Sébastien Cueff

The phase-dependent plasmonic response of a metamaterial platform composed of VO2 nanocrystals (NCs) embedded in an SiO2 matrix was investigated using ultrafast time-resolved spectroscopic ellipsometry. Localized surface plasmon resonances (LSPR) emerged upon excitation with 35 fs laser pulses at λ = 500 nm, which triggered the insulator-to-metal transition in the VO2 NCs. The study reveals the ultrafast dynamics mapped at the LSPR underlying the photoinduced optical tunability of the system, highlighting the potential of VO2 NCs for reconfigurable photonic applications such as tunable linear switching.

This work presents the design, fabrication, and characterization of a reconfigurable near‐perfect absorber based on a Metal– Sb2S3–Metal trilayer structure for applications across the visible and near‐infrared spectrum. Unlike conventional phase‐change materials such as GST, which suffer from high optical absorption, the use of Sb2S3 provides significantly reduced dissipation and enables broadband tunability of the resonant absorption peaks through Fabry–Pérot cavity effects. We experimentally demonstrate wavelength‐selective behavior with strong agreement between measured and simulated results. The proposed design combines spectral selectivity, polarization sensitivity, angle‐independence, and structural simplicity, validating Sb2S3 as a next‐generation material for tunable nanophotonic devices and establishing a new route for reconfigurable absorbers in integrated photonics.

Phase-change materials (PCM) have gained considerable interest in the photonics community as nonvolatile tunable materials for reconfigurable devices. Here, we experimentally demonstrate the continuous electrical control of the crystalline fraction of Ge2Sb2Te5 (GST) via voltage pulses in platinum micro-heaters. Using co-designed electrical micro-heaters and a simple optical stack of thin films, including GST, we demonstrate the programming of more than 50 distinct levels of optical reflection in the near-infrared, spanning a three orders of magnitude reflection modulation. This continuous electrically programmed phase transition is a promising strategy for PCM integration in industrial foundries and shows promise for individual pixel addressing in multi-level applications such as neuromorphic computing or spatial light modulators.

In recent years, silicon has emerged as a promising platform for quantum photonics, driven by its technological maturity and compatibility with large-scale photonic integration. Among the various approaches to implementing quantum emitters in silicon, color centers have gained significant attention due to their ability to operate as single-photon sources in the near-infrared, making them highly relevant for quantum communication and information processing. However, to fully exploit their potential, efficient integration into silicon photonic structures is essential to enhance photon extraction, control emission properties, and enable scalable architectures. This review provides a comprehensive overview of the progress in integrating color centers into silicon photonic structures. The most promising color centers studied to date are presented, along with the various methods developed for their creation. Strategies for coupling these emitters to photonic structures, such as waveguides and resonant cavities, are examined, highlighting their impact on emission properties, including enhanced radiative rates via the Purcell effect and improved control over emission directivity. Finally, key challenges and future directions are discussed to further advance silicon-based quantum emitters toward practical applications in quantum technologies.

Photonic integrated devices are progressively evolving beyond passive components into fully programmable systems, notably driven by the progress in chalcogenide phase‐change materials (PCMs) for non‐volatile reconfigurable nanophotonics. However, the stochastic nature of their crystal grain formation results in strong spatial and temporal crystalline inhomogeneities. Here, the concept of spatially‐controlled planar guided crystallization is proposed, a novel method for programming the growth of optically homogeneous low‐loss Sb ₂ S ₃ PCM, leveraging the seeded directional and progressive crystallization within confined channels. This guided crystallization method is experimentally shown to circumvent the current limitations of conventional PCM‐based nanophotonic devices, including a multilevel non‐volatile optical phase‐shifter exploiting a silicon nitride‐based Mach–Zehnder interferometer, and a programmable metasurface with spectrally reconfigurable bound state in the continuum. Precisely controlling the growth of PCMs to ensure optically uniform crystalline properties across devices is the cornerstone for the industrial development of non‐volatile reconfigurable photonic integrated circuits.

We report the first application of broadband time-resolved pump-probe ellipsometry to study the ultrafast dynamics of the photoinduced insulator-to-metal transition (IMT) in vanadium dioxide (VO₂) thin films driven by 35 fs laser pulses. This novel technique enables the direct measurement of the time-resolved evolution of the complex pseudodielectric function of VO₂ during the IMT. We have identified distinct thermal and nonthermal dynamics in the photoinduced IMT, which critically depends on the pump wavelength and fluence, while providing a detailed temporal and spectral phase map. A comparison of the pseudodielectric function of the VO₂ thin film during thermally and photoinduced phase transitions reveals that the primary differences in the IMT pathways occur within the first picosecond after the pump, driven by nonequilibrium dynamics in this ultrafast time scale. The ultrafast spectroscopic ellipsometry introduced in this work offers a complementary probe to study phase changes in condensed matter and emerging photonic device materials.

Phase change materials (PCMs) have gained a tremendous interest as a means to actively tune nanophotonic devices through the large optical modulation produced by their amorphous to crystalline reversible transition. Recently, materials such as Sb₂S₃ emerged as particularly promising low loss PCMs, with both large refractive index modulations and transparency in the visible and near‐infrared. Controlling the local and reversible phase transition in this material is of major importance for future applications, and an appealing method to do so is to exploit pulsed lasers. Yet, the physics and limits involved in the optical switching of Sb₂S₃ are not yet well understood. Here, the reversible laser‐induced phase transition of Sb₂S₃ is investigated, focusing specifically on the mechanisms that drive the optically induced amorphization, with multi‐physics considerations including the optical and thermal properties of the PCM and its environment. The laser energy threshold for reversibly changing the phase of the PCM is determined through both theoretical analysis and experimental investigation, not only between fully amorphous and crystalline states but also between partially recrystallized states. Then, the non‐negligible impact of the material's polycrystallinity and anisotropy on the power thresholds for optical switching is revealed. Finally, the challenges related to laser amorphization of thick Sb₂S₃ layers are addressed, as well as strategies to overcome them. These results enable a qualitative and quantitative understanding of the physics behind the optically‐induced reversible change of phase in Sb₂S₃ layers.

We introduce the Optical Materials Express feature issue on Hybrid Photonics: Integration, Design and Devices. This issue comprises a collection of ten papers including six invited and four contributed papers from well-established research groups and prominent scientists in the field. These papers cover the development, characterization, control, technologies and applications of hybrid devices for photonics.