Showing papers in "Optics Express in 2021"
TL;DR: In this paper, a hybrid metamaterial absorber comprising a conventional magnetic absorbing material and a multi-layered meta-structure predesigned with graphene films is proposed to realize wideband absorption performance starting from ultra-low frequencies (0.79-20.9 GHz and 25.1-40.0 GHz).
Abstract: Developing high-efficiency microwave absorbers remains challenging in the broadband range, particularly in the low-frequency range containing the L band and even lower. To overcome this challenge, a hybrid metamaterial absorber comprising a conventional magnetic absorbing material and a multi-layered meta-structure predesigned with graphene films is proposed to realize wideband absorption performance starting from ultra-low frequencies (0.79-20.9 GHz and 25.1-40.0 GHz). The high absorption ability of the proposed device originates from fundamental resonance modes and their coupling. The experimental results agree well with the simulated ones, proving the effectiveness of our design method. In addition, owing to the use of low-density polymethylacrylimide foam and graphene films with outstanding mechanical properties, our design is lightweight and environmentally adaptable, which reflects its engineering value.
100 citations
TL;DR: In this article, a switchable bi-functional metamaterial device based on a hybrid gold-vanadium dioxide (VO2) nanostructure was proposed, which can be thermally switched for circularly polarized light in the near-infrared region.
Abstract: In this paper, we propose a switchable bi-functional metamaterial device based on a hybrid gold-vanadium dioxide (VO2) nanostructure. Utilizing the property of a metal-to-insulator transition in VO2, perfect absorption and asymmetric transmission (AT) can be thermally switched for circularly polarized light in the near-infrared region. When VO2 is in the metallic state, the designed metamaterial device behaves as a chiral-selective plasmonic perfect absorber, which can result in an optical circular dichroism (CD) response with a maximum value ∼ 0.7. When VO2 is in the insulating state, the proposed metamaterial device exhibits a dual-band AT effect. The combined hybridization model and electromagnetic field distributions are presented to explain the physical mechanisms of chiral-selective perfect absorption and AT effect, respectively. The influences of structure parameters on CD response and AT effect are also discussed. Moreover, the proposed switchable bi-functional device is robust against the incident angle for obtaining perfect absorption and strong CD response as well as the AT effect. Our work may provide a promising path for the development of multifunctional optoelectronic devices, such as thermal emitters, optical modulators, CD spectroscopy, optical isolator, etc.
78 citations
TL;DR: In this paper, a terahertz bifunctional absorber with switchable functionalities of broadband absorption and multiband absorption was proposed, which obtains a broadband absorption with absorptance >90% from 3.25 THz to 7.08 THz.
Abstract: Based on the phase-transition property of vanadium dioxide (VO2), a terahertz bifunctional absorber is proposed with switchable functionalities of broadband absorption and multiband absorption. When VO2 is metal, the system is regarded as a broadband absorber, which is composed of VO2 patch, topas spacer, and VO2 film with metallic disks inserted. The system obtains a broadband absorption with absorptance >90% from 3.25 THz to 7.08 THz. Moreover, the designed broadband absorber has a stable performance within the incident angle range of 50°. When VO2 is dielectric, multiband absorption with six peaks is realized in the designed system. Graphene and the metallic disk-shaped array play the dominant role in the mechanism of multiband absorption. Through changing the Fermi energy level of graphene, the performance of multiband absorption can be dynamically adjusted. Because of the switchable functionalities, the proposed design may have potential application in the fields of intelligent absorption and terahertz switch.
71 citations
TL;DR: In this article, the authors proposed a chiral metasurface (CMS) that exhibits asymmetric transmission of double circularly and linearly polarized waves at the same frequency band.
Abstract: We propose a chiral metasurface (CMS) that exhibits asymmetric transmission (AT) of double circularly and linearly polarized waves at the same frequency band. In order to realize the manipulation of electromagnetic (EM) waves in the whole space, the unit cell of CMS consists of three layers of dielectric substrate and four layers of metal patches. The Z-shaped chiral micro-structure and a grating-like micro-structure are proposed and designed to achieve AT. The simulated results show that the x-polarized wave that is incident along one direction can be transmitted into the right-hand circularly polarized (RHCP) wave and the left-hand circularly polarized (LHCP) wave that is incident along the opposite direction can be reflected as the LHCP wave in the frequency band of 4.69GHz-5.84 GHz. The maximum chirality response can be reflected by AT and circular dichroism (CD) and they can reach up to 0.38 and 0.75, respectively. In addition, we also produced the sample of CMS, and the experimental results are in good agreement with the simulated results.
68 citations
TL;DR: A bifunctional metamaterial is proposed based on a hybrid graphene and vanadium dioxide (VO2) configuration, which can realize a dynamic switch between beam steering and broadband absorption, which may show great potential in applications such as terahertz switching and modulation.
Abstract: A bifunctional metamaterial is proposed based on a hybrid graphene and vanadium dioxide (VO2) configuration, which can realize a dynamic switch between beam steering and broadband absorption. The structure consists of a VO2 square, graphene patch, topas spacer, VO2 film, topas spacer, and metal substrate. When VO2 is in the metallic state, the structure serves as a coding metamaterial. By engineering different sizes of the top VO2 square and adjusting the Fermi energy level of graphene, the incident wave is scattered in different patterns. When VO2 is in the dielectric state, the structure serves as a broadband absorber. By changing the Fermi energy level of graphene from 0.0 eV to 0.9 eV, absorptance can be gradually changed and working bandwidth widens. There is an absorption band with near 100% absorptance from 0.9 THz to 1.35 THz when the Fermi energy level is 0.73 eV. And the designed broadband absorber is polarization-insensitive within the incident angle of 50°. Our work may show great potential in applications such as terahertz switching and modulation.
66 citations
TL;DR: An ultra-highly sensitive light-induced thermoelastic spectroscopy (LITES)-based carbon monoxide (CO) sensor exploiting custom quartz tuning forks (QTFs) as a photodetector, a multi-pass cell and a mid-infrared quantum cascade laser for the first time is reported on.
Abstract: In this paper, we report on an ultra-highly sensitive light-induced thermoelastic spectroscopy (LITES)-based carbon monoxide (CO) sensor exploiting custom quartz tuning forks (QTFs) as a photodetector, a multi-pass cell and a mid-infrared quantum cascade laser (QCL) for the first time. The QCL emitting at 4.58 µm with output power of 145 mW was employed as exciting source and the multi-pass cell was employed to increase the gas absorption pathlength. To reduce the noise level, wavelength modulation spectroscopy (WMS) and second harmonic demodulation techniques were exploited. Three QTFs including two custom QTFs (#1 and #2) with different geometries and a commercial standard QTF (#3) were tested as photodetector in the gas sensor. When the integration time of the system was set at 200 ms, minimum detection limits (MDLs) of 750 part-per-trillion (ppt), 4.6 part-per-billion (ppb) and 5.8 ppb were achieved employing QTF #1 #2, and #3, respectively. A full sensor calibration was achieved using the most sensitive QTF#1, demonstrating an excellent linear response with CO concentration.
64 citations
TL;DR: A broadband omnidirectional OTS in a heterostructure composed of a Cr layer and a 1D PhC containing layered hyperbolic metamaterials with an angle-insensitive photonic band gap is realized and broadband wide-angle absorption can be achieved.
Abstract: Recently, broadband optical Tamm states (OTSs) in heterostructures composed of highly lossy metal layers and all-dielectric one-dimensional (1D) photonic crystals (PhCs) have been utilized to realize broadband absorption. However, as the incident angle increases, the broadband OTSs in such heterostructures shift towards shorter wavelengths along the PBGs in all-dielectric 1D PhCs, which strongly limits the bandwidths of wide-angle absorption. In this paper, we realize a broadband omnidirectional OTS in a heterostructure composed of a Cr layer and a 1D PhC containing layered hyperbolic metamaterials with an angle-insensitive photonic band gap. Assisted by the broadband omnidirectional OTS, broadband wide-angle absorption can be achieved. High absorptance (A > 0.85) can be remained when the wavelength ranges from 1612 nm to 2335 nm and the incident angle ranges from 0° to 70°. The bandwidth of wide-angle absorption (0°-70°) reaches 723 nm. The designed absorber is a lithography-free 1D structure, which can be easily fabricated under the current magnetron sputtering or electron-beam vacuum deposition technique. This broadband, wide-angle, and lithography-free absorber would possess potential applications in the design of photodetectors, solar thermophotovoltaic devices, gas analyzers, and cloaking devices.
63 citations
TL;DR: In this article, a polarization-independent and angle-insensitive broadband THz graphene metamaterial absorber based on the surface plasmon-polaritons resonance was designed.
Abstract: In this paper, we design a polarization-independent and angle-insensitive broadband THz graphene metamaterial absorber based on the surface plasmon-polaritons resonance. Full-wave simulation is conducted, and the results show that the designed metamaterial absorber has an absorption above 99% in the frequency range from 1.23 THz to 1.68 THz, which refers to a very high standard. Furthermore, the absorber has the properties of tunability, and the absorption can be nearly adjusted from 1% to 99% by varying the Fermi energy level of the graphene from 0 eV to 0.7 eV. In the simulation, when the incident angles of TE and TM waves change from 0° to 60°, the average absorption keeps greater than 80%. The proposed absorber shows promising performance, which has potential applications in developing graphene-based terahertz energy harvesting and thermal emission.
60 citations
TL;DR: In this article, a low fiber-chip optical coupler based on thin-film lithium niobate on insulator (LNOI) is presented. But the coupler is not suitable for high optical power and temperature variation.
Abstract: Lithium niobate (LN) devices have been widely used in optical communication and nonlinear optics due to its attractive optical properties. The emergence of the thin-film lithium niobate on insulator (LNOI) improves performances of LN-based devices greatly. However, a high-efficient fiber-chip optical coupler is still necessary for the LNOI-based devices for practical applications. In this paper, we demonstrate a highly efficient and polarization-independent edge coupler based on LNOI. The coupler, fabricated by a standard semiconductor process, shows a low fiber-chip coupling loss of 0.54 dB/0.59 dB per facet at 1550 nm for TE/TM light, respectively, when coupled with an ultra-high numerical aperture fiber (UHNAF) of which the mode field diameter is about 3.2 μm. The coupling loss is lower than 1dB/facet for both TE and TM light in the wavelength range of 1527 nm to 1630 nm. A relatively large tolerance for optical misalignment is also proved, due to the coupler's large mode spot size up to 3.2 μm. The coupler shows a promising stability in high optical power and temperature variation.
59 citations
TL;DR: By monitoring the real-time temperature, the accuracy of glucose concentration detection was improved and the proposed dual channel SPR optical fiber sensor has a simple and compact structure, and it is suitable for sensing glucose solution or other analyte solutions that need temperature compensation.
Abstract: The accuracy of the surface plasmon resonance (SPR) optical fiber sensor is affected by the change of ambient temperature Therefore, we propose a simple dual channel SPR optical fiber sensor, which can measure both glucose concentration and ambient temperature The proposed sensor is a two-channel structure based on a no-core optical fiber (NCF): one channel is coated with gold film and polydimethylsiloxane (PDMS) to sense the ambient temperature, and the other is coated with silver film to sense glucose concentration The experimental results show that the sensor's sensitivity for sensing glucose concentration is 2882 nm / %, and for sensing temperature is -2904 nm / °C By monitoring the real-time temperature, the accuracy of glucose concentration detection was improved The proposed sensor has a simple and compact structure, and it is suitable for sensing glucose solution or other analyte solutions that need temperature compensation
59 citations
TL;DR: In this article, a U-bent single-mode fiber fixed in a magnetic-fluid-filled vessel is used to measure the direction and intensity of magnetic field via whispering gallery modes supported by the Ubent fiber structure with suitable bending radius, and the results show the feasibility for realizing vector magnetic sensors based on other bending structures (such as bending multimode interference, bending SPR structure) in the future.
Abstract: A novel, compact, and easy fabrication vector magnetic field sensor has been proposed and investigated. The proposed sensor consists of a U-bent single-mode fiber fixed in a magnetic-fluid-filled vessel. Neither mechanical modification nor additional fiber grating is needed during the sensor fabrication. The results show that the response of magnetic fluid to magnetic field can be used to measure the direction and intensity of magnetic field via whispering gallery modes supported by the U-bent fiber structure with suitable bending radius. The sensitivity of direction is 0.251 nm/°, and the maximum magnetic field intensity sensitivity is 0.517 nm/mT. Besides, the results of this work prove the feasibility for realizing vector magnetic sensors based on other bending structures (such as bending multimode interference, bending SPR structure) in the future.
TL;DR: This work presents an efficient polarization conversion device via using a hollow graphene metasurface that can simultaneously realize a series of excellent performances, including the broadband x-to-y cross polarization conversion (CPC) function with near unity polarization conversion ratio (PCR), dual-frequency linear- to-circular polarization Conversion (LTC) function, and highly sensitive polarization conversion function manipulation under wide oblique incidence angle range.
Abstract: In this work, we present an efficient polarization conversion device via using a hollow graphene metasurface. The platform can simultaneously realize a series of excellent performances, including the broadband x-to-y cross polarization conversion (CPC) function with near unity polarization conversion ratio (PCR), dual-frequency linear-to-circular polarization conversion (LTC-PC) function, and highly sensitive polarization conversion function manipulation under wide oblique incidence angle range. For instance, the proposed device obtains an x-to-y CPC function with the bandwidth up to 1.83 THz (χPCR ≥98.8%). Moreover, the x-to-y CPC function can be switched to LTC-PC function via artificially tuning the Fermi energy of graphene. The maximal frequency shift sensitivity (S) of polarization conversion function reaches 23.09 THz/eV, suggesting a frequency shift of 2.309 THz for the LTC-PC function when the chemical potential is changed by 0.1 eV. Based on these superior performances, the polarization converter can hold potential applications in integrated and compact devices, such as polarization sensor, switches and other optical polarization control components.
TL;DR: In this paper, a single-beam atomic magnetometer (AM) capable of measuring a three-axis magnetic field with high-sensitivity, achieved by applying a small DC offset field and a high frequency modulation field.
Abstract: We demonstrate a single-beam atomic magnetometer (AM) capable of measuring a three-axis magnetic field with high-sensitivity, achieved by applying a small DC offset field and a high frequency modulation field. To satisfy the miniaturization demand of AMs, an elliptically polarized light detuned by 50 GHz from the resonance transition center is employed. The circularly polarized component is used to polarize the alkali-metal atoms, while the linearly polarized light is used to detect the dynamics of the polarized spin under a magnetic field. Based on theoretical analysis, parameters that significantly affect the performance are optimized, and a sensitivity of 20 fT/Hz1/2 in x-axis, 25 fT/Hz1/2 in y-axis, 30 fT/Hz1/2 in z-axis is achieved with a miniature 4 × 4 × 4 mm 87Rb vapor cell. Moreover, we also verify that the operation principle of AMs can be used to null background magnetic fields in-situ with isotropic compensation resolution of 6.7 pT, which provides an effectively precise method for zeroing ambient magnetic field. The high-sensitivity operating of an elliptically-polarized-laser-based magnetometer provides prospective futures for constructing a compact, low-cost AM, which is particularly applicable for non-invasive bio-magnetic imaging such as array-based magnetoencephalography (MEG) and magnetocardiography (MCG).
TL;DR: In this article, a novel all-dielectric metasurface made of arrayed circular slots etched in a silicon layer is proposed and theoretically investigated, which is designed to support both Mie-type multipolar resonances and symmetry-protected bound states in the continuum (BIC).
Abstract: In this work, a novel all-dielectric metasurface made of arrayed circular slots etched in a silicon layer is proposed and theoretically investigated. The structure is designed to support both Mie-type multipolar resonances and symmetry-protected bound states in the continuum (BIC). Specifically, the metasurface consists of interrupted circular slots, following the paradigm of complementary split-ring resonators. This configuration allows both silicon-on-glass and free-standing metasurfaces and the arc length of the split-rings provides an extra tuning parameter. The nature of both BIC and non-BIC resonances supported by the metasurface is investigated by employing the Cartesian multipole decomposition technique. Thanks to the non-radiating nature of the quasi-BIC resonance, extremely high Q-factor responses are calculated, both by fitting the simulated transmittance spectra to an extended Fano model and by an eigenfrequency analysis. Furthermore, the effect of optical losses in silicon on quenching the achievable Q-factor values is discussed. The metasurface features a simple bulk geometry and sub-wavelength dimensions. This novel device, its high Q-factors, and strong energy confinement open new avenues of research on light-matter interactions in view of new applications in non-linear devices, biological sensors, and optical communications.
TL;DR: In this paper, the existence, bifurcation and stability of two-dimensional optical solitons in the framework of fractional nonlinear Schrodinger equation, characterized by its Levy index, with self-focusing and self-defocusing saturable nonlinearities was studied.
Abstract: We study existence, bifurcation and stability of two-dimensional optical solitons in the framework of fractional nonlinear Schrodinger equation, characterized by its Levy index, with self-focusing and self-defocusing saturable nonlinearities. We demonstrate that the fractional diffraction system with different Levy indexes, combined with saturable nonlinearity, supports two-dimensional symmetric, antisymmetric and asymmetric solitons, where the asymmetric solitons emerge by way of symmetry breaking bifurcation. Different scenarios of bifurcations emerge with the change of stability: the branches of asymmetric solitons split off the branches of unstable symmetric solitons with the increase of soliton power and form a supercritical type bifurcation for self-focusing saturable nonlinearity; the branches of asymmetric solitons bifurcates from the branches of unstable antisymmetric solitons for self-defocusing saturable nonlinearity, featuring a convex shape of the bifurcation loops: an antisymmetric soliton loses its stability via a supercritical bifurcation, which is followed by a reverse bifurcation that restores the stability of the symmetric soliton. Furthermore, we found a scheme of restoration or destruction the symmetry of the antisymmetric solitons by controlling the fractional diffraction in the case of self-defocusing saturable nonlinearity.
TL;DR: In this article, a novel fiber Michelson interferometer (FMI) based on parallel dual polarization maintaining fiber Sagnac interferometers (PMF-SIs) is proposed and experimentally demonstrated for temperature sensing.
Abstract: A novel fiber Michelson interferometer (FMI) based on parallel dual polarization maintaining fiber Sagnac interferometers (PMF-SIs) is proposed and experimentally demonstrated for temperature sensing. The free spectral range (FSR) difference of dual PMF-SIs determines the FSR of envelope and sensitivity of the sensor. The temperature sensitivity of parallel dual PMF-SIs is greatly enhanced by the Vernier effect. Experimental results show that the temperature sensitivity of the proposed sensor is improved from -1.646 nm/°C (single PMF-SI) to 78.984 nm/°C (parallel dual PMF-SIs), with a magnification factor of 47.99, and the temperature resolution is improved from ±0.03037°C to ±0.00063°C by optimizing the FSR difference between the two PMF-SIs. Our proposed ultrasensitive temperature sensor is with easy fabrication, low cost and simple configuration which can be implemented for various real applications that need high precision temperature measurement.
TL;DR: In this article, a bi-directional gated recurrent unit neural network based nonlinear equalizer (bi-GRU NLE) was proposed for coherent optical communication systems with a transmission distance of 375 km.
Abstract: We propose a bi-directional gated recurrent unit neural network based nonlinear equalizer (bi-GRU NLE) for coherent optical communication systems The performance of bi-GRU NLE has been experimentally demonstrated in a 120 Gb/s 64-quadrature amplitude modulation (64-QAM) coherent optical communication system with a transmission distance of 375 km Experimental results show that the proposed bi-GRU NLE can significantly mitigate nonlinear distortions The Q-factors can exceed the hard-decision forward error correction (HD-FEC) limit of 852 dB with the aid of bi-GRU NLE, when the launched optical power is in the range of -3 dBm to 3 dBm In addition, when the launched optical power is in the range of 0 dBm to 2 dBm, the Q-factor performances of the bi-GRU NLE and bi-directional long short-term memory neural network based nonlinear equalizer (bi-LSTM NLE) are similar, while the number of parameters of bi-GRU NLE is about 202% less than that of bi-LSTM NLE, the average training time of bi-GRU NLE is shorter than that of bi-LSTM NLE, the number of multiplications required for the bi-GRU NLE to equalize per symbol is about 245% less than that for bi-LSTM NLE
TL;DR: In this paper, a trace gas detection technique of quartzenhanced photoacoustic-photothermal spectroscopy (QEPA-PTS) is demonstrated, where water vapor (H2O) with a volume concentration of 1.01% was selected as the analyte gas to investigate the sensor performance.
Abstract: A trace gas detection technique of quartz-enhanced photoacoustic-photothermal spectroscopy (QEPA-PTS) is demonstrated. Different from quartz-enhanced photoacoustic spectroscopy (QEPAS) or quartz-enhanced photothermal spectroscopy (QEPTS), which detected only one single kind of signal, QEPA-PTS was realized by adding the photoacoustic and photothermal signals generated from two quartz tuning forks (QTFs), respectively. Water vapor (H2O) with a volume concentration of 1.01% was selected as the analyte gas to investigate the QEPA-PTS sensor performance. Compared to QEPAS and QEPTS, an enhanced signal level was achieved for this QEPA-PTS system. Further improvement of such a technique was proposed.
TL;DR: In this paper, an absorber based on hybrid metamaterial with vanadium dioxide and graphene has been proposed to achieve dynamically switchable dual-broadband absorption property in the terahertz regime.
Abstract: An absorber based on hybrid metamaterial with vanadium dioxide and graphene has been proposed to achieve dynamically switchable dual-broadband absorption property in the terahertz regime. Due to the phase transition of vanadium dioxide and the electrical tunable property of graphene, the dynamically switchable dual-broadband absorption property is implemented. When the vanadium dioxide is in the metallic phase, the Fermi energy level of graphene is set as zero simultaneously, the high-frequency broadband from 2.05 THz to 4.30 THz can be achieved with the absorptance more than 90%. The tunable absorptance can be realized through thermal control on the conductivity of the vanadium dioxide. The proposed device acts as a low-frequency broadband absorber if the vanadium dioxide is in the insulating phase, for which the Fermi energy level of graphene varies from to 0.1 eV to 0.7 eV. The low-frequency broadband possesses high absorptance which is maintained above 90% from 1.10 THz to 2.30 THz. The absorption intensity can be continuously adjusted from 5.2% to 99.8% by electrically controlling the Fermi energy level of graphene. The absorption window can be further broadened by adjusting the geometrical parameters. Furthermore, the influence of incidence angle on the absorption spectra has been investigated. The proposed absorber has potential applications in the terahertz regime, such as filtering, sensing, cloaking objects, and switches.
TL;DR: In this article, a hydrothermal method for preparing vertical MoS2 nanosheets composed of in situ grown AuNPs and chemically reduced AgNPs with large size to achieve the synergistic enhancement of surface enhanced Raman scattering (SERS) and photoelectrocatalytic (PEC) performance was presented.
Abstract: MoS2-based heterostructures have received increasing attention for not only surface-enhanced Raman scattering (SERS) but also for enhanced photoelectrocatalytic (PEC) performance. This study presents a hydrothermal method for preparing vertical MoS2 nanosheets composed of in situ grown AuNPs with small size and chemically reduced AgNPs with large size to achieve the synergistic enhancement of SERS and PEC properties owing to the size effect of the plasmonic structure. Compared with pristine MoS2 nanosheets and unitary AuNPs or AgNPs composited with MoS2 nanosheets, the ternary heterostructure exhibited the strongest electromagnetic field and surface plasmon coupling, which was confirmed by finite-difference time-domain (FDTD) simulation and absorption spectra. In addition, the experimental results confirmed the outstanding SERS enhancement with an EF of 1.1×109, and the most efficient hydrogen evolution reaction (HER) activity with a sensitive photocurrent response, attributing to the multiple surface plasmonic coupling effects of the Au-Ag bimetal and efficient charge-transfer process between MoS2 and the bimetal. That is, it provides a robust method for developing multi-size bimetal-semiconductor complex nanocomposites for high-performance SERS sensors and PEC applications.
TL;DR: A new method to impose minimum-area and minimum-enclosed-area constraints, and simultaneously adapt previous techniques for minimum linewidth, linespacing, and curvature is introduced, all of which are implemented without any additional re-parameterizations.
Abstract: We present a unified density-based topology-optimization framework that yields integrated photonic designs optimized for manufacturing constraints including all those of commercial semiconductor foundries. We introduce a new method to impose minimum-area and minimum-enclosed-area constraints, and simultaneously adapt previous techniques for minimum linewidth, linespacing, and curvature, all of which are implemented without any additional re-parameterizations. Furthermore, we show how differentiable morphological transforms can be used to produce devices that are robust to over/under-etching while also satisfying manufacturing constraints. We demonstrate our methodology by designing three broadband silicon-photonics devices for nine different foundry-constraint combinations.
TL;DR: In this paper, the authors investigated the skin effect in a ring resonator array which can be mapped into the square root of a SSH model with non-Hermitian asymmetric coupling.
Abstract: We investigate the topological skin effect in a ring resonator array which can be mapped into the square root of a Su-Schrieffer-Heeger (SSH) model with non-Hermitian asymmetric coupling. The asymmetric coupling is realized by integrating the same amount of gain and loss into the two half perimeters of linking rings that effectively couple two adjacent site rings. Such a square-root topological insulator inherits the properties from its parent Hamiltonian, which has the same phase transition points and exhibits non-Bloch features as well. We show the band closing points for open chain are different from that of periodic chain as a result of the skin effect. Moreover, the square-root insulator supports multiple topological edge modes as the number of band gaps is doubled compared to the original Hamiltonian. The full-wave simulations agree well with the theoretical analyses based on a tight-binding model. The study provides a promising approach to investigate the skin effect by utilizing ring resonators and may find potential applications in light trapping, lasers, and filters.
TL;DR: In this article, a distributed optical fiber hydrophone (DOFH) based on heterodyne coherent detection (Het Φ-OTDR) is demonstrated and tested in the field.
Abstract: In this letter, a distributed optical fiber hydrophone (DOFH) based on Φ-OTDR is demonstrated and tested in the field. The specially designed sensitized optical cable with sensitivity up to −146 dB rad/µPa/m is introduced, and an array signal processing model for DOFH is constructed to analyze the equivalence and specificity of the distributed array of acoustic sensors. In the field test, a 104-meter-long optical cable and a Φ-OTDR system based on heterodyne coherent detection (Het Φ-OTDR) is utilized, and underwater acoustic signal spatial spectrum estimation, beamforming and motion trajectory tracking with high accuracy can be realized. As far as we know, this is the first report on the field trial of DOFH based on Φ-OTDR. The DOFH has the potential to achieve an array range of tens of kilometers, with elements spaced up to the meter level and flexible configuration, which has a broad application prospect for marine acoustic detection.
TL;DR: In this article, a mono-layer metamaterial comprising four graphenestrips and one graphene-square ring is proposed to realize triple plasmon-induced transparency (PIT), and theoretical results based on the coupled mode theory are in agreement with the simulation results obtained using the finite-difference time-domain (FDTD).
Abstract: A mono-layer metamaterial comprising four graphene-strips and one graphene-square-ring is proposed herein to realize triple plasmon-induced transparency (PIT). Theoretical results based on the coupled mode theory (CMT) are in agreement with the simulation results obtained using the finite-difference time-domain (FDTD). An optical switch is investigated based on the characteristics of graphene dynamic modulation, with modulation degrees of the amplitude of 90.1%, 80.1%, 94.5%, and 84.7% corresponding to 1.905 THz, 2.455 THz, 3.131 THz, and 4.923 THz, respectively. Moreover, the proposed metamaterial is insensitive to the change in the angle of polarized light, for which the triple-PIT is equivalent in the cases of both x- and y-polarized light. The optical switch based on the proposed structure is effective not only for the linearly polarized light in different directions but also for left circularly polarized and right circularly polarized light. As such, this work provides insight into the design of optoelectronic devices based on the polarization characteristics of the incident light field on the optical switch and PIT.
TL;DR: In this article, an adaptive weighted Gerchberg-Saxton (GS) algorithm is proposed to solve the issue of iteration divergence by replacing the conventional feedback with the designed feedback, the convergence can be ensured in the proposed method.
Abstract: In the conventional weighted Gerchberg-Saxton (GS) algorithm, the feedback is used to accelerate the convergence. However, it will lead to the iteration divergence. To solve this issue, an adaptive weighted GS algorithm is proposed in this paper. By replacing the conventional feedback with our designed feedback, the convergence can be ensured in the proposed method. Compared with the traditional GS iteration method, the proposed method improves the peak signal-noise ratio of the reconstructed image with 4.8 dB on average. Moreover, an approximate quadratic phase is proposed to suppress the artifacts in optical reconstruction. Therefore, a high-quality image can be reconstructed without the artifacts in our designed Argument Reality device. Both numerical simulations and optical experiments have validated the effectiveness of the proposed method.
TL;DR: In this article, a microheater platform that employs optically transparent and electrically conductive indium-tin-oxide (ITO) bridges for the fast and reversible electrical switching of the GST phase between crystalline and amorphous states is introduced.
Abstract: Inducing a large refractive-index change is the holy grail of reconfigurable photonic structures, a goal that has long been the driving force behind the discovery of new optical material platforms Recently, the unprecedentedly large refractive-index contrast between the amorphous and crystalline states of Ge-Sb-Te (GST)-based phase-change materials (PCMs) has attracted tremendous attention for reconfigurable integrated nanophotonics Here, we introduce a microheater platform that employs optically transparent and electrically conductive indium-tin-oxide (ITO) bridges for the fast and reversible electrical switching of the GST phase between crystalline and amorphous states By the proper assignment of electrical pulses applied to the ITO microheater, we show that our platform allows for the registration of virtually any intermediate crystalline state into the GST film integrated on the top of the designed microheaters More importantly, we demonstrate the full reversibility of the GST phase between amorphous and crystalline states To show the feasibility of using this hybrid GST/ITO platform for miniaturized integrated nanophotonic structures, we integrate our designed microheaters into the arms of a Mach-Zehnder interferometer to realize electrically reconfigurable optical phase shifters with orders of magnitude smaller footprints compared to existing integrated photonic architectures We show that the phase of optical signals can be gradually shifted in multiple intermediate states using a structure that can potentially be smaller than a single wavelength We believe that our study showcases the possibility of forming a whole new class of miniaturized reconfigurable integrated nanophotonics using beyond-binary reconfiguration of optical functionalities in hybrid PCM-photonic devices
TL;DR: In this paper, a high-sensitivity fiber-optic relative humidity (RH) sensor based on sensitivity amplification and a reduction mechanism, employing an internal-external Fabry-Perot cavity (IEFPC) Vernier effect and a chitosan film as a sensing cavity, was constructed using cascaded FP interferometers.
Abstract: This study experimentally demonstrates a high-sensitivity fiber-optic relative humidity (RH) sensor based on sensitivity amplification and a reduction mechanism, employing an internal–external Fabry–Perot cavity (IEFPC) Vernier effect and a chitosan film as a Fabry–Perot (FP)-sensing cavity. The proposed sensor is constructed using cascaded FP interferometers comprised of an air cavity formed by a hollow-core fiber (HCF), a chitosan cavity, and an air–chitosan hybrid cavity. The chitosan cavity is fabricated by dipping the HCF into a chitosan solution to form a thin chitosan film. Thus, the thickness of the chitosan film could be controlled precisely based on dipping time and capillary effect. As the optical path lengths of an air–chitosan hybrid cavity and an air cavity are similar, the IEFPC Vernier effect is generated, amplifying the air–chitosan hybrid cavity’s low sensitivity to the chitosan cavity’s high sensitivity. The experimental results agree with the theoretical analysis, supporting the fact that the sensor’s sensitivity is related only to the thickness of the chitosan film. The sensitivity of the sensor reaches up to 7.15 nm/% RH, ranging 40%–92% RH at 25°C. Fabrication of the proposed sensor is cost-effective. The proposed sensor also exhibits superior stability performance, a low-temperature cross-sensitivity of 0.0068% RH/°C, and repeatable fabrication. The proposed IEFPC Vernier effect model functions well for cascaded cavities, which plays a guiding role in the sensitivity improvement of such a structure within a fiber-optic sensing context.
TL;DR: In this article, a femtosecond laser point-by-point (PbP) technology was used to construct high-temperature-resistant UWFBG arrays.
Abstract: Ultra-weak fiber Bragg grating (UWFBG) arrays are key elements for constructing large-scale quasi-distributed sensing networks for structural health monitoring. Conventional methods for creating UWFBG arrays are based on in-line UV exposure during fiber drawing. However, the UV-induced UWFBG arrays cannot withstand a high temperature above 450 °C. Here, we report for the first time, to the best of our knowledge, a new method for fabricating high-temperature-resistant UWFBG arrays by using a femtosecond laser point-by-point (PbP) technology. UWFBGs with a low peak reflectivity of ∼ - 45 dB (corresponding to ∼ 0.0032%) were successfully fabricated in a conventional single-mode fiber (SMF) by femtosecond laser PbP inscription through fiber coating. Moreover, the influences of grating length, laser pulse energy, and grating order on the UWFBGs were studied, and a grating length of 1 mm, a pulse energy of 29.2 nJ, and a grating order of 120 were used for fabricating the UWFBGs. And then, a long-term high-temperature annealing was carried out, and the results show that the UWFBGs can withstand a high temperature of 1000 °C and have an excellent thermal repeatability with a sensitivity of 18.2 pm/°C at 1000 °C. A UWFBG array consisting of 200 identical UWFBGs was successfully fabricated along a 2 m-long conventional SMF with an interval of 10 mm, and interrogated with an optical frequency domain reflectometer (OFDR). Distributed high-temperature sensing up to 1000 °C was demonstrated by using the fabricated UWFBG array and OFDR demodulation. As such, the proposed femtosecond laser-inscribed UWFBG array is promising for distributed high-temperature sensing in hash environments, such as aerospace vehicles, nuclear plants, and smelting furnaces.
TL;DR: In this article, an upper bound on the amount of quantum enhancement that is possible in a two-photon transition system was derived using a quantum-theoretic derivation of entangled TPA (ETPA) and the derived bounds indicated that in order to observe ETPA the experiments would need to operate at a combination of significantly higher rates of EPP illumination, molecular concentrations, and conventional TPA cross sections than are achieved in typical experiments.
Abstract: When a low flux of time-frequency-entangled photon pairs (EPP) illuminates a two-photon transition, the rate of two-photon absorption (TPA) can be enhanced considerably by the quantum nature of photon number correlations and frequency correlations. We use a quantum-theoretic derivation of entangled TPA (ETPA) and calculate an upper bound on the amount of quantum enhancement that is possible in such systems. The derived bounds indicate that in order to observe ETPA the experiments would need to operate at a combination of significantly higher rates of EPP illumination, molecular concentrations, and conventional TPA cross sections than are achieved in typical experiments.
TL;DR: In this paper, a probabilistic shaping (PS) based constellation encryption scheme is proposed in which two bit-level encryption operations are firstly performed according to chaotic sequences and hash values.
Abstract: The physical layer security of radio-over-fiber (RoF) system is a very important problem for future communication. In this paper, a novel probabilistic shaping (PS) based constellation encryption scheme is proposed in which two bit-level encryption operations are firstly performed according to chaotic sequences and hash values. The chaotic sequences are generated by hyperchaotic system and hash values are obtained by SHA-512. Then PS is applied to enhance transmission performance. After PS-16-quadrature amplitude modulation (QAM), constellation encryption is implemented aiming at maintaining overall shaping distribution unchanged and improving security. An encrypted PS-16-QAM orthogonal frequency division multiplexing (OFDM) signal is successfully transmitted over 50 km standard single-mode fiber (SSMF) and 5 m wireless channel in our experiment. The results demonstrate that the key space of 10121 is achieved to defend malicious attacks. Moreover, the proposed PS-based encryption scheme can obtain approximately 2.4 dB gain at a BER of 10-3 compared with traditional OFDM signal. Thus, the proposed scheme has a good application prospect in the future OFDM-RoF system due to the dominant BER and security performance.