Temporally-topological defect modes in photonic time crystals.

In this paper, we investigate the properties of temporally-topological defect modes (TTDMs) (or temporally-topological interface states) in the topological photonic time crystal (PTC) systems. The PTC systems are constructed by the cascade of multiple sub-PTCs that possess temporal inversion symmetries and different topologies. The cases of two-, three-, and multiple-sub-PTC for the topological PTC system are studied. By transfer matrix method, we find that the TTDMs appear when the topological signs of the corresponding gaps in the sub-PTCs are different. The positions of TTDMs can be adjusted by changing the modulation strength of the refractive index, the time duration, and the period of the sub-PTCs. Moreover, the number of TTDMs is one less than the number of sub-PTCs. In addition, the robustness of the systems is also studied. We find that the topological PTC systems have good robustness, especially on the random configuration of the refractive index and time duration for the temporal slabs in the systems. Such research may provide a new degree of freedom for PTC applications, such as novel PTC lasers, tunable band-stop or band-suppression PTC filters, and many others, in the field of integrated photonic circuits for optical communications.

Superbound state in photonic bandgap and its application to generate complete tunable SBS-EIT, SBS-EIR and SBS-Fano.

Bound states in continua (BICs) have high-quality factors that may approach infinity. However, the wide-band continua in BICs are noise to the bound states, limiting their applications. Therefore, this study designed fully controlled superbound state (SBS) modes in the bandgap with ultra-high-quality factors approaching infinity. The operating mechanism of the SBS is based on the interference of the fields of two phase-opposite dipole sources. Quasi-SBSs can be obtained by breaking the cavity symmetry. The SBSs can also be used to produce high-Q Fano resonance and electromagnetically-induced-reflection-like modes. The line shapes and the quality factor values of these modes could be controlled separately. Our findings provide useful guidelines for the design and manufacture of compact and high-performance sensors, nonlinear effects, and optical switches.

Two Individual Super-Bound State Modes within Band Gap with Ultra-High Q Factor for Potential Sensing Applications in the Terahertz Wave Band.

Bound states in the continuum (BICs) garnered significant research interest in the field of sensors due to their exceptionally high-quality factors. However, the wide-band continuum in BICs are noise to the bound states, and it is difficult to control and filter. Therefore, we constructed a top-bottom symmetric cavity containing three high permittivity rectangular columns. The cavity supports a symmetry-protected (SP) superbound state (SBS) mode and an accidental (AC) SBS mode within the bandgap. With a period size of 5 × 15, the bandgap effectively filters out the continuum, allowing only the bound states to exist. This configuration enabled us to achieve a high signal-to-noise ratio and a wide free-spectral-range. The AC SBS and the SP SBS can be converted into quasi-SBS by adjusting different parameters. Consequently, the cavity can function as a single-band sensor or a dual-band sensor. The achieved bulk sensitivity was 38 µm/RIU in terahertz wave band, and a record-high FOM reached 2.8 × 108 RIU-1. The effect of fabrication error on the performance for sensor application was also discussed, showing that the application was feasible. Moreover, for experimental realization, a 3D schematic was presented. These achievements pave the way for compact, high-sensitivity biosensing, multi-wavelength sensing, and other promising applications.

Terahertz absorber with dynamically switchable dual-broadband based on a hybrid metamaterial with vanadium dioxide and graphene.

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.

Anisotropic asymmetric transmission of circularly polarized terahertz waves in a three-dimensional spline assembly.

Asymmetric transmission (AT) for circularly polarized (CP) electromagnetic (e-m) waves in chiral metamaterial (CMM) is a well-known phenomenon. However, most of the CMMs exhibit AT along only one direction. In this work, AT for CP waves with a magnitude of more than 0.5 along three principal directions of a newly made three-dimensional (3D) spline assembly is reported at terahertz frequencies. Surface current analysis is presented to explain the mechanism of AT for CP waves in the proposed 3D assembly.

High-speed amplitude modulator with a high modulation index based on a plasmonic resonant tunable metasurface.

High-speed optical amplitude modulation is important for optical communication systems and sensors. Moreover, nano-optical modulators are important for developing optical-communication-aided high-speed parallel-operation processors and micro-biomedical sensors for inside-blood-capillary examinations or microsurgery operations. In this paper, we have designed a plasmonic resonant tunable metasurface with barium titanate (BTO) as a nanoscale optical modulator with a high modulation index and high speed. The BTO operated well in the VIS and near-IR ranges, enabling tunable optical devices with zero dispersion and high speed. The results obtained by rigorous finite-element method simulations have shown that the hypothesized device has good potential for fast modulation in related applications, e.g., modulators in nano-optical systems, nano-optical switches and nanosensors.

Sapphire fiber Bragg gratings inscribed with a femtosecond laser line-by-line scanning technique.

We propose and demonstrate the fabrication of single-crystal sapphire fiber Bragg gratings (SFBGs) using a femtosecond laser line-by-line scanning technique. This approach provides a robust method for producing SFBGs at various Bragg wavelengths with an acceptable reflectivity. The spectrum characteristics of the SFBGs with various fiber diameters, track lengths, and grating pitch quantities were investigated. An SFBG with a reflectivity of 6.3% was obtained via optimization of fabrication parameters. Additionally, a serial array consisting of five SFBGs at different wavelengths was successfully constructed. The high-temperature response of these SFBGs was tested and the experimental results showed the SFBGs could withstand a high temperature of 1612°C. Moreover, a temperature sensitivity of 36.5 pm/°C was achieved in the high-temperature region. Such SFBGs could be developed for promising high-temperature sensors in aero engines.

Tunable Nanosensor Based on Fano Resonances Created by Changing the Deviation Angle of the Metal Core in a Plasmonic Cavity.

In this paper, a type of tunable plasmonic refractive index nanosensor based on Fano resonance is proposed and investigated. The sensor comprises a metal-insulator-metal (MIM) nanocavity with a center-deviated metal core and two side-coupled waveguides. By carefully adjusting the deviation angle and distance of the metal core in the cavity, Fano resonances can be obtained and modulated. The Fano resonances can be considered as results induced by the symmetry-breaking or geometric effect that affects the field distribution intensity at the coupling region between the right waveguide and the cavity. Such a field-distribution pattern change can be regarded as being caused by the interference between the waveguide modes and the cavity modes. The investigations demonstrate that the spectral positions and modulation depths of Fano resonances are highly sensitive to the deviation parameters. Furthermore, the figure of merit (FOM) value is calculated for different deviation angle. The result shows that this kind of tunable sensor has compact structure, high transmission, sharp Fano lineshape, and high sensitivity to the change in background refractive index. This work provides an effective method for flexibly tuning Fano resonance, which has wide applications in designing on-chip plasmonic nanosensors or other relevant devices, such as information modulators, optical filters, and ultra-fast switches.

Electro-optical coupling of a circular Airy beam in a uniaxial crystal.

This paper investigates theoretically and numerically on the electro-optical coupling (EOC) for a circular Airy beam (CAB) propagating along the optical axis of a uniaxial crystal after deducing the wave coupling equations of EOC. For a circularly polarized incident CAB, EOC can be used to generate vortex beam by coupling the incident left-handed component into the right-handed vortex component with a vortex topological charge of 2. What's more, EOC plays important role in enhancing or suppressing the abrupt autofocusing, the most important property of CABs, for both left-handed and right-handed components. Near the focal plane, EOC can result in electrically controllable optical "needle" and "cage", which shall be interesting in micromanipulation. In addition, EOC can influence or even forbid the exchange between spin angular momentum (SAM) and orbit angular momentum (OAM). For a linearly polarized incident CAB, its two Cartesian field components of the beam cannot only couple their powers to each other, but also lead to the changes of the intensity pattern and polarization distributions. The polarization state becomes spatially inhomogeneous, and possesses vortex phase with a topological charge of 2 during propagation. EOC presents a new way to control an Airy beam fast and efficiently.

Improved cumulative probabilities and range accuracy of a pulsed Geiger-mode avalanche photodiode laser ranging system with turbulence effects.

There exists a performance limitation in a pulsed Geiger-mode avalanche photodiode laser ranging system because of the echo intensity random fluctuation caused by turbulence effects. To suppress the influence of turbulence effects, we present a cumulative pulse detection technique with the ability to achieve improved cumulative probabilities and range accuracy. Based on the modulated Poisson model, the cumulative probabilities, range accuracy, and their influencing factors are investigated for a cumulative Q-switched laser pulse train. The results show that the improved cumulative probabilities and range accuracy can be obtained by utilizing cumulative pulse detection, with the condition that the echo intensity is 10, the echo pulse width is 10 ns, and the turbulence degree is 3, the target detection probability increases by 0.4, the false alarm probability decreases by 0.08, and the accuracy and precision increase by 46 cm and 27 cm, respectively.

Wide-angle broadband terahertz metamaterial absorber with a multilayered heterostructure.

In this paper, a wide-angle broadband perfect absorber is composed of a periodical metamaterial heterostructure. The structure is designed according to the concept that the metamaterial absorber's resonant frequency range can be manipulated by adjusting the filling factor of a bi-insulator heterostructure. The calculated results reveal that the four-layer herostructure has four perfect absorption peaks at the range of the terahertz frequency band. The related absorption bandwidth is 300 GHz and the average absorptivity is 98.6%. At the same time, the structure is insensitive to the incident angle.

Star-type polarizer with equal-power splitting function for each polarization based on polarization-dependent defects in two-dimensional photonic-crystal waveguides.

We propose a star-type polarizer with equal-power splitting function for each polarization based on polarization-dependent defects (PDDs) in two-dimensional photonic-crystal waveguides (PCWs). The structure is designed by combining two Y-type PCWs, and two types of PDDs are introduced into the PCWs respectively to provide polarization functions. By using finite-element method and optimizing the parameters of the PDDs, it is demonstrated that different polarizations can only transmit through their own PCWs and output with identical power distributions, i.e., the structure can function as polarizer and equal-power splitter for each polarization at the same time. In addition, by scanning the wavelength of the structure, it is proved that the proposed splitter can work in a wide range of wavelength while keeping high output transmission for both the TE and TM polarizations. Such a structure is useful for polarization-relative multi-channel signal processing for optical communications in the mid- and far-infrared wavelength regions.

Three-visible-light wave combiner based on photonic crystal microcavities.

We propose a three-visible-light wave combiner based on two-dimensional square-lattice photonic crystal (PhC) microcavities. A coupled-cavity waveguide is introduced to reduce the insertion losses for the three waves in the combiner. The transmission characteristic of light waves in PhCs with point defects is analyzed. As an example, a combiner for combining light waves of 488, 532, and 635 nm, which are commonly used as the three primary colors in laser display systems, is designed and demonstrated through the finite-difference time-domain method. The three visible light waves of 488, 532, and 635 nm are output at the same output port with transmittances of 97.6%, 98.1%, and 90.0%, respectively. The results show that the proposed device can perform efficient synthesis and the designing method can be applied in building other combiners based on PhCs made of dispersion materials.

False alarm suppression of multipulsed laser ranging system with Geiger-mode detector.

The false alarm probability is of great concern when designing and evaluating the performance of a multipulsed laser ranging system with a Geiger-mode avalanche photodiode. In this paper, based on the statistical distribution difference of the arrival time of the echo photons and noise in the time histogram, a false alarm suppression algorithm is presented. According to the data-processing method of the algorithm, the theoretical model of target detection and false alarm probability with a Poisson statistic and the system working at long dead time is established. With typical system design parameters, the target detection probability under different echo intensity and detection number is analyzed, and the influence of four main factors, namely, detection number, echo intensity, noise, and echo position, on the false alarm probability is investigated. The results show that multipulsed detection can improve the target detection probability, and using this developed algorithm, the false alarm probability can be effectively suppressed, to obtain an appropriate false alarm probability; it is suitable that the detection number is selected as 8; and stronger echo intensity, lower noise level, and a more frontal echo position can result in a lower false alarm probability.

Compact photonic crystal circulator with flat-top transmission band created by cascading magneto-optical resonance cavities.

A new type of compact three-port circulator with flat-top transmission band (FTTB) in a two-dimensional photonic crystal has been proposed, through coupling the cascaded magneto-optical resonance cavities to waveguides. The coupled-mode theory is applied to investigate the coupled structure and analyze the condition to achieve FTTB. According to the theoretical analysis, the structure is further optimized to ensure that the condition for achieving FTTB can be satisfied for both cavity-cavity coupling and cavity-waveguide coupling. Through the finite-element method, it is demonstrated that the design can realize a high quality, nonreciprocal circulating propagation of waves with an insertion loss of 0.023 dB and an isolation of 23.3 dB, covering a wide range of operation frequency. Such a wideband circulator has potential applications in large-scale integrated photonic circuits for guiding or isolating harmful optical reflections from load elements.

All-optical sensitive phase shifting based on nonlinear out-of-plane coupling through 1-D slab photonic crystal with a layer of graphene.

We realize all-optical sensitive phase shifting based on nonlinear out-of-plane coupling to a slab waveguide through Fano resonance of a slab 1-D photonic crystal (PhC). We use a graphene layer as the nonlinear material and change its refractive index by the input light intensity through Kerr nonlinear effect to obtain a shift in the Fano resonance frequency. The Fano resonance and self-focusing effect lead to light-intensity enhancement on the graphene in the PhC, reinforcing the nonlinear effect of refractive index in the graphene. Through finite-difference time-domain simulation, we demonstrate that the phase changing sensitivity obtained can be 4 orders higher than that by a single graphene under the same input light intensity. Moreover the threshold pump intensity for all-optical sensitive phase shifting in the coupled light to the waveguide is as low as ~4 MW per square centimeter. The results are applicable in micro optical integrated circuits for phase shifters, phase modulators, power limiters, and phase logic elements for optical computation, digital phase shift keying in communication systems, and non-contact sensitive signal detectors.

Three-visible-light wave combiner based on photonic crystal waveguides.

We present a three-visible-light wave combiner based on two-dimensional photonic crystal waveguides whose widths are not integral multiples of the lattice period. The proposed device consists of two cascaded directional couplers. It combines three visible light waves with different wavelengths from three input ports into a single output port. As an example, a combiner for combining light waves of 635, 532, and 488 nm, which are commonly used as the three primary colors in laser display systems, is designed and demonstrated through the finite-difference time-domain method. The results show that the proposed device can perform efficient synthesis for three visible light waves with transmittance exceeding 89% for each wavelength and high ability in preventing the backward coupling of waves from different waveguides. The method for designing the combiner is useful for designing other waveguide couplers based on photonic crystals made of dispersion materials.

Recent advances in two-dimensional perovskite materials for light-emitting diodes.

Light-emitting diodes (LEDs) are an indispensable part of our daily life. After being studied for a few decades, this field still has some room for improvement. In this regard, perovskite materials may take the leading role. In recent years, LEDs have become a most explored topic, owing to their various applications in photodetectors, solar cells, lasers, and so on. Noticeably, they exhibit significant characteristics in developing LEDs. The luminous efficiency of LEDs can be significantly enhanced by the combination of a poor illumination LED with low-dimensional perovskite. In 2014, the first perovskite-based LED was illuminated at room temperature. Furthermore, two-dimensional (2D) perovskites have enriched this field because of their optical and electronic properties and comparatively high stability in ambient conditions. Recent and relevant advancements in LEDs using low-dimensional perovskites including zero-dimensional to three-dimensional materials is reported. The major focus of this article is based on the 2D perovskites and their heterostructures (i.e., a combination of 2D perovskites with transition metal dichalcogenides, graphene, and hexagonal boron nitride). In comparison to 2D perovskites, heterostructures exhibit more potential for application in LEDs. State-of-the-art perovskite-based LEDs, current challenges, and prospects are also discussed.

Photonic-crystal structures with polarized-wave-guiding property and their applications in the mid and far infrared wave bands.

Photonic crystal (PhC) structures with polarized-wave-guiding property (PhC polarization waveguides) are proposed, demonstrated and applied to construct several new kinds of compact and efficient micro polarization devices in the mid and far infrared wave bands, including TE polarizers, TM polarizers, TE-downward T-shaped polarization-beam splitters (PBSs), TM-downward T-shaped PBSs and lying-T-shaped PBSs. Theoretical models for the operating mechanism of the structures are presented. The polarization devices built as applications of the PhC polarization waveguides are demonstrated by the finite-element method with the dispersion of materials being considered. Furthermore, optimized parameters are obtained by investigating the extinction ratio (EXR), the degree of polarization (DOP) and insertion loss. Moreover, structures based on PhC slabs derived from the 2D ones, together with woodpile PhC covers and substrates are suggested for the 3D version of the proposed devices for implementation. An example of the 3D-version structures shows a performance as good as that of the 2D structure. The devices proposed have relatively wide ranges of operating wavelength. Meanwhile, they are very compact in their structures and convenient for connection or coupling of signals among different optical elements, so they have the potential for wide applications in mid-and-far infrared optical devices or circuits, which are useful in remote sensing, image and vision, positioning and communications with infrared waves. Furthermore, the principle can be applied to build polarizers and PBSs in other wave bands.

All-optical tunable filters based on optomechanical effects in two-dimensional photonic crystal cavities.

All-optical tunable filters are basic elements for various micro-optical circuits. Obtaining all-optical tunability remains a challenge for micro-optical circuits. Optical forces with significant effects in nanophotonic systems provide new ways for wavelength tuning. In this Letter, the optomechanical effects in two-dimensional photonic crystal cavities are investigated. Simulations based on the finite element method demonstrate that forces arise in single and coupled cavities with movable rods inside. The optical force controls the positions of the movable rods and, thus, the resonance wavelength of the cavity, based on which tunable filter is designed. The operating wavelength of the cavity or the filter for the signal can be tuned by a control light with a different frequency. The results have potential applications for various integrated circuits.