Abstract:
In spite of tremendous advancements in modern diagnostics, there is a dire need for reliable, label-free detection of highly contagious pathogens like viruses. In view of the limitations of existing diagnostic techniques, the present theoretical study proposes a novel scheme of detecting virus-like particles employing whispering gallery and quasi-whispering gallery resonant modes of a composite optical system. Whereas whispering gallery mode (WGM) resonators are conventionally realized using micro-disk, -ring, - toroid or spherical structures, the present study utilizes a rotationally symmetric array of silicon nanowires which offers higher sensitivity compared to the conventional WGM resonator while detecting virus-like particles. Notwithstanding the relatively low quality factor of the system, the underlying multiple-scattering mediated photon entrapment, coupled with peripheral total-internal reflection, results in high fidelity of the system against low signal-to-noise ratio. Finite difference time domain based numerical analysis has been performed to correlate resonant modes of the array with spatial location of the virus. The correlation has been subsequently utilized for statistical analysis of simulated test cases. For a single virus of 160 nm diameter, more than 8 nm shift of the resonant mode and nearly 100% change of quality factor are attained with the proposed nanowire array based photonic structure. Taking the dimension and geometry of the Coronavirus as a reference, resonant wavelength shift and quality factor change of the whispering gallery mode of the nanowire array in the presence of one and two viruses are evaluated. For a single coronavirus having protein spike, the maximum shift of resonant wavelength is found to be about 5 nm, whereas for two viruses the shift can be as high as 9 nm. However, interestingly, for two viruses the shift appears to change periodically depending on relative location of the virus particles. In addition, the prospect of detecting virus-like particles using single and multiple arrays of rotationally symmetric array of silicon nanowires is compared. As far as multiple arrays for virus detection is concerned, the quality factor of the most strongly confined whispering gallery mode appears to have been reduced by 50% or more because of mode-leakage resulting from coupling between the nanowire arrays. Such weaker confinement of the optical field though may have a diminishing effect on the signal to noise ratio during practical measurements, the detection sensitivity appears to have improved significantly because of the sustenance of whispering gallery resonant modes in the adjacent nanowire arrays.
