Photonic Crystal Optical Frequency Combs

Nanophotonics, driven by low processing power and high-density integration, is emerging as the next logical step for photonic integrated circuits. Microwave photonics is a diverse research topic that is set to transform traditional microwave electronics. In this thesis, the two topics are combined by investigating the ability for nanophotonic devices to harbour and generate microwave photonic signals.

Generating an optical frequency comb (OFC) plays a vital role in integrated microwave photonics. By investigating the different OFC generation methods as well as the capabilities offered in photonic crystal (PhC) devices, a method to produce an OFC from nanoscale devices is proposed. The proposal is modelled using calculations based on temporal coupled mode theory. Analysis shows that a broadband OFC can be produced using nanoscale devices that favour high-density integration.

The experimental results in this thesis are based around three fundamental areas: PhC device fabrication, optical characterisation and microwave photonic characterisation. Each chapter builds towards the overarching theme of microwave photonic signal processing at telecommunication wavelengths in a nanophotonic device. A new fabrication process for etching PhC structures in nonthermalised InP samples is developed. The developed process has excellent applications in fabrication where the use of thermal grease or a high sample stage temperature is impractical. Optical characterisation of the fabricated samples shows the effects of lithographic and photothermal tuning on the cavity mode frequency. Through this analysis, resonant cavity modes can be designed for a working wavelength within the telecommunication bandwidth. Temporal analysis shows that carrier lifetimes from a QD ensemble that spectrally and spatially overlaps with the PhC cavity are greatly reduced.

Finally, a new measurement set-up is proposed and analysed for the characterisation of microwave photonic signals in nanoscale devices. It is shown that the integration of an ultra-fast laser and a two-arm Mach-Zehnder interferometer can generate a microwave signal within the spectrum of the ultra-fast laser. This signal is integrated into the standard optical characterisation set-up, where it is used to excite PhC optical devices. The results show that the microwave signal present in the ultra-fast laser can be resolved in the emission spectrum of QDs weakly coupled to a PhC cavity.