Evaluating the Frequency Response of Nanoscale Thermocouples Using Temperature Oscillations in Nanoscale Heaters

We investigate the frequency-dependent response of nanoscale thermocouples (NTCs) thermally connected to nanoscale heaters. The heaters are energized using current at frequency w, which results in temperature oscillations of the heater at a frequency 2 ω (two heating cycles per current cycle). Furthermore, the voltage measured across the heater contains a 3 w component that is due to the product of the excitation current at w and the heat-induced resistance change of the heater at 2 ω. We simultaneously monitor the thermocouple response at 2 ω (in response to the heater temperature) and the temperature oscillations of the heater by measuring the third harmonic (the so-called 3 w method). In the experiment presented here, the upper limit of the heater frequency was defined as the point above which we can no longer accurately determine the 3 ω signal in the heater circuit. For our structures, this occurs at a current-excitation frequency of 4 MHz, where the upper limit of temperature oscillations of the heater is measured to be 8 MHz. We observe that the NTC response closely follows that of the heater, and conclude that the measured frequency response of the NTC is not limited by its own thermal inertia, but rather that of the heater, and is able to detect thermal oscillations with a frequency of at least 22 MHz.