This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Photonic integrated circuits route information with optical signals instead of relying only on electrical currents. Silicon (Si) and silicon nitride (SiNx) are excellent photonic platforms for waveguides, but they cannot efficiently perform the ever-increasing tasks required for fully integrated optical systems.
A heterogeneous photonic integration platform capable of interfacing different optical materials with high performance is thus an ongoing challenge for both academia and industry. Conventional approaches use heteroepitaxy to realize thin-film functional materials on target optical substrates. However, this method confronts fundamental challenges in lattice matching and process compatibility, and substantially deteriorated epilayer material quality is often observed in mismatched photonic chips.
In our recent work published in Nature, a group of researchers from Washington University in St. Louis (WUSTL), the Swiss Federal Institute of Technology Lausanne (EPFL), and the Massachusetts Institute of Technology (MIT) demonstrated a different strategy: preparing desired thin-film materials on their most suitable parent substrates, then delaminating them into freestanding single-crystalline nanomembranes for unbridled heterogeneous photonic integration on arbitrary photonic templates. Our method leverages van der Waals (vdW) forces from artificially defined freestanding 3D films—weak interactions compared with chemical bonds that allow exceptionally thin layers to adhere without the strict atomic matching required by conventional crystal growth.
A wide library of films, acting as designer Lego bricks, was heterogeneously integrated through either vertical stacking or lateral stitching to infuse previously inaccessible optical functionalities, such as electro-optical (EO) modulation, magneto-optical (MO) isolation and wide-band photodetection, into established photonic platforms such as Si and silicon nitride SiN photonics. Single-crystalline barium titanate (BaTiO3, BTO), a ferroelectric material with a strong linear EO (Pockels) response, was transferred onto a prefabricated Si photonic chip. By controlling the crystallographic orientations of high-quality BTO nanomembranes in a hybrid Mach-Zehnder modulator (MZM), we realized a half-wave voltage-length product (VπL) of about 0.29 V·cm, corresponding to a giant effective Pockels coefficient reff of about 950 pm/V, more than 30 times higher than established EO platform materials such as lithium niobate and lithium tantalate.
Another separately optimized traveling-wave MZM device achieved a 3-dB EO bandwidth above 23 GHz, showing that preserving high crystal quality and controlling the ferroelectric domain orientations of thin-film barium titanate can deliver both high efficiency and high-speed operation with minimized trade-offs. In addition, using remote epitaxy and epitaxial lift-off, single-crystalline gallium arsenide (GaAs) and gallium nitride (GaN) nanomembranes were also vdW-integrated on silicon nitride (SiNx) photonics, showing an expanded photodetection range from near-ultraviolet to near-infrared wavelengths. Thin-film cobalt ferrite (CoFe2O4, CFO) single crystals were heterogeneously integrated with silicon microring resonators as well.
The CFO membrane produced a giant extracted Faraday rotation coefficient of about 33,800 degrees per centimeter as an MO material, enabling on-chip nonreciprocal optical transmission with an extinction ratio above 18 dB. The CFO magnetization can remain after external magnets are removed. Finally, to illustrate the high degree of freedom in the physical assembly of different functional 3D layers, analogous to the construction of 2D vdW heterostructures, we vertically stacked BTO and CFO nanomembranes above a single silicon microring.
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