Flat multifunctional liquid crystal elements through multi-dimensional information multiplexing

Flat optical elements have attracted enormous attentions and act as promising candidates for the next generation of optical components.As one of the most outstanding representatives,liquid crystal(LC)has been widely applied in flat panel display industries and inspires the wavefront modulation with the development of LC alignment techniques.However,most LC elements perform only one type of optical manipulation and are difficult to realize the multifunctionality and light integration.Here,flat multifunctional liquid crystal elements(FMLCEs),merely composed of anisotropic LC molecules with space-variant orientations,are presented for multichannel information manipulation by means of polarization,space and wavelength multiplexing.Specifically,benefiting from the unique light response with the change of the incident polarization,observation plane,and working wavelength,a series of FMLCEs are demonstrated to achieve distinct near-and far-field display functions.The proposed strategy takes full advantage of basic optical parameters as the decrypted keys to improve the information capacity and security,and we expect it to find potential applications in information encryption,optical anti-counterfeiting,virtual/augmented reality,etc.

All-optical object identification and threedimensional reconstruction based on optical computing metasurface

Object identification and three-dimensional reconstruction techniques are always attractive research interests in machine vision,virtual reality,augmented reality,and biomedical engineering.Optical computing metasurface,as a two-dimensional artificial design component,has displayed the supernormal character of controlling phase,amplitude,polarization,and frequency distributions of the light beam,capable of performing mathematical operations on the input light field.Here,we propose and demonstrate an all-optical object identification technique based on optical computing metasurface,and apply it to 3D reconstruction.Unlike traditional mechanisms,this scheme reduces memory consumption in the processing of the contour surface extraction.The identification and reconstruction of experimental results from high-contrast and low-contrast objects agree well with the real objects.The exploration of the all-optical object identification and 3D reconstruction techniques provides potential applications of high efficiencies,low consumption,and compact systems.

Photonic spin Hall effect:fundamentals and emergent applications

The photonic spin Hall effect(SHE)refers to the transverse spin separation of photons with opposite spin angular momentum,after the beam passes through an optical interface or inhomogeneous medium,manifested as the spin-dependent splitting.It can be considered as an analogue of the SHE in electronic systems:the light’s right-circularly polarized and left-circularly polarized components play the role of the spin-up and spin-down electrons,and the refractive index gradient replaces the electronic potential gradient.Remarkably,the photonic SHE originates from the spin-orbit interaction of the photons and is mainly attributed to two different geometric phases,i.e.,the spin-redirection Rytov-Vlasimirskii-Berry in momentum space and the Pancharatnam-Berry phase in Stokes parameter space.The unique properties of the photonic SHE and its powerful ability to manipulate the photon spin,gradually,make it a useful tool in precision metrology,analog optical computing and quantum imaging,etc.In this review,we provide a brief framework to describe the fundamentals and advances of photonic SHE,and give an overview on the emergent applications of this phenomenon in different scenes.

Broadband Pulsed Fiber Laser Generation with Topological Insulator: Towards the Mid-Infrared Regime

In recent years, topological insulators have aroused the attention of a great number of scientists due to their unique electronic structures and peculiar physical properties. Triggered by the similar electronic structures as graphene, the broadband nonlinear absorption properties of topological insulator were investigated. Moreover, the mode-locked or Q-switched fiber lasers based on topological insulator were realized for broadband operating wavelength. Here, we present an overview of the preparation, transferring, linear and nonlinear optical properties and their applications of topological insulators in pulsed fiber lasers. The pulsed fiber lasers towards mid- infrared regimes have been proposed.

Extremely powerful and frequency-tunable terahertz pulses from a table-top laser–plasma wiggler

The production of broadband,terawatt terahertz(THz) pulses has been demonstrated by irradiating relativistic lasers on solid targets.However,the generation of extremely powerful,narrow-band and frequency-tunable THz pulses remains a challenge.Here,we present a novel approach for such THz pulses,in which a plasma wiggler is elaborated by a table-top laser and a near-critical density plasma.In such a wiggler,the laser-accelerated electrons emit THz radiations with a period closely related to the plasma thickness.The theoretical model and numerical simulations predict that a THz pulse with a laser-THz energy conversion of over 2.0%,an ultra-strong field exceeding 80 GV/m,a divergence angle of approximately 200 and a center frequency tunable from 4.4 to 1.5 THz can be generated from a laser of 430 mJ.Furthermore,we demonstrate that this method can work across a wide range of laser and plasma parameters,offering potential for future applications with extremely powerful THz pulses.

Giant photonic spin Hall effect in momentum space in a structured metamaterial with spatially varying birefringence

The photonic spin Hall effect(SHE)in the reflection and refraction at an interface is very weak because of the weak spin-orbit interaction.Here,we report the observation of a giant photonic SHE in a dielectric-based metamaterial.The metamaterial is structured to create a coordinate-dependent,geometric Pancharatnam–Berry phase that results in an SHE with a spin-dependent splitting in momentum space.It is unlike the SHE that occurs in real space in the reflection and refraction at an interface,which results from the momentum-dependent gradient of the geometric Rytov–Vladimirskii–Berry phase.We theorize a unified description of the photonic SHE based on the two types of geometric phase gradient,and we experimentally measure the giant spin-dependent shift of the beam centroid produced by the metamaterial at a visible wavelength.Our results suggest that the structured metamaterial offers a potential method of manipulating spin-polarized photons and the orbital angular momentum of light and thus enables applications in spin-controlled nanophotonics.

Highly stable femtosecond pulse generation from a MXene Ti_3C_2T_x(T = F, O, or OH) mode-locked fiber laser

Ultrafast fiber lasers are in great demand for various applications, such as optical communication, spectroscopy,biomedical diagnosis, and industrial fabrication. Here, we report the highly stable femtosecond pulse generation from a MXene mode-locked fiber laser. We have prepared the high-quality Ti_3C_2 T_x nanosheets via the etching method, and characterized their ultrafast dynamics and broadband nonlinear optical responses. The obvious intensity-and wavelength-dependent nonlinear responses have been observed and investigated. In addition, a highly stable femtosecond fiber laser with signal-to-noise ratio up to 70.7 dB and central wavelength of 1567.3 nm has been delivered. The study may provide some valuable design guidelines for the development of ultrafast, broadband nonlinear optical modulators, and open new avenues toward advanced photonic devices based on MXenes.

Broadband ultrafast nonlinear optical response of few-layers graphene: toward the mid-infrared regime

Gapless linear energy dispersion of graphene endows it with unique nonlinear optical properties, including broadband nonlinear absorption and giant nonlinear refractive index. Herein, we experimentally observed that fewlayers graphene has obvious nonlinear absorption and large nonlinear refraction, as investigated by the Z-scan technique in the mid-infrared（mid-IR） regime. Our study may not only, for the first time to our knowledge, verify the giant nonlinear refractive index of graphene(～10-7cm2∕W) at the mid-IR, which is 7 orders of magnitude larger than other conventional bulk materials, but also provide some new insights for graphene-based mid-IR photonics,potentially leading to the emergence of several new conceptual mid-IR optoelectronics devices.

Generation of arbitrary vector vortex beams on hybrid-order Poincaré sphere

We propose theoretically and verify experimentally a method of combining a q-plate and a spiral phase plate to generate arbitrary vector vortex beams on a hybrid-order Poincaré sphere. We demonstrate that a vector vortex beam can be decomposed into a vector beam and a vortex, whereby the generation can be realized by sequentially using a q-plate and a spiral phase plate. The generated vector beam, vortex, and vector vortex beam are verified and show good agreement with the prediction. Another advantage that should be pointed out is that the spiral phase plate and q-plate are both fabricated on silica substrates, suggesting the potential possibility to integrate the two structures on a single plate. Based on a compact method of transmissive-type transformation, our scheme may have potential applications in future integrated optical devices.

Ti2CTx MXene-based all-optical modulator

MXenes,a new class of 2D transition metal carbides,nitrides,and carbonitrides,have attracted much attention due to their outstanding properties.Here,we report the broadband spatial self-phase modulation of Ti2CTx MXene nanosheets dispersed in deionized water in the visible to near-infrared regime,highlighting the broadband nonlinear optical(NLO)response of Ti2CTx MXene.Using ultrafast pulsed laser excitation,the nonlinear refractive index n2 and the thirdorder nonlinear susceptibility χ^(3)monolayer of Ti2CTx MXene were measured to be^10^−13 m^2/W and ~10^−10 esu,respectively.Leveraging the large optical nonlinearity of Ti2CTx MXene,an all-optical modulator in the visible regime was fabricated based on the spatial cross-phase modulation effect.This work suggests that 2D MXenes are ideal broadband NLO materials with excellent prospects in NLO applications.

Ultrafast nonlinear absorption and nonlinear refraction in few-layer oxidized black phosphorus

We experimentally investigated the nonlinear optical response in few-layer oxidized black phosphorus(OBP) by the femtosecond Z-scan measurement technique, and found that OBP not only possesses strong ultrafast saturable absorption but also a nonlinear self-defocusing effect that is absent in black phosphorus(BP). The saturable absorption property originates mainly from the direct band structure, which is still maintained in OBP. The emergence of self-defocusing might originate from the combined consequences of the oxygen-induced defects in BP. Our experimental findings might constitute the first experimental evidence on how to dynamically tune its nonlinear property, offering an inroad in tailoring its optical properties through chemical modification(oxidation, introducing defects, etc.). The versatile ultrafast nonlinear optical properties(saturable absorption and self-defocusing) imply a significant potential of the layered OBP in the development of unprecedented optoelectronic devices, such as mode lockers, optical switches, laser beam shapers, and wavelength converters.

High-order nonreciprocal add-drop filter

Topological photonics have led to robust optical behavior of optical devices, which has alleviated the influence of manufacturing defects and perturbations on the device performance. Meanwhile, temporal coupled-mode theory(t-CMT) has been developed and applied widely. However, the t-CMT of cascaded coupled cavities(CCC) system and its corresponding high-order filter has yet to be established. Here, the t-CMT of CCC system is established based on the existing t-CMT. By combining the CCC with topological waveguides, a versatile design scheme of high-order nonreciprocal add-drop filter(HONAF) is proposed. The relationship between the coupling effect of cavities and transmission and the filtering performance of HONAF is analyzed quantitatively. Then, a method to improve the transmission efficiency and quality factor of the filter is given. The proposed HONAF is based on the combination of gyromagnetic photonic crystals and decagonal Penrose-type photonic quasicrystals. The transmission and filtering performance of the HONAF are numerically analyzed, which verifies the consistency between theoretical prediction and numerical simulation. The established t-CMT of CCC system can be widely used in coupled resonator optical waveguides and their related systems. The proposed HONAF with excellent performance can also be applied to wavelength division multiplexing/demultiplexing systems.

Observation of tiny polarization rotation rate in total internal reflection via weak measurements

In this paper,we examine the tiny polarization rotation effect in total internal reflection due to the spin–orbit interaction of light.We find that the tiny polarization rotation rate will induce a geometric phase gradient,which can be regarded as the physical origin of photonic spin Hall effect.We demonstrate that the spin-dependent splitting in position space is related to the polarization rotation in momentum space,while the spin-dependent splitting in momentum space is attributed to the polarization rotation in position space.Furthermore,we introduce a quantum weak measurement to determine the tiny polarization rotation rate.The rotation rate in momentum space is obtained with 118 nm,which manifests itself as a spatial shift,and the rotation rate in position space is achieved with 38 μrad∕λ,which manifests itself as an angular shift.The investigation of the polarization rotation characteristics will provide insights into the photonic spin Hall effect and will enable us to better understand the spin–orbit interaction of light.

Photonic Hall effect and helical Zitterbewegung in a synthetic Weyl system

Systems supporting Weyl points have gained increasing attention in condensed physics,photonics and acoustics due to their rich physics,such as Fermi arcs and chiral anomalies.Acting as sources or drains of Berry curvature,Weyl points exhibit a singularity of the Berry curvature at their core.It is,therefore,expected that the induced effect of the Berry curvature can be dramatically enhanced in systems supporting Weyl points.In this work,we construct synthetic Weyl points in a photonic crystal that consists of a honeycomb array of coupled rods with slowly varying radii along the direction of propagation.The system possesses photonic Weyl points in the synthetic space of two momenta plus an additional physical parameter with an enhanced Hall effect resulting from the large Berry curvature in the vicinity of the Weyl point.Interestingly,a helical Zitterbewegung(ZB)is observed when the wave packet traverses very close to a Weyl point,which is attributed to the contribution of the non-Abelian Berry connection arising from the near degenerate eigenstates.

Photonic spin Hall effect on the surface of anisotropic two-dimensional atomic crystals

We examine the spin-orbit interaction of light and photonic spin Hall effect on the surface of anisotropic two-dimensional atomic crystals. As an example, the photonic spin Hall effect on the surface of black phosphorus is investigated. The photonic spin Hall effect manifests itself as the spin-dependent beam shifts in both transverse and in-plane directions. We demonstrate that the spin-dependent shifts are sensitive to the orientation of the optical axis, doping concentration, and interband transitions. These results can be extensively extended to other anisotropic two-dimensional atomic crystals. By incorporating the quantum weak measurement techniques, the photonic spin Hall effect holds great promise for detecting the parameters of anisotropic two-dimensional atomic crystals.

Weak-value amplification for the optical signature of topological phase transitions

We show that weak measurements can be used to measure the tiny signature of topological phase transitions.The signature is an in-plane photonic spin Hall effect,which can be described as a consequence of a Berry phase.It is also parallel to the propagation direction of a light beam.The imaginary part of the weak value can be used to analyze ultrasmall longitudinal phase shifts in different topological phases.These optical signatures are related to the Chern number and bandgaps;we also use a preselection and postselection technique on the spin state to enhance the original signature.The weak amplification technique offers a potential way to determine the spin and valley properties of charge carriers,Chern numbers,and topological phases by direct optical measurement.

Broadband mid-infrared nonlinear optical modulator enabled by gold nanorods:towards the mid-infrared regime

Mid-infrared pulsed lasers operating around the 3 μm wavelength regime are important for a wide range of applications including sensing, spectroscopy, imaging, etc. Despite the recent advances in technology, the lack of a nonlinear optical modulator operating in the mid-infrared regime remains a significant challenge. Here, we report the third-order nonlinear optical response of gold nanorods(GNRs) ranging from 800 nm to the mid-infrared regime(2810 nm) enabled by their size and overlapping behavior-dependent longitudinal surface plasmon resonance. In addition, we demonstrate a wavelength-tunable Er3+-doped fluoride fiber laser modulated by GNRs, which can deliver pulsed laser output, with the pulse duration down to 533 ns, tunable wavelength ranging from 2760.2 to 2810.0 nm, and spectral 3 d B bandwidth of about 1 nm. The experimental results not only validate the GNRs’ robust mid-infrared nonlinear optical response, but also manifest their application potential in high-performance broadband optoelectronic devices.