Photon-impenetrable, electron-permeable: the carbon nanotube forest as a medium for multiphoton thermal-photoemission.

Combining the photoelectric and thermionic mechanisms to generate free electrons has been of great interest since the early days of quantum physics as exemplified by the Fowler-DuBridge theory, and recently proposed for highly efficient solar conversion. We present experimental evidence of this combined effect over the entire range spanning room-temperature photoemission to thermionic emission. Remarkably, the optical stimulus alone is responsible for both heating and photoemission at the same time. Moreover, the current depends on optical intensity quadratically, indicating two-photon photoemission, for intensities of ca. 1-50 W/cm(2), which are orders of magnitude below the intensities required for two-photon photoemission from bulk metals. This surprising behavior appears to be enabled by the internal nanostructure of the carbon nanotube forest, which captures photons effectively, yet allows electrons to escape easily.