Transient cavity-cavity strong coupling at terahertz frequency on LiNbO3 chips.

Terahertz (THz) microcavities have garnered considerable attention for their ability to localize and confine THz waves, allowing for strong coupling to remarkably enhance the light-matter interaction. These properties hold great promise for advancing THz science and technology, particularly for high-speed integrated THz chips where transient interaction between THz waves and matter is critical. However, experimental study of these transient time-domain processes requires high temporal and spatial resolution since these processes, such as THz strong coupling, occur in several picoseconds and microns. Thus, most literature studies rarely cover temporal and spatial processes at the same time. In this work, we thoroughly investigate the transient cavity-cavity strong-coupling phenomena at THz frequency and find a Rabi-like oscillation in the microcavities, manifested by direct observation of a periodic energy exchange process via a phase-contrast time-resolved imaging system. Our explanation, based on the Jaynes-Cummings model, provides theoretical insight into this transient strong-coupling process. This work provides an opportunity to deeply understand the transient strong-coupling process between THz microcavities, which sheds light on the potential of THz microcavities for high-speed THz sensor and THz chip design.

Omni-polarized Faraday isolator based on non-Hermitian Faraday system.

Non-Hermitian systems have recently attracted significant attention in photonics due to the realization that the interplay between gain and loss can lead to entirely new and unexpected features. Here, we propose and demonstrate a non-Hermitian Faraday system capable of non-reciprocal omni-polarizer action at the exceptional point. Notably, both forward and backward propagating light with arbitrary polarization converge to the same polarization state. Leveraging the robustness and non-reciprocity of the non-Hermitian Faraday system, we realize an omni-polarized Faraday isolator that can effectively isolate any polarized light without the need for a polarizer at the incident port of backward propagation. Remarkably, under the given parameter configuration, the isolator achieves a maximum isolation ratio of approximately 100 dB and a minimum isolation ratio of around 45 dB for various polarized light, accompanied by near-zero insertion loss. Furthermore, our research reveals the remarkable tolerance of the non-Hermitian Faraday isolator to nonlinear effects. This unique characteristic allows us to harness nonlinear effects to achieve various optical functions, all while maintaining excellent isolation performance. The proposed non-Hermitian Faraday system paves the way for the realization of magnetically or optically switchable non-reciprocal devices.

Topological super-modes engineering with acoustic graphene plasmons.

Acoustic graphene plasmons (AGPs) in a graphene-dielectric-metal structure possess extreme field localization and low loss, which have promising applications in strong photon-matter interaction and integrated photonic devices. Here, we propose two kinds of one-dimensional crystals supporting propagating AGPs with different topological properties, which is confirmed by the Zak phase calculations and the electric field symmetry analysis. Moreover, by combining these two plasmonic crystals to form a superlattice system, the super-modes exist because of the coupling between isolated topological interface states. A flat-like dispersion of super-modes is observed by designing the superlattice. These results should find applications in optical sensing and integrating photonic devices with plasmonic crystals.

Controllable nonlinear propagation of partially incoherent Airy beams.

The self-accelerating beams such as the Airy beam show great potentials in many applications including optical manipulation, imaging and communication. However, their superior features during linear propagation could be easily corrupted by optical nonlinearity or spatial incoherence individually. Here we investigate how the interaction of spatial incoherence and nonlinear propagation affect the beam quality of Airy beam, and find that the two destroying factors can in fact balance each other. Our results show that the influence of coherence and nonlinearity on the propagation of partially incoherent Airy beams (PIABs) can be formulated as two exponential functions that have factors of opposite signs. With appropriate spatial coherence length, the PIABs not only resist the corruption of beam profile caused by self-focusing nonlinearity, but also exhibits less anomalous diffraction caused by the self-defocusing nonlinearity. Our work provides deep insight into how to maintain the beam quality of self-accelerating Airy beams by exploiting the interaction between partially incoherence and optical nonlinearity. Our results may bring about new possibilities for optimizing partially incoherent structured field and developing related applications such as optical communication, incoherent imaging and optical manipulations.

Frequency modulation of terahertz microcavity via strong coupling with plasmonic resonators.

Tunable terahertz (THz) microcavities are crucial for the compact on-chip THz devices, aiming to future cloud-based computing, and artificial-intelligence technologies. However, the solutions to effectively modulate THz microcavities remain elusive. Strong coupling has been widely demonstrated in many configurations at different ambient conditions to date and may serve as a promising tool to modulate THz microcavities. Here, we schematically design a microcavity-plasmon hybrid system, and propose an effective approach to modulating the resonant frequencies of THz microcavities by the microcavity-resonator strong coupling. In this case, we observed the strongly coupling states, where the resultant two-polariton branches exhibit an anti-crossing splitting in the frequency domain, experimentally exhibiting a ∼6.2% frequency modulation to the microcavity compared to the uncoupled case. This work provides an efficient approach to modulating chip-scale THz microcavities, thereby facilitating the development and application of compact THz integrated devices, further empowering the evolution of future information processing and intelligent computing system.

On-chip ytterbium-doped lithium niobate waveguide amplifiers with high net internal gain.

Integrated optical systems based on lithium niobate on insulator (LNOI) have shown great potential in recent years. However, the LNOI platform is facing a shortage of active devices. Considering the significant progress made in rare-earth-doped LNOI lasers and amplifiers, the fabrication of on-chip ytterbium-doped LNOI waveguide amplifiers based on electron-beam lithography and inductively coupled plasma reactive ion etching was investigated. The signal amplification at lower pump power (<1 mW) was achieved by the fabricated waveguide amplifiers. A net internal gain of ∼18 dB/cm in the 1064 nm band was also achieved in the waveguide amplifiers under a pump power of 10 mW at 974 nm. This work proposes a new, to the best of our knowledge, active device for the LNOI integrated optical system. It may become an important basic component for lithium niobate thin-film integrated photonics in the future.

Electrically tuned coupling of lithium niobate microresonators.

Microresonators coupled with integrated waveguides operate stably but usually lack tunability for an optimal coupling state. In this Letter, we demonstrate a racetrack resonator with an electrically modulated coupling on an X-cut lithium niobate (LN) platform by introducing a Mach-Zehnder interferometer (MZI) with two balanced directional couplers (DCs) to realize light exchange. This device provides a wide-range coupling regulation, from under-coupling and critical coupling to deep over-coupling. Importantly, it has a fixed resonance frequency when the DC splitting ratio is 3 dB. The measured optical responses of the resonator exhibit a high extinction ratio, exceeding 23 dB, and an effective half-wave voltage length Vπ·L of 0.77 V·cm, suitable for CMOS compatibility. Microresonators with tunable coupling and a stable resonance frequency are expected to find application in nonlinear optical devices on LN-integrated optical platforms.

Second-harmonic and cascaded third-harmonic generation in generalized quasiperiodic poled lithium niobate waveguides.

Lithium niobate (LN) thin film has recently emerged as an important platform for nonlinear optical investigations for its large χ(2) nonlinear coefficients and ability of light localization. In this Letter, we report the first, to the best of our knowledge, fabrication of LN-on-insulator ridge waveguides with generalized quasiperiodic poled superlattices using the electric field polarization technique and microfabrication techniques. Benefiting from the abundant reciprocal vectors, we observed efficient second-harmonic and cascaded third-harmonic signals in the same device, with normalized conversion efficiency of 1735% W-1 cm-2 and 0.41% W-2 cm-4, respectively. This work opens a new direction for nonlinear integrated photonics based on LN thin film.

On-chip erbium-ytterbium-co-doped lithium niobate microdisk laser with an ultralow threshold.

Erbium-ion-doped lithium niobate (LN) microcavity lasers working in the communication band have attracted extensive attention recently. However, their conversion efficiencies and laser thresholds still have significant room to improve. Here, we prepared microdisk cavities based on erbium-ytterbium-co-doped LN thin film by using ultraviolet lithography, argon ion etching, and a chemical-mechanical polishing process. Benefiting from the erbium-ytterbium co-doping-induced gain coefficient improvement, laser emission with an ultralow threshold (∼1 µW) and high conversion efficiency (1.8 × 10-3%) was observed in the fabricated microdisks under a 980-nm-band optical pump. This study provides an effective reference for improving the performance of LN thin-film lasers.

Photonic Realization of a Generic Type of Graphene Edge States Exhibiting Topological Flat Band.

Cutting a honeycomb lattice (HCL) ends up with three types of edges (zigzag, bearded, and armchair), as is well known in the study of graphene edge states. Here, we propose and demonstrate a distinctive twig-shaped edge, thereby observing new edge states using a photonic platform. Our main findings are (i) the twig edge is a generic type of HCL edge complementary to the armchair edge, formed by choosing the right primitive cell rather than simple lattice cutting or Klein edge modification; (ii) the twig edge states form a complete flat band across the Brillouin zone with zero-energy degeneracy, characterized by nontrivial topological winding of the lattice Hamiltonian; (iii) the twig edge states can be elongated or compactly localized at the boundary, manifesting both flat band and topological features. Although realized here in a photonic graphene, such twig edge states should exist in other synthetic HCL structures. Moreover, our results may broaden the understanding of graphene edge states, as well as new avenues for realization of robust edge localization and nontrivial topological phases based on Dirac-like materials.

Stationary Charge Radiation in Anisotropic Photonic Time Crystals.

Time metamaterials offer a great potential for wave manipulation, drawing increasing attention in recent years. Here, we explore the exotic wave dynamics of an anisotropic photonic time crystal (APTC) formed by an anisotropic medium whose optical properties are uniformly and periodically changed in time. Based on a temporal transfer matrix formalism, we show that a stationary charge embedded in an APTC emits radiation, in contrast to the case of isotropic photonic time crystals, and its distribution in momentum space is controlled by the APTC band structure. Our approach extends the functionalities of time metamaterials, offering new opportunities for radiation generation and control, with implications for both classical and quantum applications.

Equilibrium Dynamics of Mutually Confined Waves with Signed Analogous Masses.

We report the first experimental realization of equilibrium dynamics of mutually confined waves with signed analogous masses in an optical fiber. Our Letter is mainly demonstrated by considering a mutual confinement between a soliton pair and a dispersive wave experiencing opposite dispersion. The resulting wave-packet complex is found robust upon random perturbation and collision with other waves. The equilibrium dynamics are also extended to scenarios of more than three waves. Our finding may trigger fundamental interest in the dynamics of many-body systems arising from the concept of negative mass, which is promising for new applications based on localized nonlinear waves.

Giant Second Harmonic Generation from Membrane Metasurfaces.

Metasurfaces have emerged as a fascinating framework for nonlinear optics, which have advantages of a compact footprint and unprecedented flexibility in manipulating light. But their nonlinear responses are generally limited by the short interaction lengths with light. Therefore, further enhancement is highly desired for building high-efficiency nonlinear devices. Here, we experimentally demonstrate a record high second harmonic generation (SHG) efficiency of 2.0 × 10-4 using lithium niobate (LN) membrane metasurfaces. Benefiting from the large refractive index contrast in the vertical direction and high fabrication quality, distinct spectral resonances and tight field confinements in the LN layer were achieved. Strong SHG peaks resulting from pump resonances of the metasurfaces were observed. Our nonlinear efficiency is more than 2 orders of magnitude larger than previously reported LN metasurfaces. The results inspire a way to improve the efficiency of nonlinear metasurfaces for ultracompact nonlinear light sources in applications of nonlinear holography, Li-Fi, beam shaping, etc.

Phase-shift-mediated sensitive detection of propagating ultra-confined graphene plasmons.

The ultra-confined plasmon field supported by graphene provides an ideal platform for enhanced light-matter interactions and studies of fundamental physical phenomena. On the other hand, the intrinsic ultra-short plasmon wavelength obstructs in-plane detectability of plasmon behaviors, like wavelength variations induced by biomolecule or dragging current. The detection of plasmon wavefront and its spatial shift relies on scattering-type scanning near-field microscopy with a spatial resolution of 20 nm. Here we propose a configuration which can efficiently separate ultra-confined plasmon region from detection region, guaranteeing both field confinement and in-plane sensitive detection of wavelength variations. As an example, the application in detecting Fizeau drag effect is demonstrated. Our study can be applied for detecting strong light-matter interactions, including fundamental physical studies and biosensing applications.

Selective trapping of chiral nanoparticles via vector Lissajous beams.

We report selective trapping of chiral nanoparticles via vector Lissajous beams. Local optical chirality densities appear in these beams by properly choosing the values of two parameters (p,q) that determine the polarization vectors of light. For a particular set of parameter (p,q) = (2,1) which is found preferable for the selective trapping, the resulting vector beam has two dominant intensity spots with opposite chirality. In the transverse plane, one spot traps a chiral particle while the other one repels the same particle under appropriate conditions, which can be reversed for a particle of opposite chirality. Various chiral parameters and radii of a particle are considered for analyzing this selective trapping effect. The longitudinal forces that are found non-conservative are also discussed. The achieved functionality of identifying and separating different chiral particles may find applications in enantiomer separation and drug delivery in pharmaceutics.

Plasmon-induced transparency in a reconfigurable composite valley photonic crystal.

We propose a new kind of reconfigurable topological valley photonic crystal (TVPC), and a novel topological waveguide can be formed by constructing a domain wall between two TVPCs with opposite valley-Chern indices. The topological waveguide mode in the composite TVPC has large group refractive index. A topologically protected coupled waveguide cavity system is then designed by introducing a hexagonal ring cavity at the center of the straight domain wall of a combined TVPC, in which a narrow plasmon induced transparency window rises at 3.8848 GHz with a Q-factor of 1387 and a maximum group refractive index as high as 186. We propose a notch filter with a resonant frequency of 3.8852 GHz and a very high Q-factor of 10224. By changing the refractive index of liquid crystals via an external voltage applied between two parallel metal plates, the filter can be switched between band-pass and band-stop based on the reconfigurable topological interface state.

Measuring saturable nonlinearity in atomic vapor via direct spatial mapping.

We demonstrate a scheme to measure the saturable nonlinearity of atomic vapor by mapping its nonlinear response function onto a light beam profile. Our analysis shows that a part of a nonlinear optical solution solved in a model governing the nonlinear beam dynamics in atomic vapor can be used to perform this measurement, even in the presence of large absorption. A desired beam profile is achieved by an evolution of a well-known structured beam, namely the Airy beam. Using this simple yet effective method, we retrieve the saturable nonlinear response function of rubidium (Rb) atomic vapor in experiment, and employ it in light propagation simulation that reproduces well observed nonlinear dynamics, which nevertheless cannot be fitted in a strong nonlinear regime with an ideal Kerr approximation. Our method is applicable to a broad spectrum of materials featured with saturable nonlinearities.

Asymmetric reflection based on asymmetric coupling in single-layer extrinsic chiral metasurfaces.

We propose and experimentally demonstrate that giant asymmetric reflection of circularly polarized light based on asymmetric coupling can be achieved in single-layer extrinsic chiral metasurfaces at oblique incidence. The asymmetric coupling and asymmetric reflection in the extrinsic chiral metasurfaces are caused by extrinsic chirality, allowing them to have extremely high values. An asymmetric reflection of approximately 40% is measured. Furthermore, the asymmetric reflection of extrinsic chiral metasurfaces is demonstrated not only in intensity but also in phase retardation, which induces asymmetric polarization state conversion. An approximately 14° asymmetric reflected polarization offset from the symmetry axis is achieved. Our research provides an effective new method for constructing huge asymmetric coupled systems to manipulate electromagnetic waves.

Broadband second-harmonic generation in step-chirped periodically poled lithium niobate waveguides.

Periodically poled lithium niobate (PPLN) structures on a chip enable efficient second-order nonlinear optical effects, benefiting from the tight light confinement and the utilization of d33. Here, we report a broadband second-harmonic (SH) generation in a step-chirped PPLN waveguide on X-cut lithium niobate on insulator (LNOI). The high fidelity of the poling period is demonstrated over the whole length of 7 mm using a non-destructive technique of piezoresponse force microscopy. The SH signal was continuously observed in the step-chirped PPLN waveguides while scanning the wavelength of the pump laser from 1550 nm to 1660 nm. The SH conversion efficiency was measured to be 9.6 % W-1 cm-2 at 1642 nm. This work will benefit wavelength conversions of light sources with wideband spectra.

Metasurfaces with high-Q resonances governed by topological edge state.

Achieving high-quality (Q)-factor resonances in metasurfaces is essential for various applications, including nano-lasers, nonlinear optics, and quantum optics. In this work, we propose a high-Q metasurface using a topological strategy: constructing the metasurface by stacking two conjugated nanopillar arrays with different topological invariants. Our study shows that a topological edge state steadily appears at the interfaces of the nanopillars, and a sharp transmission resonance with a Q-factor of more than 1000 can be obtained. The sensing application of such high-Q topological metasurface is also demonstrated, whose figure of merit reaches approximately 145. The proposed strategy and underlying theory can open up new avenues to realize ultrasharp resonances, which can promote numerous potential applications, such as biosensing, optical modulation, and slow-light devices.