Barium Titanate Glass Microspheres Research Articles

Initial results from SEM-EDX and LA-ICP-MS chemical characterisation of barium titanate glass microspheres

Barium titanate glass microspheres are used in a variety of applications, particularly for retroreflective materials, owing to their high refractive index. These microspheres can be released into the environment or found at crime scenes due to damage or decomposition. Chemical characterisation of barium titanate glass microspheres using SEM-EDX and LA-ICP-MS showed a good discrimination of the studied materials from each other. The ubiquitously used material has the potential to characterise microtraces at crime scenes or sources of anthropogenically induced particle release in environmental studies, making it a valuable feature.

Super-resolution imaging based on cascaded microsphere compound lenses.

In this paper, a cascaded microsphere compound lens (CMCL) is introduced, in which a 20-µm-diameter barium titanate glass (BTG) primary microsphere and a 250-nm-diameter or 200-nm-diameter polystyrene (PS) secondary microsphere array constitute CMCL1 and CMCL2, respectively. The field of view (FOV) depends on the size of the BTG microsphere, while the waist of the photon nanojet (PNJ) can be adjusted by the size of the PS microsphere. The narrower the waist of the PNJ, the higher the imaging resolution. In the experiment, a 200-nm-diameter hexagonally close-packed PS nanoparticle array is successfully observed by the CMCL with a high magnification of ∼11.6× and a FOV of ∼14µm, while the single BTG microsphere is incapable of observing the array. The point spread function is used to quantify the resolution of the CMCL. A well-designed CMCL can improve the imaging performances of a microsphere-assisted microscope.

Whispering Gallery Mode Microresonator Sensor Integrated in Exposed Core Fiber for Label-Free DNA Hybridization Detection

A novel optical fiber sensor using exposed-core fiber (ECF) to excite whispering gallery modes (WGMs) within a high refractive index barium titanate glass microsphere is proposed for achieving label-free real-time specific DNA detection. The microsphere is encapsulated into the exposed channel of the ECF allowing for physical stability while excited WGMs via the evanescent field. The structure enables the sensing area to be exposed to a large amount of aqueous solution and better contact with the sample to be analyzed. Using a transmission measurement, a high Q-factor in the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> is obtained in an aqueous environment. To the best of our knowledge, this is the best Q-factor level obtained by the in fiber WGM sensor in an aqueous environment. Specific sequences of probe DNA are then bound to the surface of the microsphere so that specific detection of complementary DNA can be realized by tracking the wavelength shift the WGM resonance. Experimental results show a red shift of 5.6 pm for complementary DNA hybridization detection and less than 1 pm fluctuation for non-complementary DNA detection. The portable integrated sensor with miniature size has great potential for aqueous environmental sensing applications in the fields of diseases diagnostics, chemical analysis, and environmental protection.

Microsphere Coupled Off-Core Fiber Sensor for Ultrasound Sensing.

A compact fiber ultrasound-sensing device comprising a commercially available Barium Titanate (BaTiO3) glass microsphere coupled to an open cavity off-core Fabry–Perot interferometer (FPI) fiber sensor is proposed and demonstrated. The open cavity is fabricated through splicing two segments of a single mode fiber (SMF-28) at lateral offsets. The lateral offset is matched to the radius of the microsphere to maximize their coupling and allow for an increased sensing response. Furthermore, the microsphere can be moved along the open-air cavity to allow for tuning of the reflection spectrum. The multiple passes of the FPI enabled by the high refractive index microsphere results in a 40 dB enhancement of finesse and achieves broadband ultrasound sensing from 0.1–45.6 MHz driven via a piezoelectric transducer (PZT) centered at 3.7 MHz. The goal is to achieve frequency detection in the MHz range using a repeatable, cost effective, and easy to fabricate FPI sensor design.

Optoplasmonic-Enhanced Imaging of Monolayer Polystyrene Nanoparticle Arrays by Barium Titanate Glass Microsphere-Assisted Microscopy: Implications for Nanoparticle Characterization

Low-contrast nanoparticles are difficult to detect optically because they weakly interact with photons. In this work, we have addressed this challenge by developing optoplasmonic hybrid structures composed of barium titanate glass (BTG) microspheres and Blu-ray disk (BD) substrates deposited with multilayer metal/dielectric coatings. A 300 nm and a 250 nm hexagonally close-packed polystyrene (PS) nanoparticle monolayer sample self-assembled on both a BD substrate and a metal one (glass slide deposited with the same multilayer coatings) can be discerned by BTG microsphere-assisted microscopy, and the contrast of the virtual images of the sample on the BD substrate is enhanced compared to that of the metal substrate. When the size of BTG microspheres is 20–30 μm, the contrast of the observed virtual images is the best. Moreover, the Talbot images of 300 nm PS nanoparticle arrays assembled on the BD substrate can also be experimentally observed. We can also distinguish 200 nm PS nanoparticles assembled into a one-dimensional stripe structure on the BD substrate. Numerical simulations reveal that the electric field among the gaps of PS nanoparticles and the contacting areas between the nanoparticles and a metal/BD substrate is enhanced, and the enhanced electric field of the sample placed on the BD substrate is stronger than that of the sample placed on the metal substrate. In addition, the enhanced electric field is the strongest when the BTG microsphere size is 20–30 μm in optoplamonic structures. The simulated results fit well with the experimental ones, indicating that the enhanced electric field improves the resolution of microsphere-assisted microscopy in imaging low-contrast PS nanoparticle samples.

Coated High-Refractive-Index Barium Titanate Glass Microspheres for Optically Trapped Microsphere Super-Resolution Microscopy: A Simulation Study

As water is normally used as the immersion medium in optically trapped microsphere microscopy, the high-refractive-index barium titanate glass (BTG) microsphere shows a better imaging performance than the low-index polystyrene (PS) or melamine formaldehyde (MF) microsphere, but it is difficult to be trapped by single-beam optical trapping due to its overly high refractive index. In this study, coated BTG microspheres with a PS coating have been computationally explored for the combination of optical trapping with microsphere-assisted microscopy. The PS coating thickness affects both the optical trapping efficiency and photonic nanojet (PNJ) property of the coated BTG sphere. Compared to the uncoated BTG sphere, the coated BTG sphere with a proper PS coating thickness has a highly improved trapping efficiency which enables single-beam optical trapping, and a better PNJ with a higher optical intensity Imax and a narrower full width at half maximum (FWHM) corresponding to better imaging performance. These coated BTG spheres also have an advantage in trapping efficiency and imaging performance over conventional PS and MF spheres. The coated BTG microsphere is highly desirable for optically trapped microsphere super-resolution microscopy and potentially beneficial to other research areas, such as nanoparticle detection.

Determination of Microsphere-Lens Magnification Using Micro-Robotic Scanning Superlens Nanoscopy

Microsphere-assisted nanoscopy has shown great potential in recent developments in the field of super-resolution imaging. The precise control of microspheres is leading to new discoveries that can help verify the theories behind the super-resolution imaging mechanism. However, microsphere imaging involves multiple planes that have different magnification factors, which affect the determination of the overall resolution of the image. In this study, we present a flexible probe–lens assembly scheme that uses a barium titanate glass microsphere, as well as various scanning stages that can be used to freely investigate the sample surface and perform large-area super-resolution imaging (80 μm × 60 μm). The obtained resolution using this assembly under water immersion condition is 130 nm. By investigating the relationship between the magnification factors and the corresponding focus position of the different feature patterns, a remarkable difference in the focusing characteristics between arbitrary and periodic patterns was revealed. Results demonstrate the universality of the proposed method for the quantitative selection of the best focused plane and determination of the corresponding magnification factor and resolution of a microsphere virtual image for any feature pattern. The findings provide additional insights into the interpretation of arbitrary nanostructures through 3D optical imaging.

Large field-of-view super-resolution image obtained by manipulating submerged microsphere

Images with super-resolution can be obtained by dielectric microspheres with appropriate refractive index. However, in practical applications, the microsphere must be moved to the desired position and can scan the large specific area. Here we present a simple method of positioning a microsphere by using a four-dimensional precision translation stage. The simulations have been performed to confirm that 16 μm ϕ barium titanate glass (BTG) microsphere submerged in alcohol can obtain high quality images and the influence of the probe and the flowing liquid on imaging can be ignored. Therefore, by driving the translation stage, the submerged microsphere can scan the sample surface along the X and Y axis at the speed of 10 μm/s and the large field-of-view image can be achieved by stitching the pictures of different positions. This work has provided a new technique in super-resolution imaging and can be widely applied in field of scientific research.

Turning a normal microscope into a super-resolution instrument using a scanning microlens array

We report dielectric microsphere array-based optical super-resolution microscopy. A dielectric microsphere that is placed on a sample is known to generate a virtual image with resolution better than the optical diffraction limit. However, a limitation of such type of super-resolution microscopy is the restricted field-of-view, essentially limited to the central area of the microsphere-generated image. We overcame this limitation by scanning a micro-fabricated array of ordered microspheres over the sample using a customized algorithm that moved step-by-step a motorized stage, meanwhile the microscope-mounted camera was taking pictures at every step. Finally, we stitched together the extracted central parts of the virtual images that showed super-resolution into a mosaic image. We demonstrated 130 nm lateral resolution (~λ/4) and 5 × 105 µm2 scanned surface area using a two by one array of barium titanate glass microspheres in oil-immersion environment. Our findings may serve as a basis for widespread applications of affordable optical super-resolution microscopy.

Development and characterization of retro-reflective colored tiles for advanced building skins

RR materials, able to reflect the solar radiation mainly toward the incident direction, are a strategy to mitigate the Urban Heat Island (UHI) phenomenon, by reducing the heat trapped inside the urban canyon.This paper introduces new ceramic tiles for outdoor applications properly treated to get RR optic properties.Traditional colored tiles were covered with a UV resistant, transparent paint containing RR microspheres for exterior applications, through an ad-hoc continuous tape casting process, developed for this specific application. The goal is to produce RR tiles through industrial continuous process in order to ensure the penetration of such innovative building materials in the market.Three types of microspheres were tested: i) clear barium titanate glass microspheres; ii) hemispherically aluminum coated barium titanate glass microspheres; iii) hemispherically aluminum coated barium titanate glass microspheres with an additional fluoropolymer coating. The obtained tiles were investigated with a spectrophotometric analysis, a study of the spatial distribution of the reflected radiation, a colorimetric analysis to evaluate changes in tiles’ original color. Reflectance is increased by clear barium titanate glass microspheres, while is almost halved by aluminum-coated spheres. All the three types of microspheres provide strong RR behavior for incident light directions from 0° to 60° with respect to the surface normal. Nevertheless, the aluminum-coated microspheres change completely the tiles’ original color, making the application of the aluminum RR tiles very limited. Clear barium microspheres instead improve the optic properties of the original tiles, with negligible effects on the color.

Contrast enhancement of microsphere-assisted super-resolution imaging in dark-field microscopy

We report a method of boosting the imaging contrast of microsphere-assisted super-resolution visualization by utilizing dark-field illumination (DFI). We conducted experiments on both 10-µm-diameter silica (SiO2) microspheres with refractive index n ∼ 1.46 under no and partial immersion in ethyl alcohol (n ∼ 1.36) and 20-µm-diameter barium titanate glass (BTG, n ∼ 1.9) microspheres with full immersion to show the super-resolution capability. We experimentally demonstrated that the imaging contrast and uniformity were extraordinarily improved in the DFI mode. The intensity profiles in the visualization also numerically confirm the enhanced sharpness for a better imaging quality when applying DFI.

Super-Resolution Imaging of a Dielectric Microsphere Is Governed by the Waist of Its Photonic Nanojet.

Dielectric microspheres with appropriate refractive index can image objects with super-resolution, that is, with a precision well beyond the classical diffraction limit. A microsphere is also known to generate upon illumination a photonic nanojet, which is a scattered beam of light with a high-intensity main lobe and very narrow waist. Here, we report a systematic study of the imaging of water-immersed nanostructures by barium titanate glass microspheres of different size. A numerical study of the light propagation through a microsphere points out the light focusing capability of microspheres of different size and the waist of their photonic nanojet. The former correlates to the magnification factor of the virtual images obtained from linear test nanostructures, the biggest magnification being obtained with microspheres of ∼6-7 μm in size. Analyzing the light intensity distribution of microscopy images allows determining analytically the point spread function of the optical system and thereby quantifies its resolution. We find that the super-resolution imaging of a microsphere is dependent on the waist of its photonic nanojet, the best resolution being obtained with a 6 μm Ø microsphere, which generates the nanojet with the minimum waist. This comparison allows elucidating the super-resolution imaging mechanism.

Super‐resolution microscopy by movable thin‐films with embedded microspheres: Resolution analysis

Microsphere‐assisted imaging has emerged as an extraordinary simple technique of obtaining optical super‐resolution. This work addresses two central problems in developing this technology: i) methodology of the resolution measurements and ii) limited field‐of‐view provided by each sphere. It is suggested that a standard method of resolution analysis in far‐field microscopy based on convolution with the point‐spread function can be extended into the super‐resolution area. This allows developing a unified approach to resolution measurements, which can be used for comparing results obtained by different techniques. To develop the surface scanning functionality, the high‐index (n ∼ 2) barium titanate glass microspheres were embedded in polydimethylsiloxane (PDMS) thin‐films. It is shown that such films adhere to the surface of nanoplasmonic structures so that the tips of embedded spheres experience the objects’ optical near‐fields. Based on rigorous criteria, the resolution ∼λ/6‐λ/7 (where λ is the illumination wavelength) is demonstrated for arrays of Au dimers and bowties. Such films can be translated along the surface of investigated samples after liquid lubrication. It is shown that just after lubrication the resolution is diffraction limited, however the super‐resolution gradually recovers as the lubricant evaporates. image

Spectrum analysis of liquid immersion to transparent microsphere based optical nanoscopy

Recently, it was experimentally shown that barium titanate (BaTiO3) glass microsphere immersed in the liquid can be used for super-resolution imaging. However the inherent physical mechanism is still not clear. In this paper, spectrum analysis method is adopted to investigate the effect of liquid immersion to microsphere based optical super-resolution imaging. By tracking the propagation of part of the evanescent wave of the object's spectrum, the range of immersed-liquid's refractive index to achieve super resolution can be derived. To verify this analysis, we theoretically model the imaging process with finite-difference time-domain (FDTD) and angular spectrum propagation method. The result shows that only the BaTiO3 microsphere immersed in the liquid with moderate refractive index, the super-resolution can be achieved. And the lower refractive index of liquid corresponds to larger magnification.

Dependence of focal position on the microscale spherical lens imaging

In this paper, hexagonal close-packed colloidal crystal samples are prepared by a self-assembly method. The image properties of three kinds of microspheres (silica, polystyrene and barium titanate glass) on the colloidal samples are studied. These microspheres are placed in air, semi-immersed and full-immersed in ethanol respectively. We find that all these microspheres can magnify the colloidal crystal samples and have the super-resolution imaging ability. Our experimental results indicate that the position of the focal point of the microsphere can affect the super-resolution imaging property, therefore microspheres with different material need different observing ways to get the best imaging quality. The silica microspheres and the polystyrene microspheres need semi-immersion, while the barium titanate glass microspheres need full-immersion to make the focal point close to the microsphere, so that the best imaging quality can be obtained. The mechanisms are also discussed with numerical simulations.

Optical super-resolution imaging by high-index microspheres embedded in elastomers.

We demonstrated feasibility of super-resolution imaging through high-index microspheres embedded in transparent elastomers. We performed imaging, with resolution improvement by a factor of two, by using implanted barium titanate glass microspheres (diameters ∼30-150  μm and refractive index ∼1.9-2.1) in a thin film of polydimethylsiloxane elastomer placed over the specimen. Microsphere-assisted imaging technique is a promising candidate for applications in cancer research. As a proof-of-principle, we used microsphere-assisted imaging technique for the observation of radiation-induced γ-H2AX foci formation in U87 human glioblastoma cells irradiated by clinical proton beams.

Immersed transparent microsphere magnifying sub-diffraction-limited objects

The resolution of an optical microscope is restricted by the diffraction limit, which is approximately 200 nm for a white light source. We report that sub-diffraction-limited objects can be resolved in immersion liquids using a microsphere optical nanoscopy (MONS) technique. Image magnifications and resolutions were obtained experimentally and compared in different immersion liquids. We show that a 100 μm diameter barium titanate (BaTiO(3)) glass microsphere combined with a standard optical microscope can image sub-diffraction-limited objects with halogen light in three different media: water, 40% sugar solution, and microscope immersion oil. In this paper, the super-resolution imaging performance has been described with the three immersion liquid types and the mechanisms are discussed with Mie theory calculation in the field of a Poynting vector.

Optical super-resolution by high-index liquid-immersed microspheres

It is experimentally shown that barium titanate glass microspheres with diameters (D) in the range 2–220 μm and with high refractive index (n ∼ 1.9–2.1) can be used for super-resolution imaging of liquid-immersed nanostructures. Using micron-scale microspheres, we demonstrate an ability to discern the shape of a pattern with a minimum feature size of ∼λ/7, where λ is the illumination wavelength. For spheres with D &amp;gt; 50 μm, the discernible feature sizes were found to increase to ∼λ/4. Detailed data on the resolution, magnification, and field-of-view are presented. This imaging technique can be used in biomedical microscopy, microfluidics, and nanophotonics applications.

Fiber coupling to BaTiO_3 glass microspheres in an aqueous environment

Compact microspheres with high-quality (Q) whispering gallery modes are required for many applications involving liquid immersion, such as sensing nanoparticles and studying resonant radiative pressure effects. We show that high-index (1.9 and 2.1) barium titanate glass (BTG) microspheres are perfect candidates for these applications due to their high-Q (∼10(4) in the 1100-1600 nm range) resonances evanescently excited in spheres with diameters of 4-15 μm. By reattaching the spheres at different positions along a tapered optical fiber, we show that the coupling constant exponentially increases with thinner fiber diameters. We demonstrate the close to critical coupling regime with intrinsic Q=3×10(4) for water immersed 14 μm BTG spheres.