Whispering-Gallery Mode Resonators for Detecting Cancer

Abstract

Optical resonators are sensors well known for their high sensitivity and fast response time. These sensors have a wide range of applications, including in the biomedical fields, and cancer detection is one such promising application. Sensor diagnosis currently has many limitations, such as being expensive, highly invasive, and time-consuming. New developments are welcomed to overcome these limitations. Optical resonators have high sensitivity, which enable medical testing to detect disease in the early stage. Herein, we describe the principle of whispering-gallery mode and ring optical resonators. We also add to the knowledge of cancer biomarker diagnosis, where we discuss the application of optical resonators for specific biomarkers. Lastly, we discuss advancements in optical resonators for detecting cancer in terms of their ability to detect small amounts of cancer biomarkers.

Label-Free Optical Resonator-Based Biosensors

“…Most works employing WGM resonators cite high Q-factors, high sensitivity, and rapid results, within micro- to nanoscale feature sizes. WGM-based biosensors are composed of a closed, radial cavity critically coupled with a prism, waveguide, or a fiber on its side or on the tip of a fiber as shown in Figure 16 [ 103 , 104 , 105 , 106 , 107 , 108 , 109 ]. WGM resonators confine light waves in any radial or polygonal enclosed surface, including rings, disks, spheres, cylinders, fibers, rods, or toroids, for example.…”

“…These are capable of being pumped remotely and collecting their spectra separately. Passive WGM must be directly coupled with a tunable laser source and detectors via fiber or waveguide [ 106 , 107 ]. These types do not generate light, but their resonance spectrum is tracked from the output transmission.…”

“…Physiochemical changes, such as those in bio-detection, modify the optical path length. The increased optical path length affects incoming light interactions with existing WGMs [ 17 , 24 , 104 , 107 , 110 ]. One or more detectors may be used to monitor spectral shifts, which are detectable when binding occurs on the surface of the resonator.…”

“…The resonant wavelength shift due to such binding events can be expressed as where λ and Δ λ are the resonant wavelength and resonant wavelength shifts, respectively, while α ex is the excess polarizability of molecules and σ is the surface density due to binding activity. The permittivity in a vacuum is ε 0 , r is the radius of the curvature, and n r and n b are the resonator and buffer refractive indices, respectively [ 107 ].…”

Whispering Gallery Mode Resonators for Precision Temperature Metrology Applications

“…During the last decade, whispering gallery mode (WGM) resonators have attracted considerable attention in sensing applications because of their high sensitivity. They were employed as gas sensors, biosensors and for pressure and force measurements [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. In this article, a sapphire-based whispering gallery mode resonator is employed as a thermometer with promising results in terms of sensitivity, resolution and accuracy.…”