Achieving Smart Photochromics Using Water-Processable, High-Contrast, Oxygen-Sensing, and Photoactuating Thiazolothiazole-Embedded Polymer Films.

Water-soluble dipyridinium thiazolo[5,4-d]thiazole (TTz) compounds are incorporated into inexpensive poly(vinyl alcohol) (PVA)/borax films and exhibit fast (<1 s), high-contrast photochromism, photofluorochromism, and oxygen sensing. Under illumination, the films change from clear/yellow TTz2+ to purple TTz•+ and then blue TTz0. The contrast and speed of the photochromism are dependent on the polymer matrix redox properties and the concentration of TTz2+. The photoreduced films exhibit strong, near-infrared light (1000-1500 nm) absorbances in addition to visible color changes. Spectroscopic ellipsometry was used to establish the complex dielectric function for the TTz2+ and TTz0 states. Incorporating non-photochromic dyes yields yellow-to-green and pink-to-purple photochromism. Additionally, when illuminated, reversible photoactuation occurs, causing mechanical contraction in the TTz-embedded films. The blue film returns to its colorless state via exposure to O2, making the films able to sense oxygen and leak direction for smart packaging. These films show potential for use in self-tinting smart windows, eyeglasses, displays, erasable memory devices, fiber optic communication, and oxygen sensing.

Student ultrasound education - current views and controversies.

As an extension of the clinical examination and as a diagnostic and problem-solving tool, ultrasound has become an established technique for clinicians. A prerequisite for high-quality clinical ultrasound practice is adequate student ultrasound training. In light of the considerable heterogeneity of ultrasound curricula in medical studies worldwide, this review presents basic principles of modern medical student ultrasound education and advocates for the establishment of an ultrasound core curriculum embedded both horizontally and vertically in medical studies.

Towards Two-Photon Polymerization-Compatible Diffractive Optics for Micro-Mechanical Applications.

Diffractive optics are structured optical surfaces that manipulate light based on the principles of interference and diffraction. By carefully designing the diffractive optical elements, the amplitude, phase, direction, and polarization of the transmitted and reflected light can be controlled. It is well-known that the propagation of light through diffractive optics is sensitive to changes in their structural parameters. In this study, a numerical analysis is conducted to evaluate the capabilities of slanted-wire diffraction gratings to function opto-mechanically in the infrared spectral range. The slanted wire array is designed such that it is compatible with fabrication by two-photon polymerization, a direct laser-writing approach. The modeled optical and mechanical capabilities of the diffraction grating are presented. The numerical results demonstrate a high sensitivity of the diffracted light to changes in the slant angle of the wires. The compressive force by which desired slant angles may be achieved as a function of the number of wires in the grating is investigated. The ability to fabricate the presented design using two-photon polymerization is supported by the development of a prototype. The results of this study suggest that slanted-wire gratings fabricated using two-photon polymerization may be effective in applications such as tunable beam splitting and micro-mechanical sensing.

Mechanical Control of the Optical Bandgap in One-Dimensional Photonic Crystals.

Over the last several years, two-photon polymerization has been a popular fabrication approach for photonic crystals due to its high spatial resolution. One-dimensional photonic crystals with photonic bandgap reflectivities over 90% have been demonstrated for the infrared spectral range. With the success of these structures, methods which can provide tunability of the photonic bandgap are being explored. In this study, we demonstrate the use of mechanical flexures in the design of one-dimensional photonic crystals fabricated by two-photon polymerization for the first time. Experimental results show that these photonic crystals provide active mechanically induced spectral control of the photonic bandgap. An analysis of the mechanical behavior of the photonic crystal is presented and elastic behavior is observed. These results suggest that one-dimensional photonic crystals with mechanical flexures can successfully function as opto-mechanical structures.

Terahertz anisotropic response of additively manufactured one-dimensional photonic crystals.

A polymer-based, one-dimensional photonic crystal exhibiting anisotropic responses was demonstrated in the terahertz frequency range. The photonic crystal was composed of alternating compact and low-density polymethacrylate layers. The low-density layers consisted of sub-wavelength sized columns, which were slanted 45° with respect to the substrate surface normal to achieve form-birefringence. Normal incidence polarized terahertz transmission measurements were carried out for characterization of the fabricated photonic crystals in the range from 82 to 125 GHz. The experimental data revealed a 2 GHz shift in the center frequency of the photonic bandgap as a function of in-plane orientation, well demonstrating the anisotropic behavior of the fabricated crystal. The transmission data were analyzed using stratified optical layer model calculations. A good agreement was found between the relevant model parameters and the corresponding design parameters.

Tunable cavity-enhanced terahertz frequency-domain optical Hall effect.

Presented here is the development and demonstration of a tunable cavity-enhanced terahertz (THz) frequency-domain optical Hall effect (OHE) technique. The cavity consists of at least one fixed and one tunable Fabry-Pérot resonator. The approach is suitable for the enhancement of the optical signatures produced by the OHE in semi-transparent conductive layer structures with plane parallel interfaces. Tuning one of the cavity parameters, such as the external cavity thickness, permits shifting of the frequencies of the constructive interference and provides substantial enhancement of the optical signatures produced by the OHE. A cavity-tuning optical stage and gas flow cell are used as examples of instruments that exploit tuning an external cavity to enhance polarization changes in a reflected THz beam. Permanent magnets are used to provide the necessary external magnetic field. Conveniently, the highly reflective surface of a permanent magnet can be used to create the tunable external cavity. The signal enhancement allows the extraction of the free charge carrier properties of thin films and can eliminate the need for expensive superconducting magnets. Furthermore, the thickness of the external cavity establishes an additional independent measurement condition, similar to, for example, the magnetic field strength, THz frequency, and angle of incidence. A high electron mobility transistor (HEMT) structure and epitaxial graphene are studied as examples. The tunable cavity-enhancement effect provides a maximum increase of more than one order of magnitude in the change of certain polarization components for both the HEMT structure and epitaxial graphene at particular frequencies and external cavity sizes.

Si(bzimpy)2 - a hexacoordinate silicon pincer complex for electron transport and electroluminescence.

A neutral hexacoordinate silicon complex containing two 2,6-bis(benzimidazol-2'-yl)pyridine (bzimpy) ligands has been synthesized and explored as a potential electron transport layer and electroluminescent layer in organic electronic devices. The air and water stable complex is fluorescent in solution with a λmax = 510 nm and a QY = 57%. Thin films grown via thermal evaporation also fluoresce and possess an average electron mobility of 6.3 × 10-5 cm2 V-1 s-1. An ITO/Si(bzimpy)2/Al device exhibits electroluminescence with λmax = 560 nm.

Nanomaterials as stationary phases and supports in liquid chromatography.

The development of various nanomaterials over the last few decades has led to many applications for these materials in liquid chromatography (LC). This review will look at the types of nanomaterials that have been incorporated into LC systems and the applications that have been explored for such systems. A number of carbon-based nanomaterials and inorganic nanomaterials have been considered for use in LC, ranging from carbon nanotubes, fullerenes and nanodiamonds to metal nanoparticles and nanostructures based on silica, alumina, zirconia and titanium dioxide. Many ways have been described for incorporating these nanomaterials into LC systems. These methods have included covalent immobilization, adsorption, entrapment, and the synthesis or direct development of nanomaterials as part of a chromatographic support. Nanomaterials have been used in many types of LC. These applications have included the reversed-phase, normal-phase, ion-exchange, and affinity modes of LC, as well as related methods such as chiral separations, ion-pair chromatography and hydrophilic interaction liquid chromatography. Both small and large analytes (e.g., dyes, drugs, amino acids, peptides and proteins) have been used to evaluate possible applications for these nanomaterial-based methods. The use of nanomaterials in columns, capillaries and planar chromatography has been considered as part of these efforts. Potential advantages of nanomaterials in these applications have included their good chemical and physical stabilities, the variety of interactions many nanomaterials can have with analytes, and their unique retention properties in some separation formats.

In-situ terahertz optical Hall effect measurements of ambient effects on free charge carrier properties of epitaxial graphene.

Unraveling the doping-related charge carrier scattering mechanisms in two-dimensional materials such as graphene is vital for limiting parasitic electrical conductivity losses in future electronic applications. While electric field doping is well understood, assessment of mobility and density as a function of chemical doping remained a challenge thus far. In this work, we investigate the effects of cyclically exposing epitaxial graphene to controlled inert gases and ambient humidity conditions, while measuring the Lorentz force-induced birefringence in graphene at Terahertz frequencies in magnetic fields. This technique, previously identified as the optical analogue of the electrical Hall effect, permits here measurement of charge carrier type, density, and mobility in epitaxial graphene on silicon-face silicon carbide. We observe a distinct, nearly linear relationship between mobility and electron charge density, similar to field-effect induced changes measured in electrical Hall bar devices previously. The observed doping process is completely reversible and independent of the type of inert gas exposure.

Optical Hall effect-model description: tutorial.

The optical Hall effect is a physical phenomenon that describes the occurrence of magnetic-field-induced dielectric displacement at optical wavelengths, transverse and longitudinal to the incident electric field, and analogous to the static electrical Hall effect. The electrical Hall effect and certain cases of the optical Hall effect observations can be explained by extensions of the classic Drude model for the transport of electrons in metals. The optical Hall effect is most useful for characterization of electrical properties in semiconductors. Among many advantages, while the optical Hall effect dispenses with the need of electrical contacts, electrical material properties such as effective mass and mobility parameters, including their anisotropy as well as carrier type and density, can be determined from the optical Hall effect. Measurement of the optical Hall effect can be performed within the concept of generalized ellipsometry at an oblique angle of incidence. In this paper, we review and discuss physical model equations, which can be used to calculate the optical Hall effect in single- and multiple-layered structures of semiconductor materials. We define the optical Hall effect dielectric function tensor, demonstrate diagonalization approaches, and show requirements for the optical Hall effect tensor from energy conservation. We discuss both continuum and quantum approaches, and we provide a brief description of the generalized ellipsometry concept, the Mueller matrix calculus, and a 4×4 matrix algebra to calculate data accessible by experiment. In a follow-up paper, we will discuss strategies and approaches for experimental data acquisition and analysis.

Transient Absorption Measurements on Anisotropic Monolayer ReS2.

Anisotropic optical and transport properties of monolayer ReS2 fabricated by mechanical exfoliation are reported. Transient absorption measurements with different polarization configurations and sample orientations reveal that the absorption coefficient and transient absorption are both anisotropic, with maximal and minimal values occurring when the light polarization is parallel and perpendicular to the Re atomic chains, respectively. The maximal values are about a factor of 2.5 of the minimal values. By resolving the spatiotemporal dynamics of excitons, it is found that the diffusion coefficient of excitons moving along Re atomic chains is about 16 cm(2) s(-1) at room temperature, which is about a factor of three larger than those moving perpendicular to that direction. An exciton lifetime of 40 ps is also extracted. These findings establish monolayer ReS2 as an anisotropic 2D transition metal dichalcogenide.

The retention of liquid by columnar nanostructured surfaces during quartz crystal microbalance measurements and the effects of adsorption thereon.

HYPOTHESIS: A surface comprising spatially coherent columnar nanostructures is expected to retain intercolumnar liquid during a quartz crystal microbalance measurement due to the surface structure. Part of the liquid retained by the nanostructures may then be displaced by adsorbate. EXPERIMENTS: Slanted columnar nanostructure thin films were designed to vary in height but remain structurally similar, fabricated by glancing angle deposition, and characterized by generalized ellipsometry. A frequency overtone analysis, introduced here, was applied to analyze quartz crystal microbalance data for the exchange of isotope liquids over the nanostructured surfaces and determine the areal inertial mass of structure-retained liquid. The adsorption of cetyltrimethylammonium bromide onto nanostructures was investigated by simultaneous quartz crystal microbalance and generalized ellipsometry measurements. FINDINGS: The areal inertial mass of structure-retained liquid varies linearly with nanostructure height. The proportionality constant is a function of the surface topography and agrees with the generalized ellipsometry-determined nanostructure film porosity, implying that nearly all intercolumnar liquid is retained. We report that for adsorption processes within porous nanostructured films, the quartz crystal microbalance is sensitive not to the combined areal inertial mass of adsorbate and retained liquid but rather to the density difference between adsorbate and liquid due to the volume exchange within the nanostructure film.

Multilayer graphitic coatings for thermal stabilization of metallic nanostructures.

We demonstrate that graphitic coatings, which consist of multilayer disordered graphene sheets, can be used for the thermal protection of delicate metal nanostructures. We studied cobalt slanted nanopillars grown by glancing angle deposition that were shown to melt at temperatures much lower than the melting point of bulk cobalt. After graphitic coatings were conformally grown over the surfaces of Co nanopillars by chemical vapor deposition, the resulting carbon-coated Co nanostructures retained their morphology at elevated temperatures, which would damage the uncoated structures. Thermal stabilization is also demonstrated for carbon-coated Ti nanopillars. The results of this study may be extended to other metallic and possibly even nonmetallic nanostructures that need to preserve their morphology at elevated temperatures in a broad range of applications.

Protein adsorption on and swelling of polyelectrolyte brushes: A simultaneous ellipsometry-quartz crystal microbalance study.

With a coupled spectroscopic ellipsometry-quartz crystal microbalance with dissipation (QCM-D) experimental setup, quantitative information can be obtained about the amount of buffer components (water molecules and ions) coupled to a poly(acrylic acid) (PAA) brush surface in swelling and protein adsorption processes. PAA Guiselin brushes with more than one anchoring point per single polymer chain were prepared. For the swollen brushes a high amount of buffer was found to be coupled to the brush-solution interface in addition to the content of buffer inside the brush layer. Upon adsorption of bovine serum albumin the further incorporation of buffer molecules into the protein-brush layer was monitored at overall electrostatic attractive conditions [below the protein isolectric poimt (IEP)] and electrostatic repulsive conditions (above the protein IEP), and the shear viscosity of the combined polymer-protein layer was evaluated from QCM-D data. For adsorption at the "wrong side" of the IEP an incorporation of excess buffer molecules was observed, indicating an adjustment of charges in the combined polymer-protein layer. Desorption of protein at pH 7.6 led to a very high stretching of the polymer-protein layer with additional incorporation of high amounts of buffer, reflecting the increase of negative charges on the protein molecules at this elevated pH.

Generalized far-infrared magneto-optic ellipsometry for semiconductor layer structures: determination of free-carrier effective-mass, mobility, and concentration parameters in n-type GaAs.

We report for the first time on the application of generalized ellipsometry at far-infrared wavelengths (wave numbers from 150 cm(-1) to 600 cm(-1)) for measurement of the anisotropic dielectric response of doped polar semiconductors in layered structures within an external magnetic field. Upon determination of normalized Mueller matrix elements and subsequent derivation of the normalized complex Jones reflection matrix r of an n-type doped GaAs substrate covered by a highly resistive GaAs layer, the spectral dependence of the room-temperature magneto-optic dielectric function tensor of n-type GaAs with free-electron concentration of 1.6 x 10(18) cm(-3) at the magnetic field strength of 2.3 T is obtained on a wavelength-by-wavelength basis. These data are in excellent agreement with values predicted by the Drude model. From the magneto-optic generalized ellipsometry measurements of the layered structure, the free-carrier concentration, their optical mobility, the effective-mass parameters, and the sign of the charge carriers can be determined independently, which will be demonstrated. We propose magneto-optic generalized ellipsometry as a novel approach for exploration of free-carrier parameters in complex organic or inorganic semiconducting material heterostructures, regardless of the anisotropic properties of the individual constituents.