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Radial reflector enhances biosensing

08 Jun 2005

Scientists make an ultra-sensitive biochemical sensor by etching circular micro-cavities in a semiconductor.

Researchers in the US have developed an ultra-sensitive biochemical sensor based on circular Bragg micro-cavities. According to the Caltech team, the compact structure is ten times more sensitive than conventional resonators and could become an integral part of future lab-on-a-chip devices.

The big attraction of annular Bragg resonators (ABRs) is their large interaction volume, which can be exploited in sensing applications to boost sensitivity. Made from a 250 nm thick membrane of InGaAsP active material, the Caltech device consists of a series of concentric Bragg mirror rings and a circular guiding defect.

When the rear side of the ABR is optically pumped, light that is resonant with the structure is emitted from its front surface. The value of the resonant wavelength is determined by a combination of the design and the ambient refractive index. To obtain the emission spectrum, a 20X objective lens is placed in front of the sensor to couple light from the ABR into a multimode fiber.

The researchers tested their device by immersing the sensor in several index-matching fluids to simulate biochemical samples. They found that a change in refractive index of 0.08 gave a shift in the ABR's resonance wavelength of more than 10 nm. Able to resolve resonance shifts as small as 0.1 nm, the scientists claim that their sensor can measure changes in refractive index down to 5x10-4.

Motivated by their excellent results, the researchers are now focussing on manufacturability. "Right now the devices are fabricated using E-beam lithography and optically pumped with an external [Ti:Sapphire] laser," Caltech scientist Jacob Scheuer told Optics.org. "The next steps are to develop electrically pumped ABR diode lasers and shift fabrication to more conventional photolithography techniques [to suit mass production]."

Looking ahead, Scheuer believes that these hurdles could be overcome within the next few years, leaving a clear path towards commercialization.

This research was presented at CLEO/QELS 2005.

James Tyrrell is reporter on Optics.org and Opto & Laser Europe magazine.

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