SIMULATION OF ULTRAFAST FIELD-EMISSION PLASMONIC NANOANTENNAS FOR PHz PROCESSING OF OPTICAL WAVEFORMS

SIMULATION OF ULTRAFAST FIELD-EMISSION PLASMONIC NANOANTENNAS FOR PHz PROCESSING OF OPTICAL WAVEFORMS This thesis project studies the behaviour of ultrafast nanoscale plasmonic electron emitting antennas in free-space and focuses on the design and optimization of coupling to waveguide modes for potential operation within an integrated photonics platform. The main aim of the device is to process impinging ultrafast optical pulses and to extract quantitative information on their features, in particular the carrier-envelope phase (CEP). The electron emitters consist of a series of electrically interconnected 20nm-thick Au antennas, shaped as isosceles nanotriangles, with the sharpest tip facing a perpendicular collector nanowire, at ~45nm distance. The device exploits plasmonic and geometric enhancement effects that convey electric field at the sharpest tip of the triangles. The enhanced field lowers the metal work-function at the tips and can drive optical-field photoemission toward the collector wire. The emission can be approximated by a quasi-static tunnelling rate and is calculated by a quasi-static Fowler-Nordheim (FN) emission model. The analysed optical waveform that illuminates the antennas is an ultrafast 10fs laser pulse (~2.5 optical cycles FWHM), polarized along the altitudes of the triangles, with central wavelength of λ = 1177nm (or 1550nm when the device is integrated to a waveguide). In this configuration, current is emitted when the field is directed from the tip to the wire and not vice versa, because the side of the wire does not undergo enough plasmonic and geometric enhancement to cause photoemission. Initially, the behaviour of interconnected antennas is studied in free-space. As a first step, the fundamental interaction of light with the device is analysed, in order to understand its electromagnetic response. This is done by a frequency-domain simulation which studies the device frequency response, in particular its field enhancement at the antennas tips due to plasmonic effects and the extinction plot, that represents the power absorbed by the plasmonic resonance. Subsequently, the antennas are studied as totally integrated within an integrated photonics platform, in particular on top of a SiN-core, SiO2-cladding waveguide. The geometry of the antennas is modified by substituting the collector wire with another triangle, so that the sharpest tips of each triangle are facing each other, at ~45nm distance, to form a bowtie-like structure. The device is hence composed of a series of electrically interconnected bow-tie antennas on top of a SiN waveguide. The spacing at which the bow-tie antennas are placed along the waveguide is studied, taking into account CEP shift due to material and waveguide dispersion, in order to obtain the same CEP value at each antenna and eventually have a coherent CEP-sensitive current. In this configuration, photoemission occurs from both triangles tips due to field directed from one tip to the other and vice versa which leads to higher CEP sensitivity. A frequency-domain analysis is performed on the device in order to extract field enhancement at tips. This information is important because when the pulse impinges the antennas, its different frequency components are enhanced in different ways and this impacts on the total emitted current. Finally, a time-domain simulation is done to compute the enhanced pulse and the FN emission model is used for studying the photocurrent.