The group has found that these junctions can be used to rectify AC voltages for frequencies that go from the infrared up to the visible. This opens up the possibility of building optical diodes to couple photonic and electronic circuits. It also provides a solution for converting the energy of solar radiation into useful DC current.
The rectification properties of the junctions considered in this work can be traced to their geometrical asymmetry. The electric field of the incident radiation is magnified by the sharp tip, which results in a significant circulation of current in the junction when the field is oriented downwards. The flat metal that faces the tip, however, does not magnify the field of the incident radiation and a smaller circulation of current is achieved when the field is oriented upwards. There is therefore a net flow of DC current when these junctions are subject to an oscillating field.
This idea works as long as the electrons can cross the junction before the field present in the junction changes sign. When the gap spacing is of a few nanometres only, the time taken by electrons to cross the junction is of the order of a femtosecond (this is the typical estimate of tunnelling times). This makes it possible to rectify radiation with frequencies up to 1015 Hz.
Simulation and analysis
To explore the concept in more detail, the researchers performed quantum-mechanical simulations of asymmetric metal-vacuum-metal junctions. They considered the rectification of monochromatic radiation as well as the rectification of a full distribution of frequencies in order to simulate a focused beam of solar radiation. The scientists found that a significant rectification of incident radiation does indeed occur for frequencies ranging from the infrared up to the visible.
The team also analysed the efficiency with which the energy of incident radiation is converted by the device. They obtained quantum efficiencies as high as 25% for this energy conversion and the results suggest that even better performances can be expected if larger protrusions are considered.
The occurrence of polarization resonances in the tip also improves the rectification properties of the junction. Therefore, it appears that the dependence of these polarization resonances on the material and the physical dimensions of the tip could be used to control the frequencies at which the device is especially efficient for the energy conversion of electromagnetic radiation. This opens up the possibility to build diodes of the type presented in this study for the rectification and energy conversion of infrared and optical radiations.
Further details can be found in Nanotechnology.