Selective plasmonic trapping of nano-particles by Archimedes metalens.

Optical tweezer is a non-invasive method for optical force tool applied in various fields like biology, physics, and lab on chip manipulation. The Archimedean helix shape is ideal for creating chiral nanostructures, and being able to generate plasmonic focused hotspot field for optical trapping. Here we design a metal disk with the Archimedean shape to own the ability of selective trapping nanoparticles based on the spin-orbit interactions with circularly polarized light. The plasmonic near field on the metalens can be designed by adjusting the geometric parameter flexibly. We numerically analyze the optimal size and screw pitch of the metal disk to realize the switch modulation of hotspot generation, and then demonstrate the novel switchable optical trapping ability in the view of optical force and potential well analysis under the circularly polarized light excitation by a 532 nm laser. The work shows significant potential for on-chip optical trapping in various fields.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Data transmission with up to 100 orbital angular momentum modes via commercial multi-mode fiber and parallel neural networks.

This work presents an artificial intelligence enhanced orbital angular momentum (OAM) data transmission system. This system enables encoded data retrieval from speckle patterns generated by an incident beam carrying different topological charges (TCs) at the distal end of a multi-mode fiber. An appropriately trained network is shown to support up to 100 different fractional TCs in parallel with TC intervals as small as 0.01, thus overcoming the problems with previous methods that only supported a few modes and could not use small TC intervals. Additionally, an approach using multiple parallel neural networks is proposed that can increase the system's channel capacity without increasing individual network complexity. When compared with a single network, multiple parallel networks can achieve the better performance with reduced training data requirements, which is beneficial in saving computational capacity while also expanding the network bandwidth. Finally, we demonstrate high-fidelity image transmission using a 16-bit system and four parallel 14-bit systems via OAM mode multiplexing through a 1-km-long commercial multi-mode fiber (MMF).

Energy flow inversion in an intensity-invariant focusing field.

Dependence of light intensity on energy flow is the most intuitive presentation of an optical field. This dependence, however, also limits the applications to the interaction of the light field with matter. For further insight into this, we demonstrate a novel case of the optical field, named as the counterintuitive chiral intensity field (CCIF), in the highly focusing situation: the energy flow reverses during the propagation but the intensity distribution pattern is kept approximately invariant. Our results show that, in this process, the mode correlation decreases rapidly while the intensity correlation remains invariant in the focus area. Furthermore, this property is still valid even if the pattern helicity and number of spiral arms are changed. This work deepens the understanding of the relationship between energy flow and field intensity, and it will offer diversified operations in many applications, such as optical micromanipulation, optical fabrication, etc.

Controllable transport of nanoparticles along waveguides by spin-orbit coupling of light.

Waveguide optical tweezers can capture and transport nanoparticles, and have important applications in biology, physics, and materials science. However, traditional waveguide optical tweezers need to couple incident light into one end of the waveguide, which causes problems such as difficulty in alignment and low efficiency. Here, we propose a new type of waveguide optical tweezers based on spin-orbit coupling of light. Under the effect of spin-orbit coupling between the waveguide and nearby particles illuminated by a circularly polarized light, the particles experience a lateral recoil force and a strong optical gradient force, which make particles in a large area to be trapped near the waveguide and then transmitted along the waveguide, avoiding the coupling of light into one end of the waveguide. We further demonstrate that the particles can be transmitted along a curved waveguide and even rotated along a ring-shaped waveguide, and its transmission direction can be simply switched by adjusting the spin polarization of incident light. This work has significance in the research of optical on-chip nano-tweezers.

Active tuning of longitudinal strong coupling between anisotropic borophene plasmons and Bloch surface waves.

Strong coupling between the resonant modes can give rise to many resonant states, enabling the manipulation of light-matter interactions with more flexibility. Here, we theoretically propose a coupled resonant system where an anisotropic borophene localized plasmonic (BLP) and Bloch surface wave (BSW) can be simultaneously excited. This allows us to manipulate the spectral response of the strong BLP-BSW coupling with exceptional flexibility in the near infrared region. Specifically, the strong longitudinal BLP-BSW coupling occurs when the system is driven into the strong coupling regime, which produces two hybrid modes with a large Rabi splitting up to 124 meV for borophene along both x- and y-directions. A coupled oscillator model is employed to quantitatively describe the observed BSW-BLP coupling by calculating the dispersion of the hybrid modes, which shows excellent agreement with the simulation results. Furthermore, benefited from the angle-dependent BSW mode, the BSW-BLP coupling can be flexibly tuned by actively adjusting the incident angle. Such active tunable BLP-SBW coupling with extreme flexibility offered by this simple layered system makes it promising for the development of diverse borophene-based active photonic and optoelectronic devices in the near infrared region.

Nonlinear modulation on optical trapping in a plasmonic bowtie structure.

Surface plasmon optical tweezers based on micro- and nano-structures are capable of capturing particles in a very small spatial scale and have been widely used in many front research fields. In general, distribution of optical forces and potential wells exerted on the particles can be modulated by controlling the geometric parameters of the structures. However, these fabricated structures are irreversible once processed, which greatly limits its application in dynamic manipulation. The plasmonic field in these structures can be enhanced with orders of magnitude compared to the excitation light, offering a possibility to stimulate nonlinear responses as a new degree of freedom for dynamic modulation. Here, we theoretically demonstrate that the optical force and potential well can be modulated on account of the nonlinear Kerr effect of a gold bowtie structure under a pulsed laser with high peak power. The results verify that the trapping states, including the position, width, and depth of the potential well, can be dynamically modulated by changing intensity of the incident laser. It provides an effective approach for stable trapping and dynamic controlling of particles on nanostructure-based plasmonic trapping platforms and thus has great application potential in many fields, such as enhanced Raman detection, super-resolution imaging, and optical sensing.

Nonlinearity-modulated single molecule trapping and Raman scattering analysis.

Single molecule detection and analysis play important roles in many current biomedical researches. The deep-nanoscale hotspots, being excited and confined in a plasmonic nanocavity, make it possible to simultaneously enhance the nonlinear light-matter interactions and molecular Raman scattering for label-free detections. Here, we theoretically show that a nanocavity formed in a tip-enhanced Raman scattering (TERS) system can also achieve valid optical trapping as well as TERS signal detection for a single molecule. In addition, the nonlinear responses of metallic tip and substrate film can change their intrinsic physical properties, leading to the modulation of the optical trapping force and the TERS signal. The results demonstrate a new degree of freedom brought by the nonlinearity for effectively modulating the optical trapping and Raman detection in single molecule level. This proposed platform also shows a great potential in various fields of research that need high-precision surface imaging.

Few-layer metamaterials for a spontaneous emission enhancement.

Multilayer hyperbolic metamaterials consisting of alternating metal and dielectric layers have important applications in spontaneous emission enhancement. In contrast to the conventional choice of at least dozens of layers in multilayer structures to achieve tunable Purcell effect on quantum emitters, our numerical calculations reveal that multilayers with fewer layers and thinner layers would outperform in the Purcell effect. These discoveries are attributed to the negative contributions by an increasing layer number to the imaginary part of the reflection coefficient and the stronger coupling between surface plasmon polariton modes on a thinner metal layer. This work could provide fundamental insights and a practical guide for optimizing the local density of optical states enhancement functionality of layered metamaterials.