Volume 14, Issue 3

Terahertz (THz) technology has attracted significant global interest, particularly in sensing applications, due to its nonionizing feature and sensitivity to weak interactions. Recently, owing to the advantages of low optical loss and the capability to support both electric and magnetic high-quality factor (high-Q) resonances, dielectric metasurfaces have emerged as a powerful platform for multiscenario terahertz sensing applications. This review summarizes recent advancements in dielectric metasurface-assisted THz sensing. We begin with an overview of the mechanisms and properties of dielectric metasurfaces with high-Q factors. Next, we discuss typical fabrication techniques for these terahertz dielectric metasurfaces. We then explore the diverse terahertz sensing applications across various scenarios, including biomolecule sensing, biomedical detection, environmental monitoring, and chiral sensing. Finally, we provide perspectives on the future development of this promising research field.

Bragg gratings are crucial components in passive photonic signal processing, with wide-ranging applications including biosensing, pulse compression, photonic computing, and addressing. However, the design of integrated waveguide Bragg gratings (WBGs) for arbitrary wavelengths presents significant challenges, especially when dealing with highly asymmetric layer stacks and large refractive index contrasts. Convenient approximations used for fiber Bragg gratings generally break down in these cases, resulting in nontrivial design challenges. In this work, we introduce a general simulation and design framework for WBGs, which combines coupled mode theory with three-dimensional finite-element method eigenfrequency computations. This approach allows for precise design and optimization of WBGs across a broad range of device layer stacks. The design flow is applicable to further layer stacks across nearly all wavelengths of interest, given that the coupling between the forward and backward propagating mode is dominant.

Total internal reflection generally occurs at incident angles beyond the critical angle, confining electromagnetic waves in dielectrics with higher refractive indices. In this work, we present a metasurface-based solution to transform such total reflection into high-efficiency transmission. We demonstrate that a phase-gradient antireflection metasurface designed on the dielectric surface not only compensates for the transverse wave vectors of the incident and transmitted waves but also addresses the impendence mismatch between the two media, eventually achieving high-efficiency transmission with flexibly-controlled wavefronts beyond the critical angle. The design of this unique metasurface is enabled by applying the reciprocity principle to circumvent the traditional limitation of total internal reflection. The theory and functionalities of the phase-gradient antireflection metasurfaces are verified through both simulations and microwave experiments. Our work opens a new avenue for high-efficiency radiation manipulation beyond the critical angle, enabling rich applications such as high-efficiency waveguide-to-free-space couplers, high-radiation-efficiency quantum dots, and high-radiation-efficiency light-emitting diodes.

Metalenses, with their compact form factor and unique optical capabilities, hold tremendous potential for advancing computer vision applications. In this work, we propose a high-resolution, large field-of-view (FOV) metalens intelligent recognition system, combining the latest YOLO framework, aimed at supporting a range of vision tasks. Specifically, we demonstrate its effectiveness in scanning, pose recognition, and object classification. The metalens we designed to achieve a 100° FOV while operating near the diffraction limit, as confirmed by experimental results. Moreover, the metalenses weigh only 0.1 g and occupy a compact volume of 0.04 cm 3 , effectively addressing the bulkiness of conventional lenses and overcoming the limitations of traditional metalens in spatial frequency transmission. This work highlights the transformative potential of metalenses in the field of computer vision, The integration of metalenses with computer vision opens exciting possibilities for next-generation imaging systems, offering both enhanced functionality and unprecedented miniaturization.

Low-dimensional material-based heterogeneous silicon photonics has attracted significant attention due to their applications in developing integrated optoelectronic devices from the telecommunication band to mid-infrared wavelengths. However, the study of waveguide components integrated with low-dimensional materials for mode-division multiplexing (MDM) applications mostly remains in its infancy. In this paper, we demonstrated waveguide-integrated spatial mode filters by integrating subtly designed ten-layer PtSe 2 nanoribbons on an ultrathin silicon waveguide with a deep-subwavelength thickness to eliminate modal crosstalk. To be specific, the undesirable propagating mode can be filtered out due to its strong interaction with the PtSe 2 nanoribbons on the silicon waveguide surface. Our results show that TE 1 -to-TE 0 and TE 2 -to-TE 0 modal extinction ratios of 12 dB and 14.5 dB were measured in 100 and 75-μm-long PtSe 2 -on-silicon waveguides at 2200-nm wavelengths. Our study paves the intriguing approach to developing waveguide-integrated spatial mode filters for on-chip MDM applications for optical interconnects and optical communications.

Color centers in hexagonal boron nitride (hBN) are promising candidates as quantum light sources for future technologies. In this work, we utilize a scattering-type near-field optical microscope (s-SNOM) to study the photoluminescence (PL) emission characteristics of such quantum emitters in metalorganic vapor phase epitaxy grown hBN. On the one hand, we demonstrate direct near-field optical excitation and emission through interaction with the nanofocus of the tip resulting in a subdiffraction limited tip-enhanced PL hotspot. On the other hand, we show that indirect excitation and emission via scattering from the tip significantly increases the recorded PL intensity. This demonstrates that the tip-assisted PL (TAPL) process efficiently guides the generated light to the detector. We apply the TAPL method to map the in-plane dipole orientations of the hBN color centers on the nanoscale. This work promotes the widely available s-SNOM approach to applications in the quantum domain including characterization and optical control.

Bound states in the continuum (BICs) are localized states within the radiative continuum that exhibit high quality-factor (Q-factor) resonance, which significantly boosts light–matter interactions. However, out-of-plane radiation losses can arise from inherent material absorption and inevitable technological imperfections during fabrication process. Merging BICs have been introduced as a solution to address the issue of out-of-plane radiation losses. By merging BICs, it is possible to expand the area of high Q-factor resonance in momentum space, thereby enhancing the system’s robustness against external perturbations. However, achieving this enhancement is contingent upon altering the geometrical parameters of the structure, which inherently restricts its dynamic tunability. Here, we propose an emerging approach that integrates phase change materials (PCMs) into photonic crystal slabs (PCs) metasurface, enabling dynamically tuning of merged BICs. By utilizing low-loss Sb 2 S 3 as a tunable PCMs, we demonstrate that altering its phase state can merge BICs, leading to a substantial increase in the high Q-factor across an extended range of wave vectors space. Furthermore, this study validates the universality and robustness of merging BICs against common unit-cell topology fabrication defects. Additionally, by twisting the square holes to break in-plane symmetry, asymmetric merging and inversion of topological charge at the Γ -point are achieved. This approach leverages phase-transition states of PCMs to enable reconfigurable polarization distribution of radiation field without scale and parameter changes, which is tunable and offers promising potential applications in optical vortices and nano-lasers.

Ultrafast pulses, particularly those with durations under 100 fs, are crucial in achieving unprecedented precision and control in light–matter interactions. However, conventional on-chip photonic platforms are not inherently designed for ultrafast time-domain operations, posing a significant challenge in achieving essential parameters such as high peak power and high temporal resolution. This challenge is particularly pronounced when propagating through integrated waveguides with nonlinear and high-dispersion profiles. In addressing this challenge, we present a design methodology for ultrafast pulse propagation in dispersive integrated waveguides, specifically focused on enhancing the time-domain characteristics of one-dimensional grating waveguides (1DGWs). The proposed methodology aims to determine the optimal structural parameters for achieving maximum peak power, enhanced temporal resolution, and extended pulse storage duration during ultrafast pulse propagation. To validate this approach, we design and fabricate two specialized 1DGWs on a silicon-on-insulator (SOI) platform. A digital finite impulse response (FIR) model, trained with both transmission and phase measurement data, is employed to obtain ultrafast time-domain characteristics, enabling easy extraction of these results. Our approach achieves a 2.8-fold increase in peak power and reduces pulse broadening by 24 %, resulting in a smaller sacrifice in temporal resolution. These results can possibly pave the way for advanced light–matter interactions within dispersive integrated waveguides.

In the past decade, the field of topological photonics has gained prominence exhibiting consequential effects in quantum information science, lasing, and large-scale integrated photonics. Many of these topological systems exhibit protected states, enabling robust travel along their edges without being affected by defects or disorder. Nonetheless, conventional topological structures often lack the flexibility for implementing different topological models and for tunability postfabrication. Here, we present a method to implement magnetic-like Hamiltonians supporting topologically protected edge modes on a general-purpose programmable silicon photonic mesh of interferometers. By reconfiguring the lattice onto a two-dimensional mesh of ring resonators with carefully tuned couplings, we show robust edge state transport even in the presence of manufacturing tolerance defects. We showcase the system’s reconfigurability by demonstrating topological insulator lattices of different sizes and shapes and introduce edge and bulk defects to underscore the robustness of the photonic edge states. Our study paves the way for the implementation of photonic topological insulators on general-purpose programmable photonics platforms.

Arrays of atomic emitters have proven to be a promising platform to manipulate and engineer optical properties, due to their efficient cooperative response to near-resonant light. Here, we theoretically investigate their use as an efficient metalens. We show that, by spatially tailoring the (subwavelength) lattice constants of three consecutive two-dimensional arrays of identical atomic emitters, one can realize a large transmission coefficient with arbitrary position-dependent phase shift, whose robustness against losses is enhanced by the collective response. To characterize the efficiency of this atomic metalens, we perform large-scale numerical simulations involving a substantial number of atoms ( N ∼ 5 × 10 5 ) that is considerably larger than comparable works. Our results suggest that low-loss, robust optical devices with complex functionalities, ranging from metasurfaces to computer-generated holograms, could be potentially assembled from properly engineered arrays of atomic emitters.

Circular dichroism – the spin-selective absorption of light – finds diverse applications in medicine, antennas and microwave devices. In this work, we propose and experimentally demonstrate an ultrathin electronically reconfigurable chiral metasurface which exploits the intrinsic symmetries of the meta-molecule to realize any spin absorption based on the handedness of the chirality chosen. We construct the left-chiral and right-chiral states by reconfiguring the meta-molecule into two enantiomeric states, which achieve strong circular dichroism exceeding 82 % at the design frequency of 9.5 GHz. The meta-molecule can be switched into a third (non-chiral) state which is isotropic and transparent. The achieved circular dichroism characteristics remain insensitive to incidence angles up to ±45°. The proposed reconfigurable chiral metasurface achieves left- and right- circular dichroism at the same frequency and with high efficiency, and is an attractive candidate for wide-ranging practical applications in imaging, wireless communication and medicine.

Chirality of exciton-polaritons can be tuned by the chirality of photons, excitons, and their coupling strength. In this work, we propose a general analytical model based on coupled harmonic oscillators to describe the chirality of exciton-polaritons. Our model predicts the degree of circular polarization (DCP) of exciton-polaritons, which is determined by the DCPs and weight fractions of the constituent excitons and photons. At the anticrossing point, the DCP of exciton-polaritons is equally contributed from both constituents. Away from the anticrossing point, the DCP of exciton-polaritons relaxes toward the DCP of the dominant constituent, with the relaxation rate decreasing as the coupling strength increases. We validate our model through simulations of strongly coupled topological edge states and excitons, showing good agreement with model predictions. Our model provides a valuable tool for designing the chirality of strong coupling systems and offers a framework for the inverse design of exciton-polaritons with tailored chirality.

The perfect vector vortex beams (PVVBs) have played an important role in various fields due to their advantages of unique vortex features, flexible polarization distribution and multiple degrees of freedom (DoFs). The simultaneous and precise control over multiple DoFs, such as the polarization distribution, beam shape and position which greatly influence various characteristics of PVVBs, holds paramount importance. However, it is still difficult to manipulate various DoFs in a multiplexing way and the control precision of polarization distribution only reaches the half-integer level, notably hindering the further application and development of PVVBs. Here, an approach that integrates holographic technique with geometric phase metasurfaces, experimentally demonstrates the multiplexing control of PVVBs over three DoFs, i.e., enabling the independent manipulation of non-uniform polarization distributions, beam shapes and spatial positions. Furthermore, non-integer polarization order of the generated PVVBs can be arbitrary non-integer numbers with a high resolution of 0.1, largely improving the control precision. With such multiplexing manipulation of PVVBs with high precision, we can provide abundant processing dimensions for information science and technologies, exhibiting broad application potentials in fields such as information encryption, high-speed optical communication, and precise particle manipulation.