Plasmon Resonances in Metallic Nanostructures for Photodetection and Signal Modulation
Nordlander, Peter J.
Doctor of Philosophy
Metallic nanostructures can strongly interact with the light and exhibit fascinating optical properties due to inherent collective oscillations of electrons in metals, also known as plasmon resonances. Since the plasmons are capable of confining light into a small regime and meanwhile significantly enhancing its field intensity, the metallic nanostructures can be widely used for light harvesting and manipulation. By placing gold gratings on top of a silicon substrate, hot electrons created from plasmon decay can efficiently go across the Schottky barrier and be harvested by the silicon, leading to a substantial photocurrent. This yields a good photodetector which not only possesses a narrowband photoresponse due to the plasmon resonances but also has the ability to work at wide frequency range even below the bandgap of the silicon. Moreover, instead of the top-gratings structures, embedding gold nanostructures into the semiconductors will effectively increase the photoresponsivity. Theoretical calculation shows that the embedment can lead to an increase in the surface area of the Schottky barrier and at the meantime broaden the directional range of the emitted hot electrons able to transport across the Schottky barrier. More importantly, the vertical Schottky barrier is found to be the predominant area where photoemission take places. Aside from creating hot electrons, the plasmons can also influence the performance of the photodetection by facilitating the generation of electron-hole pairs directly in the semiconductors. Here, the aluminum gratings are demonstrated to serve as good color filters when they are integrated with metal-semiconductor-metal photodiodes. The interference of plasmon near-field and incident field could either block or assist the light going through the aluminum gratings to hit the photodiodes. As for light manipulation, the metallic nanostructures act just like optical nanoantennas whose photoresponse can be modulated by loading optical materials in them. The corresponding modulation process can be described in terms of optical nanocircuitry in which various materials are represented by capacitors, inductors, and resistors. With the help of the optical nanocircuitry theory, optical nanofilters become convenient and straightforward to design and build. In addition, substrate also can strongly modify the optical response of the nanoantenna. It has been proven that a conductive substrate will blueshift and reduce the original plasmon resonances and meanwhile bring in a new charge transfer mode appearing at low energy level. Given that plasmon resonances can effectively harvest light and modulate optical signal, they may have promising applications in sensing, imaging and communication systems in the near future.
Plasmonics; Photodetector; Metallic Nanostructures; Optical Signal Modulation