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The silk fibers produced by Bombyx mori, the domestic silkworm, has been prized for millennia as a strong yet lightweight and luxurious material. Although synthetic polymers like nylon and polyester are less costly, they do not compare to silk’s natural qualities and mechanical properties. And according to research from the University of Pittsburgh’s Swanson School of Engineering, silk combined with carbon nanotubes may lead to a new generation of biomedical devices and so-called transient, biodegradable electronics.

“Silk is a very interesting material. It is made of natural fibers that humans have been using for thousands of years to make high quality textiles, but we as engineers have recently started to appreciate silk’s potential for many emerging applications such as flexible bioelectronics due to its unique biocompatibility, biodegradability and mechanical flexibility,” noted Mostafa Bedewy, assistant professor of industrial engineering at the Swanson School and lead author of the paper. “The issue is that if we want to use silk for such applications, we don’t want it to be in the form of fibers. Rather, we want to regenerate silk proteins, called fibroins, in the form of films that exhibit desired optical, mechanical and chemical properties.”

As explained by the authors in the video below, these regenerated silk fibroins (RSFs) however typically are chemically unstable in water and suffer from inferior mechanical properties, owing to the difficulty in precisely controlling the molecular structure of the fibroin proteins in RSF films. Bedewy and his NanoProduct Lab group, which also work extensively on carbon nanotubes (CNTs), thought that perhaps the molecular interactions between nanotubes and fibroins could enable “tuning” the structure of RSF proteins.
“One of the interesting aspects of CNTs is that, when they are dispersed in a polymer matrix and exposed to microwave radiation, they locally heat up,” Dr. Bedewy explained. “So we wondered whether we could leverage this unique phenomenon to create desired transformations in the fibroin structure around the CNTs in an “RSF-CNT” composite.”
According to Dr. Bedewy, the microwave irradiation, coupled with a solvent vapor treatment, provided a unique control mechanism for the protein structure and resulted in a flexible and transparent film comparable to synthetic polymers but one that could be both more sustainable and degradable. These RSF-CNT films have potential for use in flexible electronics, biomedical devices and transient electronics such as sensors that would be used for a desired period inside the body ranging from hours to weeks, and then naturally dissolve.
“We are excited about advancing this work further in the future, as we are looking forward to developing the science and technology aspects of these unique functional materials,” Dr. Bedewy said. ” From a scientific perspective, there is still a lot more to understand about the molecular interactions between the functionalization on nanotube surfaces and protein molecules. From an engineering perspective, we want to develop scalable manufacturing processes for taking cocoons of natural silk and transforming them into functional thin films for next generation wearable and implantable electronic devices.”

For more information: https://www.sciencedaily.com/releases/2018/10/181030121927.htm

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Congratulations to our new paper “Detecting the thermoplasmonic effect using ellipsometry parameters for self-assembled gold nanoparticles within a polydimethylsiloxane matrix” by Maher Abdulfadhil Gatea, Hussein A. Jawad,· S. M. Hamidi.
Light-to-heat conversion using active plasmonic materials is essential in wide-ranging applications, such as sensing, photonics, drug delivery, biomedical imaging, photothermal tumor therapy, and optoelectronics. In this work, we studied the thermoplasmonic effect and performed an optical analysis of different concentrations of self-assembled gold nanoparticles in transparent dielectric polydimethylsiloxane polymer medium under continuous-wave radiations. Composite samples were prepared and investigated experimentally by ellipsometry method and thermal photography. The gold-nanoparticle content of each composite film directly increased the generated temperature. The structure and optical properties of the samples under ambient conditions were obtained efficiently depending on the ellipsometry parameters for each polarized light. Results showed that the prolongation of the incubation time led to a lower phase value for p polarization than that for s polarization. This finding was due to the change in ellipsometry parameters and, thus, the thermoplasmonic effect. This new method of investigating the thermoplasmonic effect can provide new insights into the medical applications of
plasmonic media.

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RIVERSIDE, Calif., Jan. 8, 2019 — An international research team has discovered a new mechanism for ultra-efficient charge and energy flow in pristine graphene. The team was co-led by professor Nathaniel Gabor from the University of California, Riverside.
The researchers fabricated graphene with no impurities (pristine graphene) into different geometric shapes, connecting narrow ribbons and crosses of graphene to rectangular regions of the material. They found that when light was shined on constricted areas of the graphene, a large photocurrent was created. The photocurrent occurred in a parameter regime that was different from previously observed photothermoelectric or photovoltaic photocurrents in graphene. In the pristine graphene, the photocurrent emerged exclusively at the charge neutrality point. 



Although graphene has been studied vigorously for more than a decade, new measurements on high-performance graphene devices have revealed yet another unusual property. In ultraclean graphene sheets, energy can flow over great distances, giving rise to an unprecedented response to light. Courtesy of Max Grossnickle and QMO Labs, UC Riverside.

“We found that photocurrents may arise in pristine graphene under a special condition in which the entire sheet of graphene is completely free of excess electronic charge,” Gabor said. “Generating the photocurrent requires no special junctions and can instead be controlled, surprisingly, by simply cutting and shaping the graphene sheet into unusual configurations, from ladder-like linear arrays of contacts, to narrowly constricted rectangles, to tapered and terraced edges.”
In most solar energy harvesting devices, a photocurrent arises only in the presence of a junction between two dissimilar materials, such as p-n junctions. The electrical current is generated in the junction region and moves through the distinct regions of the two materials. In the pristine graphene, the photocurrent emerged near the edges and corners of the material.

In principle, graphene can absorb light at any frequency, making it suitable for IR and other types of photodetection. The discovery that pristine graphene can convert light into electricity efficiently could lead to more efficient, faster photodetectors, and potentially more efficient solar panels. 

The researchers have found evidence that the use of pristine graphene to generate photocurrent engenders a greatly enhanced photoresponse in the IR regime and results in ultrafast operation speeds. “We plan to further study this effect in a broad range of IR and other frequencies, and measure its response speed,” said researcher Qiong Ma from the Massachusetts Institute of Technology. 

For more information:
https://doi.org/10.1038/s41565-018-0323-8

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ROME, Jan. 3, 2019 — Scientists at the European Molecular Biology Laboratory (EMBL) in Rome have used light to stop itch — at best an annoyance and at worst an uncomfortable chronic symptom — in mice. They used NIR light to activate a phototoxic agent that selectively targets itch-sensing cells, which are located in the upper surface of the skin. When the agent is injected into a mouse’s affected skin area and the area is illuminated with the NIR light, the itch-sensing cells withdraw from the skin, reducing itch-associated behaviors in the mouse and allowing the skin to heal. The researchers said that the effect of the treatment can last several months.

The light-sensitive agent binds only to those nerve cells that sense itch. Other types of nerve cells in the skin, which cause sensations like pain, vibration, cold, or heat, are not affected by the light treatment.

The researchers said that the method works well in mice with eczema (atopic dermatitis) and in mice with amyloidosis (familial primary localized cutaneous amyloidosis), a genetic skin disease for which there is currently no cure. “For me, the most exciting part of this project was seeing the improvements in the animals’ health,” said researcher Linda Nocchi. “Their skin looked much better after treatment and they scratched less.” “We hope that one day, our method will be able to help humans suffering from a disease like eczema, which causes chronic itching,” said group leader Paul Heppenstall. It is already known that mice and humans have the same target molecule for itch therapy — a small protein called interleukin 31 (IL-31). One of the team’s next steps will be to test the light therapy in human tissues.

Previously, the Heppenstall group published a method to manage chronic pain with light. “We think that the mechanism we’ve discovered might be a general method for controlling sensation through the skin. Our goal now is to take these therapies further. We want to collaborate with industry partners to develop therapies for humans, but also for veterinary medicine, as itch is a major problem in dogs as well,” Heppenstall said.

For more information: https://doi.org/10.1038/s41551-018-0328-5

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Plasmons, collective oscillations of electron systems, can efficiently couple light and electric current, and thus can be used to create sub-wavelength photodetectors, radiation mixers, and on-chip spectrometers. Despite considerable effort, it has proven challenging to implement plasmonic devices operating at terahertz frequencies. The material capable to meet this challenge is graphene as it supports long-lived electrically tunable plasmons. Here we demonstrate plasmon-assisted resonant detection of terahertz radiation by antenna-coupled graphene transistors that act as both plasmonic Fabry-Perot cavities and rectifying elements. By varying the plasmon velocity using gate voltage, we tune our detectors between multiple resonant modes and exploit this functionality to measure plasmon wavelength and lifetime in bilayer graphene as well as to probe collective modes in its moiré minibands. Our devices offer a convenient tool for further plasmonic research that is often exceedingly difficult under non-ambient conditions (e.g. cryogenic temperatures) and promise a viable route for various photonic applications.

Graphene-based THz detectors. a Schematics of the encapsulated BLG FET used in this work. b 3D rendering of our resonant photodetector. THz radiation is focused to a broadband bow-tie antenna by a hemispherical silicon lens yielding modulation of the gate-to-source voltage, as indicated in ac Optical photograph of one of our photodetectors. Scale bar is 200 μm. d Conductance of one of our BLG FETs as a function of the gate voltage Vg, measured at a few selected temperatures. Inset: zoomed-in photograph of c showing a two-terminal FET with gate and source terminals connected to the antenna. Scale bar is 10 μm.

Broadband operation

We intentionally start the photoresponse measurements at the low end of the sub-THz domain, where the plasma oscillations are overdamped (see below). This allows us to compare the performance of our detectors with previous reports.

Plasmon-assisted THz photodetection. a Responsivity measured at f = 130 GHz and three representative temperatures. Orange rectangle highlights an offset stemming from the rectification of incident radiation at the p-n junction between the p-doped graphene channel and the n-doped area near the contact. Upper inset: FET-factor F as a function of Vg at the same T. Lower inset: maximum Ra as a function of Tb Gate dependence of responsivity recorded under 2 THz radiation. The upper inset shows a zoomed-in region of the photovoltage for electron doping. Resonances are indicated by black arrows. Lower inset: resonant responsivity at liquid-nitrogen temperature.

Resonant responsivity is a universal phenomenon in ultra-clean graphene devices and is expected to be independent of the physical mechanisms behind the rectification of the ac field into a dc photovoltage. Nevertheless, it is important to establish possible nonlinearities responsible for the rectification, for example, in order to be able to increase the magnitude of responsivity. We first note that the aforementioned asymmetry in Ra (Vg) between electron and hole doping indicates rectification at the pn junction formed in vicinity of the contacts. This rectification usually appears due to the thermoelectric effect arising as a result of non-uniform sample heating and the difference between the Seebeck coefficients in the graphene channel and contact regions.

For more information: https://www.nature.com/articles/s41467-018-07848-w

 

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Wavelength selective spin current is generated in the mid infrared rage by combining plasmonic metamaterial absorbers with platinum/yttrium-iron-garnet spintronic devices. The wavelength selectivity is attributed to the plsmonic resonance of the metamaterial absorber.

For more information: https://www.osapublishing.org/abstract.cfm?uri=CLEO_QELS-2018-FF2F.8

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Scientists at a Chinese university have designed an infrared sensor that exploits the plasmonic properties of graphene to detect multiple wavelengths.

Exploring plasmons and graphene

Many experiments with nanoscale sensor design have explored the possibilities of surface plasmon polaritons (SPPs)—electromagnetic surface waves stimulated by light. The resonance of the surface plasmons can shift dramatically due to small changes in the refractive index of the sensing medium. SPP experiments often use noble metals as plasmonic surfaces, but these materials respond only to visible wavelengths of light. Some previous sensors designed with patterned metamaterials detect only one particular frequency that is fixed at the time the sensor is built. The team at China Jiliang University in Hangzhou turned to graphene, which interacts well with infrared light, and proposed a design that employs it as the plasmonic material atop a dielectric substrate of calcium fluoride. Using computer simulations, the scientists explored what would happen if they shaped the graphene into 40-nm-radius disks, with each disk containing a small, off-center circular defect in its crystalline structure. An ion-gel layer on top of the graphene disks delivers a bias voltage to the setup.

Introducing defects

The presence of the defect stimulates a phenomenon called plasmon hybridization, which produces dual-band resonance peaks in the mid-infrared spectrum. Moving the location of the defects within the disks in the x-y plane and measuring the resulting changes in the transmission spectra shows that the array is polarization-sensitive. Scientists can change the sensitivity of the sensor to detect different substances by adjusting the applied voltage. Putting the sensor near a substance of interest changes the refractive index of the sensor, thus registering a detection.

The China Jiliang team reported that the sensitivity of their proposed design reached 550 cm–1 per refractive index unit. Researchers from Zhejiang University of Technology in China and the Technical University of Denmark also contributed to the study.

For more information: doi: 10.1364/OME.9.000035

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Congratulations to our new paper ” Unexpected large transverse magneto-optic Kerr effect at quasi-normal incidence in commercial disk-based magnetoplasmonic crystals” by Cichelero, M. A. Oskuei, M. V. Kataja, S.M. Hamidi, G. Herranz.

We investigate the transverse magneto-optic Kerr effect (TMOKE) of magnetoplasmonic crystals grown on top of commercial disks. From full angle-resolved scans we can identify Wood’s anomalies related to the excitation of plasmons of different orders. From these maps we also detect a wide range of wavelengths and angles of incidence for which the TMOKE signal is increased due to the interaction of light with plasmons. Remarkably, conditions are established for unexpectedly large responses at quasi-normal incidence, where, by fundamental symmetry reasons, the intrinsic TMOKE is insignificant. The key towards this unexpected outcome is to engineer the geometry of magnetoplasmonic crystals, so that first-order plasmon dispersion lines run up towards quasi-normal angles of incidence. These results provide general rules for magneto-optic enhancement and, in particular, show the potential of standard commercial disks as platforms for enhanced magneto-optic devices.

 

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Congratulations to our new paper “Rectangular plasmonic interferometer for high sensitive glycerol sensor” by Zahra Khajemiri , Dukhyung Lee, Seyedeh Mehri Hamidi , and Dai-Sik Kim.

A novel plasmonic interferometric sensor intended for application to biochemical sensing has been investigated experimentally and theoretically. The sensor was included a slit surrounded by rectangular grooves using a thick gold film. A three-dimensional finite difference time-domain commercial software package was applied to simulate the structure. The Focused ion beam milling has been used as a mean to fabricate series of rectangular plasmonic interferometer with varying slit-groove distance L. Oscillation behavior is shown by transmission spectra in a broadband wavelength range between 400 nm and 800 nm in the distance between slit and grooves. Red-shifted interference spectrum is the result of increasing refractive indices. The proposed structure is functional from visible to near-infrared wavelength range and yields a sensitivity of 4923 nm/RIU and a figure of merit as high as 214 at 729 nm wavelength. In conclusion, this study indicates the possibility of fabricating a low cost, compact, and real-time high-throughput plasmonic interferometer.

 

Figure 2. (a) Microfluidic fabrication process. (b)  SEM image of the 7 × 7 fabricated plasmonic interferometer array. The center-to-center distance between each interferometer is 2 μm, and the sensor array footprint is 14 × 14 μm2. Scale bar: 10 μm. (c) One of the interferometers .Scale bar: 2 μm. (d)The shematic of our measurement. (e) Normalized transmission as a function of number of grooves, ‘n’ at period of 200 nm for L= 850 nm.

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Strong coupling between two resonance modes leads to the formation of new hybrid modes exhibiting disparate characteristics owing to the reversible exchange of information between different uncoupled modes. Here, we realize the strong coupling between the localized surface plasmon resonance and surface plasmon polariton Bloch wave using multilayer nanostructures. An anticrossing behavior with a splitting energy of 144 meV can be observed from the far-field spectra. More importantly, we investigate the near-field properties in both the frequency and time domains using photoemission electron microscopy. In the frequency domain, the near-field spectra visually demonstrate normal-mode splitting and display the extent of coupling. Importantly, the variation of the dephasing time of the hybrid modes against the detuning is observed directly in the time domain. These findings signify the evolution of the dissipation and the exchange of information in plasmonic strong coupling systems and pave the way to manipulate the dephasing time of plasmon modes, which can benefit many applications of plasmonics.

Structural characterization. The structure designed to realize the strong coupling is shown in Fig. 1. A 20-nm-thick gold film is deposited on an indium-tin-oxide (ITO)-coated glass substrate to support the SPP-Bloch wave. The ITO layer has a thickness of 150 nm, which makes the entire substrate surface suitably conductive for PEEM measurements. Then, a 25-nm-thick Al2O3 spacer is deposited using the atomic layer deposition technique.
The gold square nanoblock arrays are fabricated on the Al2O3 spacer via electron-beam lithography (EBL), followed by metal sputtering and lift-off, to support the LSPR modes. The sectional view (Fig. 1b) and top view (Fig. 1c) of the sample are acquired by a scanning transmission electron microscope (STEM) and a scanning electron microscope (SEM), respectively. In addition, energy-dispersive X-ray spectroscopy (EDS) is used to mark
different elements with a distinct color in the sectional view (Fig. 1d). The nanoblocks of different sizes (side lengths) are designed (100–160 nm) to tune the LSPR energy. Beyond that, the nanoblock array can provide the additional wave vector for the excitation light (K0) to excite the SPP supported on the thin metal by compensating the momentum mismatch between the excitation light and the SPP modes.

Experimental far-field spectral property. The measured extinction spectra of samples with different nanoblock sizes and fixed periods (400 or 500 nm) are presented in Fig. 2a, b, respectively. For the period of 400 nm, the left peak is almost entirely unshifted, and the right peak undergoes a redshift as the nanoblock size increases. The left and right peaks can be assigned to the SPP Bloch wave and LSPR mode, respectively. Moreover, in this case the two modes cannot couple well with each other, as is clearly shown by the dispersion curves of the two modes (Fig. 2c), where the SPP modes are kept unchanged while the nanoblock sizes change. Similarly, the dissipation of the LSPR mode (γLSPR = 98 meV) and the SPP-Bloch waves (γSPP = 38 meV) can be calculated from the experimental line widths with a period of 400 nm and a nanoblock size of 135 nm. For the period of 500 nm, the dispersion curves (Fig. 2d) extracted from the extinction spectra show an anticrossing behavior and can be fitted by the coupled oscillator model9,22,26 (details are shown in the Supplementary Note 1). The
splitting energy is calculated as 144 meV at ELSPR = ESPP. Then, we can determine that the interaction potential (V) is 78 meV.

Simulation results. To further understand these modes, we use the finite-difference time domain (FDTD) method to simulate the mode distribution. With the large nanoblock size (150 nm) and the small period (400 nm), two peaks appear on the extinction spectrum (blue line in Fig. 3a). Peak 1 has a narrow line width, and the electric field is confined mainly on the lower surface of the Au film. Peak 2 has a broad line width, and the electric field is located mainly at the interface between the nanoblocks and Al2O3, with much greater field enhancement, as shown in Fig. Therefore, we recognize that peaks 1 and 2 represent the SPPBloch wave and LSPR mode, respectively, and that the detuning between the LSPR mode and SPP-Bloch wave is large (~396 meV) in this case. The energy exchange gives rise to a higher near-field enhancement than the single SPP-Bloch wave and a longer oscillation time than the single LSPR mode, which demonstrates that the coupling between the LSPR mode and SPP-Bloch wave modifies the field distribution resulting from the normal-mode splitting with the small detuning. Notably, the near-field enhancement of  the two coupled modes (peaks 3 and 4) is both present and, in fact, slightly higher than that of the LSPR mode only (peak 2), which is shown by Fig.

For more information: DOI: 10.1038/s41467-018-07356-x


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