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Compact Microspectrometer Achieves High Resolution, Wide Bandwidth
Jun 2011
ATLANTA, June 22, 2011 — A new microspectrometer architecture that uses compact disk-shaped resonators could address the challenges of integrated lab-on-chip sensing systems that now require a large off-chip spectrometer to achieve high resolution.

Spectrometers have conventionally been expensive and bulky benchtop instruments used to detect and identify the molecules inside a sample by shining light on it and measuring different wavelengths of the emitted or absorbed light. Previous efforts toward miniaturizing spectrometers have reduced their size and cost, but these reductions have typically resulted in lower-resolution instruments.

A micrograph shows a microspectrometer developed by Ali Adibi and colleagues at Georgia Tech. The instrument achieved 0.6-nm resolution over a spectral range of more than 50 nm with a footprint <1 mm2. (Image: Zhixuan Xia)

“For spectrometers, it is better to be small and cheap than big and bulky — provided that the optical performance targets are met,” said Ali Adibi, a professor in the School of Electrical and Computer Engineering at Georgia Institute of Technology. “We were able to achieve high resolution and wide bandwidth with a compact single-mode on-chip spectrometer through the use of an array of microdonut resonators, each with an outer radius of two microns.”

The 81-channel on-chip spectrometer designed by Georgia Tech engineers achieved 0.6-nm resolution over a spectral range of more than 50 nm with a footprint <1 mm2. The simple instrument — with its ultrasmall footprint — can be integrated with other devices, including sensors, optoelectronics, microelectronics and microfluidic channels for use in biological, chemical, medical and pharmaceutical applications.

The experimental setup used to test the 81-channel on-chip microspectrometer is shown. (Photo: Zhixuan Xia)

The microspectrometer architecture was described in a paper published June 20 in the journal Optics Express.

“This architecture is promising because the quality factor of the microdonut resonators is higher than that of microrings of the same size,” said Richard Soref of the US Air Force Research Laboratory at Hanscom Air Force Base in Massachusetts. Soref, a research scientist not directly involved in the research, added that having such small resonators also is advantageous because they can be densely packed on a chip, enabling the sampling of a large spectrum.

Adibi’s group is developing the next generation of these spectrometers, which are being designed to contain up to 1000 resonators and achieve 0.15-nm resolution with a spectral range of 150 nm and a footprint of 200 mm2.

A scanning electron microscope image of a microdonut resonator shows an outer radius of 2 μm and center hole with 0.6-μm radius. An array of these resonators was used to create a high-resolution, wide-bandwidth microspectrometer. (Image: Zhixuan Xia)

Adibi, current graduate student Zhixuan Xia, research engineer Ali A. Eftekhar, and former research engineers Babak Momeni and Siva Yegnanarayanan designed and implemented the microspectrometer using CMOS-compatible fabrication processes. The key building element they used to construct the device was an array of miniaturized microdonut resonators, which were essentially microdisks perforated in their centers. This research built on former Georgia Tech graduate student Mohammad Soltani’s work to develop miniature microresonators, which was published in the Sept. 13, 2010, issue of Optics Express.

The researchers adjusted the resonance wavelengths of different microdonut resonators by engineering their geometry. Although the resonance was very sensitive to variations in the outer radius, fine-tuning could be achieved by adjusting the inner radius. The microdonut resonators were designed so that each of the resonators tapped only a small portion of the incoming spectrum, enabling measurement of the entire spectrum of desired wavelengths in real time.

A key advantage of this microspectrometer design, according to the researchers, is the ability to independently control and configure the resolution and operating bandwidth of each channel for different applications. The device can cover a wide range of wavelengths from ~1 to 3 μm. Extending this concept to the silicon nitride platform also enables spectrometers for visible light applications.

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1. In optics, the ability of a lens system to reproduce the points, lines and surfaces in an object as separate entities in the image. 2. The minimum adjustment increment effectively achievable by a positioning mechanism. 3. In image processing, the accuracy with which brightness, spatial parameters and frame rate are divided into discrete levels.
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