultaneous multi-channel (i.e., multi-species) detection. We present an alternative monitoring scheme that allows for the spatial multiplexing of whispering gallery mode resonators with the simultaneous observation of the resonance spectra from each of them. By imaging arrays of microspheres and monitoring the glare spot intensities through image processing routines, resonance spectra from multiple resonators may be simultaneously recorded without interference or confounding effects of serial excitation/detection. We demonstrate our multiplexed imaging approach with bulk refractive index variations and virus-antibody binding.

Keywords: HPV; microcavity; morphology-dependent resonance; photonic sensor; resonator; virus; whispering gallery mode.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
An illustration of a first-radial-order WGM bound state (red curve) within the potential well (black curve) given by Equation (3) with the parameters k0a = 394.439, ℓ = 575, n = 1.46. The evanescent tail of the mode can be seen decaying away beyond the surface of the microsphere. This tail reaches out into the surrounding medium, permitting it to interact with nearby analytes, resulting in a shift in the resonance wavelength.
Figure 2
Figure 2
(A) Microspherical resonators are positioned atop a prism at the location where the tunable laser strikes the surface from the underside. The light undergoes total internal reflection and the tail of the evanescent field allows light to couple with the whispering gallery modes of the microspheres. An o-ring around the spheres defines the sample volume, which is filled with solution. A microscope looks from above to image the microsphere array and records light emission at the glare spots on the edge of the resonators on a CCD camera (not shown). (B,C) show two different renderings of a single microsphere inside the o-ring sample volume and held in position by the fiber stem. The red band around the sphere is the light circulating in the resonance, while the beam directed upward is the glare spot signal.
Figure 3
Figure 3
Images of a prism-coupled microsphere: (A) on-resonance and (B) off-resonance. The glare spot indicated results from leakage of the circulating light. This is the region monitored to produce the resonance spectrum.
Figure 4
Figure 4
(A) Image showing two microspheres situated on the prism and illuminated by the evanescent field of the tunable DFB laser. The sphere on the left shows a bright spot on the right edge corresponding to light leakage from a WGM resonance. At this point, the sphere on the right is not in resonance and does not exhibit a bright spot on its right edge. (B) WGM resonance spectra obtained simultaneously by imaging two spatially separated microspheres. Both spectra exhibit narrow peaks that may be used for trace species detection in the WGM biosensor. A video of two spheres undergoing resonance as the laser is tuned may be downloaded from the Supplementary Materials.
Figure 5
Figure 5
Resonance spectra from a single 250 μm diameter sphere immersed in water (Run 1, black) and then a water/methanol solution (Run 2, red). The change in the bulk refractive index surrounding the sphere results in a ∼0.26 nm shift in the WGM resonances. For clarity, an arrow indicates the shift in one of the resonances. Videos of the two runs, in which the sphere is observed to resonate as the laser is tuned, may be downloaded from the Supplementary Materials.
Figure 6
Figure 6
Resonance spectra showing the effect of viral binding to an antibody-coated microsphere. The black curve in the WGM spectrum was taken prior to the introduction of a Human Papillomavirus solution. After five minutes of diffusion through the sample cell and binding to the microsphere surface, the red curve was recorded. A definite shift is observed due to the response of the microsphere to the mass loading of the virus.
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