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Photoimprinted controllable Fano resonance in the terahertz regime
Get PDFThe emergence of metamaterials, including the recently proposed metasurfaces, provides unprecedented opportunities in the manipulation as well as the generation of terahertz (THz) waves. Various actively controllable THz devices such as optical switches and phase modulators have been achieved by incorporating external-stimuli-responsive media (typically semiconductors) in the configurations. Compared to predesigned metallic-resonator-based metamaterials, photoimprinted photonic structures offer us an all-optical route toward reconfigurable functionalities with superior flexibility. Here, we propose to photoimprint some specific patterns on a thin film of semiconductor to excite Fano-like resonances, which result from the coupling between dark and bright elements. Experimental measurements, performed with THz time-domain spectroscopy, demonstrate counterintuitive tunable deep features in extinction spectra positioned around the expected resonance frequency. Our simulations are in excellent agreement with the experiments, by resembling realistic conditions of low contrast photoimprinted patterns generated with a spatial light modulator and a finite detection time window. This work takes the first step toward realizing switchable Fano resonances via an all-optical approach and, therefore, paves the way to more versatile manipulations of THz waves
Integrated nano-opto-electro-mechanical sensor for spectrometry and nanometrology
Get PDFSpectrometry is widely used for the characterization of materials, tissues, and gases, and the
need for size and cost scaling is driving the development of mini and microspectrometers.
While nanophotonic devices provide narrowband filtering that can be used for spectrometry,
their practical application has been hampered by the difficulty of integrating tuning and
read-out structures. Here, a nano-opto-electro-mechanical system is presented where the
three functionalities of transduction, actuation, and detection are integrated, resulting in a
high-resolution spectrometer with a micrometer-scale footprint. The system consists of an
electromechanically tunable double-membrane photonic crystal cavity with an integrated
quantum dot photodiode. Using this structure, we demonstrate a resonance modulation
spectroscopy technique that provides subpicometer wavelength resolution. We show its
application in the measurement of narrow gas absorption lines and in the interrogation of
fiber Bragg gratings. We also explore its operation as displacement-to-photocurrent transducer,
demonstrating optomechanical displacement sensing with integrated photocurrent
read-out
Softening and Yielding of Soft Glassy Materials
Get PDFSolids deform and fluids flow, but soft glassy materials, such as emulsions, foams, suspensions, and pastes, exhibit an intricate mix of solid-and liquid-like behavior. While much progress has been made to understand their elastic (small strain) and flow (infinite strain) properties, such understanding is lacking for the softening and yielding phenomena that connect these asymptotic regimes. Here we present a comprehensive framework for softening and yielding of soft glassy materials, based on extensive numerical simulations of oscillatory rheological tests, and show that two distinct scenarios unfold depending on the material's packing density. For dense systems, there is a single, pressure-independent strain where the elastic modulus drops and the particle motion becomes diffusive. In contrast, for weakly jammed systems, a two-step process arises: at an intermediate softening strain, the elastic and loss moduli both drop down and then reach a new plateau value, whereas the particle motion becomes diffusive at the distinctly larger yield strain. We show that softening is associated with an extensive number of microscopic contact changes leading to a non-analytic rheological signature. Moreover, the scaling of the softening strain with pressure suggest the existence of a novel pressure scale above which softening and yielding coincide, and we verify the existence of this crossover scale numerically. Our findings thus evidence the existence of two distinct classes of soft glassy materials - jamming dominated and dense - and show how these can be distinguished by their rheological fingerprint
Optoelectronic Enhancement of Ultrathin CuIn1–xGaxSe2 Solar Cells by Nanophotonic Contacts
Get PDFCuIn1–xGaxSe22 (CIGSe) solar cells have achieved record efficiency values as high as 22.6% for small areas, with module efficiency values of 16.5%. However, for economic viability these values must be achieved with reduced material consumption (especially indium), which requires reducing the CIGSe absorber thickness from 2000–3000 nm to below 500 nm. Soft-imprinted SiOx nanoparticles (NPs) beneath a conformal CIGSe layer enable this thickness reduction. Optically, they enhance the absorption of light through Fabry–Pérot and waveguided resonances within the CIGSe layer, preventing current loss. For CIGSe solar cells on ITO with an absorber thickness of only 390 nm and a nanophotonic contact the current density (Jsc) increases from 25.7 to 32.1 mA cm−2. At the same time, the nanopatterned contact reduces the back barrier, leading to an increased open-circuit voltage (518 to 558 mV) and fill factor (50.7% to 55.2%). Combined, these effects increase the efficiency value from 6.8% to 10.0% for this initial demonstration. With the addition of an antireflection coating, the champion NP-enhanced cell achieves a (Jsc of 34.0 mA cm−2, corresponding to 93% of the (Jsc achieved by the thick world-record cell. This result shows that optoelectronic nanopatterning provides a path to high efficiency cells with reduced materials consumption
Molecular Structure of Hydrophobins Studied with Site-Directed Mutagenesis and Vibrational Sum-Frequency Generation Spectroscopy
Get PDFHydrophobins are surface-active fungal proteins that adsorb to the water–air interface and self-assemble into amphiphilic, water-repelling films that have a surface elasticity that is an order of magnitude higher than other molecular films. Here we use surface-specific sum-frequency generation spectroscopy (VSFG) and site-directed mutagenesis to study the properties of class I hydrophobin (HFBI) films from Trichoderma reesei at the molecular level. We identify protein specific HFBI signals in the frequency region 1200–1700 cm–1 that have not been observed in previous VSFG studies on proteins. We find evidence that the aspartic acid residue (D30) next to the hydrophobic patch is involved in lateral intermolecular protein interactions, while the two aspartic acid residues (D40, D43) opposite to the hydrophobic patch are primarily interacting with the water solvent
