Three-dimensional Finite-difference Time-domain Method Research Articles

Design and analysis of Si photonic NaCl sensors based on suspended microdisk resonators

In this paper, two optical refractive index sensors, respectively based on a suspended microdisk resonator (SMDR) and a sub-wavelength Bragg grating suspended microdisk resonator (SWG-SMDR) have been designed on a silicon on insulator (SOI) platform, and their optical and sensing performances have been analyzed using the three-dimensional finite-difference time-domain method. It has been shown that the Bragg grating increases the optical confinement factor in the sensing region and, consequently, improves the light–matter interaction and sensitivity. Considering concentrations of 0 to 25% for the NaCl dissolved in the deionized water, the enhanced sensitivity and FOM of the SWG-SMDR sensor with the miniaturized optimal geometrical dimensions are 163.77 nm RIU−1 and 10.81 RIU−1, respectively. This compact optical sensor enhances the sensitivity and FOM by combining effects of the Bragg grating and suspension configuration in improving the light–matter interaction.

Ultra-fast pulse shaping using a graphene hybrid plasmonic temporal differentiator

This paper presents the design and analysis of an ultra-compact temporal differentiator (DIFF) based on a graphene hybrid plasmonic all-pass microring resonator (GHP-APMRR) for ultra-short signal processing. Due to the use of graphene hybrid plasmonic waveguide and small ring with a radius of 1033 nm, the overall footprint of the temporal DIFF is equal to 4 × 2.586 μm2. This circuit has a very low insertion loss and a wide 3 dB bandwidth (2.058 to 2.303 THz) for various differentiation orders; so that it is able to process an ultra-short Gaussian input pulse with FWHM of 360 fs. In order to investigate performance of the proposed DIFF for the first and fractional-orders differentiation, its numerical results have been compared with those of the ideal mathematical DIFF in the frequency and time domains. On the other hand, performance of the temporal DIFF as an optical pulse shaper for generating some special pulses such as flat-top, parabolic, and triangular waveforms has been evaluated in detail. By comparing the optical results obtained based on the three-dimensional finite-difference time-domain method with the mathematical functions of the target pulses and the results calculated for the ideal mathematical first order DIFF, it has been illustrated that the GHP-APMRR can accurately generate the flat-top, parabolic, and triangular ultra-short pulses with the temporal FWHM of 600, 500 and 400 fs, respectively.

Improvement of light extraction efficiency in AlGaInP-based vertical miniaturized-light-emitting diodes via surface texturing.

AlGaInP-based light-emitting diodes (LEDs) suffer from a low external quantum efficiency (EQE), which is mainly restrained by the poor light extraction efficiency. Here, we demonstrate AlGaInP-based vertical miniaturized-LEDs (mini-LEDs) with a porous n-AlGaInP surface using a wet etching process to boost light extraction. We investigated the effects of etching time on the surface morphology of the porous n-AlGaInP surface. We found that as the etching time is prolonged, the density of pores increases initially and decreases subsequently. In comparison with the vertical mini-LED with a smooth n-AlGaInP surface, the vertical mini-LEDs with the porous n-AlGaInP surface reveal improvement in light output power and EQE, meanwhile, without the deterioration of electrical performance. The highest improvement of 38.9% in EQE measured at 20 mA is observed from the vertical mini-LED with the maximum density of the pores. Utilizing a three-dimensional finite-difference time-domain method, we reveal the underlying mechanisms of improved performance, which are associated with suppressed total internal reflection and efficient light scattering effect of the pores.

Optimizing broadband antireflection with Au micropatterns: a combined FDTD simulation andtwo-beam LIL approach.

Broadband antireflection (AR) is highly significant in a wide range of optical applications, and using a gold (Au) micropattern presents a viable method for controlling the behavior of light propagation. This study investigates a novel, to the best of our knowledge, methodology to achieve broadband AR properties in Au micropatterns. It employed the three-dimensional finite-difference time-domain (FDTD) method to simulate and optimize the design of micropatterns. In contrast, the fabrication of Au micropatterns was carried out using two-beam laser interference lithography (LIL). The fabricated Au micropatterns were characterized by a scanning electron microscope (SEM) and spectroscope to validate their antireflection and transmission properties and evaluate their performance at various wavelengths. The optimized Au micropatterns had a high transmittance rating of 96.2%. In addition, the device exhibits a broad-spectrum antireflective property, covering wavelengths ranging from 400 to 1100nm. The simulation data and experimentally derived results show comparable patterns. These structures can potentially be employed in many optical devices, such as solar cells and photodetectors, whereby achieving optimal device performance reduced reflection and enhanced light absorption.

A dual-purpose processor based on graphene hybrid plasmonic concentric resonators

This paper presents systematic design and analysis of a dual-purpose integrated processor based on graphene hybrid plasmonic concentric add-drop microring resonators for fast differentiation and integration. The footprint of this processor is equal to 4 × 4.358 μm2, containing two concentric rings with small radii of 1679 and 1204 nm. Performance of the designed dual-purpose processor for the first and fractional-orders differentiation and integration has been analyzed by the three-dimensional finite-difference time-domain method in the frequency and time domains and the accuracy of the results has been confirmed using the formulas of the ideal math differentiator and integrator. From the point of view of the performance specifications, the designed dual-purpose temporal processor has excellent 3 dB bandwidth, insertion loss, energy efficiency, and accuracy in the first and fractional-orders differentiation and integration.

Low loss and ultra-broadband design of an integrated 3  dB power splitter centered at 2  µm.

Because chemical gas is sensitive to absorption in the 2µm band, and 2µm matches the absorption band of the remote sensing material, many remote sensors and optical sensors are designed to operate in the 2µm wavelength region. In this paper, we designed an integrated 3dB power splitter centered at 2µm. The study of this device is built on a silicon-on-insulator (SOI) platform. We introduced a subwavelength grating (SWG) to improve the performance of the device. We used the three-dimensional finite-difference time-domain (3D FDTD) method to analyze the effect of the structure on the power splitter. The insertion loss (IL) of the fundamental TE mode is only 0.04dB at 2µm and its bandwidth of IL <0.45d B is 940nm (1570-2510nm). It is suitable for multidomain and all-band photonic integrated circuits at 2µm.

Simultaneous measurement of relative humidity and temperature using EIT-like effect in photonic crystal cavity coupled system integrated with PVA and graphene

In this work, we propose a sensor based on a photonic crystal nanobeam cavity (PCNC) coupled system integrated with Polyvinyl alcohol (PVA) and graphene, which uses an electromagnetically induced transparency-like (EIT-like) effect for the simultaneous measurement of relative humidity (RH) and temperature (T). The structure consists of a bus waveguide and two types of one-dimensional PCNC supporting the dielectric and air mode resonance, respectively. The RH-sensitive material PVA is used in the dielectric mode cavity to increase the RH sensitivity and anti-interference ability. We demonstrate that the EIT-like transparency window can be actively and flexibly modulated by adjusting the Fermi level of graphene. In addition, the ability of the system to simultaneously detect RH and T is proved by using the three-dimensional finite-difference time-domain (3D-FDTD) method and the sensor matrix. The RH sensitives for the dielectric and air modes are −542.0 pm/%RH and −258.2 pm/%RH, respectively. The T sensitives for the dielectric and air modes are −14.6 pm/K and 35.4 pm/K, respectively. The sensor achieves a strong anti-interference ability and its footprint is only 12 μm × 2.02 μm. Therefore, our proposed sensor has potential applications in food storage, preservation of museum collections, human health, and biological and chemical experiments.

Machine learning-driven grating-like SERS Platform toward ultra-sensitive detection of forsythin

Due to the complexity of existing substrate preparation processes and the poor reproducibility of substrates prepared from a single material, a highly reliable and low-cost Ag20/V–Ti20/DVD multi-stage composite SERS substrate was constructed in this study. The grating-like SERS substrate has an enhancement factor of 8.44×106 for R6G and a detection limit as low as 1×10−10 M. Results indicated that the highly regular grating-like SERS substrate exhibited excellent reproducibility with the relative standard deviation (RSD) values less than 10%. The three-dimensional finite-difference time-domain (3D-FDTD) method simulated the spatial distribution of the electric field intensity of the substrate, and a large number of “hot spot” regions were distributed in the Ag nanogap in the grating structure nanostructure. The optimized substrate was subsequently utilized for the discrimination of R6G with different concentrations using SERS and machine learning methods. These outstanding SERS properties of the Ag20/V–Ti20/DVD multi-stage SERS sensing platform suggested its potential application in the identification of pharmacodynamic substance in forsythin when the concentration as low as 1×10−7 g/l. At the same time, the highly ordered grating structure provided a new idea for the preparation of regular nanoarray substrates, and the combination of SERS and machine learning paved a new way toward the ultra-sensitive detection of other pharmacodynamic substances in traditional Chinese medicine (TCM).

Combined simulation and experimental study on spectral absorbance of partially disordered MoSe2 nanospheres

It is important to clarify the role and possible applicability of partially disordered structures in photonics, but there is still a lack of an effective method for it. Here, we investigate partially disordered MoSe2 nanospheres experimentally regarding their morphology and absorption spectrum in broadband wavelengths and propose an optical simulation with three-dimensional finite-difference time-domain method to explain the crucial impacts of morphological parameters on optical responses. The experimental spectral absorbance of MoSe2 nanospheres reveals a strong light-absorbing character in broadband wavelengths. The simulated spectral curves coincide with the experimental results by adjusting morphological parameters, i.e. the statistics of size and the number of layer, and the linear correlation coefficient between the simulated and experimental spectral curves is up to 0.94. The disorder plays a key role in the high light-absorption feature, and the feature originates from anti-reflection, defective state absorption, multiple light scattering and coherent diffusion effects. The results not only deepen the understanding of disordered photonics in semiconductor nanostructures, but also provide a simulation approach to optimize experimental designs.

Optical Reading of Nanoscale Magnetic Bits in an Integrated Photonic Platform

In this paper, we propose a compact integrated hybrid plasmonic-photonic device for optical reading of nanoscale magnetic bits with perpendicular magnetic anisotropy in a magnetic racetrack on top of a photonic waveguide on the indium phosphide membrane on silicon platform. The hybrid device is constructed by coupling a doublet of V-shaped gold plasmonic nanoantennas on top of the indium phosphide waveguide. By taking advantage of the localized surface plasmons, our hybrid device can enable detection of the magnetization state in magnetic bits beyond the diffraction limit of light and enhance the polar magneto-optical Kerr effect (PMOKE). We further illustrate how combining the hybrid device with a plasmonic polarization rotator provides magneto-optical read-out by transforming the PMOKE-induced polarization change into an intensity variation of the waveguide mode. According to the simulation results based on a three-dimensional finite-difference time-domain method, the hybrid device can detect the magnetization states in targeted bits in a magnetic racetrack medium down to ~ 100x100 nm2, regardless of the magnetization state of the rest of the racetrack with a relative intensity contrast of greater than 0.5% for a ~ 200x100 nm2 magnetic bit. We believe our hybrid device can be an enabling technology that can connect integrated photonics with nanoscale spintronics, paving the way toward ultrafast and energy efficient advanced on-chip applications.

Controllable terahertz beam focusing and rotating by graphene-assisted microring metasurfaces

This research proposes a terahertz (THz) metalens for focusing THz waves with an electrically controllable focal length. The dynamic of tuning of THz metalenses with graphene provides high controllability, fast responses, and compact structures for beam focusing. An array of graphene microrings has been presented with significant phase variations possibility. The three-dimensional finite-difference time-domain method has been employed for the analysis of the proposed structure. Applying a voltage to the graphene layer changes the graphene Fermi level, and the focal length can be controlled. As the chemical potential of graphene varies from 0.1 to 0.3 eV, the focal length changes 20.5λ (from 4 to 12.2 mm at the frequency of 0.75 THz). In addition, by biasing every array of graphene microrings with different gate voltages, we can electrically rotate the focal beam up to 32.02 deg. An equivalent circuit model (CM) is proposed for fast analysis and the verification of the FDTD results as well. The achieved results of the CM comply very well with those derived by the FDTD method. The proposed structure has the potential to be used in THz imaging.

Three-dimensional finite-difference time-domain simulation of moving sound source and receiver with directivity

In this paper, the implementation of a moving sound source and receiver with directivity in the three-dimensional finite-difference time-domain method is described. Fundamental solutions of moving monopole, dipole, and cardioid sources are theoretically derived. Some numerical experiments were performed on the three-dimensional sound field for a moving source and receiver. The numerical experiments showed that the effect of moving velocity on amplitude differs for the monopole and dipole or cardioid sources. Furthermore, it was found that the directivity characteristics of dipole and cardioid sources vary depending on the beam and moving directions. The present method can be accurately applied to the moving sound source and receiver with directivity.

An integrated device for electro-optic modulation and dense wavelength division multiplexing based on photonic crystals

We proposed an integrated device for electro-optic (EO) modulation and dense wavelength division multiplexing (DWDM) based on photonic crystals (PhCs). The transmittance performance of the designed structure was modelled and analyzed according to the time-domain coupled mode theory (CMT), using three-dimensional finite-difference time-domain (3D-FDTD) method to calculate the parameters of the proposed device for verification. The numerical results show that the proposed integrated device can realize the functions of “on”, “off” modulation and DWDM with a channel spacing of 0.8 nm at the operating wavelengths of 1555 nm and 1554.2 nm. The minimum insertion loss, channel crosstalk and modulation voltage are 0.7 dB, − 29.04 dB and 0.88 V, respectively. The maximum extinction ratio and modulation speed are 21.51 dB and 29.4 GHz. The designed integrated device has the advantages of low loss, compact size (41.28μm×11.18μm), narrow channel spacing, large extinction ratio, which can be easily expanded the number of channels and used in highly integrated large-capacity optical communication systems and data center.

Enhancing light extraction efficiency of GaN-based light-emitting diodes with reticulated photonic crystals

We have proposed reticulated photonic crystals (PhCs) on the top surface of conventional light-emitting diodes (LEDs) to increase the light extraction efficiency (LEE). The rigorous electromagnetic simulations based on the three-dimensional finite-difference time-domain method were carried out to investigate the optical properties of reticulated PhCs LEDs and mechanisms of reticulated PhCs improving the LEE of LEDs. Results from these simulations showed that more than 115% enhancement of LEE had been achieved for LEDs with reticulated PhCs period of 200 nm and depth of 200 nm. This study provides an innovative approach to the creation of high-efficiency LEDs of the upcoming generation.

Theoretical research and simulation of GaAs nanowire arrays in reflection-type photon-enhanced thermionic emission solar converters

In this paper, GaAs was used as the cathode of the photon-enhanced thermionic emission (PETE) solar converter. Based on the one-dimensional continuity equation and boundary conditions, a reflection-type PETE theoretical model was established to simulate the effects of solar concentration, electron affinity, cathode temperature and electron recombination velocity on the current density and the conversion efficiency. For optimizing the performance of the solar converter, we used GaAs nanowire (NW) arrays as the cathode and calculated the influences of NW arrays’ period, duty ratio, and height on the absorbance and the conversion efficiency by combining the theoretical model with the three-dimensional finite-difference time-domain (3D FDTD) method. The results were used to design the structure size of NW arrays, which improved the conversion efficiency of the solar converter to 23.07%. It shows that the GaAs NW arrays are promising PETE cathode materials.

Mitigation of Insulator Lightning-Induced Voltages by Installing Parallel Low-Voltage Surge Arresters

In this paper, we propose a mitigation method for reducing lightning-induced insulator voltages based on the installation of low-voltage surge arresters aligned parallelly to the insulator. The three-dimensional finite-difference time-domain (FDTD) method is applied to numerically model a real surge arrester residual voltage evaluation system. The application of a transient current pulse, typical of lightning discharges, is considered in our numerical model. We considered cases with one or two surge arresters installed per phase, in three different geometric and parametric configurations for installing distribution surge arresters. In addition to the Kirchhoff current division, which reduces both the absorbed energy and the thermal stress, the results associated with the installation of two surge arresters parallelly aligned to the insulator show that the interaction of magnetic fields generated by the surge arresters’ currents can produce an additional strong reduction in lightning-induced voltage over the insulator, as presented in this paper. Conditions for maximum voltage reduction are also identified. A brief cost-effectiveness analysis is also provided.

Suppressing optical crosstalk for GaN/InGaN based flip-chip micro light-emitting diodes by using an air-cavity patterned sapphire substrate as a light filter.

In this work, by using three-dimensional finite-difference time-domain (3D FDTD) method, the effect of conventional nano-patterned sapphire substrate (NPSS) on the optical crosstalk and the light extraction efficiency (LEE) for InGaN/GaN-based flip-chip micro light-emitting diodes (µ-LEDs) are systematically studied. We find that the conventional NPSS is not suitable for µ-LEDs. It is because the inclined mesa sidewall for µ-LEDs possesses a good scattering effect for µ-LEDs, but the introduced conventional NPSS causes part of the light be off escape cone between sapphire and air and become the guided light. To suppress the guided light and improve the optical crosstalk, a thick air layer between the n-GaN layer and the sapphire substrate can be used as a light filter to prevent the guided light from propagating into the sapphire. However, in reality, it is challenging to make the aforementioned air layer from point of fabrication view. Therefore, we propose the air-cavity patterned sapphire substrate (AC-PSS) as the light filter. Our results show that the crosstalk ratio can be decreased to the value even lower than 10%. The LEE can also be enhanced simultaneously due to combination effects of the filtering effect of the AC-PSS and the scattering effect of the inclined mesa sidewall.

LEMP and ground conductivity impact on the direct lightning performance of a medium-voltage line

• Assessment of the LEMP influence on direct lightning overvoltages of medium-voltage distribution lines of the type equipped with a shield wire and surge arresters. • Comparison between the results obtained by a 3D-FDTD method and the LIOV-EMTP code. • The LEMP contribution on the overvoltages across insulators increases with increasing values of the soil conductivity. • The appropriate modeling of the LEMP effects is important for the lightning performance assessment of the type of lines considered in this paper. In medium-voltage distribution lines equipped with a multi-grounded shield wire and with surge arresters installed along the line at relatively short intervals, lightning outages associated with indirect lightning are less frequent than those associated with direct ones. The protection for this type of distribution lines is therefore dependent on the response to direct strikes. This paper presents and analyses the response to direct lightning overvoltages of lines of such a type located over a lossy soil considering the influence of the lightning electromagnetic pulse (LEMP), which is typically disregarded in the calculation of overvoltages due to direct strikes. The calculation results, performed by means of a three-dimensional finite-difference time domain (3D-FDTD) method and of the LIOV-EMTP code, indicate that the voltage across the insulator is enhanced by LEMP effects. In addition, it is shown that the enhancement due to the LEMP increases with the increase of the soil conductivity value. The modeling of the LEMP effects is therefore important for the accurate assessment of the lightning performance of medium-voltage distribution lines of the type considered in this paper, even when direct strikes are considered.

Integrated device for electro-optic modulation and wavelength-mode division hybrid multiplexing based on graphene and photonic crystal

An integrated device for graphene electro-optic (EO) modulation and wavelength-mode division hybrid multiplexing (WMDM) is proposed. The device is composed of three parts: EO modulator, wavelength division multiplexer (WDM), and mode division multiplexer (MDM). The EO modulator and WDM consist of a one-dimensional photonic crystal nanobeam cavity coupled with a nanowire waveguide. The modulation of a certain wavelength can be achieved by utilizing the characteristic that the chemical potential of graphene can be adjusted with voltage. The MDM adopts nanowire waveguides to realize mode conversion. The performance parameters of the integrated device are analyzed using the three-dimensional finite-difference time-domain method of numerical analysis. The results show that the integrated device can simultaneously complete the modulation and WMDM of the fundamental transverse electric (TE0) modes and first-order TE (TE1) modes at 1570.0 and 1573.2 nm, respectively. The extinction ratio is greater than 23.2 dB, the insertion loss is <0.36 dB, and the channel crosstalk is < − 22.7 dB. In addition, the modulation speed reaches 18 GHz.

Multi-Directional Cloak Design by All-Dielectric Unit-Cell Optimized Structure.

In this manuscript, we demonstrate the design and experimental proof of an optical cloaking structure that multi-directionally conceals a perfectly electric conductor (PEC) object from an incident plane wave. The dielectric modulation around the highly reflective scattering PEC object is determined by an optimization process for multi-directional cloaking purposes. Additionally, to obtain the multi-directional effect of the cloaking structure, an optimized slice is mirror symmetrized through a radial perimeter. The three-dimensional (3D) finite-difference time-domain method is integrated with genetic optimization to achieve a cloaking design. In order to overcome the technological problems of the corresponding devices in the optical range and to experimentally demonstrate the proposed concept, our experiments were carried out on a scale model in the microwave range. The scaled proof-of-concept of the proposed structure is fabricated by 3D printing of polylactide material, and the brass metallic alloy is used as a perfect electrical conductor for microwave experiments. A good agreement between numerical and experimental results is achieved. The proposed design approach is not restricted only to multi-directional optical cloaking but can also be applied to different cloaking scenarios dealing with electromagnetic waves at nanoscales as well as other types such as acoustic waves. Using nanotechnology, our scale proof-of-concept research will take the next step toward the creation of "optical cloaking" devices.