Reconfigurable Laser-Stimulated Lock-In Thermography for Surface Micro-Crack Detection.

Surface crack detection and sizing is essential for the manufacturing and maintenance of engines, run parts, and other metal elements of aircrafts. Among various non-destructive detection methods, the fully non-contact and non-intrusive technique based on laser-stimulated lock-in thermography (LLT) has recently attracted a lot of attention from the aerospace industry. We propose and demonstrate a system of reconfigurable LLT for three-dimensional surface crack detection in metal alloys. For large area inspection, the multi-spot LLT can speed up the inspection time by a factor of the number of spots. The minimum resolved size of micro-holes is ~50 µm in diameter limited by the magnification of the camera lens. We also study the crack length ranging from 0.8 to 3.4 mm by varying the modulation frequency of LLT. An empirical parameter related to the thermal diffusion length is found to show the linear dependence with the crack length. With the proper calibration, this parameter can be used to predict the sizing of the surface fatigue cracks. Reconfigurable LLT allows us to quickly locate the crack position and accurately measure its dimensions. This method is also applicable to the non-destructive detection of surface or sub-surface defect in other materials used in various industries.

Schrödinger's red pixel by quasi-bound-states-in-the-continuum.

While structural colors are ubiquitous in nature, saturated reds are mysteriously absent. This long-standing problem of achieving Schrödinger's red demands sharp transitions from "stopband" to a high-reflectance "passband" with total suppression of higher-order resonances at blue/green wavelengths. Current approaches based on nanoantennas are insufficient to satisfy all conditions simultaneously. Here, we designed Si nanoantennas to support two partially overlapping quasi-bound-states-in-the-continuum modes with a gradient descent algorithm to achieve sharp spectral edges at red wavelengths. Meanwhile, high-order modes at blue/green wavelengths are suppressed via engineering the substrate-induced diffraction channels and the absorption of amorphous Si. This design produces possibly the most saturated and brightest reds with ~80% reflectance, exceeding the red vertex in sRGB and even the cadmium red pigment. Its nature of being sensitive to polarization and illumination angle could be potentially used for information encryption, and this proposed paradigm could be generalized to other Schrödinger's color pixels.

Nanoscale mapping of optically inaccessible bound-states-in-the-continuum.

Bound-states-in-the-continuum (BIC) is an emerging concept in nanophotonics with potential impact in applications, such as hyperspectral imaging, mirror-less lasing, and nonlinear harmonic generation. As true BIC modes are non-radiative, they cannot be excited by using propagating light to investigate their optical characteristics. In this paper, for the 1st time, we map out the strong near-field localization of the true BIC resonance on arrays of silicon nanoantennas, via electron energy loss spectroscopy with a sub-1-nm electron beam. By systematically breaking the designed antenna symmetry, emissive quasi-BIC resonances become visible. This gives a unique experimental tool to determine the coherent interaction length, which we show to require at least six neighboring antenna elements. More importantly, we demonstrate that quasi-BIC resonances are able to enhance localized light emission via the Purcell effect by at least 60 times, as compared to unpatterned silicon. This work is expected to enable practical applications of designed, ultra-compact BIC antennas such as for the controlled, localized excitation of quantum emitters.

In vivo sensing of rabbit cornea by terahertz technology.

A Novel scalable approach using Terahertz (THz) waves together with the electromagnetic field simulation was applied to investigate four rabbits of eight rabbit corneas in vivo. One eye of each rabbits' corneas was edema induced; the other eye of the corneas served as the control. The simulation revealed the propagation of THz waves at a certain distance along the sub-surface of the cornea. THz spectra have been collected close to the corneal surface by deviating the direct reflection of the THz beam for the edema cornea, the reflected wave intensity for edema corneas is generally larger compared with the control cornea. Upon edema becomes severe at the end of the observation, the reflected wave intensities obtained by detector corresponding to the corneal deep stroma layer approach to the same value for all observed corneas. Good correlation is observed between central corneal thickness measurements and THz wave reflection signal intensities. Our results demonstrated that THz spectroscopy technique could obtain the information from different corneal sublayers.

Silicon Nanoantenna Mix Arrays for a Trifecta of Quantum Emitter Enhancements.

Dielectric nanostructures have demonstrated optical antenna effects due to Mie resonances. Previous work has exhibited enhancements in absorption, emission rates and directionality with practical limitations. In this paper, we present a Si mix antenna array to achieve a trifecta enhancement of ∼1200-fold with a Purcell factor of ∼47. The antenna design incorporates ∼10 nm gaps, within which fluorescent molecules strongly absorb the pump laser energy through a resonant mode. In the emission process, the antenna array increases the radiative decay rates of the fluorescence molecules via a Purcell effect and provides directional emission through a separate mode. This work could lead to novel CMOS-compatible platforms to enhance fluorescence for biological and chemical applications.

Polyelectrolyte-Graphene Oxide Multilayer Composites for Array of Microchambers which are Mechanically Robust and Responsive to NIR Light.

Development of composite polymer/graphene oxide (GO) materials attracts significant attention due to their unique properties. In this work, highly ordered arrays of hollow microchambers made of composite polyelectrolyte/GO multilayers (PEGOMs) are successfully fabricated via layer-by-layer assembly on sacrificial or sustainable templates having imprinted patterns of microwells on their surface. Mechanical and optical properties of PEGOMs are studied by nanoindentation and near-infrared (NIR) absorption spectroscopy. Incorporation of three GO layers in between the polyelectrolyte multilayer stacks increases Young's modulus and critical stress of the microchambers by a factor of 5.6 and 2.6, respectively. Optical density of this PEGOM film is found to decrease gradually from 0.14 at λ = 800 nm to 0.06 at λ = 1500 nm. Remote opening of PEGOM microchambers with NIR laser beam is also demonstrated. One of the possible applications of the developed structures includes micropackaging and delivery systems in biological tissues with remote triggering.

Photothermally responsive gold nanoparticle conjugated polymer-grafted porous hollow silica nanocapsules.

Polymer-grafted porous hollow silica nanoparticles prepared by reversible addition-fragmentation chain transfer polymerisation have an upper critical solution temperature of 45 °C. Conjugation of 5 nm gold nanoparticles onto polymer-grafted porous hollow silica nanoparticles enables remarkable specific photothermally-induced controlled release of encapsulated Rhodamine B by laser-stimulation at physiological temperature.

Highly emissive, solution-processable and dynamic Eu(III)-containing coordination polymers.

A new class of soluble Eu(III) coordination polymers based on a tridentate ditopic pybox ligand has been developed displaying high metal emission quantum yields of up to 73% as well as a unique dynamic behaviour in solution.

Transient absorption spectroscopy on spiropyran monolayers using nanosecond pump-probe Brewster angle reflectometry.

Self-assembled monolayers of 11-(3',3'-dimethyl-6,8-dinitrospiro[chromene-2,2'-indoline]-1'-yl) undecanoic acid (amphiphilic spiropyran) at the air-water interface are studied using Brewster angle reflectometry. Transient kinetics of the spiropyran to merocyanine conversion are recorded in a UV-pump, VIS-probe configuration. By varying the probe wavelength using an optical parametric oscillator, we are able to reconstruct absorption spectra of intermediate states with a time-resolution of 10 nanoseconds, limited by the temporal convolution of the two laser pulses. After UV irradiation, spiropyran converts to merocyanine in two stages. The first occurs within a timescale of several tens of nanoseconds and is heavily convoluted with the system response time, whereas the second stage occurs over a few hundred nanoseconds. During the rise time there is a small red shift in the transient absorption spectrum of ~20 nm. We assign the red shift and the slower kinetics to the isomerization of a merocyanine isomer cis about the central methine bond to those that are trans about the same bond.

Individually addressable patterned multilayer microchambers for site-specific release-on-demand.

Patterned arrays of light-responsive microchambers are suggested as candidates for site-specific release of chemicals in small and precisely defined quantities on demand. A composite film is made of poly(allylammonium)-poly(styrene sulfonate) multilayers and gold nanoparticles incorporated between subsequent stacks of polyelectrolytes. The film shaped as microchambers is loaded with colloid particles or oil-soluble molecules. The microchambers are sealed onto a glass slide precoated with an adhesive poly(diallyldimethylammonium)-poly(styrene sulfonate) multilayer film. A focused laser beam is used for remote addressing the individual microchambers and site-specific release of the loaded cargo.

Highly wrinkled cross-linked graphene oxide membranes for biological and charge-storage applications.

Inspired by the amphiphilicity of graphene oxide (GO), the surface of water is used as a template for the assembly of a GO film. Methacrylate-functionalized GO sheets can be cross-linked instantaneously at the water-air interface to form a highly wrinkled membrane spreading over an extended area. The multiple covalent linkages amongst the GO sheets enhances the in-plane stiffness of the film compared to noncovalently bonded GO films. The highly convoluted GO membrane can be used in two applications: the promoting of spontaneous stem-cell differentiation towards bone cell lineage without any chemical inducers, and for supercapacitor electrodes. Due to reduced van der Waals restacking, capacitance values up to 211 F g(-1) can be obtained. The scalable and inexpensive nature of this assembly route enables the engineering of membranes for applications in regenerative medicine and energy-storage devices where secondary structures like nanotopography and porosity are important performance enhancers.

Delayed onset of photochromism in molybdenum oxide films caused by photoinduced defect formation.

We report the photochromic properties of amorphous MoO3 films deposited by dc sputtering with different O2 flow rates. The kinetics of film coloration under UV light irradiation is determined using optical transmission spectroscopy. Changes in the absorbance and refractive index were derived from the analysis of transmittance spectra. The absorbance spectra exhibited a growing broad peak centered around 830 nm, which was induced by the UV irradiation. In the early stages of irradiation, the absorbance of the films did not change but their refractive indices did change. This induction time was correlated with the O2 partial pressure during the film deposition, which was controlled by the O2 flow rate. The origins of this observation are discussed.

Cooperative photoinduced two-dimensional condensation in Langmuir films observed using nanosecond pump-probe Brewster angle microscopy.

Two-dimensional condensation was initiated in a self-assembled mixed monolayer of spiropyran and octadecanol by a nanosecond laser pulse. The dynamics of the process were monitored using nanosecond pump-probe Brewster angle microscopy. Domain growth followed a power law with a growth exponent of 0.47 at a velocity approaching 20 ms(-1). This represents a limit for the rate of longitudinal signaling of pressure waves through a self-assembled amphiphilic layer at an interface and adds to our understanding of signal transmission rates in biomimetic membranes where morphological change in one region can be signaled to a more remote region.

Transient Brewster angle reflectometry of spiropyran monolayers.

Brewster angle reflectometry has been developed as a tool for determining the absorbance and refractive index changes in molecular monolayers containing spiropyran. The method is sensitive to changes in both the real and imaginary parts of the refractive index in the monolayers. It was used to monitor the conversion of spiropyran to merocyanine and the reversal of this reaction when the molecules were immobilised on quartz using silane coupling. An analytical solution of Fresnel formula allowed the transient reflectometry data to be converted into transient absorption information. Absorbances of transients as low as approximately 10(-6) were possible using the current apparatus with a single laser pulse transient measurement. It was found that spiropyran photoconverted to merocyanine with an efficiency of approximately 0.1. The photochemical reversion of converted merocyanine to spiropyran occurred with efficiencies of 0.03-0.2 and this was probably site dependent. It was found that the thermal conversion from merocyanine to spiropyran was slow and even after 10 min there was no significant thermal reversion. This measurement was possible because the real part of the refractive index of the monolayer could be monitored with time using an off-resonance probe at a wavelength where the merocyanine did not absorb light meaning that the probe did not photobleach the sample. Thus our method also provides a non-intrusive method for probing changes in molecules in thin films.

Time-resolved brewster angle microscopy for photochemical and photothermal studies on thin-films and monolayers.

Transient events in thin films and interfaces have been studied using the technique of time resolved pump-probe nanosecond Brewster angle microscopy. For p-polarized light there is a minimum reflectivity at the Brewster angle. When the interface is viewed with light that is both incident and reflected at the Brewster angle the resulting image is dark. Subsequent small changes is refractive index will then cause an increase in the reflectivity in affected regions providing high contrast images of an altered interface with a dark background level. This is the basis of Brewster angle microscopy. In the present work two synchronized nanosecond pulsed lasers were used in the pump-probe configuration in order to induce changes at an air-liquid interface and to monitor the resulting morphology changes over a range of time delays from nanosecond to milliseconds after laser-excitation. This method can be used to observe morphological changes in phase altering thin-films and molecular monolayers. Further it can be used to obtain information about transient photochemistry even in optically thin materials and nano-films. In the current work the method is used to monitor laser induced processes in phase separating binary liquid mixtures as well as in monolayers of photo-responsive amphiphilic molecules derived from spiropyran on water. The two systems are quite different but provide valuable comparisons.

Infrared multiple photon dissociation of chloromethyltrifluorosilane.

Infrared multiphoton absorption and dissociation of chloromethyltrifluorosilane molecules under the action of pulsed transversely excited atmospheric pressure CO2 laser were experimentally studied. Dissociation products were analyzed. The dissociation proceeds via chlorine atom transfer from carbon to silicone. High degrees of silicon isotope separation were achieved. The presence of alpha-chlorine atom in a silicon organic compound brings about a significant improvement in multiple photon dissociation characteristics and an essential increase in isotopic selectivity.

Water expansion dynamics after pulsed IR laser heating.

A nanosecond pulsed IR (1.9 microm) laser rapidly heated water, in an open vessel, to temperatures well below the boiling point. The subsequent dynamics of volume expansion were monitored using time-resolved interferometry in order to measure the increase in the water level in the heated area. The water expanded at less than the speed of sound, taking just less than 100 ns to increase its height by approximately 500 nm at surface temperature jumps of 20 K. The initial expansion was followed by an apparent contraction and then a re-expansion. The first expansion phase occurred more slowly than the timescale for bulk H-bond re-structuring of the water, as determined from vibrational bands in the Raman spectra, and represents the limit to the rate at which the overpressure caused by sudden heating can be released. The second phase of the expansion was caused by hydrodynamic effects and is accompanied by morphological changes resulting in light scattering as well as droplet spallation.