Mapping strain fields in visually opaque structural composites -for which failure is often sudden, irreparable, and even catastrophic -requires techniques to locate and record regions of stress, fatigue, and incipient failure. Many composite materials are transparent in the terahertz spectral region, but their strain history is often too subtle to recover. Here, terahertz metamaterials with strain-severable junctions are introduced that can identify structurally compromised regions of composite materials. Specifically, multi-layer arrays of aluminum meta-atoms were designed and fabricated as strip dipole antennas with a terahertz frequency resonance and a strong response to cross polarized radiation that disappears when local stress irreversibly breaks their bowtie-shaped junction. By spatially mapping the local polarimetric response of this metamaterial as a function of global strain, the regions of local stress extrema experienced by a visually opaque material may be visualized. This proof-of-concept demonstration heralds the opportunity for embedding metamaterial laminates within composites to record and recover their strain-dependent history of fatigue.The widespread proliferation of composite materials in civilian, industrial, and military sectors has created a need for tools to monitor their structural health and warn of incipient failure. Many techniques have been tried, including embedded sensors, laser surface mapping, acoustic transducers, x-ray imaging, and terahertz imaging concepts. [1][2][3][4][5][6] The opacity of most composites prevents the use of common optical polarimetric techniques for measuring photoelasticity, while stress-induced birefringence produces weak refractive index anisotropies at terahertz frequencies that are difficult to measure. [7,8] Many of these techniques can identify damaged regions, but none Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))
Abstract. We have developed a portable, breast margin assessment probe leveraging diffuse optical spectroscopy to quantify the morphological landscape of breast tumor margins during breast conserving surgery. The approach presented here leverages a custom-made 16-channel annular photodiode imaging array (arranged in a 4 × 4 grid), a raster-scanning imaging platform with precision pressure control, and compressive sensing with an optimized set of eight wavelengths in the visible spectral range. A scalable Monte-Carlo-based inverse model is used to generate optical property [μ 0 s ðλÞ and μ a ðλÞ] measures for each of the 16 simultaneously captured diffuse reflectance spectra. Subpixel sampling (0.75 mm) is achieved through incremental x , y raster scanning of the imaging probe, providing detailed optical parameter maps of breast margins over a 2 × 2 cm 2 area in ∼9 min. The morphological landscape of a tumor margin is characterized using optical surrogates for the fat to fibroglandular content ratio, which has demonstrated diagnostic utility in delineating tissue subtypes in the breast.
The light enhancement phenomena in InGaN/GaN multi-quantum wells (MQWs) infiltrated with metal nanoparticles (NPs) are studied using resonant and off-resonant localized plasmon interactions. The emission and recombination characteristics of carriers in InGaN/GaN MQW structures with inverted hexagonal pits (IHPs) are modified distinctly depending on the nature of their interaction with the metal NPs and with the pumping and emitted photons. It is observed that the emission intensity of light is significantly enhanced when the emission energy is off-resonant to the localized plasmon frequency of the metal nanoparticles. This results in enhanced emission from MQW due to Au nanoparticles and from IHPs due to Ag nanoparticles. At resonant-plasmon frequency of the Ag NPs, the emission from MQWs is quenched due to the re-absorption of the emitted photons, or due to the drift carriers from c-plane MQWs towards the NPs because of the Coulomb forces induced by the image charge effect
This paper presents an optical element capable of multiplexing two diffraction patterns for two orthogonal linear polarizations, based on the use of non-resonant metamaterial cross elements. The metamaterial cross elements provide unique building blocks for engineering arbitrary birefringence. As a proof-of-concept demonstration, we present the design and experimental characterization of a polarization multiplexed blazed diffraction grating and a polarization multiplexed computer-generated hologram, for the telecommunication wavelength of λ = 1.55 μm. A quantitative study of the polarization multiplexed grating reveals that this approach yields a very large polarization contrast ratio. The results show that metamaterials can form the basis for a versatile and compact platform useful in the design of multi-functional photonic devices.
The enhancement of light from semiconductors
due to surface plasmons
coupled resonantly to its emission is limited because of dissipation
in the metal and is also restricted by the dielectric characteristics
and homogeneity of the metal–semiconductor interface. We report
a new mechanism based on electrostatic interactions of carriers and
their image charges in metals to generate more photons from optical
sources at frequencies that are off-resonant to the localized plasmon
frequency. Coulomb catalysis of carrier accumulation resulting from
the inhomogeneity of metal nanodroplets on a semiconductor’s
surface can result in an enhancement of light that is nondissipative
and does not require resonant coupling of plasmons to the emission
wavelength. The enhancement occurs because of an increase in the ratio
of radiative to nonradiative recombination in the vicinity of metal
nanoparticles. It is equally effective with any type of metal and
enhances radiation at any frequency, a property that is of principal
importance for the realization of widely tunable semiconductor emitters.
This fundamental mechanism provides a new perspective for improving
the efficiency of light emitters and controlling carrier concentration
on the nanoscale. The structural characteristics of the hybrid metal–semiconductor
emitters are studied using electron microscopy and atomic force microscopy.
We demonstrate the electrostatic mechanism by studying steady-state
and transient photoluminescence from two-dimensional semiconductors,
such as GaAs/AlGAs quantum wells, and bulk semiconductors, such as
ZnO thin films, emitting in the near-IR and UV wavelength regimes,
respectively.
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