Collaborative Research: All-Optical Fabrication of Low-Loss, High-Index-Contrast, Silicon-in-Silicon Waveguides

Title: All-optical fabrication of “silicon-in-silicon” waveguides

The goal of this project is to develop a direct laser writing method to produce three-dimensional (3D) optical waveguides embedded inside silicon with low propagation loss. Although low-loss 3D waveguides have been demonstrated inside glass, the typical loss for waveguides written inside silicon so far is more than one order of magnitude higher. Two main challenges faced by the research team are insufficient energy deposition and random material change inside silicon, which are tackled through novel beam delivery and fundamental understanding of material response under intense laser irradiation.

The direct laser writing method developed in this project simplifies fabrication procedures for silicon photonic devices, increases communication bandwidth, facilitates device miniaturization, and significantly enhances on-chip and chip-to-chip data processing capabilities. This method has the capability to be integrated with selective wet etching to fabricate microfluidic channels, enabling the integration of photonic, electronic and fluidic functionalities in a single chip.

The team’s strong connection with local and national photonics industries enhances the societal impact of the project by expediting lab-to-fab transition with the proposed technology. The research is tightly integrated with education through undergraduate and graduate student training, classroom teaching modules, and K-12 outreach events for future workforce development.

This grant supports basic research on laser-induced phase transformation in confined environment, with the goal to create low-loss, high-index-contrast, three-dimensional (3D) waveguides deep inside silicon (“Si-in-Si”). Current Si-in-Si waveguides have large loss and low contrast of refractive indices, making them unsuitable to be used in most photonics applications.

The poor performance is due to micro- and nano-scale inhomogeneities consisting of mixed Si phases driven by local temperature in the laser focal region. In this project, femtosecond-nanosecond laser pulses will be used to achieve energy density required for the transition to amorphous and high-pressure phases. Modelling, simulation and experiments will be conducted to identify transition pathways leading to thermodynamically stable Si phases.

A 3D splitter will be fabricated as a testing structure and its optical performance will be measured and compared with theory and simulation. This project will advance the understanding of (1) space-time confinement of ultrashort laser pulses in Si which exhibits high nonlinearity and strong two-photon absorption; (2) pressure-induced phase transition of Si, especially toward uncommon high-pressure phases; and (3) optical performance of waveguides with 3D architecture, such as bending radius and mode quality.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.