Phonon Spectrometer

Careful control of heat flow at the micro-scale is crucial to many applications in energy systems and in microelectronic devices. In semiconductors and insulators, all or most heat is carried by lattice vibrations (phonons) of wavelength 1 to 100 nm. The Robinson group is engineering a variety of nano-scale structures such as nano-wires, nano-sheets and meshes with spacings of 30 to 100 nm, to study the effect of nanoscale geometry on phonon heat transport. In this way we seek to improve the efficiency of thermoelectric materials, making possible large-scale thermoelectric coolers and recovery of waste heat into useable electricity.

To advance this work we are designing and building a microscale phonon spectrometer to cover the range of frequencies from 100 GHz to 1 THz. Heat flowing conventionally through a material includes phonons from a broad range of this spectrum. The spectrometer will instead probe the behavior of narrow bands of frequencies and wavelengths, allowing us to pinpoint the effects of the nanoscale geometry. In our prototype spectrometer, we emit phonons of controlled wavelength into a silicon microstructure and detect those that traverse the sample ballistically.

This work involves a broad range of advanced experimental techniques. We are developing the components for our spectrometer using photolithography, electron-beam lithography, thin-film deposition and plasma etching at the Cornell NanoScale Science & Technology Facility (CNF). Operation of the spectrometer requires high-precision, low-noise measurements of the weak electrical signals developed by our phonon detector. The measurements are made at temperatures below 1 Kelvin in a liquid-helium cryostat to reduce the thermal background level and to enable the use of superconductors to generate and detect phonons.

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