All-dielectric phase-change-material metagratings for non-volatile reconfiguration of infrared light beams

Chalcogenide phase change materials (PCMs) have unique properties that make them ideal for use in data storage devices. When these materials are heated, they can be transformed from a disordered amorphous phase to a highly ordered crystalline phase, which changes their electrical and optical properties. The change in properties can be used to store information in the form of binary digits (0s and 1s), which are read by detecting the difference in optical or electrical properties between the amorphous and crystalline phases. These materials are typically composed of a mixture of elements from the chalcogenide group of the periodic table, such as sulfur, selenium, and tellurium, along with other elements like germanium, antimony, and silver.

One of the main advantages of using chalcogenide PCMs in data storage is their ability to rapidly switch between amorphous and crystalline states, which allows for high-speed read and write operations. Additionally, these materials can retain their states for long periods of time, making them ideal for non-volatile memory applications. As a scalable technology, chalcogenide PCMs have shown promise to enhance or replace traditional data storage devices, from small-scale USB drives, HDDs or SSDs to large-scale data centers. As part of their revolutionary impact on the electronics industry, chalcogenide PCMs have also inspired further research in integrated photonics, especially the study of reconfigurability/tunability with different components for programmable photonics.

However, typical chalcogenide PCMs like GST are more suitable for the near-infrared or visible part of the spectrum since these can be effectively switched with stability between the amorphous and crystalline phases only for thicknesses up to about 300 nm. Nevertheless, there is a growing need for reconfigurable beam manipulation in the mid-wavelength to long-wavelength infrared (MWIR-LWIR) spectral regime. Previous findings revealed that a particular alloy of the Ge-Sb-Se-Te family (GSS4T1), developed by the group of Juejun Hu at MIT, has a wider band gap with a fully dielectric response with no residual free-carrier absorption for free space wavelengths greater than 5 µm. GSS4T1 can operate at larger thicknesses of about 1 µm and further exhibits an astonishingly large refractive index shift, of more than 1.4, when switched from the amorphous to the crystalline phase.

GSS4T1 appeared to have all the ideal characteristics as a PCM of choice for all-dielectric MWIR/LWIR reconfigurable platforms. “I was very excited to hear about this new PCM, GSS4T1. I was curious to explore what capabilities can it bring about when paired with the powerful beam tailoring capabilities of metagratings. Prof. Tsitsas had previously shown with colleagues that a metagrating can bend a beam back into the negative direction. I wondered whether we can use a metagrating to switch such a negative beam steering on and off for an infrared light beam,” said Dr. Foteinopoulou.

On this account, Dr. Stavroula Foteinopoulou from University of New Mexico together with Professor Nikolaos Tsitsas from Aristotle University of Thessaloniki proposed a new all-dielectric grating paradigm system which comprises GSS4T1 and is functional in the LWIR-MWIR spectrum. This paradigm leverages the aforementioned capabilities of the newly discovered GSS4T1 together with the power redirection capabilities of metagratings. The authors showed that due to the incorporation of the all-dielectric diffractive metagratings, the platform exhibited both reconfigurable beam splitting and beam steering. The physical principles and key attributes responsible for these phenomena were analyzed. Their work is currently published in the research journal, Optical Materials Express.

The functionality of this paradigm mainly relied on the lossless dielectric properties characterized by higher permittivity shifts between the GSS4T1 phases. Moreover, the non-volatile reconfigurable capabilities of the platform were attributed to the different physical responses of the grating structure upon switching between the GSS4T1 phases (amorphous and crystalline). The operating wavelength of the reconfigurable beam manipulation could be tuned across the 5 – 10 µm spectrum by appropriately scaling the feature sizes of the metagrating.

As an effective homogenous slab, the amorphous-GSS4T1 grating allowed light through without path perturbation. In contrast, the crystalline-GSS4T1 grating promoted leaky Floquet-Bloch modes. These modes permitted destructive interference into the primary light path and constructive interference into the back-bent diffraction path, all simultaneously at a certain wavelength during transmission, thus steering the beam into the negative direction. It was found that such an interference effect possessed some similar characteristics to Friedrich-Wintgen interference associated with quasi-BIC (bound states in continuum) phenomena. At a slightly detuned wavelength, the output power could be split evenly between the primary light path and the back-bent diffraction channel. These phenomena endowed the proposed GSS4T1-based metagrating platform with non-volatile reconfigurable beam steering and splitting capabilities.

In summary, Foteinopoulou and Tsitsas reported a novel all-dielectric metagrating paradigm for non-volatile reconfigurable photonics in the 5 – 10 µm spectral range. Overall, the results were relevant to a wide range of MWIR/LWIR devices and applications. The findings about the Friedrich-Wintgen interference effects offer important transferable insights for the design of related metasurfaces. In a statement to Advances in Engineering, the authors explained their study could potentially inspire the design of new means for programmable and reconfigurable photonics across the spectrum.