“Nanostructures are so small that their temperature can change quite rapidly,” Dan Schmidt told nanotechweb.org. “We needed to develop thermometry techniques that can fit on a nanoscale device and that are fast enough (10-100 MHz) to keep up with the rate of temperature change.”
To achieve this measurement speed, Schmidt and colleagues connected the tunnel junction to an electrical resonant circuit. The tunnel junction itself contained a 90 nm thick aluminium layer (which acted as the superconductor) and a 90 nm thick copper layer (which acted as the normal metal). The overlap area was 0.3 x 1.0 square microns. At temperatures sufficiently below the superconducting transition temperature the junction’s resistance depended exponentially on the temperature, making the device very sensitive.
“Previous results were only able to measure the average temperature at timescales associated with low audio frequencies (milliseconds),” said Schmidt. “Our thermometer can track the instantaneous temperature at radio frequency timescales (10-100 nanoseconds). This is fast enough that the thermal time constant of the device is the limiting factor, not the time constant of the measuring circuit.”
According to Schmidt, determining the thermal time constant permits measurement of the heat capacity of the nanodevice, i.e. how much energy is required to change its temperature. The researchers say that by making devices with integrated heaters, they have measured the smallest heat capacity to date - about 1 femtojoule/Kelvin. Now they plan to reduce that heat capacity by orders of magnitude and “get into the regime where it might be possible to measure the heat capacity of a single spin”.
Another possible application is in radiation detection - specifically in counting far-infrared photons, an area for which Schmidt says a technology has yet to be demonstrated.
“We can make devices that heat by a measurable fraction of a degree when the energy from a single photon from the far-infrared portion of the electromagnetic spectrum is absorbed,” he explained. “The arrival of a photon would produce a fast but measurable blip in the temperature. Keep track of the blips and you can count photons.”
The researchers, who reported their work in Applied Physics Letters, are beginning to characterize devices with antennas to facilitate coupling in photon-counting experiments.