Self-confocal NIR-II fluorescence microscopy for multifunctional in vivo imaging

Fluorescence imaging in the second near-infrared window(NIR-II,900–1880 nm)with less scattering background in biological tissues has been combined with the confocal microscopic system for achieving deep in vivo imaging with high spatial resolution.However,the traditional NIR-IIfluorescence confocal microscope with separate excitation focus and detection pinhole makes it possess low confocal e±ciency,as well as di±cultly to adjust.Two types of upgraded NIR-IIfluorescence confocal microscopes,sharing the same pinhole by excitation and emission focus,leading to higher confocal e±ciency,are built in this work.One type is-ber-pinhole-based confocal microscope applicable to CW laser excitation.It is constructed forfluorescence intensity imaging with large depth,high stabilization and low cost,which could replace multiphotonfluorescence microscopy in some applications(e.g.,cerebrovascular and hepatocellular imaging).The other type is air-pinhole-based confocal microscope applicable to femtosecond(fs)laser excitation.It can be employed not only for NIR-IIfluorescence intensity imaging,but also for multi-channelfluorescence lifetime imaging to recognize different structures with similarfluorescence spectrum.Moreover,it can be facilely combined with multiphotonfluorescence microscopy.A single fs pulsed laser is utilized to achieve up-conversion(visible multiphotonfluorescence)and down-conversion(NIR-II one-photonfluorescence)excitation simultaneously,extending imaging spectral channels,and thus facilitates multi-structure and multi-functional observation.

Additive and interfacial control for efficient perovskite light-emitting diodes with reduced trap densities

Metal halide perovskite semiconductors show excellent optoelectronic properties including tunable bandgaps[1,2],narrow emission bandwidths[3]and high luminescence quantum efficiencies[4],making them an ideal candidate for light-emitting diode(LED)applications.Perovskite LEDs(PeLEDs)have attracted considerable attention since the initial report of room-temperature electroluminescence(EL)from halide perovskites in 2014[5].

Four-channel CWDM transmitter chip based on thin-film lithium niobate platform

Multi-lane integrated transmitter chips are key components in future compact optical modules to realize high-speed optical interconnects.Thin-film lithium niobate(TFLN)photonics have emerged as a promising platform for achieving high-performance chip-scale optical systems.Combining a coarse wavelength-division multiplexing(CWDM)devices using fabrication-tolerant angled multimode interferometer structure and high-performance electro-optical modulators,we demonstrate monolithic on-chip four-channel CWDM transmitter on the TFLN platform for the first time.The four-channel CWDM transmitter enables high-speed transmissions of 100 Gb/s data rate per wavelength channel(i.e.,an aggregated date rate of 400 Gb/s).

Quantification methodologies on organization and morphology features of fiber-like structures:A review

Among all the structural formations,fiber-like structure is one of the most common modalities in organisms that undertake essential functions.Alterations in spatial organization of fibrous structures can refiect information of physiological and pathological activities,which is of significance in both researches and clinical applications.Hence,the quantification of subtle changes in fiber-like structures is potentiallymeaningful in studying structure-function relationships,disease progression,carcinoma staging and engineered tissue remodeling.In this study,we examined a wide range of methodologies that quantify organizational and morphological features of fibrous structures,including orientation,alignment,waviness and thickness.Each method was demonstrated with specific applications.Finally,perspectives of future quantification analysis techniques were explored.

Three-dimensional remodeling of collagen fibers within cervical tissues in pregnancy

The cervix is a collagen-rich connective tissue that must remain closed during pregnancy while undergoing progressive remodeling in preparation for delivery,which begins before the onset of the preterm labor process.Therefore,it is important to resolve the changes of collagen flbers during cervical remodeling for the prevention of preterm labor.Herein,we assessed the spatial organization of collagen flbers in a three-dimensional(3D)context within cervical tissues of mice on day 3,9,12,15 and 18 of gestation.We found that the 3D directional variance,a novel metric of alignment,was higher on day 9 than that on day 3 and then gradually decreased from day 9 to day 18.Compared with two-dimensional(2D)approach,a higher sensitivity was achieved from 3D analysis,highlighting the importance of truly 3D quantification.Moreover,the depthdependent variation of 3D directional variance was investigated.By combining multiple 3D directional variance-derived metrics,a high level of classification accuracy was acquired in distinguishing different periods of pregnancy.These results demonstrate that 3D directional variance is sensitive to remodeling of collagen fibers within cervical tissues,shedding new light on highly-sensitive,early detection of preterm birth(PTB).

Waveguide-integrated optical modulators with two-dimensional materials

Waveguide-integrated optical modulators are indispensable for on-chip optical interconnects and optical computing.To cope with the ever-increasing amount of data being generated and consumed,ultrafast waveguide-integrated optical modulators with low energy consumption are highly demanded.In recent years,two-dimensional(2D)materials have attracted a lot of attention and have provided tremendous opportunities for the development of high-performance waveguide-integrated optical modulators because of their extraordinary optoelectronic properties and versatile compatibility.This paper reviews the state-of-the-art waveguide-integrated optical modulators with 2D materials,providing researchers with the developing trends in the field and allowing them to identify existing challenges and promising potential solutions.First,the concept and fundamental mechanisms of optical modulation with 2D materials are summarized.Second,a review of waveguide-integrated optical modulators employing electro-optic,all-optic,and thermo-optic effects is provided.Finally,the challenges and perspectives of waveguide-integrated modulators with 2D materials are discussed.

Lipid droplets imaging with three-photon microscopy

Lipid droplets(LDs)participate in many physiological processes,the abnormality of which will cause chronic diseases and pathologies such as diabetes and obesity.It is crucial to monitor the distribution of LDs at high spatial resolution and large depth.Herein,we carried three-photon imaging of LDs in fat liver.Owing to the large three-photon absorption cross-section of the luminogen named NAP-CF_(3)(1:67×10^(-79) cm^(6) s^(2)),three-photon fluorescence fat liver imaging reached the largest depth of 80μm.Fat liver diagnosis was successfully carried out with excellent performance,providing great potential for LDs-associated pathologies research.

Swept source intraoperative OCT angiography

To accurately guide surgical instruments during ophthalmic procedures,some necessary intraoperative depth perception is required,which standard surgical microscopes supply limit-edly.Intraoperative optical coherence tomography(iOCT),combining optical coherence to-mography(OCT)technology and surgical microscope,enables noninvasive,real-time and high-resolution cross-sectional imaging.Currently,though iOCT enables structural imaging,little research has been done on intraoperative angiography.In this work,we presented a swept-source intraoperative OCT angiography(SS-iOCTA)system based on a standard surgical microscope,which provides both structural and angiographic images.The feasibility of the proposed SS-iOCTA was confirmed through deep anterior lamellar keratoplasty(DALK)of ex vivo porcine eyes and blood perfusion imaging of in vivo rat cortex.High-resolution intraoperative feedback,including sub-surface structure and angiogram of biological tissue,can be visualized simulta-neously with the SS-iOCTA system,which expand the surgeon's capabilities and could be widely used in clinical surgery.

Toward calibration-free Mach-Zehnder switches for next-generation silicon photonics

Silicon photonic Mach–Zehnder switches(MZSs)have been extensively investigated as a promising candidate for optical systems.However,conventional 2×2 MZSs are usually prone to the size variations of the arm waveguides due to imperfect fabrication,resulting in considerable random phase imbalance between the two arms,thereby imposing significant challenges for further developing next-generation N×N MZSs.Here we propose a novel design toward calibration-free 2×2 and N×N MZSs,employing optimally widened arm waveguides,enabled by novel compact tapered Euler S-bends with incorporated mode filters.With standard 180 nm CMOS foundry processes,more than thirty 2×2 MZSs and one 4×4 Benes MZS with the new design are fabricated and characterized.Compared with their conventional counterparts with 0.45-μm-wide arm waveguides,the present 2×2 MZSs exhibit significant reduction in the random phase imbalance.The measured extinction ratios of the present 2×2 and 4×4 MZSs operating in the all-cross state are 27-49 dB and∼20dB across the wavelength range of∼60nm,respectively,even without any calibrations.This work paves the way toward calibration-free large-scale N×N MZSs for next-generation silicon photonics.

Hot-band absorption of indocyanine green for advanced anti-stokes fluorescence bioimaging

Bright anti-Stokes fluorescence(ASF)in the first near-infrared spectral region(NIR-I,800 nm–900 nm)under the excitation of a 915 nm continuous wave(CW)laser,is observed in Indocyanine Green(ICG),a dye approved by the Food and Drug Administration for clinical use.The dependence of fluorescence intensity on excitation light power and temperature,together with fluorescence lifetime measurement,establish this ASF to be originated from absorption from a thermally excited vibrational level(hot-band absorption),as shown in our experiments,which is stronger than the upconversion fluorescence from widely-used rare-earth ion doped nanoparticles.To test the utility of this ASF NIR-I probe for advanced bioimaging,we successively apply it for biothermal sensing,cerebral blood vessel tomography and blood stream velocimetry.Moreover,in combination with L1057 nanoparticles,which absorb the ASF of ICG and emit beyond 1100 nm,these two probes generate multi-mode images in two fluorescent channels under the excitation of a single 915 nm CW laser.One channel is used to monitor two overlapping organs,urinary system&blood vessel of a live mouse,while the other shows urinary system only.Using in intraoperative real-time monitoring,such multi-mode imaging method can be beneficial for visual guiding in anatomy of the urinary system to avoid any accidental injury to the surrounding blood vessels during surgery.

Ultrahigh-resolution on-chip spectrometer with silicon photonic resonators

A compact spectrometer on silicon is proposed and demonstrated with an ultrahigh resolution.It consists of a thermally-tunable ultra-high-Q resonator aiming at ultrahigh resolution and an array of wideband resonators for achieving a broadened working window.The present on-chip spectrometer has a footprint as compact as 0.35 mm^(2),and is realized with standard multi-project-wafer foundry processes.The measurement results show that the on-chip spectrometer has an ultra-high resolution Δλ of 5 pm and a wide working window of 10 nm.The dynamic range defined as the ratio of the working window and the wavelength resolution is as large as 1940,which is the largest for on-chip dispersive spectro-meters to the best of our knowledge.The present high-performance on-chip spectrometer has great potential for high-resolution spectrum measurement in the applications of gas sensing,food monitoring,health analysis,etc.

Advancing lasers in silicon photonics

he integration of lasers has long been a significant challenge,impeding advancements in silicon photonic integrated circuits(PICs).Although the majority of applications necessitate lasers,most silicon PICs still need external off-chip lasers,as on-chip lasers have yet to match the performance of discrete counterparts.Additionally,stable laser operation requires adequate isolation from downstream reflections,which can negatively influence the performance;thus,isolators are typically introduced between a laser and a silicon PIC.

Anisotropic emission of orientation-controlled mixed-dimensional perovskites for light-emitting devices

Perovskite light-emitting diodes(PeLEDs)are attracting increasing attention owing to their impressive efficiencies and high luminance across the full visible light range.Further improvement of the external quantum efficiency(EQE)of planar PeLEDs is limited by the light out-coupling efficiency.Introducing perovskite emitters with directional emission in PeLEDs is an effective way to improve light extraction.Here,we report that it is possible to achieve directional emission in mixed-dimensional perovskites by controlling the orientation of the emissive center in the film.Multiple characterization methods suggest that our mixed-dimensional perovskite film shows highly orientated transition dipole moments(TDMs)with the horizontal ratio of over 88%,substantially higher than that of the isotropic emitters.The horizontally dominated TDMs lead to PeLEDs with exceptional high light out-coupling efficiency of over 32%,enabling a high EQE of 18.2%.

On-Chip Sub-Picometer Continuous Wavelength Fiber-Bragg-Grating Interrogator

Miniaturized fiber-Bragg-grating(FBG)interrogators are of interest for applications in the areas where weight and size controlling is important,e.g.,airplanes and aerospace or in-situ monitoring.An ultra-compact high-precision on-chip interrogator is proposed based on a tailored arrayed waveguide grating(AWG)on a silicon-on-insulator(SOI)platform.The on-chip interrogator enables continuous wavelength interrogation from 1544 nm to 1568 nm with the wavelength accuracy of less than 1 pm[the root-mean-square error(RMSE)is 0.73 pm]over the whole wavelength range.The chip loss is less than 5 dB.The 1×16 AWG is optimized to achieve a large bandwidth and a low noise level at each channel,and the FBG reflection peaks can be detected by multiple output channels of the AWG.The fabricated AWG is utilized to interrogate FBG sensors through the center of gravity(CoG)algorithm.The validation of an on-chip FBG interrogator that works with sub-picometer wavelength accuracy in a broad wavelength range shows large potential for applications in miniaturized fiber optic sensing systems.

Ultracompact silicon on-chip polarization controller

On-chip polarization controllers are extremely important for various optical systems.In this paper,a compact and robust silicon-based on-chip polarization controller is proposed and demonstrated by integrating a special polarization converter and phase shifters.The special polarization converter consists of a 1×1 Mach–Zehnder interferometer with two polarization-dependent mode converters at the input/output ends.When light with an arbitrary state of polarization(SOP)is launched into the chip,the TE_(0)and TM_(0)modes are simultaneously excited.The polarization extinction ratio(PER)and the phase difference for the TE_(0)∕TM_(0)modes are tuned by controlling the first phase shifter,the polarization converter,and the second phase shifter.As a result,one can reconstruct the light SOP at the output port.The fabricated polarization controller,as compact as~150μm×700μm,exhibits an excess loss of less than 1 dB and a record PER range of>54 dB for arbitrary input light beams in the wavelength range of 1530–1620 nm.

High-performance silicon−graphene hybrid plasmonic waveguide photodetectors beyond 1.55μm

Graphene has attracted much attention for the realization of high-speed photodetection for silicon photonics over a wide wavelength range.However,the reported fast graphene photodetectors mainly operate in the 1.55μm wavelength band.In this work,we propose and realize high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon−graphene hybrid plasmonic waveguide,which enables efficient light absorption in graphene at 1.55μm and beyond.When operating at 2μm,the present photodetector has a responsivity of ~70 mA/W and a setup-limited 3 dB bandwidth of >20 GHz.When operating at 1.55μm,the present photodetector also works very well with a broad 3 dB bandwidth of >40 GHz(setup-limited)and a high responsivity of ~0.4 A/W even with a low bias voltage of−0.3 V.This work paves the way for achieving highresponsivity and high-speed silicon-graphene waveguide photodetection in the near/mid-infrared ranges,which has applications in optical communications,nonlinear photonics,and on-chip sensing.

Efficiency breakthrough for all-perovskite tandem solar cells

The power conversion efficiency(PCE)of single-junction perovskite solar cells(PSCs)has rapidly boosted to 25.2%[1],approaching the Shockley-Queisser limit.A potential strategy to further elevate the PCE of single-junction PSCs is to fabricate all-perovskite tandem solar cells[2,3],which is composed of a wide-bandgap(1.7–1.9 eV)top sub-cell and a low-bandgap(0.9–1.2 eV)bottom sub-cell.

First demonstration of an on-chip quadplexer for passive optical network systems

An on-chip quadplexer is proposed and demonstrated with four wavelength-channels of 1270, 1310, 1490, and 1577 nm. The present quadplexer consists of four cascaded filters based on multimode waveguide grating(MWG), which are composed of a two-mode(de)multiplexer and an MWG. For the fabricated quadplexer on silicon, all four wavelength channels have flat-top responses with low excess losses of <0.5 dB as well as the desired bandwidths, which are about 16, 38, 19, and 6 nm, respectively. The cross-talk for both upstream channels and downstream channels is less than -24 dB. Moreover, the data transmission of 10 Gb∕s of the present silicon quadplexer is also successfully demonstrated.

Four-channel CWDM device on a thin-film lithium niobate platform using an angled multimode interferometer structure

A compact and high-performance coarse wavelength-division multiplexing(CWDM) device is introduced with a footprint of 2.1 mm × 0.02 mm using an angled multimode interferometer structure based on a thin-film lithium niobate(TFLN) platform.The demonstrated device built on a 400 nm thick x-cut TFLN shows ultra-low insertion losses of <0.72 dB.Measured 3 dB bandwidths are 12.1 nm for all channels,and cross talks from adjacent channels are better than 18 dB.Its peak wavelength positions comply with the CWDM standard with a channel spacing of 20 nm.The filter bandwidth of the proposed CWDM device can be tuned by adjusting the structural parameters.This demonstrated CWDM device will promote future realization of multi-channel and multi-wavelength transmitter chips on TFLN.

Theoretical and practical guide for an axial superresolved focus via Gouy phase steering

Achieving an axial superresolved focus with a single lens by simply inserting a modulation mask in the pupil plane is preferred due to its compact configuration and general applicability. However, lack of a universal theoretical model to manifest the superresolved focusing mechanism vastly complicates the mask design and hinders optimal resolution. Here we establish an interference model and find out that the axial resolution closely relates to the Gouy phase gradient(GPG) at the focal point. Using a GPG tuning-based optimization approach, the axial resolution of a ring-mask-modulated beam is readily improved to attain superresolved focal depth for multiple types of pupil function and polarization. In experiment, a focus with an axial resolution of 27% improved from the diffraction limit and 11% finer than the previously reported record is demonstrated for the radially polarized beam. In simulations, a spherical focus with 3D isotropic resolution and a superoscillation-like axial modulation behavior toward extremely high axial resolution is also presented. This approach can be applied for varied types of pupil function, wavelength, and polarization, and can be easily transferred to other traditional or superresolution microscopes to upgrade their axial resolution.