New optical configuration for nondestructive measurements of refractive index profiles of LiNbO(3) waveguides

A direct and nondestructive measurement technique for the determination of the refractive index profiles of one- and two-dimensional LiNbO(3) waveguides is presented for the first time to our knowledge. The technique generalizes the refracted near-field method, which is well known for optical fiber characterization. The spatial resolution and accuracy are 0.1 and 0.4 μm, respectively. The refractive-index calibration is done by an analysis of the near-field light power distribution, and its resolution is approximately 2 × 10(-4). The proposed experimental setup permits sample installation and data acquisition in a few minutes.     

Effect of interfacial refractive index on optical molecular orientation measurements

The sensitivity of optical molecular orientation measurements to assumptions regarding thin film refractive index was investigated. Specifically, the influence of the interfacial refractive index on second harmonic generation (SHG) and linear dichroism measurements made in a total internal reflection (TIR) geometry was probed for five distinct molecular systems. The five molecular thin films ranged from weakly adsorbed species in equilibrium with solution to covalently bound molecules. Polarization data from the two techniques were fit using a range of assumed interfacial refractive indices. Surprisingly, a linear relationship between the difference in calculated apparent orientation angle and the difference in solvent-prism refractive index was observed. The trend indicates that for a TIR geometry, the error introduced by the thin film refractive index is negligible when the difference in solvent and prism refractive indices is less than approximately 0.08. However, there are clearly cases, such as a glass/air interface, in which assumptions regarding the thin film refractive index can result in significant error in the extracted orientation angle.     

Temperature-dependent refractive index measurements of wafer-shaped InAs and InSb

An experimental method is introduced to measure the refractive index and its temperature dependence for wafer-shaped infrared materials over a continuous temperature range. Using a combination of Michelson interferometry, Fabry-Perot interferometry, and a temperature-controlled cryostat in a laser micrometer, refractive index values and their temperature coefficients can be measured for any specific temperature within a desired temperature range. Measurements are reported for InAs and InSb for a laser wavelength of 10.59 microm.     

Extending ATOFMS measurements to include refractive index and density

An absolute calibration of the light scattering region in an aerosol time-of-flight mass spectrometer (ATOFMS) has been performed enabling a direct comparison of the average experimentally measured light scattering intensity to theory. A fitting procedure allows for the determination of both refractive index and density for spherical homogeneous particles. The scattering information has been correlated with the other single-particle information measured by the ATOFMS. Size, chemical composition, and scattering intensity can all be linked to establish a better understanding of the relationships between the chemical and physical properties of aerosol particles. Currently, inputs into climate models are derived from data acquired from bulk aerosol composition measurements, and therefore, assumptions must be made regarding the chemical associations within individual particles (mixing state) to enable modelers to calculate the relevant aerosol optical properties. These new measurements aim for the goal of directly testing the model assumptions by utilizing single-particle chemical information to derive the optical properties of the different particle classes.     

Optoelectronic refractive index measurements: application to smart processing

In the last decade, polymeric-based composites, in light of their low weight/mechanical strength ratio, have been widely used in many industrial areas, such as automotive, aeronautic, and aerospace areas. Because of the dependence of their properties on the manufacturing stage, real-time monitoring of the processing stage has been indicated as the key point for improving the quality and reducing manufacturing process costs through an intelligent control of the manufacture itself. To this aim, optimal monitoring systems should be non-intrusive, real-time, and able to measure a physical property changing during the process development. Refractive index is a suitable state parameter being directly correlated to the material density. In this work, the assessment of the performances of a fiber-optic refractometer has been presented. Experimental results demonstrate the capability of the sensor system to monitor the cure kinetics of a polymeric thermoset and to measure its glassy transition temperature.     

Boosting the figure-of-merit of LSPR-based refractive index sensing by phase-sensitive measurements

Localized surface plasmon resonances possess very interesting properties for a wide variety of sensing applications. In many of the existing applications, only the intensity of the reflected or transmitted signals is taken into account, while the phase information is ignored. At the center frequency of a (localized) surface plasmon resonance, the electron cloud makes the transition between in- and out-of-phase oscillation with respect to the incident wave. Here we show that this information can experimentally be extracted by performing phase-sensitive measurements, which result in linewidths that are almost 1 order of magnitude smaller than those for intensity based measurements. As this phase change is an intrinsic property of a plasmon resonance, this opens up many possibilities for boosting the figure-of-merit (FOM) of refractive index sensing by taking into account the phase of the plasmon resonance. We experimentally investigated this for two model systems: randomly distributed gold nanodisks and gold nanorings on top of a continuous gold layer and a dielectric spacer and observed FOM values up to 8.3 and 16.5 for the respective nanoparticles.     

Construction of glass waveguide refractive index profiles by the effective-index finite-difference method

A numerical method is applied to construct the refractive index profiles of optical waveguides from the measured effective indices (EI). The method is based on choosing a proper analytical function for the refractive index profile and searching its unknown parameters using the simplex search algorithm. Simultaneously, the finite-difference method (FDM) is used to solve the semi-vectorial Helmholtz equation for the guided modes effective indices. The method is applied successfully to two particular Ag⁺---Na⁺ ion-exchanged glass slab waveguides. The results are as accurate as those obtained from from commonly used IWKB-based method. The EI-FDM in principle can be applied to both slab and channel waveguides and does not require that the index profiles are monotonically decreasing, like most of IWKB-based methods. The relation between the induced refractive index and silver concentration profile, measured by SIMS, is found to be almost linear.     

Simultaneous determination of thickness and refractive index based on time-of-flight measurements of terahertz pulse

We present a simple technique for simultaneous determination of thickness and refractive index of plane-parallel samples in the terahertz radiation domain. The technique uses time-of-flight measurements of the terahertz pulse. It has been employed on nine different polymers and semiconductor materials, which are transparent for terahertz frequencies. Our results of thickness measurement are in good agreement with micrometer reading. The accuracy in the determination of refractive index is on the order of two decimal points.     

Fabrication of Maxwell fish-eye spherical lenses and research on distribution profiles of gradient refractive index

We establish the equation of distribution of the refractive index of Maxwell fish-eye spherical lenses and report fabrication of Maxwell fish-eye spherical lenses using optical glasses. The distribution profiles are discussed for the process of ion exchanging, and the experimental results accord with the theoretical analysis.     

Quantitative phase and refractive index measurements with point-source digital in-line holographic microscopy

Point-source digital in-line holographic microscopy with numerical reconstruction is ideally suited for quantitative phase measurements to determine optical path lengths and to extract changes in refractive index within accuracy close to 0.001 on the submicrometer length scale. This is demonstrated with simulated holograms and with detailed measurements on a number of different micrometer-sized samples such as suspended drops, optical fibers, as well as organisms of biological interest such as E. coli bacteria, HeLa cells, and fibroblast cells.     

Estimation of complex refractive index of polydisperse particulate systems from multiple-scattered ultraviolet-visible-near-infrared measurements

A method to extract the complex refractive index of spherical particles from a polydisperse suspension at concentrations where multiple light-scattering effects are significant is presented. The optical constants are estimated from total diffuse reflectance and transmittance measurements and inverting the measurements using the radiative transfer equation (RTE) and the Mie theory for scattering by polydisperse spherical particles. The method is tested by applying it to three different polydisperse polystyrene suspensions and extracting the optical constants of polystyrene particles in the wavelength range of 450-1200 nm. The effect of particle size, concentration, and polydispersity on the estimated values of the optical constants is also discussed.     

Vector wave analysis for nonnormal incident rays in epimicroscopic refractive index profile measurements

We have investigated the effects of nonnormal incident rays in calculating the refractive index profile of a dielectric sample using the reflectance measurement data obtained with a scanning confocal epimicroscope and also by solving three-dimensional vector wave equations for linearly polarized light. The numerically calculated reflection data of tightly focused Gaussian beams with different numerical apertures (NAs) on planar surfaces with various refractive indices confirm that the reflectance increases with an increase in the NA of a focusing objective lens. This is due to the nonnormal incident ray components of a Gaussian beam. We have found that the refractive index obtained with the assumption of a normal incident beam is far from the real value when the NA of a focusing lens becomes larger than 0.5, and thus the variation in the reflectance for different angular components in a Gaussian beam must be taken into consideration while using a larger NA lens. Errors in practical refractive index calculation for an optical fiber based on a normal incident beam in reflectance measurements can be as large as 1% in comparison to real values calculated by our three-dimensional vector wave equations.     

Influence of internal refractive index gradients on size measurements of spherically symmetric particles by phase Doppler anemometry

A model based on geometric optics for predicting the response of interferometric (phase Doppler) instruments for size measurements of particles with radially symmetric but inhomogeneous internal refractive index profiles is developed. The model and results are important for applications in which heat or mass transfer from the particles or droplets is significant, for example, in liquid-fuel combustion. To quantify the magnitude of potential bias errors introduced by the classical assumption of uniform internal properties on phase Doppler measurements, we compute calibration curves for a sequence of times during the evaporation of a decane droplet immersed in an environment of T = 2000 K and p = 10 bars. The results reveal considerable effects on the relation between phase difference and droplet diameter caused by the refractive index gradients present. The model provides an important tool to assess sizing uncertainties that can be expected when applying conventional (based on uniform properties) phase Doppler calibration curves in spray combustion and similar processes.     

折射率浅释

折射是光入射介质的多种光学现象之一,而折射率是表征各种介质(材料)物理、化学性质的重要参数。科学界对其了解伴随着光学发展的整个历程。文章从折射率的定义出发,介绍了它的物理含义、测量方法、应用以及关于"负折射率"的研究发展。     

Near band-edge field-dependent absorption coefficient and refractive index determined by photocurrent and transmittance measurements

A technique is proposed for the extraction of precise values of field-dependent absorption coefficient alpha and refractive index n from photocurrent and transmittance measurements of optical modulator structures. The technique uses approximate results of alpha and n extracted from a simplified device as the initial input into an iterative procedure that utilizes the consistency between alpha and n to obtain successively better estimates of these parameters. The technique was applied to results that were measured experimentally, and we verified the accuracy by using synthetic data. Errors caused by measurement inaccuracy are also investigated. It is shown that the absorption coefficient has a modest sensitivity whereas the refractive index is insensitive to these errors.     

Temperature-dependent refractive index of semiconductors

A single-oscillator Lorentz model is applied to four different semiconductors having diamond-like crystal structure to describe the temperature dependence of their refractive index between 300 and 600 K. Theoretical results are compared to previous experiments and to experiments carried out in this study for Si, Ge, GaAs, and InP. An efficient experimental method is also presented, enabling fast measurements of the refractive index of materials. Using the Yu-Brooks formalism and the energy bandgap at the X-point of the Brillouin zone, the temperature-dependent refractive indices are calculated and they agree well with experiments, particularly, considering the simplicity of the Lorentz model. However, there are discrepancies between the theory and experiment at high temperatures (near 600 K) in certain cases. This discrepancy may be due to the single-oscillator approximation. Additionally the effect of “self-energy” on the temperature dependence of the energy bandgap, such as the temperature-dependent damping of the oscillation of electrons, can be significant at higher temperatures.     

Ellipsometry and optical measurements on amorphous thin films of As2Se3

Thin films of amorphous AsSe_3/2 have been prepared by thermal evaporation of the material under a vacuum of 1.33×10^?3 Pa. Reflectivity, transmission and ellipsometric measurements of the films have been carried out. The optical energy gap and the absorption coefficient as a function of wavelength were obtained. Two absorption bands were observed and interpreted in terms of defects in the AsSe_3/2 system (homopolar bonds). Analysis of reflection and transmission spectra shows that the electron density at band tails of both conduction and valency bands follows N(E)?E^1/2 (Taue plots). No considerable variations were observed on changing the film thickness.     

Determination of the refractive index and size distribution of aerosol from dual-scattering-angle optical particle counter measurements

A method is presented for inferring both the refractive index and the size distribution of aerosol from observations of a dual-scattering-angle optical particle counter (OPC). An existing prototype of an OPC with 60 degree and 90 degree dual-scattering angles was used for the experiments. Based on the high sensitivity of the OPC response to the refractive index of particulates, two families of size distribution curves may be calculated. The solution of the refractive index corresponds to the superposition of the two size distributions. This method was applied to the simulation and to the field measurements conducted in Beijing and Hefei, and the results of both are presented.