Ultrahigh-Q chip created

In an advance that holds promise for integrating previously disparate functions on a chip, Caltech applied physicists have created a disk smaller than the diameter of a human hair that can store light energy at extremely high efficiency. The disk, called a “microtoroid” for its doughnut shape, can be integrated into microchips for many potential applications.

Reporting in the February 27 issue of Nature, Jenkins Professor of Information Science and Technology and Professor of Applied Physics Kerry Vahala and graduate students Deniz Armani, Tobias Kippenberg, and Sean Spillane describe the optical resonator, which has a “Q factor” (quality factor) more than 10,000 times better than any previous similar chip-based device. A figure used to characterize resonators, Q is the approximate number of light oscillations within the device’s storage time.

Resonators store optical energy by resonant recirculation at the toroid’s exterior boundary, achieving Q factors in excess of 100 million. Examples of resonators include TV tuners and quartz crystals in wristwatches, which operate at radio frequencies; and optical-frequency versions used in filters, sensors, and quantum optics.

Attaining ultrahigh-Q and fabricating the resonators on a chip have so far been mutually exclusive, as very few structures have exhibited the atomically smooth surfaces needed for ultrahigh-Q. Due to a novel fabrication step, it is now possible to achieve both high Q and atomically smooth surfaces at the same time, bringing two worlds together.

The fabrication procedure uses lithography and etching techniques on a silicon wafer, in a process similar to that used in making microprocessors and memories. Thus, the resonators can be integrated with a chip’s circuitry, with lab-on-a-chip functions, or with other optical components. Wafer-scale processing methods also enable the production of wafers in large quantities, an important feature in many applications such as biosensing, where low-cost, field-deployable sensors are envisioned.

“This is the first time an optically resonant device with an ultrahigh-Q has been fabricated on a chip,” Vahala says. The group is exploring ways to further increase the devices’ Q value while reducing their size. He believes Q values in excess of 1 billion in even more compact toroids will soon be possible.

The work was supported by Caltech’s Lee Center for Advanced Networking and by DARPA.
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