congratulations to Ms. Asgari , Ms. Mahboubi, Ms. Gachilou and Mr. kouhestanian, for defending your dissertation at approved times.
Organic/metal nanowire heterostructures for the study of orientation dependent exciton-plasmon coupling. (A) Numerically simulated |E| 2 distribution of SPPs at the end of a 200-nm-diameter and 6 μm-long AgNW, where SPPs are launched by a dipole oriented along three coordinate axes x, y, and z, respectively. The dipole is positioned at the middle of the wire with a distance of 20 nm. (B) Schematic illustration for the proposed heterostructure with orderly arranged molecules around a partially embedded AgNW. (C) Oriented Frenkel typeexciton
dipoles created around the AgNW by irradiation of an incident light at the junction. SPPs can be efficiently launched by the exciton dipoles, which will subsequently propagate along the AgNW and scatter into free space at the distal ends. (D) SPPs coupling by multiple exciton dipoles. The cross angle between the AgNW and the polarization of dipoles are 0°, 45° and 90°.
The excitation of surface plasmons by optical emitters based on exciton-plasmon coupling is important for plasmonic devices with active optical properties. It has been theoretically demonstrated that the orientation of exciton dipole can significantly influence the coupling strength, yet systematic study of the coupling process in nanostructures is still hindered by the lack of proper material systems. In this work, researchers have experimentally investigated the orientation-dependent exciton-plasmon coupling in a rationally designed organic/metal nanowire heterostructure system. The heterostructures were prepared by inserting silver nanowires into crystalline organic waveguides during the self-assembly of dye molecules. Structures with different exciton orientations exhibited varying coupling efficiencies. The near-field exciton-plasmon coupling facilitates the design of nanophotonic devices based on the directional surface plasmon polariton propagations.
. Pump-laser-polarization dependent SHG mapping. (a) SEM image of single-crystalline monolayer WSe2 flake on trenches with a pitch of 910 nm. (b) Simulated electric field distribution at a plane 1 nm above the surface of gold substrate with pump laser polarized perpendicular (left panel) and parallel (right panel) to the trench. The dotted line outlines the geometry of the trench. (c,d) Corresponding experimental SHG mappings of the exact WSe2 flake on trenches as shown in the SEM image in (a) under resonant and non-resonant excitations, respectively. White dashed lines outline the WSe2 flake. The white arrows show the polarization directions of the pump laser.
Monolayer two-dimensional transition metal dichalcogenides (2D TMDCs) exhibit promising characteristics in miniaturized nonlinear optical frequency converters, due to their inversion asymmetry and large second-order nonlinear susceptibility. However, these materials usually have a very short light interaction lengths with the pump laser because they are atomically thin, such that second-harmonic generation (SHG) is generally inefficient. In this research, Joel.K.W.Yangs group fabricated a judiciously structured 150-nm-thick planar surface consisting of monolayer tungsten diselenide and sub-20-nm-wide gold trenches on flexible substrates, reporting ~7000-fold SHG enhancement without peak broadening or background in the spectra as compared to WSe2 on as-grown sapphire substrates. their proof-of-concept experiment yields effective second-order nonlinear susceptibility of 2.1 × 104 pm/V. Three orders of magnitude enhancement is maintained with pump wavelength ranging from 800 nm to 900 nm, breaking the limitation of narrow pump wavelength range for cavity-enhanced SHG. In addition, SHG amplitude can be dynamically controlled via selective excitation of the lateral gap plasmon by rotating the laser polarization. Such fully open, flat and ultrathin profile enables a great variety of functional samples with high SHG from one patterned silicon substrate, favoring scalable production of nonlinear converters. The surface accessibility also enables integration with other optical components for information processing in an ultrathin and flexible form.
A long-standing question debated among the nanophotonics community is whether size matters and helps to reduce the threshold ofmicrometre- and submicrometre-sized lasers, and whether the presence of metal interfacing the gain medium harms or improves the laser performance. In a work published in Nature Communications, Ren- Min Ma and colleagues1 address this issue through a thorough experimental study, and conclude that when the device dimensions approach the diffraction limit, plasmonic (metal-based) lasers have superior performance over traditional photonic lasers as they are faster and have lower threshold and lower power consumption (Fig. 1). A laser has two major components: (i) a gain medium providing for stimulated emission and light amplification, and (ii) a resonator facilitating stimulated emission feedback (loosely speaking, reflecting generated photons to the place of their origin and, in many cases, enabling a coherence of laser radiation). The most basic laser cavity supporting standing-wave oscillation modes consists of two parallel mirrors, the distance between which is equal to an integer number of ‘half-wavelengths’ (λ/2) of laser radiation. Therefore, the minimum distance between the mirrors is equal to λ/2, which is equivalent to ~250 nm in the visible part of the spectrum — an order of magnitude larger than the typical size of a modern transistor. This hinders the dream of keeping up with the Moore’s law by replacing electronic circuits with much faster optical circuits2, which would require laser-based sources and amplifiers of
coherent light. A novel solution to the size problem was put forward in 2003 by Bergman and Stockman3, who proposed to change the feedback mechanism and replace a set of large (by the nanoworld standards) mirrors with a nanoscopic metallic structures
that support resonant oscillations of free electrons (weakly) coupled to modes of electromagnetic radiation — the phenomenon known as a localized surface plasmon. The proposed device, termed spaser, which can be as small as a few nanometres, was primarily intended to generate surface plasmons (rather than photons) and be directly integrated into optical frequency circuits4. The first experimental demonstration, in 2009, of the spaser-based nanolaser5, in which the 14-nm Au plasmonic nanoparticle, providing for a stimulated emission feedback, was surrounded by the 44-nm dye-doped silica shell, providing for gain, was followed by a rapid development of a variety of micrometre- and submicrometre-sized plasmonic lasers (or spasers)6, bringing the dream of nanocircuitry operating at optical frequency closer to reality. Besides the very possibility of having a laser whose size is not limited by λ/2 — which, not coincidentally, is close to the diffraction limit for light (the minimum area into which the light can be focused) — the heuristic expectation that a smaller volume laser can have a lower power consumption is one of the prime motivations for laser miniaturization1. This poses the following dilemma: on one hand, surface plasmons, supported by metallic particles and structures, allow lasers to be small, giving the hope of a low power consumption and high speed. On the other hand, metals are known to have large optical loss, which tends to increase the threshold pumping power (the laser threshold) and the overall power consumption. Therefore, do metals and surface plasmons help or harm miniature lasers and does the answer to this
question depend on the laser size? Ma and co-authors fabricated and characterized an impressive sum of 170 optically pumped plasmonic and photonic lasers based on rectangular CdSe slabs placed on top of MgF2/Au and SiO2 substrates, respectively (with the thickness of the slabs varied between 50 nm and 1,000 nm, and their length varied between 0.8 μm and 6 μm). The key difference between the metal-assisted lasers in this work and the spaser3 is that while the volume of the mode is comparable or less than λ3, the demonstrated lasers are subwavelength only in one vertical dimension, while in-plane they are larger than λ and exhibit standard multiple resonances due to reflections from the cavity edges. As a result, only a small fraction of light energy penetrates into the metal and the losses are substantially reduced in comparison to the metallic structures that are sub-wavelength in all three dimensions7. The stimulated emission threshold power density Pth/S (kW cm–2), the power consumption at the threshold Pth (mW) and the emission lifetime τ (ns) have been studied as the function of the CdSe slab’s volume V (measured in units of λ3). Furthermore, the emission lifetime τ was studied and correlated with the threshold power density Pth/S for multiple slab thicknesses T. It has been shown that although Pth and Pth/S in large (V ≥ 5λ3) photonic lasers are comparable or even superior to those in plasmonic counterparts, these quantities increase dramatically at smaller laser volumes (particularly if the CdSe slab’s thickness approaches the diffraction limit). At the same time, in small plasmonic lasers (T ≤ diffraction limit), the growth of Pth/S with the reduction of V is much less dramatic and the power consumption Pth decreases with the reduction of V, justifying the quest for laser miniaturization. This allowed Ma and co-workers to demonstrate a low lasing threshold of ~10 kW cm–2 in a plasmonic laser operating below the diffraction limit (V ~ λ3 and T ~ 100 nm). According to Purcell8, spontaneous emission lifetime in a cavity (in the absence of non-radiative decay) is roughly proportional to the mode volume Vm and, since the emitter is broadband, inversely proportional to the quality factor Q, defined as Q = ω/Δωsp, where ω is the frequency and Δωsp is the spontaneous emission bandwidth. Hence, the lifetime is predicted to decrease with the reduction of the physical volume of the CdSe slabs, in both photonic and plasmonic lasers1. This prediction was in good agreement with the experimental emission lifetimes measured in lasers of different sizes. Furthermore, the threshold was experimentally demonstrated to grow with the reduction of the spontaneous emission lifetime, in good agreement with ‘old school’ laser science9. Importantly, it has been experimentally shown that sub-diffraction plasmonic lasers can have shorter lifetimes than photonic lasers, for the same threshold value. Therefore, plasmonic lasers can be faster and, at the same time, have lower threshold than photonic lasers when the cavity volume approaches or becomes smaller than the diffraction limit cubed. The results reported by Ma and coauthors1 are of high importance, as they demonstrate the advantage of plasmonic lasers over photonic lasers (of the same sub-diffraction size) and pave the road to their further miniaturization. The next critical step in this direction would be an experimental study of the size dependence of plasmonic lasers, which are sub-diffraction in all three dimensions, and a comparison of the results with the theoretical predictions10.
In the long term, however, achieving electrically pumped plasmonic nanolaser operation will truly open the doors for practical applications of these devices.
more information on : https://www.nature.com/articles/nmat5065.
research on noble metal nanoparticles has always remained interesting because of their optical and electronic properties. Gold nanoparticles (AuNPs), in particular, have been intensively studied for their fascinating localized surface plasmon resonance (LSPR) peak in the visible region of electromagnetic spectrum. The tunable nature of LSPR of AuNPs leads to a large number of applications of AuNPs in the fields of plasmonics and bioscience.
The LSPR peaks of AuNPs can be tuned from the visible to the near-infrared region by controlling the shape, size, and structure of the particles. Another way of changing the optical properties is to change the surrounding of AuNPs as the LSPR peak is also very sensitive to the dielectric properties of the surrounding medium. The later method becomes an easy and superior one when the nanoparticles are embedded in a dielectric matrix. If the surrounding matrix is a transparent one, at least in the region of the spectrum where LSPR occurs, it becomes an added advantage for many photonic and plasmonic applications.
In order to explore and control the effects of shape, size, structure and dielectric media on LSPR properties, several preparation methods are used to form hybrid structures of nanoparticles and dielectric interfaces. In the present work, different ways of tuning LSPR positions of metal-dielectric nanocomposite thin films have been demonstrated with AuNPs embedded indium oxide (Au:IO) thin films as a case study. Au:IO thin films have been prepared by a sequence-specific sandwich method. The films were characterized by glancing angle x-ray diffraction (GXRD), optical absorption, high-resolution transmission electron microscopy (HRTEM) and Rutherford backscattering spectrometry (RBS). The advantages of the sandwich method have been shown by comparing the optical properties of Au:IO thin films so formed with that of Au:IO films formed by the most commonly used co-sputtering method.
Importance of the work
The work in the publication titled “Tailoring plasmonic properties of metal nanoparticles embedded dielectric thin films: The sandwich method of preparation” by Ranjit Laha et al. reports the experimental ways of tuning plasmonic positions (following a derived working formula) of AuNP embedded metal oxide thin films. The novelty lies in followings
This study, Tailoring plasmonic properties of metal nanoparticle-embedded dielectric thin films: the sandwich method of preparation was recently published in the Journal of Nanoparticle Research.
The accidental discovery of a novel aluminium alloy that reacts with water in a highly unusual way may be the first step to reviving the struggling hydrogen economy. It could offer a convenient and portable source of hydrogen for fuel cells and other applications, potentially transforming the energy market and providing an alternative to batteries and liquid fuels.
“The important aspect of the approach is that it lets you make very compact systems,” says Anthony Kucernak, who studies fuel cells at Imperial College London and wasn’t involved with the research. “That would be very useful for systems which need to be very light or operate for long periods on hydrogen, where the use of hydrogen stored in a cylinder is prohibitive.”
The discovery came in January, when researchers at the US Army Research Laboratory at Aberdeen Proving Ground, Maryland, were working on a new, high-strength alloy, says physicist Anit Giri. When they poured water on it during routine testing, it started bubbling as it gave off hydrogen.
That doesn’t normally happen to aluminium. Usually, when exposed to water, it quickly oxidises, forming a protective barrier that puts a stop to any further reaction. But this alloy just kept reacting. The team had stumbled across the solution to a decades-old problem.
Hydrogen has long been touted as a clean, green fuel, but it is difficult to store and move around because of its bulk. “The problem with hydrogen is always transportation and pressurisation,” says Giri.
If aluminium could be made to effectively react with water, it would mean hydrogen on demand. Unlike hydrogen, aluminium and water are easy to carry – and both are stable. But previous attempts to drive the reaction required high temperatures or catalysts, and were slow: obtaining the hydrogen took hours and was around 50 per cent efficient.
The new alloy, which the team is in the process of patenting, is made of a dense powder of micron-scale grains of aluminum and one or more other metals arranged in a particular nanostructure. Adding water to the mix produces aluminium oxide or hydroxide and hydrogen – lots of it. “Ours does it to nearly 100 per cent efficiency in less than 3 minutes,” says team leader Scott Grendahl. Moreover, the new material offers at least an order of magnitude more energy than lithium batteries of the same weight. And unlike batteries, it can remain stable and ready for use indefinitely.
The army team has used the material to power a small, radio-controlled tank. Grendahl doesn’t see any practical issues with scaling up production to produce hundreds of tonnes of the stuff as it can be made from scrap aluminium, which is relatively cheap. The new material could power everything from laptops to buses and cars.
“In principle, the process should work,” says Robert Steinberger-Wilckens, who directs a fuel cell programme at the University of Birmingham, UK.
But he cautions that a repeat experiment is needed to show that the reaction works the way it should. “There’s a lot of stuff that works in the laboratory but not in the field.”
If it does pan out, the powder could also be used as the raw material for 3D printing. The researchers have put forward proposals – now being considered by the army – for small air or ground robots that use their own structure as fuel. These self-cannibalising machines would be useful for one-way intelligence-gathering missions, burning themselves up at the end to leave no trace.
more information: https://www.newscientist.com/article/2142693-nano-aluminium-offers-fuel-cells-on-demand-just-add-water/
Graphene is an attractive material for optoelectronics1 and
photodetection applications2–6 because it offers a broad spectral
bandwidth and fast response times. However, weak light
absorption and the absence of a gain mechanism that can generate
multiple charge carriers from one incident photon have
limited the responsivity of graphene-based photodetectors to
∼1022 AW21. Here, this group demonstrate a gain of ∼108 electrons
per photon and a responsivity of∼107 AW21 in a hybrid photodetector
that consists of monolayer or bilayer graphene covered
with a thin film of colloidal quantum dots. Strong and tunable
light absorption in the quantum-dot layer creates electric
charges that are transferred to the graphene, where they
recirculate many times due to the high charge mobility of
graphene and long trapped-charge lifetimes in the quantumdot
layer. The device, with a specific detectivity of 731013
Jones, benefits from gate-tunable sensitivity and speed,
spectral selectivity from the short-wavelength infrared to the
visible, and compatibility with current circuit technologies.
more information: Gerasimos Konstantatos et al. nature nanotechnology, DOI: 10.1038/NNANO.2012.60.
Enhanced Faraday rotation in a new type of Magneto-plasmonic structure with the capability of Fano resonance, has been reported theoretically. A magneto-plasmonic structure composed of a Gold corrugated layer deposited on a magneto-optically active layer was studied by means of Lumerical software based on finite-difference time-domain. In our proposed structure, plasmonic Fano resonance and localized surface plasmon induced enhancement in magneto-optical Faraday rotation. It is shown that the influence of geometrical parameters in Au layer offers a desirable platform for engineering spectral position of Fano resonance and enhancement of Faraday rotation.
Biophotonic application of plasmonic gold nanoparticles has become a highly active field of research in recent years due to their unique chemical and physical properties, such as high absorption cross sections and spectral tunability1. Many of the unique properties of gold nanoparticles are governed by the surface plasmon resonance (SPR) effect, a collective oscillation of electrons on the nanoparticle surface that occurs when excited with light at an appropriate wavelength. The SPR results in a strongly enhanced electromagnetic field near the particle surface, which causes unique, shape- and material-dependent spectral variations in light absorption and scattering. These properties allow nanoparticles to be used not only as therapeutic agents2,3,4, but also for diagnostic imaging5,6. The therapeutic effects produced by laser-nanoparticle interaction can occur through a variety of mechanisms. Photothermal transduction causes rapid heating in a localized area around the irradiated nanoparticles and has been used for the treatment of solid tumors in vivo, with both continuous-wave and nanosecond-pulsed lasers7,8,9. Photomechanical effects, such as cavitation, can occur when exposing plasmonic nanoparticles to pulsed laser light (nanosecond-to-femtosecond), and the resultant bubbles are capable of disrupting cancer cell membranes10,11,12. Photochemical effects, like the production of reactive oxygen species, have also been observed from laser irradiation of gold nanoparticles, with pulsed lasers having a greater effect than continuous-wave lasers13,14. The properties that make gold nanoparticles so effective for laser-based therapeutics can also lead to unintended side-effects in diagnostic procedures.
One of the most promising diagnostic techniques that may employ gold nanoparticle contrast agents is photoacoustic imaging (PAI), a rapidly maturing biomedical modality capable of macro- and micro-scale imaging5. In PAI, tissue is illuminated with a nanosecond pulsed laser, typically a Q-switched Nd:YAG with pulse duration in the range of 5–10 ns, at exposures below standard safety limits15. This exposure causes rapid optical absorption and thermal expansion that produces acoustic waves, which can be detected using an ultrasonic transducer. The use of gold nanoparticles as contrast agents for PAI has the potential to generate photothermal, photomechanical, or photochemical effects that result in tissue injury. Furthermore, the spectral changes brought about by nanoparticle photomodification can cause spectral shifts in absorption that degrade PAI-nanoparticle product performance16.
While there are well-established laser safety standards for the skin and eye, these standards are insufficient when exogenous chromophores such as nanoparticles are present. Additionally, no standards exist for evaluating the performance of laser-nanoparticle combination products for medicine. In recent years, a number of studies have contributed knowledge on the laser-nanoparticle interaction processes and resultant bioeffects, as well as test methods that could form the basis of new standards. One key step in elucidating laser-nanoparticle interactions is to understand the photostability of nanoparticles and characterize laser induced photomodification processes, including photothermally-induced melting/reshaping effects that have been documented for different types of metallic nanoparticles16,17,18,19,20,21,22. they chose to study spherical gold nanoparticles as their surface plasmon resonance is close to the wavelength (532 nm) of the second-harmonic Q-switched Nd:YAG laser, which is broadly employed in biophotonics and photomedicine23,24,25, and because these nanoparticles possess well-defined morphological properties and their spherical shape is well-suited for theoretical investigation.
Several prior studies have evaluated the effect of pulsed laser irradiation on gold nanospheres – more accurately gold “pseudospheres” since most of the particles studied, as well as commercially available versions, have faceted rather than smooth edges. One such study by Takami et al. looked at the size reduction of approximately 50 nm gold nanospheres at different pulse energies18. Werner et al. investigated the effect of irradiation wavelength on 55 nm gold nanospheres and was mostly concerned with modeling the difference between interband and intraband excitation22. Another study used dynamic light scattering (DLS), electron microscopy and UV-Vis spectroscopy to examine the impact of a single laser pulse on particles of different sizes26. While results were presented for a range of energy levels, specific damage thresholds were not determined and comparison with theoretical results was not presented. These prior studies have provided significant insights, yet are deficient in terms of generating a standardized test methodology as they either only used a single nanoparticle size or provided a qualitative description of the damage with no well-defined threshold.
Our overall objective was to facilitate the optimization of safety and effectiveness in emerging biophotonic products incorporating nanoparticles. Previous works have not established a common approach for assessing laser damage thresholds in nanoparticles, and information on the role of particle size in nanosphere damage is typically spread across multiple reports with inconsistent experimental conditions. Therefore, the goals of this proof-of-concept study were to implement and assess methodologies to quantitatively determine nanoparticle photomodification thresholds, and to generate data on the melting process and damage thresholds that can be used to improve understanding of this process. Specifically, we have conducted quantitative experimental and analytical investigation of the interaction between nanosecond laser pulses at 532 nm and plasmonic gold nanoparticles with diameters from 20 to 100 nm over a wide range of laser radiant exposures.
More information: Andrew M. Fales et al. Quantitative evalution of nanosecond pulsed laser-induced photomodification of plasmonic gold nanoparticles, Scientific Reports(2017). 10.1038/s41598-017-16052-7.
Researchers at ETH Zurich have developed the first opto-electronic circuit component that works without glass and is instead made of metal. The component, referred to as a modulator, converts electrical data signals into optical signals. It is smaller and faster than current modulators, and much easier and cheaper to make.
Optical components for microelectronics must be made of glass. Metals are not suitable for this purpose, since optical data can propagate only across roughly a distance of 100 micrometres. This was the general view of scientists until recently. A team of researchers headed by Juerg Leuthold, professor in the Department of Information Technology and Electrical Engineering, has now succeeded in doing what was thought to be impossible and developed a light-processing component made of metal. The researchers have presented their findings in the latest issue of the journal Science.
They accomplished this feat by building a small enough component: at just 3 x 36 micrometres, it is within a size range in which both optical and electrical information can propagate in metals.
Component for fibre optic networks
The component is a modulator: modulators convert electrical data signals into optical signals. They are installed in modern internet routers used for fibre optic networks and enable fibre optic data connections between computer units in data centres. However, the standard components used today function differently than the new modulators.
The new component works by aiming the light from a fibre optic source at the modulator, causing the electrons on its surface to oscillate. Experts refer to this as a surface plasmon oscillation. This oscillation can be changed indirectly by electrical data pulses. When the oscillation of the electrons is converted back into light, the electrical information is now encoded onto the optical signal. This means that the information is converted from an electrical into an optical data pulse that can be transmitted via fibre optics.
Schematic representation of the metallic modulator: Left: a continuous beam of light strikes a metallic lattice that deflects the light onto the chip. Right: an optical data pulse exits the component.
Faster and smaller
Two years ago, Leuthold and his colleagues developed one of these plasmonic modulators. At the time, it was the smallest and fastest modulator ever built, but the semiconductor chip still had various glass components.
By replacing all the glass components with metallic ones, the scientists have succeeded in building an even smaller modulator that works up to highest speed. “In metals, electrons can move at practically any speed, whereas the speed in glass is limited due to its physical properties,” says Masafumi Ayata, a doctoral student in Leuthold’s group and lead author of the study. In the experiment, the researchers succeeded in transmitting data at 116 gigabits per second. They are convinced that with further improvements, even higher data transfer rates will be possible.
Etched from a gold layer
The modulator prototype tested by the ETH researchers is made of a gold layer that lies on a glass surface. The scientists emphasised that the glass has no function. “Instead of the glasslayer, we could also use other suitable smooth surfaces,” says Leuthold. It might also be possible to use less expensive copper instead of gold for industrial applications. The important point is that only one metallic coating is required for the new modulators. “This makes them much easier and cheaper to fabricate,” says Leuthold.
The researchers are already working with an industrial partner in order to put the new modulator into practice, and talks with other partners are in progress. However, Leuthold believes that further development may be required before the technology is ready for the market; for example, he expects that the current loss of signal strength during modulation can be reduced further.
For computers and autonomous vehicles
The new modulator could one day be used not only for telecommunications applications, but for computers as well. “The computer industry is considering using fibre optics to transfer data between the individual chips inside computers,” says Leuthold. However, this would require tiny modulators – such as Leuthold and his team have developed.
Ultimately, it is also conceivable that the modulators could be used in displays – including bendable ones – and optical sensors, such as those in the Lidar system for distance measurement that are used in (semi-) autonomous cars.
More information: Masafumi Ayata et al. High-speed plasmonic modulator in a single metal layer, Science (2017). DOI: 10.1126/science.aan5953