Search for: All records

For more than a decade, the linear and nonlinear optical responses of materials and composites exhibiting an epsilon-near-zero (ENZ) region have been of keen interest to the community. Among the variety of effects realized, achieving index modulation near unity on the picosecond (or less) timescale has generated the most significant impact. As a long-sought combination of strength and speed, ENZ nonlinearities have reignited interest in nonlinear processes that appear to go beyond the typical perturbative expansion (e.g., non-perturbative) as well as in time-varying nonlinear processes. Here, we aim to take a physical and intuitive look at the nonlinear index modulation in Drude-like ENZ films and highlight the physical limits of tuning. We will focus particularly on their connection (or lack thereof) with non-perturbative effects and time-varying processes and provide our opinions as to the strengths and weaknesses of ENZ films in these areas. 

Photonic time-varying systems have attracted significant attention owing to their rich physics and potential opportunities for new and enhanced functionalities. In this context, the duality of space and time in wave physics has been particularly fruitful to uncover interesting physical effects in the temporal domain, such as reflection/refraction at temporal interfaces and momentum-bandgaps in time crystals. However, the characteristics of the temporal/frequency dimension, particularly its relation to causality and energy conservation ( $\mathrm{\hslash <\#comment/>}\mathrm{\omega <\#comment/>}$is energy, whereas $\mathrm{\hslash <\#comment/>}\mathit{k}$is momentum), create challenges and constraints that are unique to time-varying systems and are not present in their spatially varying counterparts. Here, we overview two key physical aspects of time-varying photonics that have only received marginal attention so far, namely temporal dispersion and external power requirements, and explore their implications. We discuss how temporal dispersion, an inherent property of any physical causal material, makes the fields evolve continuously at sharp temporal interfaces and may limit the strength of fast temporal modulations and of various resulting effects. Furthermore, we show that changing the refractive index in time always involves large amounts of energy. We derive power requirements to observe a time-crystal response in one of the most popular material platforms in time-varying photonics, i.e., transparent conducting oxides, and we argue that these effects are almost always obscured by less exotic nonlinear phenomena. These observations and findings shed light on the physics and constraints of time-varying photonics, and may guide the design and implementation of future time-modulated photonic systems. 

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally adopted in optical material modeling. The growing interest in plasmonic, polaritonic, and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials—metamaterials and metasurfaces—can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In metasurfaces, in particular, nonlocality engineering has emerged as a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials and metamaterials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward—toward new scientific discoveries and technological advancements. 

We show that concept of parity-time (PT) symmetry can be expanded to include mixed photon-exciton modes by demonstrating that eigenmodes of active (pumped) strongly coupled cavity polaritons with population inversion exhibit characteristics that are remarkably akin to those of coupled photonic structures with parity-time symmetry. The exceptional point occurs when the Rabi splitting of polariton branches inherent in passive polaritonic systems decreases with increase in pumping, leading to population inversion, and eventually two polaritonic modes merge into a single mode, thus manifesting the frequency pulling effect inherent to all lasers. But, remarkably, this exceptional point occurs below the lasing threshold. Furthermore, unlike most manifestations of PT symmetry in optics, which are observed in the interaction between two analogous photonic modes in waveguides or cavities, in this work the exceptional point is found in interaction between two very dissimilar modes—one photonic and one material excitation (exciton). Aside from fundamentally noteworthy expansion of the concept of PT symmetry to new systems, there is a prospect of using the exceptional point in polaritons for practical applications, such as sensing. 

Optical isolators, reliably integrated on-chip, are crucial components for a wide range of optical systems and applications. We introduce a new class of wideband nonmagnetic and linear optical isolators based on nonlinear frequency conversion and spectral filtering among the pump, signal, and idler wavelengths. The scheme is experimentally demonstrated using difference-frequency generation in periodically poled thin-film lithium niobate integrated devices and short- and long-pass optical filters. We demonstrate a wide bandwidth of more than 150 nm, limited only by the measurement setup, and an optical isolation ratio of up to 18 dB for the involved idler and signal waves. The difference of transmittance at the signal wavelength between forward and backward propagation is 40 dB. We also discuss pathways for substantial isolation improvement using appropriate anti-reflection coatings. The integrable isolator, operating in the telecommunication band, is characterized by a perfectly linear output versus input power dependence and can be incorporated into high-speed telecom and datacom systems as well as a variety of other applications. 

The conversion of a photon’s frequency has long been a key application area of nonlinear optics. It has been discussed how a slow temporal variation of a material’s refractive index can lead to the adiabatic frequency shift (AFS) of a pulse spectrum. Such a rigid spectral change has relevant technological implications, for example, in ultrafast signal processing. Here, we investigate the AFS process in epsilon-near-zero (ENZ) materials and show that the frequency shift can be achieved in a shorter length if operating in the vicinity of $\mathrm{R}\mathrm{e}\left\{{\mathrm{\epsilon <\#comment/>}}_r\right\}=0$. We also predict that, if the refractive index is induced by an intense optical pulse, the frequency shift is more efficient for a pump at the ENZ wavelength. Remarkably, we show that these effects are a consequence of the slow propagation speed of pulses at the ENZ wavelength. Our theoretical predictions are validated by experiments obtained for the AFS of optical pulses incident upon aluminum zinc oxide thin films at ENZ. Our results indicate that transparent metal oxides operating near the ENZ point are good candidates for future frequency conversion schemes.