Photo-thermal Self-oscillations in Cavity-Coupled Carbon Nanotube pn-Devices

We observe photothermal self-oscillations in individual, suspended, quasi-metallic carbon nanotube (CNT) pn devices irradiated with focused CW 633nm light. Here, the bottom of the trench forms an optical cavity with an anti-node at lambda/4. Oscillations arise from the optical heating of the nanotube, which causes thermal contraction of the nanotube (negative thermal expansion coefficient). This, in turn, moves the CNT out of the anti-node (maximum field intensity), where the nanotube cools to a lower temperature. It then expands and returns to the maximum field intensity anti-node where it is optically heated once again. The oscillations are observed through a change of the tunneling current in the CNT device. A pn-junction, established by two electrostatic gates positioned beneath the nanotube, results in Zener tunneling, which depends strongly on temperature. A Zener tunneling model with oscillating temperature shows good agreement with our measured I-V curves, providing further evidence that these oscillations are photothermal in nature.