Integrated system of traditional THz time-domain spectroscopy and asynchronous optical sampling

Terahertz time-domain spectroscopy(THz-TDS)system,as a new means of spectral analysis and detection,plays an increasingly pivotal role in basic scientific research.However,owing to the long scanning time of the traditional THz-TDS system and the complex control of the asynchronous optical scanning(ASOPS)system,which requires frequent calibration,we combine traditional THz-TDS and ASOPS systems to form a composite system and propose an all-fiber trigger signal generation method based on the time overlapping interference signal generated by the collinear motion of two laser pulses.Finally,the time-domain and frequency-domain spectra are obtained by using two independent systems in the integrated systems.It is found that the full width at half maximum(FWHM)of the time-domain spectra and the spectral width of the frequency-domain spectra are almost the same,but the sampling speed of the ASOPS system is significantly faster than that of the traditional THz-TDS system,which conduces to the study of the transient characteristics of substances.

A novel phase-sensitive scanning near-field optical microscope

Phase is one of the most important parameters of electromagnetic waves. It is the phase distribution that determines the propagation, reflection, refraction, focusing, divergence, and coupling features of light, and further affects the intensity distribution. In recent years, the designs of surface plasmon polariton （SPP） devices have mostly been based on the phase modulation and manipulation. Here we demonstrate a phase sensitive multi-parameter heterodyne scanning near-field opti- cal microscope （SNOM） with an aperture probe in the visible range, with which the near field optical phase and amplitude distributions can be simultaneously obtained. A novel architecture combining a spatial optical path and a fiber optical path is employed for stability and flexibility. Two kinds of typical nano-photonic devices are tested with the system. With the phase-sensitive SNOM, the phase and amplitude distributions of any nano-optical field and localized field generated with any SPP nano-structures and irregular phase modulation surfaces can be investigated. The phase distribution and the interference pattern will help us to gain a better understanding of how light interacts with SPP structures and how SPP waves generate, localize, convert, and propagate on an SPP surface. This will be a significant guidance on SPP nano-structure design and optimization.

Optical polarization response at gold nanosheet edges probed by scanning near-field optical microscopy

Optical properties of metallic edge-like structures known as knife-edges are a topic of interest and possess potential applications in enhanced Raman scattering, optical trapping, etc. In this work, we investigate the near-field optical polar- ization response at the edge of a triangular gold nanosheet, which is synthesized by a wet chemical method. A homemade scanning near-field optical microscope （SNOM） in collection mode is adopted, which is able to accurately locate its probe at the edge during experiments. An uncoated straight fiber probe is used in the SNOM, because it s611 preserves the prop- erty of light polarization though it has the depolarization to some extent. By comparing near-field intensities at the edge and glass substrate, detected in different polarization directions of incident light, the edge-induced depolarization is found, which is supported by the finite differential time domain （FDTD） simulated results. The depolarized phenomenon in the near-field is similar to that in the far-field.

Influence of the probe-sample interaction on scanning near-field optical microscopic images in the far field

We have studied the influence of probe-sample interaction in a scanning near-field optical microscopy （SNOM） in the far field by using samples with a step structure. For a sample with a step height of - λ/4, the SNOM image contrast between the two sides of the step changes periodically at different scan heights. For a step height of-λ/2, the image contrast remains approximately the same. The probe-sample interaction determines the SNOM image contrast here. The influence of different refractive indices of the sample has been also analysed by using a simple theoretical model.

Endoscopic Optical Doppler Tomography Based on Two-Axis Scanning MEMS Mirror

We study the feasibility of endoscopic optical Doppler tomography with a micro-electro-mechanical system(MEMS) mirror based probe. The additional phase shifts introduced by the probe are tracked and formulated.The suppression method of the probe phase shifts is proposed and validated by fluid flow detection experiments.In vivo blood flow detection is also implemented on a hairless mouse. The velocities of the blood flow in two directions are obtained to be-8.1 mm/s and 6.6 mm/s, respectively.

Adaptive optics scanning laser ophthalmoscopy in fundus imaging, a review and update

Adaptive optics scanning laser ophthalmoscopy（AOSLO） has been a promising technique in funds imaging with growing popularity. This review firstly gives a brief history of adaptive optics（AO） and AO-SLO. Then it compares AO-SLO with conventional imaging methods（fundus fluorescein angiography, fundus autofluorescence, indocyanine green angiography and optical coherence tomography） and other AO techniques（adaptive optics flood-illumination ophthalmoscopy and adaptive optics optical coherence tomography）. Furthermore, an update of current research situation in AO-SLO is made based on different fundus structures as photoreceptors（cones and rods）, fundus vessels, retinal pigment epithelium layer, retinal nerve fiber layer, ganglion cell layer and lamina cribrosa. Finally, this review indicates possible research directions of AO-SLO in future.

Automated cone photoreceptor cell identication in confocal adaptive optics scanning laser ophthalmoscope images based on object detection

Cone photoreceptor cell identication is important for the early diagnosis of retinopathy.In this study,an object detection algorithm is used for cone cell identication in confocal adaptive optics scanning laser ophthalmoscope(AOSLO)images.An effectiveness evaluation of identication using the proposed method reveals precision,recall,and F_(1)-score of 95.8%,96.5%,and 96.1%,respectively,considering manual identication as the ground truth.Various object detection and identication results from images with different cone photoreceptor cell distributions further demonstrate the performance of the proposed method.Overall,the proposed method can accurately identify cone photoreceptor cells on confocal adaptive optics scanning laser ophthalmoscope images,being comparable to manual identication.

Automated retinal layer segmentation in optical coherence tomography images with intraretinal fluid

We propose a novel retinal layer segmentation method to accurately segment 10 retinal layers in optical coherence tomography(OCT)images with intraretinal fluid.The method used a fan filter to enhance the linear information pertaining to retinal boundaries in an OCT image by reducing the effect of vessel shadows and fluid regions.A random forest classifier was employed to predict the location of the boundaries.Two novel methods of boundary redirection(SR)and similarity correction(SC)were combined to carry out boundary tracking and thereby accurately locate retinal layer boundaries.Experiments were performed on healthy controls and subjects with diabetic macular edema(DME).The proposed method required an average of 415 s for healthy controls and of 482 s for subjects with DME and achieved high accuracy for both groups of subjects.The proposed method requires a shorter running time than previous methods and also provides high accuracy.Thus,the proposed method may be a better choice for small training datasets.

Laser synthesis and functionalization of nanostructures

This article summarizes work at the Laser Thermal Laboratory and discusses related studies on the laser synthesis and functionalization of semiconductor nanostructures and two-dimensional(2D)semiconductor materials.Research has been carried out on the laser-induced crystallization of thin films and nanostructures.The in situ transmission electron microscopy(TEM)monitoring of the crystallization of amorphous precursors in nanodomains is discussed herein.The directed assembly of silicon nanoparticles and the modulation of their optical properties by phase switching is presented.The vapor-liquid-solid mechanism has been adopted as a bottom-up approach in the synthesis of semiconducting nanowires(NWs).In contrast to furnace heating methods,laser irradiation offers high spatial selectivity and precise control of the heating mechanism in the time domain.These attributes enabled the investigation of NW nucleation and the early stage of nanostructure growth.Site-and shape-selective,on-demand direct integration of oriented NWs was accomplished.Growth of discrete silicon NWs with nanoscale location selectivity by employing near-field laser illumination is also reported herein.Tuning the properties of 2D transition metal dichalcogenides(TMDCs)by modulating the free carrier type,density,and composition can offer an exciting new pathway to various practical nanoscale electronics.In situ Raman probing of laser-induced processing of TMDC flakes was conducted in a TEM instrument.

Optical tomography based on nonredundant array scanning holography

A novel technique termed nonredundant array scanning holography based on the principle of optical heterodyne scanning holography and the tomographic technique of coded aperture imaging is proposed. The system designed in terms of this technique codes an object optically and decodes its coded image digitally. It can realize optical tomograms of three-dimensional objects. It also has potentially practical value due to its compact structure. The computer simulations present the principle of the technique. Some experiments at the proof-principle level are performed to test the principle.

Resolution enhancement of optical scanning holography with a spiral modulated point spread function

In optical scanning holography, one pupil produces a spherical wave and another produces a plane wave. They interfere with each other and result in a fringe pattern for scanning a three-dimensional object. The resolution of the hologram reconstruction is affected by the point spread function(PSF) of the optical system. In this paper, we modulate the PSF by a spiral phase plate, which significantly enhances the lateral and depth resolution. We explain the theory for such resolution enhancement and show simulation results to verify the efficacy of the approach.

Clustering study in Eu(DBM)_3Phen-doped polymer optical fibers by optical properties and near-field scanning microscopy

The clusters of Eu^3＋ ion in Eu（DBM）3Phen-doped polymethyl methacrylate （PMMA） have been studied by three means. The relative fluorescence intensity ratio of the ^5D0 → ^7F2 to ^5D0 → ^7F1 transitions with different concentrations of Eu^3＋ in Eu（DBM）3Phen-doped PMMA and metastable-state （^5D0） lifetime dependence on Eu^3＋ concentration axe analyzed. The analysis indicates that there axe no clustering effects in Eu^3＋ ions of Eu（DBM）3Phen-doped PMMA when the Eu^3＋ doping concentration is up to 1.0 wt.-%. At the same time, the clustering effect has not been observed by the scanning neax-field optical microscopy （SNOM） in Eu（DBM）3Phen-doped PMMA with 1.0 wt.-% of Eu^3＋ ions. The analysis reveals that a high concentration of Eu^3＋ can be incorporated into polymer optical fiber （POF） without clustering effect.

Contrast Reversal of Topography Artifacts in a Transmission SNOM

We demonstrate the contrast reversal behaviour of topography artifacts by changing the diameter of the collection diaphragm in a transmission scanning near-field optical microscopy （SNOM）. This originates from the change of the approach curves. Such contrast reversal phenomenon is used to distinguish the artifact signal from the true optical signal of the SNOM image. We also show that continuously changing the diaphragm to a proper diameter can greatly reduce topography artifacts.

Nanobridged rhombic antennas supporting both dipolar and high-order plasmonic modes with spatially superimposed hotspots in the mid-infrared

Mid-infrared antennas(MIRAs)support highly-efficient optical resonance in the infrared,enabling multiple applications,such as surface-enhanced infrared absorption(SEIRA)spectroscopy and ultrasensitive mid-infrared detection.However,most MIRAs such as dipolar-antenna structures support only narrow-band dipolar-mode resonances while high-order modes are usually too weak to be observed,severely limiting other useful applications that broadband resonances make possible.In this study,we report a multiscale nanobridged rhombic antenna(NBRA)that supports two dominant reson-ances in the MIR,including a charge-transfer plasmon(CTP)band and a bridged dipolar plasmon(BDP)band which looks like a quadruple resonance.These assignments are evidenced by scattering-type scanning near-field optical micro-scopy(s-SNOM)imaging and electromagnetic simulations.The high-order mode only occurs with nanometer-sized bridge(nanobridge)linked to the one end of the rhombic arm which mainly acts as the inductance and the resistance by the circuit analysis.Moreover,the main hotspots associated with the two resonant bands are spatially superimposed,en-abling boosting up the local field for both bands by multiscale coupling.With large field enhancements,multiband detec-tion with high sensitivity to a monolayer of molecules is achieved when using SEIRA.Our work provides a new strategy possible to activate high-order modes for designing multiband MIRAs with both nanobridges and nanogaps for such MIR applications as multiband SEIRAs,IR detectors,and beam-shaping of quantum cascade lasers in the future.

Analysis of Near-Field Diffraction Pattern of a Metallic Probe Tip with the Boundary Diffraction Wave Method

The boundary diffraction wave theory is introduced to analyse a near-field diffraction （NFD） pattern of a metallic probe tip of apertureless scanning near-field microscopy. This method is simple and can give a clear physical picture. The polarization effect of the incident light and the different shapes of the metallic probe tip are simulated. The results show that the NFD pattern of the metallic probe tip is directly related to those factors.

Polarization-insensitive plasmon nanofocusing with broadband interference modulation for optical nanoimaging

Delivering light to the nanoscale using a flexible and easily integrated fiber platform holds potential in various fields of quantum science and bioscience.However,rigorous optical alignment,sophisticated fabrication process,and low spatial resolution of the fiber-based nanoconcentrators limit the practical applications.Here,a broadband azimuthal plasmon interference nanofocusing technique on a fiber-coupled spiral tip is demonstrated for fiber-based near-field optical nanoimaging.The spiral plasmonic fiber tip fabricated through a robust and reproducible process can reverse the polarization and modulate the mode field of the surface plasmon polaritons in three-dimensionally azimuthal direction,resulting in polarization-insensitive,broad-bandwidth,and azimuthal interference nanofocusing.By integrating this with a basic scanning near-field optical microscopy,a high optical resolution of 31 nm and beyond is realized.The high performance and the easy incorporation with various existing measurement platforms offered by this fiber-based nanofocusing technique have great potential in near-field optics,tip-enhanced Raman spectroscopy,nonlinear spectroscopy,and quantum sensing.

Electronic properties of wurtzite GaAs： A correlated structural, optical, and theoretical analysis of the same polytypic GaAs nanowire

III-V compound semiconductor nanowires are generally characterized by the coexistence of zincblende and wurtzite structures. So far, this polytypism has impeded the determination of the electronic properties of the metastable wurtzite phase of GaAs, which thus remain highly controversial. In an effort to obtain new insights into this topic, we cross-correlate nanoscale spectral imaging by near-field scanning optical microscopy with a transmission electron microscopy analysis of the very same polytypic GaAs nanowire dispersed onto a Si wafer. Thus, spatially resolved photoluminescence spectra could be unambiguously assigned to nanowire segments whose structure is known with lattice-resolved accuracy. An emission energy of 1.528 eV was observed from extended zincblende segments, revealing that the dispersed nanowire was under uniaxial strain presumably due to interaction with its supporting substrate. These crucial information and the emission energy obtained for extended pure wurtzite segments were used to perform envelope function calculations of zincblende quantum disks in a wurtzite matrix as well as the inverse structure. In these calculations, we varied the fundamental bandgap, the electron mass, and the band offset between zincblende and wurtzite GaAs. From this multi-parameter comparison with the experimental data, we deduced that the bandgap between the F8 conduction and A valence band ranges from 1.532 to 1.539 eV in strain-free wurtzite GaAs, and estimated values of 1.507 to 1.514 eV for the F7-A bandgap.

Automated superpixels-based identification and mosaicking of cone photoreceptor cells for adaptive optics scanning laser ophthalmoscope

An automated superpixels identification/mosaicking method is presented for the analysis of cone photoreceptor cells with the use of adaptive optics scanning laser ophthalmoscope(AO-SLO) images. This is an image oversegmentation method used for the identification and mosaicking of cone photoreceptor cells in AO-SLO images.It includes image denoising, estimation of the cone photoreceptor cell number, superpixels segmentation, merging of superpixels, and final identification and mosaicking processing steps. The effectiveness of the presented method was confirmed based on its comparison with a manual method in terms of precision, recall, and F1-score of 77.3%, 95.2%, and 85.3%, respectively.

Bimorph deformable mirror based adaptive optics scanning laser ophthalmoscope for retina imaging in vivo

A bimorph deformable mirror （DM） with a large stroke of more than 30 μm using 35 actuators is presented and characterized for an adaptive optics （AO） confocal scanning laser ophthalmoscope application. Facilitated with a Shack-Hartmann wavefront sensor, the bimorph DM-based AO operates closed-loop AO corrections for hu- man eyes and reduces wavefront aberrations in most eyes to below 0.1 μm rms. Results from living eyes, including one exhibiting ~5D of myopia and ~2D of astigmatism along with notable high-order aberrations, reveal a prac- tical efficient aberration correction and demonstrate a great benefit for retina imaging, including improving resolution, increasing brightness, and enhancing the contrast of images.

Development and prospect of near-field optical measurements and characterizations

Scanning near-field optical microscopy （SNOM） is an ideal experimental measuring system in nano-optical measurements and characterizations. Besides microscopy with resolution beyond the diffraction limit, spectroscope with nanometer resolution and other instruments with novel performances have been indispensable for researches in nano-optics and nanophotonics. This paper reviews the developing history of near-field optical （NFO） measuring method and foresees its prospects in future. The development of NFO measurements has gone through four stages, including optical imaging with super resolution, near-field spectroscopy, measurements ofnanooptical parameters, and detections of near-field interac- tions. For every stage, research objectives, technological properties and application fields are discussed.