Topological state transitions of skyrmionic beams under focusing configurations.

The recent emerging appearance of optical analogs of magnetic quasiparticles, i.e., optical skyrmions constructed via spin, field, and Stokes vectors, has garnered substantial interest from deep-subwavelength imaging and quantum entanglement. Here, we investigate systematically the topological state transitions of skyrmionic beams constructed by the Stokes vectors in the focusing configuration. We theoretically demonstrated that in the weak focusing, the skyrmion topological number is protected. Whereas, in the tight focusing, a unique topological transformation with skyrmion number variation is exhibited for the optical skyrmion, anti-skyrmion, and 2nd-order skyrmion structures. The significant difference between the topological state transitions of these two cases originates from the transformation from the paraxial optical system to the nonparaxial optical system, and the approximate two-dimensional polarization structure to the three-dimensional polarization structure. The results provide new insights into the topological state transitions in topological structures, which promote applications in information processing, data storage, and free-space optical communications.

Deep learning facilitated superhigh-resolution recognition of structured light ellipticities.

Elliptical beams (EBs), an essential family of structured light, have been investigated theoretically due to their intriguing mathematical properties. However, their practical application has been significantly limited due to the inability to determine all their physical quantities, particularly the ellipticity factor, a unique parameter for EBs of different families. In this paper, to our knowledge, we proposed the first high-accuracy approach that can effectively distinguish EBs with an ellipticity factor difference of 0.01, equivalent to 99.9% field similarities. The method is based on a transformer deep learning (DL) network, and the accuracy has reached 99% for two distinct families of exemplified EBs. To prove that the high performance of this model can dramatically extend the practical aspect of EBs, we used EBs as information carriers in free-space optical communication for an image transmission task, and an error bit rate as low as 0.22% is achieved. Advancing the path of such a DL approach will facilitate the research of EBs for many practical applications such as optical imaging, optical sensing, and quantum-related systems.

Transverse spin dynamics in structured electromagnetic guided waves.

Spin-momentum locking, a manifestation of topological properties that governs the behavior of surface states, was studied intensively in condensed-matter physics and optics, resulting in the discovery of topological insulators and related effects and their photonic counterparts. In addition to spin, optical waves may have complex structure of vector fields associated with orbital angular momentum or nonuniform intensity variations. Here, we derive a set of spin-momentum equations which describes the relationship between the spin and orbital properties of arbitrary complex electromagnetic guided modes. The predicted photonic spin dynamics is experimentally verified with four kinds of nondiffracting surface structured waves. In contrast to the one-dimensional uniform spin of a guided plane wave, a two-dimensional chiral spin swirl is observed for structured guided modes. The proposed framework opens up opportunities for designing the spin structure and topological properties of electromagnetic waves with practical importance in spin optics, topological photonics, metrology and quantum technologies and may be used to extend the spin-dynamics concepts to fluid, acoustic, and gravitational waves.

Metastability of photonic spin meron lattices in the presence of perturbed spin-orbit coupling.

Photonic skyrmions and merons are topological quasiparticles characterized by nontrivial electromagnetic textures, which have received increasing research attention recently, providing novel degree of freedom to manipulate light-matter interactions and exhibiting excellent potential in deep-subwavelength imaging and nanometrology. Here, the topological stability of photonic spin meron lattices, which indicates the invariance of skyrmion number and robustness of spin texture under a continuous deformation of the field configuration, is demonstrated by inducing a perturbation to break the C4 symmetry in the presence spin-orbit coupling in an optical field. We revealed that amplitude perturbation would result in an amplitude-dependent shift of spin center, while phase perturbation leads to the deformation of domain walls, manifesting the metastability of photonic meron. Such spin topology is verified through the interference of plasmonic vortices with a broken rotational symmetry. The results provide new insights on optical topological quasiparticles, which may pave the way towards applications in topological photonics, optical information storage and transfer.

Transversal optical singularity induced precision measurement of step-nanostructures.

Optical singularities indicate zero-intensity points in space where parameters, such as phase, polarization, are undetermined. Vortex beams such as the Laguerre-Gaussian modes are characterized by a phase factor eilθ, and contain a phase singularity in the middle of its beam. In the case of a transversal optical singularity (TOS), it occurs perpendicular to the propagation, and its phase integral is 2π in nature. Since it emerges within a nano-size range, one expects that TOSs could be sensitive in the light-matter interaction process and could provide a great possibility for accurate determination of certain parameters of nanostructure. Here, we propose to use TOSs generated by a three-wave interference to illuminate a step nanostructure. After interaction with the nanostructure, the TOS is scattered into the far field. The scattering direction can have a relation with the physical parameters of the nanostructure. We show that by monitoring the spatial coordinates of the scattered TOS, its propagation direction can be determined, and as consequence, certain physical parameters of the step nanostructure can be retrieved with high precision.

Design of a dynamic multi-topological charge graphene orbital angular momentum metalens.

Traditional OAM generation devices are bulky and can generally only create OAM with one specific topological charge. Although metasurface-based devices have overcome the volume limitations, no tunable metasurface-based OAM generators have been demonstrated to date. Here, a dynamically tunable multi-topological charge OAM generator based on an ultrathin integrable graphene metalens is demonstrated by simulation using the detour phase technique and spatial multiplexing. Different topological charges can be designed on different focal planes. Stretching the OAM graphene metalens allows the focal plane and the topological values to be changed dynamically. This design method paves an innovative route toward miniaturization and integrating OAM beam-type photonic devices for practical applications.

Temporal effect of the spin-to-orbit conversion in tightly focused femtosecond optical fields.

Spin and orbital angular momenta are two of the most fundamental physical quantities that describe the complex dynamic behaviors of optical fields. A strong coupling between these two quantities leads to many intriguing spatial topological phenomena, where one remarkable example is the generation of a helicity-dependent optical vortex that converts spin to orbital degrees of freedom. The spin-to-orbit conversion occurs inherently in lots of optical processes and has attracted increasing attention due to its crucial applications in spin-orbit photonics. However, current researches in this area are mainly focused on the monochromatic optical fields whose temporal properties are naturally neglected. In this work, we demonstrate an intriguing temporal evolution of the spin-to-orbit conversion induced by tightly-focused femtosecond optical fields. The results indicate that the conversion in such a polychromatic focused field obviously depends on time. This temporal effect originates from the superposition of local fields at the focus with different frequencies and is sensitive to the settings of pulse width and central wavelength. This work can provide fundamental insights into the spin-orbit dynamics within ultrafast wave packets, and possesses the potential for applications in spin-controlled manipulations of light.

Near-field manipulation of Tamm plasmon polaritons.

Tamm plasmon polaritons (TPPs) arise from electromagnetic resonant phenomena which appear at the interface between a metallic film and a distributed Bragg reflector. They differ from surface plasmon polaritons (SPPs), since TPPs possess both cavity mode properties and surface plasmon characteristics. In this paper, the propagation properties of TPPs are carefully investigated. With the aid of nanoantenna couplers, polarization-controlled TPP waves can propagate directionally. By combining nanoantenna couplers with Fresnel zone plates, asymmetric double focusing of TPP wave is observed. Moreover, radial unidirectional coupling of the TPP wave can be achieved when the nanoantenna couplers are arranged along a circular or a spiral shape, which shows superior focusing ability compared to a single circular or spiral groove since the electric field intensity at the focal point is 4 times larger. In comparison with SPPs, TPPs possess higher excitation efficiency and lower propagation loss. The numerical investigation shows that TPP waves have great potential in integrated photonics and on-chip devices.

Effect of the focused gap-plasmon mode on tip-enhanced Raman excitation and scattering.

As a powerful molecular detection approach, tip-enhanced Raman scattering (TERS) spectroscopy has the advantages of nanoscale spatial resolution, label-free detection and high enhancement factor, therefore has been widely used in fields of chemistry, materials and life sciences. A TERS system enhanced by the focused gap-plasmon mode composed of Surface Plasmon Polariton (SPP) focus and the metal probe has been reported, however, its underlying enhancement mechanism for Raman excitation and scattering remains to be deeply explored. Here, we focus on the different performances of optical focus and SPP focus in the TERS system, and verify that the cooperation of these two focuses can produce maximum enhancement in a local electromagnetic field. Further, the Purcell effect on sample scattering in such a system is studied for the enhancement of Raman scattering collection in the far field. Finally, the local field enhancement and the sample far-field scattering enhancement are combined to show a full view of the whole process of TERS enhancement. This research can be applied to optimize the excitation and collection of Raman signals in TERS systems, which is of great value for the research and development of TERS technology.

Motion screening of fiducial marker for improved localization precision and resolution in SMLM.

Single-molecule localization microscopy (SMLM) provides unmatched high resolution but relies on accurate drift correction due to the long acquisition time for each field of view. A popular drift correction is implemented via referencing to fiducial markers that are assumed to be firmly immobilized and remain stationary relative to the imaged sample. However, there is so far lack of efficient approaches for evaluating other motions except sample drifting of immobilized markers and for addressing their potential impacts on images. Here, we developed a new approach for quantitatively assessing the motions of fiducial markers relative to the sample via mean squared displacement (MSD) analysis. Our findings revealed that over 90% of immobilized fluorescent beads in the SMLM imaging buffer exhibited higher MSDs compared to stationary beads in dry samples and displayed varying degrees of wobbling relative to the imaged field. By excluding extremely high-MSD beads in each field from drift correction, we optimized drift correction and experimentally measured localization precision. In SMLM experiments of cellular microtubules, we also found that including only relatively low-MSD beads for drift correction significantly improved the image resolution and quality. Our study presents a simple and effective approach to assess the potential relative motions of fiducial markers and emphasizes the importance of pre-screening fiducial markers for improved image quality and resolution in SMLM imaging.

Ultraviolet metalens for photoacoustic microscopy with an elongated depth of focus.

Ultraviolet photoacoustic microscopy (UV-PAM) can achieve in vivo imaging without exogenous markers and play an important role in pathological diagnosis. However, traditional UV-PAM is unable to detect enough photoacoustic signals due to the very limited depth of focus (DOF) of excited light and the sharp decrease in energy with increasing sample depth. Here, we design a millimeter-scale UV metalens based on the extended Nijboer-Zernike wavefront-shaping theory which can effectively extend the DOF of a UV-PAM system to about 220 µm while maintaining a good lateral resolution of 1.063 µm. To experimentally verify the performance of the UV metalens, a UV-PAM system is built to achieve the volume imaging of a series of tungsten filaments at different depths. This work demonstrates the great potential of the proposed metalens-based UV-PAM in the detection of accurate diagnostic information for clinicopathologic imaging.

Switchable rotation of metal nanostructures in an intensity chirality-invariant focus field.

Light-induced rotation is a fundamental motion form that is of great significance for flexible and multifunctional manipulation modes. However, current optical rotation by a single optical field is mostly unidirectional, where switchable rotation manipulation is still challenging. To address this issue, we demonstrate a switchable rotation of non-spherical nanostructures within a single optical focus field. Interestingly, the intensity of the focus field is chiral invariant. The rotation switch is a result of the energy flux reversal in front and behind the focal plane. We quantitatively analyze the optical force exerted on a metal nanorod at different planes, as well as the surrounding energy flux. Our experimental results indicate that the direct switchover of rotational motion is achievable by adjusting the relative position of the nanostructure to the focal plane. This result enriches the basic motion mode of micro-manipulation and is expected to create potential opportunities in many application fields, such as biological cytology and optical micromachining.

All-optically controlled holographic plasmonic vortex array for multiple metallic particles manipulation.

Due to the sub-diffraction-limited size and giant field enhancement, plasmonic tweezers have a natural advantage in trapping metallic particles. However, the strict excitation condition makes it difficult to generate an arbitrary plasmonic field in a controllable manner, thus narrowing its practical applications. Here, we propose an all-optical plasmonic field shaping method based on a digital holographic algorithm and generate plasmonic vortex arrays with controllable spot numbers, spatial location, and topological charge. Our experimental results demonstrate that multiple gold particles can be stably trapped and synchronously rotated in the vortex arrays, and the particles' kinestate can be dynamically switched. The proposed holographic plasmonic vortex tweezers are suitable for a broadband particle trapping, and this method can be generalized to other surface electromagnetic waves like Bloch surface wave.

High Spatial and Temporal Resolution NIR-IIb Gastrointestinal Imaging in Mice.

Conventional biomedical imaging modalities, including endoscopy, X-rays, and magnetic resonance, are invasive and insufficient in spatial and temporal resolutions for gastrointestinal (GI) tract imaging to guide prognosis and therapy. Here we report a noninvasive method based on lanthanide-doped nanocrystals with ∼1530 nm fluorescence in the near-infrared-IIb window (NIR-IIb, 1500-1700 nm). The rational design of nanocrystals have led to an absolute quantum yield (QY) up to 48.6%. Further benefiting from the minimized scattering through the NIR-IIb window, we enhanced the spatial resolution to ∼1 mm in GI tract imaging, which is ∼3 times higher compared with the near-infrared-IIa (NIR-IIa, 1000-1500 nm) method. The approach also realized a high temporal resolution of 8 frames per second; thus the moment of mice intestinal peristalsis can be captured. Furthermore, with a light-sheet imaging system, we demonstrated a three-dimensional (3D) imaging on the GI tract. Moreover, we successfully translated these advances to diagnose inflammatory bowel disease.

Controllable transportation of microparticles along structured waveguides by the plasmonic spin-hall effect.

With the nanoscale integration advantage of near field photonics, controllable manipulation and transportation of micro-objects have possessed plentiful applications in the fields of physics, biology and material sciences. However, multifunctional optical manipulation like controllable transportation and synchronous routing by nano-devices are limited and rarely reported. Here we propose a new type of Y-shaped waveguide optical conveyor belt, which can transport and route particles along the structured waveguide based on the plasmonic spin-hall effect. The routing of micro-particles in different branches is determined by the optical force components difference at the center of the Y junction along the two branches of the waveguide. The influence of light source and structural parameters on the optical forces and transportation capability are numerically studied. The results illustrate that the proposed structured waveguide optical conveyor belt can transport the microparticles controllably in different branches of the waveguide. Due to the selective transportation ability of microparticles by the 2D waveguide, our work shows great application potential in the region of on-chip optical manipulation.

Enhancing electromagnetic field gradient in tip-enhanced Raman spectroscopy with a perfect radially polarized beam.

Tip-enhanced Raman spectroscopy (TERS) is a promising label-free super-resolving imaging technique, and the electric field gradient of nanofocusing plays a role in TERS performance. In this paper, we theoretically investigated the enhancement and manipulation of the electric field gradient in a bottom-illumination TERS configuration through a tightly focused perfect radially polarized beam (PRPB). Improvement and manipulation in electric field enhancement and field gradient of the gap-plasmon mode between a plasmonic tip and a virtual surface plasmons (SPs) probe are achieved by adjusting the ring radius of the incident PRPB. Our results demonstrate that the method of optimizing the ring radius of PRPB is to make the illumination angle of incident light as close to the surface plasmon resonance (SPR) excitation angle as possible. Under the excitation of optimal parameters, more than 10 folds improvement of field enhancement and 3 times of field gradient of the gap-plasmon mode is realized compared with that of the conventional focused RPB. By this feat, our results indicate that such a method can further enhance the gradient Raman mode in TERS. We envision that the proposed method, to achieve the dynamic manipulation and enhancement of the nanofocusing field and field gradient, can be more broadly used to control light-matter interactions and extend the reach of tip-enhanced spectroscopy.

Detecting cylindrical vector beams with an on-chip plasmonic spin-Hall metalens.

In recent years, singular optical beams, including optical vortex (OV) beams with phase singularities and cylindrical vector beams (CVBs) with polarization singularities, have brought new degrees of freedom for many applications. Although there have been various microscale devices for OV detection, the detection of CVBs with a microscale device is still a challenge. Here, we propose a new method for detection of CVBs with a designed on-chip plasmonic spin-Hall metalens structure. The focal position of the metalens and the splitting effect of at focus are studied in both an analytical model and numerical simulation. The results demonstrate that the metalens can not only detect different polarization orders of incident CVBs but also have an ability to distinguish radial, azimuthal and other vectorial polarization states under the same order of CVBs. This method has potential applications in compact integrated optical communication and processing systems.

Optical singularity assisted method for accurate parameter detection of step-shaped nanostructure in coherent Fourier scatterometry.

Accurate determination of the physical parameters of nanostructures from optical far-field scattering is an important and challenging topic in the semiconductor industry. Here, we propose a novel metrology method to determine simultaneously the height and side-wall angle of a step-shaped silicon nanostructure. By employing an optical singular beam into a typical coherent Fourier scatterometry system, both parameters can be retrieved through analyzing the intensity profile of the far-field scattering pattern. The use of singular beam is shown to be sensitive to slight changes of the parameters of the step. By changing the relative direction between the singularity and structure, the height and side-wall angle can both be retrieved with high precision. This new method is robust, simple, and can provide valuable means for micro-and-nano- metrologies.

Bloch surface waves assisted active modulation of graphene electro-absorption in a wide near-infrared region.

Light modulation has been recognized as one of the most fundamental operations in photonics. In this paper, we theoretically designed a Bloch surface wave assisted modulator for the active modulation of graphene electro-absorption. Simulations show that the strong localized electrical field generated by Bloch surface waves can significantly enhance the graphene electro-absorption up to 99.64%. Then by gate-tuning the graphene Fermi energy to transform graphene between a lossy and a lossless material, electrically switched absorption of graphene with maximum modulation depth of 97.91% can be achieved. Meanwhile, by further adjusting the incident angle to tune the resonant wavelength of Bloch surface waves, the center wavelength of the modulator can be actively controlled. This allows us to realize the active modulation of graphene electro-absorption within a wide near-infrared region, including the commercially important telecommunication wavelength of 1550 nm, indicating the excellent performance of the designed modulator via such mechanism. Such Bloch surface waves assisted wavelength-tunable graphene electro-absorption modulation strategy opens up a new avenue to design graphene-based selective multichannel modulators, which is unavailable in previous reported strategies that can be only realized by passively changing the structural parameters.

Time-varying orbital angular momentum in tight focusing of ultrafast pulses.

The orbital angular momentum (OAM) of light has important applications in a variety of fields, including optical communication, quantum information, super-resolution microscopic imaging, particle trapping, and others. However, the temporal properties of OAM in ultrafast pulses and in the evolution process of spin-orbit coupling has yet to be revealed. In this work, we theoretically studied the spatiotemporal property of time-varying OAM in the tightly focused field of ultrafast light pulses. The focusing of an incident light pulse composed of two time-delayed femtosecond sub-pulses with the same OAM but orthogonal spin states is investigated, and the ultrafast dynamicsa time delay of OAM variation during the focusing process driven by the spin-orbit coupling is visualized. Temporal properties of three typical examples, including formation, increase, and transformation of topological charge are investigated to reveal the non-uniform evolutions of phase singularities, local topological charges, self-torques, and time-varying OAM per photon. This work could deepen the understanding of spin-orbit coupling in time domain and promote many promising applications such as ultrafast OAM modulation, laser micromachining, high harmonic generation, and manipulation of molecules and nanostructures.