Multi-port Silicon Photonics Using Antimony Tri-selenide Achieves Low-loss, Reconfigurable Operations for Next-generation Technologies

Reconfigurable photonic devices represent a crucial advancement for technologies ranging from complex simulations to next-generation computing, and researchers are actively seeking materials that enable compact and efficient control of light. Thomas W. Radford and Idris A. Ajia, from the University of Southampton, alongside Latif Rozaqi, Priya Deoli, Xingzhao Yan, and Mehdi Banakar, demonstrate a significant step forward by utilising antimony tri-selenide, a material exhibiting exceptionally low optical loss. The team successfully encodes multi-port operations onto a silicon photonic chip, creating devices capable of manipulating light with unprecedented precision and scalability. This innovative approach achieves simultaneous control over numerous optical pathways, programming with 90% accuracy and maintaining stable performance across crucial communication wavelengths, paving the way for photonic processors that occupy dramatically smaller areas than existing technologies.

Reconfigurable Photonics with Phase Change Materials

The field of reconfigurable silicon photonics is rapidly advancing, with researchers focusing on phase-change materials (PCMs) like antimony tri-selenide and antimony tri-sulfide to create dynamic, programmable photonic integrated circuits. This research emphasizes the potential of these materials to control light propagation within a photonic circuit, moving beyond static designs and offering adaptability for computing, communications, and sensing. Phase-change materials switch between amorphous and crystalline states when heated and cooled, altering their refractive index. Antimony tri-selenide and antimony tri-sulfide are promising candidates due to their low optical loss, significant refractive index contrast, and relatively fast switching speeds.

Silicon photonics provides an ideal platform for integration and miniaturization, allowing for the creation of compact devices. A significant portion of this work focuses on inverse design techniques, where algorithms determine the optimal structure to achieve a desired optical function rather than relying on pre-defined designs. Several optimization methods are employed, including topology optimization, genetic algorithms, and increasingly, deep learning with neural networks. These algorithms create digital metamaterials, allowing for precise control over light propagation and enabling the development of freeform designs.

Characterizing these devices requires specialized techniques like ultrafast photomodulation spectroscopy to measure switching speed and performance. Perturbation maps assess light flow in complex circuits, while cycling endurance, the number of times the material can be switched without degradation, is a critical parameter. Researchers are actively working to improve cycling endurance and minimize optical loss through the development of low-loss PCMs and integration techniques. Future research focuses on scaling up these prototypes for mass manufacturing and seamlessly integrating PCM-based PICs with CMOS electronics to create complex, integrated systems. Reducing energy consumption during switching is also a key goal. The team pioneered a method for encoding multi-port operations onto a compact multimode interferometer architecture fabricated on a 220 nanometer silicon-on-insulator platform. Devices were etched into the silicon layer, and a thin layer of antimony tri-selenide was selectively deposited onto active regions using a lift-off process following sputter coating. A protective capping layer of zinc sulfide with silica was applied to prevent oxidation and enhance switching performance.

Prior to depositing the antimony tri-selenide, an argon-ion etch removed surface oxide, ensuring vertical single-mode operation by directly embedding the PCM onto the waveguide structure. The team developed an iterative optimization algorithm to predict the pixel patterns required to implement target transmission matrices. Starting with an unperturbed device, the algorithm randomly switches pixels between crystalline and amorphous phases, simulating the resulting transmission matrix and evaluating its agreement with the target. This process repeats for each pixel, pursuing a global minimum of a cost function that quantifies the difference between the simulated and target matrices.

Devices were fabricated using a UK silicon-on-insulator platform, featuring multimode regions ranging from 6×40 μm² to 8×40 μm², and tapered waveguides to reduce coupling losses. Switching of the antimony tri-selenide was achieved through direct laser writing using a current-modulated diode laser, enabling high-resolution programming with pixel sizes around 0. 7μm. A three-axis stage assembly accurately positioned the laser spot across the PCM region. To address limitations in accessing multiple device ports, scientists constructed a customized setup using free-space prisms for in- and out-coupling.

A fibre-coupled tunable laser source was focused through an input prism, and output signals were collected via a separate objective projecting the image onto an InGaAs camera. This setup enabled simultaneous measurement of multiple grating outputs, allowing quantification of the relative intensity distribution across output ports. The team achieved simultaneous control of up to 25 matrix elements with programming accuracy of 90% relative to simulated patterns, and patterned devices remained stable across the C-band wavelengths.

Antimony Selenide Tunes Silicon Photonics with High Accuracy

Scientists have achieved a breakthrough in reconfigurable photonics by demonstrating a platform for compact, efficient, and scalable devices using antimony tri-selenide. This work establishes a method for encoding multi-port operations onto the transmission matrix of a multimode interferometer architecture fabricated on a standard 220 nanometer silicon photonics platform. The team successfully integrated a thin film of antimony tri-selenide, a phase-change material, onto the silicon structures, enabling non-volatile and reversible tuning of optical properties. Experiments revealed the ability to control up to 25 matrix elements simultaneously in multi-port devices ranging from 2×2 to 5×5 couplers, achieving a programming accuracy of 90% relative to simulated patterns.

The researchers employed direct laser writing to locally perturb the refractive index of the antimony tri-selenide film, creating precise control over light propagation. Devices fabricated using this method maintain stable performance across the C-band wavelengths, demonstrating reliability for practical applications. A key demonstration involved a 2×2 multi-port switch, initially exhibiting significant light scattering and insertion loss. Through programmed perturbations, the device was reconfigured to implement an identity transformation and its inverse, guiding light towards targeted output waveguides.

Measurements confirm a splitting ratio of approximately 92%:8% for both states, corresponding to a port extinction of 10. 7 decibels. These results demonstrate the potential for implementing matrix operations on areas three orders of magnitude smaller than those required by conventional interferometer meshes, paving the way for highly integrated photonic circuits.