Asymmetric dual-core liquid crystal channel-based tunable mode converter.

In this work, a higher order-to-fundamental mode converter is reported and analyzed based on an asymmetric dual channel waveguide (ADC-WG) on silicon. In the reported structure, one of the two waveguides is infiltrated with nematic liquid crystal (NLC) material to add temperature tunability while the other one is a solid BK7 waveguide. The modal characteristics are obtained using the full vectorial finite difference method (FVFDM). In addition, the structural parameters and optical characteristics of the employed materials are investigated to achieve good wavelength selectivity with a short device length (LD). Thus, a compact mode converter that can work at different wavelengths including the telecommunication wavelength i.e., 1.55 µm with LD ~ 482.31 µm and a low crosstalk of - 19.86 dB is presented. To prove the thermal tunability of the suggested mode converter, its operation is tested through a temperature range between 20 and 35 °C and the results show that the mode conversion process is achieved at each temperature with different phase matching wavelengths (λPMW) but with quite similar coupling length (LC). The proposed device can therefore be effectively utilized in integrated photonic circuits.

Efficient MIR crosstalk reduction based on silicon-on-calcium fluoride platform with Ge/Si strip arrays.

Reduction of the crosstalk (CT) between contiguous photonic components is still a big challenge in fabricating high packing density photonic integrated circuits (PICs). Few techniques to accomplish that goal have been offered in recent years but all in the near-IR region. In this paper, we report a design for realizing a highly efficient CT reduction in the MIR regime, for the first time to the best of our knowledge. The reported structure is based on the silicon-on-calcium-fluoride (SOCF) platform with uniform Ge/Si strip arrays. Using Ge strips shows better CT reduction and longer coupling length (Lc) than the conventional Si based devices over a wide bandwidth in the MIR region. The effect of adding a different number of Ge and Si strips with different dimensions between two adjacent Si waveguides on the Lc and hence on the CT is analyzed using both full vectorial finite element method and 3D finite difference time domain method. An increase in the Lc by 4 orders of magnitude and 6.5 times are obtained using Ge and Si strips, respectively, compared to strips-free Si waveguides. Consequently, crosstalk suppression of - 35 dB and - 10 dB for the Ge and Si strips, respectively, is shown. The proposed structure is beneficial for high packing density nanophotonic devices in the MIR regime, such as switches, modulators, splitters, and wavelength division (de)multiplexers, which are important for MIR communication integrated circuits, spectrometers, and sensors.

Overview of Optical Biosensors for Early Cancer Detection: Fundamentals, Applications and Future Perspectives.

Conventional cancer detection and treatment methodologies are based on surgical, chemical and radiational processes, which are expensive, time consuming and painful. Therefore, great interest has been directed toward developing sensitive, inexpensive and rapid techniques for early cancer detection. Optical biosensors have advantages in terms of high sensitivity and being label free with a compact size. In this review paper, the state of the art of optical biosensors for early cancer detection is presented in detail. The basic idea, sensitivity analysis, advantages and limitations of the optical biosensors are discussed. This includes optical biosensors based on plasmonic waveguides, photonic crystal fibers, slot waveguides and metamaterials. Further, the traditional optical methods, such as the colorimetric technique, optical coherence tomography, surface-enhanced Raman spectroscopy and reflectometric interference spectroscopy, are addressed.

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.

Mid-infrared optical modulator based on silicon D-shaped photonic crystal fiber with VO2 material.

Recently, photonic crystal fibers (PCFs) have become of significant interest due to their various applications, especially in the mid-infrared (mid-IR) regime. In this work, an optical mid-IR modulator based on silicon D-shaped PCF (Si-D-PCF) with vanadium dioxide (VO2) as a phase changing material (PCM) is presented and analyzed. Thanks to the phase transition of the VO2 material between insulating (ON) and conducting (OFF) states, the modulation process can be attained. The well-known full vectorial finite element method is utilized to numerically analyze the proposed design. Further, the propagation of light through the suggested structure is studied using the 3D finite difference time domain method. The optical losses of the fundamental TM mode supported by the Si-D-PCF structure in both ON and OFF states are investigated. The obtained results reveal that the extinction ratio (ER) of the reported modulator approaches 236 dB, while the insertion loss (IL) is less than 1.3 dB over the studied wavelength range 3-7 µm at a device length (LD) of 0.5 mm. Additionally, the ER of the proposed modulator is higher than 56 dB through the whole studied wavelength range. Therefore, the proposed modulator could be utilized in photonic integrated circuits that require high ER, low IL, and large bandwidth. To the best of the authors' knowledge, this is the first time an infrared optical modulator based on Si-D-PCF with VO2 material has been presented.

Tunable liquid crystal asymmetric dual-core photonic crystal fiber mode converter.

In this study, a compact mode converter based on an asymmetric dual-core photonic crystal fiber (ADC-PCF) infiltrated with nematic liquid crystal (NLC) is reported. The full vectorial finite difference method is used to compute the modal characteristics of the studied transverse magnetic (TM) mode. In this investigation, the geometrical and material parameters of the proposed mode converter are studied to achieve high wavelength selectivity with a compact device length. The proposed mode converter has a compact device length of 403.6 µm at λ=1.3µm. In addition, the reported device is simulated under different temperature levels from 15°C to 45°C to show the thermal tunability. Therefore, the proposed design can be used efficiently in integrated photonic circuits.

Analysis of highly efficient quad-crescent-shaped Si nanowires solar cell.

Nanostructured semiconductor nanowires (NWs) present a smart solution for broadband absorption solar cells (SCs) with high efficiency and low-cost. In this paper, a novel design of quad crescent-shaped silicon NW SC is introduced and numerically studied. The suggested NW has quad crescent shapes which create a cavity between any adjacent NWs. Such a cavity will permit multiple light scattering with improved absorption. Additionally, new modes will be excited along the NWs, which are highly coupled with the incident light. Further, the surface reflection from the crescent NWs is decreased due to the reduced surface filling ratio. The finite difference time domain method is utilized to analyze the optical characteristics of the reported structure. The proposed NW offers short circuit current density (Jsc) of 27.8 mA/cm2 and ultimate efficiency (ηul) of 34% with an enhancement of 14% and volume reduction of 40% compared to the conventional NWs. The Jsc and ηul are improved to 35.8 mA/cm2 and 43.7% by adding a Si substrate and back reflector to the suggested crescent NWs.

Light absorption enhancement in ultrathin film solar cell with embedded dielectric nanowires.

A novel design of thin-film crystalline silicon solar cell (TF C-Si-SC) is proposed and numerically analyzed. The reported SC has 1.0 µm thickness of C-Si with embedded dielectric silicon dioxide nanowires (NWs). The introduced NWs increase the light scattering in the active layer which improves the optical path length and hence the light absorption. The SC geometry has been optimized using particle swarm optimization (PSO) technique to improve the optical and electrical characteristics. The suggested TF C-Si-SC with two embedded NWs offers photocurrent density ([Formula: see text]) of 32.8 mA cm-2 which is higher than 18 mA cm-2 of the conventional thin film SC with an enhancement of 82.2%. Further, a power conversion efficiency of 15.9% is achieved using the reported SC.

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.

Hybrid Si-VO2 modulator with ultra-high extinction ratio based on slot TM mode.

Nowadays, the development of modern optical systems relies on optical device size minimization and operating power reduction. Optical modulator based on silicon on insulator (SOI) platform is a key element in different optical systems. Therefore, the optical modulator with compact size and low insertion loss could improve the optical system efficiency. In this work, a novel compact optical modulator based on hybrid plasmonic/silicon layers is introduced. The full vectorial finite element method (FV-FEM) is used to numerically analyze the proposed design. Vanadium dioxide (VO2) is also utilized as a cap layer to control the modulation process. The insertion loss (IL) and extinction ratio (ER) of the suggested modulator are equal to 2.1 dB/µm and 28 dB/µm, respectively, at the operating wavelength 1.55 µm. Consequently, high figure-of-merit (FoM) =ER/IL = 13.5 is achieved with an optical bandwidth (ER > 3 dB) greater than 1 µm, which is large in comparison to pervious designs.

CMOS-compatible hybrid bi-metallic TE/TM-pass polarizers based on ITO and ZrN.

Transverse-magnetic (TM) and transverse-electric (TE) pass polarizers based on a silicon-on-insulator platform are studied and analyzed. The proposed structures are CMOS-compatible based on indium tin oxide and zirconium nitride as alternative plasmonic materials. The bi-metallic combination of the plasmonic materials exhibit large coupling between one of the modes (TE or TM) in the silicon core and the surface plasmon mode, while the other mode can propagate with low losses. The numerical simulations for the TE-pass polarizer predict 32.7 dB extinction ratio (ER) and 0.13 dB insertion loss (IL) at a compact device length of 1.5 µm. Additionally, the TM-pass polarizer has 31.5 dB ER and 0.17 dB IL at a device length of 2 µm at an operating wavelength of 1.55 µm.

Quantum Effects In Imaging Nano-Structures Using Photon-Induced Near-Field Electron Microscopy.

In this paper, we introduce the quantum mechanical approach as a more physically-realistic model to accurately quantify the electron-photon interaction in Photon-induced near-field electron microscopy (PINEM). Further, we compare the maximum coupling speed between the electrons and the photons in the quantum and classical regime. For a nanosphere of radius 2.13 nm, full quantum calculations show that the maximum coupling between photon and electron occurs at a slower speed than classical calculations report. In addition, a significant reduction in PINEM field intensity is observed for the full quantum model. Furthermore, we discuss the size limitation for particles imaged using the PIMEN technique and the role of the background material in improving the PINEM intensity. We further report a significant reduction in PINEM intensity in nearly touching plasmonic particles (0.3 nm gap) due to tunneling effect.

Optimized tapered dipole nanoantenna as efficient energy harvester.

In this paper, a novel design of tapered dipole nanoantenna is introduced and numerically analyzed for energy harvesting applications. The proposed design consists of three steps tapered dipole nanoantenna with rectangular shape. Full systematic analysis is carried out where the antenna impedance, return loss, harvesting efficiency and field confinement are calculated using 3D finite element frequency domain method (3D-FEFD). The structure geometrical parameters are optimized using particle swarm algorithm (PSO) to improve the harvesting efficiency and reduce the return loss at wavelength of 500 nm. A harvesting efficiency of 55.3% is achieved which is higher than that of conventional dipole counterpart by 29%. This enhancement is attributed to the high field confinement in the dipole gap as a result of multiple tips created in the nanoantenna design. Furthermore, the antenna input impedance is tuned to match a wide range of fabricated diode based upon the multi-resonance characteristic of the proposed structure.

Funnel-shaped silicon nanowire for highly efficient light trapping.

In this Letter, funnel-shaped silicon nanowires (SiNWs) are newly introduced for highly efficient light trapping. The proposed designs of nanowires are inspired by the funnel shape, which enhances the light trapping with reduced reflections in the wavelength range from 300 to 1100 nm. Composed of both cylindrical and conical units, the funnel nanowires increase the number of leaky mode resonances, yielding an absorption enhancement relative to a uniform nanowire array. The optical properties of the suggested nanowires have been numerically investigated using the 3D finite difference time domain (FDTD) method and compared to cylindrical and conical counterparts. The structural geometrical parameters are studied to maximize the ultimate efficiency and hence the short-circuit current. Carefully engineered structure geometry is shown to yield improved light absorption useful for solar cell applications. The proposed funnel-shaped SiNWs offer an ultimate efficiency of 41.8%, with an enhancement of 54.8% relative to conventional cylindrical SiNWs. Additionally, short-circuit current of 34.2 mA/cm2 is achieved using the suggested design.

Ultra-high tunable liquid crystal-plasmonic photonic crystal fiber polarization filter.

A novel ultra-high tunable photonic crystal fiber (PCF) polarization filter is proposed and analyzed using finite element method. The suggested design has a central hole infiltrated with a nematic liquid crystal (NLC) that offers high tunability with temperature and external electric field. Moreover, the PCF is selectively filled with metal wires into cladding air holes. Results show that the resonance losses and wavelengths are different in x and y polarized directions depending on the rotation angle φ of the NLC. The reported filter of compact device length 0.5 mm can achieve 600 dB / cm resonance losses at φ = 90° for x-polarized mode at communication wavelength of 1300 mm with low losses of 0.00751 dB / cm for y-polarized mode. However, resonance losses of 157.71 dB / cm at φ = 0° can be achieved for y-polarized mode at the same wavelength with low losses of 0.092 dB / cm for x-polarized mode.

Ultrashort silica liquid crystal photonic crystal fiber polarization rotator.

In this Letter, an ultra-compact polarization rotator (PR) based on silica photonic crystal fiber with liquid crystal core is introduced and analyzed using full-vectorial finite difference approaches. The analyzed parameters of the suggested PR are the conversion length, modal hybridness, power conversion and crosstalk. In addition, the fabrication tolerance analysis of the reported design is investigated in detail. The proposed PR has an ultra-compact device length of 4.085 µm and an almost 100% polarization conversion ratio.

Design of passive polarization rotator based on silica photonic crystal fiber.

We propose and analyze a novel (to the best of our knowledge) design of a polarization rotator (PR) based on silica photonic crystal fiber. The proposed design has a rectangular core region with a slanted sidewall. The simulation results are obtained using the full vectorial finite difference method as well as the full vectorial finite difference beam propagation method. The numerical results reveal that the suggested PR can provide a nearly 100% polarization conversion ratio with a device length of 3102 µm.

Does postdural puncture headache left untreated lead to subdural hematoma? Case report and review of the literature.

The patient was a 39-year-old pregnant woman who was scheduled for cesarean section. Spinal anesthesia was induced using a 26-gauge needle with an atraumatic bevel. Postoperatively, the patient developed cranial subdural hematoma manifesting as severe non-postural headache, associated with right eye tearing, fifth cranial nerve palsy and left hemiparesis. The diagnosis was confirmed by computed tomography scan. The patient was managed by careful neurological follow-up associated with conservative treatment and recovered fully after 12 weeks. Our report reviews the literature on 46 patients who developed a postdural puncture headache complicated by subdural hematoma following spinal or epidural anesthesia. It is possible that postdural puncture headache left untreated may be complicated by the development of subdural hematoma. Patients developing a postdural puncture headache unrelieved by conservative measures, as well as the change from postural to non-postural, require careful follow-up for early diagnosis and management of possible subdural hematoma.

Does postdural puncture headache left untreated lead to subdural hematoma? Case report and review of the literature.

The patient was a 39-year-old pregnant woman who was scheduled for cesarean section. Spinal anesthesia was induced using a 26-gauge needle with an atraumatic bevel. Postoperatively, the patient developed cranial subdural hematoma manifesting as severe non-postural headache, associated with right eye tearing, fifth cranial nerve palsy and left hemiparesis. The diagnosis was confirmed by computed tomography scan. The patient was managed by careful neurological follow-up associated with conservative treatment and recovered fully after 12 weeks. Our report reviews the literature on 46 patients who developed a postdural puncture headache complicated by subdural hematoma following spinal or epidural anesthesia. It is possible that postdural puncture headache left untreated may be complicated by the development of subdural hematoma. Patients developing a postdural puncture headache unrelieved by conservative measures, as well as the change from postural to non-postural, require careful follow-up for early diagnosis and management of possible subdural hematoma.

[Intervertebral biopolymer implant for arthrodesis. A study of 45 cases]. / Implant intervertébral au biocopolymère (B.O.P.) pour arthrodèse. Etude de 45 cas.

The B.O.P. biocopolymer is a bioresorbable, biocompatible product which can ossify. It has been used in 45 spinal arthrodesis, 42 times in cervical region but three in lumbar spine. The authors report their results after a follow-up period of 8 to 28 months. Persistent cervical pain has been noted in 9 cases. This was the most important postoperative complaint. Graft ossification has nevertheless been observed only in 40% of cases, as proved by simple X rays or/and scanner examination over a 26 months period. An osseous overproduction with posterior osteophytes could otherwise be observed. The authors remains cautious about using moreover this kind of graft, at least in its actual presentation, though the clinical results can be compared with these obtained with osseous autograft.