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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.

Electro optic modulators being key for many signal processing systems must adhere to requirements given by both electrical and optical constraints. Distinguishing between charge driven (CD) and field driven (FD) designs, we answer the question of whether fundamental performance benefits can be claimed of modulators based on emerging electro-optic materials. Following primary metrics, we compare the performance of emerging electro-optic and electro-absorption modulators such as graphene, transparent conductive oxides, and Si, based on charge injection with that of the ‘legacy’ FD modulators, such as those based on lithium niobate and quantum confined Stark effect. We show that for rather fundamental reasons and when considering energy and speed only, FD modulators always outperform CD ones in the conventional wavelength scale photonic waveguides. However, for waveguides featuring a sub-wavelength optical mode, such as those assisted by plasmonics, the emerging CD devices are indeed highly competitive especially for applications where component-density on-chip is a factor.

High refractive index dielectrics enable nanoscale integration of optical components with practically no absorption loss. Hence, high index dielectrics are promising for many emerging applications in nanophotonics. However, the lack of a complete library of high index dielectric materials poses a significant challenge to understanding the full potential for dielectric nanophotonics. Currently, it is assumed that the absorption edge and the sub‐bandgap refractive index of a semiconductor exhibit a rigid trade‐off, popularly known as the Moss rule. Thus, the Moss rule appears to set an upper limit on the refractive index of a dielectric for a given operating wavelength. However, there are many dielectric materials that surpass the Moss rule, referred to here as super‐Mossian dielectrics. Here, the general features of super‐Mossian dielectrics and their physical origin are discussed to facilitate the search for high index dielectrics. As an example, iron pyrite, an outstanding super‐Mossian material with index nearly 40% higher than the Moss rule prediction, is developed. The local dielectric resonances in iron pyrite nanoresonators are experimentally observed, and the impact of super‐Mossian materials on nanophotonics is demonstrated.

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.

As lasers get more and more miniaturized and their dimensions become comparable to the wavelength, two interconnected phenomena take place: the fraction of spontaneous radiation going into a specific laser mode (β‐factor) increases and can ultimately reach unity, while the radiative lifetime gets shortened by the Purcell factorF_p. Often it is assumed that an increase of these two factors, along with the quality factor (Q‐factor), almost invariably causes reduction of the lasing threshold. This assumption is tested on various photonic and plasmonic lasers, demonstrating that, while there is obvious correlation between the aforementioned factors and the laser threshold, the dependence is far from being straightforward and omnipresent. Depending on specific laser material and geometry, the threshold can decrease, increase, or stay unchanged whenβ‐factor,Q‐factor, andF_pincrease. For the most part, the reduction of threshold is achieved simply by reducing the laser volume and this volume reduction can concurrently cause the increase inβ‐factor and/or Purcell factor, but it would be imprudent to say that the increase in either of these factors is the cause of the threshold reduction.

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$\mathrm{R}\mathrm{e}\{{\mathrm{\epsilon <\#comment/>}}_r\}=0$. 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.