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The Mueller matrix is a mathematical description of how light is altered by an optical element or a sample under study. It describes both intensity (irradiance) and polarization changes, including a reduction of the total polarization. Mueller matrix spectroscopic ellipsometry has gained recent popularity in the optics and semiconductor communities as an effective means to characterize complex sample structures and anisotropic materials. While this method is not new, its recent expansion to new fields has left many users with only a pedestrian understanding of the data they collect. This tutorial provides an overview of Mueller matrix spectroscopic ellipsometry with focus on practical aspects for those new to the technique.

There is an emerging recognition that successful utilization of chiral degrees of freedom can bring new scientific and technological opportunities to diverse research areas. Hence, methods are being sought for creating artificial matter with controllable chirality in an uncomplicated and reproducible manner. Here, we report the development of two straightforward methods for fabricating wafer-scale chiral architectures of ordered carbon nanotubes (CNTs) with tunable and giant circular dichroism (CD). Both methods employ simple approaches, (i) mechanical rotation and (ii) twist-stacking, based on controlled vacuum filtration and do not involve any sophisticated nanofabrication processes. We used a racemic mixture of CNTs as the starting material, so the intrinsic chirality of chiral CNTs is not responsible for the observed chirality. In particular, by controlling the stacking angle and handedness in (ii), we were able to maximize the CD response and achieve a record-high deep-ultraviolet ellipticity of 40 $\pm$ 1 mdeg/nm. Our theoretical simulations using the transfer matrix method reproduce the salient features of the experimentally observed CD spectra and further predict that a film of twist-stacked CNTs with an optimized thickness will exhibit an ellipticity as high as 150 mdeg/nm. The created wafer-scale objects represent a new class of synthetic chiral matter consisting of ordered quantum wires whose macroscopic properties are governed by nanoscopic electronic signatures such as van Hove singularities. These artificial structures with engineered chirality will not only provide playgrounds for uncovering new chiral phenomena but also open up new opportunities for developing high-performance chiral photonic and optoelectronic devices., Main text: 4 figures, 21 pages; SI: 9 figures, 1 table, 11 pages

Large-scale CO2 capture through membrane separation is the only viable method for cost-effective emission reduction. However, the fabrication of CO2 separation membranes with sufficient flux and se...

Tunable mid-wave infrared (MWIR) filters are highly desirable in a wide variety of imaging and spectroscopic applications; with the waveband containing numerous invaluable spectral absorption signatures. We demonstrate spectrally-tunable, all solid-state, thin-film bandpass filters operating across the MWIR by utilizing the phase-change material (PCM) Ge2Sb2Te5 as a tunable cavity medium between two Bragg mirrors. The induced PCM refractive index modulation increases the cavity’s optical path length, thereby tuning the filter passband. Our filters have 300 nm spectral tunability, ~70% transmission efficiencies, and ~60 nm passbands. We further show multispectral thermal imaging and dynamic CO2 gas plume sensing using the filters.

We demonstrate spectrally-tunable Fabry-Perot bandpass filters operating across the MWIR by utilizing the phase-change material GeSbTe (GST) as a tunable cavity medium between two (Ge:Si) distributed Bragg reflectors. The induced refractive index modulation of GST increases the cavity's optical path length, red-shifting the passband. Our filters have spectral-tunability of ~300 nm, transmission efficiencies of 60-75% and narrowband FWHMs of 50-65 nm (Q-factor ~70-90). We further show multispectral thermal imaging and gas sensing. By matching the filter's initial passband to a CO$_{2}$ vibrational-absorption mode (~4.25 $\mu$m), tunable atmospheric CO$_{2}$ sensing and dynamic plume visualization of added CO$_{2}$ is realized., Comment: 13 pages, 5 figures

This work reports on Mueller matrix spectroscopic ellipsometry characterization of various flexible plastic substrates that are optically anisotropic with varying degrees of birefringence. The samples are divided into three groups according to the suggested characterization strategy: low birefringence, high birefringence, and twisted birefringence. The first group includes poly(methyl methacrylate) and cyclic olefin copolymer substrates. These are modeled with biaxial anisotropy for the real part of the refractive index while the imaginary part is approximated as isotropic due to small light absorption. The second group includes polyethylene terephthalate and polyethylene naphthalate substrates, which are modeled with biaxial anisotropy for both real and imaginary refractive indices. Lastly, a polyimide substrate is described as two birefringent layers with twisted in-plane orientation.

Spectroscopic ellipsometry characterization of coatings on polymeric substrates can be challenging due to the substrate optical anisotropy. We compare four characterization strategies for thin coating layers on anisotropic polymeric substrates with regard to accuracy of the resulting layer thickness and coating optical constants. Each strategy differs in measured data type, model construction, implementation complexity, and inherent capabilities and sensitivity to the coating properties. Best practices and limitations are discussed for each strategy.

We report the optical constants of graphene oxide and reduced graphene oxide determined by spectroscopic ellipsometry. The dynamic changes in optical properties and thickness of a drop-cast graphene oxide layer during reduction by long-term exposure to focused broad-band white light are monitored in situ. The anisotropic optical constants of the graphene oxide layer and the isotropically averaged optical constants of the reduced layer are precisely determined from a multiple-location analysis of spatially resolved data across the exposed location and a multiple-time-step analysis of the dynamic data, respectively. Observed inter-band transitions in the graphene oxide layer are discussed in relation to theoretical predictions for different coverage levels of the graphene oxide sheets with oxygen containing functional groups. The derived optical constants of the reduced graphene oxide layer are compared to reported values of graphene and thermally reduced graphene oxide.

The multi-pass photoacoustic spectrometer (PAS) is an important tool for the direct measurement of light absorption by atmospheric aerosol. Accurate PAS measurements heavily rely on accurate calibration of their signal. Ozone is often used for calibrating PAS instruments by relating the photoacoustic signal to the absorption coefficient measured by an independent method such as cavity ring down spectroscopy (CRD-S), cavity-enhanced spectroscopy (CES) or an ozone monitor. We report here a calibration method that uses measured absorption coefficients of aerosolized, light-absorbing organic materials and offer an alternative approach to calibrate photoacoustic aerosol spectrometers at 404 nm. To implement this method, we first determined the complex refractive index of nigrosin, an organic dye, using spectroscopic ellipsometry and then used this well-characterized material as a standard material for PAS calibration.

Concentrator photovoltaic (CPV) systems, where incident direct solar radiation is tightly concentrated onto high-efficiency multi-junction solar cells by geometric optical elements, exhibit the highest efficiencies in converting the sun’s energy into electric power. Their energy conversion efficiencies are greatly limited, however, due to Fresnel reflection losses occurring at three air/optics interfaces in the most sophisticated dual-stage CPV platforms. This paper describes a facile one-step wet-etching process to create a nanoporous surface with a graded-index profile on both flat and curved glasses, with capabilities of achieving ~99% average transmission efficiency in a wide wavelength range from 380 nm to 1.3 µm and for a wide range of incident angles up to ±40° regardless of the polarization state of incident sunlight. The simplicity of the etching process remarkably increases their versatility in various optical elements that require unconventional form factors such as Fresnel lenses and microlens arrays, and/or demanding curvatures along with much reduced dimensions such as ball lenses. Etched glass surfaces on two-stage optical concentrating systems yield enhancements in total optical transmission efficiencies by 13.8% and in the photocurrent by 14.3%, as experimentally determined by measurements on microscale triple-junction solar cells. The presented strategy can be widely adapted in a variety of applications such as image sensors, display systems, and other optoelectronic devices.

The growing demand for multifunctional nanophotonic devices has led to the exploration, and utilization, of a plethora of exotic electro-optical materials. Recently, chalcogenide glass based phase change materials (PCMs) have shown utility as a tuning material for a range of nanophotonic devices. Owing to their low loss, ultrafast switching speeds and wide waveband operation, PCMs are integrated in an increasing number of next-generation tunable components, including integrated photonic switches, metasurface optics and tunable spectral filters. Nonetheless, modelling of PCM-based devices is challenging—both in terms of accurate representation of experimentally derived material properties in different phase states, and standardization of results across the research community. Further, as each device requires optimization of specific performance metrics dependent on their respective application, any inaccuracies will lead to erroneous outcomes. In this work, we introduce PCM-net (http://nekocloud.com/pnet/): an online database of the complex refractive indices of a variety of chalcogenide glass PCMs (such as GeSbTe), as an accessible and indexed repository for data sharing across the PCM community. Refractive indices (n) and extinction coefficients (k) between amorphous and crystalline states are directly extracted from experimentally derived data in numerous academic research articles, and collated into the material resource database. Due to the inaccuracies associated with our data collection methods, this data is supplemented with additional computationally generated data, obtained through WVASE® —a commercial ellipsometry analysis software package. To demonstrate the utility of PCM-net, we provide a NASA application-driven device optimization example using the optical properties of PCMs collected with our database. We anticipate the database providing great use to the PCM community and coordinated research efforts enabled by PCM-net will promote the shared repository for the selection of appropriate PCMs for tunable nanophotonic device design for a range of applications.

We report the optical properties and solar reflectance of catalyst-free vertical graphene (VG) sheets grown on two types of substrate, copper and silicon, over a wide range of wavelengths and for the first time, a method for the separation of the sheets from the substrate. The index of refraction of the VG sheets was close to unity in the visible and infrared range, while the coefficient of absorption was higher than those of forests of carbon nanotubes (CNTs). A total reflectance (diffuse and specular) of less than 10% was observed for the VG sheets in the visible range. For VG sheets grown on a copper substrate, the specific absorption (absorption divided by the areal density) at a wavelength of 633 nm was measured to be two times higher than previously reported results for VG sheets grown on a silicon substrate and much higher than that of a CNT forest.

The authors introduce an ellipsometric data analysis strategy for a flexible polymeric circular polarizing filter consisting of a thin linear polarizer and quarter-wave plate sandwich. The circular polarizing filter is an inhomogeneous optical system exhibiting different optical responses depending on the propagation direction of light. If light enters from the linear polarizer, the transmitted beam is linearly polarized before entering the quarter-wave plate. The orientation of the quarter-wave plate is rotated 45° from the linear polarizer axis. The emerging light from the quarter-wave plate is circularly polarized. If light is circularly polarized and enters from the reserve side, the quarter-wave plate converts it into linearly polarized light. The following linear polarizer either transmits or absorbs the beam depending on the handedness of the original circular polarization. The optical response in the forward direction is utilized in photography to reduce unwanted reflections in the image. The optical response in the reverse direction is utilized in 3D eyeglasses, which consist of two orthogonal circular polarizing filters to separate the left and right images. Mueller matrix spectroscopic ellipsometry is used to observe the optical responses of a circular polarizing filter in both directions. The authors demonstrate data analysis procedures for individual layers to find the optical constants in a wide spectral range from 400 to 1700 nm. The circular polarizing filter measured in the forward direction enables ellipsometry to determine the included angle between the linear polarizer and the quarter-wave plate. The ellipsometric data analysis result is used to predict transmitted light intensity versus rotation angle (deg) with respect to any input polarization state.The authors introduce an ellipsometric data analysis strategy for a flexible polymeric circular polarizing filter consisting of a thin linear polarizer and quarter-wave plate sandwich. The circular polarizing filter is an inhomogeneous optical system exhibiting different optical responses depending on the propagation direction of light. If light enters from the linear polarizer, the transmitted beam is linearly polarized before entering the quarter-wave plate. The orientation of the quarter-wave plate is rotated 45° from the linear polarizer axis. The emerging light from the quarter-wave plate is circularly polarized. If light is circularly polarized and enters from the reserve side, the quarter-wave plate converts it into linearly polarized light. The following linear polarizer either transmits or absorbs the beam depending on the handedness of the original circular polarization. The optical response in the forward direction is utilized in photography to reduce unwanted reflections in the image. The opti...

Hyperbolic metamaterials are optical materials characterized by highly anisotropic effective permittivity tensor components having opposite signs along orthogonal directions. The techniques currently employed for characterizing the optical properties of hyperbolic metamaterials are limited in their capability for robust extraction of the complex permittivity tensor. Here we demonstrate how an ellipsometry technique based on total internal reflection can be leveraged to extract the permittivity of hyperbolic metamaterials with improved robustness and accuracy. By enhancing the interaction of light with the metamaterial stacks, improved ellipsometric sensitivity for subsequent permittivity extraction is obtained. The technique does not require any modification of the hyperbolic metamaterial sample or sophisticated ellipsometry set-up, and could therefore serve as a reliable and easy-to-adopt technique for characterization of a broad class of anisotropic metamaterials.

The optical constants of various substrates and coating materials employed widely for solar-cell fabrication are summarized. The substrate materials described here include various SiO2 (soda-lime glass, borosilicate glass, fused silica, and quartz), metal (stainless) and plastic (polycarbonate, PEN, PET, PMMA, Kapton HN) substrates. For PEN, PET and Kapton HN substrates, which exhibit strong biaxial optical anisotropy, full data sets are provided. In solar cell devices, a protection coating (EVA) and anti-reflection (MgF2/LiF) layers are quite important, and the optical constants of these coating layers are also described here. The refractive index spectra of the transparent materials were parameterized using the Sellmeier model, whereas the parameterization of absorbing polymer substrates was carried out assuming several transition peaks calculated by the Tauc-Lorentz model. In this chapter, the model parameters for all the substrates and coating materials are summarized, together with the tabulated optical constants.

We demonstrate a new measurement procedure using attenuated total internal reflection ellipsometry for accurately extracting the permittivity of hyperbolic metamaterials. This technique enhances the interaction of probe light with the HMM, thus providing sufficient ellipsometric parameters for the subsequent permittivity extraction.

Multi-pass photoacoustic spectrometer (PAS) is an important tool for the direct measurement of light absorption by atmospheric aerosol. And accurate PAS measurements require an accurate calibration of its signal. Ozone is often used for calibrating the PAS by relating the instrument signal to the absorption coefficient measured by an independent method such as cavity ring down spectroscopy (CRD), cavity enhanced spectroscopy (CES) or an ozone monitor. We report here a calibration method that uses measured absorption coefficients of aerosolized, light absorbing organic materials and offer an alternative approach to calibrate the PAS at 404 nm. To implement this method we first determined the complex refractive index of an organic dye using spectroscopic ellipsometry, and then we use this well characterized material as a standard material for PAS calibration.

The development of novel neutron detectors requires an understanding of the entire neutron detection process, a process which depends strongly on material properties. Here we present accurate measurements of the neutron detection efficiency of an unenriched 640 nm thick boron carbide solid state neutron detector grown by plasma enhanced chemical vapor deposition as a function of the neutron wavelength at a time-of-flight facility. The data were compared to that obtained simultaneously by a calibrated nitrogen detector over the same wavelength range. The measured spectra of both detectors fit a Maxwell–Boltzmann wavelength distribution, thereby indicating that the boron carbide detector can be used as a reliable beam monitor. Measurements of the material properties (density, thickness and elemental composition) of the semiconducting boron carbide enable a precise calculation of the ideal expected neutron detection efficiency. The calculated neutron detection efficiency for the effective moderator temperature (obtained from a fit to the Maxwell–Boltzmann distribution) showed excellent agreement with the experimentally determined neutron detection efficiency of 1.25%. Higher efficiencies may be obtained either by increased film thickness and/or 100% 10B enrichment of the boron carbide source molecule.

Optical properties from 0.2 to 6.5 eV of epitaxial ${\mathrm{NbO}}_{2}$ $(4{d}^{1} \text{system})$ films in their insulating states have been investigated by spectroscopic ellipsometry. The optical spectra are compared to that of epitaxial ${\mathrm{VO}}_{2}$ $(3{d}^{1} \text{system})$ in its insulating phase. Both compounds are insulators at room temperature and undergo temperature-induced metal-insulator transitions. We find a ${d}_{||}\ensuremath{-}{e}_{\mathrm{g}}^{\ensuremath{\pi}}$ orbital splitting energy of $\ensuremath{\sim}1.6$ eV in ${\mathrm{NbO}}_{2}$ compared to $\ensuremath{\sim}1.3$ eV in ${\mathrm{VO}}_{2}$; orbital splitting is a key ingredient that stabilizes the cation-dimerized insulating states of both materials. The edge of the $\text{O}2p$-like valence band is estimated to be 3.2 eV below the Fermi level through x-ray photoelectron spectroscopy measurements, suggesting that electrons from $\text{O}2p$ states do not contribute to absorption below this energy. This allows us to also assign an observed optical peak at $\ensuremath{\sim}3.0$ eV to the $\mathrm{Nb}\phantom{\rule{0.28em}{0ex}}4{d}_{||}\ensuremath{\rightarrow}4{d}_{||}^{*}$ transition.

To explore the relationship between coronary collateral circulation following percutaneous coronary intervention (PCI) for a single left anterior descending artery and the recovery of cardiac function.A total of 625 patients with coronary heart disease were retrospectively analyzed, who received selective coronary angiography demonstrating lesions involving a single left anterior descending artery and underwent stent placement between January, 2010 and December, 2012. According to Rentrop's classification, the patients were divided into group A (n=280) with Rentrop grades 1-3 and group B (n=325) with Rentrop grade 0. Group A were further divided into 3 subgroups according to the source of collateral circulation, namely group A1 (n=200) with contralateral collateral circulation, group A2 (n=44) with contralateral+ ipsilateral collateral circulation, and group A3 (n=36) with ipsilateral collateral circulation. The outcomes of cardiac function recovery were compared between groups A and B and between the 3 subgroups in group A.Compared with patients without collateral circulation, patients with collateral coronary circulation showed greater left ventricular ejection fraction increment and reduction in brain natriuretic peptide and red cell volume distribution width with also lower expansion left ventricular end-diastolic volume. Among the 3 subgroups in group A, cardiac function improvement was the most obvious in patients with contralateral+ ipsilateral collateral circulation (group A2) followed by those in group A3, and was the worst in group A1.The presence of collateral coronary circulation promotes cardiac function recovery in patients receiving PCI for lesions involving a single left anterior descending artery. Patients with contralateral+ipsilateral collateral circulation have the best cardiac function improvement followed by those with contralateral collateral circulation.

Organic-based electronic devices are rapidly increasing in popularity, making it essential to understand and characterize the interface between organic materials and metallic electrodes. This work reports on the characterization of the interface between thin films of an emerging organic ferroelectric, vinylidene fluoride (VDF) oligomer, and Co, an important high Curie temperature ferromagnet. Using a wide battery of experimental techniques, it is shown that VDF oligomer thin films as thin as 15 nm can halt, or prevent, Co oxidization in atmospheric conditions, a necessary condition for device applications. Selectivity of magnetic properties, such as remanent magnetization, is enabled by the clarification of the time scale of Co oxidation, a topic on which there are many conflicting reports. Furthermore, this work shows evidence of chemical bonding at the interface between VDF oligomer and Co, a result with important implications for organic spintronic devices. These results establish the suitability of VDF oligomer for organic-based electronic devices.

Organic thin films have numerous advantages over inorganics in device processing and price. The large polarization of the organic ferroelectric oligomer vinylidene fluoride (VDF) could prove useful for both device applications and the investigation of fundamental physical phenomena. A VDF oligomer thin film vacuum deposition process, such as thermal evaporation, preserves film and interface cleanliness, but is challenging, with successful deposition occurring only within a narrow parameter space. We report on the optimal deposition parameters for VDF oligomer thin films, refining the parameter space for successful deposition, resulting in a high yield of robust ferroelectric films. In particular, we investigate the influence of deposition parameters on surface roughness, and the role that roughness plays in sample yield. The reliable production of ferroelectric films allowed us to perform detailed measurements of previously unreported properties, including the Curie temperature, the temperature and thickness dependence of the coercive field, the melting temperature, and the index of refraction. The ability to successfully grow robust, switchable, well-characterized films makes VDF oligomer a viable candidate in the field of organic ferroelectrics.

The wide band gap, temperature stability, high resistivity, and robustness of semiconducting boron carbide make it an attractive material for device applications. Undoped boron carbide is p type; Ni acts as a n-type dopant. Here we present the results of controlled doping of boron carbide with Ni on thin film samples grown using plasma enhanced chemical vapor deposition. The change in the dopant concentration within the thin film as a function of the dopant flow rate in the precursor gas mixture was confirmed by x-ray photoelectron spectroscopy measurements; with increasing dopant concentration, current-voltage (I-V) curves clearly establish the trend from p-type to n-type boron carbide.