一种基于相位测量的角漂移自适应结构表面等离子体共振气体折射率测量系统

提出了一种可抑制角漂移的表面等离子体传感器结构,并结合相位测量设计了相应的气体折射率测量系统。分析表明激光入射角、金膜厚度以及反射光p、s分量的相位差与气体折射率之间的固有非线性是影响相位响应度与折射率测量精度的主要因素。计算表明所提出的自适应结构将角漂移引起的误差降低了一个数量级并很大程度提高了测量灵敏度。同时分析设计了金膜厚度参数,并评估了反射光p、s分量的相位差与气体折射率之间固有非线性带来的误差。应用于空气折射率的测量比对实验显示其测量精度达到了10-6量级。     

Critical Angle Refractometry for Simultaneous Measurement of Particles in Flow: Size and Relative Refractive Index

The principle of the optical technique critical angle refractometry, used to determine the size and refractive index of spherical particles (with relative refractive index below unity) in liquid flows, was investigated. This technique is based on the observation of the particle scattering pattern around the critical angle. Similarly to the recent technique developed for rainbow scattering pattern analysis for droplet temperature and size measurements, it is shown that the relative particle refractive index (m_r<1) and size can be determined from the position of the primary diffraction fringe and from the angular spacing between two fringes. Explicit equations for refractive index and particle size measurement were derived from the first-order term of the physical optics approximation. An experimental validation test and numerical computations based on the Lorenz-Mie theory were used to validate the principle of the proposed technique and to estimate its sensitivity, which was shown to be of the same order as that of the rainbow technique. This technique is considered to be useful for various applications in liquid multiphase flows where the particles size and material are to be characterized.     

Optical diffraction tomography microscopy with transport of intensity equation using a light-emitting diode array

Optical diffraction tomography (ODT) is an effective label-free technique for quantitatively refractive index imaging, which enables long-term monitoring of the internal three-dimensional (3D) structures and molecular composition of biological cells with minimal perturbation. However, existing optical tomographic methods generally rely on interferometric configuration for phase measurement and sophisticated mechanical systems for sample rotation or beam scanning. Thereby, the measurement is suspect to phase error coming from the coherent speckle, environmental vibrations, and mechanical error during data acquisition process. To overcome these limitations, we present a new ODT technique based on non-interferometric phase retrieval and programmable illumination emitting from a light-emitting diode (LED) array. The experimental system is built based on a traditional bright field microscope, with the light source replaced by a programmable LED array, which provides angle-variable quasi-monochromatic illumination with an angular coverage of ±37 degrees in both x and y directions (corresponding to an illumination numerical aperture of ∼0.6). Transport of intensity equation (TIE) is utilized to recover the phase at different illumination angles, and the refractive index distribution is reconstructed based on the ODT framework under first Rytov approximation. The missing-cone problem in ODT is addressed by using the iterative non-negative constraint algorithm, and the misalignment of the LED array is further numerically corrected to improve the accuracy of refractive index quantification. Experiments on polystyrene beads and thick biological specimens show that the proposed approach allows accurate refractive index reconstruction while greatly reduced the system complexity and environmental sensitivity compared to conventional interferometric ODT approaches.     

Large refractive index with vanishing absorption in optical fibers

We propose and demonstrate a novel scheme for realizing large refractive index with vanishing absorption in a three-level quantum system consisting of the energy levels of Er³⁺-doped ZrF₄–BaF₂–LaF₃–AlF₃–NaF (ZBLAN) optical fiber. It is found that the refractive index of the probe field can be controlled via the coherent coupling field and incoherent pumping field. The switching from negative refractive index to positive refractive index and manipulation of the value of the index of refraction while maintaining vanishing absorption of the probe field can be achieved via tuning the detuning of the coherent coupling field. Our scheme may provide some new possibility for technological applications in fiber communication.     

Liquid refractive index sensor based on slow light in slotted photonic crystal waveguide

A high sensitive and compact refractive index sensor based on slotted photonic crystal waveguide (S-PhCW) is demonstrated. This design is worked on a Mach–Zehnder interferometer (MZI) configuration with S-PhCW as the measuring arm, which can be used to detect any changes in refractive index that correspond to different concentration of the measuring liquid. Combining the slow light enhancement in photonic crystal waveguide (PhCW) with the advantage of excellent optical confinement in slot waveguide, the sensitivity of this simple scheme can reach to 2.3 × 10⁹ nm/RIU with the active region of only 1 mm long.     

Particle nature of light waves in dielectric media

Wave-particle duality is a foundation for modern science. The speed of light waves in dielectric media is less than c. The corresponding particles thus have mass. Combining wave-particle duality with the theory of relativity, an exactly solvable problem was proposed, concerning the transition from photons in vacuum to particles in dielectric media. The rest mass, the momentum, and the total energy of material particles are shown to be the functions of the refractive index of the medium and the wavelength of the incident light. The proposed relationships were applied to study the wavelength-dependent index of refraction of dielectrics and the correlation of the refractive indices of anisotropic crystals, which were confirmed by the experimental results. Variation of the refractive index with wavelength is found to obey the proposed relation. The refractive indices of anisotropic crystals are shown to be the correlated quantities.     

An electromagnetic parameters extraction method for metamaterials based on phase unwrapping technique

Abstract

A new method of extracting effective permittivity and permeability of metamaterials has been developed in this paper. Scattering parameters are used to represent effective wave impedance and refractive index of the metamaterials. Considering analytical continuation of effective refractive index of the metamaterials in the upper half of a complex angular frequency plane, a phase unwrapping technique has been implemented to solve branch ambiguity of the refractive index caused by complex inverse cosine function. An empirical formula to estimate the starting frequency in the phase unwrapping technique has been developed to improve the efficiency of the parameter retrieval procedure. Numerical results including five slabs of rods and split-ring resonators (SRRs) and a structure composed of two SRRs and four capacitively loaded strips are given to demonstrate good accuracy, high efficiency, and noise insensitivity of the proposed retrieval method.     

Measurement of small differences in refractive indices of solutions with interferometric optical method

Based on the principles of both surface plasmon resonance (SPR) and heterodyne interferometry, an optical method for measuring small differences in refractive indices of solutions was proposed. On a specially designed probe, two light beams are incident on both reference and test solutions. The phase differences between the p- and s-polarizations of each reflected light under SPR condition are measured simultaneously with heterodyne interferometric technique. The phase values are substituted into special equations derived from Fresnel's equations. Finally, the difference between the refractive indices of these two solutions can be estimated. The feasibility of this method was demonstrated and the measurement sensitivity of refractive index can reach a value of at least 8.57×10⁻⁷. This method should bear the merits of a simple structure, easy operation, high sensitivity and rapid measurement.     

A fourier optics method for the simulation of measurement-volume-effect by the slit constraint

A new method is introduced to analyze the accuracy of phase-Doppler anemometry (PDA) for sizing large particles in two-phase flows. The method is based on Fourier optics theory (FOT) and geometrical optics theory (GOT) to calculate the intensity ratio of refractive and reflective light scattered by a sphere under a slit constraint. To examine the accuracy in calculating the light intensity, the results from the GOT method were compared with those of the direct simulation based on the generalized Lorenz-Mie theory (GLMT). This newly developed FOT/GOT method also predicts the results of phase jump due to the slit constraint. The performance of dual-mode phase-Doppler anemometry can also be simulated by this method.     

外混合气溶胶粒子光散射的等效性

以两种典型的气溶胶粒子组成的单分散和多分散处理混合气溶胶粒子系统的光散射的各效率因子，各散射截面和散射相函数分析了以等效折射率描述由具有不同折射率的各种粒子组成的混合气溶胶粒子系统的适用性，结果表明，对单分散系统，本不同的混合比下对于许多尺度参数吸收效率因子和散射相函数的等效性很差，对多分散系统，在不同的混合比下等效性较稳定但各散射光学量的余差很大，因而对多分散系外混合气溶胶粒子系统如使用等效折射     

Phase-Doppler Anemometry with the Dual Burst Technique for measurement of refractive index and absorption coefficient simultaneously with size and velocity

The principle of the dual burst technique (DBT) based on phase-Doppler anemometry (PDA) is proposed for simultaneous particle refractive index, size and velocity measurements. This technique used the trajectory effects in PDA systems to separate the two contributions of the different scattering processes. In the case of forward scattering and refracting particles, it is shown that from the phase of the reflected contribution, the particle diameter can be deduced, whereas from the refracted contribution the particle refractive index and velocity can be obtained. Furthermore, the intensity ratio of these two scattering processes can be used for absorption measurements. Simulations based on generalized Lorenz-Mie theory and experimental tests using monodispersed droplets of different refractive indices and absorption coefficients have validated this technique.     

A Polydisperse Sphere Model Describing the Propagation of Light in Biological Tissue

A polydisperse sphere model with the complex refractive index is employed to describe the propagation of light in biological tissue. The scattering coefficient, absorption coefficient and scattering phase function are calculated. At the same time, the inverse problem on retrieving the particles size distribution, imaginary part of the refractive index and number density of scatterers is investigated. The result shows that the retrieval scheme together with the Chahine algorithm is effective in dealing with such an inverse problem. It is also clarified that a group of parameters including the scattering coefficient, absorption coefficient and phase function are associated with another group including the refractive index, particle size distribution and number density of scatterers, which is a problem described in two different ways and the anisotropy factor is not an independent variable, but is determined by the phase function.     

Particle levitation and laboratory scattering

Measurements of light scattering from aerosol particles can provide a non-intrusive in situ method for characterising particle size distributions, composition, refractive index, phase and morphology. When coupled with techniques for isolating single particles, considerable information on the evolution of the properties of a single particle can be gained during changes in environmental conditions or chemical processing. Electrostatic, acoustic and optical techniques have been developed over many decades for capturing and levitating single particles. In this review, we will focus on studies of particles in the Mie size regime and consider the complimentarity of electrostatic and optical techniques for levitating particles and elastic and inelastic light scattering methods for characterising particles. In particular, we will review the specific advantages of establishing a single-beam gradient force optical trap (optical tweezers) for manipulating single particles or arrays of particles. Recent developments in characterising the nature of the optical trap, in applying elastic and inelastic light scattering measurements for characterising trapped particles, and in manipulating particles will be considered.     

Surface plasmon resonance interferometer for bio- and chemical-sensors

An interferometric method for the detection of the phase shifts of reflected light under Surface Plasmon Resonance (SPR) conditions due to refractive index changes is proposed and experimentally realized. The sensitivity threshold of the method to a refractive index variation Δn is estimated to be 4×10⁻⁸. The proposed SPR-interferometer provides spatial phase resolution and thus enables to take into account the peculiarities of refractive index distribution over the surface of an SPR-supporting film. It can be successfully applied in bio- and chemical-sensor systems.     

Diffraction and external reflection by dielectric faceted particles

The scattering of light by dielectric particles much larger than the wavelength of incident light is attributed to diffraction, external reflection and outgoing refracted waves. This paper focuses on diffraction and external reflection by faceted particles, which can be calculated semi-analytically based on physical optics. Three approximate methods; the surface-integral method (SIM), the volume-integral method (VIM), and the diffraction plus reflection pattern from ray optics (DPR) are compared. Four elements of the amplitude scattering matrix in the SIM and the VIM are presented in an explicit form. Of interest is that diffraction and external reflection are separable in the SIM, whereas they are combined in the VIM. A feature of zero forward reflection is noticed in the SIM. The applicability of the DPR method is restricted to particles with random orientations. In the manner of van de Hulst, we develop a new technique to compute the reflection pattern of randomly oriented convex particles using spheres with the same refractive index, resulting in an improvement in the precision of the reflection calculation in near-forward and near-backward directions. The accuracy of the aforementioned three methods is investigated by comparing their results with those from the discrete-dipole-approximation (DDA) method for hexagonal particles at the refractive index of 1.3+i1.0. For particles with fixed orientations, it is found that the SIM and the VIM are comparable in accuracy and applicable when the size parameter is on the order of 20. The ray-spreading effect on the phase function is evident from the results of various size parameters. For randomly oriented particles, the DPR is more efficient than the SIM and the VIM.     

Light scattering by an erythrocyte based on discrete sources method: Shape and refractive index influence

An efficient method for the fast detection of properties of a single erythrocyte from its scattering characteristics is needed in practice. To develop such a method a detailed investigation of the light scattering properties of the erythrocyte and their dependence on its shape and refractive index is of great interest. In this paper the influence of the real erythrocyte's shape with deep concavities and refractive index on the scattering characteristics is analyzed based on an updated scheme of the discrete sources method. Realistic shape models of an erythrocyte, calculated from minima of membrane potential energy are considered. The numerical scheme of discrete sources method has been adjusted for shape profiles given numerically. An improved algorithm allows increasing of an accuracy of calculations.     

Effect of absorption on light scattering by agglomerated debris particles

We study the influence of material absorption on light scattering by agglomerated debris particles whose sizes are comparable with the wavelength. We find that the angular profile of linear polarization is extremely sensitive to the imaginary part of refractive index, and there are some unique features that may assist in the retrieval of physical properties of particles using remote-sensing techniques. Most notably, the position of the positive polarization maximum α_max changes monotonically with the imaginary part of refractive index, allowing it to be used to characterize this property. In addition, the amplitude of the negative polarization branch (NPB) is significantly greater for dielectric particles than for non-dielectric particles. It disappears in the transition region between dielectric and conducting particles before reappearing as the imaginary part of the refractive index is increased further. Further increasing the imaginary part of the refractive index may see the NPB disappearing and reappearing in quasi-periodic fashion. This recurrent NPB has a much smaller amplitude than that of dielectric particles. This suggests that the cometary circumnuclear haloes, which have significant NPBs, cannot contain significant quantities of absorbing particles. In addition, combined observations suggest that the polarization maximum of circumnuclear haloes are relatively small P_max～12%, and occur at relatively small phase angles α_max～60°, which is also consistent with dielectric particles.     

Automated interferometric technique for express analysis of the refractive indices in isotropic and anisotropic optical materials

This paper deals with the technique for the refractive index measurements based on the interferometry of a rotated parallel plate (IRPP). The device consists of the Michelson interferometer, the sample rotation system and the optoelectronic registration system. A refractive index of parallel plates is determined by their rotation through measuring simultaneously a shift of interference fringes. Although the IRPP technique is known from long ago [Shumate MS. Appl Opt 1966;5:327] several considerable improvements have been done in order to improve the accuracy of the method. The measuring process is completely automated. The method has been tested on the model crystals of the lithium niobate giving the magnitudes for ordinary and extraordinary refractive indices as n₀=2.2865±0.0007 and n_e=2.2034±0.0007. A considerable increase of accuracy is reached in our case by an automation of the measuring procedure, development of a new software as well as implementing the interferometric method for a precise determination of a sample zero position. The automated refractometer is offered for use in research laboratories and industry.     

Development of a time-resolved white-light interference microscope for optical phase measurements during fs-laser material processing

A modified Mach–Zehnder interferometer set-up combined with microscope objectives has been developed for the measurement of phase changes in the processed material sample, like modification and melting of glass. The white light is generated by focusing ultrafast laser radiation (t _p=80 fs) in a sapphire crystal using a micro-lens array to minimize temporal and spatial fluctuations in the white-light continuum. Lateral and coaxial pump-probe measurements of the phase changes during material processing are performed using two coupled ultrafast laser sources at different repetition rates (f _rep=1 Hz–1 MHz). The optical phase shift and therefore the refractive index of the material are calculated from the interference images using two approaches. The knowledge of the refractive index during the laser processing with a temporal resolution in the ps-range and a spatial resolution of several microns leads to a better understanding of the initial processes for the permanent material modifications.     

On chemiluminescent emission from an infiltrated chiral sculptured thin film

The theory describing the far-field emission from a dipole source embedded inside a chiral sculptured thin film (CSTF), based on a spectral Green function formalism, was further developed to allow for infiltration of the void regions of the CSTF by a fluid. In doing so, the extended Bruggeman homogenization formalism — which accommodates constituent particles that are small compared to wavelength but not vanishingly small — was used to estimate the relative permittivity parameters of the infiltrated CSTF. For a numerical example, we found that left circularly polarized (LCP) light was preferentially emitted through one face of the CSTF while right circularly polarized (RCP) light was preferentially emitted through the opposite face, at wavelengths within the Bragg regime. The centre wavelength for the preferential emission of LCP/RCP light was red shifted as the refractive index of the infiltrating fluid increased from unity, and this red shift was accentuated when the size of the constituent particles in our homogenization model was increased. Also, the bandwidth of the preferential LCP/RCP emission regime decreased as the refractive index of the infiltrating fluid increased from unity.