On-Chip Fabrication-Tolerant Exceptional Points Based on Dual-Scatterer Engineering.

The intriguing and anomalous optical characteristics of exceptional points (EPs) in optical resonators have attracted significant attention. While EP-related phenomena have been observed by perturbing resonators with off-chip components, implementing EPs fully on-chip remains challenging due to their extreme susceptibility to fabrication errors. In this Letter, we propose a succinct and compact approach to reach EP in an on-chip integrated silicon microring resonator by manipulating the evolution of backscatterings with two nanocylinders of disparate diameters. The theoretical analysis unveils that the fabrication constraints could be significantly relieved by increasing the difference in diameters of the nanocylinders. The evolution from non-EP to EP is traced experimentally through the step-by-step tuning of the angular and radial positions of nanocylinders. The proposed method opens a pathway toward the on-chip high-density integration of non-Hermitian devices.

Widefield functional speckle-correlation optical scattering mesoscopy toward hemodynamic imaging.

Speckle-correlation optical scattering imaging (SCOSI) has shown the potential for non-invasive biomedical diagnostic applications, which directly utilizes the scattering patterns to reconstruct the deep and non-line-of-sight objects. However, the course of the translation of this technique to preclinical biomedical imaging applications has been postponed by the following two facts: 1) the field of view of SCOSI was significantly limited by the optical memory effect, and 2) the molecular-tagged functional imaging of the biological tissues remains largely unexplored. In this work, a proof-of-concept design of the first-generation widefield functional SCOSI (WF-SCOSI) system was presented for simultaneously achieving mesoscopic mapping of fluid morphology and flow rate, which was realized by implementing the concepts of scanning synthesis and fluorescence scattering flowmetry. The ex vivo imaging results of the fluorescence-labeled large-scale blood vessel network phantom underneath the strong scatters demonstrated the effectiveness of WF-SCOSI toward non-invasive hemodynamic imaging applications.

Thermal-tagging photoacoustic remote sensing flowmetry.

Ultrasound coupling is one of the critical challenges for traditional photoacoustic (or optoacoustic) microscopy (PAM) techniques transferred to the clinical examination of chronic wounds and open tissues. A promising alternative potential solution for breaking the limitation of ultrasound coupling in PAM is photoacoustic remote sensing (PARS), which implements all-optical non-interferometric photoacoustic measurements. Functional imaging of PARS microscopy was demonstrated from the aspects of histopathology and oxygen metabolism, while its performance in hemodynamic quantification remains unexplored. In this Letter, we present an all-optical thermal-tagging flowmetry approach for PARS microscopy and demonstrate it with comprehensive mathematical modeling and ex vivo and in vivo experimental validations. Experimental results demonstrated that the detectable range of the blood flow rate was from 0 to 12 mm/s with a high accuracy (measurement error:±1.2%) at 10-kHz laser pulse repetition rate. The proposed all-optical thermal-tagging flowmetry offers an effective alternative approach for PARS microscopy realizing non-contact dye-free hemodynamic imaging.

Optical noise characteristics of injection-locked epitaxial quantum dot lasers on silicon.

This work theoretically investigates the relative intensity noise (RIN) and spectral linewidth characteristics of epitaxial quantum dot (QD) lasers on silicon subject to optical injection. The results show that the RIN of QD lasers can be reduced by optical injection, hence a reduction of 10 dB is achieved which leads to a RIN as low as -167.5 dB/Hz in the stable injection-locked area. Furthermore, the spectral linewidth of the QD laser can be greatly improved through the optical injection locked scheme. It is reduced from 556.5 kHz to 9 kHz with injection ratio of -60 dB and can be further reduced down to 1.5 Hz with injection ratio of 0 dB. This work provides an effective method for designing low intensity noise and ultra-narrow linewidth QD laser sources for photonics integrated circuits on silicon.

Limited view correction in low-optical-NA photoacoustic microscopy.

Photoacoustic microscope (PAM) with a low-optical NA suffers from a limited view along the optical axis, due to the coherent cancellation of acoustic pressure waves after being excited with a smoothly focused beam. Using larger-NA (NA > 0.3) objectives can readily overcome the limited-view problem, while the consequences are the shallow working distance and time-consuming depth scanning for large-volume imaging. Instead, we report an off-axis oblique detection strategy that is compatible with a low-optical-NA PAM for turning up the optical-axis structures. Comprehensive photoacoustic modeling and ex vivo phantom and in vivo mouse brain imaging experiments are conducted to validate the efficacy of correcting the limited view. Proof-of-concept experiment results show that the visibility of optical-axis structures can be greatly enhanced by making the detection angle off the optical axis larger than 45°, strongly recommending that off-axis oblique detection is a simple and cost-effective alternative method to solve the limited-view problems in low-optical-NA PAMs.

Subwavelength structure enabled ultra-long waveguide grating antenna.

Because of the high index contrast, current silicon photonics based optical phased arrays cannot achieve small beam divergence and large field-of-view simultaneously without increasing fabrication complexity. To resolve the dilemma, we propose an ultra-long waveguide grating antenna formed by placing subwavelength segments within the evanescent field of a conventional strip waveguide. Bound state in the continuum effect is leveraged to suppress the sidewall emission. As a proof of concept, we theoretically demonstrated a millimeter-long through-etched waveguide grating antenna with a divergence angle of 0.081° and a feature size compatible with current silicon photonics foundries.

Fiber-optic multipoint laser-ultrasonic excitation transducer using coreless fibers.

Photoacoustic ultrasound excitation has great potential in structural nondestructive testing and applications for medical treatments as a promising alternative to electrical ultrasound. This study proposes and demonstrates a multipoint optical fiber laser-ultrasonic transducer system, wherein the fiber-optic ultrasonic transducer is fabricated by a coreless fiber segment's fusion with single-mode fibers at each end. Simulation and experiment results show that the transducer coupling ratio is dependent on the coreless fiber's length. The structure of such an ultrasonic transducer is easily manufactured. Thus, the structures of these optical fiber ultrasonic transducers with different coupling ratios are connected in the order of small to large coupling ratios. In this manner, multipoint ultrasonic excitation with equal intensities at each excitation point can be obtained using this simple and low-cost method. Using laser guidance through the optical fiber to generate ultrasound can efficiently solve some shortcomings of traditional ultrasonic transducers, such as large volume, small bandwidth, and electromagnetic interference. Moreover, this type of fiber-optic ultrasound transducer has higher mechanical strength than other fiber-optic ultrasound transducers and is expected to be useful in structural health-monitoring of buildings.

Frequency spacing switchable multiwavelength Brillouin erbium fiber laser utilizing cascaded Brillouin gain fibers.

A new hybrid Brillouin erbium fiber laser scheme that employs cascaded multiple Brillouin gain fibers in a ring cavity to realize multiwavelength laser output with switchable frequency spacing is proposed and experimentally investigated. The multiple frequency downshifting processes introduced by multiple stimulated Brillouin scattering (SBS) effects in one round-trip of the cavity make it possible to realize multiwavelength output with frequency spacing that is an integer multiple of the SBS frequency shifting. With two cascaded SBS fibers, the frequency spacing can be switched between single and double SBS frequency shifting by properly adjusting the Brillouin pump power. Multiwavelength outputs with triple or quadruple SBS frequency spacing are also demonstrated by employing three or four SBS gain fibers, respectively.

Optimization of one-third harmonic generation in the presence of nonlinear phase modulations and power attenuation.

We investigate the one-third harmonic generation (OTHG) in optical microfibers with power attenuation considered by analytically analyzing and numerically solving the coupled mode equations (CMEs). Both the strength and effective length of signal power growing in nonlinear media, which are extremely sensitive to the relative phase between the interaction waves, contribute to the final conversion efficiency. The relative phase and its evolution along the propagating direction play crucial roles in highly efficient OTHG. In order to obtain high conversion efficiency, the general expressions of optical threshold conditions are derived and discussed for choosing proper initial parameters. Numerically simulations are performed with both partial and absolute phase matching, which are corresponding to the microfibers with uniform and non-uniform diameters, respectively. Optimizations of relative phase and phase compensation are suggested by the simulations and provide significant enhancement of conversion efficiencies.

Study on the crucial conditions for efficient third harmonic generation using a metal-hybrid-metal plasmonic slot waveguide.

We provide a comprehensive study on the efficient third harmonic generation (THG) in a lossy metal-hybrid-metal asymmetric plasmonic slot waveguide (MHM) to develop a method for efficient THG by focusing on the modal phase-matching condition (PMC), the third-order nonlinear susceptibility of the nonlinear interactive material, and the pump-harmonic modal overlap in conjunction with reasonable linear propagation loss. In addition to the PMC and the nonlinear material, the stimulated THG process can be greatly enhanced by the large pump-harmonic modal overlap. With 1 W pump power, simulation results present that THG conversion efficiency up to 2.79 × 10(-4) within 4.5 ����m MHM can be achieved.

Third harmonic generation from mid-IR to near-IR regions in a phase-matched silicon-silicon-nanocrystal hybrid plasmonic waveguide.

The conversion efficiency of third harmonic generation (THG) from mid-IR (3600 nm) to near-IR (1200 nm) regions in a silicon-silicon-nanocrystal hybrid plasmonic waveguide (SSHPW) was calculated. The required modal phase-matching condition (PMC) between the 0-th mode at fundamental wave (FW) and the 2-nd mode at third harmonic (TH) is achieved by carefully designing the waveguide geometry. Benefiting from the hybridized surface plasmon polariton (SPP) nature of the two guided modes, the SSHPW is capable of achieving both high THG nonlinear coefficient |I6| and reasonable linear propagation loss, thereby resulting in large figure-of-merits (FOMs) for both FW and TH. According to our simulation, THG conversion efficiency up to 0.823% is achieved at 62.9 ����m SSHPW with pump power of 1 W.

Efficient phase-matched third harmonic generation in an asymmetric plasmonic slot waveguide.

An asymmetric plasmonic slot waveguide (APSW) for efficient phase-matched third harmonic generation (THG) is proposed and demonstrated theoretically. Nonlinear organic material DDMEBT polymer is integrated into the bottom of the metallic slot, while silicon is used to fill the top of the slot. We introduce the rigorous coupled-mode equations of THG in the lossy APSW and apply them to optimize the waveguide geometry. Taking advantage of the surface plasmon polaritons (SPPs), the electric fields can be tightly confined in the metallic slot region and the nonlinear effect is greatly enhanced accordingly. Then, we investigate the relationships between THG efficiency and parameters such as slot width and height, phase matching condition (PMC), modal overlap related nonlinear parameter, figure-of-merit, pump power and detuning. With the proposed asymmetric waveguide, we demonstrate a high THG conversion efficiency of 4.88 × 10(-6) with a pump power of 1 W and a detuning constant of -36 m(-1) at a waveguide length of 10.65 ����m.

Efficient one-third harmonic generation in highly Germania-doped fibers enhanced by pump attenuation.

We provide a comprehensive study on one-third harmonic generation (OTHG) in highly Germania-doped fiber (HGDF) by analyzing the phase matching conditions for the step index-profile and optimizing the design parameters. For stimulated OTHG in HGDF, the process can be enhanced by fiber attenuation at the pump wavelength which dynamically compensates the accumulated phase-mismatch along the fiber. With 500 W pump and 35 W seed power, simulation results show that a 31% conversion efficiency, which is 4 times higher than the lossless OTHG process, can be achieved in 34 m of HGDF with 90 mol. % GeO2 doping in the core.

Information distribution on regions of speckle patterns for imaging of multimode fiber.

Multimode fibers (MMF) have been extensively investigated for transmitting images. The transmitting images are distorted into speckle patterns by MMFs, which can be reconstructed by neural networks. We studied the information distribution of MMF speckle patterns for image reconstruction. The speckle patterns, segmented by three methods of segmentation, as Centering (1), Quartering (2) and Surrounding (3), are reconstructed into input images by Complex Artificial Neural Network (CANN). Experimental results show that only about one third of full speckle patterns is enough to reconstruct the original images. The quality of reconstructed image is related to the cropping method with different frequency components in speckle patterns, under the same cropped size, Centering segmentation has 4% performance improvement compared to Surrounding segmentation. Optimized segmentation will improve the quality of reconstructed images.

A robust and dither-free technique for controlling driver signal amplitude for stable and arbitrary optical phase modulation.

We propose a robust and dither-free technique using a delay line interferometer, a balanced detector and simple signal processing to adjust the amplitude of the driver signal of an optical phase modulator automatically for stabilizing the modulated phase of an optical carrier at any arbitrary value. The technique is analytically shown to be robust against practical device imperfections. A stable 45 degrees phase shift with deviation less than ± 0.8 degrees is experimentally demonstrated.

Condition for Gaussian Schell-model beam to maintain the state of polarization on the propagation in free space.

In the free space optical communication system with circle polarization shift keying (CPolSK) modulation, the changes of polarization state of light beam have significant influence on the system performance. Keeping the state of polarization (SOP) unchanged on propagation can reduce the bit error rate. Based on the unified theory of coherence and polarization, we derive the sufficient condition for Gaussian Schell-model (GSM) beam to keep the SOP unchanged. We found that when the three spectral correlation widths (delta(xx), delta(yy) and delta(xy)) equal to each other and sigma(x) = sigma(y), the GSM beam maintains the SOP on propagation. This conclusion can be helpful for the design of the transmitter in the CPolSK system.

Multiwavelength Erbium-doped fiber laser employing nonlinear polarization rotation in a symmetric nonlinear optical loop mirror.

A new multiwavelength Erbium-doped fiber laser is proposed and demonstrated. The intensity-dependent loss induced by nonlinear polarization rotation in a power-symmetric nonlinear optical loop mirror (NOLM) suppresses the mode competition of an Erbium-doped fiber and ensures stable multiwavelength operation at room temperature. The polarization state and its evolution conditions for stable multiwavelength operation in the ring laser cavity are discussed. The number and spectra region of output wavelength can be controlled by adjusting the work states of NOLM.

MAC mode atomic force microscopy studies of living samples, ranging from cells to fresh tissue.

Magnetic AC mode (MAC mode) atomic force microscopy (AFM), a novel type of tapping mode AFM in which the cantilever is driven directly by a magnetic field, is a powerful tool for imaging with high spatial resolution and better signal-to-noise in liquid environment. It may largely extend the application of AFM to living samples, especially those are sensitive to cantilever forces, even to multilayer tissue samples. However, there are few reports on the imaging of living cells by MAC mode AFM previously. In our present study, we explore the optimal imaging conditions of MAC mode AFM on living astrocytes and fresh arterial intima surface. We also used nude tips for PicoTREC panel (i.e., Aux in BNC, a new data collecting channel) to image living samples and discussed its difference with phase imaging. We show that living biological samples can be imaged by MAC mode AFM at details of comparable resolution as those by high resolution scanning electron microscopy. Furthermore, the combination of height, amplitude, phase and TREC panel signals provide abundant informations for the characteristics of living samples, such as topography, profile, stiffness and adhesion.

Cytoskeletal response of microvessel endothelial cells to an applied stress force at the submicrometer scale studied by atomic force microscopy.

Cytoskeleton fibers form an intricate three-dimensional network to provide structure and function to microvessel endothelial cells. During accommodation to blood flowing, stress fiber bundles become more prominent and align with the direction of blood flow. This network either mechanically resists the applied shear stress (lateral force) or, if deformed, is dynamically remodeled back to a preferred architecture. However, the detailed response of these stress fiber bundles to applied lateral force at submicrometer scales are as yet poorly understood. In our in vitro study, the tip, topography probe in lateral force microscopy of atomic force microscopy, acted as a tool for exerting quantitative vertical and lateral force on the filaments of the cytoskeleton. Moreover, the authors developed a formula to calculate the value of lateral force exerted on every point of the filaments. The results show that cytoskeleton fibers of healthy tight junctions in rat cerebral microvessel endothelial cells formed a cross-type network, and were reinforced and elongated in the direction of scanning under lateral force of 15-42 nN. Under peroxidation (H(2)O(2) of 300 micromol/L), the cytoskeleton remodeled at intercellular junctions, and changed over the meshwork structures into a dense bundle, that redistributed the stress. Once mechanical forces were exerted on an area, the cells shrank and lost morphologic tight junctions. It would be useful in our understanding of certain pathological processes, such as cerebral ischemia/reperfusion injury, which maybe caused by biomechanical forces and which are overlooked in current disease models.

[ICCD-based fast fluorescence micro-imaging technique and preliminary application in living cells].

A fast fluorescence micro-imaging system using mainly intensified charge couple device (ICCD), argon-ion laser, and xenon lamp was set up, and its preliminary application in living cells was presented. Real-time observation and imaging of fast concentration and distribution changing of intracellular Ca2+ labeled by Fluo-3, a fluorescent Ca2+ indicator, in the proliferation process of rat cerebral micro-vessels endothelial cells (rCMECs) were carried out, and curves of artificial intensity versus imaging sequence of four typical points were obtained. It is shown that the ICCD-based fast fluorescence micro-imaging system is a powerful tool for recording the real-time fast processes in living cells.