Optically-Triggered Nanoscale Memory Effect in a Hybrid Plasmonic-Phase Changing Nanostructure

Nanoscale devices, such as all-optical modulators and electro-optical transducers, can be implemented in heterostructures that integrate plasmonic nanostructures with functional active materials. Here we demonstrate all-optical control of a nanoscale memory effect in such a heterostructure by coupling the localized surface plasmon resonance (LSPR) of gold nanodisk arrays to a phase-changing material (PCM), vanadium dioxide (VO<inf>2</inf>). By latching the VO<inf>2</inf> in a distinct correlated metallic state during the insulator-to-metal transition (IMT), while concurrently exciting the hybrid nanostructure with one or more ultraviolet optical pulses, the entire phase space of this correlated state can be accessed optically to modulate the plasmon response. We find that the LSPR modulation depends strongly but linearly on the initial latched state, suggesting that the memory effect encoded in the plasmon resonance wavelength is linked to the strongly correlated electron states of the VO<inf>2</inf>. The continuous, linear variation of the electronic and optical properties of these model heterostructures opens the way to multiple design strategies for hybrid devices with novel optoelectronic functionalities, which can be controlled by an applied electric or optical field, strain, injected charge, or temperature.Department of Applied Physic

Antenna-assisted picosecond control of nanoscale phase transition in vanadium dioxide

Nanoscale devices in which the interaction with light can be configured using external control signals hold great interest for next-generation optoelectronic circuits. Materials exhibiting a structural or electronic phase transition offer a large modulation contrast with multi-level optical switching and memory functionalities. In addition, plasmonic nanoantennas can provide an efficient enhancement mechanism for both the optically induced excitation and the readout of materials strategically positioned in their local environment. Here, we demonstrate picosecond all-optical switching of the local phase transition in plasmonic antenna-vanadium dioxide (VO2) hybrids, exploiting strong resonant field enhancement and selective optical pumping in plasmonic hotspots. Polarization- and wavelength-dependent pump-probe spectroscopy of multifrequency crossed antenna arrays shows that nanoscale optical switching in plasmonic hotspots does not affect neighboring antennas placed within 100 nm of the excited antennas. The antenna-assisted pumping mechanism is confirmed by numerical model calculations of the resonant, antenna-mediated local heating on a picosecond time scale. The hybrid, nanoscale excitation mechanism results in 20 times reduced switching energies and 5 times faster recovery times than a VO2 film without antennas, enabling fully reversible switching at over two million cycles per second and at local switching energies in the picojoule range. The hybrid solution of antennas and VO2 provides a conceptual framework to merge the field localization and phase-transition response, enabling precise, nanoscale optical memory functionalities

4D Imaging and Diffraction Dynamics of Single-Particle Phase Transition in Heterogeneous Ensembles

In this Letter, we introduce conical-scanning dark-field imaging in four-dimensional (4D) ultrafast electron microscopy to visualize single-particle dynamics of a polycrystalline ensemble undergoing phase transitions. Specifically, the ultrafast metal–insulator phase transition of vanadium dioxide is induced using laser excitation and followed by taking electron-pulsed, time-resolved images and diffraction patterns. The single-particle selectivity is achieved by identifying the origin of all constituent Bragg spots on Debye–Scherrer rings from the ensemble. Orientation mapping and dynamic scattering simulation of the electron diffraction patterns in the monoclinic and tetragonal phase during the transition confirm the observed behavior of Bragg spots change with time. We found that the threshold temperature for phase recovery increases with increasing particle sizes and we quantified the observation through a theoretical model developed for single-particle phase transitions. The reported methodology of conical scanning, orientation mapping in 4D imaging promises to be powerful for heterogeneous ensemble, as it enables imaging and diffraction at a given time with a full archive of structural information for each particle, for example, size, morphology, and orientation while minimizing radiation damage to the specimen

Bacterial Ghosts of Escherichia coli Drive Efficient Maturation of Bovine Monocyte-Derived Dendritic Cells.

Bacterial ghosts (BGs) are empty cell envelopes derived from Gram-negative bacteria. They not only represent a potential platform for development of novel vaccines but also provide a tool for efficient adjuvant and antigen delivery system. In the present study, we investigated the interaction between BGs of Escherichia coli (E. coli) and bovine monocyte-derived dendritic cells (MoDCs). MoDCs are highly potent antigen-presenting cells and have the potential to act as a powerful tool for manipulating the immune system. We generated bovine MoDCs in vitro from blood monocytes using E. coli expressed bovine GM-CSF and IL-4 cytokines. These MoDCs displayed typical morphology and functions similar to DCs. We further investigated the E. coli BGs to induce maturation of bovine MoDCs in comparison to E. coli lipopolysaccharide (LPS). We observed the maturation marker molecules such as MHC-II, CD80 and CD86 were induced early and at higher levels in BG stimulated MoDCs as compared to the LPS stimulated MoDCs. BG mediated stimulation induced significantly higher levels of cytokine expression in bovine MoDCs than LPS. Both pro-inflammatory (IL-12 and TNF-α) and anti-inflammatory (IL-10) cytokines were induced in MoDCs after BGs stimulation. We further analysed the effects of BGs on the bovine MoDCs in an allogenic mixed lymphocyte reaction (MLR). We found the BG-treated bovine MoDCs had significantly (p<0.05) higher capacity to stimulate allogenic T cell proliferation in MLR as compared to the LPS. Taken together, these findings demonstrate the E. coli BGs induce a strong activation and maturation of bovine MoDCs

Effects of high-density lipoproteins on pancreatic beta-cell insulin secretion.

OBJECTIVE: Type 2 diabetes is characterized by impaired beta-cell secretory function, insulin resistance, reduced high-density lipoprotein (HDL) levels, and increased cardiovascular risk. Given the current interest in therapeutic interventions that raise HDLs levels, this study investigates the effects of HDLs on insulin secretion from beta-cells. METHODS AND RESULTS: Incubation of Min6 cells and primary islets under basal or high-glucose conditions with either apolipoprotein (apo) A-I or apoA-II in the lipid-free form, as a constituent of discoidal reconstituted HDLs (rHDLs), or with HDLs isolated from human plasma increased insulin secretion up to 5-fold in a calcium-dependent manner. The increase was time and concentration dependent. It was also K(ATP) channel and glucose metabolism dependent under high-glucose, but not low-glucose, conditions. The lipid-free apolipoprotein-mediated increase in insulin secretion was ATP binding cassette (ABC) transporter A1 and scavenger receptor-B1 dependent. The rHDL-mediated increase in insulin secretion was ABCG1 dependent. Exposure of beta-cells to lipid-free apolipoproteins also increased insulin mRNA expression and insulin secretion without significantly depleting intracellular insulin or cholesterol levels. CONCLUSIONS: These results establish that lipid-free and lipid-associated apoA-I and apoA-II increase beta-cell insulin secretion and indicate that interventions that raise HDLs levels may be beneficial in type 2 diabetes

Tracking the evolution of electronic and structural properties of VO₂ during the ultrafast photoinduced insulator-metal transition

We present a detailed study of the photoinduced insulator-metal transition in VO2 with broadband time-resolved reflection spectroscopy. This allows us to separate the response of the lattice vibrations from the electronic dynamics and observe their individual evolution. When we excite VO2 above the photoinduced phase transition threshold, we find that the restoring forces that describe the ground-state monoclinic structure are lost during the excitation process, suggesting that an ultrafast change in the lattice potential drives the structural transition. However, by performing a series of pump-probe measurements during the nonequilibrium transition, we observe that the electronic properties of the material evolve on a different, slower time scale. This separation of time scales suggests that the early state of VO2, immediately after photoexcitation, is a nonequilibrium state that is not well defined by either the insulating or the metallic phase

Ultrafast Dynamics of the VO

Non-degenerate pump (1.5 eV)-probe (0.4 eV) transmission spectroscopy on vanadium dioxide films grown on glass and three different sapphire substrates shows systematic variations with substrate that correlate with VO2 grain size and laser fluence. Temperature dependent measurements showed changes in the electronic response that is proportional to the metallic fraction

Plasma-Corona-Processed Nanostructured Coating for Thermoregulative Textiles

A rapid increase in the atmospheric temperature has been reported in recent years worldwide. The lack of proper aid to protect from exposure to the sun during working hours has raised the number of sunburn cases among workers. It is important to promote productive workplaces without compromising safety and health concerns. In the present work, we report the low-temperature plasma (LTP)-assisted tailoring of the surface properties of fabrics to reflect IR radiation from the sun. The LTP technique can be adapted for thermally sensitive materials such as fabrics and textiles due to its lower working temperature range of 30 °C. We have modified various substrates such as commercially available fabric, regular, and boron nitride-incorporated electrospun PET surfaces with tetraethoxy orthosilicate (TEOS) plasma. TEOS plasma treatment can deposit a reactive plasma-polymerized silane nanolayer on the surface of these substrates. The plasma-processed silane nanolayer was systematically characterized using scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, Keyence 3D-microscopic imaging, and transmission electron microscopy (TEM). From the SEM and TEM data, the size of the nanoparticles was observed in the range 100–200 nm. The thermal regulation coating thickness was examined with a Keyence 3D imaging technique. The IR reflection potential of the surface was analyzed by using an FLIR thermal imaging system. The data revealed that the plasma-modeled nanosurface shows higher reflective potential toward IR rays, and it seems to be cooler than the unprocessed surface by approximately 15 °C. The stability and efficiency of the plasma-modified electrospun nanolayer in water were satisfactorily examined with SEM and IR imaging. Taken together, these results suggest the excellent potential of plasma processing to develop IR reflective coatings

Ultrafast Phase Transition in Vanadium Dioxide Driven by Hot-Electron Injection

We present a novel all-optical method of triggering the phase transition in vanadium dioxide by means of ballistic electrons injected across the interface between a mesh of Au nanoparticles coveringd VO2 nanoislands. By performing non-degenerate pump-probe transmission spectroscopy on this hybrid plasmonic/phase-changing nanostructure, structural and electronic dynamics can be retrieved and compared