Development of antimony selenide-based optical phase-change technology for silicon photonics

Abstract

In this thesis, the phase change chalcogenide antimony selenide (Sb2Se3) is optimised for use in silicon photonic integrated chips. Trialling a novel waveguide structure where the silicon waveguide is etched prior to deposition of Sb2Se3, both the etch depth and deposition thickness are investigated to improve the modulation contrast when Sb2Se3 is switched.

Sb2Se3 is a member of the chalcogenide family of novel phase change materials optimised for optical applications, which is of particular interest for its valuable refractive index contrast and low losses. When integrating this material into silicon photonic devices, it is typically deposited on the surface of waveguides and modulates light via evanescent coupling. In this thesis, Sb2Se3 is embedded directly into the waveguide, resulting in an improved overlap between the device structure and the mode of the light. This enables greater modulation when the material is switched between its two solid states. Optical switching is enacted by using laser pulses at 639 nm, with the optimal pulses for crystallisation requiring a duration of milliseconds, and those of amorphisation requiring nanoseconds.

Optical characterisation is performed to demonstrate the improved overlap between device and mode of light created by embedding Sb2Se3 into the waveguide. Initial spectral analysis showed a change in propagation loss between the amorphous and crystalline states of 0.8 dB through a straight waveguide. Further spectral analysis found that increasing the thickness of Sb2Se3 increases the propagation loss per unit length of Sb2Se3 on a waveguide from 0.003 dB per μm for a 20 nm thickness to 0.280 dB per μm for a 100 nm thickness. Optical switching of Sb2Se3 on a Mach-Zehnder interferometer caused a wavelength shift of 17.5 nm for initial crystallisation and a reversible wavelength shift of 6 nm for an 80 nm thickness of Sb2Se3, which is combined with earlier loss measurements into a figure of merit to take into account both loss and phase shift due to change of material state. Finally, selective switching of Sb2Se3 embedded in the multimode region of MMIs showed a change in the splitting ratio between the output couplers from 50 : 50 to 80 : 20, with the closest agreement to theory found where crystalline pixels were written on an amorphous background. This is complemented by simulation of the propagation of light through various configurations of Sb2Se3 in silicon waveguide devices and characterisation using SEM and EDX methods.

It is shown by this thesis that using a thicker layer of Sb2Se3 in silicon photonic waveguide devices allows for greater modulation of light travelling through such devices, with a trade-off due to larger volumes of Sb2Se3 increasing the propagation loss through the device. Embedding Sb2Se3 into silicon photonic devices rather than layering on top allows a larger thickness of the material to be included, so greater modulation is possible. This means that fewer areas are required to be switched, reducing the complexity of a memory device designed in this way. This work will enable progress in the realms of photonic computing and silicon chip design as a novel way to improve optical memory storage and tunability on-chip.

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Identifiers

ORCID for Ioannis Zeimpekis: orcid.org/0000-0002-7455-1599

ORCID for Otto Muskens: orcid.org/0000-0003-0693-5504

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