The physical concept of synthetic dimensions has recently been introduced into optics. The fundamental physics and applications are not yet fully understood, and this report explores an approach to optical neural networks using synthetic dimension in time domain, by theoretically proposing to utilize a single resonator network, where the arrival times of optical pulses are interconnected to construct a temporal synthetic dimension. The set of pulses in each roundtrip therefore provides the sites in each layer in the optical neural network, and can be linearly transformed with splitters and delay lines, including the phase modulators, when pulses circulate inside the network. Such linear transformation can be arbitrarily controlled by applied modulation phases, which serve as the building block of the neural network together with a nonlinear component for pulses. We validate the functionality of the proposed optical neural network for the deep learning purpose with examples handwritten digit recognition and optical pulse train distribution classification problems. This proof of principle computational work explores the new concept of developing a photonics-based machine learning in a single ring network using synthetic dimensions, which allows flexibility and easiness of reconfiguration with complex functionality in achieving desired optical tasks.

On-chip optical nonreciprocal devices are vital components for integrated photonic systems and scalable quantum information processing. Nonlinear optical isolators and circulators have attracted considerable attention because of their fundamental interest and their important advantages in integrated photonic circuits. However, optical nonreciprocal devices based on Kerr or Kerr-like nonlinearity are subject to dynamical reciprocity when the forward and backward signals coexist simultaneously in a nonlinear system. Here, we theoretically propose a method for realizing on-chip nonlinear isolators and circulators with dynamic nonreciprocity. Dynamic nonreciprocity is achieved via the chiral modulation on the resonance frequency due to coexisting self- and cross-Kerr nonlinearities in an optical ring resonator. This work showing dynamic nonreciprocity with a Kerr nonlinear resonator can be an essential step toward integrated optical isolation.

We reveal a special subset of non-degenerate Akhmediev breather (AB) solutions of Manakov equations that only exist in the focusing case. Based on exact solutions, we present the existence diagram of such excitations on the frequency-wavenumber plane. Conventional single-frequency modulation instability leads to simultaneous excitation of three ABs with two of them being non-degenerate.

Using one-dimensional semiconductor Bloch equations, we investigate the multiband dynamics of electrons in a cutoff extension scheme employing an infrared pulse with additional UV injection. An extended three-step model is firstly validated to play a dominant role in emitting harmonics in the second plateau. Surprisingly, further analysis employing the acceleration theorem shows that, though harmonics in both the primary and secondary present positive and negative chirps, the positive (negative) chirp in the first region is related to the so-called short (long) trajectory, while that in the second region is emitted through ‘general’ trajectory, where electrons tunneling earlier and recombining earlier contribute significantly. The novel characteristics deepen the understanding of high harmonic generation in solids and may have great significance in attosecond science and reconstruction of band dispersion beyond the band edge.

Topological materials are often characterized by unique edge states which are in turn used to detect different topological phases in experiments. Recently, with the discovery of various higher-order topological insulators, such spectral topological characteristics are extended from edge states to corner states. However, the chiral symmetry protecting the corner states is often broken in genuine materials, leading to vulnerable corner states even when the higher-order topological numbers remain quantized and invariant. Here, we show that a local artificial gauge flux can serve as a robust probe of the Wannier type higher-order topological insulators, which is effective even when the chiral symmetry is broken. The resultant observable signature is the emergence of the cyclic spectral flows traversing one or multiple band gaps. These spectral flows are associated with the local modes bound to the artificial gauge flux. This phenomenon is essentially due to the cyclic transformation of the Wannier orbitals when the local gauge flux acts on them. We extend topological Wannier cycles to systems with $C_{2}$ and $C_{3}$ symmetries and show that they can probe both the bulk and the edge Wannier centers, yielding rich topological phenomena.

We present a design and development of a high-performance light-emitting diode (LED)-side-pumped Nd:YAG rod laser with strong pulse energy, high efficiency, and consistency, very good beam quality, and high uniform pumping intensity in the active area which reduces the effects of thermal gradient significantly. A five-dimensional 810 nm LED array with a full width of 30 nm at half maximum was intended to achieve high coupling efficiency by putting the LED array as close as possible to the side of the Nd:YAG laser rod for overcoming the large pumped divergence. Under 2.25 J pump energy, maximum single pulse energy of 35.86 mJ with duration of 1.24 µs at 1063.68 nm was obtained, equivalent to optical efficiency of 1.59% and a slope efficiency of 2.53%. The laser was set to repeat at a rate of 10 Hz with a beam quality factor of $M_{x}^{2} = 2.94$ and $M_{y}^{2} = 3.35$, as well as with the output power stability of $ < $ 4.1% (root mean square) and $ < $ 7.3% (peak to peak). To the best of our ability, this is the highest performance for an LED-side-pumped Nd:YAG rod laser oscillator with a 10-mJ-level output ever reported.

Realizing single light solitons that are stable in high dimensions is a long-standing goal in research of nonlinear optical physics. Here, we address a scheme to generate stable two-dimensional solitons in a cold Rydberg atomic system with a parity-time (PT) symmetric moiré optical lattice. We uncover the formation, properties, and their dynamics of fundamental and two-pole gap solitons as well as vortical ones. The PT symmetry, lattice strength, and the degrees of local and nonlocal nonlinearity are tunable and can be used to control solitons. The stability regions of these solitons are evaluated in two numerical ways: linear-stability analysis and time evolutions with perturbations. Our results provide an insightful understanding of solitons physics in combined versatile platforms of PT-symmetric systems and Rydberg–Rydberg interaction in cold gases.

Predicting thermal conductance of complex networks poses a formidable challenge in the field of materials science and engineering. This challenge arises due to the intricate interplay between the parameters of network structure and thermal conductance, encompassing connectivity, network topology, network geometry, node inhomogeneity, and others. Our understanding of how these parameters specifically influence heat transfer performance remains limited. Deep learning offers a promising approach for addressing such complex problems. We find that the well-established convolutional neural network models AlexNet can predict the thermal conductance of complex network efficiently. Our approach further optimizes the calculation efficiency by reducing the image recognition in consideration that the thermal transfer is inherently encoded within the Laplacian matrix. Intriguingly, our findings reveal that adopting a simpler convolutional neural network architecture can achieve a comparable prediction accuracy while requiring less computational time. This result facilitates a more efficient solution for predicting the thermal conductance of complex networks and serves as a reference for machine learning algorithm in related domains.

In optical systems, it is necessary to investigate the propagation of optical solitons in optical fiber systems for fiber-optic communications. By means of the bilinear method, we obtain the two-soliton solution of the variable coefficient higher-order coupled nonlinear Schrödinger equation. According to the obtained two-soliton solution, a novel two-soliton interaction structure of the system is constructed, and their interactions are studied. Two optical solitons occur with elastic interaction under certain conditions, and their amplitudes, shapes and velocities remain unchanged before and after the action. In addition to the elastic interaction, splitting action and polymerization also occur. The present study on the dynamic behavior of interaction of optical solitons may be valuable for research and applications in optical communication and other fields.

Electromagnetic metasurface with chaos patterned surface could bring rich interaction modes contributing to fully disordered random motions in deterministic systems, which preform uncertainty, irreducibility and unpredictability. We investigate the influence of the correlation function (CF) properties of surface random patterns on the wave absorption performance. The complicated correlation function provides a fully developed random state, broadening the absorption bandwidth significantly and is helpful for reaching higher absorption rate. With the increasing number of peaks in the correlation function, the absorption band at $-15$ dB reflectivity widens significantly, band at $-20$ dB reflectivity begins to emerge. As the first peak's distance from the original point in the CF is enlarged, the absorption trough is gradually formed and deepened to $-35$ dB level. The results give in-depth understanding of the relation between absorption behavior and controlling parameters including correlation, image information and foam spacer layer thickness. This high absorption absorber has great application potential in customizable radio communication compatibility device and anechoic testing chamber.

We present optical frequency combs with a spectral emission of 48 cm$^{-1}$ and an output power of 420 mW based on a single-core quantum cascade laser at $\lambda \sim 8.7$ µm. A flat top spectrum sustains up to 130 comb modes delivering $\sim$ 3.2 mW of optical power per mode, making it a valuable tool for dual comb spectroscopy. The homogeneous gain medium, relying on a slightly diagonal bound-to-continuum structure, promises to provide a broad and stable gain for comb operating. Remarkably, the dispersion of this device is measured within 300 fs$^{2}$/mm to ensure stable comb operation over 90% of the total current range. The comb is observed with a narrow beatnote linewidth around 2 kHz and has weak dependence on the applied current for stable comb operation.

We design a multilayer cylindrical structure to realize superscattering of underwater sound. Because of the near degeneracy of resonances in multiple channels of the structure, the scattering contributions from these resonances can overlap to break the single-channel limit of subwavelength objects. However, tuning the design parameters to achieve the target response is an optimization process that is tedious and time-consuming. Here, we demonstrate that a well-trained tandem neural network can deal with this problem efficiently, which can not only forwardly predict the scattering spectra of the multilayer structure with high precision, but also inversely design the required structural parameters efficiently.

Nonlinear optical properties are investigated using the static and time-resolved second harmonic generation in the topological material molybdenum phosphide (MoP) with three-component fermions. Giant second harmonic generation signals are detected and the derived $\chi^{(2)}$ value is larger than that of the typical electro–optic material. Upon optical excitation, no photoinduced change of the symmetry is detected in MoP, which is quite different from previous observations in several other topological materials.

Fiber lasers with different net dispersion cavity values can produce some types of solitons, and rich nonlinear dynamics phenomena can be achieved by selecting different saturable absorbers. A new layered high-entropy van der Waals material (HEX) (Mn,Fe,Co,Ni)PS$_{3}$ was selected as a saturable absorber to achieve a high-power laser output of 34 mW. In addition, the wavelength can be dynamically tuned from 1560 nm to 1531 nm with significant dual-wavelength phenomena at 460 fs pulse duration.

The theory of knots and links focuses on the embedding mode of one or several closed curves in three-dimensional Euclidean space. In an electromagnetic field system, all-optical knots or links composed of phase or polarization singularities have been verified theoretically and experimentally. Recent studies have shown that robust topological all-optical coding can be achieved by using optical knots and links. However, in the current design of optical knots and links based on phase or polarization singularities, the amplitude of light between adjacent singularities is relatively weak. This brings great pressure to detection of optical knots and links and limits their applications. Here, we propose a new optimization method in theory. Compared with the existing design methods, our design method improves the relative intensity distribution of light between adjacent singularities. We verify the feasibility of our design results in experiments. Our study reduces the detection difficulty of optical knots and links, and has a positive significance for promotion of applications of optical knots and links.

Accurate and fast prediction of thermal radiation properties of materials is crucial for their potential applications. However, some models assume that the media are made up of pure water droplets, which do not account for the increasing deviations caused by volcanic eruptions, pollution, and human activities that exacerbate dust production. The distinct radiation properties of water and dust particles make it challenging to determine the thermal radiation properties of water droplets containing dust particles. To address this issue, we investigate the influence of dust particles on light transmission and energy distribution in water droplets using the multiple sphere T-matrix method. By considering different droplet and dust diameters, volume fractions, and position distributions, we analyze how extinction regulation is achieved in dust-containing water droplets. Our results reveal the significant role of dust particles in the thermal radiation effect and provide insights into the electromagnetic properties of colloidal suspensions. Moreover, the dust-induced reestablishment of energy balance raises concerns about environmental management and climate change. This research highlights the importance of accounting for dust particles in atmospheric models and their potential impact on radiative balance.

We theoretically study optomechanically induced transparency in a spinning Kerr-nonlinear resonator. The interplay between the optical Kerr effect and the Sagnac effect provides a flexible tool for modifying the optomechanically induced transparency windows of the signal comparing to the system of a single spinning resonator. It is found that the system will exhibit distinct transparency phenomenon and fast-to-slow light effects. More importantly, a symmetric transparency window appears by adjusting the rotation-induced Sagnac frequency shift to compensate for the Kerr-induced frequency shift. These results open up a new way to explore novel light propagation of optomechanically induced transparency devices in spinning resonators with Kerr nonlinearity.

Topological insulator materials, including Bi$_{2}$Te$_{3}$, Sb$_{2}$Te$_{3}$, Sb$_{2}$Te$_{3}$, and Bi$_{2}$Se$_{3}$, have attracted some attention due to their narrow band gaps, high carrier mobility, wide spectral absorption ranges and other characteristics. We report a new multi-compound topological insulator material BiSbTeSe$_{2}$ that, compared with the traditional topological insulator composed of two elements, can integrate the physical advantages of each element, helpful to build an experimental platform with rich physical properties. The nonlinear optical characteristics of the quaternary material BiSbTeSe$_{2}$ is obtained in the erbium-doped fiber laser. Using the BiSbTeSe$_{2}$ as a saturable absorber material, the passive Q-switched and mode-locked fiber lasers are achieved. The pulse duration and signal-to-noise ratio (SNR) of the Q-switched fiber laser are 854 ns and 70 dB, respectively. Meanwhile, the pulse duration and SNR of the mode-locked fiber laser are 259 fs and 87.75 dB, respectively. This work proves that the BiSbTeSe$_{2}$ has a considerable application prospect as a saturable absorber in fiber lasers, and provides a new reference for selection of high-performance saturable absorber materials.

Chiral metasurfaces have been proven to possess great potential in chiroptical applications. However, the multiband chiral metasurface with near-perfect circular dichroism has not been well studied. Also, the widely used bilayer metasurface usually suffers from the interlayer alignment and weak resonance. Here, we propose a twisted Moiré metasurface which can support three chiral bands with near-unity circular dichroism. The Moiré metasurface can remove the restriction of interlayer alignment, while maintaining a strong monolayer resonance. The two chiral bands in the forward direction can be described by two coupled-oscillator models. The third chiral band is achieved by tuning the interlayer chiral mode on resonance with the intralayer mode, to eliminate the parallel and converted components simultaneously. Finally, we study the robustness and tunability of the triple-layer Moiré metasurface in momentum space. This work provides a universal method to achieve three near-unity circular dichroism bands in one metasurface, which can promote applications of chiral metasurfaces in multiband optical communication, chiral drug separation, sensing, optical encryption, chiral laser, nonlinear and quantum optics, etc.

Thermal cloaks offer the potential to conceal internal objects from detection or to prevent thermal shock by controlling external heat flow. However, most conventional natural materials lack the desired flexibility and versatility required for on-demand thermal manipulation. We propose a solution in the form of homogeneous multilayer thermodynamic cloaks. Through an ingenious design, these cloaks achieve exceptional and extreme parameters, enabling the distribution of multiple materials in space. We first investigate the effects of important design parameters on the thermal shielding effectiveness of conventional thermal cloaks. Subsequently, we introduce an autonomous tuning function for the thermodynamic cloak, accomplished by leveraging two phase transition materials as thermal conductive layers. Remarkably, this tuning function does not require any energy input. Finite element analysis results demonstrate a significant reduction in the temperature gradient inside the thermal cloak compared to the surrounding background. This reduction indicates the cloak's remarkable ability to manipulate the spatial thermal field. Furthermore, the utilization of materials undergoing phase transition leads to an increase in thermal conductivity, enabling the cloak to achieve the opposite variation of the temperature field between the object region and the background. This means that, while the temperature gradient within the cloak decreases, the temperature gradient in the background increases. This work addresses a compelling and crucial challenge in the realm of thermal metamaterials, i.e., autonomous tuning of the thermal field without energy input. Such an achievement is currently unattainable with existing natural materials. This study establishes the groundwork for the application of thermal metamaterials in thermodynamic cloaks, with potential extensions into thermal energy harvesting, thermal camouflage, and thermoelectric conversion devices. By harnessing phonons, our findings provide an unprecedented and practical approach to flexibly implementing thermal cloaks and manipulating heat flow.

We investigate the crystal-momentum-resolved contributions to high-order harmonic generation in laser-driven graphene by semi-conductor Bloch equations in the velocity gauge. It is shown that each harmonic is generated by electrons with the specific initial crystal momentum. The higher harmonics are primarily contributed by the electrons of larger initial crystal momentum because they possess larger instantaneous energies during the intra-band motion. Particularly, we observe circular interference fringes in the crystal-momentum-resolved harmonics spectrum, which result from the inter-cycle interference of harmonic generation. These circular fringes will disappear if the inter-cycle interference is disrupted by the strong dephasing effect. Our findings can help to better analyze the mechanism of high harmonics in graphene.

Chirp-rate-tunable microwave waveforms (CTMWs) with dynamically tunable parameters are of basic interest to many practical applications. Recently, photonic generation of microwave signals has made their bandwidths wider and more convenient for optical fiber transmission. An all-optical method for generation of multiband CTMWs is proposed and demonstrated on all-fiber architecture, relying on dual temporal cavity solitons with agile repetition rate. In the experiment, the triangular optical chirp microwave waveforms with bandwidth above 0.45 GHz (ranging from 1.45 GHz to 1.9 GHz) are obtained, and the chirp rate reaches 0.9 GHz/ms. The reconfigurability is also demonstrated by adjusting the control signal. This all-optical approach provides a technical basis for compact, multi-band reconfigurable microwave photonics transmission and reception systems.

High-order harmonics are ideal probes to resolve the attosecond dynamics of strong-field recollision processes. An easy-to-implement phase mask is utilized to covert the Gaussian beam to TEM01 transverse electromagnetic mode, allowing the realization of two-source interferometry of high-order harmonics. We experimentally measure the intensity dependence of dipole phase directly with high-order harmonic interferometry, in which the driving laser intensity can be precisely adjusted. The classical electron excursion simulations reproduce the experimental findings quite well, demonstrating that Coulomb potential plays subtle roles on movement of electrons for harmonics near the ionization threshold. This work is of great importance for precision measurements of ultrafast dynamics in strong-field physics.

A D-shaped fiber is coated with a new two-dimensional nanomaterial, violet phosphorus (VP), to create a saturable absorber (SA) with a modulation depth of 3.68%. Subsequently, the SA is inserted into a fiber laser, enabling successful generation of dark solitons and bright–dark soliton pairs through adjustment of the polarization state within the cavity. Through further study, mode-locked pulses are achieved, proving the existence of polarization-locked vector solitons. The results indicate that VP can be used as a polarization-independent SA.

The increasing demands for electronic devices to achieve high miniaturization, functional integration, and wide bandwidth will exacerbate the heat generation and electromagnetic interference, which hinders the further development of electronic devices. Therefore, both the issues of microwave absorption and heat dissipation of materials need to be addressed simultaneously. Herein, a multifunctional composite material is proposed by periodic arrangement of copper pillars in a matrix, based on the wave-absorbing material. As a result, the equivalent thermal conductivity of the composite structure is nearly 35 times higher than the wave-absorbing matrix, with the area filling proportion of the thermal conductivity material being 3.14%. Meanwhile, the reflectivity of the composite structure merely changes from $-15.05$ dB to $-13.70$ dB. It is proved that the designed composite structure possesses both high thermal conduction and strong microwave absorption. The measured results accord well with the simulation results, which demonstrates that the thermal conductivity of the composite structure can reach more than 10 W$\cdot$m$^{-1}\cdot$K$^{-1}$ without significant deterioration of the absorption performance.

Phonon lasers, as a novel kind of lasers for generating coherent sound oscillation, has attracted extensive attention. Here, we theoretically propose a nonreciprocal phonon laser in a hybrid optomechanical system, which is composed of an asymmetric Fabry–Pérot cavity, an ensemble of $N$ identical two-level atoms, and a mechanical oscillator. The effective driving amplitude related to driving direction leads to an obvious difference in mechanical gain and threshold power, bringing about a nonreciprocal phonon laser. In addition, the dependence of the phonon laser on the atomic parameters is also discussed, including the decay rate of the atoms and the coupling strength between the atoms and the cavity field, which provides an additional degree of freedom to control the phonon laser action. Our work provides a path to realizing a phonon laser in an atoms-cavity optomechanical system and may aid the manufacture of directional coherent phonon sources.

Based on the numerical solution of the time-dependent Dirac equation, we propose a method to observe in real time the ac Stark shift of a vacuum driven by an ultra-intense laser field. By overlapping the ultra-intense pump pulse with another zeptosecond probe pulse whose photon energy is smaller than $2mc^2$, electron–positron pair creation can be controlled by tuning the time delay between the pump and probe pulses. Since the pair creation rate depends sensitively on the instantaneous vacuum potential, one can reconstruct the ac Stark shift of the vacuum potential according to the time-delay-dependent pair creation rate.

Metamaterial absorbers with carbon fiber reinforced polymer (CFRP) substrates, which are called meta-CFRPs, have recently gained recognition for their excellent mechanical and electromagnetic performance. Different from traditional metamaterial absorbers with an isotropic substrate, meta-CFRPs with a highly anisotropic CFRP substrate are facing challenges in acquiring polarization-insensitive absorption. Here, a lay-up-oriented structure design method is proposed to solve this problem. Considering the lay-up configuration of CFRP laminates, metallic patterns are designed under corresponding polarization angles and then united together to form an integral structure. A meta-CFRP with a typical CFRP lay-up configuration([0$^{\circ}/45^{\circ}/90^{\circ}/-45^{\circ}$]$_{3s}$) is designed and tested. The experimental results exhibit over 99% microwave absorptivity at 2.44 GHz for all polarization angles. The maximum shift among the resonance peaks of the curves at all polarization angles is only 0.021 GHz. Further studies show that when there are cross-ply laminates in the first few layers of the CFRP substrate, the lay-up-oriented design method can be effectively simplified by ignoring the subsequent lay-up orientations after the first cross-ply layers. Our method can not only provide an effective way for acquiring polarization-insensitive microwave response on meta-CFRPs but also be expected to be promoted to metamaterial absorbers with other anisotropic materials.