Compact, power-efficient, dynamic control of light at the nanoscale across the ultraviolet, visible, and infrared (UV-Vis-IR) electromagnetic spectrum bands is one of the most ubiquitous grand challenges in photonics today. Furthermore, when integrated with chip and off-chip platforms such as metamaterials, optical fibers, and waveguides, reconfigurable photonic technologies hold the key to realizing the promise of emerging quantum and neuromorphic telecommunication and computing paradigms. Notably, these reconfigurable photonic platforms rely on engineering strong light-matter coupling to functional material systems and, due to these rapidly emerging applications, have seen an explosion of research activity globally across the material science, physics, chemistry, and photonics communities. Many different materials and device architectures are being reported almost daily with different functionalities and operational parameters that continue to open exciting new pathways.

Various volatile and non-volatile tuning mechanisms inherent to different material platforms are being explored and integrated into free-space and integrated architectures. As showcased in this collection, these include optical nonlinearities and phase transitions in chalcogenide glasses and liquid crystals, carrier modulation in metal oxides and nitrides, thermo-optic effects in silicon and its derivates as well as magneto-optical effects in garnets to name a few. Here, we present a brief overview of the ensuing sections of this collection, their fundamental importance, and their relation to other contributions.

Phase transition as a tuning mechanism is explored in several contributions and can be further classified into various types, namely insulator-metallic phase transition, amorphous–crystalline phase transition, and phase transitions between various ordered configurations in liquid crystals. Strongly correlated materials like vanadium oxide undergo reversible, volatile insulator-to-metal phase transitions, while certain alloys like chalcogenide glasses undergo non-volatile amorphous to crystalline phase transitions using an electrical, optical or thermal stimulus. Due to the non-volatile nature of their phase transitions and high refractive indices across the telecommunication bands, chalcogenide phase change materials (PCM’s) have become a material of choice in emerging photonic integrated circuits as data storage elements or as various weighting or programmable layers in neuromorphic computing and imaging architectures. The amorphous-to-crystalline transition in chalcogenides is an annealing process that can be initiated by increasing the ambient temperature or locally by laser- or electrical current- induced heating to a temperature above the alloy’s glass-transition point, T_g, but below its melting point, T_m. The reverse transition, a melt-quenching process, is driven by shorter, higher energy pulsed excitation that momentarily brings the material to a temperature above T_m. Therefore, understanding the optical and thermal properties is of utmost importance for device design and is discussed by Aryana et al [1] and Lawson et al [2] for low-loss sulphide and selenide alloys. Furthermore, exploring the role of device architectures to realize control over coupling efficiency or guided modes in silicon photonic circuits using inverse-design techniques is discussed by Ye and Mandal et al, respectively [3,4]. Additionally, Nobile et al showcase the prospect of using Bragg resonant nanostructuring to tune non-volatile reconfigurability in PCM-loaded circuits [5]. Notably, this is presented in a rewriteable configuration by Lai et al, using photosensitive arsenic selenide waveguides; chalcogenide glasses also exhibit high levels of optical nonlinearity, as evident from their high nonlinear refractive indices across the visible and telecommunication bands. Together with their photodarkening properties, this is utilized in arsenic selenide waveguides to show reversible Bragg gratings and stimulated Brillouin scattering in planar waveguide architectures [6]. In all these cases, the growth and nanopatterning techniques used to create the reconfigurable phase change layer is of utmost importance to final device performance, and this is best exemplified in liquid crystal-based devices; liquid crystals offer the capability of tuning their orientation and through that optical birefringence by electrical or optical stimulus in a volatile manner. Therefore, the alignment of liquid crystals on surfaces plays a central role in optimizing their performance. To this end, Zhang et al, present a cutting-edge nano-lithography-based method and subsequent electro-optic studies to control the local orientation of thermotropic liquid crystals [7]. Dynamic orientation control of liquid crystals using standing surface acoustic waves is also demonstrated in this issue by Ma et al, through the demonstration of tunable resonant plasmonic devices based on the hybridization of a lithium niobate (LiNbO₃) substrate (due to its favourable surface acoustic wave propagation properties) with liquid crystal-covered plasmonic gold nanostructures [8]. The recent surge in the utilization of lithium niobate in reconfigurable integrated chip-based platforms due to its broad transparency window, high refractive index and large electro-optical effect, in part resulting from recent breakthroughs in the growth and patterning of this alloy, is further demonstrated by Banwitz et al, who propose FPGA-controlled thin film lithium niobate-on-insulator (TFLN) electro-optic modulators (EOMs) as high-speed, low-energy, smart transceivers that merge the process of data transmission with multiply-accumulate- operations suitable for emerging in-memory photonic computing applications [9]. Additionally, lithium niobate crystals can also be doped with rare earth ions as an optical gain mechanism that can lead to active on-chip laser sources, as demonstrated by Yin et al, through an electro-optically tunable Fabry-Perot (FP) cavity on an Er³⁺-doped TFLN [10].

Volatile reconfigurability using electrical stimuli to induce carrier-modulation effects is also highly sought after due to its natural compatibility with CMOS processes and is widely integrated into various field-effect devices that use gating architectures to induce modulation effects. This is being widely pursued in transparent conductive oxides (TCO’s) as well as more recently nitrides such as titanium nitride due to their low optical loss and favourable epsilon-near-zero (ENZ) properties and potentially high switching speeds. Notably for integrated platforms, the electro-optically tunable ENZ properties of TCO’s enabled through carrier modulation for emerging neuromorphic computing applications facilitate the demonstration by Arenas et al of a dual phase and amplitude modulator implemented by integrating a transparent conducting oxide (TCO) with a silicon waveguide structure [11]. Doped iron garnets have also been a class of metal oxides traditionally used for volatile magneto-optic modulation in photonic devices. Shoji et al extends this concept by demonstrating the use of cerium-substituted yttrium iron garnet as a magnetic, non-volatile waveguide-integrated photonic switch [12].

Additionally, the increasingly important role of nitrides in reconfigurable platforms is showcased on free-space metasurface platforms using titanium nitride for polarization-dependent absorption switching by Wang et al for telecommunication and imaging applications [13] and for broadband solar absorber applications by Nga et al, where a series of simple design rules are also offered [14]. Metal-nitrides, in particular, titanium nitride, have also had a long-standing history as an inert electrode/heater material of choice in various reconfigurable optoelectronic platforms and used as such by Zhao et al in self-configurable quadrilateral mach-zender interferometer networks relying on thermo-optic modulation of silicon facilitated by these electrodes [15]. Notably, silicon, through hybridization with diamond, will play a significant role in emerging reconfigurable quantum photonic platforms as well. Among the numerous diamond color centers, the negatively charged silicon-vacancy center (SiV) is less susceptible to spectral diffusion than the nitrogen-vacancy (NV) centers. However, obtaining high-quality colour centers in small photonic structures is challenging, as properties such as spin population lifetimes can be affected by the transition from a bulk to a nanostructured crystal host. Lutz et al present and study several ways to prepare such diamond samples containing silicon vacancy centers [16].

Finally, this theme of hybridization of existing material and circuit-level platforms to achieve unprecedented new functionalities is further exemplified by Chen et al, who present a blueprint for drastically increasing the space-bandwidth product of devices making use of spatial light modulation by merging energy-efficient modulators and grating couplers in photonic integrated circuits (PICs) with free-space beam aggregators based on metasurface technology [17].

It should be clear from this feature issue that research into reconfigurable photonic platforms represents an intensely multidisciplinary field with versatile applications that act as enablers for introducing adaptive and smart functionality to a wide range of systems with wide-ranging applications ranging from telecommunication and computing to sensing and imaging. This makes this field's current status and future directions of utmost importance to optoelectronics, nanophotonics and material sciences and the emerging artificial intelligence and quantum technologies communities.

While it may be impossible to showcase the full impact that reconfigurable photonic platforms have on all these communities, we hope this feature issue offers a timely overview of the field and will stimulate further research and new insights and pathways for development in this increasingly critical area. We hope that you enjoy this sweeping collection of articles from imminent researchers across the globe and express our thanks to all contributors and reviewers and, most notably, to the editors of Optical Materials Express and Optica staff members without whom this feature issue would not have come to fruition.