Cola, Baratunde A.

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    An Optical Rectenna
    (Georgia Institute of Technology, 2015-08-25) Cola, Baratunde A.
    An optical rectenna – that is, a device that directly converts free‐propagating electromagnetic waves at optical frequencies to d.c. electricity – was first proposed over 40 years ago, yet this concept has not been demonstrated experimentally due to fabrication challenges at the nanoscale. Realizing an optical rectenna requires that an antenna be coupled to a diode that operates on the order of 1 petahertz (switching speed on the order of a femtosecond). Ultralow capacitance, on the order of a few attofarads, enables a diode to operate at these frequencies; and the development of metal‐insulator‐metal tunnel junctions with nanoscale dimensions has emerged as a potential path to diodes with ultralow capacitance, but these structures remain extremely difficult to fabricate and couple to a nanoscale antenna reliably. Here we demonstrate an optical rectenna by engineering metal‐insulator‐metal tunnel diodes, with ultralow junction capacitance of approximately 2 attofarads, at the tips of multiwall carbon nanotubes, which act as the antenna and metallic electron field emitter in the diode. This demonstration is achieved using very small diode areas based on the diameter of a single carbon nanotube (about 10 nanometers), geometric field enhancement at the carbon nanotube tips, and a low work function semi‐transparent top metal contact. Using vertically‐aligned arrays of the diodes, we measure d.c. open‐circuit voltage and short‐circuit current at visible and infrared electromagnetic frequencies that is due to a rectification process, and quantify minor contributions from thermal effects. In contrast to recent reports of photodetection based on hot electron decay in plasmonic nanoscale antenna, a coherent optical antenna field is rectified directly in our devices, consistent with rectenna theory. Our devices show evidence of photon‐assisted tunneling that reduces diode resistance by two orders of magnitude under monochromatic illumination. Additionally, power rectification is observed under simulated solar illumination. Numerous current‐voltage scans on different devices, and between 5‐77 degrees Celsius, show no detectable change in diode performance, indicating a potential for robust operation.