In-fiber photoelectric device based on graphene-coated tilted fiber grating

Graphene and related two-dimensional materials have attracted great research interests due to prominently optical and electrical properties and flexibility in integration with versatile photonic structures.Here,we report an in-fiber photoelec-tric device by wrapping a few-layer graphene and bonding a pair of electrodes onto a tilted fiber Bragg grating(TFBG)for photoelectric and electric-induced thermo-optic conversions.The transmitted spectrum from this device consists of a dense comb of narrowband resonances that provides an observable window to sense the photocurrent and the electrical injection in the graphene layer.The device has a wavelength-sensitive photoresponse with responsivity up to 11.4 A/W,allowing the spectrum analysis by real-time monitoring of photocurrent evolution.Based on the thermal-optic effect of electrical injection,the graphene layer is energized to produce a global red-shift of the transmission spectrum of the TF-BG,with a high sensitivity approaching 2.167×10^(4)nm/A^(2).The in-fiber photoelectric device,therefore as a powerful tool,could be widely available as off-the-shelf product for photodetection,spectrometer and current sensor.

Broadband and continuous wave pumped second-harmonic generation from microfiber coated with layered GaSe crystal

The conversion-efficiency for second-harmonic(SH)in optical fibers is significantly limited by extremely weak second-order nonlinearity of fused silica,and pulse pump lasers with high peak power are widely employed.Here,we propose a simple strategy to efficiently realize the broadband and continuous wave(CW)pumped SH,by transferring a crystalline GaSe coating onto a microfiber with phase-matching diameter.In the experiment,high efficiency up to 0.08%W-1mm-1 is reached for a C-band pump laser.The high enough efficiency not only guarantees SH at a single frequency pumped by a CW laser,but also multi-frequencies mixing supported by three CW light sources.Moreover,broadband SH spectrum is also achieved under the pump of a superluminescent light-emitting diode source with a 79.3 nm bandwidth.The proposed scheme provides a beneficial method to the enhancement of various nonlinear parameter processes,development of quasi-monochromatic or broadband CW light sources at new wavelength regions.

Graphene-empowered dynamic metasurfaces and metadevices

Metasurfaces,with extremely exotic capabilities to manipulate electromagnetic(EM)waves,have derived a plethora of advanced metadevices with intriguing functionalities.Tremendous endeavors have been mainly devoted to the static metasurfaces and metadevices,where the functionalities cannot be actively tuned in situ post-fabrication.Due to the in-trinsic advantage of active tunability by external stimulus,graphene has been successively demonstrated as a favorable candidate to empower metasurfaces with remarkably dynamic tunability,and their recent advances are propelling the EM wave manipulations to a new height:from static to dynamic.Here,we review the recent progress on dynamic metasur-faces and metadevices enabled by graphene with the focus on electrically-controlled dynamic manipulation of the EM waves covering the mid-infrared,terahertz,and microwave regimes.The fundamentals of graphene,including basic ma-terial properties and plasmons,are first discussed.Then,graphene-empowered dynamic metasurfaces and met-adevices are divided into two categories,i.e.,metasurfaces with building blocks of structured graphene and hybrid metasurfaces integrated with graphene,and their recent advances in dynamic spectrum manipulation,wavefront shap-ing,polarization control,and frequency conversion in near/far fields and global/local ways are elaborated.In the end,we summarize the progress,outline the remaining challenges,and prospect the potential future developments.

High-Q resonances governed by the quasi-bound states in the continuum in all-dielectric metasurfaces

The realization of high-Q resonances in a silicon metasurface with various broken-symmetry blocks is reported. Theoretical analysis reveals that the sharp resonances in the metasurfaces originate from symmetry-protected bound in the continuum(BIC) and the magnetic dipole dominates these peculiar states. A smaller size of the defect in the broken-symmetry block gives rise to the resonance with a larger Q factor. Importantly, this relationship can be tuned by changing the structural parameter, resulting from the modulation of the topological configuration of BICs. Consequently, a Q factor of more than 3,000 can be easily achieved by optimizing dimensions of the nanostructure. At this sharp resonance, the intensity of the third harmonic generation signal in the patterned structure can be 368 times larger than that of the flat silicon film. The proposed strategy and underlying theory can open up new avenues to realize ultrasharp resonances, which may promote the development of the potential meta-devices for nonlinearity, lasing action, and sensing.

Photovoltaic effects in reconfigurable heterostructured black phosphorus transistors

We demonstrate a reconfigurable black phosphorus electrical field transistor,which is van der Waals heterostructured with few-layer graphene and hexagonal boron nitride flakes.Varied homojunctions could be realized by controlling both source–drain and top-gate voltages.With the spatially resolved scanning photocurrent microscopy technique,photovoltaic photocurrents originated from the band-bending regions are observed,confirming nine different configurations for each set of fixed voltages.In addition,as a phototransistor,high responsivity(～800 mA/W)and fast response speed(～230μs)are obtained from the device.The reconfigurable van der Waals heterostructured transistors may offer a promising structure towards electrically tunable black phosphorus-based optoelectronic devices.

On the use of deep learning for phase recovery

Phase recovery(PR)refers to calculating the phase of the light field from its intensity measurements.As exemplified from quantitative phase imaging and coherent diffraction imaging to adaptive optics,PR is essential for reconstructing the refractive index distribution or topography of an object and correcting the aberration of an imaging system.In recent years,deep learning(DL),often implemented through deep neural networks,has provided unprecedented support for computational imaging,leading to more efficient solutions for various PR problems.In this review,we first briefly introduce conventional methods for PR.Then,we review how DL provides support for PR from the following three stages,namely,pre-processing,in-processing,and post-processing.We also review how DL is used in phase image processing.Finally,we summarize the work in DL for PR and provide an outlook on how to better use DL to improve the reliability and efficiency of PR.Furthermore,we present a live-updating resource(https://github.com/kqwang/phase-recovery)for readers to learn more about PR.

Highly robust spatiotemporal wavefront prediction with a mixed graph neural network in adaptive optics

The time-delay problem,which is introduced by the response time of hardware for correction,is a critical and nonignorable problem of adaptive optics(AO)systems.It will result in significant wavefront correction errors while turbulence changes severely or system responses slowly.Predictive AO is proposed to alleviate the time-delay problem for more accurate and stable corrections in the real time-varying atmosphere.However,the existing prediction approaches either lack the ability to extract non-linear temporal features,or overlook the authenticity of spatial features during prediction,leading to poor robustness in generalization.Here,we propose a mixed graph neural network(MGNN)for spatiotemporal wavefront prediction.The MGNN introduces the Zernike polynomial and takes its inherent covariance matrix as physical constraints.It takes advantage of conventional convolutional layers and graph convolutional layers for temporal feature catch and spatial feature analysis,respectively.In particular,the graph constraints from the covariance matrix and the weight learning of the transformation matrix promote the establishment of a realistic internal spatial pattern from limited data.Furthermore,its prediction accuracy and robustness to varying unknown turbulences,including the generalization from simulation to experiment,are all discussed and verified.In experimental verification,the MGNN trained with simulated data can achieve an approximate effect of that trained with real turbulence.By comparing it with two conventional methods,the demonstrated performance of the proposed method is superior to the conventional AO in terms of root mean square error(RMS).With the prediction of the MGNN,the mean and standard deviation of RMS in the conventional AO are reduced by 54.2%and 58.6%at most,respectively.The stable prediction performance makes it suitable for wavefront predictive correction in astronomical observation,laser communication,and microscopic imaging.

Cylindrical vector beam generator on photonic crystal cavity integrated with metal split ring nanoresonators

We propose a chip-integratable cylindrical vector[CV]beam generator by integrating six plasmonic split ring resonators[SRRs]on a planar photonic crystal[PPC]cavity.The employed PPC cavity is formed by cutting six adjacent air holes in the PPC center,which could generate a CV beam with azimuthally symmetric polarizations.By further integrating six SRRs on the structure defects of the PPC cavity,the polarizations of the CV beam could be tailored by controlling the opening angles of the SRRs,e.g.,from azimuthal to radial symmetry.The mechanism is governed by the coupling between the resonance modes in SRRs and PPC cavity,which modifies the far-field radiation of the resonance mode of the PPC cavity with the SRR as the nano-antenna.The integration of SRRs also increases the coupling of the generated CV beam with the free-space optics,such as an objective lens,promising its further applications in optical communication,optical tweezer,imaging,etc.

Linear and nonlinear photonic spin Hall effect induced by analog circular birefringence of Bessel-like beams

The spin Hall effect of a light beam is essentially a product of circular birefringence but is rarely demonstrated.Here,we provide a scheme for initiating off-axis circular birefringence based on the spin-dependent wave vector bifurcation of Bessel beams via a single liquid crystal Pancharatnam–Berry phase element.The tilted Bessel beam shows a detectable photonic spin Hall effect.By introducing the nonlinear propagation trajectories,the spin Hall effect is greatly enhanced.More surprisingly,the two spin states exactly propagate along the scaled trajectories,enabling flexible control of the spin separation.This phenomenon is also applicable to other Bessel-like beams with nonlinear trajectories,which have been already reported.

Highly efficient generation of arbitrary vector beams with tunable polarization, phase, and amplitude

We propose an efficient and robust method to generate tunable vector beams by employing a single phasetype spatial light modulator(SLM). With this method, a linearly polarized Gaussian beam can be converted into a vector beam with arbitrarily controllable polarization state, phase, and amplitude. The energy loss during the conversion is greatly reduced and depends mainly on the reflectivity of the SLM. We experimentally demonstrate that conversion efficiency of about 47% is achieved by using an SLM with reflectivity of 62%.Several typical vector beams, including cylindrical vector beams, vector beams on higher order Poincaré spheres,and arbitrary vector beams attached with phases and with tunable amplitude, are generated and verified experimentally. This method is also expected to create high-power vector beams and play important roles in optical fabrication and light trapping.

Interference-assisted kaleidoscopic meta-plexer for arbitrary spin-wavefront manipulation

Achieving simultaneous polarization and wavefront control,especially circular polarization with the auxiliary degree of freedom of light and spin angular momentum,is of fundamental importance in many optical applications.Interferences are typically undesirable in highly integrated photonic circuits and metasurfaces.Here,we propose an interference-assisted metasurface-multiplexer(meta-plexer)that counterintuitively exploits constructive and destructive interferences between hybrid meta-atoms and realizes independent spin-selective wavefront manipulation.Such kaleidoscopic meta-plexers are experimentally demonstrated via two types of single-layer spinwavefront multiplexers that are composed of spatially rotated anisotropic meta-atoms.One type generates a spinselective Bessel-beam wavefront for spin-down light and a low scattering cross-section for stealth for spin-up light.The other type demonstrates versatile control of the vortex wavefront,which is also characterized by the orbital angular momentum of light,with frequency-switchable numbers of beams under linearly polarized wave excitation.Our findings offer a distinct interference-assisted concept for realizing advanced multifunctional photonics with arbitrary and independent spin-wavefront features.A variety of applications can be readily anticipated in optical diodes,isolators,and spin-Hall meta-devices without cascading bulky optical elements.

Graphene-supported manipulation of surface plasmon polaritons in metallic nanowaveguides

We investigate the electrically controlled light propagation in the metal–dielectric–metal plasmonic waveguide with a sandwiched graphene monolayer. The theoretical and simulation results show that the propagation loss exhibits an obvious peak when the permittivity of graphene approaches an epsilon-near-zero point when adjusting the gate voltage on graphene. The analog of electromagnetically induced transparency(EIT) can be generated by introducing side-coupled stubs into the waveguide. Based on the EIT-like effect, the hybrid plasmonic waveguide with a length of only 1.5 μm can work as a modulator with an extinction ratio of ～15.8 d B, which is 2.3times larger than the case without the stubs. The active modulation of surface plasmon polariton propagation can be further improved by tuning the carrier mobility of graphene. The graphene-supported plasmonic waveguide system could find applications for the nanoscale manipulation of light and chip-integrated modulation.

Polarization oscillating beams constructed by copropagating optical frozen waves

Polarization oscillating beams, namely, polarization standing waves, commonly formed by a pair of coherent counterpropagating light waves with orthogonal polarizations, oscillate their states of polarization periodically within a wavelength interval, offering conceptual and practical interests in light-matter interactions such as the nonreciprocal magnetoelectric effect, and impressive applications in optical imaging, sensing, and chirality detection. Here, we propose a new class of polarization oscillating beams that longitudinally vary states of polarization with spatial intervals within centimeters via the superposition of two copropagating optical frozen waves with preshaped longitudinal intensity profiles and transverse phase structures. The flexibility and manipulability are demonstrated by creating several polarization oscillating beams with different polarization structures. This work paves a new way to manipulate other waves and may be useful for applications of optical standing waves in optical manipulation, light guiding of atoms, polarization-sensitive sensing, etc.

Phase-matching-induced near-chirp-free solitons in normal-dispersion fiber lasers

Direct generation of chirp-free solitons without external compression in normal-dispersion fiber lasers is a long-term challenge in ultrafast optics.We demonstrate near-chirp-free solitons with distinct spectral sidebands in normaldispersion hybrid-structure fiber lasers containing a few meters of polarization-maintaining fiber.The bandwidth and duration of the typical mode-locked pulse are 0.74 nm and 1.95 ps,respectively,giving the time-bandwidth product of 0.41 and confirming the near-chirp-free property.Numerical results and theoretical analyses fully reproduce and interpret the experimental observations,and show that the fiber birefringence,normal-dispersion,and nonlinear effect follow a phase-matching principle,enabling the formation of the near-chirp-free soliton.Specifically,the phasematching effect confines the spectrum broadened by self-phase modulation and the saturable absorption effect slims the pulse stretched by normal dispersion.Such pulse is termed as birefringence-managed soliton because its two orthogonal-polarized components propagate in an unsymmetrical“X”manner inside the polarization-maintaining fiber,partially compensating the group delay difference induced by the chromatic dispersion and resulting in the selfconsistent evolution.The property and formation mechanism of birefringence-managed soliton fundamentally differ from other types of pulses in mode-locked fiber lasers,which will open new research branches in laser physics,soliton mathematics,and their related applications.

Plasmonic tip internally excited via an azimuthal vector beam for surface enhanced Raman spectroscopy

The synergy of a plasmonic tip and fiber-based structure light field excitation can provide a powerful tool for Raman examination. Here, we present a method of Raman spectrum enhancement with an Ag-nanoparticles(Ag-NPs)-coated fiber probe internally excited via an azimuthal vector beam(AVB), which is directly generated in a few-mode fiber by using an acoustically induced fiber grating. Theoretical analysis shows that gap mode can be effectively generated on the surface of the Ag-NPs-coated fiber probe excited via an AVB. The experimental result shows that the intensity of Raman signal obtained with analyte molecules of malachite green by exciting the Ag-NPs-coated fiber probe via an AVB is approximately eight times as strong as that via the linear polarization beam(LPB), and the activity of the AVB-excited fiber probe can reach 10^-11 mol∕L, which cannot be achieved by LPB excitation.Moreover, the time stability and reliability are also examined, respectively.

Magnetic plasmon resonances in nanostructured topological insulators for strongly enhanced light-MoS_(2) interactions

Magnetic resonances not only play crucial roles in artificial magnetic materials but also offer a promising way for light control and interaction with matter.Recently,magnetic resonance effects have attracted special attention in plasmonic systems for overcoming magnetic response saturation at high frequencies and realizing high-performance optical functionalities.As novel states of matter,topological insulators(TIs)present topologically protected conducting surfaces and insulating bulks in a broad optical range,providing new building blocks for plasmonics.However,until now,high-frequency(e.g.visible range)magnetic resonances and related applications have not been demonstrated in TI systems.Herein,we report for the first time,to our knowledge,a kind of visible range magnetic plasmon resonances(MPRs)in TI structures composed of nanofabricated Sb_(2)Te_(3) nanogrooves.The experimental results show that the MPR response can be tailored by adjusting the nanogroove height,width,and pitch,which agrees well with the simulations and theoretical calculations.Moreover,we innovatively integrated monolayer MoS_(2) onto a TI nanostructure and observed strongly reinforced light-MoS_(2) interactions induced by a significant MPR-induced electric field enhancement,remarkable compared with TI-based electric plasmon resonances(EPRs).The MoS_(2) photoluminescence can be flexibly tuned by controlling the incident light polarization.These results enrich TI optical physics and applications in highly efficient optical functionalities as well as artificial magnetic materials at high frequencies.

High-efficiency second-order nonlinear processes in an optical microfibre assisted by few-layer GaSe

The centrosymmetric nature of silica fibre precludes the realisation of second-order nonlinear processes in optical fibre systems.Recently,the integration of 2D materials with optical fibres has opened up a great opportunity to develop allfibre active devices.Here,we demonstrate high-efficiency second-order nonlinear frequency conversions in an optical microfibre assisted with few-layer gallium selenide(GaSe)nanoflakes.Attributed to the strong evanescent field of the microfibre and ultrahigh second-order nonlinearity of the GaSe nanoflakes,second harmonic generation(SHG)and sum-frequency generation(SFG)are effectively achieved with only sub-milliwatt continuous-wave(CW)lasers in the wavelength range of 1500–1620 nm,covering the C and L telecom bands.The SHG intensity from the microfibre is enhanced by more than four orders of magnitude with the assistance of the GaSe nanoflakes on fibre nonlinear processes.Moreover,in the SFG process,the intensity transfer between different frequencies can be effectively manipulated by changing the wavelengths and powers of two pump lasers.The realised strong second-order nonlinearity in the GaSe-integrated microfibre might expand the applications of all-fibre devices in all-optical signal processing and new light source generation at awkward wavelengths.

Dynamically measuring the holo-information of light fields in three-dimensional space using a periodic polarization-structured light

Spatially structured light field has attracted great attention due to its novel properties and application potential in numerous fields.Among them,the most striking one is the polarization-structured light,known as the vector beam.Here,using a periodic polarization-structured light,we propose a method to dynamically measure the holo-information of light fields,including the amplitude,phase,and polarization distributions,in three-dimensional(3D)space.The measurement system is composed of a Mach-Zender interferometer involving a liquid crystal polarized grating in the reference arm,which is simple,stable,and easy to operate.Featuring the single-shot measurement,this method supports observing the dynamic variation of object light fields.The accuracy,3D polarimetry,and dynamic observation of this method are validated by measuring a calibrated quarter-wave plate,a vector vortex beam,a Poincarébeam,and a stressed polymethyl methacrylate sample.

Generation of polarization and phase singular beams in fibers and fiber lasers

Cylindrical vector beams and vortex beams,two types of typical singular optical beams characterized by axially symmetric polarization and helical phase front,possess the unique focusing property and the ability of carrying orbital angular momentum.We discuss the formation mechanisms of such singular beams in few-mode fibers under the vortex basis and show recent advances in generating techniques that are mainly based on long-period fiber gratings,mode-selective couplers,offset-spliced fibers,and tapered fibers.The performances of cylindrical vector beams and vortex beams generated in fibers and fiber lasers are summarized and compared to give a comprehensive understanding of singular beams and to promote their practical applications.