Enhanced Multiphoton Processes in Perovskite Metasurfaces.

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light-matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.

Nonlinear Holographic All-Dielectric Metasurfaces.

Nonlinear holographic metasurfaces have been intensively studied due to their potentials in practical applications. So far, nonlinear holographic metasurfaces have only been realized with plasmonic nanoantennas, suffering from high absorption loss and low damage threshold. Herein we propose and experimentally demonstrate a novel mechanism for nonlinear holographic metasurfaces. In contrast with conventional studies, the all-dielectric metasurface is composed of C-shaped Si nanoantennas. The incident laser is enhanced by their fundamental resonance, whereas the generated third-harmonic generation (THG) signals are redistributed to the air gap region via the higher order resonance, significantly reducing the absorption loss at short wavelength and resulting in an enhancement factor as high as 230. After introducing abrupt phase changes from 0 to 2π to the C elements, high-efficiency cyan and blue THG holograms have been experimentally generated with the Si metasurface for the very first time. This research shall shed light on the advances of nonlinear all-dielectric metasurfaces.

Multi-Mode Damage and Fracture Mechanisms of Thin-Walled Tubular Parts with Cross Inner Ribs Manufactured via Flow Forming.

In order to study the multi-mode damage and fracture mechanisms of thin-walled tubular parts with cross inner ribs (longitudinal and transverse inner ribs, LTIRs), the Gurson-Tvergaard-Needleman (GTN) model was modified with a newly proposed stress state function. Thus, tension damage and shear damage were unified by the new stress state function, which was asymmetric with respect to stress triaxiality. Tension damage dominated the modification, which coupled with the shear damage variable, ensured the optimal prediction of fractures of thin-walled tubular parts with LTIRs by the modified GTN model. This included fractures occurring at the non-rib zone (NRZ), the longitudinal rib (LIR) and the interface between the transverse rib (TIR) and the NRZ. Among them, the stripping of material from the outer surface of the tubular part was mainly caused by the shearing of built-up material in front of the rollers under a large wall thickness reduction (ΔT). Shear and tension deformation were the causes of fractures occurring at the NRZ, while axial tension under a large TIR interval (l) mainly resulted in fractures on LIRs. Fractures at the interface between the TIR and NRZ were due to the shearing applied by rib grooves and radial tension during the formation of ribs. This study can provide guidance for the manufacturing of high-performance aluminum alloy thin-walled tubular components with complex inner ribs.

Experimental Investigation of Current Intensity and Feed Speed in Electrically Assisted Necking and Thickening of 5A02 Aluminum Alloy Tubes.

In order to further explore the forming limits of thin-wall tube necking and thickening, and obtain sufficient thickness of the tube in the thickening area, local electric pulse-assisted forming experiments were carried out to study the effects of current intensity and feed speed on the necking and thickening forming of thin-wall tube. The experimental results show that with the increase in current intensity, the temperature in the forming area of the tube increases, and the forming load for necking and thickening decreases. However, with the increase in feed speed, the overall forming load for necking and thickening increases in general, and the smaller feed speed is more conducive to forming. Taking into account the forming efficiency and electrode loss, the corresponding forming process window is obtained for the manufacturing of good parts. That is, during the necking stage, the current intensity shall not be less than 300 A, and the feed speed shall not exceed 10 mm/min. During the thickening stage, the current intensity should not be less than 1400 A, and the feed speed should not exceed 1 mm/min. The target part is finally formed, with an average wall thickness of 5.984 mm in the thickening zone and a thickening rate of 303.2%.

Advance of large-area achromatic flat lenses.

A new framework of light coherence optimization is proposed to design non-ideal broadband achromatic lenses, enabling large-scale flat lenses' implementation and high performance. The strategy paves the way for practical planar optical devices and full-color imaging systems.

Highly Controllable Etchless Perovskite Microlasers Based on Bound States in the Continuum.

Lead halide perovskites have been promising materials for lasing applications. Despite that a series of perovskite microlasers have been reported, their lasing modes are confined by either the as-grown morphology or the etched boundary. The first one is quite random and incompatible with integration, whereas the latter one strongly spoils the laser performances. Herein, we propose and experimentally demonstrate a robust and generic mechanism to realize well-controlled perovskite microlasers without the etching process. By patterning a one-dimensional polymer grating onto a perovskite film, we show that the symmetry-protected bound states in the continuum (BICs) can be formed in it. The intriguing properties of BICs including a widely spread mode profile and high Q factor, associated with the exceptional gain of perovskite, produce single-mode microlasers with high repeatability, controllability, directionality, and a polarization vortex. This mechanism can also be extended to two-dimensional nanostructures, enabling BIC lasers with different topological charges.

Direct observation of chaotic resonances in optical microcavities.

Optical microcavities play a significant role in the study of classical and quantum chaos. To date, most experimental explorations of their internal wave dynamics have focused on the properties of their inputs and outputs, without directly interrogating the dynamics and the associated mode patterns inside. As a result, this key information is rarely retrieved with certainty, which significantly restricts the verification and understanding of the actual chaotic motion. Here we demonstrate a simple and robust approach to directly and rapidly map the internal mode patterns in chaotic microcavities. By introducing a local index perturbation through a pump laser, we report a spectral response of optical microcavities that is proportional to the internal field distribution. With this technique, chaotic modes with staggered mode spacings can be distinguished. Consequently, a complete chaos assisted tunneling (CAT) and its time-reversed process are experimentally verified in the optical domain with unprecedented certainty.

[Clinical value of biomarkers in diagnosis and treatment of idiopathic pulmonary fibrosis].

Idiopathic pulmonary fibrosis (IPF) is a chronic interstitial pneumonia characterized by progressive accumulation of fibroblastic foci and destruction of the alveolar structure. Due to an incomplete understanding of the mechanism of the occurrence and progression of IPF, currently no effective means have been available for its early screening or treatment. With a poor overall prognosis, the patients with IPF have a median survival of only 2-4 years. In recent years, several studies have confirmed that dozens of molecules are involved in the development of IPF and can be used as potential biomarkers. These biomarkers play important roles in early diagnosis (such as SP-D, MMP-7, and osteopontin), prognostic evaluation (such as telomerase length, KL-6, mtDNA, HSP-70, LOXL2, CXCL13, miRNA, ICAM-1, and CCL18), and guiding treatment of IPF (such as TOLLIP rs3750920 genotype, SAMS score, and SP-D), and also provide potential therapeutic targets (such as TERT, TERR, RTEC, and PARN).

Lead halide perovskite vortex microlasers.

Lead halide perovskite microlasers have been very promising for versatile optoelectronic applications. However, most perovskite microlasers are linearly polarized with uniform wavefront. The structured laser beams carrying orbital angular momentum have rarely been studied and the applications of perovskites in next-generation optical communications are thus hindered. Herein, we experimentally demonstrate the perovskite vortex microlasers with highly directional outputs and well-controlled topological charges. High quality gratings have been experimentally fabricated in perovskite film and the subsequent vertical cavity surface emitting lasers (VCSELs) with divergent angles of 3o are achieved. With the control of Archimedean spiral gratings, the wavefront of the perovskite VCSELs has been switched to be helical with topological charges of q = -4 to 4. This research is able to expand the potential applications of perovskite microlasers in hybrid integrated photonic networks, as well as optical computing.

Achieving Circularly Polarized Surface Emitting Perovskite Microlasers with All-Dielectric Metasurfaces.

Micro- and nanolasers are miniaturized light sources with great potential in optical imaging, sensing, and communication. While various micro- and nanolasers have been synthesized, they are mostly linearly polarized and thus strongly restricted in many new applications, e.g., chiral resolution in synthetic chemistry, cancerous tissue imaging, information storage, and processing. Herein, we experimentally demonstrate the circularly polarized surface emitting perovskite lasers by integrating the as-grown perovskite microcrystals with an all-dielectric metalens. The perovskite microcrystal serves as an optical microcavity and produces linearly polarized laser emission, which is collected by a geometric phase based TiO2 metalens. The left-handed circularly polarized components are collimated by the metalens into a directional laser beam with a divergent angle of <0.9°, whereas the right-handed components are strongly diverged by the same metalens. Consequently, the right-handed circularly polarized components are filtered out, and perovskite lasers with high directionality and pure circular polarization have been experimentally realized.

Stretchable All-Dielectric Metasurfaces with Polarization-Insensitive and Full-Spectrum Response.

Mechanical stretching has been an effective way to achieve widely tunable optical response in artificial nanostructures. However, the typical stretchable optical devices produce exactly the reverse effects for two orthogonal linear polarizations, significantly hindering their practical applications in many emerging systems. Herein, we demonstrate an approach for a mechanically tunable all-dielectric metasurface with polarization insensitivity and full-spectrum response in the visible range from 450 to 650 nm. By embedding a TiO2 metasurface in a polydimethylsiloxane substrate and stretching it in one direction, we find that the distinct reflection colors of two orthogonal linear polarizations can be tuned across the entire visible spectrum simultaneously. Encryption and display of information have also been realized with the same technique. The corresponding calculations show that the spectral responses of light with polarizations perpendicular and parallel to the strain are determined by two different mechanisms, that is, the near-field mutual interaction and the grating effects. This research shall shed light on stretchable and wearable photonics.

Ultrafast control of vortex microlasers.

The development of classical and quantum information-processing technology calls for on-chip integrated sources of structured light. Although integrated vortex microlasers have been previously demonstrated, they remain static and possess relatively high lasing thresholds, making them unsuitable for high-speed optical communication and computing. We introduce perovskite-based vortex microlasers and demonstrate their application to ultrafast all-optical switching at room temperature. By exploiting both mode symmetry and far-field properties, we reveal that the vortex beam lasing can be switched to linearly polarized beam lasing, or vice versa, with switching times of 1 to 1.5 picoseconds and energy consumption that is orders of magnitude lower than in previously demonstrated all-optical switching. Our results provide an approach that breaks the long-standing trade-off between low energy consumption and high-speed nanophotonics, introducing vortex microlasers that are switchable at terahertz frequencies.

TiO2 metasurfaces: From visible planar photonics to photochemistry.

TiO2 metasurfaces have been intensively studied in the past few years. To date, the TiO2 metadevices only used their high reflective index (n). The controllable light extinction coefficient (k) of TiO2 has not been exploited yet. Here, we converted TiO2 metasurfaces to black TiO2 metasurfaces and explored their new opportunities in photochemistry. A complementary metal oxide semiconductor (CMOS)-compatible technique has been developed to reversibly and precisely control the absorption of TiO2 metasurfaces without spoiling their internal nanostructures. Consequently, two types of black TiO2 metasurfaces were realized for photochemical experiments. The metasurface with an ultrawide absorption band can substantially enhance the white light absorption and accelerate the solar-based photochemistry process by a factor of 18.7. The other metasurface with an absorption band of <20 nm only responded to the resonant wavelengths, making the photochemistry process capable of being monitored in real time. In addition, the reversible switch between normal and black states makes TiO2 metasurfaces suitable for dynamic metadevices as well.

Surface-Emitting Perovskite Random Lasers for Speckle-Free Imaging.

Random lasers have been ideal illumination sources for speckle-free and high-speed imaging. Despite their successes, the real applications of random lasers are facing a long-standing challenge, i.e., the cumbersome size of the illuminating system. Herein, we demonstrate perovskite-based surface emitting random lasers (SERLs) and explore their applications in speckle-free imaging. The random lasers are generated by multiple scattering in a perovskite polycrystalline film sandwiched by two distributed Bragg reflectors. Owing to the tight confinement in vertical direction and large number of random resonances, the wavevectors of random lasers are dominated by their vertical components, and thus, multimode SERLs with a divergence angle of ∼3-5° and low spatial coherence are produced. By directly illuminating the patterns with the SERLs, the notable speckle noises of conventional optical images have been dramatically suppressed. This research shall provide a strategy toward the integrated spectral-free imaging systems.

All-optical control of lead halide perovskite microlasers.

Lead halide perovskites based microlasers have recently shown their potential in nanophotonics. However, up to now, all of the perovskite microlasers are static and cannot be dynamically tuned in use. Herein, we demonstrate a robust mechanism to realize the all-optical control of perovskite microlasers. In lead halide perovskite microrods, deterministic mode switching takes place as the external excitation is increased: the onset of a new lasing mode switches off the initial one via a negative power slope, while the main laser characteristics are well kept. This mode switching is reversible with the excitation and has been explained via cross-gain saturation. The modal interaction induced mode switching does not rely on sophisticated cavity designs and is generic in a series of microlasers. The switching time is faster than 70 ps, extending perovskite microlasers to previously inaccessible areas, e.g., optical memory, flip-flop, and ultrafast switches etc.

Resonance-enhanced three-photon luminesce via lead halide perovskite metasurfaces for optical encoding.

Lead halide perovskites have emerged as promising materials for photovoltaic and optoelectronic devices. However, their exceptional nonlinear properties have not been fully exploited in nanophotonics yet. Herein we fabricate methyl ammonium lead tri-bromide perovskite metasurfaces and explore their internal nonlinear processes. While both of third-order harmonic generation and three-photon luminescence are generated, the latter one is less affected by the material loss and has been significantly enhanced by a factor of 60. The corresponding simulation reveals that the improvement is caused by the resonant enhancement of incident laser. Interestingly, such kind of resonance-enhanced three-photon luminescence holds true for metasurfaces with a small period number of 4, enabling promising applications of perovskite metasurface in high-resolution nonlinear color nanoprinting and optical encoding. The encoded information 'NANO' is visible only when the incident laser is on-resonance. The off-resonance pumping and the single-photon excitation just produce a uniform dark or photoluminescence background.

Formation of Lead Halide Perovskite Based Plasmonic Nanolasers and Nanolaser Arrays by Tailoring the Substrate.

Hybrid plasmonic nanolasers are intensively studied due to their nanoscale mode confinement and potentials in highly integrated photonic and quantum devices. Until now, the characteristics of plasmonic nanolasers are mostly determined by the crystal facets of top semiconductors, such as ZnO nanowires or nanoplates. As a result, the spasers are isolated, and their lasing wavelengths are random and difficult to tune. Herein, we experimentally demonstrate the formation of lead halide perovskite (MAPbX3) based hybrid plasmonic nanolasers and nanolaser arrays with arbitrary cavity shapes and controllable lasing wavelengths. These spasers are composed of MAPbX3 perovskite nanosheets, which are separated from Au patterns with a 10 nm SiO2 spacer. In contrast to previous reports, here, the spasers are determined by the boundary of Au patterns instead of the crystal facets of MAPbX3 nanosheets. As a result, whispering gallery mode based circular spasers and spaser arrays were successfully realized by patterning the Au substrate into circles and gratings, respectively. The standard wavelength deviation of spaser arrays is as small as 0.3 nm. Meanwhile, owing to the anion-exchangeable property of MAPbX3 perovskite, the emission wavelengths of spasers were tuned more than 100 nm back and forth by changing the stoichiometry of perovskite postsynthetically.