In yet another application for micro- and nanostructures, recent experiments have demonstrated the potential for microresonators to serve as ultrasensitive temperature sensors. Last year, Ashok Pandey, Oded Gottlieb, and Eyal Buks of the Technion–Israel Institute of Technology showed that the resonant frequency of a suspended, microfabricated gold–palladium beam, hundreds of microns long but just a micron wide and a fraction of a micron high and supported at each end, was a strong function of temperature. The dominant contribution came from the temperature dependence of the tension in the beam, which is due to the difference between the thermal expansion coefficients of the beam and of the silicon substrate below it. The researchers could measure temperature changes by monitoring the relative frequency shift; as a temperature sensor, the beam's sensitivity was about a third that of commonly used, macroscopic platinum sensors. More recently, a team led by Anja Boisen of the Technical University of Denmark has reported aluminum microresonators, such as the ones seen here, whose resonant frequencies are even more sensitive to temperature. The improvement, of more than an order of magnitude, arises primarily from the larger difference in thermal expansion coefficients and from the Al beam's smaller initial tension compared with that of the Au--Pd beam. With their high quality factors, microresonator-based sensors would potentially have exceptional temperature resolution as well. (A. K. Pandey et al., Appl. Phys. Lett. 96, 203105, 2010; T. Larsen et al., Appl. Phys. Lett. 98, 121901, 2011.)—Richard J. Fitzgerald
