Refractive Index Gradient Research Articles

A bond-free PPLN thin film ridge waveguide

Lithium niobate thin film waveguide is attractive in nonlinear frequency conversion, especially in the second harmonic generation (SHG). However, the conventional fabrication method of lithium niobate thin film waveguide requires expensive bonding equipment and the prepared film is easy to peel off. In this paper, a novel fabrication method of PPLN thin film ridge waveguide with a trapezoidal substrate is presented, based on glue bonding. The PPLN waveguide together with its trapezoidal substrate is surrounded with low refractive glue to modulate the vertical refractive index gradient along the waveguide length, which was proved to be useful for broadband SHG with a theoretical bandwidth above 25 nm by changing the substrate thickness from 3 μm to 17 μm. The proposed structure was verified by using a C-band ASE laser as the pump source, where the SHG laser bandwidth is about 11 nm. The power of SHG is 2.55 μw, and the normalized conversion efficiency is 10.5 %/W∙cm2. This work is beneficial to simplify the fabrication process of lithium niobate thin film ridge waveguide, and has certain promotion significance for photonic chip integration.

Dispersion controlled nanocomposite gradient index lenses

The degrees of freedom afforded by nanocomposite materials and additive manufacturing allow for the precise control over the chromatic properties of gradient index (GRIN) optics. The ability to engineer nanocomposite optical materials using blends of three or more constituents makes it possible to independently specify the refractive index gradient and the dispersion of optical materials. The refractive index spectra of the primary nanocomposite feedstock are defined relative to one another using various concentrations of monomers and nanofillers. Inkjet deposition is then used to print-compose specific feedstock to form refractive index gradients with precise control over dispersion. Arrays of 4-mm-diameter spherical GRIN lenses were fabricated using different nanomaterial compositions. The ability to positively and negatively control dispersion and to obtain achromatic performance was demonstrated. Control over partial dispersion is also shown.

Obtaining a Scattering Structure by the Method of Optical Breakdown in Special Fibers

The paper describes a technique for manufacturing samples of fiber diffusers based on fibers with a gradient refractive index profile and a double cladding. Fiber diffusers can be used in medicine, for example, as sources of input radiation in photodynamic therapy or heat sources in phototherapeutic devices such as dressings or blankets. The paper describes experiments on creating quasi-periodic and non-periodic structures from microdefects inside optical fibers by means of optical breakdown and subsequent melting of the fiber core by a moving plasma spark. On the basis of experimental data, the optimal types of fibers were determined to create diffusers with the required properties. The energy characteristics of the laser diode necessary for writing structures from microdefects were estimated, and the results of measuring the parameters of microdefects were presented.

Growth and characterization of optical and thermal properties of LiGdF4 single crystal

The opportunity of growing of several centimeters in length LiGdF4 single crystals by Bridgman–Stockbarger technique was demonstrated and the optically perfect LiGdF4 single crystals with 8 mm and 55 mm in length were grown. The phase composition, thermal conductivity, basic optical and spectral-kinetic properties of LiGdF4 single crystals were studied for the first time. For LiGdF4 crystal, the resulting linear thermal expansion coefficients α is 11% larger, than for both LiYF4 and LiYbF4 crystals. The obtained thermal conductivity dependence k(T) is more pronounced compared to LiLuF4 and LiYF4. The absorption spectrum of the LiGdF4 crystal shows absorption bands corresponding to the 8S7/2 – (6Dj, 6Ij, and 6Pj) transitions of Gd3+ ions. The measured room temperature lifetime of 6Pj multiplet of Gd3+ ions was equal to 4.25 ± 0.05 ms. The values of the temperature gradient of the LiGdF4 refractive index do not strongly depend on the wavelength.

High-performance antireflection nanostructure arrays on aluminum-doped zinc oxide film fabricated with femtosecond laser near-field processing

Transmittance is critical to the performance of optoelectronic devices, and the fabrication of nanostructure arrays with sizes and periods similar to wavelengths on optical surfaces can effectively reduce reflectance and increase transmittance. A method combining a femtosecond laser and the near-field effect of polystyrene (PS) microspheres to prepare integrated high-performance anti-reflective nanostructure arrays on aluminum-doped zinc oxide (AZO) film was proposed in this study. The near-field effect of microspheres with different diameters on a laser light field for several laser wavelengths was simulated with the finite-difference time-domain (FDTD) method. The simulation results showed that a 10-fold improvement of the intensity of a focused laser beam could be obtained with the combination of an 800-nm laser and 1-μm PS microspheres. Nano-craters on the surface of the AZO film were obtained with femtosecond laser irradiation. The depth and the area of these nano craters both increased with the increase of the laser fluences and tended toward saturation when the fluence went from 0.67 to 0.83 J/cm2. Facilitated by the gradient refractive index formed by the nano-craters, the transmittance of the sample treated with 0.83 J/cm2 increased by 8% in the 400–1100 nm band and even by 15% in the 400–600 nm band. The ablation mechanism was revealed by the time-resolved pump-probe technique, which proved that the nano-craters were formed by the phase explosion, and the smooth bottom with fine cracks was induced by the inhomogeneous resolidification of the superheated liquid phase. Our method could endow a photoelectric device with high-performance integrated anti-reflection capability, eliminating the need for additional anti-reflection coating.

Characterizing close-focus lenses for microendoscopy.

Microendoscopes are commonly used in small lumens in the body, for which a focus near to the distal tip and ability to operate in an aqueous environment are paramount for navigation and disease detection. Commercially available distal optic systems below 1mm in diameter are severely limited, and custom micro lenses are generally very expensive. Gradient index of refraction (GRIN) singlets are available in small diameters but have limited optical performance adjustability. Three-dimensional (3D) printed monolithic optical systems are an emerging option that may be suitable for enabling high performance, close-focus imaging. In this manuscript, we compared the optical performance of three custom distal optic systems; a custom-pitch GRIN singlet, 3D-printed monolithic doublet, and 3D-printed monolithic triplet, with a nominal working distance (WD) of 1.5mm, 0.5mm and 0.4mm in 0.9% saline. These short WDs are ideal for microendoscopy in collapsed or flushed lumens such as pancreatic duct or fallopian tube. The GRIN singlet had performance limited only by the fiber bundle relay over 0.9mm to 1.6 mm depth of field (DOF). The 3D printed doublet was able to achieve a comparable DOF of 0.71mm, while the 3D printed triplet suffered the most limited DOF of 0.55mm. 3D printing enables flexible design of monolithic multi-element systems with aspheric surfaces of very short WDs and relative ease of integration.

A Review of 3D Printed Gradient Refractive Index Lens Antennas

Gradient index lens antennas find their application in modern wireless communication including radar systems, satellite communication, etc. They can ensure highly directional beamforming, support multibeam operations and beam steering capabilities within a wide angle of view. The development of 3D printing technologies enables the implementation of complex gradient refractive index distribution with high accuracy as well as obtaining a significant reduction in the production time and cost. This paper presents the state of the art in gradient refractive index lens antenna design using different types of additive manufacturing processes. It shows the applicability of 3D printing to a wide range of operational frequency bands from microwave to sub-THz and THz.

Design of Highly Directive GRIN MS Lens Integrated DFHA for Deep Tissue Biomedical Imaging

A novel highly directive gradient refractive index (GRIN) metasurface (MS) lens integrated dielectric-filled horn antenna (DFHA), with an enhanced signal penetration and spatial focusing, is developed for the effective monitoring and detection of tumors deep inside the body. For this purpose, two novel GRIN MS lens topologies are proposed, where an optimized refractive index distribution along with the lens aperture is generated using an improved analytical formulation. In the first lens configuration, the core layer (CL) is sandwiched between two similar impedance matching layers (IMLs) on each side. In the second configuration, the CL is still surrounded by two IMLs on both sides, but one of the IMLs, in this case, is improved to avoid the requirement of an additional dielectric layer between the lens and the biological tissue. All the layers of the MS lens are based on an array of especially designed square ring-shaped unit cells. The proposed MS lens-based DFHA enhances the signal penetration by a maximum of 12 dB at 100 mm penetration inside the torso compared to the case without a lens. It also facilitates better spatial resolution due to the presence of an MS lens, which shrinks threefold the 3 dB beamwidth at 100 mm inside the torso than that of the DFHA without an MS lens. The proposed GRIN MS lens integrated DFHA is fabricated and tested on a realistic torso phantom to evaluate the tumor detection capability deep inside the torso. The simulation and experimental results show that the proposed GRIN MS lens-based DFHA can effectively be employed for tumor detection with high resolution deep inside the body.

Radiation-resistant graded-index multimode optical fibers based on fluorosilicate glass

Using the MCVD-technology, radiation-resistant multimode optical fibers based on fluorosilicate glass with a gradient refractive index profile were developed. Radiation-induced attenuation (RIA) of light in the fibers was compared with the literature data on analogous fibers manufactured by the PCVD-technology. It was found out that RIA in the MCVD-fibers at the wavelength λ=1310 nm under γ-irradiation at the doses of up to 10 kGy is 1-2 dB/km (19-29%) lower than RIA in the Super RadHard fiber produced by the PCVD-technology and previously supposed to have a record-high radiation resistance. Keywords: radiation-induced attenuation of light, RIA, graded-index multimode optical fiber, radiation resistance.

Радиационно-стойкие градиентные многомодовые волоконные световоды на основе фторсиликатного стекла

Using MCVD-technology, radiation-resistant multimode optical fibers based on fluorosilicate glass with a gradient refractive index profile were developed. Radiation-induced attenuation of light (RIA) in the fibers was compared with the literature data on analoguous fibers manufactured by the PCVD-technoogy. It was found out that RIA in the MCVD-fibers at the wavelength λ=1310 nm under γ-irradiation at doses of up to 10 kGy is 1-2 dB/km (19%-29%) lower than RIA in Super RadHard fiber produced by the PCVD-technology and previously supposed to have record-high radiation resistance.

Investigation of Field and Energy in a Weakly-Conducting Optical Fiber with an Arbitrary Degree of Refractive Index Profile

Introduction. We consider a weakly conductive gradient fiber in the single-mode regime and solve the equation for the electric field in the core of this fiber in a general form in the first approximation. The aim of this study is to study the field and energy in the core of a weakly conductive gradient fiber without taking into account the polarization in the single-mode regime in the case of a power-law (generally) refractive index profile. Materials and Methods. From Maxwell’s equations for dielectric media, there was derived an equation for the field in a fiber with gradient refractive index profile. Making the appropriate substitutions, replacing the zero-order Bessel function with a Gaussian function, and making the necessary approximation of the resulting equation, we arrive at an equation that we solve by the Wentzel – Kramers – Brillouin method and obtain analytical expressions for the field and energy inside waveguide for an arbitrary degree of the refractive index. Results. There was obtained a solution of the equation for the field in fiber with a powerlaw refractive index profile. Numerical calculations were carried out. A graph of the dependence of a dimensionless quantity – “normalized” energy – on the waveguide parameter for the first five powers of the profile (n = 1, 2, 3, 4, 5) was plotted. Discussion and Conclusion. It is shown that the energy increases faster for the profile with n = 1, and after this value, the energy for the profile with n = 1 increases sharply, and for n > 1, the energy growth decreases with increasing n. The results obtained in this work can be used for creating an energy-efficient core, for carrying out a possible analysis of information transmission, and for designing waveguides taking into account specific applications.

An Acoustic Flat Lens for Broadband Focusing via Cross-Shape Structure

The manipulation of refracted wavefronts is eye-catching for owning attractive applications. In this article, an airborne acoustic flat lens for broadband focusing via cross-shape structure was proposed and demonstrated, introducing the broadband manipulation of wavefronts. The designed metasurface employs gradient refractive index cells to redirect the sound wave. Based on our theory, the effective refractive indexes of our unit cells can be easily calculated. The shackle of narrowband metasurfaces is conquered, and applications in medical ultrasound imaging are just around the corner.

Preparation and refractive index characterization of chalcogenide glasses with gradient refractive index

AbstractInfrared gradient refractive index (GRIN) lenses have great application value and potential in multispectral imaging systems. This study reports various chalcogenide axial GRIN glasses prepared using the hot‐pressing diffusion method. It is worth noting that the S4–S60 GRIN sample has a difference in refractive index (RI) Δn of greater than 0.3 and a diffusion depth of about 5 mm, which is the deepest diffusion depth reported in chalcogenide glass to date. In addition, the linear portion in the profile of the GRIN sample has a RI difference of 0.15 and a thickness of 1.2 mm. The effects of the temperature, concentration difference, and diffusion time on the sample diffusion process are discussed. The dispersion properties of the GRIN samples were further calculated, providing a new option for correcting chromatic aberrations in optical systems. In addition, a method for the indirect nondestructive characterization of sample RI using the Raman intensity ratio is proposed, and the reliability of the method is verified by practical experiments, which is convenient for the subsequent measurement of the GRIN profile.

Synthesis of atomically thin yellow pearl: An impetus for nonlinear optical effect assisted light scattering application

Synthesis of 2D materials from natural resources, and having unique functionalities have created enormous research interest among material scientists. Such 2D materials can be used as impetus for light scattering to generate continuous wave (CW) pumped random laser (RL) emission at room temperature. In this work, defects-free 2D yellow pearl (2D-YP) has been synthesized from bulk south sea pearl using liquid-phase exfoliation (LPE) technique. Thereafter, 2D-YP has been employed as a passive scatterer to achieve CW pumped RL emission from a conventional gain molecule. Compared to other semiconductor (TiO2) and 2D (Graphene and hBN) passive scatterers, ∼25 times improvement in gain volume is observed for the disordered system consisting of 2D-YP, and is due to the formation of a large refractive index gradient via thermal nonlinear optical (NLO) interaction. Hence, incoherent RL (ic-RL) emission ∼591 nm is reported at a lowest threshold input pump power and a linewidth of 65 mW and 3 nm, respectively. Additionaly, a clear transition from ic-RL to mode tunable coherent RL (c-RL) emission is also demonstrated by altering the pump configuration. Therefore, the newly designed van der Waals heterostructure, i.e., 2D-YP with intrinsic photon trapping capability may pave an avenue towards developing exciting optoelectronic devices.

Three-dimensional density measurements of a heated jet using laser-speckle tomographic background-oriented schlieren

Tomographic background-oriented schlieren (TBOS) is a promising optical technique for 3D density field measurement in turbulent flows. Path-integrated information on refractive index gradients is captured by cameras looking through the flow at textured background patterns in the form of apparent displacements of the patterns. A tomographic reconstruction of the three-dimensional refractive index gradients is performed, from which the instantaneous three-dimensional density field is obtained. Most TBOS measurements to date have been limited by large temporal blurring and/or large defocus blurring. Our 15-camera experimental set-up uses a pulsed laser for both illumination and creation of a laser-speckle background pattern. The high-power laser ensures adequate exposures in nanoseconds, so temporal integration is minimised in each measurement. The beam is spread to illuminate a surface that is observed by fifteen cameras that are evenly spaced circumferentially around the flow. Laser speckle patterns are observed by the cameras due to the diffuse reflection and interference of coherent light from roughness on the surface. We develop a methodology for selecting the optimal focal length, focus distance, and lens aperture by considering the compromise between displacement sensitivity, defocus blurring and speckle size. Low temporal- and spatial-blurring 3D temperature field measurements with excellent resolution of turbulent flow features are obtained and good agreement with thermocouple. Classical behaviours of the potential core instability such as vortex rollup and radial ejections are identified. The behaviour of the potential core is examined further, which reveals that the potential core undergoes considerable deformation and fragmentation during the transition to turbulence.

GRINtrode: a neural implant for simultaneous two-photon imaging and extracellular electrophysiology in freely moving animals.

In vivo imaging and electrophysiology are powerful tools to explore neuronal function that each offer unique complementary information with advantages and limitations. Capturing both data types from the same neural population in the freely moving animal would allow researchers to take advantage of the capabilities of both modalities and further understand how they relate to each other. Here, we present a head-mounted neural implant suitable for in vivo two-photon imaging of neuronal activity with simultaneous extracellular electrical recording in head-fixed or fiber-coupled freely moving animals. A gradient refractive index (GRIN) lens-based head-mounted neural implant with extracellular electrical recording provided by tetrodes on the periphery of the GRIN lens was chronically implanted. The design of the neural implant allows for recording from head-fixed animals, as well as freely moving animals by coupling the imaging system to a coherent imaging fiber bundle. We demonstrate simultaneous two-photon imaging of GCaMP and extracellular electrophysiology of neural activity in awake head-fixed and freely moving mice. Using the collected information, we perform correlation analysis to reveal positive correlation between optical and local field potential recordings. Simultaneously recording neural activity using both optical and electrical methods provides complementary information from each modality. Designs that can provide such bi-modal recording in freely moving animals allow for the investigation of neural activity underlying a broader range of behavioral paradigms.

Optical and thermodynamic studies on binary mixtures composed of two-isothiocyanato liquid crystals

Phase diagram and optical properties of the nematic mixtures containing two-isothiocyanatobenzen liquid crystal (7CHBT and 9CHBT) were studied by refractometry and DSC measurements. The thermodynamic and optical properties of the eutectic mixture (E79) composed of 7CHBT and 9CHBT were identified and studied. The phase diagram of the eutectic system was compared with the Schroder–van Laar calculation. The excess thermodynamic properties of the eutectic mixture were calculated and interpreted in terms of intermolecular interactions. The modified four-parameter model was used to fit the temperature dependence of the nematogen parameters on the refractive index. In addition, the temperature gradient of the ordinary refractive index (dno/dT) of the nematogens was analyzed in a convenient framework. The order parameters and polarizabilities of the nematic compounds, which are important parameters, were calculated through different methods based on birefringence measurements. Finally, the eutectic mixture and one of the constituent component (7CHBT) display crossover temperatures (TCO), which is very important liquid crystal parameter for specific applications such as optical devices and nonlinear optics.

Efficient Warming Textile Enhanced by a High-Entropy Spectrally Selective Nanofilm with High Solar Absorption.

Solar and radiative warming are smart approaches to maintaining the human body at a metabolically comfortable temperature in both indoor and outdoor scenarios. Nevertheless, existing warming textiles are ineffective in frigid climates because the solar absorption of selective absorbing coating is significantly reduced when coated on rough textile surface. Herein, for the first time, high-entropy nitrides based spectrally selective film (SSF) is introduced on common cotton through a one-step magnetron sputtering method. The well-designed refractive index gradient enables destructive interference effects, offering a roughness-insensitive high solar absorptance (92.8%) and low thermal emittance (39.2%). Impressively, the solar absorptance is 9.1% higher than the reported best-performing selective nanofilm-based textile. As a result, such a textile achieves a record-high photothermal conversion efficiency (82.2% under 0.6 suns, at 0°C). This textile yields a 3.5°C drop in the set-point of indoor air-conditioner temperature. Besides, in a winter morning with an air temperature of 7.5°C, it warms up the human skin by as large as 12°C under weak sunlight (350W m-2 ). More importantly, such a superior radiative warming performance is achieved by engineering the widely used cotton without compromising its breathability and durability, showing great potential for practical applications.

Recent Applications of Antireflection Coatings in Solar Cells

The antireflection coating (ARC) suppresses surface light loss and thus improves the power conversion efficiency (PCE) of solar cells, which is its essential function. This paper reviews the latest applications of antireflection optical thin films in different types of solar cells and summarizes the experimental data. Basic optical theories of designing antireflection coatings, commonly used antireflection materials, and their classic combinations are introduced. Since single and double antireflection coatings no longer meet the research needs in terms of antireflection effect and bandwidth, the current research mainly concentrates on multiple layer antireflection coatings, for example, adjusting the porosity or material components to achieve a better refractive index matching and the reflection effect. However, blindly stacking the antireflection films is unfeasible, and the stress superposition would allow the film layer to fail quickly. The gradient refractive index (GRIN) structure almost eliminates the interface, which significantly improves the adhesion and permeability efficiency. The high-low-high-low refractive index (HLHL) structure achieves considerable antireflection efficiency with fewer materials while selecting materials with opposite stress properties improves the ease of stress management. However, more sophisticated techniques are needed to prepare these two structures. Furthermore, using fewer materials to achieve a better antireflection effect and reduce the impact of stress on the coatings is a research hotspot worthy of attention.

Structural lens for broadband triple focusing and three-beam splitting of flexural waves

In this article, we report a lens design based on a concentric circular structure with continuous changing of thickness defined in a thin plate structure for focusing a plane wave into three spots (triple focusing) and for splitting elastic waves emanating from a point source into three collimated beams of different directions (three-beam splitting). Inspired by the principle of optical graded index triple focusing lens, the governing equations of the gradient refractive index profiles necessary for achieving such structural lens were obtained. The refractive index profiles were realized by using a lens design with two concentric circular areas of different thickness variation profiles defined in a thin plate. Analytical, numerical, and experimental studies were conducted to investigate the functionalities of the variable thickness structural lens. The results showed that the lens developed in this study were able to perform triple focusing and three-beam splitting with broadband property. Furthermore, the locations of focal points and directions of collimated beams can be engineered by changing the lens thickness profiles according to the governing equations. In addition, the proposed lens is miniature and simple design, which overcome the limitations of previous triple focusing and beam splitters.