New Fabrication Approach for Photonic Circuits

Illustration of photonic integrated circuits (PICs) made of hydrogenated amorphous silicon. (Source: TU/e)

Photonic integrated circuits (PICs) carry signals via visible and infrared light. Optical materials with adjustable refractive index are essential for reconfigurable PICs as they allow for more accurate mani­pulation of light passing through the materials, leading to better PIC performance. Current programmable PIC concepts suffer from issues such as vola­tility and/or high optical signal losses – both of which negatively affect a material’s ability to keep its programmed state. Using hydro­genated amorphous silicon (a-Si:H), a material used in thin-film silicon solar cells, and the associated Staebler-Wronski effect (SWE), which describes how the optical pro­perties of a-Si:H can be changed via light exposure or heating, researchers at Eindhoven Uni­versity of Tech­nology have designed a new PIC fabri­cation process that addresses the shortfalls of current techniques and could lead to the emergence of universal programmable PICs.

According to Oded Raz, Associate Professor at the Department of Electrical Engi­neering and research lead for this project, this approach could be of paramount importance for the field of PICs. “This is the world’s first demons­tration of a reconfigurable PIC, where the material chosen for making the inte­grated optical circuit is being programmed”. Mahir Asif Mohammed also notes that the yield from existing approaches for the fabrication of PICs is typically very low. “Our method can signi­ficantly improve this yield”.

This revo­lutionary new approach could herald a wave of further inves­tigations on recon­figurable PICs and it has further advantages. “Most impor­tantly, in comparison to current methods, the time to proto­type is much shorter and much more accurate”, says Raz. “As we continue to work on the method, we predict that the time to prototype will continue to decrease”, adds Mohammed.

The researchers also point out that heaters can be placed on a pre-light exposed device to allow the user to program a PIC device as desired. The same heaters can also reset the device and return it to a state that can be easily reprogrammed. “Our approach promotes reusable and sustainable use of materials”, says Mohammed. Crucially, as pointed out by Raz, “This approach allows the user to easily program the func­tionality of a PIC and to concurrently correct for small errors in the fabri­cation process. You can just tune the func­tionality and it’s there!”

To assess the effec­tiveness of light exposure and heating a-Si:H to tune its optical properties, the researchers first considered a proof-of-concept experiment where they studied changes in the refractive index of a thin layer of a-Si:H on a silicon substrate. The material experienced cycles of heating – for four hours in the dark in a nitrogen atmo­sphere – and light soaking – via a tunable laser in the near-infrared range – treatments. The experiment showed a reversible refractive index change of about 0.001 – a key require­ment for the fabrication of recon­figurable PICs.

Next a recon­figurable optical switch based on a micro-ring resonator (MRR) which was subject to cycles of light soaking and heating treatments also showed repeatable rever­sibility. And finally to better understand the cause of the rever­sible refractive index changes, the researchers examined variations in the structure of a 1-dimensional membrane where the main contributor to the switching states of the MRR device is shown to be metastable volumetric expansion. (Source: TU Eindhoven)

Reference: M. H. Mohammed et al.: Metastable Refractive Index Manipulation in Hydrogenated Amorphous Silicon for Reconfigurable Photonics, Appl. Opt. Mat. 8, 1901680 (2020); DOI: 10.1002/adom.201901680

Link: Electro‐Optical Communication Group, Dept. of Electrical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands

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