Measuring refractive index gradient of sugar solution

To measure refractive index at a particular altitude in a solution with vertical refractive index gradient, a transparent wedge-shaped container was constructed altogether with the development of mathematical formula derived from the Snell’s law. The refractive index of the solution can be calculated by measuring the angles of incoming and outgoing laser beams relative to respective normal line. By varying height of the laser beam, the refractive index as a function of height of a sugar solution was obtained. This technique is applied to investigate Fata Morgana which is a kind of superior mirage resulting from bending of light in a medium with density gradient.

Planar refractive index patterning through microcontact photo-thermal annealing of a printable organic/inorganic hybrid material

We demonstrate proof-of-concept refractive-index structures with large refractive-index-gradient profiles, using a micro-contact photothermal annealing (CPA) process to pattern organic/inorganic hybrid materials comprising titanium oxide hydrate within a poly(vinyl alcohol) binder....

Mountain Top-Based Atmospheric Radio Occultation Observations with Open/Closed Loop Tracking: Experiment and Validation

Through Global Navigation Satellite System (GNSS) occultation measurement, the global ionosphere and atmosphere can be observed. When the navigation satellites’ signal passes through the lower atmosphere, the rapid change of the atmospheric refractive index gradient will cause serious multipath phenomena in radio wave propagation. Atmospheric doppler frequency shift and amplitude signal fluctuations increase drastically. Due to the attenuation of signal amplitude and the rapid change of the Doppler frequency, the general phase locked loop (PLL) cannot work properly. Hence, a more stable tracking technology is needed to track the occultation signal passing through the lower atmosphere. In this paper, a mountain-top based radio occultation experiment is performed, where we employ an open-loop receiver and remove the navigation bits by the internal demodulation. In the process of the experiment, we adopt the open-loop tracking technique and there is no feedback between the observed signal and the control model. Specifically, taking the pseudo-range and doppler information from models as input, three key parameters, i.e., accurate code phase, carrier doppler and code doppler, can be obtained, and furthermore, the accurate accumulation is determined by them. For the full open-loop occultation data, a closed-loop observation assisted strategy is presented to compare the tracking results between open-loop and closed-loop occultation data. Through the compared results, we can determine whether the initial phase has been reversed or not, and obtain the high consistency corrected open-loop data that can be directly used for subsequent atmospheric parameters inversion. To verify the effect of open-loop tracking and open-loop inversion, we used the company’s self-developed occult receiver system for verification. The company’s self-developed occult receiver system supports Global Position System (GPS)/Beidou satellites constellation (BD, the 2nd and 3rd generations) dual systems. We have verified GPS and BD open-loop tracking and inversion, carried out in a three-week mountain-based experiment. We used closed-loop and open-loop strategies to track and capture the same navigation star to detect its acquisition effect. Finally, we counted the results of a week (we only listed the GPS data; BD’s effect is similar). The experimental results show that the open-loop has expanded the signal-cut-off angle by nearly 20% under the condition of counting all angles, while the open-loop has increased the signal-cut-off angle value by nearly 89% when only calculating the negative angle. Finally, the atmosphere profiles retrieved from observations in open-loop tracking mode are evaluated with the local observations of temperature, humidity and pressure provided by the Beijing Meteorological Bureau, and it is concluded that the error of open-loop tracking method is within ~4% in MSER (mean square error of relative error), which meets the accuracy of its applications (<5%, in MSER).

Synthesis methods for realistic images of three-dimensional scenes containing media with a refractive index gradient

The paper presents the results of a study of the possibility of implementing an effective and physically correct stochastic ray tracing in gradient media based on the Runge-Kutta method. For implementation in the photorealistic rendering system, the specifics of the ray tracing method in complex three-dimensional scenes were considered. One of the main features of ray tracing in geometrically complex scenes is the large volume of geometric primitives that need to be tested for the intersection of the ray segment with the primitives. A method of ray propagation in voxel space of the scene is proposed. The method allows significant speeding up the process of searching for ray intersections with geometry primitives. To implement these ray tracing features the special program interface for gradient media was proposed, which can become the basic interface for a media of all types. Methods for calculating the luminance of all lighting components in gradient media were considered. The results of modeling the propagation of rays and image synthesis in a fiber with a refractive index gradient are presented.

Hybrid Adaptive Wavelet-Based Optical Flow Algorithm for Background Oriented Schlieren (BOS) Experiments

Quantitative analysis of the flow field is an effective method to study hydrodynamics. As a flow field measurement technology, the Background Oriented Schlieren (BOS) is widely used. However, it is difficult to measure the complex transparent flow field (flow field with large refractive index gradient) using the BOS experiment. In order to overcome this difficulty and improve the accuracy of the BOS experiment, this paper presents a hybrid adaptive wavelet-based optical flow algorithm for the BOS. The current algorithm is a combination of the traditional optical flow algorithm and the wavelet-based optical flow algorithm. By adding the initial value constraints, the adaptive scale constraints, and the adaptive regularization constraints, the algorithm can effectively overcome the above-mentioned difficulty and also improve its accuracy. To further illustrate the feasibility of the proposed method, this paper uses the simulation data, the data of the DNS datasets, and the data of the BOS experiment to verify the performance of the algorithm. The experiment of comparing the proposed algorithm with the existing ones is carried out on the DNS datasets and the data of the BOS experiment. Finally, the proposed method is verified by a practical BOS experiment. The results show that the proposed algorithm can effectively improve the measurement accuracy of displacements.

Thin SiNC/SiOC Coatings with a Gradient of Refractive Index Deposited from Organosilicon Precursor

In this work, optical coatings with a gradient of the refractive index are described. Its aim was to deposit, using the RF PECVD method, films of variable composition (ranging from silicon carbon-oxide to silicon carbon-nitride) for a smooth change of their optical properties enabling a production of the filter with a refractive index gradient. For that purpose, two organosilicon compounds, namely tetramethyldisilazane and hexamethyldisilazane, were selected as precursor compounds. The results reveal better optical properties of the materials obtained from the latter source. Depending on whether deposited in pure oxygen atmosphere or under conditions of pure nitrogen, the refractive index of the coatings amounted to 1.65 and to 2.22, respectively. By using a variable composition N2/O2 gas mixture, coatings of intermediate magnitudes of “n” were acquired. The optical properties were investigated using both UV-Vis absorption spectroscopy and variable angle spectroscopic ellipsometry. The chemical structure of the coatings was studied with the help of Fourier transform infrared and X-ray photoelectron spectroscopies. Finally, atomic force microscopy was applied to examine their surface topography. As the last step, a “cold mirror” type interference filter with a gradient of refractive index was designed and manufactured.

High Homogeneity of Magnesium Doped LiNbO3 Crystals Grown by Bridgman Method

A series of LiNbO3 crystals doped with various MgO concentrations (0, 3%, and 5 mol%) was simultaneously grown in one furnace by the modified vertical Bridgman method. The wet chemistry method was used to prepare the polycrystalline powders, and the growth conditions were optimized. The full width at half maximum of high-resolution X-ray rocking curves for (001) reflection of 5 mol% Mg doped lithium niobate (LN) crystal was about 8″, which meant it possessed high crystalline quality. The OH− absorption spectra shifted to 3534.7 cm−1, and the UV absorption edge violet shift indicated that 5 mol% MgO successfully doped in LN and exceeded the threshold. The extraordinary refractive index gradient of 5 mol% Mg doped LN crystal was as small as 2.5 × 10−5/cm, which exhibited high optical homogeneity.

GRADIENT ALIGNMENT MATERIALS WITH DIFFERENT PHOTOSENSITIVITY FOR TUNABLE POLARIZATION-INDEPENDENT LIQUID CRYSTAL LENS

The development of electrically tunable liquid crystal (LC) lenses is perspective and promising for a wide range of applications, for example, for imaging system, pico projectors, optical zoom systems, ophthalmology applications and other. Of particular note is the development of polarization-independent LC lenses, as eliminates polarizers from application devices that reduce the efficiency of light transmission through optical systems. Alignment benzaldehyde photosensitive materials, capable of changing the pretilt angles of nematic LC from 90 to 0 ºС in a controlled manner under UV exposure are developed. The anisotropy of the benzaldehyde alignment layers is generated by a two-stage treatment consisting of uniform rubbing with a cloth and subsequent non-polarized UV exposure. Inhomogeneous UV exposure of uniformly rubbed alignment layers allows formation of refractive index gradient inside the LC cell. The concept of tunable polarization-independent self-aligned LC lens based on gradient pretilt angle alignment materials with different photosensitivity is demonstrated. Self-alignment of two polarization-dependent sub-lens is achieved due to a single UV exposure act of two alignment layers, which are located on the same piece of glass on both sides, forming one common optical axis for a polarization-independent LC lens. The independence of the polarization of LC lenses is achieved by setting the azimuthal rubbing direction of the alignment layers of two polarizationdependent LC lenses perpendicular to each other. The sub-lens cells have uniform cell gap and are independently controlled using low-voltage driving. Devices based on gradient benzaldehyde alignment materials can be used in many modern optical and photonic devices.

Universally Applicable Fabrication Technique for Biomimetic Nanocone Arrays on Flexible Polymer Substrates for Anti-Reflection Functionality

Inspired by the anti-reflection functionality of cicada wings decorated with nanocone arrays, a facile technique to endow flexible polymer substrates of diverse chemical compositions with the same functionalities has been devised. In this universally applicable two-step technique based on a capacitatively coupled radiofrequency plasma (CCRP), first oxygen-plasma treatment (OPT) is implemented to grow arrays of vertical elongated nanostructures with almost uniform cross-sectional diameter of the polymer substrate, and then fluorocarbon polymer deposition (FPD) is carried out so that the nanostructures evolve into nanocones with small apex angle. The dependence of ion-bombardment-induced sputtering on the local impingement angle of ions taper the vertical nanostructures into the nanocones during the FPD under CCRP. The nanocone arrays exhibit low specular reflectance in a broad wavelength range and a wide incidence angle range that is quite insensitive to the polarization state of the incident light. The effective refractive index gradient of irregularly arranged nanocone arrays is characterized from the refractive index of the fluorocarbon polymer and the volume fraction considering the nanocone probability with the Gaussian distribution. The excellent broadband and omnidirectional anti-reflection properties are in consequence of a graded refractive index.