Xiao Lin

Researchers have proposed that structural disorder can be harnessed to generate coherent, directional light from the passage of a charged particle.

Interfacial Cherenkov radiation originates from the interaction of charged particles with sequential interfaces inside photonic crystals and is crucial to many applications, ranging from particle detectors and lasers to electron microscopy and spectroscopy, since it can create directional light emission at arbitrary spectra. However, the interfacial Cherenkov radiation suffers from chromatic issues and is featured with frequency-sensitive radiation angles due to the intrinsic structural dispersion of photonic crystals. Counterintuitively, here we reveal a universal mechanism to create the broadband achromatic interfacial Cherenkov radiation from photonic crystals. Our mechanism exploits the anomalous Maxwell-Garnett theory beyond the long-wavelength limit via the Brewster effect and introduces it for the first time into the context of Cherenkov radiation. Specifically, we present photonic crystal designs that mimic homogeneous achromatic media in terms of their interfacial Cherenkov radiation, emitted precisely at the Brewster angle. We find the radiation peak angle could be frequency-insensitive, even over the entire visible spectrum.

Guided mode resonance (GMR), the resonant coupling of free‐space light into leaky waveguide modes, is traditionally achieved with periodic patterned structures. However, this approach makes its key properties, such as quality factor (Q‐factor), fabrication‐dependent and non‐tunable. Here, we introduce a time grating platform, i.e., a homogeneous waveguide whose refractive index is modulated periodically in time, that allows tunable GMRs through temporal modulation engineering rather than spatial structural redesign. We show that the Q‐factors of these GMRs diverge as the modulation depth vanishes. Furthermore, unconstrained by energy conservation, the resonances exhibit near‐unity reflection for fundamental harmonics and values exceeding 40 for first‐order harmonics. Our findings not only apply to yield a giant Goos–Hänchen shift over 10 ³ times wavelength without sacrificing the reflection magnitude, but also open new avenues for related phenomena such as bound states in the continuum, unidirectional GMRs, and beyond.

It has long been thought that the reversed Cherenkov radiation is impossible in homogeneous media with a positive refractive index n. Here, we break this long-held belief by revealing the possibility of creating reversed Cherenkov radiation from homogeneous positive-index moving media. The underlying mechanism is essentially related to the Fizeau–Fresnel drag effect, which provides a unique route to drag the emitted light in the direction of the moving medium and thus enables the possibility of dragging the emitted light in the opposite direction of the moving charged particle. Moreover, we discover the existence of a threshold for the velocity vmedium of moving media, only above which, namely, vmedium>c/n2, the reversed Cherenkov radiation may emerge, where c is the velocity of light in vacuum. Particularly, we find that the reversed Cherenkov radiation inside superluminal moving media (i.e., vmedium>c/n) can become thresholdless for the velocity of moving charged particles.

The Maxwell-Garnett theory, dating back to James Clerk Maxwell-Garnett's foundational work in 1904, provides a simple yet powerful framework to describe the inhomogeneous structure as an effective homogeneous medium, which significantly reduces the overall complexity of analysis, calculation, and design. As such, the Maxwell-Garnett theory enables many practical applications in diverse realms, ranging from photonics, acoustics, mechanics, thermodynamics, to materials science. It has long been thought that the Maxwell-Garnett theory of light in impedance-mismatched periodic structures is valid only within the long-wavelength limit, necessitating either the temporal or spatial period of light to be much larger than that of structures. Here, we break this long-held belief by revealing an anomalous Maxwell-Garnett theory for impedance-mismatched photonic time crystals beyond this long-wavelength limit. The key to this anomaly lies in the Fabry–Pérot resonance. We discover that under the Fabry–Pérot resonance, the impedance-mismatched photonic time crystal could be essentially equivalent to a homogeneous temporal slab simultaneously at specific discrete wavelengths, despite the temporal period of these light being comparable to or even much smaller than that of photonic time crystals.

Motion underpins various exotic phenomena in nature, such as the Doppler effect, and enables numerous practical applications. In this study, we reveal a counterintuitive phenomenon that p-polarized surface plasmons cannot be excited for all directions and Doppler frequencies, when a complex dipolar source moves parallel and close to the planar interface that supports the propagation of surface plasmons under a newfound critical suppression condition. The underlying mechanism is that the motion provides an extra degree of freedom to reshape the near-field interference of dipolar sources themselves and could be exploited to fully suppress p-polarized waves carried by the judiciously designed dipolar source at all Doppler frequencies. To be specific, this critical polarization suppression arises when the dipolar source acts as a moving Huygens dipole, with its electric and magnetic dipole moments being in-phase and mutually perpendicular. Moreover, this Huygens-like dipole must move with a critical velocity, which is determined solely by the ratio of the source’s electric to magnetic dipole moment, but irrelevant to surface plasmons' phase velocity.

Superscattering offers an enticing route to significantly enhance the scattering cross section of subwavelength scatterers, far exceeding the single-channel scattering limit. It is of paramount importance to many applications, such as optical sensing, antennas, and imaging. However, the superscattering phenomenon is generally accompanied with nonzero forward-scattered light. Here, we find the possibility of creating the exotic phenomenon of superscattering with zero forward-scattered light by exploiting the optical gain. The underlying mechanism is that the optical gain can facilitate the realization of not only the single-channel superscattering but also the second Kerker effect, which is intrinsically related to the zero forward scattering phenomenon. Because of the importance of the second Kerker effect in regulating the directionality of superscattering, this revealed phenomenon is termed as Kerker-superscattering.

Maxwell-Garnett theory, dating back to James Clerk Maxwell-Garnett's foundational work in 1904, provides a simple yet powerful framework to describe the inhomogeneous structure as an effective homogeneous medium, which significantly reduces the overall complexity of analysis, calculation, and design. As such, the Maxwell-Garnett theory enables many practical applications in diverse realms, ranging from photonics, acoustics, mechanics, thermodynamics, to material science. It has long been thought that the Maxwell-Garnett theory of light in impedance-mismatched periodic structures is valid only within the long-wavelength limit, necessitating either the temporal or spatial period of light to be much larger than that of structures. Here, we break this long-held belief by revealing an anomalous Maxwell-Garnett theory for impedance-mismatched photonic time crystals beyond this long-wavelength limit. The key to this anomaly lies in the Fabry-Perot resonance. We discover that under the Fabry-Pérot resonance, the impedance-mismatched photonic time crystal could be essentially equivalent to a homogeneous temporal slab simultaneously at specific discrete wavelengths, despite the temporal period of these light being comparable to or even much smaller than that of photonic time crystals.