The hetero-integration of functional optical materials with photonic structures not only underpins massive high-performance optoelectronic applications, but also enables an ideal playground to explore optical coupling and polaritonic physics.
The concept of van der Waals (vdW) integration was originally proposed in two-dimensional (2D) materials communication for the physical assembly of 2D vdW building blocks to design electronic devices and heterostructures . Recent advances on 2D-materials assisted epitaxy [2, 3] and layer lift-off technology [4, 5] have enabled a wide spectrum of single-crystalline thin-films that can be delaminated from the substrate into freestanding forms with artificially defined vdW interfaces . This unveils exciting perspectives that 3D crystals can be also made into ultrathin, flexible, and transferrable to arbitrary substrates and photonic structures for advanced optical and optoelectronic applications [7, 8]. Novel vdW heterostructures are also enabled by combining those 2D and 3D freestanding nanomembrane building blocks  to prototype record-setting hetero-integrated photonic devices in previously inaccessible layouts, or exploring exotic nanophotonic physics hatching from mixed-dimensional vdW interfaces .
In a recent Review paper published on Nature Reviews Materials, a group of authors from USA, Singapore, and Korea presented a comprehensive survey on latest progress of photonic vdW integration of 2D and 3D freestanding nanomembranes. The 3D freestanding films also possess signature vdW interfaces like 2D materials. However, distinctive from 2D materials that have natural layered lattices bounded by vdW interactions, 3D freestanding films has chemical bonds throughout the film (Fig. 1, top) and they are artificially delaminated from parent substrates via diverse layer lift-off techniques [3, 4]. They can be readily transferred to prefabricated photonic structures such as optical (or plasmonic) waveguides, micro-resonators , photonic crystals, and metasurfaces , to name a few (Fig. 1, bottom).
A vast collection of currently available 3D freestanding films with diverse optical functionalities are also summarized (Fig. 2). Fundamental photonic properties of these vdW films and their hybrid heterostructures are catalogued with representative applications and exemplary functionalities, such as the optical gain, photovoltaic, electro-optical (EO) or magneto-optical (MO) medium, creating new opportunities in advanced photonic hetero-integration. Detailed guidelines from 2D and 3D vdW film preparation to device implementation are outlined as well.
Awaiting challenges in this field for scalable 2D and 3D nanomembranes manufacture and layer transfer, as well as emerging opportunities for advanced vdW hetero-integration, flexible, and bio-compatible photonic applications are also discussed based on current perspectives. For additional information, please refer to the review paper with information below.
Corresponding authors: Prof. Sang-Hoon Bae (WUSTL), Prof. Cheng-Wei Qiu (NUS), Prof. Lan Yang (WUSTL), Prof. Jin-Wook Lee (SKKU).
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