Showing papers on "Fresnel equations published in 2014"

Tunable Characteristics and Mechanism Analysis of the Magnetic Fluid Refractive Index With Applied Magnetic Field

Abstract: The tunable refractive index of the magnetic fluid (MF) is a unique optical property, which has attracted a lot of research interest in recent years. In this paper, a method based on the Fresnel reflection at the fiber end face is presented. Experimental measurements are carried out to investigate the magnetic field (intensity and direction) and temperature-dependent refractive index of the MF. For a given concentration, with the increase of the magnetic field intensity, the nMF increases gradually when H//Light, while decreases when H⊥Light. The effect of temperature on nMF is relatively insignificant with the sensitivity of -8 × 10 -5 /°C. In addition, the mechanism is analyzed from the point of the microstructure by the Monte Carlo method.

High sensitivity lossy mode resonance sensors

Abstract: We propose a sensor based on spectral as well as angular interrogation of lossy mode resonance between absorbing thin film lossy modes and the evanescent wave for measurement of variation in the refractive index of the bulk media. It is shown that a low index dielectric matching layer introduced between the prism and the lossy waveguiding layer can produce an efficient refractive index sensor. The obtained sensitivity (4000–5000 nm/RIU) is comparable to some of the best surface plasmon resonance (SPR) sensors. Unlike the SPR sensor in which surface plasmons can be excited only by p-polarized (TM) light, the proposed sensor can operate at either of these polarizations. Detailed theoretical analysis of the proposed sensor based on Fresnel reflection coefficients and transfer matrix method is presented. Effect of losses (imaginary part of refractive index) on the sensor parameters is also presented in sufficient details. Various parameters such as resonance angle, film thickness and refractive index of various layers are optimized for maximum evanescent field enhancement and increased sensitivity.

Graphene as broadband terahertz antireflection coating

Abstract: We examined the potential of stacked multilayer graphene as broadband terahertz (THz) antireflection coating based on the impedance matching effect in experiment and theory. The reflected pulses from the quartz and silicon substrates were observed to change with the layer number and doping concentration of the graphene coating. Remarkable broadband impedance matching was achieved due to optimized THz conductivity. Theoretical analysis based on Drude model and thin film Fresnel coefficients have been used to explain the experimental phenomena, which indicated the shift of Fermi level caused by chemical doping. This work paves the way for graphene-based broadband THz antireflection coating.

Observing angular deviations in light-beam reflection via weak measurements.

Abstract: An optical analog of the quantum weak measurement scheme proved to be very useful for the observation of optical beam shifts. Here we adapt the weak value amplification method to the observation of the angular Goos-Hanchen shift. We observe this effect in the case of external air-dielectric reflection, the more fundamental case in which it occurs. We show that weak measurements allow for a faithful amplification of the effect at any angle of incidence, even at Brewster's angle of incidence.

As_2S_3–silica double-nanospike waveguide for mid-infrared supercontinuum generation

Abstract: A double-nanospike As2S3–silica hybrid waveguide structure is reported. The structure comprises nanotapers at input and output ends of a step-index waveguide with a subwavelength core (1 μm in diameter), with the aim of increasing the in-coupling and out-coupling efficiency. The design of the input nanospike is numerically optimized to match both the diameter and divergence of the input beam, resulting in efficient excitation of the fundamental mode of the waveguide. The output nanospike is introduced to reduce the output beam divergence and the strong endface Fresnel reflection. The insertion loss of the waveguide is measured to be ∼2 dB at 1550 nm in the case of free-space in-coupling, which is ∼7 dB lower than the previously reported single-nanospike waveguide. By pumping a 3-mm-long waveguide at 1550 nm using a 60-fs fiber laser, an octave-spanning supercontinuum (from 0.8 to beyond 2.5 μm) is generated at 38 pJ input energy.

Microstructured gradient-index antireflective coating fabricated on a fiber tip with direct laser writing.

Abstract: We present a simple broadband gradient-index antireflective coating, fabricated directly on a single mode telecom fiber tip. A regular array of hemi-ellipsoidal protrusions significantly reduce the Fresnel reflection from the glass-air interface. The parameters of the structure were optimized with numerical simulation for the best performance at and around 1550 nm and the coating was fabricated with Direct Laser Writing. The measured reflectance decreased by a factor of 30 at 1550 nm and was below 0.28% for the 100 nm spectral band around the central wavelength. Compared to quarter wavelength antireflective coatings the demonstrated approach offers significantly reduced technological challenges, in particular processing of a single optical material with low sensitivity to imperfections in the fabrication process.

Three-dimensional facial recognition using passive long-wavelength infrared polarimetric imaging.

Abstract: We use a polarimetric camera to record the Stokes parameters and the degree of linear polarization of long-wavelength infrared radiation emitted by human faces. These Stokes images are combined with Fresnel relations to extract the surface normal at each pixel. Integrating over these surface normals yields a three-dimensional facial image. One major difficulty of this technique is that the normal vectors determined from the polarizations are not unique. We overcome this problem by introducing an additional boundary condition on the subject. The major sources of error in producing inversions are noise in the images caused by scattering of the background signal and the ambiguity in determining the surface normals from the Fresnel coefficients.

A Fresnel Reflection-Based Optical Fiber Sensor System for Remote Refractive Index Measurement Using an OTDR

Abstract: In this paper, we propose a Fresnel reflection-based optical fiber sensor system for remote refractive index measurement using the optical time domain reflectometry technique as an interrogation method. The surrounding refractive index from a long distance away can be measured easily by using this sensor system, which operates based on testing the Fresnel reflection intensity from the fiber-sample interface. This system is a simple configuration, which is easy to handle. Experimental results showed that the range of this measurement could reach about 100.8 km, and the refractive index sensitivities were from 38.71 dB/RIU to 304.89 dB/RIU in the refractive index (RI) range from 1.3486 to 1.4525.

Resolution limits of extrinsic Fabry–Perot interferometric displacement sensors utilizing wavelength scanning interrogation

Abstract: The factors limiting the resolution of displacement sensors based on the extrinsic Fabry–Perot interferometer were studied. An analytical model giving the dependency of extrinsic Fabry–Perot interferometric (EFPI) resolution on the parameters of an optical setup and a sensor interrogator was developed. The proposed model enables one to either estimate the limit of possible resolution achievable with a given setup, or derive the requirements for optical elements and/or a sensor interrogator necessary for attaining the desired sensor resolution. An experiment supporting the analytical derivations was performed, demonstrating a large dynamic measurement range (with cavity length from tens of microns to 5 mm), a high baseline resolution (from 14 pm), and good agreement with the model.

Polarization dependence of laser interaction with carbon fibers and CFRP

Abstract: A key factor for laser materials processing is the absorptivity of the material at the laser wavelength, which determines the fraction of the laser energy that is coupled into the material. Based on the Fresnel equations, a theoretical model is used to determine the absorptivity for carbon fiber fabrics and carbon fiber reinforced plastics (CFRP). The surface of each carbon fiber is considered as multiple layers of concentric cylinders of graphite. With this the optical properties of carbon fibers and their composites can be estimated from the well-known optical properties of graphite.

Total internal reflection-based planar waveguide solar concentrator with symmetric air prisms as couplers.

Abstract: We present a waveguide coupling approach for planar waveguide solar concentrator. In this approach, total internal reflection (TIR)-based symmetric air prisms are used as couplers to increase the coupler reflectivity and to maximize the optical efficiency. The proposed concentrator consists of a line focusing cylindrical lens array over a planar waveguide. The TIR-based couplers are located at the focal line of each lens to couple the focused sunlight into the waveguide. The optical system was modeled and simulated with a commercial ray tracing software (Zemax). Results show that the system used with optimized TIR-based couplers can achieve 70% optical efficiency at 50 × geometrical concentration ratio, resulting in a flux concentration ratio of 35 without additional secondary concentrator. An acceptance angle of ± 7.5° is achieved in the x-z plane due to the use of cylindrical lens array as the primary concentrator.

First Results in Near and Mid IR Lithium Niobate-Based Integrated Optics Interferometer Based on SWIFTS-Lippmann Concept

Abstract: High-resolution spectrometers are nowadays achievable in compact devices using integrated optics. The approach developed here consists in obtaining a static interferogram by means of a Fresnel reflection at the waveguide output (Lippmann interference between forward and backward beams) and then sample the fringes by periodically etching the waveguide with transverse nanogrooves, that will collect a negligible part of the flux. We present the first SWIFTS-Lippmann interferometer in the near and mid-infrared, thanks to high form factor grooves obtained by focused ion beam in lithium niobate, which opens the way to electrooptic modulation of the interferogram and thus, sampling on-chip, without any moving part. Possible applications are high-resolution spectroscopy and accurate measurement of effective refractive index of a waveguide. A measurement of the effective group refractive index of the guided mode is presented.

Bio-inspired, sub-wavelength surface structures for ultra-broadband, omni-directional anti-reflection in the mid and far IR

Abstract: Quasi-ordered moth-eye arrays were fabricated in Si using a colloidal lithography method to achieve highly efficient, omni-directional transmission of mid and far infrared (IR) radiation. The effect of structure height and aspect ratio on transmittance and scattering was explored experimentally and modeled quantitatively using effective medium theory. The highest aspect ratio structures (AR = 9.4) achieved peak transmittance of 98%, with >85% transmission for λ = 7-30 μm. A detailed photon balance was constructed by measuring transmission, forward scattering, specular reflection and diffuse reflection to quantify optical losses due to near-field effects. In addition, angle-dependent transmission measurements showed that moth-eye structures provide superior anti-reflective properties compared to unstructured interfaces over a wide angular range (0-60° incidence). The colloidal lithography method presented here is scalable and substrate-independent, providing a general approach to realize moth-eye structures and anti-reflection in many IR-compatible material systems.

Design of Weak 1-D Bragg Grating Filters in SOI Waveguides Using Volume Holography Techniques

Abstract: To answer the growing need for more versatile integrated spectral filters, we show that weak one-dimensional gratings can be designed towards any desired target spectrum. We follow a very straightforward design procedure to demonstrate the performance of these devices. Measurements and simulations show a very good correspondence with the target spectra. By analyzing the results, we also found that the design procedure can be refined by using simulated reflections, instead of relying on the calculated Fresnel reflections.

Radiative transfer equation for predicting light propagation in biological media: comparison of a modified finite volume method, the Monte Carlo technique, and an exact analytical solution

Abstract: We examine the accuracy of a modified finite volume method compared to analytical and Monte Carlo solutions for solving the radiative transfer equation. The model is used for predicting light propagation within a two-dimensional absorbing and highly forward-scattering medium such as biological tissue subjected to a collimated light beam. Numerical simulations for the spatially resolved reflectance and transmittance are presented considering refractive index mismatch with Fresnel reflection at the interface, homogeneous and two-layered media. Time-dependent as well as steady-state cases are considered. In the steady state, it is found that the modified finite volume method is in good agreement with the other two methods. The relative differences between the solutions are found to decrease with spatial mesh refinement applied for the modified finite volume method obtaining <2.4% . In the time domain, the fourth-order Runge-Kutta method is used for the time semi-discretization of the radiative transfer equation. An agreement among the modified finite volume method, Runge-Kutta method, and Monte Carlo solutions are shown, but with relative differences higher than in the steady state.

Reflective properties of randomly rough surfaces under large incidence angles

Abstract: The reflective properties of randomly rough surfaces at large incidence angles have been reported due to their potential applications in some of the radiative heat transfer research areas. The main purpose of this work is to investigate the formation mechanism of the specular reflection peak of rough surfaces at large incidence angles. The bidirectional reflectance distribution function (BRDF) of rough aluminum surfaces with different roughnesses at different incident angles is measured by a three-axis automated scatterometer. This study used a validated and accurate computational model, the rigorous coupled-wave analysis (RCWA) method, to compare and analyze the measurement BRDF results. It is found that the RCWA results show the same trend of specular peak as the measurement. This paper mainly focuses on the relative roughness at the range of 0.16<σ/λ<5.35. As the relative roughness decreases, the specular peak enhancement dramatically increases and the scattering region significantly reduces, especially under large incidence angles. The RCWA and the Rayleigh criterion results have been compared, showing that the relative error of the total integrated scatter increases as the roughness of the surface increases at large incidence angles. In addition, the zero-order diffractive power calculated by RCWA and the reflectance calculated by Fresnel equations are compared. The comparison shows that the relative error declines sharply when the incident angle is large and the roughness is small.

Laser-generated broadband antireflection structures for freeform silicon lenses at terahertz frequencies.

Abstract: We present a flexible technology to generate broadband antireflection (AR) structures for the terahertz (THz) frequency range on planar and curved surfaces of silicon optics. Ultrashort laser pulses are used to ablate the surface to form a pattern of conical pillars with a period of 30 μm. These subwavelength structures act as an effective medium with gradual transition of the refractive index from air to silicon, which reduces the Fresnel reflection losses. The characterization with the THz time-domain spectroscopy system shows an AR effect for a frequency range of 0.1-1.5 THz with a maximum enhancement of the spectral amplitude by ca. 32% at 0.4 THz for planar surfaces. In addition, we demonstrate laser-generated AR structures on convex silicon lenses of both photoconductive emitter and detector devices. Here, the THz pulse amplitude can be increased by about 28%, and single frequencies even show an improvement of the spectral amplitude up to 58%.

9-fold Fresnel–Köhler concentrator with Fresnel lens of variable focal point

Abstract: Non-uniform irradiance patterns over Multi-Junction Cells gives rise to power losses, especially when considering spectral irradiance distributions over different junctions. Thermal effects on Silicone-on-Glass lenses affect spectral irradiance distributions. A new Photovoltaic Concentrator (CPV), formed by nine optical channels, each one with a Kohler configuration, has been designed to overcome these effects at high concentrations for a large acceptance angle. A Fresnel Lens with a Variable Focal Point is proposed to prevent optical crosstalk in multichannel systems. When integrated into the concentrator, improves the acceptance angle. These designs are designed to fulfill the expected requirements of four junction CPV systems.

Modeling the incidence angle dependence of photovoltaic modules in PVsyst

Abstract: For reliable energy yield simulation, it is important to know how irradiance is transmitted through the glass front cover of a photovoltaic module. In this work, Fresnel reflection and Snell's law are used to create a physical model to determine transmission as a function of incidence angle. The physical model is used to describe glass with and without an anti-reflective coating. An approach for optimizing the ASHRAE incidence angle modifier parameter in PVsyst to match the physical model output is presented and applied to both cases and the impact on energy yield estimates is demonstrated for various geographical locations.

Mirror contamination in space I: mirror modelling

Abstract: . We present a comprehensive model that can be employed to describe and correct for degradation of (scan) mirrors and diffusers in satellite instruments that suffer from changing optical Ultraviolet to visible (UV–VIS) properties during their operational lifetime. As trend studies become more important, so does the importance of understanding and correcting for this degradation. This is the case not only with respect to the transmission of the optical components, but also with respect to wavelength, polarisation, or scan-angle effects. Our hypothesis is that mirrors in flight suffer from the deposition of a thin absorbing layer of contaminant, which slowly builds up over time. We describe this with the Mueller matrix formalism and Fresnel equations for thin multi-layer contamination films. Special care is taken to avoid the confusion often present in earlier publications concerning the Mueller matrix calculus with out-of-plane reflections. The method can be applied to any UV–VIS satellite instrument. We illustrate and verify our approach to the optical behaviour of the multiple scan mirrors of SCIAMACHY (onboard ENVISAT).

Polarized radiative transfer in an arbitrary multilayer semitransparent medium

Abstract: Polarized radiative transfer in a multilayer system is an important problem and has wide applications in various fields. In this work, a Monte Carlo (MC) model is developed to simulate polarized radiative transfer in a semitransparent arbitrary multilayer medium with different refractive indices in each layer. Two kinds of polarization mechanisms are considered: scattering by particles and reflection and refraction at the Fresnel surfaces or interfaces. The MC method has an obvious superiority in that complex mathematical derivations can be avoided in solving the polarization by Fresnel reflection and refraction in an arbitrary multilayer system. We define the vector radiative transfer matrix (VRTM), which describes the polarization characteristics of radiative transfer, and obtain four elements of Stokes vector using the VRTM. The results for the two-layer model by MC method are compared against those for coupled atmosphere–ocean model by the discrete–ordinate method available in the literature, which validates the correctness of the MC multilayer model of polarized radiative transfer. Finally, the results for three-layer, five-layer, and ten-layer models are presented in graphical form. Results show that in the multilayer system, total reflections occurring at the surfaces/interfaces have significant effects on the polarized radiative transfer, which causes abrupt changes or fluctuations like waves in the curves of the Stokes vector.

Estimation of Soil Permittivity in Presence of Antenna-Soil Interactions

Abstract: This work deals with a technique for estimating the soil permittivity starting from Ground-Penetrating-Radar (GPR) measurements collected under a reflection mode single view (one transmitting antenna) multistatic (plurality of receivers) configuration. The technique consists of two steps: i) first the soil Fresnel reflection coefficient is estimated; ii) then the soil electromagnetic parameters are inferred from the retrieved reflection coefficient. In real scenarios, the transmitting antenna couples with the soil as it is in close proximity of the air-soil interface so that mutual interactions arise. Accordingly, the transmitting antenna plane-wave spectrum depends on the electromagnetic properties of the medium under investigation. This strongly complicates the problem as the first step becomes a non-linear inverse problem. To overcome such a drawback, a pre-processing stage consisting of a suitable time gating procedure is here introduced. This allows to restore linearity of the Fresnel coefficient estimation. Moreover, it is also able to mitigate negative effect due to not matched transmitting antenna. Numerical examples are presented to assess the feasibility of the proposed technique.

The Fresnel equations for lossy dielectrics and conservation of energy

Abstract: The well-known relation for energy conservation at the boundary of two dielectrics R + T = 1 does not hold in the case of lossy systems, if for R and T the conventional relations are used. It has to be corrected to R + T = 1 + Δ. The additional term is derived for the s- and p-components from Maxwell’s equations by proper use of the Poynting vectors. Δ depends on the angle of incidence, the complex refractive indices and the polarization. The correct transmission factor for lossy transitions reads . The correction term is very small for dielectrics used in thin film technology, but becomes important for metallic layers.

Investigation of surface plasmon resonance phenomena by finite element analysis and Fresnel calculation

Abstract: In this study, a novel simulated method was developed to provide a graphical and simplistic simulation technique for the purpose of enhancing feasibility of classical surface plasmon resonance (SPR) experiment. Winspall, the software based on the Fresnel equations and the matrix formalism was used to compute the reflectivity of optical multilayer systems. In a simulation study, SPR curves were plotted using Winspall with ultrapure water, 50% ethanol aqueous solvent and pure ethanol as the environmental media. SPR angles of 67.1°, 71.7° and 72.5° were obtained, respectively. These values displayed significant disparity from the experimental data of 62.7°, 65.1° and 65.3° under the same conditions. Finite element analysis (FEA) was then evaluated for its suitability as a better alternative method. By establishing a new Kretchmann SPR model and simulating the S11 parameters, the obtained SPR angles of 62.6°, 65.0° and 65.3° agreed well with the experimental results. Thus FEA method was a more feasible alternative based on our experimental conditions. Hence, an optimal methodology for the theoretical study of SPR detection was developed in this study which contains port boundary conditions, background filed and period boundary conditions. More importantly, this study has established a new avenue of SPR chip applications for portable SPR devices.

Raman-induced gratings in atomic media.

Abstract: A novel type of electromagnetically induced gratings based on the Raman nonlinearity in the field of standing pump waves are proposed Unlike electromagnetically induced absorption gratings, these gratings are based on the spatial modulation of Raman susceptibility We present a theoretical study of the optical response of such a spatial periodically modulated three-level atomic medium It is shown that transmission and reflection of a probe Raman wave can be simultaneously amplified in the grating Transmission and reflection spectra can be controlled by varying the pump field intensity The basic mechanism responsible for all-optical control of transmission and reflection in the samples of Raman driven atoms are discussed

Millimeter-Wave Conical Fresnel Zone Lens of Flat Dielectric Rings

Abstract: Conical in shape Fresnel zone (CFZ) lens of flat dielectric rings is introduced and studied in this research. It is contrasted to a plane Fresnel zone (PFZ) lens, a CFZ lens of conical rings and a refractive plane-hyperbolic (PH) lens. For the same aperture diameter of about 25 mm and focal length of 30 mm, a 229-GHz CFZ lens of flat rings with a 30-degree opening semi-angle significantly surpasses the PFZ and PH lenses in focusing gain and efficiency. Also, it exhibits a subwavelength transverse resolution and much higher axial resolution. The cited 30-degree flat-ring CFZ lens has 4.5 times smaller weight than the PH lens, and is easy to manufacture as a plane multilayer package by means of precise machining or modern photolithographic and other microelectronic technologies. Nevertheless, the CFZ lens benefits over the corresponding PH refractive lens are attained on account of smaller frequency bandwidth and bigger lens thickness. The new flat-ring CFZ lens design can be applied in accurate imaging systems or for a creation of light and efficient microwave, terahertz and optical lens antennas.

Development of DOT System For ROI Scanning

Abstract: A simple limited measurement Diffuse Optical Tomographic (DOT) System is developed for scanning region of interest. Simulation and experimental results are presented. The approach is very encouraging and that could pave the way for the functional DOT system.

Interfacial Susceptibilities in Nanoplasmonics via Inversion of Fresnel Coefficients

Abstract: The reflection coefficients of a nanoparticle film are driven to a large extent by perpendicular and parallel interfacial susceptibilities that have the meaning of “dielectric thicknesses” which combine the actual geometry of the film and its dielectric properties. The direct determination of these parameters faces the long-standing issue of the derivation of complex optical constants from Fresnel coefficients via a unique spectroscopic measurement. The present work sets up an iterative algorithm based on inversion of the reflection coefficients recorded in the UV–visible range for two polarization states s and p and Kramers–Kronig (KK) analysis. To calculate the KK integrals over a limited energy window, the strategy was to complement measurements by spectra calculated in the framework of the spheroidal dipole approximation. The algorithm has been successfully tested on synthetic data of differential reflectivity for supported truncated spheres. These were chosen to span different dielectric behaviors, involving (a) for the particles, metals whose optical response is dominated by plasmonic excitations with a noticeable Drude behavior (Ag and Au) and (b) for the substrate, either nonabsorbing wide bandgap (alumina) or semiconducting (zincite and titania) oxides. Unlike the thin plate model, the approach was proven to apply to “dielectric thicknesses” of several tens of nanometres in cases in which, even for geometric sizes of the order of the nanometer, the classical long-wavelength dielectric approximation fails because of strong plasmon resonances. Therefore, the disentanglement of dielectric behaviors along the parallel and perpendicular directions simplifies the understanding on the interface polarization process by removing substrate contribution. The present work that deals with plasmonics in nanoparticles can be easily generalized to different morphologies as well as to other combinations of Fresnel coefficients.