Free-space Radiation Research Articles

Nanostructure-Mediated Launching and Detection of 2D Surface Plasmons

Au nanoparticles deposited on a metallic film act as nanoantenna receivers and transmitters for the coupling of free-space radiation into, and out of, 2D surface plasmons. Nanosteps, sub-10-nm gaps between metallic films of differing thickness, can also launch and detect surface plasmons. Here we use both types of structures to locally launch propagating surface plasmon waves and probe their properties. Nanoparticle-launched surface plasmons emerge as two lobes of nominally 90 degree angular width, propagating along the direction of incident polarization. Alternatively, plasmons can be launched unidirectionally, by asymmetric illumination of a nanoparticle receiver.

SPASER laser and Purcell factor

This work presents an interpretation of experiments where laser action was observed in the so-called SPASER, a laser with a nanoscale surface plasmon. The experimental implementation of SPASER was reported by M. A. Noginov et al. in the paper entitled “Demonstration of SPASER-Based Nanolaser” and by Xiang Zhang et al. in the paper entitled “Plasmon Lasers at Deep Subwavelength Scale”, both of which were published online in the advance online publication of Nature in August 2009. Nanoscale plasmonic modes experience huge losses; however, simultaneously, the probabilities of radiative processes in atoms (molecules) leading to the emission of radiation into these modes increase 1000-fold compared to the probabilities of radiation in free space. The similarity of the physical nature of the effects in SPASER with SERS is pointed out.

Numerical investigations of a multi-walled carbon nanotube-based multi-segmented optical antenna

Motivated by the fabrication potential of multi-walled carbon nanotube structures, we numerically investigated a paired structure consisting of two metallic spheres each grown on one end of a multi-walled nanotube. The paired two-segmented structure is capable to convert free-space radiation into an intense near-field, and, hence, acting as an optical antenna. Vice versa the presence of the two nanotubes enable a current source at the antenna feed to more efficiently energy into the radiation modes, resulting e.g. in correspondingly altered luminescence lifetimes when an excited single molecule is placed in the feed point. Furthermore, the structure represents a mean to localize light on a sub-wavelength scale within different materials, which is interesting in the context of a fabrication technology for integrated nanophotonic components with different material combinations. The optical properties of the nano-antenna are analyzed by means of numerical simulations using the finite element method. Our investigations have revealed that the field enhancement, the resonances, and the radiation patterns can be easily tuned since all these quantities strongly depend on the size of the nanotubes and the metallic spheres, as well as on their material properties The structure we propose here carries a great potential for bio-sensing, for tip-enhanced spectroscopy applications, and for interfacing integrated photonic nano circuits.

Surface acoustic wave mediated coupling of free-space radiation into surface plasmon polaritons on plain metal films

We propose and demonstrate a surface acoustic wave-driven converter of light into surface plasmon polaritons. An otherwise unstructured thin metal film is deformed by traveling acoustic waves on a piezoelectric substrate underneath. This spatially periodic corrugation enables to bridge the momentum gap between free-space radiation and surface-bound modes. This principle is realized with plain gold films on a ${\text{LiNbO}}_{3}$ wafer where surface acoustic waves induce surface ripples in the metal. For near-infrared light we observe efficiencies of order ${10}^{\ensuremath{-}4}$ for exciting surface plasmon polaritons.

Generation of Free Space Radiation Patterns From Non-Anechoic Measurements Using Chebyshev Polynomials

Generally, antenna radiation patterns are measured in an anechoic condition so that the presence of undesired reflections does not skew the final measurements. The objective of this communication is to search for methodologies which can extract meaningful approximate results from non-anechoic measurements. In other words, how to process the reflection contaminated data and extract valuable information when one does not have a good anechoic room and performs measurements with various undesired reflections present in the data. The Chebyshev polynomials are used to extract from the data collected under non-anechoic environment and generate a radiation pattern for the same antenna radiating in free space conditions. The problem with this method is that some <emphasis emphasistype="italic" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a priori</emphasis> information is required about the environment. Measured data has been used to illustrate the applicability of this new methodology and its improved performance over the FFT based methods.

Accurate Radiation-Pattern Measurements in a Time-Reversal Electromagnetic Chamber

In a recent paper [1], we introduced the concept of the time-reversal electromagnetic chamber (TREC), a new facility for creating coherent wavefronts within a reverberation chamber. This facility, based on the use of time-reversal techniques in a reverberating environment, is shown here to also be a useful tool for the characterization of the field radiated by an antenna under test (AUT). The time-reversal electromagnetic chamber has proven to be capable of providing real-time measurements, with accuracy typically better than fdB over the main Iobes, while-using a limited number of static probe antennas. This performance is made possible by taking advantage of the reflections over the chamber's walls in order to gain access to the field radiated along all directions, with no need to mechanically displace the probes, or to have a full range of electronically switched probes. A two-dimensional numerical validation supported this approach, proving that the proposed procedure allows the retrieval of the free-space radiation pattern of the AUT.

Tunable ultrafast control of plasmonic coupling to gold films

We demonstrate ultrafast modulation of the coupling efficiency of free-space radiation to a surface plasmon polariton mode on a gold film with a PMMA grating overlayer over a broad range of wavelengths (540--700 nm). We observe modulation magnitudes of up to 14% at the center of the resonance, or 61% at the outlying regions. The limitations of this method are explored using simulations of the nonequilibrium electron thermal dynamics of the gold and of the response of the grating structure.

Entanglement in Weisskopf–Wigner theory of atomic decay in free space

In this paper, we use the Weisskopf–Wigner theory to study the entanglement in the state of the free-space radiation field produced from vacuum due to atomic decay. We show how bipartite entanglement is shared between different partitions of the radiation modes. We investigate the role played by the size of the partitions and their detuning with the decaying atom. The dynamics of the atom-field entanglement during the atomic decay is also briefly discussed. From this dynamics, we assert that such entanglement is the physical quantity that fix the statistical atomic decay time.

Non-invasive single bunch monitoring for ps pulse radiolysis

A single-shot electro-optic (EO) diagnostic has been installed on the ELYSE photocathode RF gun accelerator to monitor the electron bunch at the place and under the conditions of the ps pulse radiolysis experiments. The EO signal is due to the coulombic field of the electron bunch and to a contribution of a free-space THz radiation generated by the same electron pulse. This signal is recorded shot-to-shot at the repetition rate of the accelerator. The jitter of the arrival time of the electron bunch is characterized for the first time with a non-invasive method and is confirmed to be around 1 ps.

Intercoupling of free-space radiation to s-polarized confined modes via nanocavities

The interaction of the s-polarized waveguide mode in the dielectric layer placed on a metal film with one-dimensional indentations in the metal is studied. The efficiency of the coupling to the out-of-plane radiation is considered. It is shown that varying the parameters of the cavities, the periodical array of resonant indentations can almost totally reflect back the incident waveguide mode or, on the contrary, efficiently transform it into a narrow-directional outgoing beam. An efficient waveguide mode launcher is examined on the basis of the results for Bragg mirror. The differences and similarities between scattering of surface plasmon polaritons and waveguide modes are discussed.

Coherent transmission of terahertz wave through randomly packed subwavelength-sized aluminium particles

We experimentally discovered that the efficient coupling of surface plasmon wave to free-space radiation beyond the interaction zone via random metallic particles. In the experiments of terahertz wave transmission through an ensemble of randomly packed aluminium particles with sub-wavelength sizes, we observed a strong dependence of the transmission on the lateral boundary conditions. Our results showed that the processes of terahertz wave propagating in the disk-shaped samples and reemitting to the free space were resulted from surface modes excitation, which is critically affected by lateral sizes of the particle ensembles.

A simple solution for an intense terahertz emitter

Terahertz (THz, in the far IR region) imaging and spectroscopy offers the ability to discover chemical composition noninvasively. Applications include medical imaging, security screening, and pharmaceutical quality control. Semiconductors can produce pulsed THz radiation when their surface is illuminated by a sub-picosecond laser pulse, rather than continuous wave sources, such as a quantum cascade laser. Yet despite several technical advances, the output power is low. Thus the development of higher power semiconductor THz emitters is of considerable practical significance.1 THz generation occurs when a semiconductor is illuminated by an ultrafast laser pulse with photon energy greater than the semiconductor bandgap. A large number of electron and hole pairs are created and accelerated in opposite directions by the electric field. The resulting charge separation forms a dipole that emits a coherent THz pulse. THz radiation generated within a material of high refractive index n has an extraction problem: at the semiconductor surface, all rays outside an ‘emission cone’ of half-angle α = sin−1 1 n to the surface normal (shown as a grey cone in Figure 1) suffer total internal reflection and do not escape from the device. The fraction of power extracted depends on the orientation of the dipole axis relative to the surface. Several methods exist to increase the extraction efficiency, such as applying an external magnetic field,2 using a coupling prism,3 or fabrication of low-dimensional nanostructures.4 For example, under a magnetic field the Lorentz force—the force on a point charge due to electromagnetic fields—can rotate the orientation of the dipole to coincide with the emission cone as shown in Figure 1(a). For the extreme case of a dipole parallel to the surface, the emitted power would increase by more than a factor of 20. But these methods require cumbersome equipment, such as a strong magnet, or special sample growth techniques. We simply changed the growth orientation of the Figure 1. (a) By applying an external magnetic field (B), the THz dipole can be rotated and the radiation pattern can be overlapped with the emission cone to enhance the free-space radiation. (b) The wurtzite (hexagonal crystal) structure of indium nitride (InN). The InN film is grown along the a-axis and the in-plane electric field is formed on the a-plane.

Loss mechanisms of surface plasmon polaritons on gold probed by cathodoluminescence imaging spectroscopy.

We use cathodoluminescence imaging spectroscopy to excite surface plasmon polaritons and measure their decay length on single crystal and polycrystalline gold surfaces. The surface plasmon polaritons are excited on the gold surface by a nanoscale focused electron beam and are coupled into free space radiation by gratings fabricated into the surface. By scanning the electron beam on a line perpendicular to the gratings, the propagation length is determined. Data for single-crystal gold are in agreement with calculations based on dielectric constants. For polycrystalline films, grain boundary scattering is identified as additional loss mechanism, with a scattering coefficient SG=0.2%.

Ultrafast control of grating-assisted light coupling to surface plasmons

We demonstrate subpicosecond control over the coupling of free-space radiation to surface-plasmon polaritons using 830 and 500 nm period gold gratings. Thermal changes to the electron distribution following irradiation by 100 fs, 810 nm pulses produce a shift of the 570 nm plasmon resonance by approximately 0.75 nm with reflectivity change up to 6% and decay time of approximately 1 ps.

Local and anisotropic excitation of surface plasmon polaritons by semiconductor nanowires

We demonstrate a novel functionality of semiconductor nanowires as local sources for surface plasmon polaritons (SPPs). Photoexcited semiconductor nanowires decay non-radiatively exciting SPPs when they are on top of a metallic surface. We have investigated the anisotropic excitation of SPPs by nanowires by placing individual InP nanowires inside gold bullseye gratings. The gratings serve to couple SPPs to free space radiation that is detected with a scanning confocal microscope. The circular geometry of the grating allows to conclude that SPPs are preferentially generated in the direction along the nanowire axis.

Excitation and Propagation of Surface Plasmons in a Metallic Nanoslit Structure

We provide an analysis of optical interactions with a nanoslit structure formed on a metal film. In particular we analyzed the excitation and coupling of surface plasmons (SPs) at the input and output apertures of the nanoslit as well as the propagation of light in the nanoslit waveguide structure. In the input coupling case, an incident light (TM polarized) is found to induce polarization charges on the surface around the aperture. These charges generate the normal component of electric field on the incident surface around the corner. This results in an inward flow (funneling) of incident power into the slit. At the output side, the slit corners again play a critical role, exciting SPs on the exit surface, which propagate away from the slit, and also allowing for decoupling of the SP power into free-space radiation. In the case of narrow slits, the incident optical power is carried exclusively via the SP mode that has an antisymmetric distribution of polarization charges on the slit walls. The materials required to support a surface-bound wave are also discussed, taking into account the dielectric loss of metal.

Fundamental role of the retarded potential in the electrodynamics of superluminal sources

We calculate the gradient of the radiation field generated by a polarization current with a superluminally rotating distribution pattern and show that the absolute value of this gradient increases as R(7/2) with distance R, within the sharply focused subbeams that constitute the overall radiation beam from such a source. In addition to supporting the earlier finding that the azimuthal and polar widths of these subbeams become narrower (as R(-3) and R(-1), respectively) with distance from the source, this result implies that the boundary contribution to the solution of the wave equation governing the radiation field does not always vanish in the limit where the boundary tends to infinity (as is commonly assumed in textbooks and the published literature). While the boundary contribution to the retarded solution for the potential can always be rendered equal to zero by means of a gauge transformation that preserves the Lorenz condition, the boundary contribution to the retarded solution of the wave equation for the field may be neglected only if it diminishes with distance faster than the contribution of the source density. In the case of a rotating superluminal source, however, the boundary term in the retarded solution for the field is by a factor of the order of R(1/2)larger than the source term of this solution, in the limit where the boundary tends to infinity. This result explains why an argument based on the solution of the wave equation governing the field in which the boundary term is neglected [such as that presented by Hannay, J. Opt. Soc. A 23, 1530 (2006)] misses the nonspherical decay of the field that is generated by a rotating superluminal source. The only way one can calculate the free-space radiation field of an accelerated superluminal source is via the retarded solution for the potential. Our findings have implications also for the observations of the pulsar emission: The more distant a pulsar, the narrower and brighter its giant pulses should be.

Near- to far-field imaging of free-space and surface-bound waves emanating from a metal nanoslit

The authors report the radiation pattern (radial and angular distribution of light intensity) of a silver nanoslit measured in the near- to far-field regimes. In most far fields, the 1∕r dependence of intensity distribution, expected from a cylindrical wave emanating from a line source, is clearly observed. The glancing angle regime is found to be governed by the presence of surface plasmons, showing higher intensity closer to the metal surface. From the radiation patterns measured with a tilted-probe, radial-scan method, a branching ratio is quantitatively determined for the free-space radiation and surface plasmon components, emerging from the nanoslit.

Grating Couplers Fabricated by Electron-Beam Lithography for Coupling Free-Space Light Into Nanophotonic Devices

Grating couplers with nanoscale periodicity have been fabricated on silicon-on-insulator (SOI) substrates by electron beam lithography and reactive ion etching. A versatile experimental apparatus has been implemented to measure the efficiency of these gratings in coupling free-space radiation into planar waveguides. This coupling efficiency has been measured as a function of grating depth and the angle and wavelength of incident radiation. Coupling efficiencies of at least 5% and as high as 20% are demonstrated for wavelengths in the vicinity of 1550 nm and incident angles around 45deg.

Frequency selective surfaces for high sensitivity terahertz sensing

An apporach for the sensing of small amounts of chemical and biochemical material is presented. A frequency selective surface made from asymmetric split ring resonators is excited with free space radiation. Due to interference effects a resonance occurs with a steep flank in the frequency response which is shifted upon dielectric loading. Utilizing a strong E-field concentration by selective loading the detection of very small amounts of probe material becomes possible. The functionality is proven by numerical simulation and the optimization of structure and loading is performed.