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Conductive hydrogels (CHs) have shown promising potential applied as wearable or epidermal sensors owing to their mechanical adaptability and similarity to natural tissues. However, it remains a great challenge to develop an integrated hydrogel combining outstanding conductive, self-healing and biocompatible performances with simple approaches. In this work, we propose a "one-pot" strategy to synthesize multifunctional CHs by incorporating two-dimensional (2D) transition metal carbides/nitrides (MXenes) multi-layer nano-flakes as nanofillers into oxidized alginate and gelatin hydrogels to form the composite CHs with various MXene contents. The presence of MXene with abundant surface groups and outstanding conductivity could improve the mechanical property and electroactivity of the composite hydrogels compared to pure oxidized alginate dialdehyde-gelatin (ADA-GEL). MXene-ADA-GELs kept good self-healing properties due to the dynamic imine linkage of the ADA-GEL network and have a promoting effect on mouse fibroblast (NH3T3s) attachment and spreading, which could be a result of the integration of MXenes with stimulating conductivity and hydrophily surface. This study suggests that the electroactive MXene-ADA-GELs can serve as an appealing candidate for skin wound healing and flexible bio-electronics.

We report on carrier recombination within self-catalyzed InAs/InAlAs core-shell nanowires (NWs), disentangling recombination rates at the ends, sidewalls, and interior of the NWs. Ultrafast optical pump-probe spectroscopy measurements were performed from 77-293 K on the free-standing, variable-sized NWs grown on lattice-mismatched Si(111) substrates, independently varying NW length and diameter. We found NW carrier recombination in the interior is nontrivial compared to the surface recombination, especially at 293 K. Surface recombination is dominated by carrier recombination at the NW sidewall, while contributions from the highly strained, impure NW base are negligible.

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The interactions between light and plasmonic charge oscillations in conducting materials are important venues for realizing nanoscale light manipulations. Conventional metal-based plasmonic devices lack tunability due to the fixed material permittivities. Here, we show that reconfigurable plasmonic functionalities can be achieved using the spatially controlled phase transitions in strongly correlated oxide films. The experimental results discussed here are enabled by a recently developed scanning probe-based technique that allows a nonvolatile, monoclinic-metal VO2 phase to be reversibly patterned at the nanoscale in ambient conditions. Using this technique, rewritable waveguides, spatially modulated plasmonic resonators, and reconfigurable wire-grid polarizers are successfully demonstrated. These structures, effectively controlling infrared lights through spatially confined mobile carriers, showcase a great potential for building programmable nanoplasmonic devices on correlated oxide platforms.

Here, we demonstrate the nanoscale manipulations of two types of charge transfer to the LaAlO3/SrTiO3 interfaces: one from surface adsorbates and another from oxygen vacancies inside LaAlO3 films. This method can be used to produce multiple insulating and metallic interface states with distinct carrier properties that are highly stable in air. By reconfiguring the patterning and comparing interface structures formed from different doping sources, effects of extrinsic and intrinsic material characters on the transport properties can be distinguished. In particular, a multisubband to single-subband transition controlled by the structural phases in SrTiO3 was revealed. In addition, the transient behaviors of nanostructures also provided a unique opportunity to study the nanoscale diffusions of adsorbates and oxygen vacancies in oxide heterostructures. Knowledge of such dynamic processes is important for nanodevice implementations.

To maximize the photovoltaic efficiency, it is highly desirable to enable the electricity conversion from low energy photons and to extract the excessive energy from hot carriers. Here we report a large photovoltage generation at the LaAlO

To improve the efficiency of GaSb-based mid-wave infrared LEDs, we investigated the enhanced spontaneous emission by a dipole in a thin slab numerically. A Purcell factor around 5 was achieved by combining a thin waveguide and plasmonic grating.

In this paper, a time- and cost-effective method of fabricating a light management structure on the surface of planar silicon (Si) substrates is developed utilizing localized surface plasmon resonance (LSPR) of silver (Ag) nanoparticles (NPs). The Ag NPs are produced by an electroless plating method and then modified in hot water. The resulting randomly distributed Ag NPs can reduce the reflection of the Si surface in the entire visible spectrum. With the help of a MATLAB-based analytical model on Mie theory, the size distribution of Ag NPs for desired optical properties is determined, and the reflection of the best performance sample decreases by up to 24.8% at a wavelength of 371 nm. An atmospheric degradation study of the Ag NPs is also reported, which demonstrates that the LSPR response of unprotected Ag NPs is markedly impaired after 14 days, while the LSPR response of aluminum oxide (Al2O3) protected Ag NPs is unchanged even after 90 days. The Al2O3 coated sample also shows a strong reflection reduction, exhibiting a reflection of as low as 7.6% at a wavelength of 662 nm and a weighted average spectral reflectance (Rave) of 12.2%.

We report an oxygen surface adsorbates induced metal–insulator transition at the LaAlO3/SrTiO3 interfaces. The observed effects were attributed to the terminations of surface Al sites and the resultant electron-accepting surface states. By controlling the local oxygen adsorptions, we successfully demonstrated the nondestructive patterning of the interface two-dimensional electron gas (2DEG). The obtained 2DEG structures are stable in air and also robust against general solvent treatments. This study provides new insights into the metal–insulator transition mechanism at the complex oxide interfaces and also a highly efficient technique for tailoring the interface properties.

Efficient trapping of the light in a photon absorber or a photodetector can improve its performance and reduce its cost. In this paper we investigate two designs for light-trapping in application to infrared absorption. Our numerical simulations demonstrate that nonabsorptive pyramids either located on top of an absorbing film or having embedded absorbing rods can efficiently enhance the absorption in the absorbing material. A spectrally averaged absorptance of 83% is achieved compared to an average absorptance of 28% for the optimized multilayer structure that has the same amount of absorbing material. This enhancement is explained by the coupled-mode theory. Similar designs can also be applied to solar cells.

United States. Defense Advanced Research Projects Agency (DARPA under contract N66604-09-C-3652)

We are studying a subwavelength slit in a lossy metal film illuminated by electromagnetic waves. Periodic grooves are placed on the output side surrounding the slit. Surface plasmons along the metallic surface on the output side are excited by the electromagnetic wave propagating through the slit. The grooves modulate the surface plasmons and couple them into propagation modes, which facilitates the beaming of the output field. We developed an efficient method to determine the geometric parameters of the grooves that are necessary to achieve oblique beaming at any angle between 0 and 70 degrees. We also designed a frequency splitter by setting appropriate parameters of the grooves.

The metal-dielectric interface supports surface plasmons. But the metal-dielectric interface with defects has not only surface plasmons but also residual waves. In this paper we calculate the fields along the metaldielectric interface with defects from Maxwell’s equations analytically using the surface impedance approximation and study the asymptotic behavior of the residual waves. These analytic results set up a solid foundation to understand various phenomena such as beaming and extraordinary optical transmission.

A surface mode is an electromagnetic field distribution bounded at a surface. It decays exponentially with the distance from the surface on both sides of the surface and propagates at the surface. The surface mode exists at a metal-dielectric interface as surface plasmon (1) or at a photonic crystal surface terminated properly (34; 35; 36). Besides its prominent near-filed properties, it can connect structures at its propagation surface and results in far-field effects. Extraordinary transmission (EOT) and beaming are two examples and they are the subjects I am studying in this thesis. EOT means the transmission through holes in an opaque screen can be much larger than the geometrical optics limitation. Based on our everyday experience about shadows, the transmission equals the filling ratio of the holes in geometrical optics. The conventional diffraction theory also proved that the transmission through a subwavelength circular hole in an infinitely thin perfect electric conductor (PEC) film converges to zero when the hole's dimension is much smaller than the wavelength (40). Recently it is discovered that the transmission can be much larger than the the filling ratio of the holes at some special wavelengths (41). This cannot be explained by conventional theories, so itmore » is called extraordinary transmission. It is generally believed that surface plasmons play an important role (43; 44) in the EOT through a periodic subwavelength hole array in a metallic film. The common theories in literatures are based on these arguments. The surface plasmons cannot be excited by incident plane waves directly because of momentum mismatch. The periodicity of the hole arrays will provide addition momentum. When the momentum-matching condition of surface plasmons is satisfied, the surface plasmons will be excited. Then these surface plasmons will collect the energy along the input surface and carry them to the holes. So the transmission can be bigger than the filling ratio. Based on this picture, they deduced naturally that when surface plasmons momentum-matching condition is satisfied, the surface plasmons are excited sufficiently and the transmission reaches its peak. I present a new theory from first principles to explain EOT through one-dimensional periodic subwavelength metallic slits in this thesis. This theory can also be extended to 2D hole arrays. I define the incident wavelengths that satisfy the momentum-matching condition as surface resonant wavelengths. I proved analytically that the transmission is actually zero at the surface resonant wavelengths. The correct logic is: When the momentum-matching condition is satisfied, the surface plasmons excited by each slit interfere constructively with each other, the total surface plasmons will go to infinity. But the law of nature forbids the infinity. The only solution is the surface plasmon excited by one slit is zero and all the energy is reflected. In my theory, the term corresponding to surface plasmons appear explicitly in the equations. So it confirms the importance of surface plasmons without any doubt. The theory divides the transmission process into two steps: energy collection process along the input surface and the propagation process in the slits. In the first process, the surface plasmons collect the energy along the input surface and carry them to the slits. This process happens efficiently at any wavelength other than the surface resonant wavelengths. So EOT can happen at almost any wavelength. After the energy enter the slits, the Fabry-Perot interference between the input and output surface decides how much energy is emitted from the slits. So the EOT wavelengths are decided by the Fabry-Perot resonances. I also use my theory to explain the data in literatures. The transmission spectra through 1D slits or 2D hole arrays in literatures agree with my theory very well. The new theory can explain a lot of experimental results published recently, such as the transmission through randomized hole arrays, the strong influence of the hole shape on the transmission peaks, and so on. Beaming is another far-field effect resulting from surface modes. Normally light coming from a subwavelength waveguide is diffracted to all angles. With the help of surface modes, we can confine the output field in a small angle interval. This phenomenon is called beaming (46). The principle of the beaming has been explained clearly in literatures (47). To achieve good beaming, a photonic crystal waveguide need a surface layer to support surface modes and a grating layer to coupling the evanescent surface modes into propagation modes. A metallic beaming structure is generally a subwavelength waveguide surrounded by periodic structures such as grooves or dielectric gratings (53; 54). The flat metal surface supports the surface mode, so additional surface layer is not necessary. The periodic structures work as the grating layer.« less

We report experiments that demonstrate the quantization of the conductance of photonic crystal waveguides. To obtain a diffusive wave, we have added all the transmitted channels for all the incident angles. The conductance steps have equal height and a width of one half the wavelength used. Detailed numerical results agree very well with the novel experimental results.

Photonic crystals with a finite size can support surface modes when appropriately terminated. We calculate the dispersion curves of surface modes for different terminations using the plane wave expansion method. These non-radiative surface modes can be excited with the help of attenuated total reflection technique. We did experiments and simulations to trace the surface band curve, both in good agreement with the numerical calculations.

We have shown that a two-layer dielectric structure can give excellent beaming and enhanced transmission simultaneously of a Gaussian source. The front surface of the layer of dielectric rods supports surface states and the rear grading layer couples the surface states to radiation modes. By repeating periodically this two-layer structure, one can obtain excellent beaming and enhanced transmission for very long distances.