Enhanced Quality Factors of Surface Lattice Resonances in Plasmonic Arrays of Nanoparticles

The experimental demonstration of narrow resonances in arrays of metallic nanoparticles was more elusive due to limitations in the quality of samples and the use of focused beams. Kravets et al. reported ultranarrow plasmonic resonances in asymmetric (different refractive indexes in the upper and lower media) arrays of Au nanoparticles. Shortly after, Auguié and Barnes and Chu et al. reported narrow resonances in symmetric arrays. The potential of narrow plasmonic resonances in arrays of nanoparticles for modifying the emission of fluorophores was also demonstrated. The origin of the narrow resonances, which are known as surface lattice resonances (SLRs), is the diffractive coupling of LSPRs through in-plane diffraction orders in symmetric media or evanescent
diffraction orders—the so-called Rayleigh anomalies (RAs)—in asymmetric media. SLRs can be described as a driven damped coupled oscillator system in which one oscillator has the natural frequency of the LSPR while the other has the frequency of the diffraction order. Nanoparticle arrays are open cavities that are easy to fabricate and offer the possibility of integration with thin films or planar structures. The remarkable properties of SLRs have led to improved surface-enhanced Raman scattering, sensitive bio/chemical sensing, plasmonic band-edge lasing, strong light–matter coupling, Bose–Einstein condensation, and optoelectronic devices. The multidisciplinary impact of SLRs has stimulated the quest toward modes with the highest possible quality (Q) factor. One strategy to obtain narrow linewidths with SLRs is by coupling multipolar resonances with different diffraction orders. However, Q-factors by these approaches vary significantly over momentum space.

a) Figure of merit of silver films, defined as the ratio of the real and imaginary components of the permittivity. b) AFM map of the surface of silver films deposited at 10 Å s−1; the surface roughness, defined in terms of the root mean square of the surface profile, is 0.567 nm. c) AFM map of a silver film deposited at 30 Å s−1 with RMS = 3.817 nm.

We have demonstrated high quality factor plasmonic resonances in arrays of Ag nanoparticles (Q > 300). These resonances, known as surface lattice resonances, emerge from the coupling of localized surface plasmon polaritons to diffraction orders in the plane of the array. The quadratic dispersion of SLRs leads to a nearly constant Q-factor over a wide range
of wave vectors or angles of incidence. We have investigated the role of the intrinsic quality of the metal in the Q-factor of SLRs. We have also iscussed the effect of the adhesion layer used between the substrate and the metal on the SLRs. The suppression of this layer can lead to SLRs with Q-factors larger than 1500. These extremely high Q-factors render arrays of metallic nanoparticles very interesting systems for plasmonic applications such in sensors, for enhanced light–matter interaction and nonlinear phenomena.

a) Extinction spectra as a function of the wave vector parallel to the surface, showing the dispersion of the degenerate SLRs resulting from the coupling of LSPRs in Ag nanoparticles to (0, ±1) RAs. The LSPR at λ = 475 nm is independent of the wave vector, whereas the white curve corresponds to the (0, ±1) RAs. b) Extinction spectra showing the dispersion of the diagonal SLRs arising from the coupling of the LSPRs to the (±1, 0) RAs (indicated
by the white lines). c) Extinction spectra from part (a), measured at different angles. d) Extinction spectra from part (b), measured at different angles. The insets in parts (a) and (b) show a schematic representation of the angle-dependent extinction measurements, where the rectangles represent the nanorods, the orange double arrow indicates the polarization of the incident light, and the white curve illustrates the rotation direction of the sample.

For more information: DOI: 10.1002/adom.201801451




Laser- synthesized TiN nanoparticles as promising plasmonic alternative for biomedical applications

Exhibiting a
red-shifted absorption/scattering feature compared to conventional plasmonic
metals, titanium nitride nanoparticles (TiN NPs) look as very promising
candidates for biomedical applications, but these applications are still
underexplored despite the presence of extensive data for conventional plasmonic
counterparts. Here, we report the fabrication of ultrapure, size-tunable TiN
NPs by methods of femtosecond laser ablation in liquids and their biological
testing. We show that TiN NPs demonstrate
strong
and broad plasmonic peak around 640–700 nm with a significant tail over 800 nm
even for small
NPs sizes (<7 nm). In vitro tests
of laser-synthesized TiN NPs on cellular models evidence their low
cytotoxicity and excellent cell uptake. We finally demonstrate a
strong photothermal therapy effect on U87–MG cancer cell cultures using TiN NPs
as sensitizers of local hyperthermia under near-infrared
laser excitation. Based on absorption band in the region of
relative tissue transparency and acceptable biocompatibility, laser-synthesized
TiN NPs promise the advancement of biomedical modalities
employing plasmonic effects, including absorption/scattering
contrast imaging, photothermal therapy,
photoacoustic
imaging and SERS.

(a) Schematics of laser ablation setup. A laser beam is focused on the surface of the TiN target, which is placed in the vessel filled with a liquid. The vessel is mounted on a moving translation stage to avoid ablation from the same area of the target. (b) Schematic of laser fragmentation setup to minimize size dispersion of NPs. Ar bubbling used optionally to remove dissolved oxygen.

For more information: https://www.nature.com/articles/s41598-018-37519-1




Biosensor Could Scale New Sensitivity Heights

Researchers in Switzerland and Australia have brought together the physics of dielectric metasurfaces and hyperspectral imaging to create an ultrasensitive, label-free biosensing platform. The team believes that the platform—reportedly capable of detecting and analyzing samples at spatial concentrations of less than three molecules per square micron—could ultimately enable compact portable diagnostics for personalized medicine. It could also, according to the researchers, offer a route to high-throughput, high-resolution optical characterization of single-atom-thick, 2-D materials such as graphene, a key requirement for advancing the technical development of those much-ballyhooed materials.

The detection platform demonstrated by scientists in Switzerland and Australia, which combines a high-Q-factor dielectric surface and hyperspectral imaging, can reportedly detect biomolecules at a density of less than three molecules per square micron of detector.

The dielectric difference

Sensors that operate via surface plasmon resonances—the subwavelength concentration and amplification of light on surfaces decorated with nanoscale metallic antennas—already have a long pedigree in biomedical diagnostics. As cells or biomolecules bind to the nanostructures, they change the local refractive index by an infinitesimal amount, which in turn leads to sharp changes in the peak wavelength of the surface plasmon resonance. Those wavelength changes can be read to track the presence, concentration and growth of the biological agent under study.

Bound states in the continuum

Originally a concept from quantum mechanics, BICs are confined waves that remain localized within a continuous spectrum of radiating waves. (More precisely, they represent discrete solutions of the single-particle Schrödinger equation from quantum mechanics, embedded within a continuum of positive energy states.) In principle, such bound states would be perfectly localized and would have an infinite quality (Q) factor. In practice, for device design, one can mathematically engineer a subwavelength optical “supercavity” that supports a “quasi-BIC” with an extremely high Q and an extremely narrow resonance width.

High Q response with a hyperspectral kicker

The researchers behind the new work realized that, by virtue of those characteristics, a surface that exploited such supercavity-enabled BICs would be very responsive to local refractive-index changes—and, thus, could enable a very sensitive biodetection platform. So they set about building one. To do so, they used electron-beam lithography to pattern a 100-nm-thick layer of amorphous silicon (on a fused-silica substrate) with an array of tilted silicon “nanobars” around 100 nm wide and 280 nm long. The specific configuration and tilting angle of the bars had been mathematically calculated to take maximum advantage of quasi-BIC states—and the high Q factors and spectrally isolated resonances they enable—in the near-infrared region. That set up a metasurface that, when illuminated with a laser of the right wavelength, was primed to respond dramatically, via changes in resonance peak, to refractive-index changes due to individual biomolecules clinging to the surface. Next, the team supercharged this dielectric-metasurface platform by combining it with another hot technique: hyperspectral imaging. To obtain the hyperspectral data from the metasurface sensor, the team used a supercontinuum laser source to illuminate the sensor, sweeping the laser through multiple, narrow-linewidth frequencies by means of a laser line tunable filter. At each frequency of incident light, a high-resolution CMOS camera captures a new image of the resonant response of the sensor.

The team believes that the combination of high-Q resonant dielectric metasurfaces and high-throughput, imaging-based data acquisition amounts to “a superior and versatile sensing platform.” And the researchers suggest that further explorations—leveraging alternative materials, other dimensions such as incident-light polarization, and machine learning—could expand the system’s flexibility still further, potentially enabling “a field-deployable high-throughput single-molecule detector for biomedical applications.”

For more information: doi: 10.1038/s41566-019-0394-6




Toward Nanoparticle “Night-Vision Goggles”

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




Magnetoplasmonics in nanocavities: Dark plasmons enhance magneto-optics beyond the intrinsic limit of magnetoplasmonic nanoantennas

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




NIR Imaging System Could Help Identify Hard-to-Detect Cancers at Earlier Stage

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




Light-controlled nanomaterials are revolutionizing sensor technology

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/




Microchannel-based plasmonic refractive index sensor for low refractive index detection

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




With a Few Tweaks, a Near-Perfect Absorber Can Become a Time-Reversed Laser

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




A New Way to Fabricate High-Performance Optical Metasurfaces for Use in Photonic Circuits

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