PhD defence of Yannick D'Mello - Harvesting solar energy on a silicon photonic chip

Abstract

An hour of sunlight can satisfy global power consumption for a year, yet solar energy contributes to only 2% of electricity generation. This gap is due to the high price per watt of capture, conversion, and retention. Solar panels capture direct sunlight, but this depends on weather conditions. A fraction of light is converted into electricity as determined by the absorption bandwidth of silicon (Si). Electrical energy is then retained in batteries whose functionality is constrained by the fermionic nature of electrons. These limitations on the electrical process of harvesting solar energy motivate us to reconsider it as an optical process.

In this thesis, we show that ambient light can be captured into confined modes, retained by exploiting the bosonic nature of photons, and also converted into kinetic energy of free electrons. Rather than converting energy from direct sunlight to bound electrons, our novel scheme converts energy from ambient light to free electrons. It employs complementary metal oxide-semiconductor technology to ensure inexpensive mass-manufacturability and leverages the maturity of the Si photonic (SiP) platform to design scale-invariant, optical devices. We present this scheme as a SiP circuit consisting of 6 devices which perform the following functions: (i) capture ambient light into confined modes, (ii) split the modes based on polarization, (iii) rotate one polarization, (iv) match the phases, (v) combine them into a single mode, and (vi) convert the energy to free electrons.

(i) To capture ambient light, we analyze the solar energy harvesting mechanisms of naturally occurring, biosilica frustules in diatoms. We find that sub wavelength structures localized in the frustule produce a combined response to enhance optical capture, redistribution, and retention in the cell by 9.83%. This shows how the silica cladding of a SiP chip can enhance free-space coupling to the devices on-chip. (ii) To split the fundamental transverse electric (TE0) and transverse magnetic (TM0) modes, we demonstrate an on-chip polarization beam splitter. Our design offers a high fabrication tolerance in a compact form factor resulting in an insertion loss of 2 dB and extinction ratio of 11.45 dB over a wavelength range of 1500-1600 nm. (iii) To rotate the TE0 mode towards TM0, we demonstrate an on chip electromagnetic coil which uses 14 mA of current to generate an alternating magnetic flux density up to 1.16 mT inside a strip waveguide. We calculate a Faraday rotation of 34.65 pico-degrees at 1550 nm over an interaction length of 1097.4 μm. Our analysis also reveals ways to increase the rotation by orders of magnitude. (iv) To phase-match both polarization branches, we design a dual polarization phase shifter to induce the Pockels effect in an electro optic polymer. Simulations show a phase shift of 1.35 radians per 20 V over an interaction length of 8 mm. (v) The two branches are then combined (demonstrated but not included). (vi) To convert light into electricity, we design an on-chip device to maximize the overlap between an exposed TM0 supermode in a slot waveguide with co-propagating free electrons in an electron microscope. We optimize the coupling efficiency over the interaction length to predict either an unprecedented acceleration gradient of 3.81 GeV/m or an energy gain of 43.68 keV. This increase in kinetic energy of the electron represents an increase in electric current.

Our novel device designs already offer direct applications to a variety of fields including telecommunications, sensing, and quantum information science. Their separate applications incentivize further development, which is supported by the modularized design of our circuit. Hence, this thesis provides a starting point on the roadmap towards harvesting solar energy on a SiP chip.