Plasmonic devices based on vanadium dioxide for optical communication

Abstract

There is an ever-increasing demand in bandwidth and data rate in optical communications, internet, sensing, computing, machine learning applications and cloud computing. For chip-level communications, existing complementary metal oxide semiconductor (CMOS) electronic-based solutions are currently challenged and strained to its maximum limit. The maximum speed of the new 5G mobile connection is 10 Gbits/s. When the bit rates increase beyond 10 Gbits/s, copper tracks that carry current between the processing chips severely distort and attenuate the data signal due to parasitic resistance, inductance, and capacitance of the track. To overcome this challenge, the industry is increasingly looking towards silicon photonics as a key technology in next-generation communications systems, and data interconnects.

Silicon photonics is an emerging technology based on silicon chips, where photons (instead of electrons) are used to carry information in a CMOS chip. Photons can carry far more data in less time than electrons because it can be pulsed at higher frequencies than electric currents resulting in high bandwidth. Since both CMOS and silicon photonics are based on silicon, it is possible to integrate photonics circuits and electronic circuits in a single chip. Conventional on-chip silicon photonics-based functional elements such as filters, waveguides, laser sources, modulators and switches are still bulky compared to electronics process technology (14 nm process technology became available in 2014 and 10 nm in 2017) and hence there is a mismatch in the integration. This mismatch in the size of photonic components and transistor gates is a bottleneck in developing functional elements integrated with electronics on a chip. This requirement has recently triggered research on new device technologies where silicon photonics is combined with plasmonics and electro-optic materials to reduce the footprint, energy consumption and insertion loss as well as to increase modulation index (modulators) and the frequency of operation.

This thesis investigates the development of new plasmonic modulators and switches combined with vanadium oxide operating in optical communication wavelengths to reduce the footprint and increase the performance. Further research is carried out to study other suitable plasmonic geometries, which are optimised in optical communication wavelengths.

Optical modulation is at the heart of silicon photonics for on-chip optical communications where there is a trade-off exists among optical loss (attenuation), size and modulation depth, speed and footprint. This demands innovative approaches to research on CMOS compatible novel high-speed modulators for silicon photonics for next-generation communication devices. We first explored the plasmonic slot modulators using Vanadium Dioxide (VO2) as modulating material realised on silicon on insulator (SOI) wafer with 200 nm by 140 nm modulating section within 1 um by 3 um device footprint. By utilising the phase transition property of VO2, the modulator can achieve a broad working wavelength range from 1100 nm to 1800 nm, with modulation depth 21.5dB/um. We also explored the hybrid design of VO2 and different dielectric materials using the same base structure. Our device geometries can have potential applications in the development of next-generation miniaturised high-frequency optical modulators.

Then, we have shown for the first time, a photonic switch using vanadium dioxide as switching material with a hexagonal nanohole array structure, with a maximum 37% transmission at optical communication C, L, U band with numerical and experimental results. The wide operating range, high transmission and compact device footprint (thickness of 125 nm) give us more flexibility and efficiency in integration and application. The results will have potential applications in developing ultra-compact photonic switches in transmission mode.

Finally, we have presented a double coaxial aperture array (DAA) in a hexagonal geometry where fine-tuning of the resonance is achieved using two coaxial annular apertures. The demonstrated effective manipulation of plasmon resonances in the visible and near-infrared frequencies using the DAAs permits the realisation of new wavelength filters for the development of modulators and switches operating in transmission mode with a sensitive response via a simple geometry control.