Chiral junctions of carbon nanotubes have the potential of serving as optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting–metallic junction. We image topography, laser absorption at 532 nm, and measure I–V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting–metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction.