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We present an experimental method that utilizes a conic probe attached to an integrating sphere for characterizing the small-spot luminance of virtual reality (VR) head-mounted displays (HMDs). This allows for the measurement of relative luminance in virtual scenes and the assessment of VR displays for visualizing grayscale medical images. We characterized the relative luminance of the VIVE Pro and Oculus Rift by displaying test patterns with a circular dark spot of varying diameter in the bright field and compared the probe performance with commercially available systems. Compared to a conventional flat-panel display, the results show a significant level of veiling glare in the HMDs using both a customized conic probe with improved optical performance and an advanced imaging photometer. We also found no significant change in the relative luminance response with the probe’s working distance for the HMDs due to their unique optical architecture for near-eye viewing conditions. Our results highlight the issue of high veiling glare in HMDs and the importance of small-spot luminance measurements to adequately evaluate the technical performance of HMDs for emerging medical applications.

Virtual reality (VR) head mounted displays (HMDs) require both high spatial resolution and fast temporal response. However, methods to quantify the VR image quality in the spatiotemporal domain when motion exists are not yet standardized. In this study, we characterize the spatiotemporal capabilities of three VR devices: the HTC VIVE, VIVE Pro, and VIVE Pro 2 during smooth pursuit. A spatiotemporal model for VR HMDs is presented using measured spatial and temporal characteristics. Among the three evaluated headsets, the VIVE Pro 2 improves the display temporal performance using a fast 120 Hz refresh rate and pulsed emission with a small duty cycle of 5%. In combination with a high pixel resolution beyond 2 k $$\times$$ × 2 k per eye, the VIVE Pro 2 achieves an improved spatiotemporal performance compared to the VIVE and VIVE Pro in the high spatial frequency range above 8 cycles per degree during smooth pursuit. The result demonstrates that reducing the display emission duty cycle to less than 20% is beneficial to mitigate motion blur in VR HMDs. Frame rate reduction (e.g., to below 60 Hz) of the input signal compared to the display refresh rate of 120 Hz yields replicated shadow images that can affect the image quality under motion. This work supports the regulatory science research efforts in development of testing methods to characterize the spatiotemporal performance of VR devices for medical use.

Scratches in polymer coatings and barrier layers negatively impact optical properties (haze, light transmission, etc.), initiate routes of degradation or corrosion (moisture permeability), and nucleate delamination of the coating. Detecting scratches in coatings on advanced materials systems is an important component of structural health monitoring but can be difficult if the defects are too small to be detected by the naked eye. The primary focus of the present work is to investigate scratch damage using fluorescence lifetime imaging microscopy (FLIM) and mechanical activation of a mechanophore (MP)-containing transparent epoxy coating. The approach utilizes a Berkovich tip to scratch MP-epoxy coatings under a linearly increasing normal load. The goal is to utilize the fluorescent behavior of activated MPs to enable the detection of microscale scratches and molecular scale changes in polymeric systems. Taking advantage of the amine functionality present in a polyetheramine/bisphenol A epoxy network, a modified rhodamine dye is covalently bonded into a transparent, thermoset polymer network. Following instrumented scratch application, subsequent fluorescence imaging of the scratched MP-epoxy reveals the extent of fluorescence activation induced by the mechanical deformation. In this work, the rhodamine-based mechanophore is used to identify both ductile and fracture-dominated processes during the scratch application. The fluorescence intensity increases linearly with the applied normal load and is sensitive to fracture dominated processes. Fluorescence lifetime and hyperspectral imaging of damage zones provide additional insight into the local (nanoscopic) environment and molecular structure of the MP around the fracture process zone, respectively. The mechanophore/scratch deformation approach allows a fluorescence microscope to probe local yielding and fracture events in a powerful way that enhances the optical characterization of damage zones formed by standard scratch test methods and leads to novel defect detection strategies.

Sub-micron-size light sources are currently extremely dim, achieving nanowatt output powers due to the current density and temperature droop. Recently, we reported a droop-free fin light-emitting diode (LED) pixel that at high current densities becomes a laser with record output power in the microwatt range. Here, we show a scalable method for selectively metallizing fins via their nonpolar side facet that allows electrical injection to sub-200 nm wide n-ZnO fins on p-GaN with at least 0.8 μm2 active area. Electrically addressable fin LEDs are fabricated in a linear array format using standard 2 μm resolution photolithography. Electroluminescence analysis across different pixels shows that the fin acts as the active region of the LED and generates a narrow-band ultraviolet emission between ≈368 and ≈390 nm. Investigating fins at high current densities, ranging from 100 to 2000 kA/cm2, shows that their emission increases without any decline even as the junction temperature reaches a range of 200-340 °C. The absence of electron leakage to p-GaN at high injection levels and an undetectable electron-hole escape from the fin at high temperatures indicate that the fin shape is highly efficient in controlling the nonradiative recombination pathways such as Auger recombination. The fin LED geometry is expected to enable the realization of high-brightness arrays of light sources at sub-micron-size regimes suitable for operation at high temperatures and high current densities.

Potential commercial applications for transition metal dichalcogenide (TMD) semiconductors such as MoS(2) rely on unique material properties that are only accessible at monolayer to few-layer thickness regimes. Therefore, production methods that lend themselves to scalable and controllable formation of TMD films on surfaces are desirable for high volume manufacturing of devices based on these materials. We have developed a new thermal atomic layer deposition (ALD) process using bis(tert-butylimido)-bis(dimethylamido)molybdenum and 1-propanethiol to produce MoS(2)-containing amorphous films. We observe self-limiting reaction behavior with respect to both the Mo and S precursors at a substrate temperature of 350 °C. Film thickness scales linearly with precursor cycling, with growth per cycle values of (≈)0.1 nm/cycle. As-deposited films are smooth and contain nitrogen and carbon impurities attributed to poor ligand elimination from the Mo source. Upon high-temperature annealing, a large portion of the impurities are removed, and we obtain few-layer crystalline 2H-MoS(2) films.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe(2) crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T′ semimetallic phase at high temperatures. Alloying MoTe(2) with WTe(2) reduces the energy barrier between these two phases, while also allowing access to the T(d) Weyl semimetal phase. The Mo(1–x) W(x)Te(2) alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe(2)-WTe(2) system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk Mo(1–x)W(x)Te(2) alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T′, and T(d) structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe(2)–WTe(2) system, including single-phase 2H, 1T′, and T(d) regions, as well as a two-phase 1T′ + T(d) region. Disorder arising from compositional fluctuations in Mo(1–x)W(x)Te(2) alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T′-MoTe(2) mode and the enhancement of a double-resonance Raman process in 2H-Mo(1–x) W(x)Te(2) alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo(1–x)W(x)Te(2) alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

Visual information is a critical component in the evaluation and communication of patient medical information. As display technologies have evolved, the medical community has sought to take advantage of advances in wider color gamuts, greater display portability, and more immersive imagery. These image quality enhancements have shown improvements in the quality of healthcare through greater efficiency, higher diagnostic accuracy, added functionality, enhanced training, and better health records. However, the display technology advances typically introduce greater complexity in the image workflow and display evaluation. This paper highlights some of the optical measurement challenges created by these new display technologies and offers possible pathways to address them.

We demonstrate that transverse chromatic aberration (TCA) measurements can be used to determine the eyebox of a virtual reality head-mounted display using a digital test pattern, which consists of red, green, and blue bars placed horizontally and vertically at ±5°, ±10°, ±15° in the field of view. The pattern also features a white cross in the center of the field of view to determine the horizontal and vertical resolution. This provides simultaneous measurements of the TCAs and resolution for a given position of the camera in the eyebox. A map of the eyebox was generated by raster-scanning the position of the camera over the eyebox. The results for the Oculus Rift show that resolution has approximately a quadratic dependence on the position in the eyebox whereas the response to TCAs is linear. Therefore, TCA provides a more sensitive eyebox measurement at small displacements allowing for repeatable centering within 0.5 mm.

High-volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultrathin films will require deposition techniques that are capable of reproducible wafer-scale growth with monolayer control. To date, TMD growth efforts have largely relied upon sublimation and transport of solid precursors with minimal control over vapor-phase flux and gas-phase chemistry, which are critical for scaling up laboratory processes to manufacturing settings. To address these issues, we report a new pulsed metal–organic chemical vapor deposition (MOCVD) route for MoS2 film growth in a research-grade single-wafer reactor. Using bis(tert-butylimido)bis(dimethylamido)molybdenum and diethyl disulfide, we deposit MoS2 films from ∼1 nm to ∼25 nm in thickness on SiO2/Si substrates. We show that layered 2H-MoS2 can be produced at comparatively low reaction temperatures of 591 °C at short deposition times, approximately 90 s for few-layer films. In addition to the growth studies performed on SiO2/Si, films with wafer-...

We demonstrate a method for measuring the transverse chromatic aberration (TCA) in a virtual reality head-mounted display. The method relies on acquiring images of a digital bar pattern and measuring the displacement of different color bars. This procedure was used to characterize the TCAs in the Oculus Go, Oculus Rift, Samsung Gear, and HTC Vive. The results show noticeable TCAs for the Oculus devices for angles larger than 5° from the center of the field of view. TCA is less noticeable in the Vive in part due to off-axis monochromatic aberrations. Finally, user measurements were conducted, which were in excellent agreement with the laboratory results.

We study the crystal symmetry of few-layer 1T′ MoTe2 using the polarization dependence of the second harmonic generation (SHG) and Raman scattering. Bulk 1T′ MoTe2 is known to be inversion symmetric; however, we find that the inversion symmetry is broken for finite crystals with even numbers of layers, resulting in strong SHG comparable to other transition-metal dichalcogenides. Group theory analysis of the polarization dependence of the Raman signals allows for the definitive assignment of all the Raman modes in 1T′ MoTe2 and clears up a discrepancy in the literature. The Raman results were also compared with density functional theory simulations and are in excellent agreement with the layer-dependent variations of the Raman modes. The experimental measurements also determine the relationship between the crystal axes and the polarization dependence of the SHG and Raman scattering, which now allows the anisotropy of polarized SHG or Raman signal to independently determine the crystal orientation.

Fiber fracture is a major contributor for the initiation of failure in fiber-reinforced polymer composites. The purpose of this study was to identify where damage occurs when a fiber fractures in a composite. Because this dynamic event occurs in less than 100 1ns, obtaining experimental evidence has been prohibitively difficult. Here, we report a novel method using a model composite, comprised of a spirolactam-based mechanophore-functionalized epoxy resin with a single embedded glass fiber. The ductile epoxy matrix is loaded under tension, which transfers load through interfacial shearing to the embedded brittle fiber, causing the fiber to break into segments. Fluorescence lifetime imaging microscopy (FLIM) is used to visualize regions of mechanophore activation and quantify changes in the dye's local environment such as relaxations on the nanosecond timescale. FLIM of these single fiber fragmentation test (SFFT) samples reveals localized damage zones in the epoxy matrix, not only at the point of fiber fragmentation, but also at distances remote from the fracture site that extend radially into the matrix up to two fiber diameters. The average mechanophore lifetime in the bulk epoxy is (1.5 ± 0.5) ns and in the damage regions is (0.75 ± 0.15) ns. This new data reveals the existence of polymer matrix yielding, which occurs during the fiber fracture event, and agrees with a dynamic model that predicts yielding behavior in the matrix during fracture. These results suggest that the damage from fiber fracture is more extensive than previously recognized. This new approach can be used to elucidate damage in other mechanical tests. The subsequent insights into composite damage mechanisms promises to dramatically accelerate the development of tough, durable, and damage tolerant composites.

We demonstrate a digital test pattern and a methodology for measuring the transverse chromatic aberrations in head-mounted displays. The test pattern was also used for psychophysics TCA measurements, which agreed with the laboratory results.

High volume manufacturing of devices based on transition metal dichalcogenide (TMD) ultra-thin films will require deposition techniques that are capable of reproducible wafer-scale growth with monolayer control. To date, TMD growth efforts have largely relied upon sublimation and transport of solid precursors with minimal control over vapor phase flux and gas-phase chemistry, which are critical for scaling up laboratory processes to manufacturing settings. To address these issues, we report a new pulsed metalorganic chemical vapor deposition (MOCVD) route for MoS2 film growth in a research-grade single-wafer reactor. Using bis(tert-butylimido)-bis(dimethylamido)molybdenum and diethyl disulfide we deposit MoS2 films from ≈ 1 nm to ≈ 25 nm in thickness on SiO2/Si substrates. We show that layered 2H-MoS2 can be produced at comparatively low reaction temperatures of 591 °C at short deposition times, approximately 90 s for few-layer films. In addition to the growth studies performed on SiO2/Si, films with wafer-level uniformity are demonstrated on 50 mm quartz wafers. Process chemistry and impurity incorporation from precursors are also discussed. This low-temperature and fast process highlights the opportunities presented by metalorganic reagents in the controlled synthesis of TMDs.

Although semiconductor defects can often be detrimental to device performance, they are also responsible for the breadth of functionality exhibited by modern optoelectronic devices. Artificially engineered defects (so-called quantum dots) or naturally occurring defects in solids are currently being investigated for applications ranging from quantum information science and optoelectronics to high-resolution metrology. In parallel, the quantum confinement exhibited by atomically thin materials (semi-metals, semiconductors and insulators) has ushered in an era of flatland optoelectronics whose full potential is still being articulated. In this Letter we demonstrate the possibility of leveraging the atomically thin semiconductor tungsten diselenide (WSe2) as a host for quantum dot-like defects. We report that this previously unexplored solid-state quantum emitter in WSe2 generates single photons with emission properties that can be controlled via the application of external d.c. electric and magnetic fields. These new optically active quantum dots exhibit excited-state lifetimes on the order of 1 ns and remarkably large excitonic g-factors of 10. It is anticipated that WSe2 quantum dots will provide a novel platform for integrated solid-state quantum photonics and quantum information processing, as well as a rich condensed-matter physics playground with which to explore the coupling of quantum dots and atomically thin semiconductors.

We demonstrate a multimodal superresolution microscopy technique based on a phase masked excitation beam in combination with spatially filtered detection. The theoretical foundation for calculating the focus from a non-paraxial beam with an arbitrary azimuthally symmetric phase mask is presented for linear and two-photon excitation processes as well as the theoretical resolution limitations. Experimentally this technique is demonstrated using two-photon luminescence from 80 nm gold particle as well as two-photon fluorescence lifetime imaging of fluorescent polystyrene beads. Finally to illustrate the versatility of this technique we acquire two-photon fluorescence lifetime, two-photon luminescence, and second harmonic images of a mixture of fluorescent molecules and 80 nm gold particles with

The structural polymorphism intrinsic to select transition metal dichalcogenides provides exciting opportunities for engineering novel devices. Of special interest are memory technologies that rely upon controlled changes in crystal phase, collectively known as phase change memories (PCMs). MoTe$_2$ is ideal for PCMs as the ground state energy difference between the hexagonal (2H, semiconducting) and monoclinic (1T’, metallic) phases is minimal. This energy difference can be made arbitrarily small by substituting W for Mo on the metal sublattice, thus improving PCM performance. Therefore, understanding the properties of Mo$_{1-x}$W$_x$Te$_2$ alloys across the entire compositional range is vital for the technological application of these versatile materials. We combine Raman spectroscopy with aberration-corrected scanning transmission electron microscopy and x-ray diffraction to explore the MoTe$_2$-WTe$_2$ alloy system. The results of these studies enable the construction of the complete alloy phase diagram, while polarization-resolved Raman measurements provide phonon mode and symmetry assignments for all compositions. Temperature-dependent Raman measurements indicate a transition from 1T’-MoTe$_2$ to a distorted orthorhombic phase (T$_d$) below 250 K and facilitate identification of the anharmonic contributions to the optical phonon modes in bulk MoTe$_2$ and Mo$_{1-x}W$_x$Te$_2$ alloys. We also identify a Raman-forbidden MoTe$_2$ mode that is activated by compositional disorder and find that the main WTe$_2$ Raman peak is asymmetric for x

We introduce a simple criterion to identify two-dimensional (2D) materials based on the comparison between experimental lattice constants and lattice constants mainly obtained from Materials-Project (MP) density functional theory (DFT) calculation repository. Specifically, if the relative difference between the two lattice constants for a specific material is greater than or equal to 5%, we predict them to be good candidates for 2D materials. We have predicted at least 1356 such 2D materials. For all the systems satisfying our criterion, we manually create single layer systems and calculate their energetics, structural, electronic, and elastic properties for both the bulk and the single layer cases. Currently the database consists of 1012 bulk and 430 single layer materials, of which 371 systems are common to bulk and single layer. The rest of calculations are underway. To validate our criterion, we calculated the exfoliation energy of the suggested layered materials, and we found that in 88.9% of the cases the currently accepted criterion for exfoliation was satisfied. Also, using molybdenum telluride as a test case, we performed X-ray diffraction and Raman scattering experiments to benchmark our calculations and understand their applicability and limitations. The data is publicly available at the website http://www.ctcms.nist.gov/~knc6/JVASP.html.

Polymer composite materials are found throughout the world both natural and artificial in origin. In the vast majority of applications in these arenas, composites serve as structural support or reinforcement. Demand for lightweight tough composites is growing in multiple application spaces such as aerospace, biomaterials, and infrastructure with physical properties as diverse as the applications. The unifying component in all composites is the presence of the interphase. Many measurement techniques and measurement tools have been developed for the study of this crucial region in composite materials. Many of these methods are great for the measurement and study of bulk properties or model systems. However, development of tools that permit the direct observation of interactions at the interphase during applied stress are needed. Here we employ fluorescence lifetime imaging and hyperspectral imaging to observe activation of a fluorogenic dye at the composite interface as a result of applied stress. The advantages of this system include commercial availability of the dye precursor, and simple one-pot functionalization. The attachment of the dye at the interface is easily monitored through emission wavelength shifts or fluorescence lifetime. Interfacial mechano-responsive dyes have potential for both fundamental studies as well as industrial use as a structural health monitoring tool.

We present a new transfer procedure for graphene using acetic acid, which removes the residue that is common in standard acetone treatments. Post-transfer samples cleaned with acetic acid and acetone were characterized using Raman spectroscopy and atomic force microscopy for comparison. We further illustrate the quality of our transfer process by using fluorescence quenching to create an optical map of surface contaminants., 3 pages, 3 figures

Solid-state quantum emitters, such as artificially engineered quantum dots or naturally occurring defects in solids, are being investigated for applications ranging from quantum information science and optoelectronics to biomedical imaging. Recently, these same systems have also been studied from the perspective of nanoscale metrology. In this letter, we study the near-field optical properties of a diamond nanocrystal hosting a single nitrogen vacancy center. We find that the nitrogen vacancy center is a sensitive probe of the surrounding electromagnetic mode structure. We exploit this sensitivity to demonstrate nanoscale fluorescence lifetime imaging microscopy (FLIM) with a single nitrogen vacancy center by imaging the local density of states of an optical antenna.

Molybdenum ditelluride (MoTe2), which can exist in a semiconducting prismatic hexagonal (2H) or a metallic distorted octahedral (1T') phases, is one of the very few materials that exhibit metal-semiconductor transition. Temperature-driven 2H – 1T’ phase transition in bulk MoTe2 occurs at high temperatures (above ~900 C) and it is usually accompanied by Te loss. The latter can exacerbate the control over reversibility of the phase transition. Here, we study effects of high-temperature annealing on phase transition in MoTe2 single crystals. First, MoTe2 were grown in sealed evacuated quartz ampoules from polycrystalline MoTe2 powder in an iodine-assisted chemical vapor transport process at 1000 C. The 2H and 1T’ phases were stabilized by controlling the cooling rate after the growth. In particular, slow cooling at 10 C/h rate yielded the 2H phase whereas the 1T’ phase was stabilized by ice-water quenching. Next, the phase conversion was achieved by annealing MoTe2 single crystals in vacuum-sealed ampoules at 1000 C with or without additional poly-MoTe2 powder followed by fast or slow cooling. Similarly to the CVT growth, slow cooling and quenching consistently produced 2H and 1T’ phases, respectively, regardless of the initial MoTe2 crystal structure. We will discuss structural and optical properties of the as-grown and phase-converted MoTe2 single crystals using TEM, SEM/EDS, XRD, XPS and Raman. Electrical characteristics of two-terminal devices made from metallic 1T’ and bottom-gated FETs made from 2H exfoliated crystals will also be presented.

We study the crystal symmetry of few-layer 1T' MoTe

We examine anharmonic contributions to the optical phonon modes in bulk $T_d$-MoTe$_2$ through temperature-dependent Raman spectroscopy. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature in agreement with the Gr\"{u}neisen model. However, below 100 K we observe nonlinear temperature-dependent frequency shifts in some modes. We demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near $T\approx 250$ K that correlate well with the $T_d \to 1T^\prime$ structural phase transition. These results suggest that phonon-phonon interactions can dominate anharmonic contributions at low temperatures in bulk $T_d$-MoTe$_2$, an experimental regime that is currently receiving attention in efforts to understand Weyl semimetals., Comment: 14 pages, 3 figures, 1 table

With a template-stripping fabrication technique, we demonstrate the mass fabrication of high-quality, uniform, ultrasharp (10 nm) metallic probes suitable for single-molecule fluorescence imaging, tip-enhanced Raman spectroscopy (TERS), and other near-field imaging techniques. We achieve reproducible single-molecule imaging with sub-20-nm spatial resolution and an enhancement in the detected fluorescence signal of up to 200. Similar results are obtained for TERS imaging of carbon nanotubes. We show that the large apex angle (70.5°) of our pyramidal tip is well suited to scatter the near-field optical signal into the far-field, leading to larger emission enhancement and hence to a larger quantum yield. Each gold or silver pyramidal probe is used on-demand, one at a time, and the unused tips can be stored for extended times without degradation or contamination. The high yield (>95%), reproducibility, durability, and massively parallel fabrication (1.5 million identical probes over a wafer) of the probes hold promise for reliable optical sensing and detection and for cementing near-field optical imaging and spectroscopy as a routine characterization technique., ACS Nano, 6 (10), ISSN:1936-0851, ISSN:1936-086X

Potential commercial applications for transition metal dichalcogenide (TMD) semiconductors such as MoS2 rely on unique material properties that are only accessible at monolayer to few-layer thickness regimes. Therefore, production methods that lend themselves to the scalable and controllable formation of TMD films on surfaces are desirable for high volume manufacturing of devices based on these materials. The authors have developed a new thermal atomic layer deposition process using bis(tert-butylimido)-bis(dimethylamido)molybdenum and 1-propanethiol to produce MoS2-containing amorphous films. They observe a self-limiting reaction behavior with respect to both the Mo and S precursors at a substrate temperature of 350 °C. Film thickness scales linearly with precursor cycling, with growth per cycle values of ≈0.1 nm/cycle. As-deposited films are smooth and contain nitrogen and carbon impurities attributed to poor ligand elimination from the Mo source. Upon high-temperature annealing, a large portion of the impurities are removed, and the authors obtain few-layer crystalline 2H-MoS2 films.