Photonic crystal enhanced silicon photonics

Abstract

With silicon waveguides providing an on-chip platform and low-loss for optical signal processing, silicon photonics has the potential to increase bandwidth density and reduce energy consumption in short-distance communications and information processing. Furthermore, it facilitates faster and more cost-effective interconnections between data centres by overcoming the limitations of traditional transceivers.

Photonic Crystals (PhCs) are periodic optical nanostructures designed to control the propagation of light waves. This periodic configuration generates a photonic band gap (PBG), which restricts the propagation of light with frequencies within the gap. Furthermore, PhCs exhibit slow-light effects, allowing for more efficient control and manipulation of light. The PhC-based devices enable strong temporal confinement of light, which is capable of meeting and even surpassing the performance of standard waveguides, Fabry-Perot cavities, Ring Resonators (RRs), and plasmonic resonators.

The long-term objective of our research group is to develop a photonic crystal modulator (PhCM) that incorporates PhC structures into a conventional silicon optical modulator. Recognizing the essential role of photonic crystal waveguides (PhCWs) in PhCMs, particularly due to their transmission and slow light properties, this project focuses on the design, fabrication, and characterization of PCWs.

An effective methodology was developed to optimize PhCW transmission and time delay properties by comparing two approaches: the Plane Wave Expansion Method in conjunction with the Finite Difference Time Domain Method. Four sets of PhCWs were thus designed using different (slab/hole) material configuration, including Si/Air, Si/SiO2, Ta2O5/Air, and Ta2O5/SiO2.

Since Ta2O5 operates within the visible light range, this property facilitates the experimental setup and testing of the fabricated PhCW. The Ta2O5-based PhCW was thus prioritized and fabricated first among the four design PCW sets. Four types of Ta2O5-based PhCWs with different cladding deposition techniques were fabricated. A key achievement during the fabrication process was the thorough analysis of how various parameters—RF platen power, ICP power, C4F8: O2 gas ratio, and chamber pressure—impact the outcome of Ta2O5 PhCW etching using Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE). This analysis was conducted through a Minitab-suggested 4-factor half-factorial Design of Experiment (DOE) for Ta2O5 etching, followed by individual parameter sweeps. As a result, a smooth and vertical sidewall profile for the Ta2O5 PhC was successfully achieved.

During transmission measurements, PhCWs posed significant challenges in optical alignment and troubleshooting compared to standard tapered waveguides. The lowest measured propagation loss occurred with a PhCW featuring a photonic crystal lattice pitch of 330 nm and a diameter of 230 nm, at 565 nm for TM polarization, with a loss of 4.1 dB/cm. A LabVIEW interface was developed and configured to facilitate the time delay measurement by sweeping the linear stage and collecting spectrometer data at each specified position. Furthermore, a specialized Python script was created and optimized to enhance the efficiency of data-processing of the zero-time delay position. In the collinear setup, the zero-time delay position was measured to be at 70.75 mm, but it is non-reproducible. However, time delay measurement is a highly complex experiment. The current process has established a foundational framework for conducting zero-time delay position measurements. Considerable work remains in troubleshooting and optimizing the experimental setup and procedures.

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Identifiers

ORCID for Wenjie Wang: orcid.org/0000-0001-5739-3085

ORCID for Harold Chong: orcid.org/0000-0002-7110-5761

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