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In this work we show the prospect of using periodic refractive index modulation – photonic crystals – to control the spatial dispersion curvature in a way that could potentially counteract the Kerr effect, thus suppressing filamentation. Numerical simulations by beam propagation method indicate that fine control of the spatial dispersion can be achieved by tuning the photonic crystal geometry and projecting to the appropriate Bloch mode by chirping the longitudinal period. Fabrication of such photonic crystals poses some unique challenges due to the small feature size and large-scale requirement. Bessel beam writing is shown to be a valid method for the fabrication of such photonic crystals.

One third of the final turning mirrors on the National Ignition Facility are laser damaged by 351 nm target backscattered light. The mirrors are spectrally complex hafnia and silica multilayer coatings with high 1 reflection for the forward propagating beam and high 3 transmission to suppress propagation of target backscatter. Alumina, a wider bandgap high index material, was selected to improve the 3 laser damage resistance despite a significantly lower refractive index resulting in increased layer count and reduced angular bandwidth. The impact on 3 laser damage fluence, coating stress, and spectral performance will be reported.

To extended the effective focusing length (f) and narrowing lateral full width at half maxima (FWHM) of photonic nanojets (PNJs) formed by microlenses with wavelength-scale size, in this paper, we study focusing characteristics of dielectric hemisphere. Refractive index (RI) of hemisphere, radius of covering Au disks on flat-surface of hemisphere and immersed materials effect on focusing characteristics are studied. Simulation study show that hemisphere with radius 4.5 μm, shows narrower focusing beam waist, and shorter f when RI of hemisphere gets bigger. When RI reaches 2.5, lateral FWHM of PNJ and f are 177nm, 324nm, respectively, under illumination of a plane wave with a 365nm wavelength. Comparing with normal hemisphere, hemisphere (RI 1.52, radius 4.5μm and illumination wavelength 365nm) with Au disk covering its flat-surface center, shows obvious smaller FWHM of PNJ and shorter f (still longer than 2 μm). it is because that the engineered hemisphere is like a lens with high numerical aperture, and it only allows incident beam far away from the axis participate in the formation of PNJ. With increasing of Au disk radius, the equivalent numerical aperture gets bigger and thus FWHM of PNJ gets smaller. FWHM of PNJ small than half of illumination wavelength when radius of Au disk gets bigger than 1800nm. The length of PNJ in above study is short, less than 2μm. When the engineered hemisphere (RI 1.52, radius of hemisphere and Au disk 4.5μm and 1800nm, respectively) is immersed in water, MgF2, et al, the f and L get longer than 11μm, 5μm, respectively. Although lateral FWHM of PNJ at this time is bigger than 400nm, it can be narrowed by replacing it with bigger Au disk, optimizing immersed material, et al. Due to low manufacture cost of these hemisphere lenses and lenses array, we believe they have potential in near-field and far-field application with resolution small than 0.5λ.

Optical interferometry technology is one of the most accurate and effective detection methods. For the refractive index distribution measurement of G-Lens, we propose a method to calculate the order of circular interference fringes produced by a Mach-Zehnder interference optical system. This method is not only limited to the refractive index measurement, but also suitable for applications which is measured via interference method. For example, the surface error measurement of optical components. Several specific works had been performed such as effective area masking, image preprocessing, fringe center relocation, central-radiation fringe fractional-level calculation and fitting of circular optical interference fringe images. Furthermore, the two-dimensional distribution of cross-sections and three-dimensional of fringe orders are also obtained. In addition, we propose a new primary and secondary location method for the gray value peaks and valleys of noisy interference fringes, and on the basis of this location, the relative gray fringe fractional-level calculation is performed, which effectively avoids uneven illumination, noise, and the influence of unfavorable factors such as poor circularity and over-density of large-order fringes on the fringe calculation. For the work content of G-Lens refractive index distribution measurement, the algorithm has good repeatability accuracy, and the specific value is about 0.3‰.

This paper has fabricated and characterized a bilayer SPR optical fiber sensor, enabled by hydrogen doped molybdenum oxide (HMO) nanodisks coated on the metal film's surface. Doped molybdenum oxide with ultra-doping and large permittivity properties can tune the plasmonic absorption in a visible region. The materials were synthesized by hydrothermal method, characterized by the atomic force microscope, Raman spectroscopy, and other analytical instruments. The electrical and optical properties of the materials are calculated and analyzed with density functional theory (DFT). The sensor measured the refractive index of a liquid solution, and a sensitivity of 2565 nm/RIU was achieved, which was approximately 1.4 times higher than that of a conventional SPR sensor without modification HMO nanodisks.

We employ a Bayesian optimization (BO) approach coupled with electromagnetic solver to optimize the photonic nanojets (PNJs) respectively generated by a 2D cuboid and a 3D concentric cylinder. The operating wavelength is selected atλ = 632.8 nm. By optimizing a five-layer cuboid, we achieved a narrow PNJ with waist at the full width at half-maximum (FWHM) about w ~ 0.2λ and an ultra-long PNJ with a beam length 123λ , respectively. Whereas by optimizing 3D concentric cylinder, we achieved a narrow PNJ with waist at FWHM about w ~ 0.22λand ultra-long PNJ with a beam length 78λ , respectively. Furthermore, we explore the physics behind the PNJ phenomenon, such as the fundamentals of extreme PNJ generation and the relation between the objective intensity profile and the refractive index gradients across the structure. The proposed approach presents an alternative approach to the design of various photonic nanostructures and may provide conventionally-inaccessible physical insight to their functionalities.

The Maxwell fish eye lens is a gradient index design from 1854 with a spherically symmetrical gradient. In one version the gradient index ball has an index of refraction of 3.0 at the center and 1.5 at its outer surface. It images perfectly any point on its surface to a point on the opposite side of the sphere. The ray paths are arcs of circles. A diagram like Fig.1 can be found everywhere in textbooks or on the internet. Fig 1. .Maxwell gradient index fish eye lens with spherical symmetry. In 1991 at the OSA annual meeting I pointed out that if a reflecting coating is placed on the outside of the ball then any ray from a point #1 on the surface will go to point #2, reflect and go back to point #1, reflect and continue around back and forth forever [1]. Then I showed by a very simple geometrical argument that such a system has a very unexpected property. Any point inside the volume of the ball is imaged perfectly, after just one reflection from the inside of the outer surface, to another point inside the ball with a magnification of -1.0 X. That means that the entire inner volume of the ball is perfectly imaged at -1.0X. After two reflections inside the ball every point is imaged perfectly back onto itself with a magnification of +1.0X. Since every point inside the gradient index ball is imaged perfectly after one inside reflection that means that any interior flat surface is also perfectly imaged to another interior flat surface. That type of perfect system had never existed before, of perfectly imaging a flat surface that is real to a flat image that is real. Table 1 shows how that new design from 1991 compared to the very few previously known perfect imaging systems. The perfect flat object and image are not in air, however, but are buried inside the gradient index ball. Point

Axial as well as lateral color aberrations in broadband optical systems can efficiently be corrected by using diffractive optical elements (DOEs). However, DOEs such as kinoforms consisting of only one material are not suitable for high-quality optics because of straylight in spurious diffraction orders. The amount of stray light can significantly be reduced by using so-called efficiency-achromatized DOEs (EA-DOEs), which consist of a material pair whose refractive indices fulfill a specific material condition. Unfortunately, manufacturing of EA-DOEs is very challenging because the grating structures are subject to tight fabrication tolerances. Therefore, only few broadband optical systems with EA-DOEs are on the market. Here we show that DOEs in broadband optical systems can surprisingly be replaced by refractive doublets made of materials that fulfill the material condition for EA-DOEs. As opposed to the EA-DOEs themselves, these purely refractive replacements do not suffer from stray light. In addition, from a theoretical point of view, our result allows for understanding the effect of DOEs in optical designs by classical refractive optical design theory.

We demonstrate a method for simultaneous measurement of thickness and refractive index of transparent materials based on spectrally-resolved interferometry. By extracting the phase from the interference spectrum, the measured optical path difference can be obtained directly without phase shift device in the measurable range. This unique advantage makes it capable of measuring physical thickness and refractive index by introducing transparent medium and processing three useful optical path differences. In the long-term stability test, the repeatability experiment of spectral resolution interferometric ranging technology using femtosecond laser is carried out within 60 min, and the standard deviation is 341nm.

(Ga0.3In0.7)2Se3 films were deposited by the thermal evaporation technique. As-deposited (Ga0.3In0.7)2Se3 films were irradiated using wideband radiation of Cu-anode X-ray tube at different exposure times. The spectral dependences of refractive index and extinction coefficient were measured by the spectral ellipsometry technique. The optical transmission spectra of X-ray irradiated (Ga0.3In0.7)2Se3 films were studied depending on irradiation time. Parameters of Urbach absorption edge for X-ray irradiated (Ga0.3In0.7)2Se3 thin films were determined and compared with nonirradiated film. The spectral dependences of refractive indices of non-irradiated and X-ray irradiated (Ga0.3In0.7)2Se3 films are described in the framework of Wemple and DiDomenico model. TThe variation of the parameters of Wemple and DiDomenico model for non-irradiated and X-ray irradiated (Ga0.3In0.7)2Se3 films was discussed.

Ultra-compact optical switch is a crucial module for densely integrated on-chip network. In recent years phase-change materials (PCMs) enable design of ultra-compact reconfigurable devices due to their drastic refractive index change during phase transition. In this work we inverse design a photonic-crystal-like (PhC-like) subwavelength optical switch assisted by novel ultralow loss PCM: Sb2Se3. At amorphous state the refractive index of Sb2Se3 is similar to that of silicon, so the device work as a MMI and light outputs at output port 2. At crystalline state the refractive index of Sb2Se3 increases to 4.05 and the subwavelength structure guides light to output port 1. The proposed switch has a compact footprint of 2.75 × 16 μm2. At the center wavelength of 1550nm, the simulated insertion losses and crosstalks are 0.36 dB, -44.36 dB, 0.49 dB, and -40.37 dB under amorphous and crystalline states respectively.

We describe high-precision measurements of the transmitted wavefront error and refractive index homogeneity of glass plates using a low-coherence interferometer in a downward-looking configuration. With the low-coherence illumination there are no spurious interference fringes even for a plane-parallel sample. Unbiased results are obtained by using a combination of four different measurement steps. The measurement performance and repeatability are shown for the lowcoherence interferometer OptoFlat with a special fixture incorporating sample holder and return mirror. Its ease of handling and supporting software-based user guidance through the measurement steps greatly facilitate wavefront and homogeneity measurements in the optics shop.

In 1884 Otto Schott, Ernst Abbe and Carl and Roderich Zeiss founded the SCHOTT & Associates Glass Technology laboratory in Jena, Germany. Otto Schott’s target was to develop new glasses on optical positions defined by Ernst Abbe and high reproducible quality that would tremendously improve the image quality of Carl Zeiss microscopes and optical instruments. Prior to 1880, optical components were made from simple crown and flint glasses. The crown glasses were soda-lime silicates with low refractive indices and moderate Abbe numbers. The flint glasses were lead silicates with relatively low Abbe Numbers. By 1884 already two dozen flint and crown glasses were available for optical system designs. Still today photonics industry relies heavily on optical glasses to realize tightest optical design requirements of today photonics applications. The difference between today and the past is that the designer can choose from a variety of more than 120 optical glasses with differences in optical position, transmittance and very specific dispersion requirements.

A theoretical model is developed to study the dynamic gain in the transmitted optical power through a 3D printed photopolymer waveguide. The given model shows that the solvent molecules diffused in the photopolymer and develops a swelling layer which acts as cladding. Formation of the cladding layer results in reduction of surface scattering losses and increase in the optical power. A methacrylate-based photopolymer waveguide is 3D printed using stereolithography as a single-step fabrication technique. Two solvents vapor: methanol and ethanol were tested with the given waveguide. An increase in the transmitted optical power is experimentally recorded and compared with the theoretical results to verify our model. This information is significant for fabricating integrated optical devices for sensing applications using photopolymers.

A framework for simulation and compensation of thermal lensing in general optical systems is presented. In high-power optical systems, the beam itself can serve as a non-neglectable heat source, altering the properties of the optical system. For estimating the influence on the optical system, a ray tracing algorithm is used and coupled with a finite element framework. Ray tracing is used to model the optical beam and evaluate the properties of the system. The finite element framework is used to solve the heat and the linear elasticity equation. From the solution of the heat equation, the inhomogeneous refractive index distribution is obtained. Refractive index gradients make it necessary to solve the ray equation iteratively on the finite element mesh. Additionally, the linear elasticity equation results in a displacement vector, which gives the deformation of the optical elements. To solve the ray equation and consider the surface deformation, efficient continuous data interpolation on the finite element mesh is crucial, as the interpolation is a limiting factor for the overall computational performance. This could be achieved by using direct interpolation on the finite element mesh, which reduces memory requirement while improving the interpolation accuracy. With the help of this simulation framework, optical setups which completely compensate for thermal lensing could be found. This could be achieved by a combination of optical materials with positive and negative temperature coefficients and suitable coating parameters, such that the overall thermal lens, resulting form deformation and temperature gradients, is completely compensated. This is a highly coupled problem, as the optical beam, the thermal gradients, and the material deformation affect each other. Hence an iterative simulation setup is necessary, which simulates the optical system until a stable state is found.

Alternatives to Ta-based absorbers are being considered for next generation lithography nodes to reduce 3D mask effects and to improve image modulation through phase interference. Low complex refractive index (n – ik) materials can provide phase shifting behavior at thicknesses less than those needed for conventional absorbers, essentially acting as attenuated phase shift mask (attPSM) films. Identifying attPSM absorber thickness and consequent phase requires determining optimum phase shift mask reflectance. Imaging with absorbers at high reflectance show better imaging performance. The absorber thickness is determined where the interference effects lead to high absorber reflectivity. Low refractive index (n) materials are therefore desired as candidate attPSM absorbers. Low – n material combinations identified using Wiener bounds and Effective media approximation (EMA) modelling are optimized for NILS and MEEF using absorber reflectivity on line-space and contact-holes patterns. Absorber candidates at optimum thickness for contact holes are compared with conventional Ta-based absorber using reflected nearfield intensity imaging.

Terahertz nondestructive testing technology is a technology widely used in samples evaluation with the merit of on-site and surface-damage free. Terahertz waves have low quantum energy and are transparent to most non-polar substances, thus, terahertz nondestructive testing has gradually become a research hotspot over the years. However when a femtosecond laser in used as a terahertz excitation source, the excess pump light in the system may cause radiation damage to the measured object. In order to eliminate the impact of residual pump light in the system on the measured object, we designed the spectral filter elements of reflect and terahertz band transmission in the wave bands of 0.80 μm and 1.56 μm, by using Nb2O5 and SiO2 as high and low refractive index materials, respectively. Then we prepared the filter membrane system electron beam evaporation through an ion beam assisted deposition. The influence of annealing process on the growth structure of the films was analyzed according to the AFM test results of Nb2O5 monolayer films under different process conditions, and the annealing process parameters were optimized to reduce the influence of surface roughness on the film spectrum. Based on the KIM vibration model (Lorentz extension model), the optical constant dispersion distributions of NB2O5 thin flims prepared under different processing conditions were accurately fitted. The influence of temperature on the refractive index and absorption of Nb2O5 thin films were analyzed. The deposition temperature was optimized to reduce the absorption of the filter in the wave bands of 0.8 μm and 1.560 μm. By using PE Lambda 1050, the reflectivity of the prepared filter in the wave bands of 0.8 μm and 1.560 μm were tested. The prepared filter was scanned by focusing point by point using a built-up transmission terahertz time domain spectral system. The transmittance of the quartz coated sample relative to the reference signal was calculated by analyzing signals in the frequency domain. The average reflectivity of the prepared spectral filter was 99.9% in both the band of 0.9 μm and 1.560 μm. The transmittance of terahertz frequency 2 THz was 68%, which was almost consistent with the theoretical design values, and could meet the requirements for the terahertz nondestructive testing system's application.

In recent years, Augmented reality (AR) technology has gained great attention. One critical component of AR devices is the see-through optical combiner. Several approaches have been proposed, such as using either refractive, reflective, diffractive, holographic optics, or a combination of them. Meta-surfaces can realize the design of more required optical element due to its superb abilities of controlling the amplitude, phase, polarization or other parameters of wave-front at subwavelength scale. Thus, we proposed a waveguide coupler based on a polarization independent doublet meta-surface, which can realize field of view (FOV) to 50° at the wavelength of 638 nm. Compare to the diffractive grating based optical combiner architecture, our doublet meta-surfaces improve the problem of FOV limited by the refractive index of the waveguide and uneven brightness in AR display devices. The polarization independent doublet meta-surfaces presented in this paper can provide a large phase mutation rate to achieve a large deflection angle. The meta-surfaces consist of a group of Silicon nanofins with different side length, but same height arranged on a rectangle glass substrate lattice with the reflection index of 1.764. Each unit structure has different electromagnetic responses to plane electromagnetic waves with different incident angles. By designing the shape and size of each silicon nanorods and other parameters, the phase and amplitude modulation we need can be achieved. Here we only consider one-dimensional eye-box expansion at a single wavelength. The design approach can be generalized to two-dimensional eye-box expansion. Besides, full-color display can be achieved by coordination of the coupling-in and coupling-out device in waveguides.

In this paper, we present a study on thermo-optical effect in core-shell silver/thiol-termination ligand nanoparticles. Nanoparticles were dissolved in Dichloromethane. Experimental measurements were carried out using a Z-scan setup. As laser sources we used two 1064 nm lasers: i) 28 ps pulse width laser with 1000 Hz pulse repetition rate; ii) 8 ns pulse width laser with changeable pulse repetition rate 200 – 40 000 Hz. To study what processes lead to refractive index changes we used the polarization-resolved Z-scan method. Comparing ps and ns results showed that response time of single pulse thermal effects for organic solvents depends on beam size while for nanoparticles it corresponds to nanoparticle size. Measurements with ns laser using different lenses and pulse repetition rate showed that thermal response scales with the ratio of laser pulse repetition rate times beam size squared.

Live cell microscopy technology, especially cell three-dimensional refractive-index reconstruction, is one of the important single-cell analysis methods, which can investigate subcellular organelles and biological mechanisms. Compared with the method of rotating the illumination light, rotating cells can obtain the complete internal structure information of the cell. However, cell rotation based reconstruction methods typically rotate cells in low operation throughput and image unstably in a low sensitivity. To bridge this gap, we demonstrate a combination strategy that uses acoustically oscillating bubble array based acoustofluidic device to simultaneously rotate cells in large-scale, and sets up a wDPM module to record cell quantitative phase maps with a high temporal and spatial phase sensitivity due to the common-path geometry and white light illumination. Result shows the phase accuracy of this system is less than 6.7%, the phase spatial consistency does not exceed 4%.

The traditional methods of measuring refractive index have their unique value and advantages. In order to study the properties of materials and the application of multi-wavelength laser interferometer, a new method of measuring refractive index and dispersion of materials is proposed. The multi-wavelength laser interferometer is designed and built based on the principle of the Fizeau interferometer. It integrates five kinds of laser bands with a wide coverage range through a splitting prism, and can quickly change the measurement wavelength during remeasurement and improve the detection efficiency. In order to further verify the refractive index measurement method, a parallel plate is taken as an example to measure the refractive index. The multi-wavelength laser interferometer combined with variable wavelength standard spherical mirror is used to measure the displacement of ray focus in the case of parallel plate or not, and the refractive index of parallel plate is calculated by geometric optics. The refractive index corresponding to each wavelength is measured, and the refractive index curve of the parallel plate material is calculated by Conrady formula and ACF formula by fitting polynomial method using the measured data, and then the dispersion coefficient of the material can be calculated. The comparison results show that the ACF formula can be used to calculate the refractive index of materials accurately in a larger band range. The experimental results also show that the multi-wavelength laser interferometer has the advantage of measuring multi-wavelength transmission wavefront and can also play a role in more measurement applications.

Quantitative phase information which can reflect the internal structure and refractive index distribution of the object is able to be obtained by diffractive and interferometry techniques. However, the phase resolution achieved by the diffraction method is lower than that of interferometry method; while the setup for interferometry method is more complex. To obtain high-resolution phase images without reference beam path, we propose an end-to-end DL based super resolved quantitative phase imaging method (AF-SRQPI) based on generative adversarial network (GAN) to transform low-resolution amplitude images into super-resolved phase images. Meanwhile, considering the inevitable out-focusing during the long hours of observing, autofocusing function is also included by the network. In the training process, out-of-focus low-resolution amplitude images are used as the inputs and corresponding super-resolved phase images obtained by structured illumination digital holographic microscopy (SI-DHM) are used as the ground truth labels. The well-trained network can reconstruct the high-resolution phase image at high speed (20fps) from a single-shot out-of-focus amplitude image. Comparing with other DL-based reconstruction schemes, the proposed method can perform autofocusing and superresolution phase imaging simultaneously. The simulation results verify that the high-resolution quantitative phase images of different biological samples can be reconstructed by using AF-SRQPI .

In this work, a simple and effective sensor using single-mode fiber (SMF) tapered structure is developed to detect different concentrations of acetylcholine solutions, and its function is to test the probe's performance. A layer of synthesized gold nanoparticles (AuNPs) and zinc oxide nanoparticles (ZnO-NPs) is used to immobilize this type of SMF-based tapered structure. The work is based on the well-known phenomenon of localized surface plasmon resonance (LSPR) principle. A tapered region of the probe with a high fraction power of evanescent wave can stimulate LSPR and produce specific absorption peaks sensitive to the refractive index variation. The sensor probes performance was examined, including their stability, repeatability, reusability, and selectivity. Furthermore, the biosensor's ability to improve performance was tested in the experiment.

Quantitative phase imaging (QPI) has quickly emerged as a powerful tool for label-free living cell morphology and metabolism monitoring. However, for current QPI techniques, interference signals from different layers overlay with each other and impede nanoscale optical sectioning. This phenomenon leads to unsatisfactory performances for optically thick or complex scattering biological samples. To address this challenge, we have developed an alternative quantitative phase microscopy with computational hyperspectral interferometry. Nanoscale optical sectioning could be achieved with Fourier domain spectral decomposition. Morphological fluctuations and refractive index distribution could be reconstructed simultaneously with 89.2 nm axial resolution and 1.91 nm optical path difference sensitivity. With this method, we established a label-free cell imaging system for long-term cellular dry mass measurement and in-situ dynamic single cell monitoring. Different intrinsic cell growth characteristics of dry mass between HeLa cells and Human Cervical Epithelial Cells (HCerEpiC) were studied. The dry mass of HeLa cells consistently increased before M phase, whereas that of HCerEpiC increased and then decreased. The maximum growth rate of HeLa cells was 11.7% higher than that of HCerEpiC. We also use the proposed method and system to explore the relationship between cellular dry mass distributions and drug effects for cancer cells. The results show that cells with higher nuclear dry mass and nuclear density standard deviations were more likely to survive the chemotherapy. The presented work shows potential values for cell growth dynamics research, cell health characterization, medication guidance and adjuvant drug development.

We employ the hybrid mask model of the Fraunhofer IISB simulator Dr.LiTHO to investigate the role of the absorber and multilayer for the observed image blur and contrast loss mechanisms. Hybrid mask model decomposes the EUV mask into the absorber and multilayer part. Both parts can be described by real 3D objects or idealized 2D objects. An idealized absorber is specified by the transmission tr and phase of the transmitted light. An idealized multilayer mirror is characterized by a certain reflectivity value, by an angular range or bandwidth (BW) where the light is reflected and by a distance Zeff from the absorber. The diffraction of the incident light by the absorber, the reflection of the diffracted light by the multilayer mirror, and the second diffraction of the reflected light by the absorber are combined by transfer matrices. The obtained superposed diffraction orders provide characteristic diffraction efficiencies and phase values of discrete orders. The presented analysis of the light diffraction from EUV mask and resulting imaging metrics provides several novel insights into the root causes of M3D effects in high NA EUV imaging. The combination of double diffraction by an ideal (thin) absorber and reflection by an ideal (thin) multilayer with a limited reflectivity range (BW

Air refractive index measurement technology based on laser interference has shown great application value in many fields, especially in laser ranging, precision manufacturing and integrated circuit manufacturing equipment. With the help of air refractive index measurement technology, the measurement problems in many fields can be solved. In this paper, the research progress of indirect measurement method, synthetic pseudo wavelength method, F-P cavity method, variable cavity length method and optical frequency comb measurement method are introduced, and the advantages and disadvantages of various measurement methods are analyzed. Compared with the indirect measurement method, the direct measurement method based on laser interference technology has higher measurement accuracy. Finally, the current application status of air refractive index measurement technology is introduced, and its future application prospect and development trend are described in detail.

Light propagation in non-homogeneous media with varying refractive index is challenging to analyse, even with knowledge of the gradient index (GRIN) structure n(x,y,z). Differential equations can be used to describe the path light rays take when travelling through a medium. Solutions to a differential equation are used to calculate the focal length of a GRIN lens with focus on quadratic GRIN profiles. Ray-tracing in GRIN media is notoriously difficult, and usually not possible without numerical methods, however for several types of GRIN lenses, the analytical solution exists. Examples of spherical GRIN lenses can be found in nature such as the lens in an octopus eye. A formula is proposed here to calculate the focal length of such a lens which will be compared to previous studies and models created on ray-tracing software, ZEMAX.

Structural, optical, and electrical properties of ultrathin chromium films manufactured using magnetron sputtering were investigated. The films showed pure metallic chromium phase yet their refractive index and extinction coefficient result very different from previously reported in literature. Structural, electrical and optical properties of ultrathin chromium layers are discussed in detail. The obtained optical constants of ultrathin chromium films show a specific trend with the film thickness increase. Precise knowledge of optical constants of ultrathin chromium films is important for many electro-optical and optical applications.

In oil and gas production, it is important to determine the type and chemical composition of the formation fluids in the pipeline in real time. Different types of the hydrocarbon formations present in the downhole environment have distinct optical characteristics, such as spectral transmittance, diffusion, fluorescence and refractive index. In the formation sampling and testing, knowing the refractive index of a downhole fluid makes it possible to identify the fluid (water, gas or oil), estimate various properties, such as salinity and crude oil density, and monitor the cleanup processes. The paper describes a new design for an on-axis inline process refractometer for continuous fluid measurements using a defocusing imaging technique. Design principles for the refractometer are discussed, the dependence of the defocus effect on the refractive index is calculated. Experimental energy distributions for different liquid samples are obtained and analysed.

In this work we present reactive sputtered SiOxNy films with a variable refractive index as a convienent solution for contrast improvement of liquid crystal diffuser multi stacks in near-to-eye AR/VR displays. The focus concerns minimization of light reflections between internal structures, in particular ITO, by optimizing internal layers through tailored properties of thin film coatings, as well as subsequent laser patterning of thin film stack. Inorganic thin films have been deposited on glass by physical vapor deposition. Corresponding refractive index, thickness, uniformity and dielectric characteristics and other electro-optical properties have been measured and their impact on the resulting optical performance of the final integrated element stack has been compared against counterparts utilizing traditional polyimide and SiOx films.

Semi-absorbent metallic layers offer some unique possibilities in thin-film coating design due to their versatile dispersion properties. However, the presence of absorbance and the dependence of the properties on thickness present significant challenges for characterisation of such films. Therefore, as of today, this is no reliable and universal technique to characterize these layers. We propose here an alternative spectrophotometric method to determine the refractive index of a semitransparent metallic thin film. The method involves the preparation of a semi-reflective silicon substrate plus a thick dielectric layer several hundred nanometers thick. A thin, semitransparent metallic film is then deposited over this dielectric layer, creating an asymmetrical Fabry-Perot structure. The resulting spectrum displays oscillatory features from the dielectric layer, which are modulated by the dispersion properties of the thin metallic layer to be determined. A numerical optimization is then used to estimate the refractive index dispersion via use of an appropriate dispersion model. The sensitivity of the spectrum to the dispersion properties of the thin metallic layer allows these properties to be determined with a higher accuracy and robustness. In this paper, we detail a numerical and experimental validation of the method in the case of titanium thin films. To model the dispersion properties of these layers, we use the combined Modified Drude and Forouhi-Bloomer models. The index dispersion was determined for a range of titanium layer thicknesses from 10 nm to 70 nm. We show that the proposed method is accurate and stable and allows determining dispersion properties that can then be used for the design of multilayer structures for purposes such as colorimetry.

Aberration-corrected holographic gratings are widely used in spectral instruments. They allow to achieve high resolution and uniformly distributed diffraction efficiency as well as to combine several functions in a single optical element. However, their performance is limited. In particular, when the optical system has a large aperture the hologram replay conditions vary significantly across its’ surface. Due to this variation the hologram aberration properties and its efficiency change locally thus leading to decrease of the resolution and efficiency of the entire system. In the present research we consider a composite volume phase holographic optical element used as a disperser in a spectrograph design. Such an optical element represent a hologram recorded by stitching of several elementary fields or zones. The refraction index modulation depth, the fringes tilt and the hologram spatial frequency may vary locally in each of the elementary fields to match the changing reconstruction conditions. This approach allows to implement a better aberrations correction and to maximize the overall diffraction efficiency. We demonstrate an exemplary spectrograph design with a composite hologram for the visible range of 400-800 nm. It is shown, that in the design as fast as f/2.1 the maximum aberrations can be decreased by factor of 1.19 and 2 in the X and Y directions, respectively, while the average diffraction efficiency increases by 15.6% at shorter wavelenghts. We continue the study by investigation of the composite hologram technological feasibility and demonstrate that it can be recorded with a standard precision of the moving sources positioning, achievable stroke of the auxiliary deformable mirror and reasonably high accuracy of the photosensitive layer’s parameters.

Anti-reflective (AR) coatings are indispensable for an excellent imaging of optical systems. Common AR coating systems consist of layer stacks of alternating low and high refractive index materials with the residual reflection depending mainly on the low refractive index (LRI) of the last layer relative to air. However, conventional LRI materials are limited to SiO2 (n = 1,46 at 532 nm) and MgF2 (n = 1,38 at 532 nm), where MgF2 is not environmentally stable. Nanostructures with an adjustable effective LRI in the range of 1.07 to 1.25 are an attractive alternative to these materials. Integrated as the last layer in the stack system, these structures significantly improve the optical performance compared to a conventional interference coating system, as a broadband AR coating can be realized, which is less sensitive to high angles of light incidence. Nanostructures can be produced using various methods - e.g. wet chemical or lithographic. However, these methods are expensive and time-consuming, as often more than one manufacturing step is necessary. At the Fraunhofer Institute IOF in Jena, self-assembling AR nanostructures have already been successfully fabricated for several years using a conventional plasma-ion-assisted-deposition (PIAD) technology. Thereby organic material is deposited on the substrate via thermal evaporation and subsequently self-assembling nanostructures are formed by an ion plasma source. Melamine and Uracil are already being used successfully as organic material. The advantage of this method is the generation of nanostructures in one process which is cost- and time-effective. In this talk, we want to introduce Xanthine, another organic material that is also highly promising for the generation of nanostructures exhibiting an LRI. We will demonstrate the formation of these nanostructures during plasma etching and investigate their optical performance for the use as an AR coating.

The refractive index is the most important property of optical glass. Therefore, refractive index measurement is a key characterization method also for process control. The requirements for refractive index control demand a fast and accurate measurement method for production control and a very high accurate measurement method for tightest requirements. For refractive index measurement two different measurement setups are common: the v-block refractometer for economic refractive index control up to the fifths digit and the spectral goniometer for high precision index measurement over a broad wavelength range from 185 nm to 2.325 μm up to the sixth digit. Providing Sellmeier dispersion equation data for individual measurements enables accurate interpolation of index data for arbitrary wavelengths for high precision applications. To cope with specific production requirements and the required high reliability of the measurements SCHOTT develops its own measurement setups. Both, the v-block system and the high precision spectral goniometer have been updated in recent time. To evaluate the accuracy SCHOTT participates in international round-robin tests. SCHOTT is also participating in the standardization of refractive index measurement methods for the optical industry. In this paper the current status of refractive index measurement of optical glasses at SCHOTT is presented.

New chalcogenide glass (FI-02) has been developed successfully, with its most effective feature being its infrared (IR) transmittance. FI-02 can transmit up to 20-μm wavelengths of IR light and has a very wide transmittance spectrum in the IR region as compared to the germanium single crystal and conventional chalcogenide glass. Its excellent transmission property is expected to improve the brightness and clarity of IR images. In addition, FI-02 has a high refractive index of 3.47 (at a wavelength of 10 μm). This is the highest refractive index among glass materials, thus enabling the fabrication of thin and high-performance IR lenses. Furthermore, unlike crystal materials, FI-02 can be processed through press molding. This enables to make high performance lens with high productivity. To investigate the performance of FI-02 as an IR lens material, we made two types of IR lens units: focal length (FL) 10.5 mm (standard type) and FL 2.04 mm (super wide-angle type). In the standard lens unit, IR images were captured with better contrast as compared to those captured using the germanium or conventional chalcogenide glass lenses. This could be attributed to the good IR transmission property of FI-02. In the super wide-angle type, IR images were captured that were wider and less distorted as compared to those of the typical wide-angle IR lens unit. This was attributed to the advantage of the high refractive index of FI-02 and its ability to form aspherical shapes through press molding.

Bifocal diffractive-refractive intraocular lenses (BDRIOL) are used in ophthalmology to replace natural lenses that are clouded by cataracts. In Russia, the REPER-NN enterprise developed a BDRIOL manufacture technology on the base of a liquid photopolymer frontal polymerization placed between two transparent fused silica moulds. One of them has a spherical shape, the other has a flat surface with a diffractive optical element (DOE). In our BDRIOL design the diffractive structure depth changes so that the energy distribution over the foci weakly depends on the pupil diameter. Zeroth and first orders efficiencies should almost the same. Measurement of this distribution is complicated due to the diffractive profile depth corresponds to the operation of a polymer lens in the eye and is significantly greater than required for working in air. We have proposed to use temporal planarization of the diffractive mould to measure energy distribution over foci. The diffractive structure is filled with a material that provides approximately the same module of refractive index difference with fused silica as between the polymer and vitreous body of the eye. This difference is adjusted by the temperature and time selection at the photoresist baking. The sign inversion of the refractive index difference in comparison with the operating conditions in human eye changes the DOE focus to positive. The paper discussed specific photoresist treatment, refractive index measurement and preliminary results of the developed method.

A new process for prototype manufacturing of integrated optical components was investigated. Sodium ions near the surface of a glass wafer are exchanged with silver ions, which creates a layer of increased refractive index. Subsequently, parts of the glass surface are ablated using a femtosecond laser. The resulting ridges determine the final optical waveguide structure. However, manufacturing-related roughness leads to high optical losses. To reduce these losses and to optimize the index profile, a second ion exchange with sodium ions is performed. These ions are introduced into the glass from all three ridge surfaces, causing the silver ions to migrate towards the ridge center. This results in a gradient index waveguide. We created a numerical model, to simulate the ion exchanges. Experiments were conducted, to determine the parameters for the ion exchange and the laser ablation. Based on the results, a process window was defined for each step, thus enabling the manufacturing of integrated optical components.

As known, lasers are used for atmospheric communication. There are advantages of an optical wave system over a radio frequency system. But there arise a number of problems in that case. Turbulent fluctuations of density in the atmosphere cause various deteriorative effectson signals. We investigated the propagation of the non-paraxial Gaussian beam through air inhomogeneity. The refractive index of air can be written in the form : n=1+n1 , n1 being a small quantity. In this case we can find the solution of the Maxwell equations. The paper represents results of computations for model inhomogeneity.

Hybrid integrated photonics open up new application perspectives due to compact size and the shift to cost-efficient components. Therefore, integration of optical and electro-optical functionalities into photonic chips has recently attracted great interest. Research has been directed towards miniaturization of demanding spectral transfer properties for individual applications. However, it remains challenging to implement highly complex transmission and reflection characteristics with few additional process steps. In this contribution, we report on our advancement in the field of optical thin-film coating fabrication, which enables a manufacturing process comparable to die assembly in electronics. We have combined a sacrificial-substrate approach with the production of miniaturized optical thin-film coatings by ion-beam sputtering. The concept is applicable to high precision coatings with more than 130 individual layers and adding up to over 26 µm total film thickness. Segmentation down to sizes of 25 μm x 25 μm pieces is realized by laser cutting of the coating. By completely removing the substrate afterwards, we achieve a freestanding thin-film and thus minimized thickness. Our measurements indicate no general performance loss compared to coatings on glass substrates. Additionally, the substrates refractive index and absorption do not have to be considered in the multilayer-coating design. Therefore, the design can be optimized and matched to the refractive index of specific waveguides on the chip. Furthermore, we demonstrate the compatibility to releasable transfer tape. With this, we aim for enabling a high-volume feed of miniaturized thin-film filters to an automated assembly process.

Spinel ferrite has the excellent characteristics of high saturation magnetization, high resistivity and low dielectric loss. It plays an important role in signal isolation, circulation, phase shift, frequency limiting, filtering, delay and amplification. So it is of great significance to study the spectral characteristics of spinel ferrite at different temperatures by using terahertz time-domain spectroscopy. The terahertz spectral characteristics of spinel ferrite under the different temperature condition were measured by terahertz time domain spectroscopy system. The transmittance of the power, absorption rate, absorption coefficient and refractive index of spinel ferrite were obtained by calculation. The experimental results show that the transmittance is positively correlated with temperature and the absorption spectrum of the samples increases with the increase of frequency in the 0-1THz band, and the refractive index of spinel ferrite increases with the increase of temperature.

Highly sensitive response of anisotropic terahertz metamaterials to electromagnetic waves has attracted considerable attention due to its potential applications in terahertz modulators and biosensing devices. We designed three microstructure samples with split ring arrays. The terahertz transmission spectra are experimentally measured at different rotation angle. It is found that as the rotation angle increases, the resonance mode has gradually evolved from a single dipole oscillation mode to the double oscillation modes accompanied with LC and dipole oscillations, we further simulated the polarization conversion characteristics of the single splitting rings. It shows that the highest conversion efficiency appears at 45 degrees. But it doesn’t have polarization conversion effect at 0° and 90°. Subsequently, we have utilized ultrashort pulse laser to optically control the electromagnetic response by exciting the photogenerated carriers in the metamaterial samples. The experimental data show that the transmittance change obviously with the pump light. Additionally, when the angle is 90 degrees, it can be observed that the LC resonance annihilates earlier than the resonance of the dipole resonance with the increase of pump power, implying the former is particularly sensitive to the variation of the dielectric environment. To further explore the influence of dielectric environment on the resonance characteristics of the terahertz metamaterials, we have further performed the simulations with the applied surface analyte of different refractive index. The simulated data show that with the increase of the refractive index of the surface analyte, the resonance frequency of the dipole oscillation has a more significant blue shift than the resonance frequency of the LC oscillation. Our obtained results could provide the idea for designing terahertz modulators and sensitive biosensing devices.

Multiple filaments generation and manipulation are crucial to numerous applications. The refractive index of medium has essentially influenced nonlinear effects in the ultrafast optical process of femtosecond laser filamentation, which is an effective method to tailor and manipulate filamentation. We have proposed a new method that using a periodic gas lattice as transmission medium to generate multiple filaments of femtosecond laser pulse. The periodic gas lattice is consisted of air waveguides, which has the special refractive index change distribution with alternating positive and negative. Our results show that the properties of multi-filament can be well controlled by changing the parameters of the gas lattice. A multi-filament array is formed by the gas lattice along the beam propagation direction, which are systemically investigated with different parameters of lattice at filamentation stages. The underlying physical mechanism of the multi-filament array formation is discussed, which has been demonstrated that the effects of extra focusing and discrete diffraction introduced by the gas lattice medium to promote a new competition and balance in the nonlinear filamentation process. The findings in this work provide a new way for many potential and promising applications that based on the controlled and optimized the femtosecond laser multi-filament array.

With strong optical response from subwavelength metamaterial structures in terahertz, plasmon-induced transparency (PIT) has attracted considerable attention in terahertz modulators and biosensing devices. Here, we tune PIT effect by the destructive interference of two bright modes. We design a terahertz metamaterials structure with triple U-shaped resonators (TUR) arrays. In the vertical direction, double U-shaped resonators (DUR) are arranged downward, single-U resonator (SUR) is upward. We change the length of the SUR, the inner arms of the DUR and the horizontal distance of the DUR in order to observe the terahertz transmission spectrum. It is found from the results that with the length of the SUR increases， the resonant frequency has an obvious red-shift, the absorption of the low-frequency resonance increases and the nonresonance absorption peak gradually decreases. As the arms of the DUR increase and the distance between them decreases in horizontal, the frequency of resonance dip has a red-shift, and the transmittance of the non-resonant region increases slightly. To explore the influence of dielectric environment on the resonance characteristics of the terahertz metamaterials, we have further performed the simulations with the applied surface analyte of different refractive index. The results show that with the increase of the refractive index of the surface of the analyte, the resonance frequency has a more significant red-shift. Our obtained results could provide the idea for designing terahertz modulators and sensitive biosensing devices.