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.

Structured light fields embody strong spatial variations of polarization, phase, and amplitude. Understanding, characterization, and exploitation of such fields can be achieved through their topological properties. Three-dimensional (3D) topological solitons, such as hopfions, are 3D localized continuous field configurations with nontrivial particle-like structures that exhibit a host of important topologically protected properties. Here, we propose and demonstrate photonic counterparts of hopfions with exact characteristics of Hopf fibration, Hopf index, and Hopf mapping from real-space vector beams to homotopic hyperspheres representing polarization states. We experimentally generate photonic hopfions with on-demand high-order Hopf indices and independently controlled topological textures, including Néel-, Bloch-, and antiskyrmionic types. We also demonstrate a robust free-space transport of photonic hopfions, thus showing the potential of hopfions for developing optical topological informatics and communications.

The control of thermal emission is of great importance for emerging applications in energy conversion and thermometric sensing. Usually, thermal emission at ambient temperature is limited to the mid- to far-infrared, according to the linear theory of Planck’s law. We experimentally demonstrate a broadband nonlinear thermal emission in the visible-NIR spectrum within a quadradic nonlinear medium, which emits visible thermal radiation through a pump-driven nonlinear upconversion from its mid-IR components even at room temperature, unlike its linear counterpart which requires ultrahigh temperature. The broadband emission is enabled by the crucial random quasi-phase-matching condition in our nonlinear nanocrystal powders. Moreover, nonlinear thermal emission also permits visible thermometry using traditional optical cameras instead of thermal ones. This scheme paves the way to understand thermal radiation dynamics with nonlinearity in many fields, such as nonlinear heat transfer and nonlinear thermodynamics.

It is experimentally verified that nonreciprocal photonic systems with continuous translation symmetry may have an ill-defined topology. The topological classification of such systems is only feasible when the material response is regularized with a spatial-frequency cutoff. We experimentally demonstrate that adjoining a small air layer to the relevant material interface may effectively imitate an idealized spatial cutoff that suppresses the nonreciprocal response for short wavelengths and regularizes the topology. Furthermore, it is experimentally verified that nonreciprocal systems with an ill-defined topology may be used to abruptly halt the energy flow in a unidirectional waveguide due to the violation of the bulk-edge correspondence. In particular, we report the formation of an energy sink that absorbs the incoming electromagnetic waves with a large field enhancement at the singularity.

Materials that exhibit visible luminescence upon X-ray irradiation show great potential in the medical and industrial fields. Pure organic materials have recently emerged as promising scintillators for X-ray detection and radiography, due to their diversified design, low cost, and facile preparation. However, recent progress in efficient radioluminescence has mainly focused on small molecules, which are inevitably associated with processability and repeatability issues. Here, a concise strategy is proposed to prepare radioluminescent polymers that exhibit multiple emission colors from blue to yellow with high brightness in an amorphous state by the radical copolymerization of negatively charged polyacrylic acid and different positively charged quaternary phosphonium salts. One of the obtained polymers exhibits excellent photostability under a high X-ray irradiation dosage of 27.35 Gy and has a detection limit of 149 nGy s - 1. This performance is superior to that of conventional anthracene-based scintillators. Furthermore, by simply drop-casting a polymer methanol solution on a quartz plate, a transparent scintillator screen was successfully fabricated for X-ray imaging with a resolution of 8.7 line pairs mm - 1. The pure organic phosphorescent polymers with a highly efficient radioluminescence were demonstrated for the first time, and the strategy reported herein offers a promising pathway to expand the application range of amorphous organic scintillators.

Optical chaos generated by perturbing semiconductor lasers has been viewed, over recent decades, as an excellent entropy source for fast physical random bit generation (RBG) owing to its high bandwidth and large random fluctuations. However, most optical-chaos-based random bit generators perform their quantization process in the electrical domain using electrical analog-to-digital converters, so their real-time rates in a single channel are severely limited at the level of Gb/s due to the electronic bottleneck. Here, we propose and experimentally demonstrate an all-optical method for RBG where chaotic pulses are quantized into a physical random bit stream in the all-optical domain by means of a length of highly nonlinear fiber. In our proof-of-concept experiment, a 10-Gb/s random bit stream is successfully generated on-line using our method. Note that the single-channel real-time rate is limited only by the chaos bandwidth. Considering that the Kerr nonlinearity of silica fiber with an ultrafast response of few femtoseconds is exploited for composing the key part of quantizing laser chaos, this scheme thus may operate potentially at much higher real-time rates than 100 Gb/s provided that a chaotic entropy source of sufficient bandwidth is available.

Programmable metasurfaces enable real-time control of electromagnetic waves in a digital coding manner, which are suitable for implementing time-domain metasurfaces with strong harmonic manipulation capabilities. However, the time-domain metasurfaces are usually realized by adopting the wired electrical control method, which is effective and robust, but there are still some limitations. Here, we propose a light-controllable time-domain digital coding metasurface consisting of a full-polarization dynamic metasurface and a high-speed photoelectric detection circuit, from which the microwave reflection spectra are manipulated by time-varying light signals with periodic phase modulations. As demonstrated, the light-controllable time-domain digital coding metasurface is illuminated by the light signals with two designed time-coding sequences. The measured results show that the metasurface can well generate symmetrical harmonics and white-noise-like spectra, respectively, under such cases in the reflected wave. The proposed light-controllable time-varying metasurface offers a planar interface to tailor and link microwaves with lights in the time domain, which could promote the development of photoelectric hybrid metasurfaces and related multiphysics applications.

Entanglement-based quantum key distribution (QKD) promises enhanced robustness against eavesdropping and compatibility with future quantum networks. Among other sources, semiconductor quantum dots (QDs) can generate polarization-entangled photon pairs with near-unity entanglement fidelity and a multiphoton emission probability close to zero even at maximum brightness. These properties have been demonstrated under resonant two-photon excitation (TPE) and at operation temperatures below 10 K. However, source blinking is often reported under TPE conditions, limiting the maximum achievable photon rate. In addition, operation temperatures reachable with compact cryocoolers could facilitate the widespread deployment of QDs, e.g., in satellite-based quantum communication. We demonstrate blinking-free emission of highly entangled photon pairs from GaAs QDs embedded in a p-i-n diode. High fidelity entanglement persists at temperatures of at least 20 K, which we use to implement fiber-based QKD between two buildings with an average raw key rate of 55 bits / s and a qubit error rate of 8.4%. We are confident that by combining electrical control with already demonstrated photonic and strain engineering, QDs will keep approaching the ideal source of entangled photons for real world applications.

Optical parametric oscillators (OPOs) have been widely applied in spectroscopy, squeezed light, and correlated photons, as well as quantum information. Conventional OPOs usually suffer from a high power threshold limited by weak high-order nonlinearity in traditional pure photonic systems. Alternatively, polaritonic systems based on hybridized exciton–photon quasi-particles exhibit enhanced optical nonlinearity by dressing photons with excitons, ensuring highly nonlinear operations with low power consumption. We report an on-chip perovskite polariton parametric oscillator with a low threshold. Under the resonant excitation at a range of angles, the signal at the ground state is obtained, emerging from the polariton–polariton interactions at room temperature. Our results advocate a practical way toward integrated nonlinear polaritonic devices with low thresholds.

Integrated photonics provides a route to both miniaturization of quantum key distribution (QKD) devices and enhancing their performance. A key element for achieving discrete-variable QKD is a single-photon detector. It is highly desirable to integrate detectors onto a photonic chip to enable the realization of practical and scalable quantum networks. We realize a heterogeneously integrated, superconducting silicon-photonic chip. Harnessing the unique high-speed feature of our optical waveguide-integrated superconducting detector, we perform the first optimal Bell-state measurement (BSM) of time-bin encoded qubits generated from two independent lasers. The optimal BSM enables an increased key rate of measurement-device-independent QKD (MDI-QKD), which is immune to all attacks against the detection system and hence provides the basis for a QKD network with untrusted relays. Together with the time-multiplexed technique, we have enhanced the sifted key rate by almost one order of magnitude. With a 125-MHz clock rate, we obtain a secure key rate of 6.166 kbps over 24.0 dB loss, which is comparable to the state-of-the-art MDI-QKD experimental results with a GHz clock rate. Combined with integrated QKD transmitters, a scalable, chip-based, and cost-effective QKD network should become realizable in the near future.

We predict theoretically a regime of photon-pair generation driven by the interplay of multiple bound states in the continuum resonances in nonlinear metasurfaces. This nondegenerate photon-pair generation is derived from the hyperbolic topology of the transverse phase matching and can enable orders-of-magnitude enhancement of the photon rate and spectral brightness, as compared to the degenerate regime. We show through comprehensive simulations that the entanglement of the photon pairs can be tuned by varying the pump polarization, which can underpin future advances and applications of ultracompact quantum light sources.

As a new-generation light source, free-electron lasers (FELs) provide high-brightness x-ray pulses at the angstrom-femtosecond space and time scales. The fundamental physics behind the FEL is the interaction between an electromagnetic wave and a relativistic electron beam in an undulator, which consists of hundreds or thousands of dipole magnets with an alternating magnetic field. We report the first observation of the laser–beam interaction in a pure dipole magnet in which the electron beam energy modulation with a 40-keV amplitude and a 266-nm period is measured. We demonstrate that such an energy modulation can be used to launch a seeded FEL, that is, lasing at the sixth harmonic of the seed laser in a high-gain harmonic generation scheme. The results reveal the most basic process of the FEL lasing and open up a new direction for the study and exploitation of laser–beam interactions.

Optical superoscillation refers to an intriguing phenomenon of a wave packet that can oscillate locally faster than its highest Fourier component, which potentially produces an extremely localized wave in the far field. It provides an alternative way to overcome the diffraction limit, hence improving the resolution of an optical microscopy system. However, the optical superoscillatory waves are inevitably accompanied by strong side lobes, which limits their fields of view and, hence, potential applications. Here, we report both experimentally and theoretically a new superoscillatory wave form, which not only produces significant feature size down to deep subwavelength, but also completely eliminates side lobes in a particular dimension. We demonstrate a new mechanism for achieving such a wave form based on a pair of moonlike sharp-edge apertures. The resultant superoscillatory wave exhibits Bessel-like forms, hence allowing long-distance propagation of subwavelength structures. The result facilitates the study of optical superoscillation and on a fundamental level eliminates the compromise between the superoscillatory feature size and the field of view.

In ultrafast optical imaging, it is critical to obtain the spatial structure, temporal evolution, and spectral composition of the object with snapshots in order to better observe and understand unrepeatable or irreversible dynamic scenes. However, so far, there are no ultrafast optical imaging techniques that can simultaneously capture the spatial–temporal–spectral five-dimensional (5D) information of dynamic scenes. To break the limitation of the existing techniques in imaging dimensions, we develop a spectral-volumetric compressed ultrafast photography (SV-CUP) technique. In our SV-CUP, the spatial resolutions in the x, y and z directions are, respectively, 0.39, 0.35, and 3 mm with an 8.8 mm × 6.3 mm field of view, the temporal frame interval is 2 ps, and the spectral frame interval is 1.72 nm. To demonstrate the excellent performance of our SV-CUP in spatial–temporal–spectral 5D imaging, we successfully measure the spectrally resolved photoluminescent dynamics of a 3D mannequin coated with CdSe quantum dots. Our SV-CUP brings unprecedented detection capabilities to dynamic scenes, which has important application prospects in fundamental research and applied science.

Tunneling ionization of atoms and molecules induced by intense laser pulses contains the contributions of numerous quantum orbits. Identifying the contributions of these orbits is crucial for exploring the application of tunneling and for understanding various tunneling-triggered strong-field phenomena. We perform a combined experimental and theoretical study to identify the relative contributions of the quantum orbits corresponding to the electrons tunneling ionized during the adjacent rising and falling quarter cycles of the electric field of the laser pulse. In our scheme, a perturbative second-harmonic field is added to the fundamental driving field. By analyzing the relative phase dependence of the signal in the photoelectron momentum distribution, the relative contributions of these two orbits are unambiguously determined. Our results show that their relative contributions sensitively depend on the longitudinal momentum and modulate with the transverse momentum of the photoelectron, which is attributed to the interference of the electron wave packets of the long orbit. The relative contributions of these orbits resolved here are important for the application of strong-field tunneling ionization as a photoelectron spectroscopy for attosecond time-resolved measurements.

Abbe’s resolution limit, one of the best-known physical limitations, poses a great challenge for any wave system in imaging, wave transport, and dynamics. Originally formulated in linear optics, the Abbe limit can be broken using nonlinear optical interactions. We extend the Abbe theory into a nonlinear regime and experimentally demonstrate a far-field, label-free, and scan-free super-resolution imaging technique based on nonlinear four-wave mixing to retrieve near-field scattered evanescent waves, achieving a sub-wavelength resolution of λ / 5.6. This method paves the way for numerous new applications in biomedical imaging, semiconductor metrology, and photolithography.

The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laser-induced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particle-in-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequency-domain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ～50 % of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90 μJ / pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.

Nonlinear holography has been identified as a vital platform for optical multiplexing holography because of the appearance of new optical frequencies. However, due to nonlinear wave coupling in nonlinear optical processes, the nonlinear harmonic field is coupled with the input field, laying a fundamental barrier to independent control of the interacting fields for holography. We propose and experimentally demonstrate high-dimensional orbital angular momentum (OAM) multiplexing nonlinear holography to overcome this problem. By dividing the wavefront of the fundamental wave into different orthogonal OAM channels, multiple OAM and polarization-dependent holographic images in both the fundamental wave and second-harmonic wave have been reconstructed independently in the spatial frequency domain through a type-II second harmonic generation process. Moreover, this method can be easily extended to cascaded χ2 nonlinear optical processes for multiplexing in more wavelength channels, leading to potential applications in multicasting in optical communications, multiwavelength display, multidimensional optical storage, anticounterfeiting, and optical encryption.

A new optical microscopy technique, termed high spatial and temporal resolution synthetic aperture phase microscopy (HISTR-SAPM), is proposed to improve the lateral resolution of wide-field coherent imaging. Under plane wave illumination, the resolution is increased by twofold to around 260 nm, while achieving millisecond-level temporal resolution. In HISTR-SAPM, digital micromirror devices are used to actively change the sample illumination beam angle at high speed with high stability. An off-axis interferometer is used to measure the sample scattered complex fields, which are then processed to reconstruct high-resolution phase images. Using HISTR-SAPM, we are able to map the height profiles of subwavelength photonic structures and resolve the period structures that have 198 nm linewidth and 132 nm gap (i.e., a full pitch of 330 nm). As the reconstruction averages out laser speckle noise while maintaining high temporal resolution, HISTR-SAPM further enables imaging and quantification of nanoscale dynamics of live cells, such as red blood cell membrane fluctuations and subcellular structure dynamics within nucleated cells. We envision that HISTR-SAPM will broadly benefit research in material science and biology.

Laser-induced breakdown spectroscopy (LIBS) is a useful tool for determination of elements in solids, liquids, and gases. For nanosecond LIBS (ns-LIBS), the plasma shielding effect limits its reproducibility, repeatability, and signal-to-noise ratios. Although femtosecond laser filament induced breakdown spectroscopy (FIBS) has no plasma shielding effects, the power density clamping inside the filaments limits the measurement sensitivity. We propose and demonstrate plasma-grating-induced breakdown spectroscopy (GIBS). The technique relies on a plasma excitation source—a plasma grating generated by the interference of two noncollinear femtosecond filaments. We demonstrate that GIBS can overcome the limitations of standard techniques such as ns-LIBS and FIBS. Signal intensity enhancement with GIBS is observed to be greater than 3 times that of FIBS. The matrix effect is also significantly reduced with GIBS, by virtue of the high power and electron density of the plasma grating, demonstrating great potential for analyzing samples with complex matrix.

Microbubbles acting as lenses are interesting for optical and photonic applications such as volumetric displays, optical resonators, integration of photonic components onto chips, high-resolution spectroscopy, lithography, and imaging. However, stable, rationally designed, and uniform microbubbles on substrates such as silicon chips are challenging because of the random nature of microbubble formation. We describe the fabrication of elastic microbubbles with a precise control of volume and curvature based on femtosecond laser irradiated graphene oxide. We demonstrate that the graphene microbubbles possess a near-perfect curvature that allows them to function as reflective microlenses for focusing broadband white light into an ultrahigh aspect ratio diffraction-limited photonic jet without chromatic aberration. Our results provide a pathway for integration of graphene microbubbles as lenses for nanophotonic components for miniaturized lab-on-a-chip devices along with applications in high-resolution spectroscopy and imaging.

Optical vortices, which carry orbital angular momentum, offer special capabilities in a host of applications. A single-laser source with dual-beam-mode output may open up new research fields of nonlinear optics and quantum optics. We demonstrate a dual-channel scheme to generate femtosecond, dual-wavelength, and dual-beam-mode tunable signals in the near infrared wavelength range. Dual-wavelength operation is derived by stimulating two adjacent periods of a periodically poled lithium niobate crystal. Pumped by an Yb-doped fiber laser with a Gaussian (lp = 0) beam, two tunable signal emissions with different beam modes are observed simultaneously. Although one of the emissions can be tuned from 1520 to 1613 nm with the Gaussian (ls = 0) beam, the other is capable of producing a vortex spatial profile with different vortex orders (ls = 0 to 2) tunable from 1490 to 1549 nm. The proposed system provides unprecedented freedom and will be an exciting platform for super-resolution imaging, nonlinear optics, multidimensional quantum entanglement, etc.

Terahertz (THz) wave generation from laser-induced air plasma generally requires a short temporal laser pulse. In contrast, it was observed that THz radiation from ionized liquid water prefers a longer pulse, wherein the mechanism remains unclear. We attribute the preference for longer pulse duration to the process of ionization and plasma formation in water, which is supported by a numerical simulation result showing that the highest electron density is achieved with a subpicosecond pulse. The explanation is further verified by the coincidence of our experimental result and simulation when the thickness of the water is varied. Other liquids are also tested to assure the preference for such a pulse is not exclusive to water.

The mode-locked fluoride fiber laser (MLFFL) is an exciting platform for directly generating ultrashort pulses in the mid-infrared (mid-IR). However, owing to difficulty in managing the dispersion in fluoride fiber lasers, MLFFLs are restricted to the soliton regime, hindering pulse-energy scaling. We overcame the problem of dispersion management by utilizing the huge normal dispersion generated near the absorption edge of an infrared-bandgap semiconductor and promoted MLFFL from soliton to breathing-pulse mode-locking. In the breathing-pulse regime, the accumulated nonlinear phase shift can be significantly reduced in the cavity, and the pulse-energy-limitation effect is mitigated. The breathing-pulse MLFFL directly produced a pulse energy of 9.3 nJ and pulse duration of 215 fs, with a record peak power of 43.3 kW at 2.8 μm. Our work paves the way for the pulse-energy and peak-power scaling of mid-IR fluoride fiber lasers, enabling a wide range of applications.

We present a fully automated laser system with low-intensity noise for coherent Raman scattering microscopy. The robust two-color system is pumped by a solid-state oscillator, which provides Stokes pulses fixed at 1043 nm. The tunable pump pulses of 750 to 950 nm are generated by a frequency-doubled fiber-feedback femtosecond optical parametric oscillator. The resulting pulse duration of 1.2 ps provides a viable compromise between optimal coherent Raman scattering signal and the necessary spectral resolution. Thus a spectral range of 1015 to 3695 cm&minus;1 with spectral resolution of <13 cm&minus;1 can be addressed.

In many optical metrology techniques, fringe pattern analysis is the central algorithm for recovering the underlying phase distribution from the recorded fringe patterns. Despite extensive research efforts for decades, how to extract the desired phase information, with the highest possible accuracy, from the minimum number of fringe patterns remains one of the most challenging open problems. Inspired by recent successes of deep learning techniques for computer vision and other applications, we demonstrate for the first time, to our knowledge, that the deep neural networks can be trained to perform fringe analysis, which substantially enhances the accuracy of phase demodulation from a single fringe pattern. The effectiveness of the proposed method is experimentally verified using carrier fringe patterns under the scenario of fringe projection profilometry. Experimental results demonstrate its superior performance, in terms of high accuracy and edge-preserving, over two representative single-frame techniques: Fourier transform profilometry and windowed Fourier transform profilometry.