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Lithium niobate Mach–Zehnder modulators (MZMs) with compact footprint and fast electro-optics (EO) responses are highly demanded for the next-generation optical interconnect systems. Here, we demonstrate slow-light (SL) effect using a coupled Bragg resonator structure on the thin-film lithium niobate (TFLN) platform, and an ultra-compact SL-MZM with length of ∼370 μm is also constructed. The fabricated SL waveguides show a large optical passband width of ∼8 nm, an insertion loss of 2.9 dB, and a maximal optical group index of 7.50, corresponding to 3.4 times as large as that of regular TFLN rib waveguide. The fabricated SL-MZM exhibits a large EO bandwidth of >50 GHz in an operating wavelength band of ∼8 nm as well. High-speed OOK transmissions at data rates of 64 Gbit/s and 80 Gbit/s are successfully achieved. To our best knowledge, it is first time to build SL waveguides and compact SL-MZMs with large EO bandwidths of >50 GHz on the monolithic TFLN platform.

Solar energy is regarded as one of the most plentiful sources of renewable energy. An extraordinary light-harvesting property of a germanium periodic nanopyramid array is reported in this Letter. Both our theoretical and experimental results demonstrate that the nanopyramid array can achieve perfect broadband absorption from 500- to 800-nm wavelength. Especially in the visible regime, the experimentally measured absorption can even reach 100%. Further analyses reveal that the intrinsic antireflection effect and slow-light waveguide mode play an important role in the ultra-high absorption, which is helpful for the research and development of photovoltaic devices.

Tunable slow light systems have gained much interests recently due to their efficient control of strong light–matter interactions as well as their huge potential for realizing tunable device applications. Here, a dynamically tunable polarization independent slow light system is experimentally demonstrated via electromagnetically induced transparency (EIT) in a terahertz (THz) metasurface constituted by plus and dimer-shaped resonators. Optical pump-power dependent THz transmissions through the metasurface samples are studied using the optical pump THz probe technique. Under various photoexcitations, the EIT spectra undergo significant modulations in terms of its resonance line shapes (amplitude and intensity contrast) leading to dynamic tailoring of the slow light characteristics. Group delay and delay bandwidth product values are modulated from 0.915 ps to 0.42 ps and 0.059 to 0.025 as the pump fluence increases from 0 to 62.5 nJ cm −2 . This results in tunable slow THz light with group velocities ranging from 2.18 × 10 5  m s −1 to 4.76 × 10 5  m s −1 , almost 54% change in group velocity. The observed tuning is attributed to the photo-induced modifications of the optoelectronic properties of the substrate layer. The demonstrated slow light scheme can provide opportunities for realizing dynamically tunable slow light devices, delay lines, and other ultrafast devices for THz domain.

This paper proposed a plasmonic resonator system, consisting of a metal-insulator-metal structure and two stubs, and a Fano resonance arose in its transmittance, which resulted from the coupling between the two stubs. On the basis of the proposed structure, a circle and a ring cavity are separately added above the stubs to create different coupled plasmonic structures, providing triple and quadruple Fano resonances, respectively. Additionally, by adjusting the geometric parameters of the system, multiple Fano Resonances obtained can be tuned. The proposed structure can be served as a high efficient refractive index sensor, yielding a sensitivity of 2000 nm/RIU and figure of merit (FOM) of 4.05 × 10 4 and performing better than most of the similar structures. It is believed that the proposed structure may support substantial applications for on-chip sensors, slow light and nonlinear devices in highly integrated photonic circuits.

A dual-frequency on-off modulator with considerable modulation depth (MD) and relatively low insertion loss (IL) is performed with patterned monolayer graphene metamaterial. Destructive interference in this structure gives rise to the dual plasmon-induced transparency (DPIT) phenomenon. The coupled mode theory, confirmed by simulated values, is comprehensively introduced to expound the physical mechanism of the DPIT effect. In addition, the influences of the Fermi level on the DPIT transmission spectrum and the carrier mobility of graphene on the on-off modulation are researched. It is found that the dual-frequency on-off modulator exhibits remarkable modulation performance on both switches and is easier to fabricate in operation than other multi-layer graphene-based modulators. In the 'on1/off1' state, the MD and IL are 93%, 0.32 dB, respectively. In the 'on2/off2' state, the MD and IL are 85%, 0.25 dB, separately. Moreover, the property of slow light reflected by the group index is analyzed. It exhibits that the group index of the proposed structure with multi-channel can reach 358. Thus, the proposed structure stretches the versatile applications in multi-function modulators and multi-channel slow light devices at the terahertz band.

The rainbow trapping effect has been demonstrated in electromagnetic and acoustic waves. In this study, rainbow trapping of ultrasonic guided waves is achieved in chirped phononic crystal plates that spatially modulate the dispersion, group velocity, and stopband. The rainbow trapping is related to the progressively slowing group velocity, and the extremely low group velocity near the lower boundary of a stopband that gradually varies in chirped phononic crystal plates. As guided waves propagate along the phononic crystal plate, waves gradually slow down and finally stop forward propagating. The energy of guided waves is concentrated at the low velocity region near the stopband. Moreover, the guided wave energy of different frequencies is concentrated at different locations, which manifests as rainbow guided waves. We believe implementing the rainbow trapping will open new paradigms for guiding and focusing of guided waves. Moreover, the rainbow guided waves with energy concentration and spatial separation of frequencies may have potential applications in nondestructive evaluation, spatial wave filtering, energy harvesting, and acoustofluidics.

Spectrally narrow optical resonances can be used to generate slow light, i.e., a large reduction in the group velocity. In a previous work, we developed hybrid 2D semiconductor plasmonic structures, which consist of propagating optical frequency surface-plasmon polaritons interacting with excitons in a semiconductor monolayer. Here, we use coupled exciton-surface plasmon polaritons (E-SPPs) in monolayer WSe 2 to demonstrate slow light with a 1300 fold decrease of the SPP group velocity. Specifically, we use a high resolution two-color laser technique where the nonlinear E-SPP response gives rise to ultra-narrow coherent population oscillation (CPO) resonances, resulting in a group velocity on order of 10 5  m/s. Our work paves the way toward on-chip actively switched delay lines and optical buffers that utilize 2D semiconductors as active elements. Slow light effects are interesting for telecommunications and quantum photonics applications. Here, the authors use coupled exciton-surface plasmon polaritons (SPPs) in a hybrid monolayer WSe 2 -metallic waveguide structure to demonstrate a 1300-fold reduction of the SPP group velocity.

Refractive index fronts propagating in waveguides are special spatiotemporal perturbations. The interaction of light with such fronts can be described in terms of an indirect transition where the frequency and wavenumber of a guided mode both are changed. In recent years, front-induced transitions have been used in dispersion-engineered waveguides for frequency conversion, optical delays, and bandwidth and pulse duration manipulation. These concepts have originated from different research areas of photonics, such as nonlinear fibre optics, slow-light waveguides, plasma physics, moving media and relativistic effects. Here, we discuss these concepts, providing a unifying theoretical description and highlight the potential of this exciting research field for light manipulation in guided optics.

Metal–insulator–metal waveguide structure, which has a fascinating feature to confine the signal far beyond the diffraction light is numerically investigated by the finite difference time domain and the finite element methods. In this study, the MIM waveguide is both coupled with a half-elliptical groove (HEG) and an elliptical cavity resonator (ECR), and it can support the propagation of light in the nanoscale regime at the visible and near-infrared ranges. The interaction between these last elements gives rise to Fano resonance modes. Thanks to its interesting characteristics, a high sensitivity value, a factor of merit and interesting value of the group index are obtained for the proposed structure. We show that the transmission of the Fano system and the group index can reach 90% and a value of 63, respectively. We also report an investigation of the influence of the both geometrical HEG and ECR’s parameters on optical properties. Hence, the proposed structure could find a potential for applications in the integrated optical circuits such as optical storage, ultrafast plasmonic switchers, high performance filters and slow light devices.

We report the experimental observation of a trapped rainbow in adiabatically graded metallic gratings, designed to validate theoretical predictions for this unique plasmonic structure. One-dimensional graded nanogratings were fabricated and their surface dispersion properties tailored by varying the grating groove depth, whose dimensions were confirmed by atomic force microscopy. Tunable plasmonic bandgaps were observed experimentally, and direct optical measurements on graded grating structures show that light of different wavelengths in the 500-700-nm region is \"trapped\" at different positions along the grating, consistent with computer simulations, thus verifying the \"rainbow\" trapping effect.