Injection of Light Improves Biosensor

A new end-fire injection technique uses a waveguide (W) that is directly connected to the edge of the microdisk. A phenomenon known as laser time-reversal creates a laser that absorbs light rather than emits it, allowing the light to efficiently enter the microdisk, which use the whispering-gallery optical effect to confine and enhance light that enters the disk. (Source: Q. Song, Harbin Inst. of Technology)

There is a continuing need for practical chip-based sensors that can be used at the point of care to detect cancer and other diseases. An inno­vative way to inject light into tiny silicon micro­disks could help meet this need by bringing down the cost and improving the perfor­mance of chip-based bio­sensors. The advance 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.

“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,” said research team leader Qinghai Song from Harbin Insti­tute of Tech­nology, China. “Our new design enables excellent device perfor­mance that works with a variety of wavelengths with low cost, higher stability and better device repeata­bility.” The researchers found a new end-fire injection confi­guration, which offers a simple, cost-effective and effi­cient way to get light into the microdisk resonator. They also show that devices using micro­disks and end-fire injection can be used to detect tempera­ture changes and the presence of nano­particles.

The researchers’ ultimate 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.

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.

“Because this confi­guration doesn’t require any parts that are smaller than 500 nanometers, it can be fabri­cated with low-cost techniques,” said Song. To test their design, the researchers fabri­cated a device that included a microdisk with a 5-micron 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 percent.

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 nano­meters to 700 nano­meters. “We show that the performance of the end-fire injec­tion technique is comparable to that of conven­tional micro­disks but with improved robust­ness and reduced cost,” said Song. “Overall, our findings show that microdisks are now ready for commer­cial appli­cations.”

The researchers also demonstrated that sensors incor­porating microdisks and end-fire injection could detect the presence of multiple large nano­particles as well as single nano­particles as small as 30 nano­meters. They are interested in using cell-derived vesicles that are around 40 to 100 nano­meters to detect cancer, which should be possible based on these results. 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. (Source: OSA)

Reference: S. Liu et al.: End-fire injection of light into high-Q silicon microdisks, Optica 5, 612 (2018); DOI: 10.1364/OPTICA.5.000612

Link: State Key Laboratory on Tunable Laser Technology, Harbin Institute of Technology, Shenzhen, China

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