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Researchers in China and the United States have reported the creation of upconversion nanoparticles (UCNPs) that can latch onto retinal photoreceptors and serve as tiny antennae for otherwise invisible near-infrared (NIR) light, converting it into a visible signal .
 The research team found that mice injected with the photoreceptor-binding nanoparticles were able not only to perceive NIR radiation, but even to distinguish between different shapes in the dark based only on their NIR signals.

A mammalian limitation

Like other mammals, including humans, mice can see only in the visible part of the spectrum, ranging from wavelengths of 400 to 700 nm. They perceive the visible world through light-absorbing pigments in retinal photoreceptors—the well-known rods, which handle low-light, monochrome vision, and the cones, which are generally responsible for color vision in comparatively brightly lit conditions.
The researchers behind the new work, led by Tian Xue and Jin Bao at the University of Science and Technology of China and Gang Han at the University of Massachusetts Medical School, USA, hit upon a different approach. Instead of using wearable goggles to convert NIR light to visible light, one could do the same thing by adding a tiny NIR antenna to the photoreceptor itself.


Upconversion nanoparticles were coated with a polymer and attached to proteins that bound to retinal photoreceptors. Tests showed that the nanoparticles, injected into the retinas of lab mice, allowed the mice to see infrared radiation as green light.


The team began by chemically creating NIR-sensitive UCNPs—core-shell nanoparticles tuned to have an excitation peak at 980 nm, firmly in the NIR, and to re-emit light at 535 nm, in the green part of the visible-light spectrum. Next, they coated the UCNPs with poly-acrylic acid, to which they then tied a protein strand, concanavalin A (ConA). The researchers chose ConA specifically for its known ability to grab onto sugar residues in the outer segments of the mouse photoreceptors. Spectroscopy confirmed that the ConA strands had bound tightly to the nanoparticles.

Next, the researchers put the mice, with their souped-up retinas, through a number of tests, to see what impact the nanoparticles had on their vision. First, they found that the pupils of mice with the nanoparticle-treated retinas contracted when illuminated with 980-nm light, while the pupils of control mice showed no reaction. Next, they trained both UCNP-treated and control mice to expect a mild shock when confronted with a 20-second green-light pulse at 535 nm, the emission wavelength of the UCNPs, and thus to show a “freezing” behavior upon such a light flash. The treated mice showed the freezing behavior when stimulated with 980-nm (NIR) light or with 535-nm (visible) light, whereas the control mice froze only when stimulated with the 535 light. That clearly indicated that the UCNPs were absorbing the 980-nm light, and converting it to a visible-light signal that could be sopped up by the attached photoreceptors and sent to the mouse brain to interpret.


In a maze test, mice treated with the nanoparticles were able to distinguish between different shapes projected in only infrared light.

The team found that one injection of the nanoparticles gave the mice some level of NIR vision for up to ten weeks after the treatment. And tests months after the treatment, according to the team, suggested that the injections did not cause any long-term damage to the animals’ retinas. While much additional work clearly remains to be done, the research team is bullish about the prospects for its technology. In addition to creating a potential alternative to night-vision goggles for some military, encryption and security applications, the team points out that, by using different kinds of UCNPs, the treatment could help repair certain vision problems, such as red color-vision defects. And for other eye diseases, the nanoparticles could, the team believes, serve as targeted drug-delivery systems that could release small molecules locally to photoreceptors on stimulation with light of a specific wavelength. Co-P.I. Geng Han, in a press release accompanying the research, pitched the potential even further. “It is very likely that the sky may look very differently both at night and in daytime,” he said. “We may have the capability to view all the hidden information from NIR and IR radiation in the universe which is invisible to our naked eyes.”

For more information: doi: 10.1016/j.cell.2019.01.038

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Enhancing magneto-optical effects is crucial for size reduction of key photonic devices based on non-reciprocal propagation of light and to enable active nanophotonics. We disclose a so far unexplored approach that exploits dark plasmons to produce an unprecedented amplification of magneto-optical activity. We designed and fabricated non-concentric magnetoplasmonic-disk/plasmonic-ring-resonator nanocavities supporting multipolar dark modes. The broken geometrical symmetry of the design enables coupling with free-space light and hybridization of dark modes of the ring nanoresonator with the dipolar localized plasmon resonance of the magnetoplasmonic disk. Such hybridization generates a multipolar resonance that amplifies the magneto-optical response of the nanocavity by ~1-order of magnitude with respect to the maximum enhancement achievable by localized plasmons in bare magnetoplasmonic nanoantennas. This large amplification results from the peculiar and enhanced electrodynamic response of the nanocavity, yielding an intense magnetically-activated radiant magneto-optical dipole driven by the low-radiant multipolar resonance. The concept proposed is general and, therefore, our results open a new path that can revitalize research and applications of magnetoplasmonics to active nanophotonics and flat optics.

Magnetoplasmonic NCRD ferromagnetic-nanoantenna/gold-nanocavity and parent Py-DI and Au-RI nanostructures. a Schematic of the NCRD hybrid structure with its four geometric characteristic parameters. b Atomic force and c SEM images of the individual NCRD nanocavity and array. SEM images of the parent single d Py-DI, e Au-RI constituents and arrays. The scale bars in panels c-e correspond to 5 μm and those in their insets to 100 nm.

Both experimental and simulated spectra of the NCRD array display two strongly marked dips located at 600 and 1650 nm and a weaker dip at 820 nm. A comparison with the spectra (simulated and experimental) of the array of bare Au-RI and a close inspection of the spectral dependence of calculated surface charge distribution maps in Fig. 2c, reveal that the two most prominent dips in the NCRD correspond to the excitation of the so-called antibonding and bonding plasmonic resonances in the Au ring portion of the nanocavity at 600 nm and 1650, respectively.

Au-RI and NCRD optical properties and electrodynamics. a Simulated and b experimental transmittance spectra for the NCRD, Py-DI and Au-RI structures. Dashed lines mark the major features in the spectra at 600, 820 and 1600 nm. The small black arrow in panel a highlights a minor feature due to the weak far-field diffractive coupling in a simulated periodic array of defect-less structures. c Surface charge density maps (see Methods) for the Au-RI and NCRD structures at 600, 820 and 1600 nm, normalized to the map at 820 nm for the NCRD for direct comparison. Simulations in panel c are carried out using linearly polarized electromagnetic radiation as indicated by the black arrow (Ei = 1V/m). The surface charge density for the Au-RI at 820 nm has been multiplied by a factor 10 for visualization purposes.

We have demonstrated that high-order multi-polar dark plasmon resonances in magnetoplasmonic nanocavities can be utilized to achieve unprecedented enhancement of the magneto-activated optical response, beyond the present limitations of magnetoplasmonic nanoantennas, enabling a far more efficient active control of the light polarization under weak magnetic fields. The superior behavior of geometrical symmetry broken magnetoplasmonic nanocavities, as compared to corresponding nanoantennas, is explained by the generation of largely enhanced magnetic-field-induced radiant dipole in the magnetoplasmonic nanoantenna driven by a hybrid low-radiant multipolar Fano resonance mode. Therefore, in this novel design, a large enhancement of the magneto-optical response, i.e., the magneto-activated electrical dipole inducing the light polarization modification, is achieved without a significant increase of the pure optical response thanks to the low-radiant character of the hybrid mode. As a result, in the NCRD magnetoplasmonic nanocavity the MOA is additionally amplified by ~1-order of magnitude with respect to the parent Py-DI structure. The novel concept unveiled here opens a fresh path towards applications of magnetoplasmonics to a variety of fields ranging from flat and active nanophotonics to sensing. Therefore, this exploratory work should catalyze future research. Tuning of dark and bright plasmon modes can be achieved by varying the design and the materials to boost both plasmonics (e.g. using silver instead of gold) and intrinsic MO activity (e.g., employing multilayers Au/Co), as well as tuning the relative spectral position and sharpness of the dark and dipolar modes, and thus of the Fano resonance line shape and intensity. Finally, this mechanism might have a huge impact on forthcoming photonic nanotechnologies based on plasmon-mediated local enhanced manipulation of electronic spin-currents opening excellent perspectives in disclosing novel opto-electronic phenomena.

For more information: https://arxiv.org/abs/1903.08392v1

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CAMBRIDGE, Mass., March 12, 2019 — An optical imaging system developed by MIT researchers could enable physicians to identify tiny tumors deep within the body, leading to earlier detection and treatment of cancer. The researchers call their system DOLPHIN, which stands for “Detection of Optically Luminescent Probes using Hyperspectral and diffuse Imaging in Near-infrared.” The team’s goal with DOLPHIN is to detect cancer earlier by finding tiny tumors in a noninvasive way. 


MIT researchers have devised a way to simultaneously image in multiple wavelengths of near-infrared light, allowing them to determine the depth of particles emitting different wavelengths. Courtesy of X. Dang, N. Bardhan, A. Belcher, et al.

DOLPHIN can be used to image very small groups of cells deep within tissue and without any kind of radioactive labeling. The system uses fluorescent probes that emit light at different NIR wavelengths, depending on the type of doping element that is used. Hyperspectral imaging is used to enable simultaneous imaging in multiple wavelengths of NIR light. Using algorithms they developed, the researchers can analyze the data from the hyperspectral scan to identify the location of fluorescent probes. By analyzing the light from the various wavelength bands within the entire NIR spectrum, the researchers can determine the depth at which a probe is located. 
According to the researchers, to date, the maximum reported depth using second-window NIR (NIR-II: 1000 to 1700 nm) fluorophores is 3.2 cm through tissue. DOLPHIN was able to track a 0.1-mm fluorescent probe through the digestive tract of a living mouse and to detect a signal to a tissue depth of 8 cm. The researchers also demonstrated that they could inject fluorescent particles into the body of a mouse or a rat and then image through the entire animal, to a depth of about 4 cm, to determine where the particles ended up. 
In ongoing work, they are using a related version of this imaging system to try to detect ovarian tumors at an early stage. “Ovarian cancer is a terrible disease, and it gets diagnosed so late because the symptoms are so nondescript,” said professor Angela Belcher. “We want a way to follow recurrence of the tumors, and eventually a way to find and follow early tumors when they first go down the path to cancer or metastasis. This is one of the first steps along the way in terms of developing this technology.” 
The researchers have also begun working on adapting DOLPHIN to detect other types of cancers such as pancreatic cancer, brain cancer, and melanoma. 

For more information:
https://doi.org/10.1038/s41598-019-39502-w

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Writing in Scientific American in 2007, Harry A. Atwater of the California Institute of Technology predicted that a technology he called “plasmonics” could eventually lead to an array of applications, from highly sensitive biological detectors to invisibility cloaks. A decade later various plasmonic technologies are already a commercial reality, and others are transitioning from the laboratory to the market.

These technologies all rely on controlling the interaction between an electromagnetic field and the free electrons in a metal (typically gold or silver) that account for the metal’s conductivity and optical properties. Free electrons on a metal’s surface oscillate collectively when hit by light, forming what is known as surface plasmon. When a piece of metal is large, the free electrons reflect the light that hits them, giving the material its shine. But when a metal measures just a few nanometers, its free electrons are confined in a very small space, limiting the frequency at which they can vibrate. The specific frequency of the oscillation depends on the size of the metal nanoparticle. In a phenomenon called resonance, the plasmon absorbs only the fraction of incoming light that oscillates at the same frequency as the plasmon itself does (reflecting the rest of the light). This surface plasmon resonance can be exploited to create nanoantennas, efficient solar cells and other useful devices.
One of the best studied applications of plasmonic materials is sensors for detecting chemical and biological agents. In one approach, researchers coat a plasmonic nanomaterial with a substance that binds to a molecule of interest—say, a bacterial toxin. In the absence of the toxin, light shining on the material is reemitted at a specific angle. But if the toxin is present, it will alter the frequency of the surface plasmon and, consequently, the angle of the reflected light. This effect can be measured with great accuracy, enabling even trace amounts of the toxin to be detected and measured. Several start-ups are developing products based on this and related approaches—among them an internal sensor for batteries that allows their activity to be monitored to assist in increasing power density and charge rate and a device that can distinguish viral from bacterial infections. Plasmonics is also working its way into magnetic memory storage on disks. For instance, heat-assisted magnetic recording devices increase memory storage by momentarily heating tiny spots on a disk during writing.
In the medical field, light-activated nanoparticles are being tested in clinical trials for their ability to treat cancer. Nanoparticles are infused into the blood, after which they concentrate inside a tumor. Then light of the same frequency as the surface plasmon is shone into the mass, causing the particles to heat by resonance. The heat selectively kills the cancer cells in the tumor without hurting surrounding healthy tissue.

For more information: https://www.scientificamerican.com/article/plasmonic-materials/

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A microchannel incorporated photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor for detection of low refractive index (RI) at near-infrared wavelength is presented in this paper. To attain a simple
and practically feasible mechanism, plasmonic material gold (Au) and sensing medium are placed outside the fiber. A thin layer of TiO2 is employed as an adhesive layer to strongly attach the Au with the silica glass. In the sensing range of 1.22 to 1.37, maximum sensitivities of 51,000 nm/RIU (RI unit) and 1872 RIU−1 are obtained with resolutions of 1.96 × 10−6 and 9.09 × 10−6 RIUs using wavelength and amplitude interrogation
methods, respectively. To the best of the authors’ knowledge, the obtained maximum wavelength sensitivity and resolution are the highest among reported PCF-based SPR sensors to date. The sensor also exhibits a maximum figure of merit of 566. Therefore, the proposed sensor would be an excellent candidate for a wide range of RI detection with higher accuracy for applications such as pharmaceutical inspection and leakage monitoring, bio-sensing, and other low RI analytes.

The surrounding medium adjacent to the sensing layer plays an important role to determine the phase-matching condition at resonance frequency. Moreover, there are other physical phenomena as well that are responsible for altering the phasematching condition of the SPR, namely, interparticle coupling, change in particle size or shape, charging of particles, and change in electron dynamics. The influence of shape can be very complex and leads to shifting resonance frequency toward shorter or longer wavelengths.

Under phase-matching point, also known as resonance condition, a sharp peak loss can be seen at which effective RIs of the fundamental core-guided
mode and SPP mode are equal.

For the analyte having RI of 1.31, Figs. 2(a) and 2(b) represent
the field distribution of the core-guided mode and
plasmonic mode, and Fig. 2(c) shows field distributions at resonance
point. It is seen that the entire optical field is confined
in the core for the core-guided mode, whereas plasmonic mode
is seen between the metal-coated microchannel and sensing
medium.

A highly sensitive PCF-SPR sensor for low RI detection has been proposed and numerically analyzed in this paper. Square lattice and two leaky channels toward the Au-TiO2 coated microchannel have been designed to enhance the resonance effects significantly. As a result, in the sensing range of 1.22–1.37, the proposed sensor exhibits maximum wavelength and amplitude sensitivities of 51000 nm/RIU and 1872 RIU−1 with corresponding resolutions of 1.96 × 10−6 and 9.09 × 10−6 RIU, respectively.
A high FOM of 566 is also exhibited by the sensor. Moreover, incorporation of a microchannel reduces the amount of Au-TiO2 film as well as analyte in order to sense the changes in RI. The lower propagation loss of the proposed sensor is another attractive feature that makes the proposed sensor a well-suited candidate for an integrated SPR sensor, such as a
lab-on-fiber technology.

For more information: https://doi.org/10.1364/AO.58.001547

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DURHAM, N.C., Feb. 19, 2019 — With small adjustments, a near-perfect absorber of electromagnetic waves can be changed into a coherent perfect absorber (CPA), a device that absorbs coherent light and shows near-zero reflectance and high absorption. A CPA, also known as a time-reversed laser, absorbs all of the energy from two identical electromagnetic waves in synchrony. The waves are absorbed as they enter the material from either side at precisely the same time. 


The width, height, and spacing of the cylinders depicted here dictate how the metamaterial described in the new paper absorbs electromagnetic energy. Courtesy of Kebin Fan, Duke University.

This metamaterial features a zirconia ceramic built into a surface dimpled with cylinders, like the face of a Lego brick. After computationally modeling the metamaterial’s properties, the researchers found that they could create a basic CPA from the metamaterial by altering the cylinder size and spacing. 


Traditional “reverse lasers” can only absorb energy when the incoming electromagnetic waves are perfectly aligned, as in the top example. Courtesy of Kebin Fan and Willie Padilla, Duke University.

In contrast to existing CPAs, which work in one mode only, the CPA created by the Duke team has two overlapping modes, enabling it to absorb both aligned and misaligned waves. By changing the material’s parameters so that the two modes no longer overlapped, the researchers were able to create a CPA just like the CPAs currently described in the literature, but with more versatility. “Typical CPAs have only one variable, the material’s thickness,” said professor Kebin Fan. “We have three: the cylinders’ radius, height, and periodicity. This gives us a lot more room to tailor these modes and put them in the frequency spectrum where we want them, giving us a lot of flexibility for tailoring the CPAs.”  By increasing the cylinder height in the metamaterial from 1.1 to 1.4 mm, the researchers gave the device the ability to switch between absorbing all phases of electromagnetic waves and absorbing only waves occurring in sync with each other. The team believes that it could be possible to engineer a material that can make this switch dynamically. “We haven’t done that yet. It is challenging, but it’s on our agenda,” said professor Willie Padilla.  In principle, the researchers said, a device could be engineered that measures not just the intensity of incoming light like a normal camera, but also its phase. “If you’re trying to figure out the properties of a material, the more measurements you have, the more you can understand about the material,” Padilla said. “And while coherent detectors do exist … they’re extremely expensive to build through other technologies.” 
The demonstrated system and theory could open the way to a new class of absorbers for future applications in hyperspectral imaging and energy harvesting. 

For more information:
https://doi.org/10.1002/adom.201801632

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LAUSANNE, Switzerland, Feb. 13, 2019 — A way to produce glass metasurfaces that can be either rigid or flexible, developed by engineers from the EPFL Laboratory of Photonic Materials and Fiber Devices, could be used to fabricate all-dielectric optical metasurfaces quickly, at low temperatures, and with no need for a cleanroom. These metasurfaces could be used to build next-generation photonic circuits. Optical circuits, which are 10 to 100 times faster than electronic circuits and more energy-efficient, could transform the performance of many devices.


The new method employs dewetting, a natural process that occurs when a thin film of material is deposited on a substrate and then heated. The heat causes the film to retract and break apart into tiny nanoparticles.

The EPFL engineers used dewetting to create dielectric glass metasurfaces, rather than metallic metasurfaces. First, they created a substrate textured with the desired architecture. Then, they deposited the material — chalcogenide glass — in thin films just tens of nanometers (nm) thick. The substrate was heated for a couple of minutes until the glass became fluid and nanoparticles began to form in the sizes and positions dictated by the substrate’s texture. 
The engineers demonstrated the ability to tailor the position, shape, and size of nano-objects with feature sizes below 100 nm and with interparticle distances down to 10 nm. They used their method to generate optical nanostructures over rigid and soft substrates that were several centimeters in size, with optical performance and resolution comparable to traditional lithography-based processes. The metasurfaces are highly sensitive to changes in ambient conditions, thus able to detect the presence of very low concentrations of bioparticles, the team said.
Metasurfaces could enable engineers to make flexible photonic circuits and ultrathin optics for a host of applications, ranging from flexible tablet computers to solar panels with enhanced light-absorption characteristics. They could also be used to create flexible sensors to be placed directly on a patient’s skin, for example, to measure things such as pulse and blood pressure or to detect specific chemical compounds. 

For more information:
https://doi.org/10.1038/s41565-019-0362-9

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Congratulations to our new paper ” Experimental study and micro-magnetic modeling of magnetization dynamics in L1⁠0-FePt thin film” by M. Shafei, M. M. Tehranchi, H. Falizkaran Yazdi, S. M. Hamidi, R. Yusupov, S. Nikitin

Among different magnetic thin films, L10 FePt due to high magnetocrystalline anisotropy is attracting much attention for applications in new generation of magnetic recording media. In this work, switching time and switching mechanism of magnetization as essential properties of L10 FePt film was studied by magneto-optical Kerr effect (MOKE) and time-resolved magneto-optical Kerr effect (TR-MOKE). For this purpose, static in plane and out of plane magnetic hysteresis loop of a L10 FePt film on (100) MgO was measured and modeled using polar and longitudinal MOKE and mumax code respectively. Furthermore, the switching time of magnetization was studied using laser induced ultrafast demagnetization and relaxation of the sample by TR-MOKE, in which for the first time, the magnetic field was applied in the plane of the sample for this measurement.

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They consider the effect of electromagnetic coupling between localized surface plasmons in

a metallic nanoparticle (NP) and excitons or weakly interacting electron-hole pairs in a semiconductor

matrix where the NP is embedded.

An expression is derived for the NP polarizability renormalized by this coupling and two possible situations are analyzed, both compatible with the conditions for Fano-type resonances:

  • a narrow-bound exciton transition overlapping with the NP surface plasmon resonance (SPR), and
  •  SPR overlapping with a parabolic absorption band due to electron-hole transitions in the semiconductor.

The absorption band line shape is strongly non-Lorentzian in both cases and similar to the typical Fano spectrum in the case (i).

However, it looks differently in the situation (ii) that takes place for gold NPs embedded in a CuO film and the use of the renormalized polarizability derived in this work permits to obtain a very good fit to the experimentally measured LSPR line shape.

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Congratulations to our new paper ” Switching time Probing in electric field assisted magnetization of PbZrTiO3/Cobalt structure ” by M. Shafei, M. M. Tehranchi, S. M. Hamidi

Electric field assisted full magnetization switching in a multiferroic heterostructure composed of a PbZrTiO3 (PZT) substrate and 100nm Cobalt (Co) layer was investigated. For this, by measuring magnetic in plane anisotropy of the sample, using magneto-optical Kerr effect (MOKE), it was shown that the sample has a uniaxial anisotropy. In addition, the coercive field of the Co layer can be tuned by applying an electric field to the PZT which can be used in electric field assisted magnetization reversal in the Co layer. Direct measurement reveals that electric field assisted magnetization switching in layers take place in about 100 µs that is in compatibility with domain wall motion. Our measurement is a promising technique for probing of switching time in electric field assisted magnetization switching elements.


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