Showing papers in "Spie Newsroom in 2015"

First Solid-state Cooling Below 100K

Abstract: : Advances in material purity and laser light absorption offer new possibilities for vibration-free cryogenic cooling. Material properties change as a function of temperature, and cryogenic refrigeration allows us to obtain very useful properties not available at higher temperatures. For instance, in the temperature range 77 150K, superconductivity, long- and mid-wave IR detectors, and ultra-stable laser cavities become usable.1 Currently, these low temperatures are reached using liquid or solid cryogens or mechanical refrigerators. Unfortunately, liquids and solids require regular attention to refill after evaporating away, and mechanical refrigerators introduce vibrational noise and mechanically wear over time. Space-based applications, and ultra-stable laser cavities in particular, cannot tolerate these drawbacks. A solid-state solution is preferable for its inherent vibration-free operation and potentially long lifetime. Optical refrigeration via anti-Stokes fluorescence is currently the only solid-state cooling technology capable of reaching cryogenic temperatures.

Simple super-resolution biological imaging

Abstract: Microscopic imaging has traditionally been constrained by the diffraction limit, which for visible light restricts spatial resolution to several hundred nanometers. Surpassing this limit would significantly benefit several disciplines, such as medical and material sciences. We developed a simple, generic superresolution imaging method that improves the resolution of optical imaging by a factor of 2 to 3. Our approach uses microscope coverslips composed of a monolayer of microspheres with high refractive index positioned in a transparent elastomer layer. We fabricated several different microscope coverslips by spin coating barium titanate glass (BTG) microspheres (refractive index n 1.9–2.1) in a polydimethylsilaxane (PDMS) layer whose thickness can be controlled by the spin speed and duration. When the slide is placed over the specimen, a magnified virtual image is formed by the microsphere and captured by the objective lens: see Figure 1(a,b). To demonstrate the principle, we used a commercial Blu-ray R disk (BD) as the imaging object, the structure of which consists of stripes 200nm wide separated by gaps 100nm wide. Conventional microscopy cannot resolve the structure of the BD; however, it can be resolved using one of our coverslips, as illustrated in Figure 1(c). Improving the spatial resolution by microsphere-assisted imaging acts by enhancing the effective numerical aperture (NA) of the system1–3 and the extraordinary focusing properties of microspheres,4–6 the so-called photonic nanojet effect.7 It should be noted that the focusing properties of our microsphereembedded films can find applications in other areas such as nano-scale patterning, spectroscopy, and photovoltaics. (Reviews are available elsewhere.1) We also investigated the feasibility of using our microsphereassisted technique to image biological structures, such as cells2 and tissue sections.3 A limiting factor in predicting patients’ Figure 1. (a) Schematic of the imaging setup (not to scale). (b) Ray diagram of virtual image formation with microsphere refractive index n and refractive index of the medium nm. (c) Image of the Blu-ray disk structure through a 50 m-diameter barium titanate glass (BTG) sphere (n 1.9) embedded in a polydimethylsilaxane (PDMS) layer.

A highly reconfigurable photonic microwave frequency mixer

Abstract: Frequency mixers represent one of the most essential components of microwave and millimeter-wave systems. Compared to conventional electrical mixers, photonic microwave frequency mixers are of great interest for wideband, multifunctional, and reconfigurable radio frequency (RF) systems. This is due to a number of distinct features, including wide bandwidth, good isolation, and immunity to electromagnetic interference. However, despite great efforts being devoted to the development of high-performance photonic microwave frequency mixers over the past two decades, the two intrinsic challenges of photonic mixers—undesirable mixing spurs (unnecessary frequency components produced due to nonlinearities in the mixer), which restrict the operation bandwidth and reduce the signal-to-interference ratio, and limited mixing functions—have not yet been overcome.

Octave-spanning semiconductor laser for frequency comb applications

Abstract: The recent development of frequency combs has revolutionized the field of high-resolution spectroscopy. These combs can be used as frequency domain ‘rulers,’ and can be realized from either a short-pulse mode-locked laser1 or via nonlinear processes.2, 3 The laser emission from a comb can be stabilized and frequency-locked to highly stable microwave oscillators. The most common—and most efficient—method of stabilizing the offset frequency of a comb is based on a self-referencing approach,4 which requires laser emission that spans at least one octave. It is therefore important to achieve an octave-spanning spectrum with any broadband laser that is used for frequency comb generation. Frequency combs have so far been demonstrated in the visible,5 mid-IR,3, 6, 7 and terahertz (THz)8, 9 regions of the electromagnetic spectrum. The effective metrology and high-precision spectroscopy measurements that the combs enable1, 10–12 have many applications in several fundamental research and industrial environment contexts. Quantum cascade lasers (QCLs)13 are based on intersubband transitions between quantized electronic energy levels in the conduction band of semiconductor heterostructures. They can be used as compact coherent sources that emit radiation across mid-IR and THz wavelengths.14 QCLs constitute an ideal platform for broadband sources and nonlinear optics as they exhibit an absence of reabsorption across the band gap. We have developed new QCLs with ultra-broad gain bandwidths. We achieve these bandwidths by exploiting the quantum engineering potential of intersubband transitions. We integrate different designs of a quantum cascade structure in Figure 1. Scanning electron microscope image of a processed 50 m dry-etched laser (top). The inset shows the electric field intensity distribution of a metal-metal waveguide. Light-current and current-voltage characteristics for a 2mm 150 m laser (thickness about 13 m) operated in continuous wave (CW) mode at different temperatures (bottom). The first power axis is normalized to a measurement made with a broad area terahertz (THz) absolute power meter (TK Instruments, aperture 55 40mm2). The second power axis shows measurements made with an Ophir THz absolute power meter with a smaller detector surface area (aperture diameter 12mm). A maximum power of 3.4mW in CW at 25K was achieved.

Measuring the mechanics of biofilms at multiple lengthscales

Abstract: Biofilms are multicellular communities of sessile, interacting microbes that form infections readily on medical devices and in chronic wounds. The films can cause biofouling of industrial equipment, which reduces efficiency and increases wear, and removing them requires knowledge of their structure and mechanics. Biofilms have intrinsic structure on a range of lengthscales covering three or more orders of magnitude, from submicron bacterial appendages, to the hundreds or thousands of microns characterizing the biomass thickness of a mature biofilm. Although they can contain both prokaryote and eukaryote cells, in this review we focus on bacterial biofilms. The mechanics of single bacterial structures affects their adhesivity, sensing, and the signaling that regulates their initiation. For mature biofilms, their mechanics impact their response to means of removal. Considering these structural varieties, researchers have developed different approaches to measuring the mechanics of surface-attached bacterial systems. Each of these methods is well adapted to some subset of the biologically relevant span of lengthscales. Biofilm formation begins when single bacterial cells sense and attach to a surface, aided by motility appendages (flagella and pili).1 Extracellular polymeric substances (EPSs) are produced, and these increase the attachment and eventually form the biofilm matrix, giving the film its mechanical properties.2 Generally speaking, the means of bacterial adhesion to surfaces can be classed as either a slip or a catch bond. Slip bonds weaken and break as more external force is applied. Catch bonds are strengthened by increased external force that induces conformational changes in the binding proteins.3 As an example, for the pathogen P. aeruginosa, adhesion to a solid surface can increase levels of cyclic-di-GMP, an intracellular signaling molecule that triggers the transition to a biofilm state, and induces Figure 1. Schematic of an experiment using an atomic force microscope cantilever to measure the forces of surface attachment. The cartoon is not drawn to scale: in reality, the bacteria are smaller than the cantilever by an order of magnitude or more. The distance over which polymers and/or appendages confer adhesion may be comparable to the cell length, as shown, or it may be smaller by an order of magnitude or more.

Automatic quality prediction of authentically distorted pictures

Abstract: Social media and rapid advances in camera and mobile device technology have led to the creation and consumption of a seemingly limitless supply of visual content. However, the vast majority of these digital images are captured by casual amateur photographers whose unsure hands and eyes often introduce annoying artifacts during acquisition. In addition, subsequent storage and transmission of visual media can further degrade their visual quality. Recent developments in visual modeling have elucidated the impact of visual distortions on perception of such pictures and videos. They have laid the foundation for automatic and accurate metrics that can identify and predict the quality of visual media as perceived by human observers.1 To address this problem, several objective blind or no-reference (NR) image quality assessment (IQA) algorithms have been developed to predict the perceptual quality of a given (possibly distorted) image without additional information.2–7 Such quality metrics could be used to monitor and control multimedia services on networks and devices or to prioritize quality of transmission over speed, for example. Real-world images are usually afflicted by mixtures of distortions that differ significantly from the single, unmixed distortions contained in restrictive and unrepresentative legacy databases.9–12 We recently designed a unique and challenging image data set with associated human opinion scores called the Laboratory for Image and Video Engineering (LIVE) authentic image quality challenge database8 (see Figure 1). Using this LIVE challenge database, we have been developing a robust blind IQA model for images suffering from real-world, authentic distortions. We call our model the ‘feature maps driven referenceless image quality evaluation engine’ (FRIQUEE) index. FRIQUEE outperforms other state-of-the-art blind IQA algorithms on both the LIVE legacy IQA9 and the LIVE challenge database8 (see Table 1). Figure 1. Sample images from the Laboratory for Image and Video Engineering (LIVE) authentic image quality challenge database.8 This collection comprises 1163 images afflicted with complex mixtures of unknown distortions, of different types and severities, from diverse camera devices, and under varied illumination conditions. The content includes pictures of faces, people, animals, close-up shots, wideangle shots, nature scenes, man-made objects, images with distinct foreground/background configurations, and images without any notable object of interest.

Control of mirror segment actuators for the European Extremely Large Telescope

Abstract: Controller design based on dynamic analysis proves compliant performance of highly accurate actuators for piston-tip-tilt correction of primary mirror segments for the European Extremely Large Telescope.

High-frequency polychromatic visual stimuli for new interactive display systems

Abstract: Brain–computer interfaces (BCIs) are intuitive operation modes that use electrical brain activity to communicate with external electronic devices. Over the past decade, BCI systems have been used for assistive living applications.1 In addition, 3D technologies are now widely available and are frequently used for virtual reality and augmented reality applications. As vision is the most dominant sense for humans, it is thought that BCI-enabled interactive displays (especially 3D displays) will also have a broad range of applications in gaming and e-learning. The steady state visual evoked potential (SSVEP) is an example of a BCI modality that can be induced by visual stimuli. The SSVEP is the natural response of the brain to repetitive stimuli that are modulated at a constant frequency. It is thought that the SSVEP may be the most suitable modality in brain–display interactions (BDIs) because of its non-intrusive, easy detection, and high information transfer rates.2 The functional architecture of BDI systems is illustrated in Figure 1. SSVEP-based BCI systems have been developed in recent years because of their attractive features. To induce strong SSVEP responses, however, most of these systems use visual stimuli in a low-frequency band (less than 20Hz).3 Unfortunately, bright lights that flicker in this frequency range can be distracting to viewers, and they can cause visual fatigue, migraine headaches, and even photosensitive epilepsy attacks. Our group is making the first steps in the development of a display-embedded SSVEP stimulus. As part of this work, we have proposed using a combination of flickering red-green lights to create an imperceptible flickering visual stimulus that can elicit an SSVEP at a basic flickering frequency.4 We have thus conducted a series of experiments to investigate whether the Figure 1. Architecture of a typical brain–display interactive system. SSVEP: Steady-state visual evoked potential.

New wide-area surveillance techniques for protection of pipeline infrastructure

Abstract: There are millions of miles of pipes buried along the length and breadth of the United States. The areas through which these pipelines run cannot be used for other activities. Furthermore, machinery—such as construction equipment and heavy vehicles—are major threats to pipeline infrastructure. Monitoring is therefore required to know whether a pipeline’s right-of-way is threatened at any time. Rapid advances in sensor technologies have enabled the use of high-end video acquisition systems to monitor the right-of-way of pipelines and have generated a huge amount of data. It is, however, very costly to employ analysts to scan through this data and to identify right-of-way threats. An automated mechanism that can detect these threats and issue warnings is therefore warranted. Several object detection and recognition algorithms have been proposed previously.1–5 Car body edges, edges of a front windshield, and shadows can be used as features for car detection.6 An alternative simple vehicle detection algorithm involves exploring four elongated edge operators.7 A top-down matching method has also been developed for vehicle detection from high-resolution aerial imagery.8 On-line boosting, with an interactive training framework, is another option for automatic car detection.9 A feature-based approach to car detection uses scale-invariant transform features and an affinity propagation algorithm.10 We previously designed a three-stage pattern recognition framework to detect construction equipment in various lighting conditions and different object orientations using monogenic signal representation.11, 12 The majority of these techniques, however, are either computationally expensive or unable to deal with the complex environments associated with aerial Figure 1. A sample result of the local textural features-based segmentation (LTFS) algorithm. (a) The original red, green, blue (RGB) image. (b) The LTFS output. The yellow circle indicates the threat object that has been identified.

Photonic hypercrystals: the best of both worlds

Abstract: “East is East, and West is West, and never the twain shall meet.” These words by Rudyard Kipling could have been used just as well to describe the primary paradigms for two new optical materials: metamaterials and photonic crystals. A metamaterial relies on the average response of the individual ‘meta-atoms.’ These building blocks of the new composite are made with dimensions much smaller than the wavelength of light, so, for the light in such a medium—which by its

Multijunction photovoltavics: integrating III–V semiconductor heterostructures on silicon

Abstract: Gallium arsenide phosphide nitride shows promise for developing highefficiency tandem solar cells on low-cost silicon substrates

3D human gesture recognition using integral imaging

Abstract: Optical image sensing and visualization technologies in 3D have been researched extensively in fields as diverse as TV broadcasting, entertainment, medical sciences, and robotics.1–4 One promising technology, integral imaging, is an autostereoscopic 3D imaging method that offers a passive and relatively inexpensive way to capture 3D information and visualize it optically or computationally.5–7 Integral imaging belongs to the broader class of multi-view imaging techniques that allow depth analysis from three points of view: stereo, time-of-flight, and structuredlight strategies.8 Integral imaging has also been used for classification tasks.9 However, we are the first to apply it to action recognition.10, 11 Integral imaging provides the 3D profile and range of the objects in a scene using an array of high-resolution imaging sensors or in a synthetic aperture mode (see Figure 1). When a single sensor captures multiple 2D images, it is possible to obtain larger field-of-view (FOV) 2D images. In the synthetic aperture integral imaging mode that we used, a series of sensors are distributed in a grid, or a single sensor is moved to the positions in the grid. The horizontal or vertical distance between two of these positions is called the pitch (p). The 3D image reconstruction can be achieved by computationally simulating the optical back-projection of the elemental images. In Figure 1, cx and cv are the horizontal and vertical sizes of the sensor and f its focal length. We used a computer-synthesized, virtual pinhole array to inversely map the elemental images into the object space (see Figure 1). Superimposing the properly shifted elemental images created the 3D reconstructed images. Our methodology is based on acquiring 3D videos of hand gestures using an integral imaging system formed by an array of 3 3 cameras. We analyzed the potential of gesture recognition using 3D integral imaging and compared the performance to 2D single-camera videos. We processed sectional reconstructed Figure 1. Synthetic aperture integral imaging acquisition and computational reconstruction method. Pitch (p) is the distance between the sensor centers (left), cx and cv represent the horizontal and vertical sizes of the sensor, and f is the sensor focal length (right).

Synthesizing carbon nanothreads from benzene

Abstract: Carbon nanomaterials—e.g., fullerenes, nanotubes, and graphene, with dimensionalities of 0, 1, and 2, respectively—possess beautiful chemical bonding arrangements and extraordinary physical and chemical properties:1–3 see Figure 1(a). As is the case with graphite, their carbon atoms are bonded to three neighbors. Diamondoid molecules and graphane (the fully hydrogenated form of graphene) have fourfold tetrahedral bonding (like diamond) and dimensionalities of 0 and 2, respectively: see Figure 1(b). The diamondoid molecule adamantane, which is capped by hydrogen atoms, comprises the smallest unit cage structure of the diamond crystal lattice. Similarly, graphane represents the thinnest possible sheet of diamond. In this context, queries arise regarding what form the thinnest possible 1D diamond thread would take. In the 1979 novel The Fountains of Paradise, Arthur C. Clarke proposed a ‘continuous pseudo-one-dimensional diamond crystal.’ It was later predicted that subjecting the molecule benzene4 (consisting of a hydrogen-capped ring of graphite) to a high-pressure chemical reaction could lead to such a 1D nanomaterial.5, 6 We have discovered that when solidified benzene is compressed to 200,000 times atmospheric pressure and then decompressed very slowly, a partially ordered material is created.7 To determine the formation of subnanometer-diameter threads with diamond-like carbon bonding, we employed a combination of first-principles calculations, nuclear magnetic resonance, Raman spectroscopy, x-ray and neutron diffraction, and electron microscopy (see Figure 2). Evidence of diamond nanowire formation inside conventional nanotubes has also been reported.8 Diamond is the strongest material known, making it reasonable to predict that diamondoid threads would have significant strength. The proposed application for Clarke’s ‘1D diamond’ was as an ultrastrong, lightweight cable for a space elevator, Figure 1. Carbon nanomaterial dimensionality and bonding geometry for (a) fullerene (C60), nanotubes, and graphene, and (b) diamondoid (admantane) and graphane. The black and off-white spheres represent carbon and hydrogen, respectively.

Searching for optical transient and variable sources with the Palomar Transient Factory

Abstract: The Palomar Transient Factory (PTF) is a comprehensive astronomical transient detection system that includes a wide-field survey camera, an automated real-time data reduction pipeline, a dedicated photometric follow-up telescope, and a full archive of all detected sources. The PTF robotic telescope is mounted on the 48-inch Samuel Oschin Telescope at the Palomar Observatory in southern California and is used to scan the sky every night for optical transient and variable astronomical sources.1, 2 The majority of synoptic optical surveys are tuned to maximize discoveries of selected source populations (e.g., microlenses, classical novae, or supernovae). These surveys are crucial for specialized science investigations, but they are not relevant to the time-domain phase space. The PTF represents a nextgeneration transient survey that can be used to systematically explore the variable sky on a number of timescales. PTF first light was achieved on 13 December 2008, and commissioning of the survey was completed on 1 March 2009. Although the original survey finished on 31 December 2012, operations are continuing until 2016 as part of the re-tooled Intermediate PTF (iPTF) survey.3 With the PTF, simultaneous discoveries of well-studied populations (e.g., classical novae or supernovae) and poorly constrained events (e.g., luminous red novae or tidal disruption flares) have been made. In addition, several phenomena—only previously predicted—have been observed for the first time (e.g., orphan afterglows of gamma-ray bursts and supernova precursor explosions). Over 2300 supernovae have been discovered with the PTF since 2009, including several examples that have been used to define a new class of astronomical transients. The discoveries that have been made with the PTF are summarized in Figure 1. Figure 1. The phase space of optical transients.4 The gray shaded areas highlight the locations of transients that were known prior to the Palomar Transient Factory (PTF). The new PTF observations have led to the classification of several new classes of explosions, including relativistic explosions that have timescales of minutes.5 The PTF discoveries are enabled by the coupling of high-performance computing resources with the workflow pipelines. iPTF: Intermediate Palomar Transient Factory. Ia: Type Ia supernovae.

Origami antennas and packaging using 3D printing technologies

Abstract: For several radio frequency (RF) applications (e.g., ‘smart’ sensors, energy harvesting devices, and communication systems), custom antenna and packaging designs are required. In the case of reconfigurable devices, or those that require easy deployment (e.g., spreading sensors with random orientations within a large area for environmental monitoring), a conventional one-antenna sensor is generally unable to communicate or harvest energy if its antenna does not face the central station. A 3D structure can be designed to accommodate multiple variably oriented antennas, thereby achieving diversity (improving the wireless link by exploiting signals arriving from different directions) and enabling operability at virtually any orientation. Additive manufacturing techniques, such as inkjet and 3D printing, can facilitate the implementation of complex structures at very low cost. Using an inkjet printer, conductive inks or dielectric materials can be deposited on a substrate, enabling the fabrication of planar circuits, antennas, or sensors.1 3D printers make it possible to build polymeric 3D structures with arbitrary heights and thicknesses. However, increasing the total printing volume requires a significantly longer fabrication time and greater material cost.2 To overcome these limitations, we have developed an origamistyle folding technique in which a 2D structure can be folded into a variety of 3D shapes. The original 2D structure constitutes either a thin, easily foldable substrate (e.g., paper), a 3Dprinted structure made of flexible polymer, or a hard 3D-printed thick planar structure with hinging features that allow folding. The latter has the advantage of mechanical stability, which is particularly important for certain packaging applications (e.g., protecting electronics). Conductive traces can be printed/shaped Figure 1. Prototype origami antenna, developed using 3D and inkjet printing, folded in the shape of a cube. For the sides, a rigid material (VeroWhite) is used. Grey60, a material rigid at room temperature but soft when heated, is used to form the hinging edges. The antenna, comprised of silver inks, is deposited onto the substrate prior to folding using an inkjet printer.

Compact optical probe for inner profile measurements of pipes

Abstract: Inner profile measurements serve an important function in the manufacture of components in many fields, such as mechanical engineering (within the car and aircraft industries) and heavy industries (e.g., for jet engine manufacturing and power plants).1–3 Conventional methods for measuring the inner diameter of circular holes involve contact-type instruments such as cylinder bore gauges or inside micrometers. When these conventional instruments are used, however, the average diameter of a circular hole is determined from only two or three measured points. This is because it is too time-consuming to find the diameter for many cross sections. In cases where the inner profiles of non-circular pipes or holes (e.g., tunnels and sewers) need to be measured, optical sectioning methods have been applied. These techniques use a rotating mirror or prism, and are based on the principle of triangulation. Optical sectioning methods, however, are difficult to use for pipes with diameters or cross sections that are less than about 100mm. Furthermore, non-contact measuring methods— that lack rotating or moving parts—are preferable for practical applications. We have recently developed a new principle for the measurement of the inner diameter and profile of pipes and tubes. The key component that we use in our technique is a ring beam device, which consists of a conical mirror and a laser diode (LD).4 The fundamental principle that underlies our technique is based on optical sectioning, without the use of any contact-type stylus. During our inner profile measurements (see Figure 1), a laser beam that is emitted from the LD hits the apex of the conical mirror. The beam is then reflected and spreads out to form a ring-like disk. When this disk-like beam reaches the inner wall of the tube or pipe, an optically sectioned profile (i.e., the peripheral cross section of the pipe) can be observed. We analyze the optically sectioned profile of a pipe-like object’s inner wall to Figure 1. Cartoon illustrating the principle of inner profile measurement for a pipe or tube. A laser beam is emitted from a laser diode and is reflected from the apex of a conical mirror. The beam reflection spreads out into a disk-like sheet. l: Baseline length between the conical mirror and the camera lens. r( ): Radial length at circular angle ( ). . /: Angle of attack, with respect to the measurement point at the circular angle.

Exploring packaging efficiency in phosphor-converted white LEDs

Abstract: Phosphor-converted white LEDs (pcW-LEDs) have ever more important roles in modern lighting,1 since their high efficiency and luminous efficacy make them suitable replacements for both traditional and specialized light sources.2 Using a well design, with a low-cost substrate such as sapphire, the technology demonstrates luminous efficacy as high as 150 lumens/W operated at 1W.3 By combining internal quantumand lightextraction efficiencies (IQE and LEE),4–6 the external quantum efficiency (EQE) of a blue die for a pcW-LED can be as high as 80%. However, to achieve this we need to consider the packaging efficiency (PkE): the ratio of the emitted light power of a pcW-LED to the blue light power escaping from the die to the packaging volume.7 Currently there is a shortage of detailed analysis of PkE, making it difficult to determine pcW-LED luminous efficacy limits in terms of correlated color temperature (CCT). Here, we analyze possible power losses at the packaging level. We establish the total packaging loss (PkL) by summing up three kinds of power reductions. First, the phosphor material’s limited quantum efficiency in wavelength conversion causes phosphor quantum loss. Second is stokes loss, where reduced photon energy is caused by down-conversion of the re-emitted photons pumped by the blue photons, and is a function of the spectrum of the white light. The third example is geometry loss, which includes all possible absorptions in surfaces or materials other than the phosphors, and depends on the packaging construction. To calculate the PkE in a real pcW-LED, we need to measure the power of the white light outside the device and the blue light from the blue die to the packaging volume. Generally, the blue die is covered by a layer comprising phosphor and the encapsulation material (such as silicone). The blue light in the packFigure 1. The seven packaging geometries of phosphor-converted white LEDs (pcW-LEDs).(a) Type I, hemisphere phosphor dome. (b) Type II, rectangle molding. (c) Type III, reflective cup. (d) Type IV, remote type of (a). (e) Type V, remote type of (b). (f) Type VI, remote type of (c). (g) Type VII, conformal coating.

Synthetic aperture radar for imaging through cloud layers

Abstract: Recent military conflicts have demonstrated the need for close air support from precision attack platforms (e.g., the AC-130 gunship) to support ground forces. These aircraft typically use IR sensors to engage moving ground targets, such as vehicles and dismounts. Under clear conditions, targets can be easily identified and effectively engaged. In degraded environments, however, the atmosphere can inhibit traditional electro-optical sensors. Close-in air support therefore can currently only be conducted during clear weather conditions. In many important parts of the world (such as the Korean Peninsula, Central America, Colombia, and the Balkans) clouds are present between 25 and 50% of the time.1 The amount of time in which US close-air-support aircraft can engage targets is therefore severely limited. In addition—even in clear weather— once targets are engaged from the AC-130, copious amounts of dust are raised. This dust—from explosions and incoming rounds—prevents targeting of adversaries from the aircraft and makes it difficult to track friendly ground forces in the area. We are currently developing a gimbal-mounted radar sensor that will provide high-frame-rate radar images. This data will be used for targeting operations when atmospheric conditions inhibit the use of electro-optic sensors. Our radar system can be used to image the ground, even through clouds and dust, at a sufficiently high resolution and frame rate to support the engagement of maneuvering targets. An artist’s depiction of our system in operation is shown in Figure 1. For our system, the radar must be able to create a highresolution background, and it must permit detection and accurate location of moving targets. Radar frequency selection has therefore been a key tradeoff decision. For synthetic aperture radar (SAR) systems with a given resolution, the frame rate is proportional to the frequency. Moving to a higher frequency therefore provides a higher frame rate (and a lower latency). Moving to a higher frequency also requires a smaller antenna Figure 1. Artist’s impression of a close-air-support aircraft with the Video Synthetic Aperture Radar (ViSAR) imaging the ground through a layer of clouds.

Static real-time capture of 3D microscopy images

Abstract: Two-dimensional widefield microscopy provides basic dynamic information about live biological specimens. However, it provides only a partial representation of the 3D biological processes and may be incomplete or even misleading. Current techniques, such as widefield, confocal, structured-illumination, or light-sheet microscopy cannot capture the 3D structure of a specimen in a single frame. In contrast, in 2D widefield microscopy, a stack of 2D depth images of the sample are recorded and a 3D digital image is computed from them. The different depth images are typically recorded with axial mechanical scanning. But mechanical movement could damage the sample, cause it to vibrate and hence introduce image distortions, or slow down image acquisition, which would make it impossible to record highly dynamic biological processes. The trivial solution is to use digital holographic microscopy, which permits the 3D complex distribution scattered by the sample to be rendered from a single frame.1 However, this system operates coherently and makes fluorescence imaging impossible. We have investigated using an electrically addressable liquid lens (LL) to acquire images at different depths. The lens is based on electrowetting technology: how a drop of water spreads on an electrically insulating surface can be modified by accumulating charge at the base of the drop. The optical power of the resulting LL can be tuned by an applied voltage.2 In 2010, our group proposed using an LL for parallel dynamic focusing of images obtained through an array of microlenses.3 More recently, other groups have applied LL technology in microscopy.4, 5 Our proposal is to insert an LL at the aperture stop of a widefield microscope, which is arranged as the telecentric coupling between a high-numerical-aperture (NA) infinitycorrected microscope objective and a low-NA tube lens.6 The insertion of the LL enables the axial position of the object plane to be controlled by the voltage while preserving the telecentric Figure 1. Scheme of the static axial scanning experimental setup. CCD: Charge-coupled device. Fob and FTL: Front focus of the objective and the tube lens, respectively. F0TL: Back focus of the tube lens. f0R1; R2: Focal lengths of the relay lenses.

Separation-rate improvement of epitaxial lift-off for III-V solar cells

Abstract: Thin-film III-V semiconductor solar cells have a number of advantages compared with other types of solar cells. For example, tuning the bandgap of III-V compound materials to match the solar spectrum gives the resulting solar cells unsurpassed conversion efficiencies. The virtues of these devices notwithstanding, the semiconductor substrate used in fabricating them is expensive, which adds to their cost. A method known as epitaxial lift-off (ELO) enables substrate reuse, which enhances affordability.1 However, the technique relies on hydrofluoric acid (HF) solution, a popular chemical etchant, and long-term exposure to the etchant increases the surface roughness of either the epilayer (i.e., the thin film containing the device) or the substrate. This roughness in turn hinders both substrate reuse and the performance of the solar cells. To solve this problem, various chemical fluids have been proposed to clean the substrate and modify the surface structure.2 But chemical cleaning is difficult to control because it is isotropic (that is, it etches at the same rate in every direction). A gallium arsenide (GaAs) solar cell on a (100) GaAs substrate consists of a 0.2 m-thick GaAs buffer layer, a 0.2 m-thick indium gallium phosphide (InGaP) etching stop layer, a 3 mthick GaAs buffer layer, a 20nm-thick aluminum arsenide (AlAs) sacrificial layer, and a 2.6 m-thick GaAs device epilayer. For ELO to be practical, etching time needs to be fast. However, arsine (AsH3) bubbles formed during the ELO process are known to obstruct the etching slits and prevent the AlAs sacrificial layer from reacting with the HF solution. Previous research3 established that oxygen is required for chemical etching of AlAs in HF solution. Blocking of the etching slits by the AsH3 bubbles Figure 1. (a) Lateral etching rate for the aluminum arsenide (AlAs) sacrificial layer during solar cell fabrication using various hydrofluoric acid (HF) solution mixtures. (b) Photograph of the sample. H2O: Water. ACE: Acetone. IPA: Isopropanol. MA: Methanol.

Quantum-like polarization metrology with classical light

Abstract: The natural world as perceived by our senses appears to be ruled by the laws of classical physics, as established by Galileo, Newton, Maxwell, and many others. Conversely, the microscopic world of atoms, molecules and elementary particles (such as electrons and photons) obeys the strange laws of quantum mechanics. Perhaps the most dazzling characteristic of the quantum world is the existence of peculiar states of either matter or light where two or more distinct particles, as two electrons or two photons, are entangled. Entanglement implies that, for example, measuring the spin of one particle allows one to predict the measured value of the spin of the other particle, no matter how distant the two particles are. Traditionally, entanglement has been regarded either as a peculiar feature of quantum mechanics or as a powerful resource especially for quantum information science.1 In particular, quantum approaches relying on entangled photon pairs have been proposed to extend optical measurements beyond the limits encountered in classical optics. However, quantum states of light are not always easy to prepare. In some cases these problems can be circumvented by using classical, as opposed to quantum, beams of light with special characteristics akin to the ones exhibited by entangled quantum systems: this brings outstanding advantages over conventional measurements in metrology and sensing. In standard Mueller matrix polarimetry, information about a sample illuminated by polarized light is deduced by analyzing the scattered light. We have shown that using beams of light with nonuniform polarization patterns enables real-time single-shot Mueller matrix polarimetry, a technique crucial to testing optical, chemical, and structural properties of materials quickly, precisely, and without damaging the sample. Key to the success of our approach is the notion that, contrary to common belief, entanglement is not necessarily a signature Figure 1. (a) Visual representation of the intensity distribution and polarization pattern of the radially polarized beam of light. (b–d) Possible equivalent decompositions of this beam. The color scale (bottom) gives the phase of the electric field. (b) Superposing electric fields of a Hermite-Gauss (HG) mode with horizontal and vertical polarizations yields a radially polarized pattern. (c) Superposing diagonal HG modes with diagonal polarizations also produces a radially polarized beam. (d) A radially polarized beam is decomposed into circular spatial modes and circular polarizations.

Concentrating photovoltaic panels for the rooftop

Abstract: For many of us, the image that comes to mind when we think of solar energy is a house with photovoltaic (PV) panels on the roof. Panels such as these are currently based on either silicon or thin-film semiconductor cells and achieve efficiencies typically between 15 and 20%. Advances in manufacturing have decreased the cost of these cells dramatically over the past few years. So much so, in fact, that in many cases they no longer make up the dominant cost component of the power that they generate, with a greater percentage of the overall cost going toward more mundane expenses, such as the inverter, mounting hardware, installation labor, and permitting fees. Because generating more power from a given panel leverages all of these costs over the system lifetime, there is a strong incentive to increase efficiency. This has motivated the combination of multijunction cells with concentrating optics. Together, these devices can enable module efficiencies of more than 35%.1 There is currently a great deal of effort aimed at the commercialization of these concentrating PV (CPV) systems. The current paradigm, however, relies on large-scale assemblies of Fresnel lenses or mirrors that must be pointed toward the Sun throughout the day to concentrate its light. Although this approach is well-suited for large, open-land areas, it is incompatible with the limited space and compact panel profile required for rooftop installation. Working together with our collaborators at the University of Illinois at Urbana-Champaign, we have developed a route toward addressing this challenge. In our design, high-efficiency microscale PV cells are embedded in the concentrator optic itself, enabling us to exploit an alternate, translation-based tracking scheme in which the cells slide laterally to track the Sun.2 This concept, outlined in Figure 1, consists of a central glass or acrylic sheet sandwiched between an upper refractive and lower reflective lenslet array. A corresponding array of microcell PVs, Figure 1. Ray-tracing diagram of the folded optical path within a unit cell of the concentrator stack. The photovoltaic (PV) cell (red rectangle) faces downward and slides laterally, together with the middle spacer sheet, to track different solar incidence angles. inc: Incidence angle of the sunlight with respect to the panel normal (i.e., perpendicular to the panel surface). rec: Range of angles for the light rays incident on the solar cell.

An optical method for early detection of Alzheimer's disease

Abstract: Alzheimer’s Disease (AD) is currently diagnosed using neuropsychological tests. These differentiate between mild cognitive impairment (MCI), AD, and other dementia types. However, the pathogenic process of AD begins several years before the clinical onset of the disease, and it induces measurable changes in the brain, cerebrospinal fluid (CSF), and the blood. There is a worldwide effort to develop treatments to prevent, cure, or delay the progression of AD. If we had a diagnostic tool for early detection of the disease based on its pathological expressions, we could significantly advance the development of such treatments, and increase their effectiveness. A promising approach for early detection of AD is based on measuring the concentration ratio of amyloid-beta and tau proteins in the CSF. Several studies using this approach showed that it is possible to predict the development of MCI to AD several years before clinical diagnosis of the disease.1, 2 However, to measure concentrations of CSF biomarkers, it is necessary to collect CSF from patients during a lumbar puncture procedure. This involves many risks and complications and may not be suitable as an annual test. To overcome these drawbacks, we proposed3 an optical method that replaces the lumbar puncture process and enables estimation of the CSF biomarkers concentration ratio. Our proposed method, described in Figure 1, has several stages. First, we inject fluorescent probes4–7 that bind specifically to the CSF biomarkers into the blood. We then insert a miniature needle with an optical fiber into the lumbar area. It passes several tissue layers until it reaches the epidural fat, without penetrating the dura mater (a membrane around the brain). Laser radiation reaches the CSF through the optical fiber in the needle and excites the fluorescent biomarkers. We measure the resultant emission and use the ratio of fluorescence intensities to estimate the biomarkers’ concentration ratio and the risk of Figure 1. The proposed setup. Stage 1: Injection of two fluorescent probes. Stage 2 : Measuring the fluorescence intensity of the two probes using a needle near the cerebrospinal fluid. Stage 3: Calculating the concentration ratio of amyloid-beta (Aˇ) and tau proteins based on the fluorescence intensity ratio and estimating the risk of developing Alzheimer’s Disease. NIR: Near-IR. PMT: Photomultiplier (detector).

Colloidal Nanoparticles for Heterogeneous Catalysis

Abstract: For many industrial processes, catalytic conversion is indispensible for reducing energy consumption, pollution, and unwanted side products. The majority of technical processes are heterogeneously catalyzed, and heterogeneous catalysts are important components in systems for exhaust gas cleaning and energy conversion or storage. Recently, nanomaterials, which often exhibit different properties from the bulk material, have attracted attention as potentially cheaper and more efficient catalysts. We have prepared a variety of heterogeneous nanoparticle catalysts on carrier particles. We used pulsed laser ablation in liquid and investigated the influence of factors such as ligands, pH, ionic strength, and ligand surface coverage on adsorption efficiency to titanium dioxide supports.1, 2 Colloidal nanoparticles offer fascinating possibilities for investigating new functional heterogeneous catalysts. The surfaces of nanomaterials such as gold or platinum nanoparticles, which are synthesized by reducing precursors, are usually covered by surfactants. However, these can reduce or even prevent catalytic activity. As a result, extensive and expensive purification methods (such as thermal treatment, centrifugation, and solvent extraction) are required.3–5 However, quantitative removal of the surfactants is very challenging. Furthermore, sintering or agglomeration, which modify the nanoparticle properties, often take place, limiting follow-up treatments. In contrast, highly pure nanoparticles can be fabricated by pulsed laser ablation of a bulk target in liquid (see Figure 1). This permits rapid nanomaterial design and can be applied to a variety of materials and solvents. In addition, such plasma-induced nanoparticle formation positively charges the nanoparticles’ surfaces, stabilizing them electrostatically without surfactants. The effective charge of nanoparticles in colloidal solution is measured by the zeta potential. Using a saline solution during the laser ablation also delivers a surface charge from anions, enabling control of the particle size.6 The increase in surface charge Figure 1. Laser ablation of a gold target in a solvent (left) and colloidal solutions with gold, silver, and nickel nanoparticles (right).

Evaluating deformation at high temperatures

Abstract: To address the need for accurate characterization of the mechanical behavior of materials at high temperatures, numerous non-contact optical measurement techniques have been developed. These techniques include moiré interferometry (MI),1, 2 digital image correlation (DIC),3 and electronic speckle pattern interferometry.4 As a basic component for deformation measurement, the quality of the deformation carriers (i.e., gratings that arise due to the periodic structure of a medium, and random patterns called speckles) directly influence the moiré fringe contrast and the speckle pattern quality. Preparing a deformation carrier that operates at high temperatures is therefore a crucial issue for both MI and DIC measurement, and for other advanced optical techniques. Existing technologies for grating or speckle fabrication do not meet all the necessary requirements simultaneously (i.e., high quality, high contrast, high temperature resistance, resistance to oxidation, and good resistance to fall-off for speckles fabricated on the specimen surface at high temperature).5 We have developed a novel moiré grating fabrication method,2 based on a thermal nanoimprint process and a high-temperature transferring technique, which enables measurement at very high temperatures. A micro-speckle particle fabrication method3 has also been developed, based on adsorbing two types of heat-resistant particles with high color contrast (zirconium dioxide and cobaltous oxide, Zr02 and CoO) on the sample surface.2, 3 Using a thermal nanoimprint technique, the holographic grating (used as a mold) can be copied onto the glass substrate with the photoresist on its surface. The hightemperature chromium film is then coated on the surface of the glass substrate. The resulting heat-resistant grating can be transferred onto the surface of a sample using high-temperature adhesive (see Figure 1). We have successfully implemented these gratings in the MI analysis of the mechanical properties of a Figure 1. A high-temperature grating.2

Detecting traces of explosives

Abstract: Detecting and identifying chemicals is critical for addressing security threats such as explosives. Those handling explosives tend to contaminate surfaces they touch with trace particles. Traditional trace detection technologies require surfaces to be swabbed and the swab placed into an instrument for analysis. For many reasons, non-contact methods are preferred, but these need to be safe to use around people, sensitive to small amounts of threat material, and sufficiently selective to distinguish chemicals of interest from the broad variety of surface types (vehicles, clothing, luggage, packages) in typical security scenarios. Many approaches to non-contact or ‘standoff’ detection of explosives have been explored. Most explosives evaporate slowly and, consequently, insufficient vapor is left in the air for ‘sniffing’ technologies. Previously deployed ‘puffer’ technologies (designed to blow particles off surfaces and collect them) were abandoned because they collected too much interfering dust. More recent non-contact approaches have been based on optical spectroscopies (such as laser-induced breakdown or Raman), but those typically use visible or UV lasers, which are not safe for our eyes. To overcome these issues, we have developed a novel method using IR spectroscopy.1–4 IR spectroscopy is based on the well-known absorption properties intrinsic to the specific chemical bonds within a molecule. IR spectrometers are ubiquitous in analytical chemistry laboratories and are a benchmark for chemical identification based on each material’s unique spectral ‘fingerprint.’ In our method, we shine specific IR wavelengths on the surface of interest while observing the target using an IR camera as a thermal imager. Where the light is absorbed, the surface will warm slightly (by 1C), which is observed in the IR image.5 This thermal emission signal also contains spectroscopic signatures of the material emitting it, which we exploit using a series of IR filters before the camera.6 By comparing images captured as a function of incident wavelength, we generate a Figure 1. Schematic of photo-thermal IR imaging spectroscopy (PT-IRIS). A tunable IR quantum cascade laser is tuned to wavelengths ( ) characteristic of absorption bands in analytes of interest, such as explosives, and directed toward a target surface. The target surface is imaged using an IR camera to measure the local heating due to absorption by potential chemical threats.

Nanostructured semiconductors for efficient water splitting

Abstract: Photoelectrochemical (PEC) water splitting, in which molecular hydrogen and oxygen are directly dissociated from water molecules using semiconductor electrodes, dates back to 1972.1 Certain factors have, however, delayed developments in this field over the last few decades. Limitations include the expense of the required materials, electrode instability against photocorrosion, and poor solar-to-hydrogen (STH) generation efficiencies.2 Recent advances in the field of nanotechnology may enable these bottlenecks to be overcome. PEC watersplitting research currently targets low-cost and Earth-abundant elements that show promising STH conversion efficiencies. The semiconductor materials used usually include inexpensive metal oxides, metal sulfides, and metal carbides. A stateof-the-art device based on tungsten-doped bismuth vanadate photoanodes3 has enabled an STH conversion efficiency of 5%. The highest efficiency to date (14%) was achieved using a titanium dioxide-coated indium phosphide photocathode.4 These recent studies show the potential for commercial PEC water-splitting devices. Surface roughness plays a vital role in improving the collection of photogenerated carriers at the semiconductor-electrolyte interface (see Figure 1).5 The water oxidation reaction at a semiconductor electrode is much more efficient if a rough surface is used instead of a smooth one. This is due to its higher surface area, which leads to more photocatalytic sites and thus to higher activity. Structures such as nanorods, nanotubes, nanoplates, and porous films can be used to introduce surface roughness in electrodes. We have developed a novel approach for the preparation of nanostructured porous tungsten trioxide (WO3) thin films.6 These textured films—see Figure 2—are composed of small individual nanoparticles and show high surface area and porosity. The films exhibit catalytic activity upon illumination with Figure 1. Minority diffusion lengths for the electron (Le , –) and hole (Lh, +) on flat and rough electrode surfaces. On the textured electrode, the distance between the surface and the location at which holes (+) are generated can be shorter in comparison with the flat semiconductor substrate.

All-optical signal processing based on space-time duality

Abstract: The extraordinary increase in data transmission rates and network complexity over recent years has led to a high demand for high-speed, real-time signal measurement. In addition, new signal manipulation techniques are called for. Remarkably, it is possible to exploit spatial-domain techniques in the temporal domain using the concept of space-time duality. This novel approach enables more sophisticated and powerful approaches for temporal processing and information characterization. All-optical signal processing functionalities can be achieved on this basis. Temporal imaging is generally implemented by using a time lens, which works by imparting a quadratic phase to the input waveform. However, in the generation of an ideal time lens, there exists a very strict requirement for a pure quadratic phase shift and flattened amplitude within the operation window.1 Using a temporal pinhole (a shutter that allows a signal to pass through), we have developed an alternative approach to temporal imaging that is in accordance with spatial pinhole imaging:2 see Figure 1. The configuration consists of two sections of dispersive fibers connected by a temporal shutter. Both theoretical analysis and experimental results show the output waveform to be a scaled profile of the input waveform. Specifically, the output waveform is reversed if the signs of the dispersion on both sides of the temporal shutter are identical. If the signs of the dispersion are opposite, the waveform is non-reversed. A small portion of the dispersed signal contains the total information of the original signal. This technique therefore has the potential to enable the detection and recovery of the original information from a fraction of the dispersed signal. The concepts of temporal Fourier transformation and temporal imaging, which incorporate space-time duality, have aroused great interest and extensive research. Temporal Fourier Figure 1. (a) Pinhole imaging in the spatial domain. An object (Ii ) placed at a distance (di ) in front of an aperture is projected to an image (Io) at a distance behind the aperture (do). (b) Pinhole imaging in the temporal domain. An input waveform with a pulse packet (1101) propagates through a section of dispersion fiber. It then undergoes a short temporal shutter, which allows a small burst of the waveform to pass through. Finally, the waveform propagates in another dispersive fiber and forms the scaled profile of the input waveform (1011).

Hollow-core photonic crystal fibers for high-power, ultrafast lasers

Abstract: Industry and science require ever faster laser systems with high repetition (MHz) rates and peak powers. As an example, fast specialized micromachining requires ultrafast lasers with average power ranging from hundreds of watts to kilowatts1. The past decade has seen dramatic progress in the development of hollow-core photonic crystal fibers (HC-PCF)2 and ultrafast lasers2–5, and we can now combine the two to enable laser systems of unsurpassed ultra-high average power with short pulse durations. Several groundbreaking laser technologies, namely slab-2, fiber-3, and thin-disk amplifiers and oscillators4, 5 have exponentially increased the available averageand peak power and pulse energy. Ultrafast sources have exceeded the milestone of kilowatt average power, and they reach pulse peak powers of tensto hundreds of megawatts in sub-picosecond pulses (see Figure 1). For scientific applications, the same parameter range is desirable for experiments with higher average photon flux and the accompanying higher signal-to-noise ratio and shorter measurement durations6, 7. However, there are few robust solutions for fiber-based beam delivery at this very high peak power, creating a hindrance for some applications. In industry there are practical challenges, such as the degradation of beam quality and limits on pulse duration and power available on target after transmission in standard silica fibers. For scientific applications, one difficulty is the relatively long pulse durations of a few hundred femtoseconds to a few picoseconds. Most targeted experiments, such as undertaken in strong-field laser physics, would benefit from efficient fiber-based solutions for simple pulse compression schemes, Figure 1. Overview of ultrafast laser sources combining high pulse energy and repetition rate, and illustration of different laser architectures. These optimize heat removal to support very high average power in the kW regime. TDL: Thin disk laser. CPA: Chirped pulse amplification.