The article comments on a recent advance in dual volatile and nonvolatile modulation for application in optical neural networks.

The article comments on a recent advance for wavelength tuning of laser particles, through a new photoelectrochemical etching method.

The article comments on a new way to design multiple microring resonators with large free spectral range and high intrinsic Q-factor, within the standard manufacturing process.

Different from single and static photonic materials, dynamically responsive materials possess numerous advantages, such as being multifunctional, dynamically responsive, and able to provide multiple channels within spatially limited platforms, thus exhibiting great potential for application in the color-on-demand areas, including imaging, optical displays, anticounterfeiting, and encoding. Photonic functional metal–organic frameworks (MOFs), with highly designable framework structures and varieties of optical functional building units, possess broad research and application prospects in the field of photonics, which make it possible to design a promising platform with multifunctional and integrated photonic performance. In this review, beyond the preparation strategies of stimuli-responsive photonic MOFs, we also summarize the stimuli-responsive photonic MOFs regarding several most representative types of external stimuli (such as light, gas, pressure, and polarization). As shown, external stimulation endows the stimuli-responsive photonic MOFs with intriguing regulatable photonic properties: intensive and tunable emission, multiphoton-excitable luminescence, nonlinear optical, circularly polarized luminescence, lasing, etc. Furthermore, their advanced representative applications, such as information encryption and anticounterfeiting display, biological imaging, chemosensing, and others, are also reviewed. The challenges are proposed and the prospects are addressed.

The collective response of macroscopic quantum states under perturbation is widely used to study quantum correlations and cooperative properties, such as defect-induced quantum vortices in Bose–Einstein condensates and the non-destructive scattering of impurities in superfluids. Superfluorescence (SF), as a collective effect rooted in dipole–dipole cooperation through virtual photon exchange, leads to the macroscopic dipole moment (MDM) in high-density dipole ensembles. However, the perturbation response of the MDM in SF systems remains unknown. Echo-like behavior is observed in a cooperative exciton ensemble under a controllable perturbation, corresponding to an initial collapse followed by a revival of the MDM. Such a dynamic response could refer to a phase transition between the macroscopic coherence regime and the incoherent classical state on a time scale of 10 ps. The echo-like behavior is absent above 100 K due to the instability of MDM in a strongly dephased exciton ensemble. Experimentally, the MDM response to perturbations is shown to be controlled by the amplitude and injection time of the perturbations.

In quantum mechanics, when an electron is quickly ripped off from a molecule, a superposition of new eigenstates of the cation creates an electron wave packet that governs the charge flow inside, which has been called charge migration (CM). Experimentally, extracting such dynamics at its natural (attosecond) timescale is quite difficult. We report the first such experiment in a linear carbon-chain molecule, butadiyne (C₄H₂), via high-harmonic spectroscopy (HHS). By employing advanced theoretical and computational tools, we showed that the wave packet and the CM of a single molecule are reconstructed from the harmonic spectra for each fixed-in-space angle of the molecule. For this one-dimensional molecule, we calculate the center of charge ⟨ x ⟩ ( t ) to obtain v_cm, to quantify the migration speed and how it depends on the orientation angle. The findings also uncover how the electron dynamics at the first few tens to hundreds of attoseconds depends on molecular structure. The method can be extended to other molecules where the HHS technique can be employed.

As an inherent degree of freedom, total angular momentum (TAM) of photons consisting of spin angular momentum and orbital angular momentum has inspired many advanced applications and attracted much attention in recent years. Probing TAM and tailoring beam’s TAM spectrum on demand are of great significance for TAM-based scenarios. We propose both theoretically and experimentally a TAM processor enabling tunable TAM manipulation. Such a processor consists of a set of quasi-symmetric units, and each unit is composed of a couple of diffraction optical elements fabricated through polymerized liquid crystals. Forty-two single TAM states are experimentally employed to prove the concept. The favorable results illustrate good TAM state selection performance, which makes it particularly attractive for high-speed large-capacity data transmission, optical computing, and high-security photon encryption systems.

Diverse spatial mode bases can be exploited in mode-division multiplexing (MDM) to sustain the capacity growth in fiber-optic communications, such as linearly polarized (LP) modes, vector modes, LP orbital angular momentum (LP-OAM) modes, and circularly polarized OAM (CP-OAM) modes. Nevertheless, which kind of mode bases is more appropriate to be utilized in fiber still remains unclear. Here, we aim to find the superior mode basis in MDM fiber-optic communications via a system-level comparison in air-core fiber (ACF). We first investigate the walk-off effect of four spatial mode bases over 1-km ACF, where LP and LP-OAM modes show intrinsic mode walk-off, while it is negligible for vector and CP-OAM modes. We then study the mode coupling effect of degenerate vector and CP-OAM modes over 1-km ACF under fiber perturbations, where degenerate even and odd vector modes suffer severe mode cross talk, while negligible for high-order degenerate CP-OAM modes based on the laws of angular momentum conservation. Moreover, we comprehensively evaluate the system-level performance for data-carrying single-channel and two-channel MDM transmission with different spatial mode bases under various kinds of fiber perturbations (bending, twisting, pressing, winding, and out-of-plane moving). The obtained results indicate that the CP-OAM mode basis shows superiority compared to other mode bases in MDM fiber-optic communications without using multiple-input multiple-output digital signal processing. Our findings may pave the way for robust short-reach MDM optical interconnects for data centers and high-performance computing.

Micro- and nanodisk lasers have emerged as promising optical sources and probes for on-chip and free-space applications. However, the randomness in disk diameter introduced by standard nanofabrication makes it challenging to obtain deterministic wavelengths. To address this, we developed a photoelectrochemical (PEC) etching-based technique that enables us to precisely tune the lasing wavelength with subnanometer accuracy. We examined the PEC mechanism and compound semiconductor etching rate in diluted sulfuric acid solution. Using this technique, we produced microlasers on a chip and isolated particles with distinct lasing wavelengths. These precisely tuned disk lasers were then used to tag cells in culture. Our results demonstrate that this scalable technique can be used to produce groups of lasers with precise emission wavelengths for various nanophotonic and biomedical applications.

Photonic bound states in the continuum (BICs) are spatially localized modes with infinitely long lifetimes, which exist within a radiation continuum at discrete energy levels. These states have been explored in various systems, including photonic and phononic crystal slabs, metasurfaces, waveguides, and integrated circuits. Robustness and availability of the BICs are important aspects for fully taming the BICs toward practical applications. Here, we propose a generic mechanism to realize BICs that exist by first principles free of fine parameter tuning based on non-Maxwellian double-net metamaterials (DNMs). An ideal warm hydrodynamic double plasma (HDP) fluid model provides a homogenized description of DNMs and explains the robustness of the BICs found herein. In the HDP model, these are standing wave formations made of electron acoustic waves (EAWs), which are pure charge oscillations with vanishing electromagnetic fields. EAW BICs have various advantages, such as (i) frequency-comb-like collection of BICs free from normal resonances; (ii) robustness to symmetry-breaking perturbations and formation of quasi-BICs with an ultrahigh Q-factor even if subject to disorder; and (iii) giving rise to subwavelength microcavity resonators hosting quasi-BIC modes with an ultrahigh Q-factor.

Precise and ultrafast control over photo-induced charge currents across nanoscale interfaces could lead to important applications in energy harvesting, ultrafast electronics, and coherent terahertz sources. Recent studies have shown that several relativistic mechanisms, including inverse spin-Hall effect, inverse Rashba–Edelstein effect, and inverse spin-orbit-torque effect, can convert longitudinally injected spin-polarized currents from magnetic materials to transverse charge currents, thereby harnessing these currents for terahertz generation. However, these mechanisms typically require external magnetic fields and exhibit limitations in terms of spin-polarization rates and efficiencies of relativistic spin-to-charge conversion. We present a nonrelativistic and nonmagnetic mechanism that directly utilizes the photoexcited high-density charge currents across the interface. We demonstrate that the electrical anisotropy of conductive oxides RuO₂ and IrO₂ can effectively deflect injected charge currents to the transverse direction, resulting in efficient and broadband terahertz radiation. Importantly, this mechanism has the potential to offer much higher conversion efficiency compared to previous methods, as conductive materials with large electrical anisotropy are readily available, whereas further increasing the spin-Hall angle of heavy-metal materials would be challenging. Our findings offer exciting possibilities for directly utilizing these photoexcited high-density currents across metallic interfaces for ultrafast electronics and terahertz spectroscopy.

Imaging three-dimensional, subcellular structures with high axial resolution has always been the core purpose of fluorescence microscopy. However, trade-offs exist between axial resolution and other important technical indicators, such as temporal resolution, optical power density, and imaging process complexity. We report a new imaging modality, fluorescence interference structured illumination microscopy (FI-SIM), which is based on three-dimensional structured illumination microscopy for wide-field lateral imaging and fluorescence interference for axial reconstruction. FI-SIM can acquire images quickly within the order of hundreds of milliseconds and exhibit even 30 nm axial resolution in half the wavelength depth range without z-axis scanning. Moreover, the relatively low laser power density relaxes the requirements for dyes and enables a wide range of applications for observing fixed and live subcellular structures.

Mode-division multiplexing (MDM) technology enables high-bandwidth data transmission using orthogonal waveguide modes to construct parallel data streams. However, few demonstrations have been realized for generating and supporting high-order modes, mainly due to the intrinsic large material group-velocity dispersion (GVD), which make it challenging to selectively couple different-order spatial modes. We show the feasibility of on-chip GVD engineering by introducing a gradient-index metamaterial structure, which enables a robust and fully scalable MDM process. We demonstrate a record-high-order MDM device that supports TE₀–TE₁₅ modes simultaneously. 40-GBaud 16-ary quadrature amplitude modulation signals encoded on 16 mode channels contribute to a 2.162 Tbit / s net data rate, which is the highest data rate ever reported for an on-chip single-wavelength transmission. Our method can effectively expand the number of channels provided by MDM technology and promote the emerging research fields with great demand for parallelism, such as high-capacity optical interconnects, high-dimensional quantum communications, and large-scale neural networks.

The erratum lists corrections to the published article.

The article provides information about the image on the cover of Advanced Photonics, Volume 5, Issue 5.