High sensitivity sensor based on circular Bragg micro-cavities

Citation

(2006), "High sensitivity sensor based on circular Bragg micro-cavities", Sensor Review, Vol. 26 No. 1. https://doi.org/10.1108/sr.2006.08726aaf.002

High sensitivity sensor based on circular Bragg micro-cavities

High sensitivity sensor based on circular Bragg micro-cavities

Keywords: Sensors, Semiconductors

A research team at the California Institute of Technology (Caltech) is developing an ultra-sensitive optical sensor based around a unique semiconductor structure. The sensor consists of a 250 nm-thick membrane of InGaAsP (indium gallium arsenide phosphide) which is etched using electron beam lithography to create circular, concentric micro-cavities and a circular guiding defect (Plate 3). These micro-cavities are termed annular Bragg resonators (ABRs). According to the Caltech group, this type of structure is around ten times more sensitive than alternative circular resonators (rings and disks), where light is confined in the guiding channel by total internal reflection. Micro-cavities employing distributed Bragg reflection as the transverse confinement mechanism, such as ABRs, exhibit unique mode profiles which can offer enhanced sensitivity because of the larger interaction volume. The rear (i.e. un-etched) side of the structure is optically pumped with an external Ti:sapphire laser and light that is resonant with the structure is emitted from its front surface. The value of the resonant wavelength is governed by a combination of the sensor's design features and the ambient refractive index. The emission spectrum of the light is determined by placing a 20 × objective lens in front of the sensor which couples the light into a multimode optical fibre.

Plate 3 The sensor, showing a circumferentially guiding defect surrounded by etched radial Bragg resonators (Credit: Caltech)

The sensor was tested by immersing it in a range of different index- matching fluids which were used to simulate biochemical samples. It was found that a change in refractive index of 0.08 gave a shift in the ABR's resonance wavelength of more than 10 nm. As it is possible to resolve resonance shifts of as little as 0.1 nm, the group claims that the sensor can measure changes in refractive index down to 5 × 10⁻⁴. Anticipating applications in high sensitivity chemical sensing and as components within “lab on a chip” devices, the group has since embedded an array of ABRs into a micro-fluidic circuit (Plate 4). The next stage of the research is to develop techniques that would allow the sensors to be mass-produced. Critical issues include replacing the external laser with an electrically pumped ABR diode laser and fabricating the sensor with conventional photolithographic techniques.

Plate 4 An array of annular Bragg reflectors embedded in a micro-fluidic circuit (Credit: Caltech)

For details contact: Jacob Scheuer. Tel: ++1-626-395-4413, E-mail: koby@caltech.edu