Third-harmonic generation and imaging with resonant Si membrane metasurface

Dielectric metasurfaces play an increasingly important role in enhancing optical nonlinear generations owing to their ability to support strong light-matter interactions based on Mie-type multipolar resonances.Compared to metasurfaces composed of the periodic arrangement of nanoparticles,inverse,so-called,membrane metasurfaces offer unique possibilities for supporting multipolar resonances,while maintaining small unit cell size,large mode volume and high field enhancement for enhancing nonlinear frequency conversion.Here,we theoretically and experimentally investigate the formation of bound states in the continuum(BICs)from silicon dimer-hole membrane metasurfaces.We demonstrate that our BIC-formed resonance features a strong and tailorable electric near-field confinement inside the silicon membrane films.Furthermore,we show that by tuning the gap between the holes,one can open a leaky channel to transform these regular BICs into quasi-BICs,which can be excited directly under normal plane wave incidence.To prove the capabilities of such metasurfaces,we demonstrate the conversion of an infrared image to the visible range,based on the Third-harmonic generation(THG)process with the resonant membrane metasurfaces.Our results suggest a new paradigm for realising efficient nonlinear photonics metadevices and hold promise for extending the applications of nonlinear structuring surfaces to new types of all-optical near-infrared imaging technologies.

Assessing pathological features of breast cancer via the multimodal information of multiphoton and Raman imaging

For unveiling the pathological evolution of breast cancer, nonlinear multiphoton microscopic(MPM) and confocal Raman microspectral imaging(CRMI) techniques were both utilized to address the structural and constitutional characteristics of healthy(H), ductal carcinoma in situ(DCIS), and invasive ductal carcinoma(IDC) tissues. MPM-based techniques,including two-photon excited fluorescence(TPEF) and second harmonic generation(SHG), visualized label-free and the fine structure of breast tissue. Meanwhile, CRMI not only presented the chemical images of investigated samples with the K-mean cluster analysis method(KCA), but also pictured the distribution of components in the scanned area through univariate imaging. MPM images illustrated that the cancer cells first arranged around the basement membrane of the duct,then proliferated to fill the lumens of the duct, and finally broke through the basement membrane to infiltrate into the stroma.Although the Raman imaging failed to visualize the cell structure with high resolution, it explained spectroscopically the gradual increase of nucleic acid and protein components inside the ducts as cancer cells proliferated, and displayed the distribution pattern of each biological component during the evolution of breast cancer. Thus, the combination of MPM and CRMI provided new insights into the on-site pathological diagnosis of malignant breast cancer, also ensured technical support for the development of multimodal optical imaging techniques for precise histopathological analysis.

Prospects of microwave-induced thermoacoustic imaging

Microwave-induced thermoacoustic imaging(MTAI)has advantages including the large imaging depth,high imaging resolution,high imaging contrast,and fast imaging speed.The thermoacoustic(TA)group of South China Normal University has dedicated to developing TA imaging for more than a decade and has made many breakthroughs.This review introduces these breakthroughs from two aspects including the improvement in techniques and the exploration of applications.On the technological level,there are ultrashort microwave pulse(USMP)-inducedTA imaging that can improve the imaging resolution,nonlinear thermoacoustic imaging(NTAI)that can improve the imaging contrast,polarized microwave-inducedthermoacoustic imaging(P-MTAI)that can obtain cellular-level alignment information,and more convenient and accurate handheld and multimodal probes.On the application side,the optimization and expansion have been carried out,mainly concentrating on breast and myocardial imaging.Finally,several current research directions are introduced,including the application of P-MTAI on joint imaging and research on whole-body imaging of small animals.

Imaging the crystal orientation of 2D transition metal dichalcogenides using polarization-resolved second-harmonic generation

We use laser-scanning nonlinear imaging microscopy in atomically thin transition metal dichalcogenides(TMDs)to reveal information on the crystalline orientation distribution,within the 2D lattice.In particular,we perform polarization-resolved second-harmonic generation(PSHG)imaging in a stationary,raster-scanned chemical vapor deposition(CVD)-grown WS2 flake,in order to obtain with high precision a spatially resolved map of the orientation of its main crystallographic axis(armchair orientation).By fitting the experimental PSHG images of sub-micron resolution into a generalized nonlinear model,we are able to determine the armchair orientation for every pixel of the image of the 2D material,with further improved resolution.This pixel-wise mapping of the armchair orientation of 2D WS2 allows us to distinguish between different domains,reveal fine structure,and estimate the crystal orientation variability,which can be used as a unique crystal quality marker over large areas.The necessity and superiority of a polarization-resolved analysis over intensity-only measurements is experimentally demonstrated,while the advantages of PSHG over other techniques are analysed and discussed.

Nonlinear images of scatterers in chirped pulsed laser beams

The bandwidth and the duration of incident pulsed beam are proved to play important roles in modifying the nonlinear image of amplitude-type scatterer. It is found that the initially positive chirp-type bandwidth can suppress the nonlinear image, while the negative one can enhance it, and that both effects are inversely proportional to the incident pulse duration. Numerical simulations further demonstrate that the location of nonlinear image is at the conjugate plane of the scatterer and that, for negatively pre-chirped pulsed beam, the nonlinear image peak intensity can be higher than that in the corresponding monochromatic case under certain conditions. Moreover the effect of group velocity dispersion on nonlinear image is found to be similar to that of chirp-type bandwidth.

The pitch-catch nonlinear ultrasonic imaging techniques for structural health monitoring

Nonlinear ultrasonic imaging techniques in pulse-echo configuration have recently shown their potential to allow the effective separation of nonlinear and linear features in a nonlinear image.In this study,two ultrasonic phased arrays are implemented to produce an image of elastic nonlinearity through the parallel-sequential subtraction of the coherently scattered components in the through-transmission acoustic field at the transmission or subharmonic frequency.In parallel mode,a physical focus at each pixel is achieved by firing the transmitters with a predefined delay law.In sequential mode,each transmitter is fired in sequence and all the receivers are employed to capture the data simultaneously.This full matrix captured data can be post-processed and focused synthetically at the target area.The images of parallel focusing and sequential focusing are expected to be linearly identical and hence any differences remained on the subtracted image can be related to the nonlinearities arising from the defects.Therefore,the imaging metric here is defined as the difference between parallel and sequentially focused amplitudes obtained from forward coherently scattered fields at each target point.Additionally,the negative influences due to the instrumentation nonlinearities are investigated by studying the remaining relative phase and amplitude at undamaged pixels.A compensation method is implemented to suppress these noises,significantly enhancing the selectivity of nonlinear scattering features.The proposed techniques are then implemented to monitor fatigue crack growth in order to explore the capability of these methods as measures of elastic nonlinearity induced by different sizes of small closed cracks.The promising results suggest that nonlinear imaging can be used to monitor crack growth and improve the detectability at early stages.

Dynamic tensile characterization of pig skin

The strain-rate dependent response of porcine skin oriented in the fiber direction is explored under tensile loading. Quasi-static response was obtained at strain rates in the range of 10-3s-1to 25 s-1. Characterization of the response at even greater strain rates is accomplished by measuring the spatio-temporal evolution of the particle velocity and strain in a thin strip subjected to high speed impact loading that generates uniaxial stress conditions. These experiments indicate the formation of shock waves; the shock Hugoniot that relates particle velocity to the shock velocity and the dynamic stress to dynamic strain is obtained directly through experimental measurements, without any assumptions regarding the constitutive properties of the material.