Light-Delivery Technique Improves Biosensors

Researchers from the Harbin Institute of Technology in China have developed an innovative way to inject light into tiny silicon microdisks that could eventually lead to a portable and low cost optical sensor for early-stage cancer diagnostics. Microdisks are a type of microscale resonator that use the whispering-gallery optical effect to confine and enhance light that enters the disk.


Just as the curved walls of a whispering gallery carry sound waves to allow whispers to be clearly heard across a room, the curved inner surface of a microdisk carries light waves across the disk, enhancing the light. This allows the microdisk to boost a light-based signal coming from a cell, protein or virus of interest, allowing more sensitive detection of subtle changes associated with diseases such as lupus, fibromyalgia, and certain heart problems.

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Research team leader Qinghai Song reports, “Although there are whispering gallery mode micro-resonators that can already be used to resolve single molecules, their application is limited by problems in device repeatability, stability and wavelength range. Our new design enables excellent device performance that works with a variety of wavelengths with low cost, higher stability and better device repeatability.”


Time Reversal


Most microdisks are designed so that light is indirectly injected into the microdisk using an optical phenomenon known as evanescent light coupling. However, this method requires very precise alignment between the waveguide and the microdisk, which increases manufacturing costs and makes devices susceptible to stability problems.


The researchers’ end-fire injection technique uses a waveguide that is directly connected to the edge of the microdisk. Although light that is exactly perpendicular to the disk’s side will bounce off the interface, using light angled just slightly less than perpendicular induces a counterintuitive phenomenon known as laser time-reversal. This creates a laser that absorbs light rather than emits it, allowing the light to efficiently enter the microdisk. Song says, “Because this configuration doesn’t require any parts that are smaller than 500 nanometers, it can be fabricated with low-cost techniques.”


To test their design, the researchers fabricated a device that included a microdisk with a 5-µm radius connected to a waveguide. To measure the end-fire injection, they incorporated a Y-splitter that allowed light passing through the splitter to be injected into the microdisk and then be transmitted out of the microdisk along the same waveguide. Recording the spectrum coming from the Y-junction showed that light could be coupled into the microdisk with an efficiency as high as 57%.


They also showed that the device exhibited a high Q-factor, a measure of how well the microdisk confines and amplifies the light. In addition, the device maintained good performance parameters even with fabrication deviations such as increasing the waveguide width from 400 nm to 700 nm.


The researchers also demonstrated that sensors using microdisks and end-fire injection could detect the presence of multiple large nanoparticles as well as single nanoparticles as small as 30 nm. They are interested in using cell-derived vesicles that are around 40 nm to 100 nm to detect cancer. The researchers are now working on other parts of the device that would be needed to use the end-fire injection technique to create a portable and low-cost sensor that can detect early indicators of cancer.


The researchers’ end goal is to use their new end-fire injection technique to create a portable and low-cost sensor that can detect changes in cells that are early indicators of cancer. However, they point out that the new light-coupling configuration could also be useful for integrated photonic circuits for communication applications and a variety of sensors such as those used in homeland security or environmental monitoring.


For greater insights and technical description, a paper authored by researchers S. Liu, W. Sun, Y. Wang, X. Yu, K. Xu, Y. Huang, S. Xiao, and Q. Song is available for your perusal: “End-fire Injection of Light into High-Q Silicon Microdisks,” Optica, Volume 5, Issue 5, 612-616 (2018).


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