Omnidirectional Band Gap Research Articles

Thermally tunable terahertz omnidirectional photonic bandgap and defect mode in 1D photonic crystals containing moderately doped semiconductor

A new one-dimensional photonic crystal (1DPC) containing moderately doped silicon (m-Si) semiconductor is proposed and its tunable properties are theoretically investigated. The tunable complex permittivity of m-Si with temperature is the central idea that makes the 1DPC thermally tunable. The 1DPC systems such as (m-Si/SiO2)12 and defective (m-Si/SiO2)6D(m-Si/SiO2)6 are examined in the spectral range of 0.4–0.8 THz with varying temperature from 40 to 100 K. Air and m-Si are considered as defect layer (D) in two different 1DPC systems. Our simulation shows that the photonic band gap (PBG) shifts to higher frequency and the omnidirectional PBG gets narrower with increasing temperature. The defect mode in both defective 1DPCs shifts to higher frequency with increasing temperature. The temperature sensitivity of peak transmission is found better in the air defective 1DPC, while that of defect frequency is better in the m-Si defective 1DPC. The angle of incidence and polarization dependency of the defect modes are studied in detail. The present work can be utilized for the development of tunable omnidirectional mirrors and narrow bandpass filters in the terahertz region as well as THz light based thermal sensors.

Truncated Metallo-Dielectric Omnidirectional Reflector: Collecting Single Photons in the Fundamental Gaussian Mode with 95% Efficiency

We propose a novel antenna structure which funnels single photons from a single emitter with unprecedented efficiency into a low-divergence fundamental Gaussian mode. Our device relies on the concept of creating an omnidirectional photonic bandgap to inhibit unwanted large-angle emission and to enhance small-angle defect-guided-mode emission. The new photon collection strategy is intuitively illustrated, rigorously verified and optimized by implementing an efficient body-of-revolution finite-difference time-domain method for in-plane dipole emitters. We investigate a few antenna designs to cover various boundary conditions posed by fabrication processes or material restrictions and theoretically demonstrate that collection efficiencies into the fundamental Gaussian mode exceeding 95% are achievable. Our antennas are broadband, insensitive to fabrication imperfections and compatible with a variety of solid-state emitters such as organic molecules, quantum dots and defect centers in diamond. Unidirectional and low-divergence Gaussian-mode emission from a single emitter may enable the realization of a variety of photonic quantum computer architectures as well as highly efficient light-matter interfaces.

A reconfigurable device based on the one-dimensional magnetized plasma photonic crystals nested with the Pell and Thue–Morse sequences

In this paper, the properties of a reconfigurable device which realizes the omnidirectional band gaps (OBGs), nonreciprocity (NR) and polarization beam splitting (PBS) based on the one-dimensional (1-D) magnetized plasma photonic crystals (MPPCs) nested with quasi-periodic Pell and Thue–Morse sequences are theoretically investigated by the transfer matrix method (TMM). The obtained results show that the OBGs, the NR region, and the scope of PBS can be notably manipulated by the plasma frequency, plasma cyclotron frequency, incident angle, and the plasma collision frequency. More specifically, the higher plasma frequency is needed for accomplishing the property of OBGs while the better performance of NR can be achieved with a larger incident angle and a higher plasma cyclotron frequency. Also, the lower plasma frequency and higher plasma cyclotron frequency are suitable for the splendid performance of PBS. The augment of the plasma collision frequency is adverse to the properties as mentioned above. Due to the Voigt magneto-optical effect generating from the magnetized plasma layers, the time-reversal symmetry is destroyed which is beneficial for obtaining the properties of NR and PBS. Besides, owing to the nested technology which breaks the spatial symmetry of the structure further, compared with the conventional 1-D single quasi-periodic structures, the proposed nested MPPCs have a preeminent strength in achieving the three functions through modulating the corresponding physical parameters. These findings provide theoretical guidance for the design and application of the multifunctional devices.

Tunable lateral shift of the reflected optical beams from a nanocomposite structurally chiral medium

The lateral shifts of the reflected and transmitted circularly polarized beams from a slab of nanocomposite structurally chiral medium are investigated at the edges of the photonic bandgaps of the structure using the transfer matrix method. The nanocomposite structurally chiral medium consists of the structurally chiral material with the silver nanoparticles randomly dispersed inside it. Due to the presence of metallic nanoparticles, this structure also contains an omnidirectional bandgap in addition to the conventional Bragg gap. By obtaining the co-polarized and cross-polarized reflection and transmission spectra of the structure, we show that only the reflected co-polarized right-handed circularly polarized beam has a considerable lateral shift at the edges of the Bragg gap. Moreover, we have shown that the circularly polarized beams do not have a significant lateral shift at the edges of the nanoparticles created bandgap. We also reveal that the reflected beam experiences a positive lateral shift at the lower edge of the Bragg gap and a negative lateral shift at the upper one. Then we examine the influences of the different parameters, such as the filling fraction of nanoparticles, incidence angle, tilt angle, and slab thickness on the lateral shift of the reflected co-polarized right-handed circularly polarized beam.

Analysis of the features of a multifunctional device based on the regulation of the magnetic field in one-dimensional photonic crystals containing only plasma with a novel quasi-periodic structure

In this paper, the features of a multifunctional device that are realized by one-dimensional magnetized plasma photonic crystals (MPPCs) containing only the plasma regulated by the external magnetic field in an innovative quasi-periodic arrangement are studied by the transfer matrix method, which possesses the characteristics of the omnidirectional bandgap (OBG), nonreciprocity (NR), and polarization separation (PS). The effects of the magnetic field and other physical parameters including the plasma frequency and incident angle on those three properties are illustrated, respectively. The computed results demonstrate that, with the gradually increasing magnetic field, the performances of OBG and NR apparently deteriorate while the PS region expands ideally. Meanwhile, the greater presentations of OBG and PS can be gotten with the proper rise of the plasma frequency. Nevertheless, the higher the plasma frequency is, the worse the quality of NR will be observed. Furthermore, an appropriate incident angle also plays a crucial part in the display of NR and PS for the proposed structure, and these properties can be enhanced well with a larger incident angle. This new kind of MPPC has potential application in designing tunable and multifunctional optical instruments.

A type of arrangement for photonic crystal structures interacting with a Terahertz wave with omnidirectional and thermal effects

Omnidirectional photonic bandgaps are a new special type of one-dimensional quasi-photonic crystals that contains semiconductor and dielectric material layers and are investigated here in the Terahertz wave range. The proposed medium is constructed with a special type of layer arrangement, which uses both the Fibonacci sequence as a quasi-periodic sequence and the absolute periodic sequence in a period. As the Terahertz bandgaps of the transmittance spectrum are essential in some devices, the tuning and manipulation of these bandgaps has been of great interest in recent years. One of the best methods of manipulating these bandgaps to reach the desired outcome is by changing their arrangement using different types of quasi-periodic sequences in the structure. The beneficial results of applying these sequences have been clearly observed. So, we propose another new type of arrangement here in order to completely satisfy the changing methods of the photonic crystal structures. According to the results of the current investigation, it has been demonstrated that the proposed arrangement could be used to achieve a wide variety of desirable states. The semiconductor could make the bandgaps tunable via temperature changes through its thermally tunable permittivity. These types of media, which can operate as tunable Terahertz filters and mirrors, offer many promising omnidirectional Terahertz components and devices.

Investigation on temperature controlling multifunctional selector in superconducting photonic crystals based on Thue–Morse sequence

In this paper, a novel one-dimensional superconducting photonic crystal exploiting the Thue–Morse arrangement is theoretically investigated by the transfer matrix method. Two transmission states are switched in utilization of ambient temperature in the case of the same structure in the terahertz regime: one is the omnidirectional photonic bandgap (OBG) characteristics in low-temperature zones (about 10 K), and the other is wide-angle broadband absorption characteristics in high-temperature zones (about 90 K). Due to the modulation of temperature-dependent superconducting complex permittivity, the proposed structure can induce the OBG and broadband absorption at different temperatures. From the numerical results, the OBG can be notably tuned by manipulating the structural parameter of high refractive index dielectric. The effects of superconducting thickness on the switchable function regions are also considered. The proposed structure can possess both environment stable zero- n ¯ OBG at 10 K and preeminent broadband absorption in the TM polarization at 90 K, which offers theoretical guidance to the design of temperature switchable function selectors.

Nonlinear wave propagation and dynamic reconfiguration in two-dimensional lattices with bistable elements

This paper examines the propagation of elastic waves of large amplitudes in two-dimensional square lattices that include an alternating pattern of linear springs and nonlinear bistable springs. Because of the presence of bistable springs, these lattices have multiple stable configurations. In this paper, a theoretical model that takes into account the non-linearity of the bistable springs while neglecting geometric nonlinearities is used to study non-linear wave propagation in these lattices. Results from numerical simulations demonstrate that, for a lattice that is initially in a deformed stable equilibrium configuration, stimuli of large amplitudes are able to cause a reconfiguration of the lattice to a configuration of lower potential energy. Even for initial stable configurations that exhibit omnidirectional or directional bandgaps for infinitesimal wave propagation, waves of large amplitudes can propagate in the lattice due to its reconfiguration; however, due to the nonlinearity of bistable springs, the transmitted response tends to have multiple spectral peaks or be broadband for narrow-band stimuli of large amplitude. Simulations are used to study how the reconfiguration of the lattice depends on the amplitude and duration of the stimulus, on damping, and on the energy barrier between the two stable equilibria of the bistable springs. These numerical results suggest that these multistable lattices can serve as reconfigurable wave guides for which the amplitude of the stimulus offers opportunities for enhanced tunability.

Temperature-switchable mode selector in superconducting photonic crystals based on the Thue–Morse sequence

In this paper, a novel one-dimensional superconducting photonic crystal based on the quasi-periodic Thue–Morse (ThM) arrangement is theoretically investigated by the transfer matrix method, which can switch two transmission properties by controlling ambient temperature. Its transmission spectrum realizes omnidirectional photonic band gap (OBG) characteristics in low-temperature zones (about 10 K) and wide-angle broadband absorption characteristics in high-temperature zones (about 90 K) as the whole structure remains the same in the terahertz regime. The intrinsic reason for switchable functions can be ascribed to the superconducting negative permittivity that is dependent on both temperature and frequency under the superconducting state, which causes an OBG corresponding to the zero-averaged (volume) refractive index (zero- n ¯ ) and broadband absorption induced by high permittivity dissipation. From the numerical results, the OBG from the Bragg gap or absorption bandwidth can be notably tuned by manipulating the periodicity of the ThM sequence and dielectric or superconducting thicknesses. The effects of incident angle and polarization modes on the proposed structure are also considered. We report that the proposed structure has a preeminent zero- n ¯ OBG ranging from 0.1 to 1.7 THz at 10 K and stable broadband absorption for a wide angle (at most 70 deg) in TM mode at 90 K, which provides theoretical guidance for the design and application of the temperature-switchable mode selector.

Tunable omnidirectional band gap and polarization splitting in one-dimensional magnetized plasma photonic crystals with a quasi-periodic topological structure

An omnidirectional band gap (OBG) can be achieved by stacked hetero-structures or conventional quasi-periodic dielectric structures, which has similar effects on both polarization waves. In this paper, a new quasi-periodic structure (QPS) is constructed to form a photonic crystal, which is composed of magnetized plasma and common dielectrics. The computed results show that different OBG features of two polarized waves can be obtained in the proposed QPS. On this basis, the polarization splitting (PS) phenomenon also can be found, and the bandwidths of both OBG and PS are ultra-wideband. The effects of plasma frequency, external magnetic field, and normal dielectrics on the performance of OBG and PS are also investigated by the transfer matrix method, respectively. It is found that the performance can be adjusted by changing those parameters. The greater OBG can be observed at a large plasma frequency and small magnetic field, while the property of PS is just the opposite. With a smaller plasma frequency or a larger magnetic field, an improved performance of PS can be obtained. These results can provide ideas for the construction of novel quasi-periodic topology, OBG devices, and polarized splitters.

Band gap engineering and applications in compound periodic structure containing hyperbolic metamaterials

Behaviours of light in materials strongly depend on the topological structure of the iso-frequency surface (IFS). The usual materials, of which the unit cell of photonic crystal is made up, are dielectrics, whose IFSs have the same closed topological structure. As a simplest photonic crystal, one-dimensional photonic crystal (1DPC) has attracted intensive attention due to its simple fabrication technique as well as numerous applications. However, in a conventional all-dielectric 1DPC, photonic band gaps (PBGs) for both transverse magnetic (TM) and transverse electric (TE) polarizations will shift toward short wavelengths (i.e. blueshift) as incident angle increases. The underlying physical reason is that the propagating phase in isotropic dielectric will decrease as incident angle increases. The blueshift property of band gap for TM and TE polarization will limit the band width of omnidirectional band gap and the range of operating incident angles in some PBG-based applications, including near-perfect absorption, polarization selection and sensitive refractive index sensing. However, for TM polarization, the propagating phase in a hyperbolic metamaterial (HMM) will increase with incident angle increasing. This special phase property of HMM provides us with a way to flexibly tune the angle-dependent property of band gap in periodic compound structure composed of alternative HMM with open IFS and dielectric with close IFS. In this review, we realize zeroshift (i.e. angle-independent) band gaps as well as redshift band gaps in 1DPCs containing HMMs, which can be utilized to realize near-perfect absorption, sensitive refractive index sensing and polarization selection working in a wide range of incident angles.

Tuning band structures of photonic multilayers with positive and negative refractive index materials according to generalized Fibonacci and Thue–Morse sequences

We investigate the photonic band structure, using the transfer-matrix method, in one-dimensional structures composed of a dispersive metamaterial A juxtaposed with a non-dispersive dielectric B according to the generalized Fibonacci and Thue–Morse sequences, which are ruled and characterized by two positive integer numbers, p and q. We present the band structures of different generations of these sequences for both TE and TM modes and discuss how they are affected by the p and q parameters and the ratio between the thicknesses of the building blocks. We obtain a new and very important analytical expression for the frequency in which the average refraction index vanishes, i.e. when the gap condition is satisfied, and we investigate, in detail, the behavior of this emergent scale insensitive gap, as a function of the thicknesses ratio, for all structures considered. For the metamaterial considered, we show that the reduced frequency for converges faster or slower to 0.41 depending on the sequence. By adjusting the p and q parameters, we can obtain a higher concentration of the pass bands inside (outside) the nA < 0 region when there are more (less) blocks A than B in the unit cell. Our results also show the presence of omnidirectional and complete band gaps, which have various practical and technological applications.

Reversible tuning of omnidirectional band gaps in two-dimensional magnonic crystals by magnetic field and in-plane squeezing

By means of the plane wave method, we study nonuniform, i.e., mode- and k-dependent, effects in the spin-wave spectrum of a two-dimensional bicomponent magnonic crystal. We use the crystal based on a hexagonal lattice squeezed in the direction of the external magnetic field wherein the squeezing applies to the lattice and the shape of inclusions. The squeezing changes both the demagnetizing field and the spatial confinement of the excitation, which may lead to the occurrence of an omnidirectional magnonic band gaps. In particular, we study the role played by propagational effects, which allows us to explain the k-dependent softening of modes. The effects we found enabled us not only to design the width and position of magnonic band gaps, but also to plan their response to a change in the external magnetic field magnitude. This allows the reversible tuning of magnonic band gaps, and it shows that the studied structures are promising candidates for designing magnonic devices that are tunable during operation.

Polarization conversion and phase modulation of terahertz electromagnetic waves via graphene-dielectric structure

In this paper, by using the 4 × 4 transfer matrix approach, the reflection and transmission of circularly polarized (CP) waves which are obliquely incident to the periodic structure comprising of graphene-dielectric layers are investigated. The results indicate that by applying a magnetic field in appropriate direction there exists an omni-directional band gap in the transmission spectrum either left or right CP wave. The given structure can convert the linearly polarized wave to CP wave in reflected and transmitted waves. Furthermore, the dependence of the ellipticity of the reflected and transmitted waves to the applied magnetic field and the chemical potential of the graphene is studied. Such structures can be used as a polarization converter and phase modulator in the terahertz frequency range.

Independently Tunable Thermal Conductance and Phononic Band Gaps of 3D Lattice Materials

Lattice materials provide unusual thermal and vibrational properties but not within the same structure. Thermal and vibrational multifunctionality is, however, crucial for thermomechanical applications such as automotive, aerospace, building, transportation, and energy infrastructure. In applications involving mobility, both high heat transfer and low mass are desired. Although there have been various efforts to design multifunctional lattice materials, the focus has largely remained on quasi‐static mechanical and thermal properties or mechanical and vibrational properties. Herein, designs of realizable lattice materials are reported, which are inherently thermally resistive, with vastly improved thermal conductance and omnidirectional phononic band gaps. By redesigning the truss structures to serve as interconnected heat pipes, a three‐order‐of‐magnitude improvement in the specific thermal conductance is found. Nodal masses at truss junctions are further used to obtain full vibrational band gaps. It is shown that it is possible to independently tune vibrational and thermal properties within the same structure. This work provides background for the design and fabrication of multifunctional lattice materials that simultaneously prevent structural vibrations and enhance heat conduction.

Microwave multichannel tunable filter based on transmission and reflection properties of 1D magnetized plasma photonic crystal heterostructures

The transmission and reflection properties of one-dimensional magnetized plasma photonic crystal heterostructures have been theoretically investigated. The proposed structure is composed of two sub-PCs containing magnetized cold plasma and lossless dielectric materials. The optical properties of the structure are suitable for multichannel tunable transmission filter and omnidirectional band-stop filters applications. The investigations have been carried out by applying transfer matrix method and employing electrostatic boundary conditions for TE and TM wave, respectively, in microwave region. The transmission spectra of the proposed structure possess external magnetic field-dependent 3N − 3 comb-like resonant peaks called as transmission channels for period number (N) > 1. Due to multiple interactions between forward and backward decaying evanescent waves in plasma and dielectric layers, respectively, 3N − 3 transmission channels are found in defect-free magnetized plasma photonic crystal heterostructure, enabling the structure to work as a multichannel filter. Next, the filter properties have been made tunable, i.e., channel frequency of each channel can either be red or blue shifted, depending upon the RHP and LHP configurations of external magnetic field under magneto-optical Faraday effect, respectively. The number of channels and their positions can also be modulated by changing the number of periods (N) and the incident angle (θ0) for TE and TM waves both besides other plasma parameters. Apart from the transmission properties of the proposed structure, we have also studied the reflection properties to obtain multichannel tunable omnidirectional photonic band gaps under the influence of external magnetic field. These results may be utilized to develop new kind of externally tunable single to multichannel omnidirectional band-stop filters.

Inverse design and deep learning for phononic crystals

In this talk, I will present some results pertaining to the inverse design of phononic crystals. First, I will consider the design of 3-D phononic crystals exhibiting large omnidirectional bandgaps within the context of topology optimization using the SIMP method. The problem will be presented in the traditional forward-inverse paradigm where the forward solver is a mixed-variational scheme implemented on multiple graphical processing units. The inverse solver is a large scale adjoint based optimization framework with highly efficient vectorized operations for sensitivity calculations. In the second part of the talk, I will consider a deep learning based surrogate model for the forward problem. The neural network architecture is based on convolutional neural networks (CNN) and is trained to predict the phononic eigenvalues of unseen unit cell configurations. I show that a CNN based model easily outperforms traditional neural network architectures such as the multi layer perceptron (MLP) on both efficiency of learning and generalization capabilities. Strong and highly efficient surrogate models such as CNN coupled with topology optimization provide a an important way forward for the inverse design of phononic crystals and metamaterials.

3D printed architected hollow sphere foams with low-frequency phononic band gaps

We experimentally and numerically investigate elastic wave propagation in a class of lightweight architected materials composed of hollow spheres and binders. Elastic wave transmission tests demonstrate the existence of vibration mitigation capability in the proposed architected foams, which is validated against the numerically predicted phononic band gap. We further describe that the phononic band gap properties can be significantly altered through changing hollow sphere thickness and binder size in the architected foams. Importantly, our results indicate that by increasing the stiffness contrast between hollow spheres and binders, the phononic band gaps are broadened and shifted toward a low-frequency range. At the threshold stiffness contrast of 50, the proposed architected foam requires only a volume fraction of 10.8% while exhibiting an omnidirectional band gap size exceeding 130%. The proposed design paradigm and physical mechanisms are robust and applicable to architected foams with other topologies, thus providing new opportunities to design phononic metamaterials for low-frequency vibration control.

Topological design of 3D phononic crystals for ultra-wide omnidirectional bandgaps

As a unique kind of artificial periodic composite materials, phononic crystals have attracted the wide attention in recent years. Unremitting efforts have been made to investigate the potential existence of phononic bandgaps, which can be used to manipulate the propagation of acoustic/elastic waves. This paper proposes a new topology optimization algorithm to achieve the optimised bandgap structures of 3D phononic crystals based on fixed-grid finite element method (FEM) and evolutionary procedure. The maximum normalised gap ratio obtained is as large as 97% between the 12th and 13th order of bands, with the optimised structure of tungsten carbide spheres embraced in epoxy matrix. Symmetric unit cells for simple cubic (SC) lattice are presented for the specified bandgaps to validate the effectiveness of the proposed optimization algorithm for designing 3D phononic bandgap crystals. Meanwhile, the proposed topology optimization represents structures with clear topologies and smooth boundaries.

Phononic band-gaps of Hoberman spherical metamaterials in low frequencies

Periodically engineered structures, having the capability to manipulate mechanical wave propagation and exhibiting omnidirectional phononic band gaps, are important for various potential applications such as wave filtering, waveguiding, acoustic cloaking, thermal management, and energy harvesting. In natural materials, vibration mitigation capability depending on inherent damping feature usually cannot be adjusted easily and broad attenuation frequency ranges are still uncommon in these materials. Here, inspired by the Hoberman sphere architecture, we propose a novel lattice system with broad and multiple omnidirectional phononic band gaps, which are attributed to the local resonances mechanism. Guided by numerical simulations, analytical formulations, and low amplitude transmission testing, we show that the proposed lattice metamaterials can exhibit broad and multiple omnidirectional band gaps over a wide range of node connectivity and constituents. The finding reported here provides a new routine to design phononic metamaterial systems with tunable band gaps, offering a wide range of potential applications in harsh environmental conditions.