Phototunable chip-scale topological photonics: 160 Gbps waveguide and demultiplexer for THz 6G communication

Flexible Terahertz Broadband Absorber Based on a Copper Composite Film

Terahertz absorbers play a crucial role in terahertz detectors, radar stealth, electromagnetic shielding, and other fields. However, the design and fabrication of flexible terahertz broadband absorbers remain a challenge at present. Here, we demonstrated a terahertz broadband absorber based on a copper composite film (CCF) consisting of a copper foam and an organic silica gel doped with Fe3O4 powder. The CCF can be fabricated by the infiltration method. The influence of the thickness and the pore size of the copper foam and the mass fraction of doped Fe3O4 powder on the absorption bandwidth were investigated. When the thickness of the CCF is 1.5 mm, the pore size of the copper foam is 95 pores per inch (ppi), and the mass fraction of Fe3O4 is 1%; a broadband absorption is achieved in the range of 0.11-3.5 THz. It is noted that the mass fraction of Fe3O4 has a significant impact on the absorption bandwidth. In addition, the thickness of the CCF and the pore size of the copper foam also have an impact on the absorption. The impedance matching theory is introduced to understand the mechanism of broadband absorption. This flexible broadband absorber has potential application in terahertz stealth, shielding, and the sixth-generation (6G) broadband wireless communication in the future.

Integrated terahertz topological valley-locked power divider with arbitrary power ratios

Integrated power dividers (PDs) are essential in terahertz (THz) communication and radar systems, but miniaturization often leads to performance degradation due to fabrication inaccuracies and sharp bends. Topological photonics offers a solution to these issues, yet creating THz power dividers with arbitrary splitting ratios remains challenging. We present a design methodology for on-chip topological THz power dividers with customizable splitting ratios using valley-locked photonic crystals. These crystals feature a tri-layered structure with two distinct valley Chern number layers and an intermediate semimetal layer. Utilizing the Jackiw-Rebbi model, we show that the characteristic impedance of the valley-locked photonic crystals, and thus the power division ratio, can be tuned by adjusting the semimetal layer width. Our approach is validated through simulations and experiments for both equal (1:1) and unequal (4:9) power ratios. This method enables efficient navigation around sharp bends and robust THz on-chip connectivity.

Ultrafast Unidirectional On-Chip Heat Transfer

Effective and rapid heat transfer is critical to improving electronic components' performance and operational stability, particularly for highly integrated and miniaturized devices in complex scenarios. However, current thermal manipulation approaches, including the recent advancement in thermal metamaterials, cannot realize fast and unidirectional heat flow control. In addition, any defects in thermal conductive materials cause a significant decrease in thermal conductivity, severely degrading heat transfer performance. Here, the utilization of silicon-based valley photonic crystals (VPCs) is proposed and numerically demonstrated to facilitate ultrafast, unidirectional heat transfer through thermal radiation on a microscale. Utilizing the infrared wavelength region, the approach achieves a significant thermal rectification effect, ensuring continuous heat flow along designed paths with high transmission efficiency. Remarkably, the process is unaffected by temperature gradients due to the unidirectional property, maintaining transmission directionality. Furthermore, the VPCs' inherent robustness affords defect-immune heat transfer, overcoming the limitations of traditional conduction methods that inevitably cause device heating, performance degradation, and energy waste. The design is fully CMOS compatible, thus will find broad applications, particularly for integrated optoelectronic devices.

3D Bulk Metamaterials with Engineered Optical Dispersion at Terahertz Frequencies Utilizing Amorphous Multilayered Split-Ring Resonators

A 3D bulk metamaterial (MM) containing amorphous multilayered split-ring resonators is proposed, fabricated, and evaluated. Experimentally, the effective refractive index is engineered via the 3D bulk MM, with a contrast of 0.118 across the frequency span from 0.315 to 0.366 THz and the index changing at a slope of 2.314 per THz within this frequency range. Additionally, the 3D bulk MM exhibits optical isotropy with respect to polarization. Moreover, the peak transmission and optical dispersion are tailored by adjusting the density of the split-ring resonators. Compared to reported conventional approaches for constructing bulk MMs, this approach offers advantages in terms of the potential for large-scale manufacturing, the ability to adopt any shape, optical isotropy, and rapid optical dispersion. These features hold promise for dispersive optical devices operating at THz frequencies, such as high-dispersive prisms for high-resolution spectroscopy.

Monolayer Vacancy-Induced MXene Memory for Write-Verify-Free Programming

The fundamental logic states of 1 and 0 in Complementary Metal-Oxide-Semiconductor (CMOS) are essential for modern high-speed non-volatile solid-state memories. However, the accumulated storage signal in conventional physical components often leads to data distortion after multiple write operations. This necessitates a write-verify operation to ensure proper values within the 0/1 threshold ranges. In this work, a non-gradual switching memory with two distinct stable resistance levels is introduced, enabled by the asymmetric vertical structure of monolayer vacancy-induced oxidized Ti3C2Tx MXene for efficient carrier trapping and releasing. This non-cumulative resistance effect allows non-volatile memories to attain valid 0/1 logic levels through direct reprogramming, eliminating the need for a write-verify operation. The device exhibits superior performance characteristics, including short write/erase times (100 ns), a large switching ratio (≈3 × 104), long cyclic endurance (>104 cycles), extended retention (>4 × 106 s), and highly resistive stability (>104 continuous write operations). These findings present promising avenues for next-generation resistive memories, offering faster programming speed, exceptional write performance, and streamlined algorithms.

Terahertz flexible multiplexing chip enabled by synthetic topological phase transitions

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.

On-chip topological beamformer for multi-link terahertz 6G to XG wireless

Terahertz (THz) wireless communication holds immense potential to revolutionize future 6G to XG networks with high capacity, low latency and extensive connectivity. Efficient THz beamformers are essential for energy-efficient connections, compensating path loss, optimizing resource usage and enhancing spectral efficiency. However, current beamformers face several challenges, including notable loss, limited bandwidth, constrained spatial coverage and poor integration with on-chip THz circuits. Here we present an on-chip broadband THz topological beamformer using valley vortices for waveguiding, splitting and perfect isolation in waveguide phased arrays, featuring 184 densely packed valley-locked waveguides, 54 power splitters and 136 sharp bends. Leveraging neural-network-assisted inverse design, the beamformer achieves complete 360° azimuthal beamforming with gains of up to 20 dBi, radiating THz signals into free space with customizable user-defined beams. Photoexciting the all-silicon beamformer enables reconfigurable control of THz beams. The low-loss and broadband beamformer enables a 72-Gbps chip-to-chip wireless link over 300 mm and eight simultaneous 40-Gbps wireless links. Using four of these links, we demonstrate point-to-4-point real-time HD video streaming. Our work provides a complementary metal-oxide-semiconductor-compatible THz topological photonic integrated circuit for efficient large-scale beamforming, enabling massive single-input multiple-output and multiple-input and multiple-output systems for terabit-per-second 6G to XG wireless communications.

Chiral Kirigami for Bend-Tolerant Reconfigurable Hologram with Continuously Variable Chirality Measures

Despite the commonality of static holograms, the holography with multiple information layers and reconfigurable grey-scale images at communication frequencies remain a confluence of scientific challenges. One well-known difficulty is the simultaneous modulation of phase and amplitude of electromagnetic wavefronts with a high modulation depth. A less appreciated challenge is scrambling of the information and images with hologram bending. Here, this work shows that chirality-guided pixelation of plasmonic kirigami sheets enables tunable multiplexed holography at terahertz (THz) frequencies. The convex and concave structures with slanted Au strips exhibit gradual variations in geometries facilitating modulation of light ellipticity reaching 40 deg. Real-time switching of 3D images of the letter "M" and the Mona Lisa demonstrates the possibility of complex grey-scale information content and importance of continuously variable mirror asymmetry. Microscale chirality measures of each pixel experiences little change with bending while retaining controllable reconfigurability upon stretching, which translates to remarkable resilience of chiral holograms to bending. Simplicity of their design with local chirality measures opens the door to information technologies with fault-tolerant THz encryption, wearable holographic devices, and new communication technologies.

Color coded metadevices toward programmed terahertz switching

Terahertz modulators play a critical role in high-speed wireless communication, non-destructive imaging, and so on, which have attracted a large amount of research interest. Nevertheless, all-optical terahertz modulation, an ultrafast dynamical control approach, remains to be limited in terms of encoding and multifunction. Here we experimentally demonstrated an optical-programmed terahertz switching realized by combining optical metasurfaces with the terahertz metasurface, resulting in 2-bit dual-channel terahertz encoding. The terahertz metasurface, made up of semiconductor islands and artificial microstructures, enables effective all-optical programming by providing multiple frequency channels with ultrafast modulation at the nanosecond level. Meanwhile, optical metasurfaces covered in terahertz metasurface alter the spatial light field distribution to obtain color code. According to the time-domain coupled mode theory analysis, the energy dissipation modes in terahertz metasurface can be independently controlled by color excitation, which explains the principle of 2-bit encoding well. This work establishes a platform for all-optical programmed terahertz metadevices and may further advance the application of composite metasurface in terahertz manipulation.

Design and analysis of 2D one-way splitter waveguide based on topological photonics

We present a new high-efficiency splitter waveguide design based on photonic topological insulators. The system's robust edge states allow electromagnetic waves to propagate in the 2D waveguide without backscattering, resulting in almost 100% transmission in the outputs. We also study resonating modes in the structure and show that introducing specific defects can create such modes. We consider four domains with rods of varying magneto-optical properties to provide edge modes in the system. By eliminating rows and columns of rods, we calculate the transmission at the outputs, revealing resonating modes in the middle of the structure with spatial symmetry. Our calculations indicate that the most promising resonating mode occurs when two rods and two columns are eliminated, with a quality factor Q = 1.02 × 106 at frequency f = 8.23 GHz and almost zero transmission at this frequency to the outputs. We further confirm our results using the transmission line resonator model as a semi-analytical model, which agrees well with our findings.

150 Gbps THz Chipscale Topological Photonic Diplexer

Photonic diplexers are being widely investigated for high data transfer rates in on-chip communication. However, dividing the available spectrum into nonoverlapping multicarrier frequency sub-bands has remained a challenge in designing frequency-selective time-invariant channels. Here, an on-chip topological diplexer is reported exhibiting terahertz frequency band filtering through Klein tunneling of topological edge modes. The silicon topological diplexer chip facilitates two high-speed channels with quadrature amplitude modulation (QAM) over a broad bandwidth of 12.5 GHz each. These channels operate at carrier frequencies of 305 and 321.6 GHz, achieving a combined diplexer capacity of 150 Gbit s-1. To ensure minimal interference between adjacent channels, a guard band is implemented. Topologically protected edge modes suppress the frequency selective fading of the broadband signals and hold promise for diverse integrated photonic applications spanning terahertz and telecommunication realms, including the design of lossless topological multiplexers, interconnects, antennas, and modulators for the sixth to X generation (6G to XG) wireless.

All-Dielectric Integrated Meta-Antenna Operating in 6G Terahertz Communication Window

Efficient transceivers and antennas at terahertz frequencies are leading the development of 6G terahertz communication systems. The antenna design for high-resolution terahertz spatial sensing and communication remains challenging, while emergent metallic metasurface antennas can address this issue but often suffer from low efficiency and complex manufacturing. Here, an all-dielectric integrated meta-antenna operating in 6G terahertz communication window for high-efficiency beam focusing in the sub-wavelength scale is reported. With the antenna surface functionalized by metagrating arrays with asymmetric scattering patterns, the design and optimization methods are demonstrated with a physical size constraint. The highest manipulation and diffraction efficiencies achieve 84.1% and 48.1%. The commercially accessible fabrication method with low cost and easy to implement has been demonstrated for the meta-antenna by photocuring 3D printing. A filamentous focal spot is measured as 0.86λ with a long depth of focus of 25.3λ. Its application for integrated imaging and communication has been demonstrated. The proposed technical roadmap provides a general pathway for creating high-efficiency integrated meta-antennas with great potential in high-resolution 6G terahertz spatial sensing and communication applications.

Large transmittance contrast via 90-degree sharp bends in square lattice glide-symmetric photonic crystal waveguides

We demonstrate an intriguing transmittance contrast in a glide-symmetric square-lattice photonic crystal waveguide with a 90-degree sharp bend. The glide-symmetry gives rise to a degeneracy point in the band structure and separates a high-frequency and a low-frequency band. Previously, a similar large transmittance contrast between these two bands has been observed in glide-symmetric triangular- or honeycomb-lattice photonic crystals without inversion symmetry, and this phenomenon has been attributed to the valley-photonic effect. In this study, we demonstrate the first example of this phenomenon in square-lattice photonic crystals, which do not possess the valley effect. Our result sheds new light onto unexplored properties of glide-symmetric waveguides. We show that this phenomenon is related to the spatial distribution of circular polarization singularities in glide-symmetric waveguides. This work expands the possible designs of low-loss photonic circuits and provides a new understanding of light transmission via sharp bends in photonic crystal waveguides.

Room-Temperature Plasmon-Assisted Resonant THz Detection in Single-Layer Graphene Transistors

Frequency-selective or even frequency-tunable terahertz (THz) photodevices are critical components for many technological applications that require nanoscale manipulation, control, and confinement of light. Within this context, gate-tunable phototransistors based on plasmonic resonances are often regarded as the most promising devices for the frequency-selective detection of THz radiation. The exploitation of constructive interference of plasma waves in such detectors promises not only frequency selectivity but also a pronounced sensitivity enhancement at target frequencies. However, clear signatures of plasmon-assisted resonances in THz detectors have been revealed only at cryogenic temperatures so far and remain unobserved at application-relevant room-temperature conditions. In this work, we demonstrate the sought-after room-temperature resonant detection of THz radiation in short-channel gated photodetectors made from high-quality single-layer graphene. The survival of this intriguing resonant regime at room temperature ultimately relies on the weak intrinsic electron-phonon scattering in monolayer graphene, which avoids the damping of the plasma oscillations present in the device channel.

Terahertz photodiode integration with multi-octave-bandwidth dielectric rod waveguide probe

Photonic integrated circuits play a vital role in enabling terahertz (THz) applications that require multi-octave bandwidth. Prior research has been limited in bandwidth due to rectangular waveguide (WRs) interconnects, which can only support single octave at low loss. To overcome this fundamental limitation, we exploit the ultra-wideband (UWB) near-field coupling between planar waveguides and silicon (Si)-based subwavelength dielectric rod waveguides (DRWs) to interconnect THz bandwidth uni-traveling-carrier photodiodes (UTC-PDs) at 0.08-1.03 THz. In a proof-of-concept experiment, the on-chip integrated UTC-PDs demonstrate a UWB operation from 0.1 THz to 0.4 THz. Furthermore, by employing Si DRWs as probes, multi-octave device-under-test characterization of UTC-PDs integrated with UWB transition is enabled with only one DRW probe. The proposed UWB interconnect technology is distinct from previously used WR-based ground-signal-ground probes or quasi-optical free-space coupling since it can provide multi-octave bandwidth and enable on-chip THz circuit integration.

An ultra-broadband frequency-agile terahertz perfect absorber with perturbed MoS2 plasmon modes

Multidomain dynamic manipulations for terahertz (THz) absorbers usually necessitate the orchestrated actions of several active elements, inevitably complicating the structural design and elongating the modulation time. Herein, we utilize the coupling between the total reflection prism and electrically-driven MoS2 to activate a tight field confinement in a deep-subwavelength interlayer, ultimately achieving frequency-agile absorption adjustments only with a gate voltage. Theoretical and simulation analysis results indicate that the redistributed electric field and susceptible dielectric response are attributed to the limited spatial near-field perturbation of surface plasmon resonances. We also demonstrate that perturbed MoS2 plasmon modes promote the formation of dual-phase singularities, significantly suppressing the attenuation of the absorption amplitude as large-scale frequency shifts, thereby extending the relative tuning range (WRTR) to 175.4%. These findings offer an efficient approach for expanding the horizon of THz absorption applications that require ultra-broadband and swift-response capabilities.

Ge2Sb2Te5-based efficient switching between a cross-polarization conversion and a circular-to-linear polarization conversion

The terahertz (THz) band has a great potential for the development of communication technology, but it has not been fully utilized due to the lack of practical devices, especially actively controllable multifunctional devices. Here, we propose and demonstrate a Ge2Sb2Te5 (GST)-based metamaterial device, where an actively controllable function is experimentally verified by inducing the crystallization process with thermal activation. Cross-polarization conversion in the reflection mode and circular-to-linear polarization conversion in the transmission mode are obtained under crystalline and amorphous GST conditions, respectively. The combination of GST and THz waves has a wide range of applications and will further advance the THz field.

Valley-conserved topological integrated antenna for 100-Gbps THz 6G wireless

The topological phase revolutionized wave transport, enabling integrated photonic interconnects with sharp light bending on a chip. However, the persistent challenge of momentum mismatch during intermedium topological mode transitions due to material impedance inconsistency remains. We present a 100-Gbps topological wireless communication link using integrated photonic devices that conserve valley momentum. The valley-conserved silicon topological waveguide antenna achieves a 12.2-dBi gain, constant group delay across a 30-GHz bandwidth and enables active beam steering within a 36° angular range. The complementary metal oxide semiconductor-compatible valley-conserved devices represent a major milestone in hybrid electronic-photonic-based topological wireless communications, enabling terabit-per-second backhaul communication, high throughput, and intermedium transport of information carriers, vital for the future of communication from the sixth to X generation.

Modified Kramers-Kronig receiver based on memory polynomial compensation for photonics-assisted millimeter-wave communications

Photonics-assisted millimeter-wave (MMW) wireless communications are advancing rapidly driven by the escalating congestion in the lower-band spectrum and the growing demand for higher data rates. Concurrently, Kramers-Kronig (KK) receivers provide an economical solution ideally suited for cost-sensitive deployment and application. However, the conventional KK receiver is subject to performance degradation due to the nonlinearity and memory effects introduced by practical electronic devices. In this work, the performance degradation of the conventional KK receiver is investigated and quantitatively simulated, showing that the KK receiver exhibits greater sensitivity to nonlinearity and memory effects compared to the conventional coherent receiver. To enhance the performance of KK receivers deployed in MMW communication systems, we propose a modified KK receiver employing memory polynomial compensation, namely MP-KK receiver, capable of effectively compensating memory effects whilst simultaneously addressing nonlinearity. Crucially, the memory polynomial model is employed prior to the KK algorithm to prevent further signal degradation caused by the nonlinear operator in the KK algorithm in the scenario of photonics-assisted MMW wireless communication based on the KK receiver. For verification, we present a 95 GHz W-band MMW wireless transmission demonstration with 20 Gb/s QPSK and 40 Gb/s 16-QAM signals. The experimental results indicate that the MP-KK receiver can achieve more than 3.5 dB improvement in EVM and 71.25% reduction in BER compared to the conventional approaches.

Transmission of sub-terahertz signals over a fiber-FSO-5 G NR hybrid system with an aggregate net bit rate of 227.912 Gb/s

Transmission of sub-terahertz (sub-THz) signals over a fiber-free-space optical (FSO)-fifth-generation (5 G) new radio (NR) hybrid system is successfully realized. It is a promising system that utilizes multiple media of optical fiber, optical wireless, and 5 G NR wireless to achieve a 227.912-Gb/s record-high aggregate net bit rate. The system concurrently transmits a 59.813-Gb/s net bit rate in the 150-GHz sub-THz frequency, 74.766-Gb/s in the 250-GHz sub-THz frequency, and 93.333-Gb/s in the 325-GHz sub-THz frequency through the fiber-FSO-wireless convergence, including 25-km single-mode fiber, 100-m FSO, and 30-m/25-m/20-m sub-THz-wave transmissions. This system achieves sufficiently low bit error rates (< hard-decision forward error correction (FEC) threshold of 3.8 × 10-3 at 16 and 20 Gbaud symbol rates; < soft-decision FEC threshold of 2 × 10-2 at 28 Gbaud symbol rate) and clear and distinct constellation diagrams, meeting the demands of 5 G NR communications in the sub-THz band. The development of fiber-FSO-5 G NR hybrid system represents a substantial development in the field of advanced communications. It has the ability to enhance the way we communicate in the future.

Topological transport in heterostructure of valley photonic crystals

We propose a heterogeneous structure, which are composed of two valley photonic crystals (VPCs) with opposite valley Chern numbers and air channel. With the increasing width of the air channel, valley-locked waveguide modes are found in topological bandgap by analyzing energy bands. Finite element method (FEM) simulation results show that the fundamental and high order modes are valley-locked, propagating unidirectionally under the excitation of chiral source, and possess higher flux compared to the valley-locked topological edge state in the domain wall. Besides, the immunity to backscattering in bend and couplers, and the robustness to random disorders are discussed in detail. We also investigate the one-way multimode interference (MMI) effect based on valley-locked waveguide modes, and design topological beam splitters. Our study provides a novel idea for topological transport with high flux, and more freedom to design valley-locked waveguide devices, including bends, couplers and splitters.

Coexistence of large-area topological pseudospin and valley states in a tri-band heterostructure system

The rapid development of topological photonics has significantly revolutionized our comprehension of electromagnetic wave manipulation in recent decades. Recent research exploiting large-area topological states inserts an additional gapless PC structure between topologically trivial and nontrivial PCs, effectively introducing the mode width degree of freedom. Nevertheless, these heterostructures mainly support only single-type waveguide states operating within a single frequency band. To address these limitations, we propose a novel, to the best of our knowledge, tri-band three-layer heterostructure system, supporting both large-area pseudospin- and valley-locked states. The system showcases tunable mode widths with different operational bandwidths. Moreover, the heterostructures exhibit inherent topological characteristics and reflection-free interfacing, which are verified in the well-designed Z-shaped channels. The proposed heterostructure system can be used to design multi-band multi-functional high-flexibility topological devices, providing great advantages for enlarging the on-chip integrated communication systems.

Reconfigurable THz metamaterial based on microelectromechanical cantilever switches with a dimpled tip

We numerically and experimentally proposed a reconfigurable THz metamaterial (MM) by employing microelectromechanical cantilevers into a ladder-shaped MM (LS-MM). A fixed-free cantilever array with a dimpled tip behaved as Ohmic switches to reshape the LS-MM so as to actively regular the transmission response of THz waves. The cantilever tip was designed to be a concave dimple to improve the operational life without sacrificing the mechanical resonant frequency (fmr), and a fmr of 635 kHz was demonstrated. The device actively achieved a 115-GHz change in transmittance resonant frequency and a 1.82-rad difference in transmission phase shift, which can practically benefit advancing THz applications such as fast THz imaging and 6 G communications.

Free-space transmission of picosecond-level, high-speed optical pulse streams in the 3 µm band

The utilization of mid-infrared (mid-IR) light spanning the 3-5 µm range presents notable merits over the 1.5 µm band when operating in adverse atmospheric conditions. Consequently, it emerges as a promising prospect for serving as optical carriers in free-space communication (FSO) through atmospheric channels. However, due to the insufficient performance level of devices in the mid-IR band, the capability of mid-IR communication is hindered in terms of transmission capacity and signal format. In this study, we conduct experimental investigations on the transmission of time-domain multiplexed ultra-short optical pulse streams, with a pulse width of 1.8 ps and a data rate of up to 40 Gbps at 3.6 µm, based on the difference frequency generation (DFG) effect. The mid-IR transmitter realizes an effective wavelength conversion of optical time division multiplexing (OTDM) signals from 1.5 µm to 3.6 µm, and the obtained power of the 40 Gbps mid-IR OTDM signal at the optimum temperature of 54.8 °C is 7.4 dBm. The mid-IR receiver successfully achieves the regeneration of the 40 Gbps 1.5 µm OTDM signal, and the corresponding regenerated power at the optimum temperature of 51.5 °C is -30.56 dBm. Detailed results pertaining to the demodulation of regeneration 1.5 µm OTDM signal have been acquired, encompassing parameters such as pulse waveform diagram, bit error rate (BER), and Q factor. The estimated power penalty of the 40 Gbps mid-IR OTDM transmission is 2.4 dB at a BER of 1E-6, compared with the back-to-back (BTB) transmission. Moreover, it is feasible by using chirped PPLN crystals with wider bandwidth to increase the data rate to the order of one hundred gigabits.

Deep Learning in the Ubiquitous Human-Computer Interactive 6G Era: Applications, Principles and Prospects

With the rapid development of enabling technologies like VR and AR, we human beings are on the threshold of the ubiquitous human-centric intelligence era. 6G is believed to be an indispensable cornerstone for efficient interaction between humans and computers in this promising vision. 6G is supposed to boost many human-centric applications due to its unprecedented performance improvements compared to 5G and before. However, challenges are still to be addressed, including but not limited to the following six aspects: Terahertz and millimeter-wave communication, low latency and high reliability, energy efficiency, security, efficient edge computing and heterogeneity of services. It is a daunting job to fit traditional analytical methods into these problems due to the complex architecture and highly dynamic features of ubiquitous interactive 6G systems. Fortunately, deep learning can circumvent the interpretability issue and train tremendous neural network parameters, which build mapping relationships from neural network input (status and specific requirements of a 6G application) to neural network output (settings to satisfy the requirements). Deep learning methods can be an efficient alternative to traditional analytical methods or even conquer unresolvable predicaments of analytical methods. We review representative deep learning solutions to the aforementioned six aspects separately and focus on the principles of fitting a deep learning method into specific 6G issues. Based on this review, our main contributions are highlighted as follows. (i) We investigate the representative works in a systematic view and find out some important issues like the vital role of deep reinforcement learning in the 6G context. (ii) We point out solutions to the lack of training data in 6G communication context. (iii) We reveal the relationship between traditional analytical methods and deep learning, in terms of 6G applications. (iv) We identify some frequently used efficient techniques in deep-learning-based 6G solutions. Finally, we point out open problems and future directions.

Terahertz topological photonic crystals with dual edge states for efficient routing

Topological photonic crystals with robust pseudo-spin and valley edge states have shown promising and wide applications in topological waveguides, lasers, and antennas. However, the limited bandwidth and intrinsic coupling properties of a single pseudo-spin or valley edge state have imposed restrictions on their multifunctional applications in integrated photonic circuits. Here, we propose a topological photonic crystal that can support pseudo-spin and valley edge states simultaneously in a single waveguiding channel, which effectively broadens the bandwidth and enables a multipath routing solution for terahertz information processing and broadcasting. We show that distorted Kekulé lattices can open two types of bandgaps with different topological properties simultaneously by molding the inter- and intra-unit cell coupling of the tight-binding model. The distinct topological origins of the edge states provide versatile signal routing paths toward free space radiation or on-chip self-localized edge modes by virtue of their intrinsic coupling properties. Such a powerful platform could function as an integrated photonic chip with capabilities of broadband on-chip signal processing and distributions that will especially benefit terahertz wireless communications.

150 Gbps multi-wavelength FSO transmission with 25-GHz ITU-T grid in the mid-infrared region

The 3∼5 µm mid-infrared (mid-IR) light has several exceptional benefits in the case of adverse atmospheric conditions compared to the 1.5 µm band, so it is a promising candidate for optical carriers for free-space communication (FSO) through atmospheric channels. However, the transmission capacity in the mid-IR band is constrained in the lower range due to the immaturity of its devices. In this work, to replicate the 1.5 µm band dense wavelength division multiplexing (DWDM) technology to the 3 µm band for high-capacity transmission, we demonstrate a 12-channel 150 Gbps FSO transmission in the 3 µm band based on our developed mid-IR transmitter and receiver modules. These modules enable wavelength conversion between the 1.5 µm and 3 µm bands based on the effect of difference-frequency generation (DFG). The mid-IR transmitter effectively generates up to 12 optical channels ranging from 3.5768 µm to 3.5885 µm with a power of 6.6 dBm, and each channel carries 12.5 Gbps binary phase shift keying (BPSK) modulated data. The mid-IR receiver regenerates the 1.5 µm band DWDM signal with a power of -32.1 dBm. Relevant results of regenerated signal demodulation have been collected in detail, including bit error ratio (BER), constellation diagram, and eye diagram. The power penalties of the 6th to 8th channels selected from the regenerated signal are lower than 2.2 dB compared with back-to-back (BTB) DWDM signal at a bit error ratio (BER) of 1E-6, and other channels can also achieve good transmission quality. It is expected to further push the data capacity to the terabit-per-second level by adding more 1.5 µm band laser sources and using wider-bandwidth chirped nonlinear crystals.

Spin-decoupled excitation and wavefront shaping of structured surface waves via on-chip terahertz metasurfaces

Surface waves (SWs) are of great importance in terahertz (THz) photonics applications due to their subwavelength properties. Hence, it is crucial to develop surface wavefront shaping techniques, which is urgent in modern information technologies. In this paper, a new scheme is proposed to realize SW excitation and spin-decoupled wavefront shaping with an ultracompact planar meta-device working in the THz range. The meta-device is composed of two parts: meta-atoms (in the center) and plasmonic metals (on the left and right sides). By carefully setting the geometry size and rotation angle of each meta-atom, the encoded spin-decoupled phase distributions for both left circularly polarized (LCP) and right circularly polarized (RCP) incident THz waves are determined. In this way, circularly polarized (CP) incident THz waves can be converted to SWs propagating along plasmonic metals with unique wavefront profiles, i.e., Bessel and focusing profiles. Full-wave simulations and THz near-field scanning experiments were performed to verify the functionalities of the meta-device, both of which are in great agreement with theoretical predictions. Our findings may provide more solutions to design THz integrated photonic devices and systems.

Thermal tunable silicon valley photonic crystal ring resonators at the telecommunication wavelength

Tunable ring resonators are essential devices in integrated circuits. Compared to conventional ring resonators, valley photonic crystal (VPC) ring resonators have a compact design and high quality factor (Q-factor), attracting broad attention. However, tunable VPC ring resonators haven't been demonstrated. Here we theoretically demonstrate the first tunable VPC ring resonator in the telecommunication wavelength region, the resonance peaks of which are tuned by controlling the temperature based on the thermal-optic effect of silicon. The design is ultracompact (12.05 µm by 10.44 µm), with a high Q-factor of 1281.00. By tuning the temperature from 100 K to 750 K, the phase modulation can reach 7.70 π, and the adjustment efficiency is 0.062 nm/K. Since thermal tuning has been broadly applied in silicon photonics, our design can be readily applied in integrated photonic circuits and will find broad applications. Furthermore, our work opens new possibilities and deepens the understanding of designing novel tunable VPC photonic devices.

Lightweight MXene-Based Hybrid Aerogels with Ultrabroadband Terahertz Absorption and Anisotropic Strain Sensitivity

MXene aerogels with a three-dimensional (3D) network structure have attracted increasing attention for lightweight electromagnetic wave absorbers. It is intriguing to expand their absorption band, i.e., to the booming terahertz (THz) region, and explore multifunctionality. Herein, we assemble MXene (Ti₃C₂T_x)-based hybrid aerogels into an aligned lamellar architecture using a bidirectional freezing technique. With air pore size and lamellar layer spacing comparable to THz wavelengths, high porosity of the aerogels allows nearly isotropic absorption of 99% and electromagnetic interference (EMI) shielding effectiveness with a remarkable value of 57.5 dB, in the ultrabroad bandwidth ranging from 0.5 to 3.0 THz. Simultaneous, strain-sensing response reflects the macroscopic anisotropy of the network structure of the aerogels. The improved sensitivity is measured for the out-of-lamellar layer plane under 0-30% strain. The corresponding long-term stability and durability persist over 120 stretching-releasing cycles. Our findings thus not only expand multiple functions of MXene in an anisotropic 3D macroscopic form but also clarify its nearly isotropic absorption in the THz band.

All-Dielectric Tunable Terahertz Metagrating for Diffraction Control

Recently, tunable metagratings have attracted substantial attention in manipulating the diffraction of electromagnetic waves with considerable flexibility, but they are usually limited to inherent ohmic loss due to the metal layers. The all-dielectric schemes can address this issue, but its design and optimization remain challenging in the terahertz regime, especially in the 6G communication window. In this work, an all-dielectric tunable terahertz metagrating is demonstrated in theoretical and experimental investigations. The metagrating operating in the 6G communication window bends the electromagnetic waves beam into the T_-1 diffraction order by optimizing the unit cell. In the experiments, more than 72.46% of the transmitted energy is concentrated in the desired diffraction order for p-polarized light and more than 66.60% for s-polarized light, which agrees well with the theoretical design. The tunability by angular deflection is reported in this all-dielectric metagrating. Then, based on the all-dielectric metagrating arrays, a metalens with numerical aperture of NA = 0.39 at 0.14 THz is demonstrated. The subwavelength scale focal spot is obtained as 2.0 mm × 2.0 mm with the focusing distance of 117.8 mm. Imaging capability of the metalens is performed utilizing the transmission imaging manner. The measured and anticipated results are satisfactorily congruous with one another, which could validate our design. This work paves the way toward designing highly efficient and tunable devices with potential applications in terahertz communications, sensors, and super-resolution imaging.