It is well known that spatial symmetry in a photonic crystal (PhC) slab is capable of creating bound states in the continuum (BICs), which can be characterized by topological charges of polarization vortices. Here, we show that when a PT-symmetric perturbation is introduced into the PhC slab, a new type of BICs ($pt$-BICs) will arise from each ordinary BIC together with the creation of rings of lasing threshold modes with $pt$-BICs embedded in these rings. Different from ordinary BICs, the $Q$-factor divergence rate of a $pt$-BIC is reduced and anisotropic in momentum space. Also, $pt$-BICs can even appear at off-high symmetry lines of the Brillouin zone. The $pt$-BICs also carry topological charges and can be created or annihilated with the total charge conserved. A unified picture on $pt$-BICs and the associated lasing threshold modes is given based on the temporal coupled mode theory. Our findings reveal the new physics arising from the interplay between PT symmetry and BIC in PhC slabs.

Time synchronization and phase shaping of single photons both play fundamental roles in quantum information applications that rely on multi-photon quantum interference. Phase shaping typically requires separate modulators with extra insertion losses. Here, we develop an all-optical built-in phase modulator for single photons using a quantum memory. The fast phase modulation of a single photon in both step and linear manner are verified by observing the efficient quantum-memory-assisted Hong–Ou–Mandel interference between two single photons, where the anti-coalescence effect of bosonic photon pairs is demonstrated. The developed phase modulator may push forward the practical quantum information applications.

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.

Two-dimensional surface-enhanced Raman scattering (SERS) substrates have drawn intense attention due to their excellent spectral reproducibility, high uniformity and perfect anti-interference ability. However, the inferior detection sensitivity and low enhancement have limited the practical application of two-dimensional SERS substrates. To address this issue, we propose that the interaction between the MoTe$_{2}$ substrate and the analyte rhodamine 6G molecules could be remarkably enhanced by the introduced p-doping effect and lattice distortion of MoTe$_{2}$ via hydrogen plasma treatment. After the treatment, the SERS is greatly improved, the enhancement factor of probe molecules reaches $1.83 \times 10^{6}$ as well as the limit of detection concentration reaches $10^{-13}$ M. This method is anticipated to afford new enhancement probability for other 2D materials, even non-metal oxide semiconductor SERS substrates.

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 theoretically study the Casimir interaction between Weyl semimetals. When the distance $a$ between semi-infinite Weyl semimetals is in the micrometer regime, the Casimir attraction can be enhanced by the chiral anomaly. The Casimir attraction depends sensitively on the relative orientations between the separations ($\boldsymbol{b}_1$, $\boldsymbol{b}_2$) of Weyl nodes in the Brillouin zone and show anisotropic behavior for the relative orientation of these separations ($\boldsymbol{b}_1$, $\boldsymbol{b}_2$) when they orient parallel to the interface. This anisotropy is quite larger than that in conventional birefringent materials. The Casimir force can be repulsive in the micrometer regime if the Weyl semimetal slabs are sufficiently thin and the direction of Weyl nodes separations ($\boldsymbol{b}_1$, $\boldsymbol{b}_2$) is perpendicular to the interface. The Casimir attraction between Weyl semimetal slabs decays slower than $1/a^4$ when the Weyl nodes separations $\boldsymbol{b}_1$ and $\boldsymbol{b}_2$ are both parallel to the interface.

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.

A photon source with high-dimensional entanglement is able to bring increasing capacity of information in quantum communication. The dimensionality is determined by the chosen degree of freedom of the photons and is limited by the complexity of the physical systems. Here we propose a new type of high-dimensional entangled photon source, generated via path-indistinguishable scheme from a two-dimensional atomic cloud, which is prepared in a magneto-optical trap. To verify the photon source, we demonstrate experimentally the quantum state of the single photons heralded by its partner photon, with homodyne tomographic technology.

We demonstrate a broadband optical parametric oscillation, using a sheet cavity, via cavity phase-matching. A 21.2 THz broad comb-like spectrum is achieved, with a uniform line spacing of 133.0 GHz, despite a relatively large dispersion of 275.4 fs$^{2}$/mm around 1064 nm. With 22.6% high slope efficiency, and 14.9 kW peak power handling, this sheet optical parametric oscillator can be further developed for $\chi^{(2)}$ comb.

Algorithms for wavefront sensing and error correction from intensity attract great concern in many fields. Here we propose Bayesian optimization to retrieve phase and demonstrate its performance in simulation and experiment. For small aberration, this method demonstrates a convergence process with high accuracy of phase sensing, which is also verified experimentally. For large aberration, Bayesian optimization is shown to be insensitive to the initial phase while maintaining high accuracy. The approach's merits of high accuracy and robustness make it promising in being applied in optical systems with static aberration such as AMO experiments, optical testing shops, and electron or optical microscopes.

Photoelectron momentum distribution in strong-field ionization has a variety of structures that reveal the complicated dynamics of this process. Recently, we identified a low-energy interference structure in the case of a super-intense extreme ultraviolet (XUV) laser pulse and attributed it to the laser-induced electron Fresnel diffraction. This structure is determined by the laser-induced electron displacement [Geng et al. Phys. Rev. A 104 (2021) L021102]. In the present work, we find that the Fresnel diffraction picture also appears in the tunneling and over-barrier regime of ionization by short pulses. However, the electron displacement is now induced by the electric field component of the laser pulse rather than the magnetic field component in the case of the super-intense XUV pulse. After corresponding modifications to our quantum and semiclassical models, we find that the same physical mechanism of the Fresnel diffraction governs the low-energy interference structures along the laser polarization. The results predicted by the two models agree well with the accurate results from the numerical solution to the time-dependent Schrödinger equation.

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.

Leidenfrost effect is a common and important phenomenon which has many applications, however there is a limited body of knowledge about the Leidenfrost effect at the nanoscale regime. We investigate the impact of substrate wettability on Leidenfrost point temperature (LPT) of nanoscale water film via molecular dynamics simulations, and reveal a new mechanism different from that at the macroscale. In the molecular dynamics simulations, a method of monitoring density change at different heating rates is proposed to obtain accurate LPT under different surface wettability. The results show that LPT decreases firstly and then increases with the surface wettability at the nanoscale, which is different from the monotonous increasing trend at the macroscale. The mechanism is elucidated by analyzing the competitive effect of adhesion force and interfacial thermal resistance, as well as different contributions of gravity on LPT at the nanoscale and macroscale. The investigations can deepen the understanding of Leidenfrost effect at the nanoscale regime and also facilitate to guide the applications of heat transfer and flow transport.

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 demonstrated a nonlinear temporal filter based on the self-diffraction (SD) process. Temporal contrast enhancement, angular dispersion and spectrum broadening properties of the SD process are investigated in experiment and simulation. Driven by spectral phase well compensated laser pulses with bandwidth of 28 nm, the filter produced clean pulses with a temporal contrast higher than $10^{10}$ and excellent spatial profile, the spectrum of which was smoothed and broadened to 64 nm. After implementing this filter into a home-made 30 TW Ti:sapphire amplifier, temporal contrast of the amplified pulses was enhanced to $10^{10}$ within the time scale of $-400$ ps.

Perfect vector beams are a class of special vector beams with invariant radius and intensity profiles under changing topological charges. However, with the limitation of current devices, the generation of these vector beams is limited in the visible and infrared wavebands. Herein, we generate perfect vector beams in the ultraviolet region assisted by nonlinear frequency conversion. Experimental and simulation results show that the radius of the generated ultraviolet perfect vector beams remains invariant and is thus independent of the topological charge. Furthermore, we measure the power of the generated ultraviolet perfect vector beams with the change of their topological charges. This study provides an alternative approach to generating perfect vector beams for ultraviolet wavebands and may promote their application to optical trapping and optical communication.

We propose a scheme to generate stable vector spatiotemporal solitons through a Rydberg electromagnetically induced transparency (Rydberg-EIT) system. Three-dimensional vector monopole and vortex solitons have been found under three nonlocal degrees. The numerical calculation and analytical solutions indicate that these solitons are generated with low energy and can stably propagate along the axes. The behavior of vector spatiotemporal solitons can be manipulated by the local and nonlocal nonlinearities. The results show a memory feature as these solitons can be stored and retrieved effectively by tuning the control field.

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.

We propose a controllable exponential-Cosine Gaussian vortex (ECGV) beam, which can evolve into the different beam profiles with three parameters: distance modulation factor (DMF), split modulation factor (SMF) and rotation modulation factor (RMF). When SMF is 0, the ECGV beam appears as a perfect single-ring vortex beam and the ring radius can be adjusted by the DMF. We deduce from mathematics and give the reason for the single-ring characteristics. When SMF is not 0, the beam splits symmetrically. DMF, SMF and RMF control the number, distance and rotation angle of the split, respectively. Our experiments verify the correctness of the theory.

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.

The complex cubic-quintic Ginzburg–Landau equation (CQGLE) is a universal model for describing a dissipative system, especially fiber laser. The analytic one-soliton solution of the variable-coefficients CQGLE is calculated by a modified Hirota method. Then, phenomena of soliton pulses splitting and stable bound states of two solitons are investigated. Moreover, rectangular dissipative soliton pulses of the variable-coefficients CQGLE are realized and controlled effectively in the theoretical research for the first time, which breaks through energy limitation of soliton pulses and is expected to provide theoretical basis for preparation of high-energy soliton pulses in fiber lasers.

Above-band-gap optical excitation of electron-hole pairs screens the doping-induced surface electric field and generates terahertz (THz) pulses via free-carrier transport. THz emission from a heavily doped silicon surface is much weaker than that of lightly doped samples. A polarity reversal of the THz electric field is observed in heavily doped p-type silicon, indicating that the doping related and carrier induced surface electric fields oppose each other. By comparing the penetration depth of the excitation laser with the thickness of the depletion layer for the doped silicon, it is shown that competition between diffusion and drift current causes the polarity reversal.

Single rare-earth ions doped in solids are one kind of the promising candidates for quantum nodes towards a scalable quantum network. Realizing a universal set of high-fidelity gate operations is a central requirement for functional quantum nodes. Here we propose geometric gate operations using the hybridized states of electron spin and nuclear spin of an ion embedded in a crystal. The fidelities of these geometric gates achieve $0.98$ in the realistic experimental situations. We also show the robustness of geometric gates to pulse fluctuations and to environment decoherence. These results provide insights for geometric phases in dissipative systems and show a potential application of high fidelity manipulations for future quantum internet nodes.

This work focuses on chirped solitons in a higher-order nonlinear Schrödinger equation, including cubic-quintic-septic nonlinearity, weak nonlocal nonlinearity, self-frequency shift, and self-steepening effect. For the first time, analytical bright and kink solitons, as well as their corresponding chirping, are obtained. The influence of septic nonlinearity and weak nonlocality on the dynamical behaviors of those nonlinearly chirped solitons is thoroughly addressed. The findings of the study give an experimental basis for nonlinear-managed solitons in optical fibers.

We demonstrate the coherent transfer of an ultrastable optical frequency reference over a 490 km noisy field fiber link. The fiber-induced phase noise power spectrum density per-unit-length at 1 Hz offset frequency can reach up to 510 rad$^2$$\cdot$Hz$^{-1}$$\cdot$km$^{-1}$, which is much higher than the fiber noise observed in previous reports. This extreme level of phase noise is mainly due to the fiber link laying underground along the highway. Appropriate phase-locked loop parameters are chosen to complete the active compensation of fiber noise by measuring the intensity fluctuation of additional phase noise and designing a homemade digital frequency division phase discriminator with a large phase detection range of $2^{12} \pi$ rad. Finally, a noise suppression intensity of approximately 40 dB at 1 Hz is obtained, with fractional frequency instability of $1.1\times10^{-14}$ at 1 s averaging time, and $3.7\times10^{-19}$ at 10000 s. The transfer system will be used for remote atomic clock comparisons and optical frequency distribution over a long-distance communication network established in China.

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.

We report an all-fiberized chirped pulse amplification system without any bulk devices. The stretcher and compressor are chirped fiber Bragg gratings inscribed in a 6/125 µm single-mode fiber and a 30/250 µm large-mode-area fiber. The fabrication system of chirped fiber Bragg gratings was designed and built by ourselves. The width of the linear exposure spot was controlled according to the different fiber sizes to improve the fabrication quality, and the parameters of chirped fiber Bragg gratings were fine-tuned during the fabrication to achieve the overall system's spectral matching. Two fiber circulators with the same fiber sizes as the chirped fiber Bragg gratings were employed to auxiliarily achieve the pulse stretching and compression. The dispersion accumulations provided by the stretcher and compressor are 129.8 ps and 90.8 ps. The power amplifiers were composed of the two-stage 10/130 µm fiber pre-amplifier and the 30/250 µm fiber main amplifier. The proposed chirped pulse amplification system with no spatial light is the true sense of an all-fiberized chirped pulse amplification structure and shows the main trend in development of ultrashort pulse fiber lasers.

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.

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.