Reconfigurable photonic devices capable of dynamically tuning their optical characteristics to adapt to different application demands on-the-fly have become a focus of intensive research and development efforts in recent years. Compared to classical tuning mechanisms relying on thermo-optic or electro-optic effects, the giant nonvolatile refractive index contrast provided by phase change materials (PCMs) upon solid-state structural transitions enables reconfigurable devices with an unprecedented compact footprint, zero static power consumption, and superior optical performance. These unique features have catalyzed phase change photonics, an emerging field characterized by a rapidly evolving research landscape. This review presents a deep dive into this dynamic field, spanning fundamental material design principles and processing techniques to PCM integration into various state-of-the-art device platforms and industry-standard foundry manufacturing processes. We also identify challenges that PCM photonics need to address and point out directions where exciting innovations will likely come to fruition in the near future.
Stimulated Raman scattering (SRS) microscopy has shown enormous potential in revealing molecular structures, dynamics, and couplings in complex systems. For most biomolecules, the detection sensitivity of SRS is fundamentally limited to the milli-molar level due to the shot noise and the small modulation depth. Additionally, the operation of SRS imaging is complicated by cross phase modulation. We recently revisited SRS from the perspective of energy deposition. Via intensity gain in the Stokes beam and loss in the pump beam, the SRS process pumps molecules to their vibrationally excited states. The thereafter relaxation heats up the surroundings and induces refractive index changes. By probing the refractive index changes with a laser beam, stimulated Raman photothermal (SRP) microscopy is developed, where a >500-fold boost of modulation depth is achieved. Moreover, SRP imaging can be operated with a noisy fiber laser for excitation and a long working distance air condenser for signal collection. Two implementations and broad biological applications are reviewed. In summary, SRP microscopy opens a new way to perform chemical imaging with ultrahigh sensitivity and long working distance optics toward clinical translation.
Spin-glass theory emerged in the 1980s as a merger between theoretical physics and condensed matter. Soon, physicists realized that spin glasses serve as a paradigm for complex systems, as underscored by the 2021 Nobel Prize in Physics, and for applications in machine learning and neuroscience, with a profound connection with the Hopfield model and Boltzmann machines, subjects of the 2024 Nobel Prize in Physics. However, the connection with optics and photonics is even more profound and fundamental; this connection was identified as early as 1982, with the first realizations of optical neural networks. Thirty years later, the first experimental demonstration of a pillar of spin-glass theory, the replica symmetry breaking, was reported in photonics. Nowadays, many scientists consider photonics as an effective solution for new hardware in artificial intelligence, capable of reducing energy consumption in training large machine-learning modules, and also more suitable for realizing fully connected models that underpin modern data-driven analysis. The substantial equivalence between linear optical propagation and a system of interacting binary spins is now well recognized, triggering the development of a new family of devices for both classical and quantum computing. This review is intended to detail the work of the past twenty years concerning the link between spin-glass theory and optics. After a simple introduction to the main ideas of spin glasses, we start from the first works aimed at finding a direct experimental proof of ideas such as the landscape and ultrametricity; then we report on “linear optical spin glasses,” which refer to the photonic simulation of various Ising models for combinatorial optimization and interlinked with quantum computers; finally, we discuss the emerging field of “nonlinear optical spin glasses,” driven by the impressive progress in the realization of coherent Ising machines with parametric oscillators, that opened an new research direction driven by the cross-fertilization of advanced theoretical physics, artificial intelligence, classical and quantum nonlinear optics.
