  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.
