Bragg Structures Research Articles

(In,Ga)N-GaN resonant Bragg structures with single and double quantum wells in the unit supercell

We study the formation of a superradiant optical mode in the room temperature reflection spectra from resonant Bragg structures (RBSs) composed of single and double (In,Ga)N quantum wells (QWs) in the unit cell. The appearance of the mode manifests itself by a significant increase in the resonant optical reflectivity due to the electromagnetic coupling of quasi-two-dimensional excitons in the QWs. The implementation of the supercells with double (In,Ga)N QWs results in an increase in the oscillator strength of the quasi-2D excitons and corresponding rise of the radiative broadening parameter to the value as high as 0.3 ± 0.02 meV. We also show that the supercells with double QWs are preferable for RBS with large number of periods due to better tolerance to deviations from the exact periodicity.

Terahertz Components by Additive Manufacturing: Material and Fabrication Characterizations Realized Through Bragg Structures

Terahertz Components by Additive Manufacturing: Material and Fabrication Characterizations Realized Through Bragg Structures

Reversible Laser Imprinting of Phase Change Photonic Structures in Integrated Waveguides.

Formation of laser-induced periodic surface structures (LIPSS) is known as a fast and robust method of functionalization of material surfaces. Of particular interest are LIPSS that manifest as periodic modulation of phase state of the material, as it implies reversibility of phase modification that constitute rewritable LIPSS, and recently was demonstrated for chalcogenide phase change materials (PCMs). Due to remarkable properties of chalcogenide PCMs─nonvolatality, prominent optical contrast and ns switching speed─such novel phase change LIPSS hold potential for exciting applications in all-optical tunable photonics. In this work we explore phase change LIPSS formation in thin films of Ge2Sb2Te5 (GST) integrated with planar and rib waveguides. We demonstrate that by fine-tuning laser radiation, the morphology of phase change LIPSS can be controlled, including their period and fill factor, and investigate the limitations of multicycle rewriting of the structures. We also demonstrate the formation of phase change LIPSS on a 1D waveguide, which has potential for use as tunable Bragg filters or structures for on-demand light decoupling into the far-field. The presented concept of applying phase change LIPSS offers a promising approach to enable fast and simple tuning in integrated photonic devices.

Investigating optical up-conversion process of erbium-doped ceria nanoparticles in photonic bilayer Bragg structure

This paper offers a thorough analysis using simulations, delving into the dual photonic impacts of using a Bragg stack in the optical upconversion (UC) process; the alteration of the local density of photon states (LDOS) and the amplification of local irradiance. We specifically investigate their influence on the upconversion quantum yield of the upconverter Er–CeO2nanoparticles (NPs). The Bragg stack being analyzed is composed of layers that alternate between TiO2 and Poly(methylmethacrylate) (PMMA), with the latter housing nanoparticles responsible for upconversion. The photonic effects are initially simulated independently and later integrated in a rate equation model to study its effect on the upconversion efficiency. This model accurately describes the kinetics associated with the upconversion (UC) processes within Er–CeO2 NPs. In order to ascertain the highest achievable upconversion efficiency across a spectrum of realistically feasible structures, adjustments are made to the refractive index of the active material within the con-fines of available materials. This work demonstrates the positive effect of photonic Bragg structure on the upconversion efficiency of Er–CeO2NPs.

Growth mechanism of bismuth-filled nanoporous silicon with improved structural stability and low thermal conductivity

Nano porous silicon (PS) is a strong candidate for thermoelectric application, but it shows low structural stability and is prone to collapse. To enhance the structural stability of PS and offer a new way to improve its electrical conductivity, PS was prepared by electrochemical etching and Bi nanowires were fabricated in the pores of PS by electrodeposition. In addition, for studying the effect of structures on the properties of Bi-filled PS composites, aperiodic single-layer (SL) structure, and periodic rugate (Rgt) structure and Bragg structure PS samples were prepared by changing the etching current waveform and etching time. The results show that the initial Bi deposition position and filling fraction of Bi nanowires in PS templates can be precisely controlled. Since Bi nanowires have significantly different continuity in Rgt and Bragg structures compared with SL structures, we proposed two growth mechanisms of Bi nanowires, including the axis-growth mechanism and transverse-axis-growth mechanism, with the supports of the experimental results of XRD, SEM, and EDS.The room temperature thermal conductivities of pristine and Bi-filled PS layers are in the ranges of 4.5 ∼ 5.0 and 4.2 ∼ 6.0 W･m−1K−1, respectively, about 1/10 of the original unetched silicon wafer. No apparent increase of thermal conductivity was observed in any Bi-filled composites. In addition, the thermal conductivities of PS layers with Bragg structure, including pristine and Bi-filled PS composites, are significantly lower than those of the other two structures.SEM images also demonstrates that Bi greatly improved the mechanical property of PS composites, especially the periodic structures composites. This study firmly demonstrates that Bi-filling in PS with periodic Bragg structure is practical to obtain nanocomposites with low thermal conductivity and improved structural stability. It offers a new dimension to optimize the property of PS for thermoelectric application by pore configuration design and controllable nanowire Bi filling.

Using three-dimensional distributed feedback to enhance selectivity of Bragg structures for free-electron lasers operating at sub-THz to THz frequencies

Using three-dimensional distributed feedback to enhance selectivity of Bragg structures for free-electron lasers operating at sub-THz to THz frequencies

High Electromechanical Coupling Coefficient of Longitudinally Excited Shear Wave Resonator Based on Optimized Bragg Structure.

In this work, a longitudinally excited shear-wave resonator (YBAR) based on single-crystalline lithium tantalate (LiTaO3, LT) thin film is proposed. The YBAR has a 200 nm X-cut thin film and molybdenum electrode. A high effective electromechanical coupling coefficient (k2eff) of up to 19% for the suspension-type structure was obtained. Furthermore, a Bragg reflector (SiO2/Pt) with optimized layer thickness ratio was employed to improve the performance of the YBAR. Compared to the acoustic wave resonators with the conventional quarter-wave (λ/4) Bragg reflector, the proposed YBAR with an optimized Bragg reflector can reflect both the longitudinal and shear waves efficiently, and resonators with spurious-free response and high quality (Q) value were achieved. This work provides a potential solution to enabling high coupling micro-acoustic resonators with high Q factor in the 5G/6G communication system.

Compact dual-band spectral analysis via multiplexed rotated chirped volume Bragg gratings.

Chirped Bragg volume gratings (CBGs) offer a useful alternative for spectral analysis, but increasing the bandwidth necessitates increasing the device area. In contrast, recently developed rotated CBGs (r-CBGs), in which the Bragg structure is rotated by 45° with respect to the device facets, require increasing only the device length to extend the bandwidth, in addition to the convenience of resolving the spectrum at normal incidence. Here, we multiplex r-CBGs in the same device to enable spectral analysis in two independent spectral windows without increasing the system volume. This new, to the best of our knowledge, device, which we term an X-CBG, allows for compact multi-band spectroscopy in contiguous or separated spectral windows for the visible and near-infrared applications in nonlinear microscopy and material identification and sensing.

Fiber-optic temperature sensor based on glass Fabry – Perot interferometer

In this work, analytical expressions are obtained that allow one to determine the shift of the central wavelength of a four-component addressable fiber Bragg structure (FAFBS). The computer simulation of the use of CHAVBS in the problems of measuring the temperature of the windings of power transformers has been carried out. Simulation showed that more accurate temperature measurement results are obtained when the circuit operates in reflection, i.e. at an additional address frequency. With the use of FAFBS, there is no need to install additional frequency filters. It is not required to use address component recovery schemes compared to the three-component AFBS option. This fact is a significant difference between the developed systems based on AFBS and systems based on two- and three-component AFBS.

Resonant optical reflection from a GaN/(Al,Ga)N excitonic Bragg structure

We experimentally demonstrate the formation of a superradiant optical mode in the room-temperature reflection spectra from a resonant Bragg structure composed of 30 equidistant GaN quantum wells separated by (Al,Ga)N barriers. The mode arises when the condition of the Bragg diffraction is fulfilled at the wavelength corresponding to the energy of the quasi-two-dimensional excitons in the quantum wells. It manifests itself as a significant increase in the amplitude and a change in the shape of the resonant optical reflection due to the electromagnetic coupling of the excitons. By modeling of the optical spectra, we evaluate the radiative and non-radiative broadening parameters of the excitonic states in the GaN quantum wells, which appear to be 0.4 ± 0.02 and 40 ± 5 meV, correspondingly, for the resonant exciton energy of 3.605 eV. The resonant Bragg structure based on the periodic sequence of the GaN quantum wells demonstrates an efficient coupling of photons and excitons at room temperature, which makes it promising for device applications.

Fabrication and characterization of polymer optical waveguide Bragg grating for pulse signal sensing.

Polymer materials have the advantages of a low Young's modulus and low-cost preparation process. In this paper, a polymer-based optical waveguide pressure sensor based on a Bragg structure is proposed. The change in the Bragg wavelength in the output spectrum of the waveguide Bragg grating (WBG) is used to linearly characterize the change in pressure acting on the device. The polymer-based WBG was developed through a polymer film preparation process, and the experimental results show that the output signal of the device has a sensitivity of 1.275 nm/kPa with a measurement range of 0-12 kPa and an accuracy of 1 kPa. The experimental results indicate that the device already perfectly responds to a pulse signal. It has significant potential application value in medical diagnostics and health testing, such as blood pressure monitoring, sleep quality monitoring, and tactile sensing.

Anti-resonant hollow-core terahertz fiber based on Bragg structure elements

An anti-resonant hollow-core terahertz fiber applying Bragg structure as the basic unit is presented, the basic unit is composed of two or three concentric tubes which is cut and connected by an outer ring. Simulations results show that the proposed fiber has the transmission characteristics of both conventional anti-resonant structure and Bragg fiber, and its confinement loss could be 2–3 orders of magnitude lower than that of an anti-resonant hollow-core terahertz fiber composed of only a single semi-tube as the anti-resonant unit, and the total transmission loss is one order of magnitude lower, which can achieve a low transmission loss of 0.5 dB/m or less with the bandwidth of 0.15 THz. In addition, the appropriated choice of the outer ring diameter can effectively suppress the mode coupling between the core and cladding, thereby reducing its confinement loss and bending loss. The proposed fibers show great potential for applications in terahertz wave communication and the fabrication of terahertz sensors.

Numerical Modelling of the Optical–Acoustical Characterization of an Anti-Resonant Bragg Hollow Core Fiber

Anti-resonant hollow core fibers (AR-HCFs) provide a promising solution for photothermal spectroscopy and photoacoustic imaging applications. Here, the AR-HCF serves as a micro platform to induce the photothermal/photoacoustic effect. Since the Bragg structure can induce multiple AR effects compared with the general AR-HCF, we proposed a novel device, the AR-BHCF (AR-HCF with Bragg cladding), to enhance the excitation efficiency. The simulation and experimental results validate that the AR-BHCF dominates in having a stronger ability to confine the optical field in the air core indeed. Then, the acoustic signal stimulated by the photoacoustic effect will propagate along with the fiber axial, and part of it will penetrate out of the AR-BHCF. The results revealed that the transmission bandwidth of the acoustic wave in the AR-BHCF ranges from 1 Hz to 1 MHz, covering infrasound to ultrasound. In particular, a constant coefficient of 0.5 exists in the acoustic wave fading process, related to the propagation frequency and time. The acoustic signal can be monitored in real time, assisted by the ultra-highly sensitive sensor head. Therefore, BHCF-based devices combined with photoacoustic techniques may accelerate their sensing applications. Meanwhile, this scheme shines a light on the theoretical foundation of novel short-haul distributed acoustic sensing.

THz Bragg structures fabricated with additive manufacturing.

The advancement of THz science and technology is desirable to facilitate the application of THz technologies in many sectors. Specialized THz photonic elements for these applications require desirable absorption and refractive characteristics in the THz regime. THz photonic elements can be created with additive manufacturing, and specifically 3D printing, forgoing the need for complex fabrication procedures and methodologies. Such THz photonic elements include periodic Bragg structures, which are capable of filtering specific THz frequencies. The authors present a THz Bragg structure fabricated with 3D printing via fused filament fabrication. The THz Bragg structure is made from high-impact polystyrene filament material, which is characterized in this paper with THz time-domain spectroscopy. The geometry and theoretical operation of the THz Bragg structure is investigated with finite-difference time-domain electromagnetic simulations. The THz Bragg structure is evaluated using a THz experimental test bed. There is agreement between the theoretical and the experimental filtering placement within the frequency domain for the THz Bragg structure. The capability of tunable frequency filtering of the presented THz Bragg structure, fabricated with 3D printing, is established and facilitates future advancements in applications of THz science and technology.

GaN/AlGaN resonant Bragg structure

Reflection spectra from a resonant Bragg structure with 30 GaN/AlGaN quantum wells have been measured at room temperature. Numerical modeling using the method of transfer matrices gave a quantitatively accurate fit of the experimental results. Defined radiative and non-radiative broadening parameters of the exciton in GaN/AlGaN quantum wells.

Investigation of Bragg resonant reflection of linear and nonlinear water waves by multiple fixed floating structures based on viscous meshfree method

The Bragg resonant reflection of linear and nonlinear water waves over a classical Bragg structures is numerically studied. The Smoothed Particle Hydrodynamics (SPH) method is a Lagrangian meshfree method, and SPH is adopted based on the code DualSPHysics, which takes the effect of viscosity into consideration. The sensitivity of particle spacing and a benchmark test to the transmission coefficient is analyzed. In the propagation of both linear and nonlinear water waves over Bragg floating structures, the Bragg resonant reflection occurs when the wavelength (λ) is about double of the spacing (L). The phase upshift can be observed in the cases of linear and nonlinear water waves, which means the minimum transmission coefficient appears when 2L/λ>1.0, and the phase upshift occurs with the increase of the width of floating in the nonlinear water waves. Besides, in the resonant case with three or more floating structures, the transmitted wave height is reduced by more than 70.9% and 73.5% relative to the incident wave height under linear and nonlinear water waves, respectively. The results give a reference for the design of wave attenuation in floating periodic structures in both linear and nonlinear water waves based on the Bragg rules.

Rotated chirped volume Bragg gratings for compact spectral analysis.

We introduce a new, to the best of our knowledge, optical component-a rotated chirped volume Bragg grating (r-CBG)-that spatially resolves the spectrum of a normally incident light beam in a compact footprint and without the need for subsequent free-space propagation or collimation. Unlike conventional chirped volume Bragg gratings in which both the length and width of the device must be increased to increase the bandwidth, by rotating the Bragg structure we sever the link between the length and width of a r-CBG, leading to a significantly reduced device footprint for the same bandwidth. We fabricate and characterize such a device in multiple spectral windows, we study its spectral resolution, and confirm that a pair of cascaded r-CBGs can resolve and then recombine the spectrum. Such a device can lead to ultracompact spectrometers and pulse modulators.

Comparative Analysis of the Methods for Fiber Bragg Structures Spectrum Modeling

The work is dedicated to a comparative analysis of the following methods for fiber Bragg grating (FBG) spectral response modeling. The Layer Sweep (LS) method, which is similar to the common layer peeling algorithm, is based on the reflectance and transmittance determination for the plane waves propagating through layered structures, which results in the solution of a system of linear equations for the transmittance and reflectance of each layer using the sweep method. Another considered method is based on the determination of transfer matrices (TM) for the FBG as a whole. Firstly, a homogeneous FBG was modeled using both methods, and the resulting reflectance spectra were compared to the one obtained via a specialized commercial software package. Secondly, modeling results of a π-phase-shifted FBG were presented and discussed. For both FBG models, the influence of the partition interval of the LS method on the simulated spectrum was studied. Based on the analysis of the simulation data, additional required modeling conditions for phase-shifted FBGs were established, which enhanced the modeling performance of the LS method.

Overview of Addressed Fiber Bragg Structures’ Development

An addressed fiber Bragg structure (AFBS) is a special type of fiber Bragg grating simultaneously performing the functions of a two-frequency radiation shaper and a sensitive element. An AFBS forms a two-frequency optical spectral response at its output, the difference frequency of which is invariant to measured physical fields and is referred to as the address frequency of the AFBS. Each of the AFBSs in the system has its own address frequency; therefore, a number of such structures can be interrogated simultaneously enabling the addressed multiplexing. In this article, we provide an overview of the theory and technology of AFBS, including the structures with three or more spectral components with various combinations of difference frequencies, both symmetrical and asymmetric. The subjects of interrogation of AFBSs, their fabrication and calibration are discussed as well. We also consider a wide range of applications in which AFBS can be used, covering such areas as oil and gas production, power engineering, transport, medicine, etc. In addition, the prospects for the further development of AFBS are proposed that mitigate the shortcomings of the current AFBSs’ state of the art and open up new possibilities of their application.

Разработка одномодового РОС-гетеролазера с выводом излучения под углом к поверхности структуры

We study a possibility of implementing a single-mode DFB heterolaser with inclined (with respect to the surface of the structure) output of the generated radiation. We find the periodic dielectric structure shape that makes it possible to realize the distributed feedback for wavebeams propagating along the structure and, at the same time, to ensure the output of up to 70% of the generated radiation power in the inclined direction. Within the framework of the semiclassical quasi-optical model, the possibility of stationary lasing regimes is shown for finite lateral dimensions of the Bragg structure.