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While spatial structured light based free space optical communication provides high-bandwidth communication with broad application prospect, severe signal distortion caused by optical scattering from ambient microparticles in the atmosphere can lead to data degradation. A deep-learning-based adaptive demodulator has been demonstrated to resolve the information encoded in the severely distorted channel, but the high generalization ability for different scattering always requires prohibitive costs on data preparation and reiterative training. Here, we demonstrate a meta-learning-based auto-encoder demodulator, which learns from prior theoretical knowledge, and then training with only three realistic samples per class can rectify and recognize transmission distortion. By employing such a demodulator to hybrid vector beams, high fidelity communication can be established, and data costs are reduced when faced with different scattering channels. In a proof-of-principle experiment, an image with 256 gray values is transmitted under severe scattering with an error ratio of less than 0.05%. Our work opens the door to high-fidelity optical communication in random media environments.

An ultra-highly sensitive and robust CH 4 sensor is reported based on a 3.3 µm interband cascade laser (ICL) and a low-noise differential photoacoustic (PAS) cell. The ICL emission wavelength targeted a fundamental absorption line of CH 4 at 2988.795 cm -1 with an intensity of 1.08 × 10 -19 cm/molecule. The double-pass and differential design of the PAS cell effectively enhanced the PAS signal amplitude and decreased its background noise. The wavelength modulation depth, operating pressure and V-T relaxation promotion were optimized to maximize the sensor detection limit. With an integration time of 90 s, a detection limit of 0.6 ppb was achieved. No additional water or air laser cooling were required and thereby allowing the realization of a compact and robust CH 4 sensor.

Surface plasmon resonance (SPR) enables strong field confinement, opening thereby new avenues for device miniaturization and reducing energy consumption. Plasmonic devices with electrical tunability attract tremendous interest for various applications. Most of the current researches achieved SPR modulation with relatively large driving voltages, or by other relatively low-speed tuning approaches, such as thermo-optic, magneto-optic, acousto-optic etc. In this paper, we propose and demonstrate an efficiently electrical SPR modulation based on lithium niobate (LN) with gold nanolayer (~81 nm) via electron-plasmon interaction. Efficient intensity modulation and wavelength shift (in visible band) of ~5.7 dB/V and ~36.3 nm/V are respectively obtained with low DC current. More importantly, modulation phenomenon of field distribution dependent is also observed and experimentally unveiled. Further performance is analyzed in terms of AC modulation and polarization characteristics. This key achievement opens up opportunities for applications such as optical interconnection, electric field sensing, electrically plasmonic modulation, etc.

In-plane photonic spin splitting effect is investigated in tunneling terahertz waves through an epsilon-near-zero metamaterial sandwiched between monolayer black phosphorus (BP). The strong in-plane anisotropy of BP layers will induce in-plane asymmetric spin splitting. The asymmetric spin splitting can be flexibly tuned by the angles between the incident plane and the armchair crystalline directions of the top and bottom BP layers, i.e., ϕ 1 and ϕ 2 . Based on this, an angle-resolved barcode-encryption scheme is discussed. For the special case of ϕ 1 = ϕ 2 = 0 or 90°, the transmitted beam undergoes Goos-Hänchen shift, which varies with the carrier density of BP. We believe these findings can facilitate the development of novel optoelectronic devices in the Terahertz region.

All-optical light amplitude tuning functionality is demonstrated in a layered tungsten disulfide (WS 2 ) nanosheets coated microfiber (MF) structure. Due to the strong light-matter interactions between WS 2 nanosheets and the evanescent field around the MF, a large variation in the transmitted power can be observed under both external and internal pump light excitations over a broadband spectrum (~100 nm). A power variation rate of ~0.3744 dB/mW is obtained under external violet pump light excitation, whereas the power variation rate of similar devices in the state of the art are usually less than 0.3 dB/mW. In terms of the response time, a moderate rise/fall time of ∼20.5/19.6 ms is achieved, which is mainly limited by the employed structure fabrication methods. These results indicate that the optical transmitted power of the WS 2 coated MF can be modulated by different pump light with the power in the order of mW, thus the proposed device might have potential applications in all optical controllable devices and sensors, etc.

The fiber geometry, fiber parameters and mode-guiding properties are crucial for realizing high-performance fiber-based sensors. In this work, we propose and demonstrate a few-mode fiber (FMF)-based surface plasmon resonance (SPR) biosensor. The FMF-SPR sensor was fabricated via side-polishing a few-mode fiber and coating a thin layer of gold film, on the basis of the optimization of fiber geometry, thickness of the gold film and mode selection, which were performed with the finite element method. The refractive index (RI) sensing performance of three such sensors with different residual fiber thicknesses were investigated. In the RI range from 1.333 to 1.404, the highest sensitivity up to 4903 nm/RIU and a figure of merit of 46.1 RIU -1 are achieved. For testing the bovine serum albumin (BSA) solution, an averaged BSA RI sensitivity of 6328 nm/RIU and an averaged BSA concentration sensitivity of 1.17 nm/(mg/ml) are realized. Benefiting from only a few modes supported in the FMF, a smaller line-width of the SPR spectrum is obtained, which further results in a higher figure of merit (FOM). Moreover, when combined with the superiority of the mode-multiplexing technology brought by the FMF, the FMF-SPR sensors may find applications in biochemical analysis with high performance and high throughputs.

In this work, a sensitivity-enhanced surface plasmon resonance (SPR) sensor, which is integrated with MoSe 2 as modification overlayer, is proposed and investigated. The sensor is constructed by physically depositing MoSe 2 onto the surface of a conventional SPR sensor based on Krestchman configuration. Thanks to the commendable properties of MoSe 2 including high carrier mobility, high refractive index (RI), large surface area, and so forth, adding an overlayer within a certain thickness can effectively improve the RI sensitivity. Experimental results show that, with the increased number of deposition cycles-which positively correlates with the duty ratio and the MoSe 2 overlayer's thickness-the sensitivity at first increases, and then declines. The highest sensitivity of 2524.8 nm/RIU is achieved experimentally, which corresponds to the 2 deposition cycles. This shows an improvement of 36.3%, compared with the case without the MoSe 2 modification. The ease of fabrication, efficiency of performance enhancement, and great potentials (such as the large surface area of MoSe 2 for linking abundant functional groups) allow the method presented in this paper to contribute to the development of performance-enhanced SPR sensors for the biological, chemical, and medical fields.

We propose an electro-optic mode deflection device based on an annealed proton exchange (APE) waveguide in lithium niobate, associated with isosceles-triangle-shaped array electrodes and a horn-shaped input waveguide. The input waveguide is tapered down to ensure that the output of the device has a good beam quality, i.e., a quasi-single mode in this case. This new device allows beam deflection at a relative low voltage and large deflection angle. At an APE-waveguide width of 80 μm, mode deflections of 0.265 and 0.240 μm/V are obtained for 1064 and 980 nm, respectively. This beam deflection device can be applied in high-speed optical switch, and beam smoothing of a high-power laser, etc.

All-optical light-control-light functionality is realized in a layered tungsten disulfide (WS 2 ) nanosheet coated microfiber knot resonator (MKR) structure. Mainly due to the photon generated excitons induced refractive index variation in WS 2 nanosheets, a large variation in the transmitted power (∆T) can be observed under external violet/red laser excitation. The ∆T variation rates can reach up to ~0.4 dB/mW under violet pump light excitation whereas the state of the art light-control-light structures usually has a variation rate of less than 0.25 dB/mW. In terms of the response time, the averaged rise/fall time is ~0.12/0.1 s. The demonstrated structure has the advantages of easy fabrication, low cost and high sensitivity, therefore, it might be a promising candidate for building future all-fiber-optics based functional devices and all-optical circuitry.

A high-sensitivity vector magnetic field is proposed and demonstrated. The sensor consists of a side-polished-fiber (SPF)-based surface plasmon resonance (SPR) structure integrated with ferrofluid. Because of the high refractive index sensitivity of the SPR scheme and the outstanding magneto optical properties of ferrofluid, the sensor shows a high sensitivity (up to 598.7 pm/Oe) to magnetic field intensity. Moreover, owing to the non-circular-symmetric geometry of the SPF and non-uniform distribution of the ferrofluid around the SPF, the sensor exhibits a sensitivity of -5.63  nm/deg to the orientation of the magnetic field. The proposed vector magnetic field sensor, integrating over magnetic, plasmonic, and fiber-optic schemes, is highly sensitive to both the intensity and orientation of the magnetic field simultaneously, and holds potential in applications in many fields, such as medicine, industry, and the military.

A pure vortex beam carrying m-order orbital angular momentum (OAM) will be deformed when transmitting through a thin slab, and "neighboring" sideband {m + 1} and {m-1} modes will emerge. The emergence of the OAM sideband is accompanied with OAM-dependent Goos-Hänchen (GH) shift. When the energies carried by the {m} mode of the transmitted beam and by the sideband modes are identical, the OAM-dependent shifts reach their upper limits, |m|w 0 /2(|m| + 1)1/2, where w 0 is the incident beam waist. The epsilon-near-zero metamaterial is found to be suitable to achieve the upper-limited OAM-dependent GH shifts. These findings provide a deeper insight into the beam shifts of vortex beams and have potential applications in the optical sensing, detection of OAM, and other OAM-based applications.

Upon reflection, a light beam embedded with m-order orbital angular momentum (OAM) will undergo the Imbert-Fedorov (IF) shift, which induces OAM sidebands. The energies of the neighboring {-m-1} and {-m+1} sideband modes of the reflected beam are always equal. Controllable OAM sidebands are theoretically achieved by introducing a monolayer graphene in a three-layer structure composed of air, hexagonal boron nitride, and metal. By modulating the Fermi energy of graphene, the OAM-dependent IF shift can be tuned from positive to negative values, and the OAM sideband modes can be suppressed or enhanced, since the reflectivity for perpendicular and parallel polarizations vary with the Fermi energy. These findings provide an alternative method for the control of optical OAM in the terahertz region.

The effective mode index (EMI) of a graphene-coated side-polished fiber (GSPF) is calculated numerically. Whereby, the influences of graphene atom layer number, residual radius of SPF, light frequency, scattering rate of graphene, and temperature on the EMI are investigated comprehensively. Two types of mechanisms for the electro-optical absorption modulation are found for such GSPF-based modulator. One mechanism is Pauli blocking effect (PBE) and the other is plasmonic attenuation effect (PAE). With the optimal design parameters, a PBE-based modulator is theoretically predicted to have a 0.0072 dB/μm modulation depth, 2.92 V driving voltage swing, 6.35 nJ/bit power consumption, and 56.2 THz optical modulation bandwidth. It is also predicted that a PAE-based modulator could have a 0.0056 dB/μm modulation depth, 0.6 V driving voltage swing, 0.27 nJ/bit power consumption, and 2.5 THz optical modulation bandwidth. By further optimization, the modulator performance such as the relatively high power consumption and the narrow operation bandwidth can be improved. Owing to their seamless connection to optical fiber networks, the GSPF-based modulators have great potential to be used in fast and high-capacity optical communication systems.

A novel fiber structure, coreless side-polished fiber (CSPF) that is wrapped by polydimethylsiloxane (PDMS), is demonstrated to be highly sensitive to temperature because of the high refractive index sensitivity of the CSPF and the large thermal optic coefficient of the PDMS. Our numerical and experimental results show that the several dips in the transmitted spectra of PDMSW-CSPF is originated from the multimode interference (MMI) in the CSPF and will blueshift with the increase of temperature. Furthermore, for such a PDMSW-CSPF, we investigate its temperature sensing characteristics and the influences of residual thickness (RT) and dip wavelength on the sensitivity both numerically and experimentally. In the temperature range of 30~85°C, the PDMSW-CSPF with RT = 43.26 μm exhibits a high temperature sensitivity of -0.4409 nm/°C, the high linearity of 0.9974, and the high stability with low standard deviation of 0.141 nm. Moreover, in the cycle experiments, where the environmental temperature was set to automatically increase and then decrease, the PDMSW-CSPF exhibits a low relative deviation of sensitivity (RSD) of down to ± 0.068%. Here, the RSD is defined as the ratio of sensitivity deviation to the average sensitivity measured in the heating/cooling cycle experiments. The lower RSD indicates that PDMSW-CSPF has better reversibility than other fiber structure. The investigations also show that the sensitivity of the PDMSW-CSPF could be enhanced further by reducing the residual thickness and choosing the dip at a longer wavelength.

The spin Hall effect (SHE) of light beams reflected from an air-chiral interface are investigated systematically. Due to the intrinsic chiral asymmetry of the medium, a horizontally polarized incident Gaussian beam will undergo asymmetric spin splitting, i.e., both the displacements and energies of two spin components of the reflected beam are different. One spin component can undergo large displacement near points of |rpp| = |rsp| (rpp and rsp are the Fresnel reflection coefficients), where the reflected beams are almost in circular polarization states. Moreover, for an incident beam carrying orbital angular momentum (OAM), the two spin components acquire additional OAM dependent shifts, which attribute to the asymmetric spin splitting. Thus, the asymmetric spin splitting of the reflected beam will vary with the incident OAM. These findings provide a deeper insight into the SHE of light, and they may have potential application in precision metrology.

The angular Goos-Hänchen shift of vortex beam is investigated theoretically when a Laguerre-Gaussian (LG) beam is reflected by an air-metamaterial interface. The upper limit of the angular GH shift is found to be half of the divergence angle of the incident beam, i.e., |Θup| = (|ℓ| + 1) 1/2 /k 0 w 0 , with ℓ, k 0 , and w 0 being the vortex charge, wavenumber in vacuum, and beam waist, respectively. Interestingly, the upper limited angular GH shift is accompanied by the upper-limited spatial IF shift. A parameter F is introduced to compare the total beam shift with the beam size. F varies with the vortex charge ℓ and the propagation distance zr. The values of F at zr = ∞ plane can approach 0.5, which are always larger than those at zr = 0 plane. These findings provide a deeper insight into optical beam shifts, and they may have potential application in precision metrology.

The spin splitting of light beams carrying orbital angular momentum (OAM) is investigated theoretically in attenuated internal reflection in the Kretschmann configuration. The excitation of the surface plasmon resonance (SPR) can significantly enhance the OAM-induced Imbert-Fedorov (IF) shift and the OAM-dependent spin splitting. The cooperation effect of these two shifts will result in an asymmetric spin splitting, which varies with the incident angle and polarization state. Specially, at the SPR angle, the OAM-induced IF shift vanishes, and the OAM-dependent spin splitting will cause a symmetric spin splitting. However, when the incident beam is horizontally polarized, the OAM-induced IF shift predominates. Thus the two spin components of the reflected will not be split; instead, they will be shifted together. This flexible control of the optical spin splitting can find applications in quantum information and precision metrology.

An orbital angular momentum (OAM)-induced spin splitting is theoretically predicted when a higher-order Laguerre-Gaussian beam is transmitted through a metamaterial slab. The upper bound of this spin splitting is found to be |ℓ|w 0 /(|ℓ|+1) 1/2 , where ℓ and w 0 are the incident OAM and beam waist, respectively. By optimizing the structure parameter of the metamaterial, as well as the incident angle, the OAM-induced spin splitting can reach more than 0.99 of the upper bound in the cases of both the horizontal and vertical incident polarization states, and the transmitted light fields turn out to be full Poincaré beams. These findings provide a deeper insight into the spin-orbit interaction, and, thereby, facilitate the development of spin-based applications.

In this paper, a side-polished fiber (SPF) coated with molybdenum diselenide (MoSe 2 ) is proposed, and its characteristic of relative humidity (RH) sensing is investigated. It is found in the experiment that an enhancement in RH sensitivity (0.321 dB/%RH) can be achieved in a very wide RH range of 32%RH to 73%RH for the proposed MoSe 2 coated SPF (MoSe 2 CSPF). It is also shown that the MoSe 2 CSPF has a rapid response of 1s and recovery time of 4s, which makes the sensor capable of monitoring human breath. The experimental results suggest MoSe 2 has a promising potential in photonics applications such as all-fiber optic humidity sensing networks.

We propose and experimentally demonstrate a new electro-optically controllable add-drop filter based on light coupling between a microfiber knot ring (MKR) and a lithium niobate (LN) microwaveguide. In our design, the MKR works as a resonator and routes the resonant light into the LN microwaveguide. The LN microwaveguide, as an excellent intermediary between electronics and optics, is a robust platform that not only enables stable support and manipulation of the MKR but also provides amplitude tunability taking advantage of its electro-optic property. Two add-drop filters with different diameters of the MKR, 1.12 mm, and 560 μm respectively, are studied, and a maximum amplitude tunability of ∼0.139  dB/V is obtained. The results show that this design can be a solution to interconnect a microstructured optical fiber with a microstructured on-chip device and provide an effective method to realize the active on-chip integration of the conventional fiber system.

A novel fiber structure, coreless side-polished fiber (CSPF), is proposed and investigated to implement multimode interference (MMI) and high sensitive refractive index (RI) sensors. For such CSPF, the part of the cladding and the core of a single-mode fiber are side-polished off so as to make the remained cladding a D-shaped multimode waveguide. The excitation and evolution of MMI in the CSPF are simulated numerically. The simulation results show that the high-order modes excited within the D-shaped multimode waveguide are mainly TE 0,1 (TM 0,1 )~TE 0,6 (TM 0,6 ) modes. Moreover, the RI sensing characteristics and the influences of residual thickness and dip wavelength on the sensitivity are investigated both numerically and experimentally. The experimental results show that the CSPF with a residual thickness of 43.1 μm can reach an ultra-high sensitivity of 28000 nm/RIU in the RI range of 1.442~1.444. It is also found that the sensitivity can be further increased by reducing the residual thickness and choosing the dip at a longer wavelength. Thanks to the ultra-high RI sensitivity and the ease of fabrication, the CSPF could provide a cost-effective platform to build high-performance fiber devices of various functions.

We show theoretically that after transmitted through a thin anisotropic ε-near-zero metamaterial, a linearly polarized Gaussian beam can undergo both transverse spatial and angular spin splitting. The upper limits of spatial and angular spin splitting are found to be the beam waist and divergence angle of incident Gaussian beam, respectively. The spin splitting of transmitted beam after propagating a distance z depends on both the spatial and angular spin splitting. By combining the spatial and angular spin splitting properly, we can maximize the spin splitting of propagated beam, which is nearly equal to the spot size of Gaussian beam w(z).