Nonlinear magnetoplasmonics aims to utilize plasmonic
excitations to control the mechanisms and tailor the efficiencies of
Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs).
Bagheri et al. (2016) employ laser interference lithography as a reliable and low-cost fabrication method to create nanowire and nanosquare arrays in photopolymers for manufacturing plasmonic perfect absorber sensors over homogeneous areas as large as 5.7 cm2. Subsequently, they transfer the fabricated patterns into a palladium layer by using argon ion beam etching. Geometry and periodicity of their large-area metallic nanostructures are precisely controlled by adjusting the interference conditions during single- and double-exposure processes, resulting in active nanostructures over large areas with spectrally selective perfect absorption of light from the visible to the near-infrared wavelength range. In addition, they demonstrate the method’s applicability for hydrogen detection schemes by measuring the hydrogen sensing performance of our polarization independent palladium-based perfect absorbers. Since palladium changes its optical and structural properties reversibly upon hydrogenation, exposure of the sample to hydrogen causes distinct and reversible changes within seconds in the absorption of light, which are easily measured by standard microscopic tools. The fabricated large-area perfect absorber sensors provide nearly perfect absorption of light at 730 and 950 nm, respectively, and absolute reflectance changes from below 1% to above 4% in the presence of hydrogen. This translates to a relative signal change of almost 400%. The large-area and fast manufacturing process makes our approach highly attractive for simple and low-cost sensor fabrication, and therefore, suitable for industrial production of plasmonic devices in the near future.
Odeta Limaj et al., report infrared plasmonic biosensor for real-time and lable-free monitoring of lipid membranes.
In this work, they present an infrared plasmonic biosensor for chemical-specific detection and monitoring of biomimetic lipid membranes in a label-free and real-time fashion. Lipid membranes constitute the primary biological interface mediating cell signaling and interaction with drugs and pathogens. By exploiting the plasmonic field enhancement in the vicinity of engineered and surface-modified nanoantennas, the proposed biosensor is able to capture the vibrational fingerprints of lipid molecules and monitor in real time the formation kinetics of planar biomimetic membranes in aqueous environments. Furthermore, they show that this plasmonic biosensor features high-field enhancement extending over tens of nanometers away from the surface, matching the size of typical bioassays while preserving high sensitivity.
Link address: http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b05316
Plasmonic Lasers Get a Sharper Focus
Lasers based on coherent surface plasmon polaritons (SPPs)—subwavelength oscillations of electrons that are excited when incident light hits a metal-dielectric interface—hold promise for ultraminiaturized, chip-scale optics, and also as a possible platform for terahertz quantum cascade lasers (QCLs). But there’s a catch: SPP lasers, precisely because of their subwavelength apertures, tend to have divergent radiation patterns, making it tough to produce a sharp, directional beam.
Now, a research team led by Sushil Kumar of Lehigh University, Penn., USA, has devised an “antenna feedback” scheme that reportedly can provide single-mode operation and strong, highly directional far-field coupling in such SPP lasers, bringing them “closer to practical applications” (Optica, doi:10.1364/OPTICA.3.000734). The team’s work includes a proof-of-concept terahertz QCL based on the scheme that, according to the study, achieved the narrowest beam yet reported for such a QCL.
The pros and cons of “spasers”
SPP lasers—also called plasmonic lasers or “spasers”—operate by confining light energy as coherent SPP oscillations in subwavelength cavities (commonly parallel-plate Fabry-Perot-type cavities in with a length greater than the subwavelength cavity width). Their subwavelength dimensions make these lasers intriguing for certain applications in integrated photonics and nanophotonics. Parallel-plate cavities with SPP modes are also used for terahertz QCLs (which have some interesting potential applications in biosensing and standoff detection of dangerous materials), as they can show low-threshold, high-temperature performance at those frequencies.
It turns out, however, that it’s difficult to extract light from the plasmonic energy trapped in the spaser cavity. And when light can be made to leak out, it tends to be low in power and highly divergent, which limits its usefulness in actual applications.
A plasmonic phased array?
Kumar’s team found a potential solution through a distributed-feedback approach that the team has dubbed “antenna feedback,” and that Kumar compares to the action of phased-array antennas in microwave communication systems. The team demonstrated by numerical modeling that a grating of slits on one side of the subwavelength Fabry-Perot resonator, spaced at a specific value, would allow a single SPP mode within the cavity to diffract outside of the cavity in the surrounding medium, through Bragg diffraction. The energy outside of the cavity builds up with positive feedback (again the result of the selection of the grating period).
As a result, a second intense SPP wave develops in the medium outside of the cavity that remains coupled to the cavity’s metal cladding but also can form a highly directional beam outside of it. “The narrow-beam emission,” the team writes, “is due in part to the cavity acting like an end-fire phased-array antenna at microwave frequencies.”
In a proof of concept, the team implemented the antenna-feedback scheme in a terahertz QCL, using a box-shaped cavity consisting of two 100-µm by 1400-µm metallic plates, separated by a distance of 10 µm. The researchers report that the resulting laser showed a beam divergence as small as 4 degrees by 4 degrees—“the narrowest beam reported for any terahertz QCL to date,” according to the study.
Applications in security and elsewhere
The researchers note that terahertz QCLs in particular have some interesting applications in security and standoff detection. At a recent innovation conference, they pointed out that “approximately 80 to 95 percent of explosives, and all commonly used ones, have unique and identifiable terahertz signatures.”
But, while their experiments focused particularly on terahertz QCLs, they stress that the antenna-feedback scheme should be applicable to plasmonic lasers of any operating wavelength that operate with Fabry-Perot cavities. That, in turn, could aid help make other applications of plasmonic lasers, in areas such as nanophotonics, more feasible, according to the scientists.
News source: http://www.osa-opn.org/home/newsroom/2016/july/plasmonic_lasers_get_a_sharper_focus/
The development of active and passive plasmonic devices is challenging due to the high level of dissipation in normal metals. One possible solution to this problem is using alternative materials. Graphene is a good candidate for plasmonics in the near-infrared region. In this paper, we develop a quantum theory of a graphene plasmon generator. Lozovic et al. account for quantum correlations and dissipation effects, thus they are able to describe such regimes of a quantum plasmonic amplifier as a surface plasmon emitting diode and a surface plasmon amplifier using stimulated radiation emission. Switching between these generation types is possible in situ with a variance of the graphene Fermi level. They provide explicit expressions for dissipation and interaction constants through material parameters, and they identify the generation spectrum and the second-order correlation function, which predicts the laser statistics.
New source: http://journals.aps.org/prb/abstract/10.1103/PhysRevB.94.035406
The optimization of plasmonic systems—i.e., those based on ‘plasmons,’ the quanta of plasma oscillations—could enable their use in a range of applications (e.g., biomedical and chemical sensing, imaging, and the development of polarization-based plasmonic metadevices). Information about the polarization properties of scattered light from plasmonic metal nanoparticles and nanostructures is key to achieving this.
The acquisition and subsequent analysis of complete polarization information from plasmonic nanostructures represents a crucial step toward the fundamental understanding of a number of recently observed intricate spin-optical effects,1 and the optimization of experimental parameters for many practical applications.2 Such studies are, however, confounded by the experimental difficulties inherent in recording the full polarization response from plasmonic nanostructures, and by the complexities of the polarization signals that are obtained from such systems. The scattering signal from plasmonic structures (which is relatively weak) is generally swamped by a large amount of unscattered background light. As a result of this, polarimetric measurements on plasmonic nanostructures must be made in a high numerical-aperture (NA) setting.
To address some of the outstanding challenges in plasmon polarimetry, we have developed a novel spectroscopic system based on a Mueller matrix (i.e., a 4×4 matrix representing the transfer function of any optical system in its interaction with polarized light).2 Our system integrates an efficient Mueller-matrix measurement scheme with a dark-field microscopic-spectroscopy arrangement. This arrangement facilitates the detection of weak scattering signals from plasmonic nanostructures. The inherent complexities of measuring polarization in high NA settings (i.e., where one must account for the complex polarization transformation that occurs because of both the focusing and collection geometry of the microscope arrangement) are dealt with by using a robust eigenvalue calibration method, which works over a broad wavelength range.2, 3 The experimental system is also enhanced by our inverse analysis models, which extract and quantify the individual polarization properties of plasmonic samples. Using this approach, the experimental Mueller matrix (M) is decomposed into a product of a depolarizing and a non-depolarizing matrix.3, 4 The latter is then further analyzed to extract the useful polarimetry parameters (i.e., the diattenuation and retardance).
We demonstrated the feasibility and potential utility of our quantitative Mueller-matrix polarimetry on a simple plasmonic system made up of gold nanorods. A schematic of the experimental system (which employs broadband white-light excitation) and the associated calibration results are shown in Figure 1.2 We constructed the wavelength-dependent scattering Mueller matrices for this sample—M(λ)—by combining 16 spectrally resolved intensity measurements (i.e., the scattering spectra) for four different combinations of the optimized-elliptical polarization-state-generator (PSG) and polarization-state-analyzer (PSA) states. We used the eigenvalue calibration method to determine the exact experimental forms of the system’s PSG and PSA matrices (and their associated wavelength dependence), and subsequently used these values to determine the Mueller matrix of the callibrating achromatic quarter waveplate.2, 3 The illustrative Mueller matrices exhibit characteristic features of a pure linear retarder over the entire wavelength (λ) range: see Figure 1(c). We also found the values for linear retardance (δ) and linear diattenuation (d) of the quarter waveplate and linear polarizer (respectively) to be in close agreement (δ∼1.60rad and d∼0.98) with the ideal values (δ=1.57rad and d=1) for the entire range: see Figure 1(d). These (and other) calibration results validate the accuracy of our Mueller-matrix measurements.2
We subsequently employed the system to record a spectroscopic-scattering Mueller matrix from a single isolated gold nanorod (with a diameter of 14±3nm and a length of 40±3nm): see Figure 2. The scattering spectra from the gold nanorod exhibits two distinct peaks, which correspond to the two electric dipolar plasmon resonances. One of the peaks occurs at λ∼525nm due to transverse resonance along the short axis, and the second occurs at λ∼650nm due to longitudinal resonance along the long axis.
The corresponding scattering Mueller matrices encode several interesting and potentially useful pieces of information. Despite considerable depolarization effects, which occur due to the high-NA imaging geometry, the decomposed non-depolarizing component (i.e., the diattenuating retarder matrix) shown in Figure 2(a) can be interpreted to extract the spectral diattenuation and retardance—d(λ) and δ(λ), respectively—of the plasmonic sample. We interpret these effects via the relative amplitudes and phases of the two orthogonal dipolar plasmon polarizations of the gold nanorod. The d(λ) parameter is a measure of the relative amplitudes of the transverse and longitudinal dipolar polarizabilities, and therefore peaks at wavelengths that correspond to their resonances (525 and 650nm for transverse and longitudinal, respectively). The δ(λ) parameter, on the other hand, quantifies the phase difference between the two orthogonal polarizabilities, and thus reaches its maximum value in the spectral overlap region of the two resonances (∼575nm): see Figure 2(b). The d(λ) and δ(λ) parameters therefore capture and quantify unique information on the relative strengths and phases (respectively) of the two dipolar plasmon polarizabilities of the gold nanorod. We can use this polarization information to probe, manipulate, and tune the interference of neighboring resonant modes (i.e., orthogonal dipolar modes in plasmonic nanorods), and the resulting spectral line shape of the plasmonic system, via polarization control.2
In summary, we have developed a novel experimental system for recording full 4×4 spectroscopic-scattering Mueller matrices from single isolated plasmonic nanoparticles or nanostructures. Inverse Mueller-matrix analysis on a single plasmonic gold nanorod yields intriguing spectral diattenuation and retardance effects that encode potentially valuable information about the relative strengths and phases of the resonant modes in plasmonic structures. These polarization parameters therefore hold considerable promise as novel experimental metrics for the analysis of a number of interesting plasmonic effects. The parameters could, for example, be used to probe, manipulate, and tune the interference of neighboring modes in complex coupled plasmonic structures, or to study spin-orbit interaction and the spin Hall effect of light.1, 5 Results from such studies could be used to develop and optimize novel polarization-controlled plasmonic sensing schemes. We are currently expanding our investigations in these directions. Our next steps include the analysis, interpretation, and control of the plasmonic Fano resonance using Mueller-matrix-based polarization analysis. We are also employing polarization-based weak-measurement schemes to observe and amplify tiny spin-optical effects in plasmonic structures. It is our hope that these studies will aid in the development of novel polarization-controlled optical metadevices for diverse applications.
Indian Institute of Science Education and Research, Kolkata
Shubham Chandel is currently pursuing his doctoral research in the Bio-Optics and Nano-Photonics Group, where his research focuses on the use of polarimetry. He obtained his MSc from the International School of Photonics, India.
Nirmalya Ghosh is a physicist with a specialization in optics and photonics. He is currently an associate professor in the Department of Physical Sciences, and heads the Bio-Optics and Nano-Photonics Group.
Physicists from the Moscow Institute of Physics and Technology (MIPT) have found that the two-dimensional form of carbon, known as graphene, might be the ideal material for manufacturing plasmonic devices capable of detecting explosive materials, toxic chemicals, and other organic compounds based on a single molecule, says an article published in Physical Review B.
Plasmons in constructing high-accuracy electronics and optics
Scientists have long been fascinated by the potential applications of a quasiparticle called the plasmon, a quantum of plasma oscillations. In the case of a solid body, plasmons are the oscillations of free electrons. Of special interest are the effects arising from the surface interactions of electromagnetic waves with plasmons—usually in the context of metals or semimetals, as they have a higher free electron density. Harnessing these effects could bring about a breakthrough in high-accuracy electronics and optics. One possibility opened up by plasmonic effects is the subwavelength light focusing, which increases the sensitivity of plasmonic devices to a point where they can distinguish a single molecule. Such measurements are beyond what any conventional (classical) optical devices can achieve. Unfortunately, plasmons in metals tend to lose energy quickly due to resistance, and for this reason they are not self-sustained, i.e. they need continuous excitation. Scientists are trying to tackle this issue by using composite materials with predefined microstructure, including graphene.
Graphene is an allotrope of carbon in the form of a two-dimensional crystal. It can be visualised as a one-atom-thick honeycomb lattice made of carbon atoms. Two MIPT graduates, Andre Geim and Konstantin Novoselov, were the first to isolate graphene, which won them a Nobel Prize in Physics. Graphene is a semiconductor with extremely high charge carrier mobility. Its electrical conductivity is also exceptionally high, which makes graphene-based transistors possible.
Theoretical physicists give the okay
Although, plasmonic devices have seemed an exciting prospect to pursue from the start, to take advantage of them, it was first necessary to find out whether the technology behind them was feasible. To do this, scientists had to find a numerical solution to the relevant quantum-mechanical equations. This was accomplished by a team of researchers at the Laboratory of Nanostructure Spectroscopy headed by Prof. Yurii Lozovik: they formulated and solved the necessary equation. Their research has led them to develop a quantum model that predicts plasmonic behaviour in graphene. As a result, the scientists described the operation of a surface-plasmon-emitting diode (SPED) and the nanoplasmonic counterpart of the laser—known as the spaser—whose construction involves a graphene layer.
Pic.: Design of the spaser with the graphene layer shown as a honeycomb lattice above the dielectric layer (blue). The spaser is optically pumped through the active (gain) medium shown in orange.
A spaser could be described as a device similar to a laser and operating on the same basic principle. However, to produce radiation, it relies on optical transitions in the gain medium, and the particles emitted are surface plasmons, as opposed to photons produced by a laser. An SPED is different from a spaser in that it is an incoherent source of surface plasmons. It also requires considerably lower pump power. Both devices would operate within the infrared region of the spectrum, which is useful for studying biological molecules.
‘The graphene spaser could be used to design compact spectral measurement devices capable of detecting even a single molecule of a substance, which is essential for many potential applications. Such sensors could detect organic molecules based on their characteristic vibrational transitions (‘fingerprints’), as the light emitted/absorbed falls into the medium infrared region, which is exactly where the graphene-based spaser operates,’ says Alexander Dorofeenko, one of the authors of the study.
Source news: https://mipt.ru/english/news/_sniffer_plasmons_could_detect_explosives?sphrase_id=143931