Topological rainbows hold significant promise for robust on-chip frequency-division multiplexing. In this Letter, we report a spectral control mechanism of topological edge states by modulating boundary onsite potentials. Shifting upper-gap modes into the lower bandgap creates engineered edge states (EESs) with phase velocities opposite to conventional lower-gap edge states (LESs), achieving enhanced localization via anomalous main-side lobe matching. Topological rainbows leveraging these states show distinct behaviors. In parallel-plate waveguides, only BZ-boundary-directed LES rainbows yield over fourfold field enhancement. In open waveguide systems, BZ-center-directed rainbows incur radiation loss upon entering the light cone. Significantly, the reversed phase velocities of EESs invert the radiation direction relative to LESs. This research establishes a disorder-immune strategy for directional radiation control and interconnection in integrated photonic devices.

Objective With the rapid development of electronic technology, the requirements for wafer dicing techniques in chip packaging have become increasingly stringent. Traditional mechanical dicing tools are limited in depth control and unsuitable for wafers with complex surface structures such as waveguides. Although laser ablation techniques have been introduced to address these limitations, they often result in residue deposition and uncontrollable crack propagation, leading to chipping and damage at the wafer edge. This study addresses the cracking issue in the wafer dicing process caused by stress damage and proposes an optimized method based on stealth dicing technology. A dual-pulse-duration laser stealth dicing system is developed to enable efficient and precise cutting of both silicon-based and advanced compound wafers, such as sapphire and lithium niobate (LiNbO3). The system aims to minimize stress-induced damage while preserving the integrity of surface waveguide structures, thereby improving the reliability and lifespan of semiconductor devices. Methods A finite element method (FEM) combined with the J-integral approach is employed to simulate the stress field near cracks in multi-layered silicon wafers. The study focuses on the effects of crack depth and force application position on the crack opening displacement (COD) and stress distribution. A two-dimensional beam model is established to analyze the mechanical response of wafers during the dicing process and its influence on surface waveguide structures. In the experimental part, a dual-pulse-duration laser stealth dicing system is constructed, integrating a nanosecond Yb-doped fiber laser and a picosecond solid-state laser. The two laser beams are combined using a polarization beam splitter (PBS), and their scanning paths are controlled by a high-precision motion platform. Laser parameters and scanning strategies are optimized for different wafer materials, such as silicon-on-insulator (SOI), sapphire, quartz, and LiNbO3 , to validate the simulation results and evaluate the dicing quality. Results and Discussions The experimental results demonstrate that the proposed dual-pulse-duration laser stealth dicing system can achieve high-quality, damage-free dicing on various wafer materials. For silicon-based wafers, a stealth dicing depth of 120 mu m and an applied force at the center of the die are selected to minimize the surface stress response and preserve the integrity of waveguide structures (Figs. 9 and 10). Scanning electron microscope (SEM) confirms that the resulting waveguide end faces are smooth and flat, without the need for post-processing polishing (Figs. 12 and 13). For low-thermal-conductivity materials such as sapphire and quartz, picosecond laser stealth dicing is adopted to avoid thermal damage and maintain the quality of surface structures. A sapphire wafer with a 730 nm thick silicon nitride waveguide is successfully cut at a depth of approximately 70 mu m, achieving excellent structural preservation (Fig. 16). Similarly, a LiNbO3 wafer with a 600 nm thick LiNbO3 layer shows smooth and undamaged waveguide end faces after processing (Fig. 17). SEM images further confirms the absence of ablation residue, micro-chipping, or structural collapse, highlighting the system effectiveness in preserving micro/nano-scale features. Moreover, the system enables simultaneous dual-side stealth dicing of heterogeneously bonded wafers, eliminating the need for repeated alignment between different dicing systems and streamlining the overall dicing workflow, thus significantly improving production efficiency. Conclusions This study systematically reveals the influence of crack depth and force application position on the stress distribution during stealth dicing. The results show that when the crack depth exceeds 25 mu m, the crack opening displacement is stabilized and the surface stress is significantly reduced, providing a theoretical basis for damage-free dicing. The proposed dual-pulse-duration laser system is capable of adapting to both silicon-based and non-silicon-based materials, achieving high-precision and low-damage dicing. Compared with conventional mechanical and single-pulse-duration laser dicing technologies, the system proposed here exhibits superior performance in processing speed, chipping reduction, material compatibility, and structural integrity, particularly for wafers with waveguides, meta surfaces, and other complex structures. This research not only contributes to the advancement of domestic stealth dicing technology but also provides a feasible technical pathway for the future development of high-performance semiconductor and photonic devices with intricate designs.

Hyperbolic metasurfaces with hyperbolic dispersion have gained significant attention due to their unprecedented capabilities to manipulate the propagation of surface plasmon polaritons, such as non-divergent diffraction and polarization-controlled signal routing based on the plasmonic spin Hall effect. However, thus far, selective routing of surface waves has only been observed with the mechanism of plasmonic spin-orbit coupling, which depends on the polarization of light and the hyperbolic iso-frequency contours. Here, we propose and experimentally demonstrate symmetry-dependent selective electric and magnetic surface wave excitation around the geometric phase transition frequency with coexisting transverse electric and transverse magnetic polarization states. Effective medium theory and S-parameter retrieval process are further utilized to analyze the electromagnetic responses and dispersion characteristics of the designed asymmetric hyperbolic metasurface. These results open up a new avenue, to our knowledge, to realize integrated plasmonic devices, holding potential for applications in areas including imaging, sensing, and quantum information science.

Chiral metasurfaces with high-quality factors can significantly enhance light-matter interactions, making them highly valuable in biomolecular detection, optical sensing, and related applications. However, most experimentally demonstrated chiral devices rely on extrinsic chirality induced by oblique incidence or fabrication techniques involving slanted etching, which limits their practical applicability. Furthermore, the operation modes and functionalities of these devices are typically fixed after fabrication, restricting their adaptability. Here, a reconfigurable intrinsic chiral metasurface consisting of bi-layer photonic crystal slabs are experimentally demonstrated. The bi-layer architecture introduces strong interlayer coupling effects, enabling the realization of a nearly unity circular dichroism (CD) value under normal incidence in the terahertz regime by leveraging tunable topological polarization singularities. Our approach allows tunable and independent manipulation of CD in multiple operation modes, significantly enhancing the flexibility of chiral devices. This work paves the way for advanced polarization and phase manipulation, with promising applications in chiral lasers, chiral filters, and beyond.

Controlling topological modes in photonic systems remains a fundamental challenge,as conventional approaches rely on global lattice modifications and lack topological phase engineering of the induced non-trivial states.Here,we reveal that staggered onsite edge potential(SOEP)modulation breaks mirror symmetry in folded edge states,inducing edge-confined Wannier function deviations and thus driving edge bands into distinct topological phases.The emergence of resulting higher-order localized modes is further confirmed.Notably,the hosted corner modes are SOEP-selective,depending on the deviation direction of the Wannier function.Using spoof surface plasmon polariton photonic crystals,we experimentally confirm two SOEP-driven evolution regimes:symmetric evolution enabling corner-mode pumping between adjacent corners and asymmetric evolution restricting corner modes to negative potentials.These results demonstrate spatial control over corner states,linking edge potential engineer-ing to higher-order topology.Our study paves the way for manipulating the band structure and modeling topological phases of photonic states.

Metasurfaces, two-dimensional metamaterials comprising subwavelength units, have the unique ability to control the phase, amplitude, and polarization of incident electromagnetic waves [1]. Metasurfaces engineered with dielectric materials, such as silicon, demonstrate high transmission efficiency, making them ideally suited for generating non-diffractive beams in the spectrally-important terahertz frequency range.

In this paper, we propose a concept of resolution enhancement for sub-terahertz (THz) images by employing the coupling between a dielectric sphere and a time-domain THz spectrometer. The terajet effect is used as a simple approach to overcome the diffraction limit. Correlation between the resolutions in the x- and y-directions and the frequency of the incident THz beam is analyzed both analytically and experimentally.

Advanced micro–nano devices commonly require precise three-dimensional (3D) fabrication solutions for pre-designing and integrating 0D to 3D configurations. The additive–subtractive hybrid manufacturing strategy dominated by femtosecond laser direct writing has become an increasingly interesting technical route for material processing. In this study, a novel approach termed femtosecond adaptive optics-assisted hybrid manufacturing was proposed, which integrates subtractive (femtosecond laser ablation) and additive (two-photon polymerization) fabrication. In this hybrid manufacturing method, the introduction of adaptive optics offers parallel direct writing and wide-area material processing capabilities. To demonstrate the validity of the hybrid approach, on-chip surface plasmon polariton waveguides with strong sub-wavelength field confinement and enhanced functionality were successfully fabricated. In comparison with the terahertz-wave devices fabricated based on the focused ion beam technique, the functional tests in terahertz near-field microscopy show a rival performance fabricated with our hybrid approach. Besides, our cost-effective solution also dramatically reduces the fabricating time of excitation regions by a factor >16. Our work provides a new inspiration in integrated photonics.

Over the past decade, orbital angular momentum has garnered considerable interest in the field of plasmonics owing to the emergence of surface-confined vortices, known as plasmonic vortices. Significant progress has been made in the generation and manipulation of plasmonic vortices, which broadly unveil the natures of plasmonic spin-orbit coupling and provide accessible means for light-matter interactions. However, traditional characterizations in the frequency domain miss some detailed information on the plasmonic vortex evolution process. Herein, an exotic spin-orbit coupling phenomenon is demonstrated. More specifically, we theoretically investigated and experimentally verified a temporally deuterogenic vortex mode, which can be observed only in the time domain and interferes destructively in the intensity field. The spatiotemporal evolution of this concomitant vortex can be tailored with different designs and incident beams. This work extends the fundamental understanding of plasmonic spin-orbit coupling and provides a unique optical force manipulation strategy, which may fuel plasmonic research and applications in the near future.

Conversion from free-space waves to surface plasmons has been well studied as a key aspect of plasmonics. In particular, efficient coupling and propagation of surface plasmons via phase gradient metasurfaces are of great current research interest. Hereby, we demonstrate a terahertz metacoupler based on a bilayer bright-dark mode coupling structure attaining near-perfect conversion efficiency (exceeding 95%) without considering absorption loss of the materials and maintaining a high conversion level even when the area of the excitation region changes. To validate our design, a fabricated metacoupler was assessed by scanning near-field terahertz microscopy. Our findings could pave the way for developing high-performance plasmonic devices encompassing ultra-thin and compact functional devices for a diverse range of applications, especially within the realm of high-speed terahertz communications. (c) 2024 Chinese Laser Press

Topological photonics provides a platform for robust energy transport regardless of sharp corners and defects. Recently, the frequency multiplexing topological devices have attracted much attention due to the ability to separate optical signals by wavelength and hence the potential application in optical communication systems. Existing frequency multiplexing topological devices are generally based on the slow light effect. However, the resulting static local spatial mode or finely tuned flat band has zero-group velocity, making it difficult for both experimental excitation and channel out-coupling. Here, we propose and experimentally demonstrate an alternative prototype of asymmetric frequency multiplexing devices including a topological rainbow and frequency router based on floating topological edge mode (instead of localized ones); hence the multiple wavelength channels can be collectively excited with a point source and efficiently routed to separate output ports. The channel separation in our design is achieved by gradually tuning the band gap truncation on a topological edge band over a wide range of frequencies. A crucial feature lies in that the topological edge band is detached from bulk states and floating within the upper and lower photonic band gaps. More interestingly, due to the sandwiched morphology of the edge band, the top and bottom band gaps will each truncate into transport channels that support topological propagation towards opposite directions, and the asymmetrical transportation is realized for the frequency multiplexing topological devices.

Terahertz integrated devices have attracted great attention due to their potential applications in communication systems. Although some functional devices based on terahertz spoof surface plasmon polariton (SPP) waveguides have been studied, multimode waveguides and multimode interference mechanism have rarely been explored. This work investigates several key functional plasmonic devices based on multi-SPP mode interference in the terahertz regime. The feasibility of multimode transmission on terahertz plasmonic waveguides and control of the propagation modes by varying the waveguide parameters are first investigated. Then, the interference mechanism between two or more propagation modes and the self-imaging principle are applied to plasmonic waveguides. Finally, a series of terahertz plasmonic functional devices such as a coupler, wavelength diplexers, and cascaded devices are proposed and simulated by exploring different interference lengths, frequencies, and cascades via the self-imaging principle. These devices have good performance in transmission response, crosstalk, bandwidth, and footprint, and are expected to play an important role in the development of terahertz on-chip communication systems.

Flexible multiplexing chips that permit reconfigurable multidimensional channel utilization are indispensable for revolutionary 6G terahertz communications,but the insufficient manipulation capability of terahertz waves prevents their practical implementation.Herein,we propose the first experimental demonstration of a flexible multiplexing chip for terahertz communication by revealing the unique mechanism of topological phase(TP) transition and perseveration in a heterogeneously coupled bilayer valley Hall topological photonic system.The synthetic and individual TPs operated in the coupled and decoupled states enable controllable on-chip modular TP transitions and subchannel switching.Two time-frequency interleaved subchannels support 10-and 12-Gbit/s QAM-16 high-speed data streams along corresponding paths over carriers of 120 and 130 GHz with 2.5-and 3-GHz bandwidths,respectively.This work unlocks interlayer heterogeneous TPs for inspiring ingenious on-chip terahertz-wave regulation,allowing functionality-reconfigurable,compactly integrated and CMOS-compatible chips.

Terahertz (THz) metasurfaces have emerged as a powerful tool for manipulating THz wavefronts, typically achieved by adjusting various geometric parameters. In this study, an approach is introduced by incorporating interlayer coupling into twisted stacking metasurface design to completely control the amplitude and phase of circularly polarized THz waves. By leveraging the interlayer coupling effect and the Pancharatnam–Berry phase, the study achieves efficient control over transmission phase and amplitude by simply adjusting the relative twist angle between paired C‐shaped split‐ring resonators. To validate the concept, a holographic metasurface is fabricated and characterized, providing experimental evidence of its THz wavefront manipulation capabilities. This design strategy presents a versatile and tunable solution for THz wave control, promising applications in a wide range of functional devices.

Metasurfaces offer an exciting opportunity to manipulate electromagnetic waves, presenting vast potential across diverse applications. In this study, we introduce a novel deep learning approach that integrates an Autoencoder with a Multi-Layer Perceptron to effectively forecast the Terahertz (THz) spectral response of metasurfaces. By harnessing a large dataset of training examples, our model adeptly captures the intricate correlation between metasurface structures and their optical responses, circumventing the traditionally time-consuming analysis of complex patterns. This proposed methodology furnishes a valuable tool for examining the THz transmission response of metasurfaces and has the potential to expedite metasurface design processes.

Valley topological photonic crystals (TPCs), which are robust against local disorders and structural defects, have attracted great research interest, from theoretical verification to technical applications. However, previous works mostly focused on the robustness of topologically protected edge states and little attention was paid to the importance of the photonic bandgaps (PBGs), which hinders the implementation of various multifrequency functional topological photonic devices. Here, by systematically studying the relationship between the degree of symmetry breaking and the working bandwidth of the edge states, we present spoof surface plasmon polariton valley TPCs with broadband edge states and engineered PBGs, where the operation frequency is easy to adjust. Furthermore, by connecting valley TPCs operating at different frequencies, a broadband multifunctional frequency-dependent topological photonic device with selectively directional light transmission is fabricated and experimentally demonstrated, achieving the functions of wavelength division multiplexing and add-drop multiplexing. We provide an effective and insightful method for building multi-frequency topological photonic devices.

Free manipulation of electromagnetic waves in the terahertz[THz]band based on metasurface functional devices has been the focus of research in recent years.Among these devices,active metasurfaces have generated extensive research inter-est due to their reconfigurability.In this work,we demonstrate a mechanically reconfigurable THz polarization converter that consists of two parallel transmissive metasurfaces with a tunable spacing.By mechanically adjusting the coupling strength between the metasurfaces,the orthogonal polarization conversion of the incident linearly polarized THz waves can be tuned.Specifically,the device can be tuned from efficient dual-frequency orthogonal polarization conversion to efficient single-frequency orthogonal polarization conversion.After a gradual decrease in efficiency,it is finally changed to a low transmission state as the gap distance increases from 150 to 800 μm.We theoretically analyze the tuning process under different spacings and experimentally verify it using a vector network analyzer.Our proposed design is straightforward and robust,with the potential to find wide applications in THz science and technology.

Efficiently converting incident free-space light into surface plasmon polaritons (SPPs) and improving the integration of on-chip plasmonic devices in the terahertz regime remain great challenges. Here, a single ultra-compact device is proposed to efficiently couple propagating waves into on-chip terahertz plasmonic waveguides. We first design a high-efficiency meta-coupler composed of isotropic meta-atoms with an absolute excitation efficiency of 82%. This scheme is further extended using anisotropic meta-atoms to realize another meta-coupler that can efficiently couple orthogonal linearly polarized light into SPPs propagating in orthogonal directions. Next, we use the same anisotropic meta-atoms to efficiently convert free-space light into an SPP beam and then focus the beam to a spot (with a focusing efficiency of approximately 59%), which is then fed into a subwavelength on-chip plasmonic waveguide with an absolute working efficiency of 53%. Finally, a meta-coupler is constructed that can achieve polarization-controlled unidirectional SPP excitation with a focused wavefront in different directions and squeeze the focused SPPs into a dual-waveguide system. Our design scheme that can efficiently couple propagating light into SPPs has potential applications in on-chip devices in the terahertz regime. (c) 2024 Optica Publishing Group

The terahertz regime is widely recognized as a fundamental domain with significant potential to address the demands of next-generation wireless communications. In parallel, mode division multiplexing based on orbital angular momentum (OAM) shows promise in enhancing bandwidth utilization, thereby expanding the overall communication channel capacity. In this study, we present both theoretical and experimental demonstrations of an on-chip terahertz OAM demultiplexer. This device effectively couples and steers seven incident terahertz vortex beams into distinct high-quality focusing surface plasmonic beams, and the focusing directions can be arbitrarily designated. The proposed design strategy integrates space-to-chip mode conversion, OAM recognition, and on-chip routing in a compact space with subwavelength thickness, exhibiting versatility and superior performance.

Metasurfaces offer remarkable capabilities for manipulating electromagnetic waves and by incorporating multiplexing techniques can significantly increase the versatility of design possibilities. Here, we designed and experimentally demonstrated a series of dual non-diffractive beam generators for terahertz radiation based on all-dielectric metasurfaces. These generators could produce switch- able Bessel beams and abruptly autofocusing beams depending on the spin and frequency of the incident terahertz waves. In addition, by further applying appropriate phase gradients in the design, these non-diffractive beams could be deflected in specified directions. It is also possible to simultaneously generate multiple non-diffractive beams with different properties. The generated non-diffractive beams were measured with near-field scanning terahertz microscopy, and the results agreed well with simulations. We believe that these metasurface-based beam generators hold tremendous potential in terahertz imaging, communications, non-destructive evaluation, and many other applications.