查看更多>>摘要：? 2021 Elsevier B.V.We investigate the topological Bloch–Zener oscillations for surface plasmon polaritons (SPPs) propagating in a Su–Schrieffer–Heeger plasmonic system, which composed of graphene dimer arrays with a potential gradient. The topological trivial and non-trivial band gaps are achieved to investigate the topological effect to plasmonic Bloch oscillations. The topological transition from trivial phase to non-trivial one through the exceptional point is realized by alternating the gain and loss in the graphene waveguide arrays. Zener tunneling of the SPP beam was observed in the parity-time (PT) symmetric region, which was prohibited in the PT-symmetric broken region. The numerical calculations demonstrated reasonable agreements with theoretical tight-binding model. Furthermore, a single dynamically stable zero-energy edge state was also found in these non-Hermitian plasmonic waveguide arrays.
查看更多>>摘要：? 2021 Elsevier B.V.Self-injection locking is an efficient technique to lock the frequency of laser at the resonant frequency of external cavity, which can be used for the all-optical frequency locking of the resonant optical gyroscope instead of the electric frequency locking. However, this technique is very sensitive to the system parameters and environmental disturbances. In this paper, we theoretically and experimentally analyze the dependences of injection ratio, intimal phase delay, operating temperature of distributed feedback (DFB) laser, and the environment temperature disturbance on the stability of self-injection locking. With the DFB laser self-injection locking to a high-Q fiber optic resonator (FOR), the frequency stability achieves less than 100 kHz in 30 min without active electrical feedback.
查看更多>>摘要：? 2021 Elsevier B.V.We demonstrate both theoretically and experimentally that the broadband THz pulses emitted by air plasma can be tuned in the spectrum domain by employing pump pulses composed of a fundamental femtosecond pulse and its detuned second harmonic (SH). Based on the plasma photocurrent model, our numerical simulations show that the central frequency of the broadband THz emission can be shifted from ～4 THz to much higher frequency (～38 THz) by red-shifting the second harmonic pulse. The relative phase of the two optical frequencies is found to present a critical role on the yield and spectrum of the THz emission. Experimentally, we obtained detuned second harmonic of the fundamental 800 nm femtosecond pulse by tilting the second harmonic generation (SHG) nonlinear crystal and achieved a tuning range from 391 nm to 403 nm. The central frequency of the corresponding THz emission was observed to shift from 4 THz to higher frequency side, in agreement with the simulation. This method therefore provides a simple mean to tune the spectrum of the broadband THz obtained from air plasma. We believe that even larger tunability can be achieved with a wavelength tunable Optical Parametric Amplification (OPA) driving laser.
查看更多>>摘要：? 2021 The AuthorsIn this paper, we proposed an ultra-high amplitude-sensitive multi-slot sub-wavelength Bragg grating (MS-SW BG) waveguide sensor on silicon on insulator (SOI) platform with a thick silicon waveguide (WG) and analyzed the design and performances. The MS-SW BG sensor with a thick waveguide supports multi-transverse modes for the first and second modes, whose stopbands (SBs) have different center wavelengths and bandwidths to form a mid-transmission band (MTB) between these SBs. By adjusting sensor parameters, the MTB can be made sharp and steep, and these spectral profiles shift in response to a refractive index change in the surrounding medium. Due to the sharp spectral feature of the MTB, ultra-high amplitude-sensitive performance can be realized. The sensor length is as short as about 10 μm for a grating period count of 21. With an additional increase of the period count the transmission peak count in the MTB region increases, and the shortest wavelength peak among them exhibits steeper slope which is suitable for higher sensitivity. The appearance of the MTB is explained by coupled mode theory and band structure approach. The structural design is investigated from viewpoints of WG thickness, WG width, duty ratio of the grating and so on to demonstrate an ultra-high amplitude sensitivity of 5000 RIU?1 with transmission loss limit up to 3 dB.
查看更多>>摘要：? 2021 Elsevier B.V.Three novel refractive index (RI) sensors based on one, two, and three H-shaped cavities (HCs) and metal–insulator–metal (MIM) waveguides are proposed in this paper. By coupling more than one HC (two and three), the Q-factor and then the FOM value of the designed RI sensors increase. A well-known method for increasing the sharpness transition between the passband and stopband of a spectrum is coupling cavities together. Fortunately, the proposed HC has such a property that increases the FOM value. The maximum obtained FOM of the proposed sensors using one, two, and three HCs reached 69.5, 100.19, and 108.36 RIU-1, respectively. Three proposed plasmonic structures which are used to design RI sensors have single-mode spectra in a wide wavelength range of 500 to 2000 nm. A single-mode spectrum is easier to cope with in circuits with higher complexity such as RI sensors. The metal and insulator materials that are used in the designed structures are silver and air, respectively. The finite-difference time-domain (FDTD) method is used for the numerical investigation of the proposed RI sensors. To verify FDTD simulations, the basic rectangular cavity which is used to design HC is analyzed using an analytical method. In summary, the benefits of the proposed RI sensors are having single-mode spectra in a wide wavelength range and also their simple structures. The proposed topology easily be redesigned and adapted with different numbers of HCs without disturbing its single-mode spectrum to achieve various FOM values. Furthermore, considering the fact that the proposed cavity is based on rectangular shapes, it has a simple fabrication process. Taking into account the mentioned advantages of the proposed structures, they can be used in integrated optical circuits for optical communication purposes.
查看更多>>摘要：? 2021 The Author(s)The coherent superposition of orthogonal modes can result in phase dependent transverse offsets, shifts of the focus position, variations of the Rayleigh length and a reduction of the beam quality factor of the coherent sum of modes in comparison to the incoherent sum. Relations for first and second order moments, the beam quality and the Rayleigh length for the superposition of Hermite–Gauss modes are derived. The Courant–Snyder formalism, which was originally developed in the context of charged particle optics, is applied to propagate an arbitrary coherent sum of orthogonal modes through a lens system. Relations of generating and observable optical functions are highlighted. In the last part of the report the elegant Hermite–Gauss solution is interpreted in terms of generating and observable functions and the solution is decomposed into a sum of standard Hermite–Gauss modes.
Andre P.S.Silverio T.Figueiredo G.Ferreira R.A.S....
6页
查看更多>>摘要：? 2021 Elsevier B.V.In contemporary society, secure communications employing chaotic communication schemes have opened new challenges. Chaotic communication schemes in which signals are spectrally spread, with low power spectral density, become useful to improve resilience against multipath fading, making them increasingly important for indoor applications in the framework of visible light communication (VLC). Here, we will explore the 16-QAM orthogonal shift key in combination with Walsh coded chaotic basis for application of a secure VLC link, used simultaneously for illumination and security-enhanced data transmission. The experimental results are carried out for a 12 Baud symbol rate, indicating a symbol error rate below the FEC hard threshold limit for an optical irradiance higher than 25 mW/cm 2, covering a 1.4 m2 operating area.
查看更多>>摘要：? 2021 Elsevier B.V.Active metasurface provides an efficient way to achieve optical response in the subwavelength range, dielectric metasurface has attracted much attention due to its low-loss mode and excitable electric/magnetic resonance mode, resulting in a far wider range of applications. However, the resonance spectrum is relatively broad due to the strong radiative loss of the symmetric dielectric metasurface, which limits its application in large modulation extinction ratio. Hence, an active metasurface integrated with the phase change material Ge2Sb2Te5 (GST) is proposed. The active metasurface can support the symmetry-protected quasi bound states in the continuum (QBIC) and excite resonance modes with extremely high quality-factors. As an active medium, the GST layer undergoes a transition from the amorphous state to the crystalline state when the temperature increases, leading to a change in the amplitude/phase of the reflection spectrum. It is demonstrated that dual reflection modulation and the figure-of-merit (FoM) can reach up to 92.0% at the wavelength of 1.45 μm and 92.5% at the wavelength of 1.52 μm as the GST layer is in the middle of nanodisks. An extremely high FoM of 98.5% is also realized when the surface of silicon nanodisks is coated with the GST layer. In addition, the modulation mechanism of the optical response of the active metasurface has been investigated, which is of great significance to the design of the active metasurface. The proposed active all-dielectric asymmetric metasurface has a huge potential in tunable nonlinear optical devices.
查看更多>>摘要：? 2021 Elsevier B.V.The goal of this research is recording of volume holograms in thick photopolymer material, based on the use of forbidden singlet–triplet transitions of the dye-sensitizer with high intensity (up to 12 GW cm?2) nanosecond pulses at λ=532nm. The material has threshold character of recording, that can lead to improved three-dimensional (3D) control of photochemical or photophysical processes. This is especially important for the microholographic approach to 3D multilayer optical data storage. The concentrations of the dye-sensitizer and electron donor in the photopolymer was optimized, and high value of refractive index change of 3.1?10?3 was achieved. The sensitivity of the material under research is much higher than the sensitivity of materials using two-photon absorption, which made it possible to record microholograms by a single laser pulse. Using the method of heterodyne detection, we also studied the spatial distribution of the optical inhomogeneity in photopolymer material.
查看更多>>摘要：? 2021 Elsevier B.V.Coherent diffraction imaging (CDI) has emerged as a thriving field promising applications in materials and biological sciences. Transmission geometry is adopted in most CDI methods, which is not suitable for opaque structures or objects of interest comprising only surfaces or interfaces. Here, we present a reflective CDI system using binary random sampling patterns, which extends the application range to bulk and opaque samples. Instead of resorting to an array of lenslets in Shack–Hartmann wavefront sensing or a reference beam in interferometry, the spatial information is captured by illuminating the object with four patterns generated with a digital micromirror device (DMD). On this basis, an adaptive algorithm for updating the support constraints in the iteration process is proposed to solve the problem of pattern deformation caused by the height variations of the sample surface. Our method achieves high-speed modulation, high-accuracy and high-fidelity quantitative phase imaging (QPI). The effectiveness of the proposed method is demonstrated by both simulated and real experiments.
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Elsevier