Effective Index Change Research Articles

Measurement accuracy in silicon photonic ring resonator thermometers: identifying and mitigating intrinsic impairments.

Silicon photonic ring resonator thermometers have been shown to provide temperature measurements with a 10 mK accuracy. In this work we identify and quantify the intrinsic on-chip impairments that may limit further improvement in temperature measurement accuracy. The impairments arise from optically induced changes in the waveguide effective index, and from back-reflections and scattering at defects and interfaces inside the ring cavity and along the path between light source and detector. These impairments are characterized for 220 × 500 nm Si waveguide rings by experimental measurement in a calibrated temperature bath and by phenomenological models of ring response. At different optical power levels both positive and negative light induced resonance shifts are observed. For a ring with L = 100 µm cavity length, the self-heating induced resonance red shift can alter the temperature reading by 200 mK at 1 mW incident power, while a small blue shift is observed below 100 µW. The effect of self-heating is shown to be effectively suppressed by choosing longer ring cavities. Scattering and back-reflections often produce split and distorted resonance line shapes. Although these distortions can vary with resonance order, they are almost completely invariant with temperature for a given resonance and do not lead to measurement errors in themselves. The effect of line shape distortions can largely be mitigated by tracking only selected resonance orders with negligible shape distortion, and by measuring the resonance minimum wavelength directly, rather than attempting to fit the entire resonance line shape. The results demonstrate the temperature error due to these impairments can be limited to below the 3 mK level through appropriate design choices and measurement procedures.

Understanding the Role of M13 Bacteriophage Thin Films on a Metallic Nanostructure through a Standard and Dynamic Model.

The dynamic and surface manipulation of the M13 bacteriophage via the meeting application demands the creation of a pathway to design efficient applications with high selectivity and responsivity rates. Here, we report the role of the M13 bacteriophage thin film layer that is deposited on an optical nanostructure involving gold nanoparticles/SiO2/Si, as well as its influence on optical and geometrical properties. The thickness of the M13 bacteriophage layer was controlled by varying either the concentration or humidity exposure levels, and optical studies were conducted. We designed a standard and dynamic model based upon three-dimensional finite-difference time-domain (3D FDTD) simulations that distinguished the respective necessity of each model under variable conditions. As seen in the experiments, the origin of respective peak wavelength positions was addressed in detail with the help of simulations. The importance of the dynamic model was noted when humidity-based experiments were conducted. Upon introducing varied humidity levels, the dynamic model predicted changes in plasmonic properties as a function of changes in NP positioning, gap size, and effective index (this approach agreed with the experiments and simulated results). We believe that this work will provide fundamental insight into understanding and interpreting the geometrical and optical properties of the nanostructures that involve the M13 bacteriophage. By combining such significant plasmonic properties with the numerous benefits of M13 bacteriophage (like low-cost fabrication, multi-wavelength optical characteristics devised from a single structure, reproducibility, reversible characteristics, and surface modification to suit application requirements), it is possible to develop highly efficient integrated plasmonic biomaterial-based sensor nanostructures.

Extremely large nondegenerate nonlinear index and phase shift in epsilon-near-zero materials [Invited

Epsilon-near-zero (ENZ) materials have emerged as viable platforms for strong nonlinear optical (NLO) interactions. The NLO phase shift in materials exhibiting an ENZ condition is extremely large; however, direct experimental measurements of the magnitude and time dynamics of this phenomenon, particularly nondegenerate NLO phase shifts, have so far been lacking. Here, we directly measure the NLO phase shift of an Indium-Tin-Oxide (ITO) thin film using three different techniques. By characterizing the excitation-induced, time-resolved beam deflection (BD) of a probe beam, we measure the nondegenerate NLO effects, allowing a separate determination of the effects of excitation and probe wavelengths on the NLO phase shift as they are varied across the ENZ region. These experiments reveal that having the probe pulse centered at ENZ greatly contributes to this enhancement; however, the NLO phase shift is less sensitive to the excitation wavelength, which only slightly enhances the nonlinearity for obliquely incident TM-polarized light. We also find that the spectral shift of the probe pulse induced by the excitation follows both the magnitude and time dynamics of the NLO phase shift measured via the BD experiments. We observe large, ultrafast cross-phase modulation in agreement with a redistribution of carriers in the conduction band. Finally, using the Z-scan method, we measure the degenerate nonlinear refraction at ENZ near normal incidence. The results of all three measurements agree, revealing a gigantic sub-picosecond NLO phase shift in ITO. At its largest, we consistently measure an effective induced index change greater than the linear index.

Direct Bragg Grating Inscription in Single Mode Step-Index TOPAS/ZEONEX Polymer Optical Fiber Using 520 nm Femtosecond Pulses.

We experimentally report fiber Bragg gratings (FBGs) in a single mode step-index polymer optical fiber (POF) with a core made of TOPAS and cladding made of ZEONEX using 520 nm femtosecond pulses and a point-by-point (PbP) inscription method. With different pulse energies between 9.7 nJ and 11.2 nJ, 12 FBGs are distributed along the cores of two pieces of POFs with negative averaged effective index change up to ~6 × 10−4 in the TOPAS. For POF 1 with FBGs 1–6, the highest reflectivity 45.1% is obtained with a pulse energy of 10.6 nJ. After inscription, good grating stability is reported. Thanks to the post-annealing at 125 °C for 24 h, after cooling the grating reflectivity increases by ~10%. For POF 2 with FBGs 7–12, similar FBG data are obtained showing good reproducibility. Then, the FBGs are annealed at 125 °C for 78 h, and the average reflectivity of the FBGs during the annealing process increases by ~50% compared to that before the annealing, which could be potentially applied to humidity insensitive high temperature measurement.

Renal Function and Pulmonary Fluid Balance in Porcine Models of Acute Lung Injury

Managing pulmonary edema is critical to maintaining blood oxygenation and perfusion for adequate tissue oxygen delivery after acute lung injury. While it is well recognized that renal failure often leads to complications of nephrogenic pulmonary edema, how the regulation of lung fluid is changed during altered renal function in conditions of traumatic injury is poorly understood. We hypothesized that control of lung water balance after a traumatic blunt force lung injury accompanied by a renal response to hemorrhage and hypotension would be different than after simple disruption of the alveolar-capillary liquid barrier interface. Hence, we compared the changes in effective lung water index (ELWI, measured by the Pulse Ion Contour Cardiac Output (PICCO), Getinge) method in a pig model of Traumatic Acute Lung Injury (TALI, n=9), and a model of diffuse alveolar-capillary barrier disruption with oleic acid (OA, n=5), versus a control group (n=10). Anesthetized, mechanically ventilated, Yorkshire cross male and female pigs (body weight 27.0 +/- 0.6 kg) were catheterized with a Swan Ganz catheter for measurement of pulmonary artery pressure and cardiac output determination by thermodilution. Pulmonary function, hemodynamics, and urinary output were examined before and after localized blunt right chest trauma (TALI), or oleic acid injection into the main pulmonary artery (OA 0.3 ml in 50% w/v ethanol). Pulmonary vasoconstriction was induced in the acute lung injury groups as indicated by an increase in pulmonary to systemic vascular resistance ratios in TALI (0.37 +/- 0.03) and OA (0.34 +/-0.08) compared to controls (0.21+/-0.02). TALI resulted in a decrease in mean arterial pressure (MAP, 46+/-4 mmHg) compared to controls (60 +/-2 mmHg) or OA (62+/-3 mm Hg). In all groups ELWI remained steady compared to pre-injury baseline. In the OA and control groups, circulating vasopressin decreased over the time course of the experiment (p<0.01) but did not decrease in the TALI group in accordance with the injury-induced hemorrhage and decreased MAP. Accordingly, urine flow decreased in the TALI group compared to OA and controls. Fractional excretion of water decreased in the OA and control groups, (p<0.01) but remained unchanged in TALI, indicating retention of water in OA and controls compared to TALI. The overall maintenance of ELWI despite differences in renal water handling in TALI compared to controls suggest that protection against pulmonary edema needs to be considered in treatment of hypotension with fluid resuscitation in traumatic lung injury.

A SiGe Slot Approach for Enhancing Strain Induced Pockels Effect in the Mid-IR Range

Strained silicon was proposed more than a decade ago promising to revolutionize the silicon photonics field by allowing efficient modulation in this platform. Despite all the efforts, still rather low χ(2) values have been measured in strained silicon devices. In addition, the way of applying strain has not barely changed since the concept was proposed, usually consisting on a silicon waveguide covered by a stressor material such as silicon nitride. In this letter, a SiGe slot approach is explored as a different route to enhance the strain induced Pockels effect in the mid-IR range. Such approach would allow effective index change values which are near to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> and improve the values expected for the most common silicon - silicon nitride structure by more than three orders of magnitude.

Performance analysis of a silicon NOEMS device applied as an optical modulator based on a slot waveguide.

In this paper, we analyse the performance of a silicon nano-opto-electro-mechanical system (NOEMS) applied as an optical modulator, based on a suspended slot waveguide driven by electrostatic forces. The analysis is carried out with the help of the finite element analysis (FEA) method involving the influences from Casimir force, optical force and electrostatic force. The performance of the modulator are analysed from aspects of actuating modes, actuating voltage, modulating frequency, effective index, phase change, and energy consumption using the FEA method. Simulation results show that a suspended slot modulator has the advantages of low actuation voltage, low power consumption, as well as large effective index and phase change compared with modulators based upon other approaches. The performance of such a modulator can fill the performance gap between the carrier-based approach and micro-opto-electro-mechanical system (MOEMS) approach for modulation.

Compact design of an optical phase shifter packaged with IST microheater used for integrated photonics

A design of compact optical phase shifter which is constructed by two cascading microring resonators and a corresponding packaged microheater is proposed. We illustrated that the footprint of the compact phase shifter could be within 25um × 10um. The effective index change efficiency under thermo-optic effect is significantly increased and the thermal distribution is highly concentrated. In addition, there is a substantial improvement in phase shift efficiency. Only 3 mW power is needed to obtain a phase shift of 2π and 56 mW is needed for 6π. The design could adapt to various optical systems, which will obtain higher integration level and lower power consumption.

Performance Estimation of Hybrid Plasmonic Waveguide in Presence of Stress

In this paper, a detailed numerical investigation has been carried out to find the effect of stress on the optical performances of hybrid plasmonic waveguides (HPWs) operating at 1550 nm wavelength. In the HPWs, the stress is induced due to mismatch in lattice constant and thermal expansion coefficient (TEC) between its different layers. In this paper, optical performances of two HPWs having rectangular and triangular cross-section have been investigated using the finite-element method (FEM) in the presence of stress. For HPW with triangular cross-sections, 25° and 35° ridge angles (α) have been chosen categorically, as they are feasible to fabricate. The effect of stress on optical performance parameters, namely effective refractive index, propagation loss, propagation length, and temperature sensitivity for the HPWs, has been investigated. In this study, different metals (Au and Ag) and dielectric materials (SiO2 and Si3N4) have been introduced in HPWs to realize different magnitude of stresses, which alter the optical performances of the waveguide. Our simulation studies reveal that for all metal and dielectric materials the triangular HPW with 35° ridge angle introduces the highest magnitude of stress as compared to the triangular waveguide with 25° ridge angle and rectangular waveguide. Further, the results show that the presence of stress in the HPW reduces propagation loss and helps in achieving larger propagation length (LP). The work also shows that the largest magnitude of stress is exerted by the triangular HPW (α = 35°) having Si3N4 and Ag materials as dielectric and metal, respectively; and the stress-induced changes in effective index, propagation loss, propagation length and temperature sensitivity of effective index, and propagation length are 0.35%, 42%, 42%, 0.141 × 10− 4/°C (1.45 × 10− 4/°C to 1.309 × 10− 4/°C), and 15.6 nm/°C (5.5 nm/°C to 21.1 nm/°C), respectively. Significant performance variation in HPW in the presence of stress emphasizes the need for the consideration of stress-optic effect while designing HPWs, which can then be employed in photonic integrated circuits.

To the Problem of Modeling Gas Flows Behind the Strong Shock Wave Front Using an Effective Adiabatic Index

In problems involving strong shock waves (the problem of a strong explosion, the motion of bodies with large supersonic speeds, and the problem of supersonic piston expansion), a significant increase in temperature occurs in the disturbed flow region. It is therefore necessary to take the real properties of the gas into account when determining the parameters of the gas flow behind the front of a strong shock wave. This significantly complicates the construction of approximate analytical solutions. However, studies show that the effect of the real properties of the gas on the gas-dynamic parameters of the flow can be taken into account by changing the adiabatic index, i.e., by introducing an effective adiabatic index. If the gas behind the shock wave is in a state of thermodynamic equilibrium, then the effective adiabatic index changes insignificantly in the entire flow zone. This allows us to simulate the flow behind the shock wave front using some perfect gas, whose adiabatic index is determined by the shock wave front depending on the Mach number and the thermodynamic state of the gas. To obtain more accurate solutions for problems with strong shock waves, the model must allow a discontinuity in the adiabatic index at the shock wave. An explicit expression for gas parameters behind the front of an intense shock wave is obtained in this study under the assumption that the adiabatic index undergoes discontinuity during the transition of gas particles through the shock wave surface. Plane and axisymmetric cases are considered.

Analysis of Leaky Modes in Photonic Crystal Fibers Using the Surface Integral Equation Method

A fully vectorial algorithm based on the surface integral equation method for the modelling of leaky modes in photonic crystal fibers (PCFs) by solely solving the complex propagation constants of characteristic equations is presented. It can be used for calculations of the complex effective index and confinement losses of photonic crystal fibers. As complex root examination is the key technique in the solution, the new algorithm which possesses this technique can be used to solve the leaky modes of photonic crystal fibers. The leaky modes of solid-core PCFs with a hexagonal lattice of circular air-holes are reported and discussed. The simulation results indicate how the confinement loss by the imaginary part of the effective index changes with air-hole size, the number of rings of air-holes, and wavelength. Confinement loss reductions can be realized by increasing the air-hole size and the number of air-holes. The results show that the confinement loss rises with wavelength, implying that the light leaks more easily for longer wavelengths; meanwhile, the losses are decreased significantly as the air-hole size d/Λ is increased.

Comprehensive description of the electro-optic effects in strained silicon waveguides

We present a novel and comprehensive analysis method that considers both the plasma-dispersion effect and the strain-induced Pockels effect to faithfully describe the electro-optic effects taking place in a strained silicon waveguide under an applied voltage. The change in carrier distribution arising from the application of a voltage leads to a redistribution of the electrostatic field which deeply affects the strain-induced Pockels effect. By simulating the strain gradient distribution inside the waveguide together with the free carrier concentration in silicon, we were able to describe that the effective index change due to the Pockels effect in strained silicon waveguides and the applied voltage have a nonlinear relationship.

Post-fabrication phase trimming of Mach–Zehnder interferometers by laser annealing of germanium implanted waveguides

We demonstrate a novel high-accuracy post-fabrication trimming technique to fine-tune the phase of integrated Mach–Zehnder interferometers, enabling permanent correction of typical fabrication-based phase errors. The effective index change of the optical mode is 0.19 in our measurement, which is approximately an order of magnitude improvement compared to previous work with similar excess optical loss. Our measurement results suggest that a phase accuracy of 0.078 rad was achievable with active feedback control.

C-band optical filters with micromechanical tuning

Tunable optical filters that cover the entire range of the C-band (1530–565 nm) are designed by utilizing the Vernier effect, i.e. series coupling of microring resonators of different sizes, and the micromechanical tuning method. The micromechanical tuning method employs lateral comb-drive actuators to control evanescent coupling between the resonators and index modulators. Single crystalline silicon is used as the material for all of the main components including bus waveguide cores, resonators, index modulators, and comb-drive actuators. A finite-difference time-domain method is used for optical analysis of the filter. The simulation results show good agreement with those by analytical methods, previously reported. The width of the index modulator is found to play an important role to the filter characteristics. A wider modulator (e.g., width: 100 nm) can cover the full tuning range of 35 nm without switching among different bands owing to stronger effective index change effect, but induces significant loss to the filter, especially when it is brought close to the resonator. While a narrow modulator (e.g., width: 50 nm), on the other hand, induces moderate loss to the filter, it requires hopping among multiple bands to cover the full range since the effective index change incurred is again moderate. In order to achieve linear tuning characteristics in the cascaded-resonator filters, the shaped-finger comb-drive actuator design method is applied. The design method based on two-dimensional slice approximation is further examined by three-dimensional finite element analysis for verification. It is shown that the design method can also work for the cascaded-resonator filters, even for the one that requires band hopping. Effects of fabrication imperfections to the designed device characteristics are studied as well.

Graphene-Assisted Electroabsorption Optical Modulator Using D-Microfiber

We report on an electroabsorption optical modulator using graphene-on-D-microfiber configuration. Two-lamellae of graphene embedded in the dielectric spacer are incorporated with a side-polished microfiber to enhance the graphene–light interaction efficiency measured in terms of effective mode index change. The effects of the side-polished microfiber on different radial distances are analyzed on an effective mode index change and energy flow through the fiber for transverse electric mode by applying bias to the graphene lamellae. The theoretical simulated results show that the effective mode index change is maximized from a value of 0.006 to 0.018, which is almost three times larger than the existing fiber optic modulator. Calculations also show that the electroabsorption optical modulator can be realized with extinction ratio as high as 17.80 dB, modulation bandwidth of 97.26 GHz and operates at C-band of optical communication with the active length of 80 μm long at the expense of 24.30 fJ/bit.

A simple equation to describe cross-sensitivity between temperature and refractive index in fiber Bragg gratings refractometers

In this paper, we propose a simple nonlinear equation to describe the cross-sensitivity between temperature and refractive index in fiber Bragg grating refractometers. A numerical analysis using Comsol Multiphysics software was performed to understand how the effective index changes with the temperature and refractive index. The sensor response to refractive index (from 1.33 to 1.45) and temperature (from 15 °C to 45 °C) was fitted using a surface equation. Even though the refractive index range considered being significantly large, and the sensor response being nonlinear, proper concordance between computer simulations and experiments point out that this multivariable equation is suitable to describe the Bragg wavelengths as a function of temperature and refractive index. When applied to multimode fiber Bragg gratings, the proposed equation can be used to construct a system of equations to calculate temperature and refractive index from the measured Bragg wavelengths.

Design of Mid-infrared electro-optic modulators based on aluminum nitride waveguides

We propose and theoretically evaluate the performance of aluminum nitride (AlN) waveguides as mid-infrared (mid-IR) electrooptic (EO) modulators. The device configurations considered include a strip waveguide modulator and a horizontal slot waveguide modulator. The device dimensions were optimized by finite element analysis of the mode field and electric fields in order to calculate the modulation efficiency and the loss. By applying 15 V on the modulator, a maximum effective waveguide index change of over 2 × 10–5 is obtained using the slot waveguide structure. The modulator has a reasonable loss of ∼2 dB/cm at 2.5 μm wavelength with oxide cladding absorption taken into account. The modulation efficiency of the slot waveguide is found to be twice of the normal strip waveguide. The results also indicate that EO effect in AlN works well over a wide range of wavelengths in mid-IR.

Toward the integration of optical sensors in smartphone screens using femtosecond laser writing.

We demonstrate a new type of sensor incorporated directly into Corning Gorilla glass, an ultraresistant glass widely used in the screen of popular devices such as smartphones, tablets, and smart watches. Although physical space is limited in portable devices, the screens have been so far neglected in regard to functionalization. Our proof-of-concept shows a new niche for photonics device development, in which the screen becomes an active component integrated into the device. The sensor itself is a near-surface waveguide, sensitive to refractive index changes, enabling the analysis of liquids directly on the screen of a smartphone, without the need for any add-ons, thus opening this part of the device to advanced functionalization. The primary function of the screen is unaffected, since the sensor and waveguide are effectively invisible to the naked eye. We fabricated a waveguide just below the glass surface, directly written without any surface preparation, in which the change in refractive index on the surface-air interface changes the light guidance, thus the transmission of light. This work reports on sensor fabrication, using a femtosecond pulsed laser, and the light-interaction model of the beam propagating at the surface is discussed and compared with experimental measurement for refractive indexes in the range 1.3-1.7. A new and improved model, including input and output reflections due to the effective mode index change, is also proposed and yields a better match with our experimental measurements and also with previous measurements reported in the literature.

Optical and magneto-optical properties of plasma-magnetic metamaterials

We investigate the optical and magneto-optical properties of a tunable left-handed material (LHM) consisting of an array of plasma and ferrite layers. It has been shown that the effective refraction index of a homogeneous composite in certain frequencies is negative. It can also be seen that the magnitude of extremum of the negative effective refraction index changes with frequency, external magnetic field and the electron density of plasma layer. In addition, a theoretical calculation of the faraday optical rotation effect of the proposed metamaterial is presented. From the obtained results, we find that there is a large faraday rotation angle in the frequency range where the system shows the left-handed property. Our outcomes demonstrate the potential applications of the device for tunable perfect lenses and active magneto-optic in micro-wave devices.

Optimization of a horizontal slot waveguide biosensor to detect DNA hybridization.

A full-vectorial H-field formulation-based finite element approach is used to optimize a biosensor structure incorporating a horizontal slot waveguide. It is designed to detect DNA hybridization through the change of the effective index of the waveguide structure. The key parameters, such as normalized power confinement, power density, change in effective index, and sensitivity are presented by optimizing the device parameters of the slot waveguide. It is shown here that a 90.0μm long compact Mach-Zehnder section can be designed with horizontal slot waveguide to detect DNA hybridization and for a ring resonator arrangement a sensitivity of 893.5nm/RIU is obtained.