Computational Investigation of Advanced Refractive Index Sensor Using 3-Dimensional Metamaterial Based Nanoantenna Array.

Photonic researchers are increasingly exploiting nanotechnology due to the development of numerous prevalent nanosized manufacturing technologies, which has enabled novel shape-optimized nanostructures to be manufactured and investigated. Hybrid nanostructures that integrate dielectric resonators with plasmonic nanostructures are also offering new opportunities. In this work, we have explored a hybrid coupled nano-structured antenna with stacked multilayer lithium tantalate (LiTaO3) and Aluminum oxide (Al2O3), operating at wavelength ranging from 400 nm to 2000 nm. Here, the sensitivity response has been explored of these nano-structured hybrid arrays. It shows a strong electromagnetic confinement in the separation gap (g) of the dimers due to strong surface plasmon resonance (SPR). The influences of the structural dimensions have been investigated to optimize the sensitivity. The designed hybrid coupled nanostructure with the combination of 10 layers of gold (Au) and Lithium tantalate (LiTaO3) or Aluminum oxide (Al2O3) (five layers each) having height, h1 = h2 = 10 nm exhibits 730 and 660 nm/RIU sensitivity, respectively. The sensitivity of the proposed hybrid nanostructure has been compared with a single metallic (only gold) elliptical paired nanostructure. Depending on these findings, we demonstrated that a roughly two-fold increase in the sensitivity (S) can be obtained by utilizing a hybrid coupled nanostructure compared to an identical nanostructure, which competes with traditional sensors of the same height, (h). Our innovative novel plasmonic hybrid nanostructures provide a framework for developing plasmonic nanostructures for use in various sensing applications.

Electrical performance of efficient quad-crescent-shaped Si nanowire solar cell.

The electrical characteristics of quad-crescent-shaped silicon nanowire (NW) solar cells (SCs) are numerically analyzed and as a result their performance optimized. The structure discussed consists of four crescents, forming a cavity that permits multiple light scattering with high trapping between the NWs. Additionally, new modes strongly coupled to the incident light are generated along the NWs. As a result, the optical absorption has been increased over a large portion of light wavelengths and hence the power conversion efficiency (PCE) has been improved. The electron-hole (e-h) generation rate in the design reported has been calculated using the 3D finite difference time domain method. Further, the electrical performance of the SC reported has been investigated through the finite element method, using the Lumerical charge software package. In this investigation, the axial and core-shell junctions were analyzed looking at the reported crescent and, as well, conventional NW designs. Additionally, the doping concentration and NW-junction position were studied in this design proposed, as well as the carrier-recombination-and-lifetime effects. This study has revealed that the high back surface field layer used improves the conversion efficiency by [Formula: see text] 80%. Moreover, conserving the NW radial shell as a low thickness layer can efficiently reduce the NW sidewall recombination effect. The PCE and short circuit current were determined to be equal to 18.5% and 33.8 mA[Formula: see text] for the axial junction proposed. However, the core-shell junction shows figures of 19% and 34.9 mA[Formula: see text]. The suggested crescent design offers an enhancement of 23% compared to the conventional NW, for both junctions. For a practical surface recombination velocity of [Formula: see text] cm/s, the PCE of the proposed design, in the axial junction, has been reduced to 16.6%, with a reduction of 11%. However, the core-shell junction achieves PCE of 18.7%, with a slight reduction of 1.6%. Therefore, the optoelectronic performance of the core-shell junction was marginally affected by the NW surface recombination, compared to the axial junction.

All-Opto Plasmonic-Controlled Bulk and Surface Sensitivity Analysis of a Paired Nano-Structured Antenna with a Label-Free Detection Approach.

Gold nanoantennas have been used in a variety of biomedical applications due to their attractive electronic and optical properties, which are shape- and size-dependent. Here, a periodic paired gold nanostructure exploiting surface plasmon resonance is proposed, which shows promising results for Refractive Index (RI) detection due to its high electric field confinement and diffraction limit. Here, single and paired gold nanostructured sensors were designed for real-time RI detection. The Full-Width at Half-Maximum (FWHM) and Figure-Of-Merit (FOM) were also calculated, which relate the sensitivity to the sharpness of the peak. The effect of different possible structural shapes and dimensions were studied to optimise the sensitivity response of nanosensing structures and identify an optimised elliptical nanoantenna with the major axis a, minor axis b, gap between the pair g, and heights h being 100 nm, 10 nm, 10 nm, and 40 nm, respectively. In this work, we investigated the bulk sensitivity, which is the spectral shift per refractive index unit due to the change in the surrounding material, and this value was calculated as 526-530 nm/RIU, while the FWHM was calculated around 110 nm with a FOM of 8.1. On the other hand, the surface sensing was related to the spectral shift due to the refractive index variation of the surface layer near the paired nanoantenna surface, and this value for the same antenna pair was calculated as 250 nm/RIU for a surface layer thickness of 4.5 nm.

Growth of 1D TiO2nanostructures on Ti substrates incorporated with residual stress through humid oxidation and their characterizations.

Different Ti substrates, such as particles (as-received and ball milled), plate and TEM grid were oxidized for the growth of one dimensional (1D) TiO2nanostructures. The Ti substrates were oxidized for 4 h at temperatures of 700 °C-750 °C in humid and dry Ar containing 5 ppm of O2. The effects of residual stress on the growth of 1D TiO2nanostructures were investigated. The residual stress inside the Ti particles was measured by XRD-sin2ψtechnique. The oxidized Ti substrates were characterized using field emission scanning electron microscope equipped with energy dispersive x-ray spectroscope, transmission electron microscope, x-ray diffractometer and x-ray photoelectron spectroscope. Results revealed that humid environment enhances the growth of 1D TiO2nanostructures. Four different types of 1D morphologies obtained during humid oxidation, e.g. stacked, ribbon, plateau and lamp-post shaped nanostructures. The presence of residual stress significantly enhances the density and coverage of 1D nanostructures. The as-grown TiO2nanostructures possess tetragonal rutile structure having length up to 10µm along the 〈1 0 1〉 directions. During initial stage of oxidation, a TiO2layer is formed on Ti substrate. Lower valence oxides (Ti3O5, Ti2O3and TiO) then form underneath the TiO2layer and induce stress at the interface of oxide layers. The induced stress plays significant role on the growth of 1D TiO2nanostructures. The induced stress is relaxed by creating new surfaces in the form of 1D TiO2nanostructures. A diffusion based model is proposed to explain the mechanism of 1D TiO2growth during humid oxidation of Ti. The 1D TiO2nanostructures and TiO2layer is formed by the interstitial diffusion of Ti4+ions to the surface and reacts with the surface adsorbed hydroxide ions (OH-). Lower valence oxides are formed at the metal-oxide interface by the reaction between diffused oxygen ions and Ti ions.

Comparison of the phase change process in a GST-loaded silicon waveguide and MMI.

In the past decades, silicon photonic integrated circuits (PICs) have been considered a promising approach to solve the bandwidth bottleneck in optical communications and interconnections. Despite the rapid advances, large-scale PICs still face a series of technical challenges, such as large footprint, high power consumption, and lack of optical memory, resulting from the active tuning methods used to control the optical waves. These challenges can be partially addressed by combining chalcogenide phase change materials (PCMs) such as Ge2Sb2Te-5 (GST) with silicon photonics, especially applicable in reconfigurable optical circuit applications due to the nonvolatile nature of the GST. We systematically investigate the phase change process induced by optical and electrical pulses in GST-loaded silicon waveguide and multimode interferometer. Using optical pulse excitation to amorphize GST has a clear advantage in terms of operation speed and energy efficiency, while electrical pulse excitation is more suitable for large-scale integration because it does not require complex optical routing. This study helps us better understand the phase change process and push forward the further development of the Si-GST hybrid photonic integration platform, bringing in new potential applications.

Characteristics of silicon nanowire solar cells with a crescent nanohole.

In recent years, newly emerging photovoltaic (PV) devices based on silicon nanowire solar cells (SiNW-SCs) have attracted considerable research attention. This is due to their efficient light-trapping capability and large carrier transportation and collection with compact size. However, there is a strong desire to find effective strategies to provide high and wideband optical absorption. In this paper, a modified circular nanowire (NW) with a nanocrescent hole is newly introduced and analyzed for solar cell applications. The crescent hole can strongly improve the light absorption through the NW due to the excitation of numbers of modes that can be coupled with the incident light. The material index, volume, and position of the nanohole are studied to significantly increase the optical absorption efficiency and hence the power conversion efficiency (PCE). The absorption performance can be further preserved by using a silicon substrate due to the coupling between the supported modes by the NW, and that of the substrate. The optical and electrical characteristics of the suggested design are investigated using finite difference time domain and finite element methods via Lumerical software packages. The reported asymmetric design offers higher optical and electrical efficiencies compared to the conventional NW counterpart. The proposed NW offers a short circuit current density (Jsc) of 33.85 (34.35) mA/cm2 and power conversion efficiency (PCE) of 16.78 (17.05) % with an enhancement of 16.3 (16.8) % and 17.3 (18.4) % for transverse magnetic (TM) and transverse electric (TE) polarizations, respectively, compared to the conventional cylindrical counterpart.

Study of low-peak-power highly coherent broadband supercontinuum generation through a dispersion-engineered Si-rich silicon nitride waveguide.

Since the first observation by Alfano and Shapiro in the 1970s [Phys. Rev. Lett.24, 584 (1970)PRLTAO0031-900710.1103/PhysRevLett.24.584], supercontinuum generation study has become an attractive research area in the field of broadband light source design, including its use in various applications associated with nonlinear optics in recent years. In this work, the numerical demonstration of ultrabroadband supercontinuum generation in the mid-infrared (MIR) region via the use of complementary metal-oxide semiconductor compatible Si-rich silicon nitride as the core in a planar waveguide design employing one of two materials, either LiNbO3 or MgF2 glass, as the top and bottom claddings is explored. A rigorous numerical investigation of broadband source design in the MIR using 2 mm long Si-rich silicon nitride waveguides is carried out in terms of waveguide structural parameter variations, input peak power variation, varying unexpected deformation of the waveguide along the core region during fabrication, and spectral coherence analysis. Among the several waveguide models studied, two promising designs are identified for wideband supercontinuum generation up to the MIR using a relatively low input peak power of 50 W. Simulation results reveal that spectral coverage spanning from 0.8 µm to 4.6 µm can be obtained by using a LiNbO3-cladded waveguide, and similar spectral coverage is also predicted for the other design, a MgF2-cladded waveguide. To the best of our knowledge, this is the widest spectral span in the MIR region employing a Si-rich silicon nitride waveguide so far. In dispersion tuning as well as in supercontinuum generation, the effect of possible unexpected waveguide deformation along the transverse directions during fabrication is also studied. No significant amount of spectral change is observed in the proposed model for a maximum of 10° inside/outside variation along the width. On the other hand, even 1° upward/downward variation along the thickness could cause substantial spectral change at the waveguide output. Finally, the obtained output spectra from the proposed waveguides are found to be highly coherent and can be applied in various MIR region applications such as optical coherence tomography, spectroscopic measurement, and frequency metrology.

Contra-directional switching enabled by Si-GST grating.

We present the design, simulation, and experimental demonstration of a Si-GST grating assisted contra-directional coupler for optical switching. The effective refractive index of the GST-loaded silicon waveguide changes significantly when the GST is switched from the amorphous state to the crystalline state, allowing for large tuning of the propagation constant. The two coupled waveguides are designed to satisfy the phase-match condition only at the amorphous state to achieve Bragg reflection at the drop-port. Experimental results show that the device insertion loss is less than 5 dB and the extinction ratio is more than 15 dB with an operation bandwidth of 2.2 nm around the 1576 nm operating wavelength. Due to the nonvolatile property of GST, there is no static power consumption to maintain the two states. It is the first demonstration of a GST-enabled grating coupler that can be switched by phase change material.

Design, optimization, and performance evaluation of GSST clad low-loss non-volatile switches.

In this paper, we investigate the performance of a self-sustained ON-OFF switch incorporating a new phase change material ${\text{Ge}_2}{\text{Sb}_2}{\text{Se}_4}{\text{Te}_1}$Ge2Sb2Se4Te1 (GSST) with a silicon rib waveguide at the telecommunication wavelength 1.55 µm. A full-vectorial $\textbf{H}$H-field finite-element method is used to find the effective index and modal loss of the quasi-TE modes in the GSST-Si waveguide. Both the electro-refraction and electro-absorption type design are studied, and the effects of GSST thickness and Si slab thickness on the device performance are presented. These results suggest that a GSST-Si rib waveguide with a 500 nm wide Si core, 60-90 nm Si slab thickness, and 40-60 nm GSST layer could be a realistic switch design that can yield a very compact, 5 µm long device with only 0.135 dB total insertion loss and more than 20 dB extinction ratio.

Machine learning approach for computing optical properties of a photonic crystal fiber.

Photonic crystal fibers (PCFs) are the specialized optical waveguides that led to many interesting applications ranging from nonlinear optical signal processing to high-power fiber amplifiers. In this paper, machine learning techniques are used to compute various optical properties including effective index, effective mode area, dispersion and confinement loss for a solid-core PCF. These machine learning algorithms based on artificial neural networks are able to make accurate predictions of above-mentioned optical properties for usual parameter space of wavelength ranging from 0.5-1.8 µm, pitch from 0.8-2.0 µm, diameter by pitch from 0.6-0.9 and number of rings as 4 or 5 in a silica solid-core PCF. We demonstrate the use of simple and fast-training feed-forward artificial neural networks that predicts the output for unknown device parameters faster than conventional numerical simulation techniques. Computation runtimes required with neural networks (for training and testing) and Lumerical MODE solutions are also compared.

High sensitivity micro-fiber Mach-Zehnder interferometric temperature sensors with a high index ring layer.

The influence of the high index ring layer (HIRL) in a tapered fiber Mach-Zehnder interferometer (MZI) on the interference observed, and thus on its potential applications in temperature sensing, has been investigated. The MZI was comprised of a tapered Ring Core Fiber (RCF), spliced between two single mode fibers (SMF). Since part of core mode from the SMF was converted into cladding modes in the RCF, due to the mismatch in the cores between the RCF and SMF, the residual power enters and then propagates along the center of the RCF (silica). The difference in phase between the radiation travelling along these different paths is separated by the HIRL to generate an interference effect. Compared with fiber interferometers based on core and cladding mode interference, the thin fiber HIRL is capable of separating the high order cladding modes and the silica core mode, under grazing incident conditions. Therefore, the optical path difference (OPD) and the sensitivity are both substantially improved over what is seen in conventional devices, showing their potential for interferometric temperature sensor applications. The optimum temperature sensitivity obtained was 186.6 pm/°C, which is ∼ 11.7 times higher than has been reported previously.

Sensitivity Enhancement of a Concave Shaped Optical Fiber Refractive Index Sensor Covered with Multiple Au Nanowires.

In the present paper, a new kind of concave shaped refractive index sensor (CSRIS) exploiting localized surface plasmon resonance (LSPR) is proposed and numerically optimized. The LSPR effect between polaritons and the core guided mode of designed CSRIS is used to enhance the sensing performance. The sensor is characterized for two types of sensing structures coated with gold (Au) film and Au nanowires (AuNWs), respectively. The influence of structural parameters such as the distance (D) of the concave shaped channel (CSC) from the core, the diameter of the nanowire (dn) and the size (s) of the CSC are investigated here. In comparison to Au film, the AuNWs are shown to significantly enhance the sensitivity and the performance of the designed sensor. An enhanced sensitivity of 4471 nm/RIU (refractive index unit) is obtained with AuNWs, for a wide range of analytes refractive index (na) varying between 1.33 to 1.38. However, for conventional Au film; the sensitivity of 808.57 nm/RIU is obtained for the same range of analytes.

Design of Ultra-Compact Optical Memristive Switches with GST as the Active Material.

In the following study, we propose optical memristive switches consisting of a silicon waveguide integrated with phase-change material Ge2Sb2Te5 (GST). Thanks to its high refractive index contrast between the crystalline and amorphous states, a miniature-size GST material can offer a high switching extinction ratio. We optimize the device design by using finite-difference-time-domain (FDTD) simulations. A device with a length of 4.7 µm including silicon waveguide tapers exhibits a high extinction ratio of 33.1 dB and a low insertion loss of 0.48 dB around the 1550 nm wavelength. The operation bandwidth of the device is around 60 nm.

Elimination of spurious modes in full-vectorial finite element method based acoustic modal solution.

Finite element method is a powerful technique for solving a wide range of engineering problems. However, the existence of the spurious solutions in full-vectorial finite element method has been a major problem for both acoustic and optic modal analyses. For emerging photonic devices exploiting light-sound interactions in high index contrast waveguides, this problem is a major limitation. A penalty function is introduced to remove these unwanted spurious modes in acoustic waveguides, which also identifies the acoustic modes more easily. Numerically simulated results also show considerably improved vector mode profiles. The proposed penalty method has been applied for the characterization of low index contrast single mode fiber and also for high index contrast silicon nanowire to demonstrate its effectiveness.

Nonvolatile waveguide transmission tuning with electrically-driven ultra-small GST phase-change material.

Low-power reconfigurable optical circuits are highly demanded to satisfy a variety of different applications. Conventional electro-optic and thermo-optic refractive index tuning methods in silicon photonics are not suitable for reconfiguration of optical circuits due to their high static power consumption and volatility. We propose and demonstrate a nonvolatile tuning method by utilizing the reversible phase change property of GST integrated on top of the silicon waveguide. The phase change is enabled by applying electrical pulses to the µm-sized GST active region in a sandwich structure. The experimental results show that the optical transmission of the silicon waveguide can be tuned by controlling the phase state of GST.

All-optical non-volatile tuning of an AMZI-coupled ring resonator with GST phase-change material.

We present a Ge2Sb2Te5 (GST)-integrated ring resonator with the tuning enabled by an all-optical phase change of GST using a sequence of optical pulses. The tuning is non-volatile and repeatable, with no static power consumption due to the "self-holding" feature of the GST phase-change material. The 2 µm long GST can be partially crystallized by controlling the number of pulses, increasing the tuning freedom. The coupling between the ring resonator and the bus waveguide is based on an asymmetric Mach-Zehnder interferometer. The coupling strength is wavelength-dependent, so that an optimal wavelength can be selected for the probe light to get more than 20 dB transmission contrast between the amorphous and crystalline GST states.

Efficient strategy to increase higher order inter-modal stability of a step index multimode fiber.

We demonstrate a novel approach to enhance the mode stability through increased effective index difference (Δneff) between the higher-order modes (LP18, LP09 and LP19) of a multimode fiber. Fibers with large diameters have bigger effective mode areas (Aeff) and can be useful for high power lasers and amplifiers. However, a large mode area (LMA) results in an increased number of modes that can be more susceptible to mode coupling. The modal effective index difference (Δneff) strongly correlates with mode stability and this increases as the modal order (m) increases. We report here that the mode spacing between the higher order modes can be further enhanced by introducing doped concentric rings in the core. In our work, we have shown a more than 35% increase in the mode spacing between the higher order modes by optimizing the doping profile of a LMA fiber. The proposed design technique is also scalable and can be applied to improve the mode spacing between different higher order modes and their neighboring antisymmetric modes, as necessary.

Error probability performance of a short-reach multicore fiber optical interconnect transmission system.

A standalone module for rectangular array multicore fiber (MCF)-based optical interconnect (OI) is realized that includes inherent intercore crosstalk and provides space division multiplexed coupling/decoupling of optical power. The module is integrated in a short-reach communication system to provide bit error probability (BEP). Next, a closed-form equation for BEP applicable to MCF OI with intercore crosstalk is derived. For characteristic parameters of the module, results obtained by two approaches agree within 1% for 40 Gbps per channel and predict an error-free transmission of aggregated data rate of 2.5 Tbps through the MCF OI under consideration.

Plastic fiber design for THz generation through wavelength translation.

We report on an all-fiber terahertz (THz) radiation source by exploiting nonlinear parametric process in a theoretically designed microstructured-core double-clad plastic fiber (MC-DCPF). The required phase-matching condition is satisfied through suitable tailoring of the fiber dispersion and nonlinear properties at the pump wavelength of a high-power CO2 laser, with a CO laser of much lower power acting as a seed concomitantly. Our simulated results reveal that a THz radiation source at the frequency of ∼3 THz could be realized with a 3-dB phase-matching band width of 2.13 GHz in a 65-m-long optimized MC-DCPF. Maximum power conversion efficiency >1% is realizable even after including the material loss.

Rigorous analysis of acoustic modes in low and high index contrast silica fibers.

The vector acoustic modes in both well-established and emerging designs of optical waveguides have been studied through use of a computer code which has been developed based on the finite element method (FEM). Dispersion curves and the displacement vectors for the transverse and longitudinal acoustic modes and the modal hybridness have been determined and these are shown for both low and high index contrast silica (SiO2) acoustic waveguides. Stimulated Brillouin scattering (SBS) frequencies are also reported for the subwavelength size SiO2 optical waveguides.