|M.Sc Student||Yosub Shay|
|Subject||Non Adiabatic Plasmonic Antennas for Enhancement|
of Detection Capability
|Department||Department of Electrical Engineering||Supervisor||Professor Meir Orenstein|
|Full Thesis text|
Collecting light efficiently from a large area into a nano-sized photodetector is a significant merit of plasmonic antennas, paving the way towards future high performance sensors as well as for contemporary applications, e.g. pixelated high quality imagers. CMOS imagers are evolving towards micron2 pixels with a much smaller sensing area (due to peripheral circuitry), and low light imagers - e.g. InGaAs/InP may rely on sensing area smaller than the pixel size to reduce noise. We show that a plasmonic antenna design for high collection efficiency, broad-band flat response and simple manufacturing is applicable to both. A starting point is one dimensional Metal-Insulator-Metal plasmonic tapers that were shown to collect efficiently light to the nano regime. However, a major hurdle in our application is due to the 2D light collection, resulting in modal cutoff when the structure is shrinking towards the smaller output port - which is not the case for the 1D taper - at least for its basic mode. Mitigating this difficulty requires engaging additional mechanisms.
The horn antenna is a well-known electromagnetic radiation collection structure in radio frequencies. It is comprised of a hollow aperture with metallic walls, which narrows adiabatically towards a smaller aperture. However, to collect light efficiently, narrow flare angle is needed, requiring tens of microns high structures that are implausible to realize in optics; therefore, a major challenge is to make the antenna planar. A flat horn antenna with no extra mechanisms is abrupt with poor collection efficiency and high back-reflections. To mitigate, we apply additional concepts for a non adiabatic horn antenna, reaching in simulation 73% efficiency at a height of 2.2μm, increasing absorption of a 5μm InGaAs detector by 6-fold. The structure was simulated using FDTD simulation and materials constants (including losses) were taken from experimental data. Experimental measurements of the non adiabatic antenna integrated with a real detector showed an average improvement of 4-fold in absorption for λ=1.55µm, compared to 5.5-fold in theory, and an average 4.6-fold enhancement over a spectrum of 1100nm-1700nm.
Finally, the horn was scaled to collect a diameter of 1 micron to a nano-size absorber. By efficiently transforming propagating modes into localized evanescent fields, and owing to additional plasmonic effects, absorption is increased to levels exceeding geometrical aperture factors, over passing diffraction limit. A factor of 22,000-fold increase in absorption was shown for 10,000:1 concentration into Si (1μm to 10nm in diameter), equivalently collecting over 200%.