Quantitative phase imaging (QPI) utilizes the fact that the phase of an imaging field is much more sensitive than its amplitude. As fields from the source interact with the specimen, local variations in the phase front are produced, which provide structural information about the sample and can be used to reconstruct its topography with nanometer accuracy. QPI techniques do not require staining or coating of the specimen and are therefore nondestructive. Diffraction phase microscopy (DPM) combines many of the best attributes of current QPI methods; its compact configuration uses a common-path off-axis geometry which realizes the benefits of both low noise and single-shot imaging. This unique collection of features enables the DPM system to monitor, at the nanoscale, a wide variety of phenomena in their natural environments. Over the past decade, QPI techniques have become ubiquitous in biological studies and a recent effort has been made to extend QPI to materials science applications. We briefly review several recent studies which include real-time monitoring of wet etching, photochemical etching, surface wetting and evaporation, dissolution of biodegradable electronic materials, and the expansion and deformation of thin-films. We also discuss recent advances in semiconductor wafer defect detection using QPI.
Electrospinning constitutes a simple and versatile approach of fabricating polymer heterostructures composed of nanofibers. A preferred alignment of polymer crystallites stems from complex shear elongational forces and generates a strong intrinsic optical anisotropy in typical electrospun fibers of semicrystalline polymers. While it can prove useful for certain devices, this intrinsic anisotropy can be extremely detrimental for other key applications such as high-performance polymer-based lighting and solar-energy harvesting platforms. We report a dramatic reduction in the intrinsic dichroism of electrospun poly(ethylene oxide) fibers resulting from the incorporation of inorganic nanoparticles in the polymer matrix. This effect is shown to originate from a controllable randomization of the orientational ordering of the crystalline domains in the hybrid nanofibers and not merely from a reduction in crystallinity. This improved understanding of the crystalline structure-optical property correlation then leads to a better control over the intrinsic anisotropy of electrospun fibers using localized surface-plasmon enhancement effects around metallic nanoparticles.
Hydrogen has attracted attention as an alternative fuel source and as an energy storage medium. However, the flammability of hydrogen at low concentrations makes it a safety concern. Thus, gas concentration measurements are a vital safety issue. Here we present the experimental realization of a palladium thin film cantilever optomechanical hydrogen gas sensor. We measured the instantaneous shape of the cantilever to nanometer-level accuracy using diffraction phase microscopy. Thus, we were able to quantify changes in the curvature of the cantilever as a function of hydrogen concentration and observed that the sensor's minimum detection limit was well below the 250 p.p.m. limit of our test equipment. Using the change in curvature versus the hydrogen curve for calibration, we accurately determined the hydrogen concentrations for a random sequence of exposures. In addition, we calculated the change in film stress as a function of hydrogen concentration and observed a greater sensitivity at lower concentrations.Keywords: hydrogen detection; imaging and sensing; interference microscopy; optomechanics; optical sensors; surface dynamics INTRODUCTIONHydrogen has always been viewed as a promising alternative to fossil fuels, and it can also function as an effective energy storage medium for intermittent energy sources. In addition, hydrogen is used in a range of other industries, including chemical production, metal refining, and food processing. A major safety concern with hydrogen is combustibility. Therefore, early leak detection and concentration determination of hydrogen have been areas of intense research 1-3 . There are various types of hydrogen sensors that use a wide range of detection mechanisms. Lundström et al. 4 proposed metal oxide semiconductor (MOS)-type hydrogen sensors 5 . However, MOS sensors suffer from drawbacks such as premature saturation of detectable hydrogen concentrations and low sensitivity. Other MOS-based devices have been used as hydrogen sensors, such as MOS field-effect transistors (FETs) 6,7 , high electron mobility transistors [8][9][10] , and Schottky diode-type FETs 11,12 . However, these devices require complicated fabrication processes and have high production costs. Optical gas sensors 13-21 not only overcome these disadvantages but also have other unique advantages, such as negligible electrical interference, no risk of ignition from an electrical spark, and the ability to work at high temperatures or in harsh environments.Palladium (Pd) can absorb up to 900 times its own weight in hydrogen gas at room temperature 22 . Compared with platinum 23 , Pd film is more popular because of its lower cost. Pd alloys with Ag, Au, Ni, and WO 3 have also been studied 24-29 because of their improved response time 24,25,27 , sensitivity 28,29 , and durability 26 as a sensing material. The alloys can avoid blistering effects 26 and the α to β phase transition at higher hydrogen concentrations 25,27 .
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