Prof. Philip Russell (Max Planck Institute for the Science of Light) discusses the development of photonic crystal fibers, in a conversation with Prof. Long Zhang (Chinese Academy of Sciences).

Label-free vibrational stimulated Raman microscopy surpasses the diffraction limit by employing structured illumination, opening new avenues for super-resolution imaging of biological targets.

The study by Shao et al.1 marks a critical advancement in quantum simulations by reporting the first experimental observation of the antiferromagnetic (AFM) phase transition in a 3D fermionic Hubbard model (FHM) using ultracold lithium-6 atoms. This experiment bridges a significant gap in our understanding of strongly correlated systems and provides a new platform to explore low-temperature quantum phases. This commentary will analyze the results, place them in the context of ongoing research, and discuss their broader implications for the field of quantum materials and technologies.

The application of light remains constrained in deep tissues, where we struggle to achieve the same or comparable high-resolution focusing and imaging as in superficial tissues. This limitation mainly arises from the spatial inhomogeneities of refractive index (RI) in biological tissues, where light propagation is severely hindered by scattering and absorption. In recent years, enhancing imaging outcomes by reducing the scattering effects in tissues has become a hot topic of research. One effective way is to increase the refractive index of the aqueous components with optical clearing agents, achieving an effective RI level closer to that of lipids and proteins. This article comments on a recent study that reported a counterintuitive yet plain approach to the barrier and achieved biocompatible transparency of live tissues. In brief, the proposed strategy enables harmless, reversible tissue clarification of living rodents, greatly increasing the penetration depth and resolution of current in-vivo imaging technologies.

In celebration of the 60th anniversary of the founding of the Shanghai Institute of Optics and Fine Mechanics, the institute launched a public call for questions and challenges in laser science and technology. Twenty questions were ultimately selected, covering the mechanisms of coherent light sources, laser applications in imaging and manufacturing, new laser materials, and so on. These questions may serve as guidance in the future development of laser physics and applications.

Recent advancements in deep learning (DL) have propelled the virtual transformation of microscopy images across optical modalities, enabling unprecedented multimodal imaging analysis hitherto impossible. Despite these strides, the integration of such algorithms into scientists’ daily routines and clinical trials remains limited, largely due to a lack of recognition within their respective fields and the plethora of available transformation methods. To address this, we present a structured overview of cross-modality transformations, encompassing applications, data sets, and implementations, aimed at unifying this evolving field. Our review focuses on DL solutions for two key applications: contrast enhancement of targeted features within images and resolution enhancements. We recognize cross-modality transformations as a valuable resource for biologists seeking a deeper understanding of the field, as well as for technology developers aiming to better grasp sample limitations and potential applications. Notably, they enable high-contrast, high-specificity imaging akin to fluorescence microscopy without the need for laborious, costly, and disruptive physical-staining procedures. In addition, they facilitate the realization of imaging with properties that would typically require costly or complex physical modifications, such as achieving superresolution capabilities. By consolidating the current state of research in this review, we aim to catalyze further investigation and development, ultimately bringing the potential of cross-modality transformations into the hands of researchers and clinicians alike.

A single molecule is the building block of the material world and the smallest independently stable unit. Exploring single-molecule properties using optical, photonic, and optoelectronic techniques holds great scientific significance in revealing the molecular dynamics, molecular structures, and molecular quantum properties. Nano-optical techniques, such as single-molecule photoluminescence and Raman scattering, not only enable a comprehensive analysis of interactions and conformational dynamics through spectral analysis but also provide unparalleled insights into elucidating the intricate structure of single molecules through atomic-resolution imaging. The research of photonics based on single-molecule electroluminescence has brought new ideas and limitless possibilities to the design and manufacture of photonic information devices. Single-molecule optoelectronics, which leverages photoexcitation to modulate electrical properties, has significant contributions to elucidating charge transport characteristics and optimizing the optoelectronic functions realized by single-molecule devices. Moreover, the optoelectronic characterization based on the interaction of ultrafast optical pulses with single molecules provides unprecedented opportunities for exploring their dynamic behavior and regulation laws on ultrafast time scales. We provide a timely and comprehensive overview of the latest significant advancements pertaining to the optical, photonic, and optoelectronic properties of single molecules, thereby presenting a fresh perspective for research across diverse fields, including single-molecule photophysics and photochemistry.

Modern optical communications rely heavily on dense wavelength-division multiplexing (DWDM) technology because of its capability of significantly increasing transmission channels. Here, we demonstrate, for the first time to the best of our knowledge, a compact photonic chip for DWDM transmitters on lithium-niobate-on-insulator (LNOI) by introducing the array of 2×2 Fabry–Perot (FP) cavity electro-optic (EO) modulators. A four-channel LNOI photonic chip for DWDM is designed and realized with a channel spacing of 1.6 nm (which is the narrowest one reported until now for LNOI optical transmitters), exhibiting a total excess loss of 1.3 dB and high 3-dB EO bandwidths of >67 GHz for all channels. Specifically, these four 2×2 FP cavities are designed with broadened LNOI photonic waveguides in the cavity sections, and they are placed very closely on the chip so that their resonance wavelengths are aligned precisely with the desired channel-spacing of ∼1.6 nm. Finally, the generation of 4×80-Gbps on–off keying and 4×100-Gbps PAM4 signals is demonstrated successfully with four channels, and the power consumption is as low as ∼5.1 fJ/bit. The present photonic chip has a compact footprint of about 0.78 mm×0.58 mm, showing great potential to work with more than four channels and to be very useful for future large-capacity optical links.

Recent advances in metasurface lenses (metalenses) have shown great potential for opening a new era in compact imaging, photography, light detection, and ranging (LiDAR) and virtual reality/augmented reality applications. However, the fundamental trade-off between broadband focusing efficiency and operating bandwidth limits the performance of broadband metalenses, resulting in chromatic aberration, angular aberration, and a relatively low efficiency. A deep-learning-based image restoration framework is proposed to overcome these limitations and realize end-to-end metalens imaging, thereby achieving aberration-free full-color imaging for mass-produced metalenses with 10 mm diameter. Neural-network-assisted metalens imaging achieved a high resolution comparable to that of the ground truth image.

Two-photon microscopy provides sectioned excitation, but in practical settings, it often suffers from contrast limitations. Here, we report the observation of a strong acousto-optic modulation of two-photon excited fluorescence. Harnessing this effect yields enhanced image detail and contrast and improves optical sectioning in deep brain tissue in vivo. Ultrasound-modulation assisted multiphoton imaging (UMAMI) is compatible with standard multiphoton microscopes, without requiring changes to the optical path or image acquisition parameters.

Terahertz (THz) communications are vulnerable to eavesdropping due to their scattering and diffraction properties, which limits their practical deployment. We propose a photonic THz chaos encryption and synchronization scheme to secure THz communications. We experimentally demonstrate the generation, encryption, and wireless transmission of a 5 Gbit/s non-return-to-zero signal at 120 GHz using flexible photonic THz chaos. In addition, we achieve high-quality chaos synchronization with a neural network, attaining a correlation coefficient of up to 90.6%. This scheme offers a viable solution for secure THz communications, showing significant potential for enhancing wireless communication privacy.

Soliton molecules, referred to as closely bounded solitons, have recently attracted considerable interest in both fundamental nonlinear physics research and refreshed application promises. To date, extensive efforts have been made on the generation of quadratic soliton molecules. These are soliton molecules whose formation exclusively involves second-order dispersion and Kerr nonlinearity. Here, for the first time, we demonstrate the realization of various third-order dispersion-supported soliton molecules, including vector dark–anti-dark solitons, vector anti-dark solitons, and vector anti-dark soliton molecules formed in a fiber laser with net cavity dispersion near the zero-group-velocity-dispersion point. High-order dispersion could greatly alter the internal soliton interaction within a soliton molecule. This finding could open exciting new avenues in soliton research.

Optical vector analysis (OVA) is an enabling technology for comprehensively characterizing both amplitude and phase responses of optical devices or systems. Conventional OVA technologies are mostly based on discrete optoelectronic components, leading to unsatisfactory system sizes, complexity, and stability. They also encounter challenges in revealing the on-chip characteristics of integrated photonic devices, which are often overwhelmed by the substantial coupling loss and extra spectral response at chip facets. In this work, we demonstrate a miniaturized OVA system based on broadband single-sideband (SSB) modulators on a thin-film lithium niobate (LN) platform. The OVA could provide a direct probe of both amplitude and phase responses of photonic devices with kilohertz-level resolution and tens of terahertz of measurement bandwidth. We perform in situ characterizations of single and coupled microring resonators fabricated on the same chip as the OVA, unfolding their intrinsic loss and coupling states unambiguously. Furthermore, we achieve the direct measurement of collective phase dynamics and density of states of the Bloch modes in a synthetic frequency crystal by in situ OVA of a dynamically modulated microring resonator. Our OVA system provides a compact, high-precision, and broadband solution for characterizing future integrated photonic devices and circuits, with potential applications ranging from optical communications, biosensing, and neuromorphic computing, to quantum information processing.

Topology is the study of geometrical properties and spatial relations unaffected by continuous changes and has become an important tool for understanding complex physical systems. Although recent optical experiments have inferred the existence of vector fields with the topologies of merons, the inability to extract the full three-dimensional vectors misses a richer set of topologies that have not yet been fully explored. We extend the study of the topology of electromagnetic fields on surfaces to a spin quasi-particle with the topology of a meron pair, formed by interfering surface plasmon polaritons (SPPs), and show that the in-plane vectors are constrained by the embedding topology of the space as dictated by the Poincaré–Hopf theorem. In addition, we explore the time evolution of the three-dimensional topology of the spin field formed by femtosecond laser pulses. These experiments are possible using our here-developed method called polarimetric photo-emission electron microscopy (polarimetric PEEM), which combines an optical pump–probe technique and polarimetry with PEEM. This method allows for the accurate generation of SPP fields and their subsequent measurement, revealing both the spatial distribution of the full three-dimensional electromagnetic fields at deep subwavelength resolution and their time evolution.

Miniaturized laser spectroscopy capable of in situ and real-time ppb-level trace gas sensing is of fundamental importance for numerous applications, including environment monitoring, industry process control, and biomedical diagnosis. Benchtop laser spectroscopy systems based on direct absorption, photoacoustic, and Raman effects exhibit high sensitivity but face challenges for in situ and real-time gas sensing due to their bulky size, slow response, and offline sampling. We demonstrate a microscale high-performance all-fiber photoacoustic spectrometer integrating the key components, i.e., the photoacoustic gas cell and the optical microphone, into a single optical fiber tip with a diameter of 125 μm. Without a long optical path to enhance the light–gas interaction, the fiber-tip gas cell with acoustic-hard boundary tightly confines and amplifies the local photoacoustic wave, compensating for the sensitivity loss during miniaturization. This localized acoustic wave is demodulated by high-sensitivity fiber-optic interferometry, enabling a ∼9 ppb detection limit for acetylene gas approaching the benchtop system. The microscale fiber spectrometer also exhibits a short response time of ∼18 ms and a subnanoliter sample volume, not only suitable for routine real-time in situ trace gas measurement but also inspiring new applications such as two-dimensional gas flow concentration mapping and in vivo intravascular blood gas monitoring as showcased.

The article provides information about the image on the cover of Advanced Photonics, Volume 6 Issue 6.