We give an introduction to the special issue on spatiotemporal optical fields and time-varying optical materials, composed of six articles.

We report the experimental and theoretical investigation of tilted spatiotemporal optical vortices with partial temporal coherence. The theoretical study shows that the instantaneous spatiotemporal optical vortex is widely variable with the statistical orbital angular momentum (OAM) direction. While decreasing temporal coherence results in a larger variability of OAM tilt, the average OAM direction is relatively unchanged.

Cascaded holography coupled with the secret-sharing scheme has recently gained considerable attention due to its enhanced information processing and encryption capabilities. Here, we propose a new holographic iterative algorithm and present the implementation of cascaded liquid crystal (LC) holography for optical encryption. Each LC layer acts as the secret key and can generate a distinct holographic image. By cascading two LC elements, a new holographic image is formed. Additionally, we showcase the dynamic optical encryption achieved by electrically switching LCs with combined electric keys. This work may offer promising applications in optical cryptography, all-optical computing, and data storage.

We demonstrate a deep-learning neural network (DNN) method for the measurement of molecular alignment by using the molecular-alignment-based cross-correlation polarization-gating frequency resolved optical gating (M-XFROG) technique. Our network has the capacity for direct measurement of molecular alignment from the FROG traces. In a proof-of-principle experiment, we have demonstrated our method in O2 molecules. With our method, the molecular alignment factor ⟨cos2θ⟩(t) of O2, impulsively excited by a pump pulse, was directly reconstructed. The accuracy and validity of the reconstruction have been verified by comparison with the simulations based on experimental parameters.

Indoor organic and perovskite photovoltaics (PVs) have been attracting great interest in recent years. The theoretical limit of indoor PVs has been calculated based on the detailed balance method developed by Shockley–Queisser. However, realistic losses of the organic and perovskite PVs under indoor illumination are to be understood for further efficiency improvement. In this work, the efficiency limit of indoor PVs is calculated to 55.33% under indoor illumination (2700 K, 1000 lux) when the bandgap (E_g) of the semiconductor is 1.77 eV. The efficiency limit was obtained on the basis of assuming 100% photovoltaic external quantum efficiency (EQE_PV) when E ≥ E_g, there was no nonradiative recombination, and there were no resistance losses. In reality, the maximum EQE_PV reported in the literature is 0.80–0.90. The proportion of radiative recombination in realistic devices is only 10^-5–10^-2, which causes the open-circuit voltage loss (ΔV_loss) of 0.12–0.3 V. The fill factor (FF) of the indoor PVs is sensitive to the shunt resistance (R_sh). The realistic losses of EQE_PV, nonradiative recombination, and resistance cause the large efficiency gap between the realistic values (excellent perovskite indoor PV, 32.4%; superior organic indoor PV, 30.2%) and the theoretical limit of 55.33%. In reality, it is feasible to reach the efficiency of 47.4% at 1.77 eV for organic and perovskite photovoltaics under indoor light (1000 lux, 2700 K) with V_OC = 1.299 V, J_SC = 125.33 µA/cm², and FF = 0.903 when EQE_PV = 0.9, EQE_EL = 10^-1, R_s = 0.5 Ω cm², and R_sh = 10⁴ kΩ cm².

Large-bandwidth, high-sensitivity, and large dynamic range electric field sensors are gradually replacing their traditional counterparts. The lithium-niobate-on-insulator (LNOI) material has emerged as an ideal platform for developing such devices, owing to its low optical loss, high electro-optical modulation efficiency, and significant bandwidth potential. In this paper, we propose and demonstrate an electric field sensor based on LNOI. The sensor consists of an asymmetric Mach–Zehnder interferometer (MZI) and a tapered dipole antenna array. The measured fiber-to-fiber loss is less than -6.7 dB, while the MZI structure exhibits an extinction ratio of greater than 20 dB. Moreover, 64-QAM signals at 2 GHz were measured, showing an error vector magnitude (EVM) of less than 8%.

The ongoing quest for higher data storage density has led to a plethora of innovations in the field of optical data storage. This review paper provides a comprehensive overview of recent advancements in next-generation optical data storage, offering insights into various technological roadmaps. We pay particular attention to multidimensional and superresolution approaches, each of which uniquely addresses the challenge of dense storage. The multidimensional approach exploits multiple parameters of light, allowing for the storage of multiple bits of information within a single voxel while still adhering to diffraction limitation. Alternatively, superresolution approaches leverage the photoexcitation and photoinhibition properties of materials to create diffraction-unlimited data voxels. We conclude by summarizing the immense opportunities these approaches present, while also outlining the formidable challenges they face in the transition to industrial applications.

Ischemic stroke causes long-term disability and results in motor impairments. Such impairments are associated with structural changes in the neuromuscular junction (NMJ), including detailed morphology and three-dimensional (3D) distribution. However, previous studies only explored morphological changes of individual NMJs after stroke, which limits the understanding of their role in post-stroke motor impairment. Here, we examine 3D distributions and detailed morphology of NMJs in entire mouse muscles after unilateral and bilateral strokes induced by photothrombosis. The results show that 3D distributions and numbers of NMJs do not change after stroke, and severe unilateral stroke causes similar levels of NMJ fragmentation and area enlargement to bilateral stroke. This research provides structural data, deepening the understanding of neuromuscular pathophysiology after stroke.

Four-channel off-axis holography is proposed to simultaneously understand the polarization states and the mode coefficients of linearly polarized (LP) modes in few-mode fiber. Far-field off-axis holograms in the four polarization directions of the fiber laser were acquired at the same moment through a four-channel holographic device. The weights, the relative phase differences, and the polarization parameters of the vector fiber laser mode can be solved simultaneously. The simulated and experimental mode analysis of the laser output by 1060-XP fiber with 6 LP modes at 632.8 nm is conducted, which shows that the similarity of the total intensity distribution of the laser before and after mode analysis is above 0.97. The mode polarization states, the mode weights, and the relative phase differences of the few-mode laser can be determined simultaneously in a single shot by four-channel off-axis holography.

Realizing high-fidelity optical information transmission through a scattering medium is of vital importance in both science and applications, such as short-range fiber communication and optical encryption. Theoretically, an input wavefront can be reconstructed by inverting the transmission matrix of the scattering medium. However, this deterministic method for retrieving light field information encoded in the wavefront has not yet been experimentally demonstrated. Herein, we demonstrate light field information transmission through different scattering media with near-unity fidelity. Multi-dimensional optical information can be delivered through either a multimode fiber or a ground glass without relying on any averaging or approximation, where their Pearson correlation coefficients can be up to 99%.

In this work, we demonstrated the double-cladding Tm/Al co-doped photonic crystal fiber (PCF) by laser additive manufacturing. The measurements show that the fiber was heavily doped with a Tm3+ concentration of 2.13% (mass fraction) without any crystallization. The splicing property of PCF was studied, and the integrity of the PCF air holes was maintained during the splicing process. The PCF with combiner pigtail has a splice loss of 0.23 dB. The all-fiber Tm/Al co-doped PCF amplifier system achieves a slope efficiency of 13% at 1948 nm with an output laser power of nearly 1.59 W. An upconversion process was also observed under laser excitation with a 1064 nm pulse. This method provides a new idea to deal with Tm-doped PCF fabrication and promotes the promising application of 2 µm fiber lasers.

A simple quasi-distributed fiber sensing interrogation system based on random speckles is proposed for weak fiber Bragg gratings (WFBGs) in this work. Without using tunable lasers or spectrometers, a piece of multimode fiber is applied to interrogate the WFBGs relying on the wavelength sensitivity of speckles. Instead of the CCD sensor, an InGaAs quadrant detector serves as the receiver to capture the fast-changing speckle patterns. A supervised deep learning algorithm of the multilayer perceptron architecture is implemented to process speckle data and to interrogate temperature changes or dynamic strains. The proposed demodulation system is experimentally demonstrated for WFBGs with 0.1% reflectivity. The experimental results demonstrate that the new system is capable of measuring temperature change with an accuracy of 1°C and achieving dynamic frequency of 100 Hz. This speckle-based interrogation system paves a new way for distributed WFBGs sensing with a simple design.

Optical frequency conversion based on the second-order nonlinearity (χ(2)) only occurs in anisotropic media (or at interfaces) and thus is intrinsically polarization-dependent. But for practical applications, polarization-insensitive or independent operation is highly sought after. Here, by leveraging polarization coupling and second-order nonlinearity, we experimentally demonstrate a paradigm of TE/TM polarization-independent frequency upconversion, i.e., sum frequency generation, in the periodically poled lithium niobate-on-insulator ridge waveguide. The cascading of quasi-phase-matched polarization coupling and nonlinear frequency conversion is exploited. With a proper transverse electric field, TE and TM mode fundamental waves can be frequency-upconverted with an equal efficiency in the frequency converter. The proposed method may find ready application in all-optical wavelength conversion and upconversion detection technologies.

A multi-direction bending sensor based on spot pattern demodulation of a dual-hole fiber (DHF) is proposed. By using the interference and scattering in a DHF, the related multidirectional variations can be captured by the optical field. Furthermore, the multi-directional bending characteristics of the fiber are quantitatively described by the pattern of the output light spot, achieving multidirectional bending sensing. In addition, considering the subtle changes in the deformation patterns over time, a convolutional neural network (CNN) model based on deep learning is introduced for accurate recognition and prediction of the bending angle. The experimental results show that the sensor can perceive different bending angles in four directions. These outstanding results indicate that the multi-directional bending sensor based on dual-hole interference pattern decoding has potential applications in multi-directional quantitative sensing and artificial intelligence perception.

Topological photonic states have promising applications in slow light, photon sorting, and optical buffering. However, realizing such states in non-Hermitian systems has been challenging due to their complexity and elusive properties. In this work, we have experimentally realized a topological rainbow in non-Hermitian photonic crystals by controlling loss in the microwave frequency range for what we believe is the first time. We reveal that the lossy photonic crystal provides a reliable platform for the study of non-Hermitian photonics, and loss is also taken as a degree of freedom to modulate topological states, both theoretically and experimentally. This work opens a way for the construction of a non-Hermitian photonic crystal platform, will greatly promote the development of topological photonic devices, and will lay a foundation for the real-world applications.