Transferring orbital angular momentum of light to plasmonic excitations in metamaterials

Metamaterial structure for OAM transfer. (A) Schematic view with the following structural parameters: inner radius (r), outer radius (R), periodicity (d), groove width (a), and number of grooves (N). The refractive indices inside the groove and outside the disk are given by ng and nout, respectively. (B) Optical image of the sample made of gold (r = 70 μm, R = 100 μm, N = 30, and a/d = 0.4). The thickness is around 100 nm. Chromium (10 nm thick) is deposited under the gold as an adhesion layer. Credit: Science Advances, doi: 10.1126/sciadv.aay1977

by Thamarasee Jeewandara
https://phys.org/news/2020-06-orbital-angular-momentum-plasmonic-metamaterials.html

The vortex beam with orbital angular momentum (OAM) is a new and ideal tool to selectively excite dipole forbidden states through linear optical absorption. The emergence of the vortex beam with OAM provides intriguing opportunities to induce optical transitions beyond the framework of electric dipole interactions. The unique feature arose from the transfer of OAM from light-to-material as demonstrated with electronic transitions in atomic systems .

In a new report on Science Advances, T. Arikawa and a team of researchers in physics, electrical engineering and cell materials science in Japan and Canada, detailed OAM transfer to electrons in solid-state systems. They used metamaterials to show how multipolar modes of surface electromagnetic excitations, also known as 'spoof' localized surface plasmons, could be selectively induced through the terahertz vortex beam. Spoof surface plasmons are a type of surface plasmon polariton (SPP) that typically propagates across dielectric and metallic interfaces at infrared and visible frequencies. However, since such polaritons cannot naturally occur in terahertz or microwave frequencies, spoof surface plasmons require artificial metamaterials for propagation in such frequencies.

The selection rules of the study were governed by the conservation of total angular momentum, which Arikawa et al. confirmed using numerical simulations. The efficient transfer of light orbital angular momentum to elementary excitations at room temperature in solid-state systems can expand the potential for experimental OAM manipulation to construct OAM-based applications, including quantum memories and OAM-based sensors.

Light-matter interactions are governed by spatial-temporal structures of a light field and via material wave functions. Researchers have used nonlinear optical methods such as two-photon absorption to selectively excite a specific dark mode, in the presence of strong light sources. The OAM (orbital angular momentum) provides a new method to selectively excite dipole-forbidden states through linear optical absorption, while deriving different selection rules. Scientists can explore such selectivity, relative to OAM transfer from light to a material, although such transitions are very small to record. In this work, Arikawa et al. investigated electrons in solids with extended wave functions as an ideal platform to study vortex light-matter interactions.

Recent studies in electromagnetic field analysis had predicted efficient OAM transfer from vortex beams to localized surface plasmons (LSPs) in a metallic disk. During simulations, multipolar modes with large angular momentum, i.e. quadrupole, hexapole, etc., can be selectively excited as a result of OAM transfer.

Experimental setup. (A) Schematic of the experimental setup. BS: beam splitter, QWP: quarter wave plate, PBS: polarizing beam splitter. (B) Magnified view of around the EO crystal (side view). (C) Electric field waveform of the incident Gaussian THz pulse. The inset shows its frequency spectrum. Credit: Science Advances, doi: 10.1126/sciadv.aay1977

In this work, the team experimentally showed selective excitation using spoof LSP (a low-frequency analog of LSP) that can exist around the surface of a periodically textured metallic disk. They built the metamaterial structure to bring the resonance frequencies down to the terahertz (THZ) frequency range for non-destructive imaging. The experimental setup allowed the scientists to visualize the characteristic patterns surrounding the corrugated disk and identify spoof LSP modes excited in the sample. To visualize near-field patterns due to LSPs, Arikawa et al. engineered corrugated gold disks on the top surface of a terahertz (THZ) detector crystal, to sample the electric field that formed a few microns away from the metallic structure. They performed the experiments at room temperature and obtained five snapshots of the THZ electric field around the sample after excitation by a linearly polarized Gaussian beam.

Time-resolved near-field imaging and mode expansion analysis.

Simulations for the vortex beam (OAM +ħ) excitation exhibit the characteristic field distribution (six zero crossing points) unique to the clockwise quadrupole mode, similar to the experimental result. Credit: Science Advances, doi: 10.1126/sciadv.aay1977

After the incident terahertz pulse passed through the sample, the team observed an electric field oscillation localized around the outer circle of the sample as a resonant excitation of spoof LSP, representing the expected electric field pattern. The work confirmed the excitation of the dipole mode by the Gaussian beam and that multiple spoof LSPs could be excited by vortex beams. To illustrate this point, Arikawa et al. performed additional analyses by focusing on the electric field along the outer circle of the sample to represent the frequency spectrum of each LSP mode. The results showed the efficient and selective excitation of multipolar modes based on the OAM of light, allowing the scientists to identify all spoof LSP modes excited in the sample.

Selective excitation of multipole spoof LSPs. Selected snapshots of the near-field evolution around the sample excited by (A) Gaussian beam, (C) vortex beam (OAM +ħ), and (E) vortex beam (OAM −2ħ). The double circle represents the position of the sample (inner and outer radius). The time origin (0 ps) is the time when the first positive peak of the incident pulse comes. The color scales are optimized at each frame for the sake of clarity. (B, D, and F) The electric field taken along the outer circle of the sample as a function of the azimuthal angle φ (red curves). The error bars are almost the same as the thickness of the traces. The dashed cosine curves are expected electric field patterns when the modes depicted on the right are excited. The solid arrows schematically represent the quasi-static electric field around each mode. The cosine functions are obtained by projecting the quasi-static field onto the polarization axis (e0, dashed up arrow) detected in the experiment. er and eφ are cylindrical unit vectors introduced to calculate quasi-static fields. a.u., arbitrary units. Credit: Science Advances, doi: 10.1126/sciadv.aay1977

The analysis additionally revealed the resonance frequency of each mode, allowing them to draw the dispersion relation i.e. the relation between the optical frequency and the propagation constants of surface plasmon polariton modes. The dispersion relation of the spoof LSPs depended on the geometrical parameters of the metallic structures, providing the scientists a powerful tool to control the resonance frequencies. The team performed additional experiments and analyses on samples with diverse dimensions of corrugation to demonstrate resonance frequency control. The results allowed them to deduce the selection rules in the system to excite multiple spoof LSPs. The observations strongly supported that the selection rules were governed by the conservation of total angular momentum (TAM), which the team then numerically confirmed for spoof LSPs using similar electromagnetic field analyses.

Mode decomposition of near-field distributions. Frequency spectra of the dipole [E(±2, f)], quadrupole [E(±3, f)], and hexapole [E(±4, f)] modes excited in the sample illuminated by (A) Gaussian beam, (B) vortex beam (+ħ), and (C) vortex beam (−2ħ). (D) Dispersion relation of the spoof LSP. The red dots represent the resonance frequencies determined in (A) to (C). The blue curve is a theoretical fitting. Credit: Science Advances, doi: 10.1126/sciadv.aay1977

In this way, T. Arikawa and colleagues observed traveling surface waves with low electron scattering to enable coherent collective motion of electrons across the entire sample. The frequency tunability of the corrugated metallic disk geometry allowed it to be a very versatile OAM receiver with wide ranging frequencies as long as the scattering in the experimental setup was sufficiently low. The team expect the OAM to transfer across to other elementary excitations in solids including Rydberg excitons, skyrmions and phonons, although they will need focusing techniques beyond the diffraction limit in such instances. The work on efficient OAM exchange between light and elementary excitations in solid-state systems will be foundational to generate novel solid-state devices for OAM applications.

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Abstract-Terahertz microscopy assisted by semiconductor nonlinearities

François Blanchard, Xin Chai, Tomoko Tanaka, Takashi Arikawa, Tsuneyuki Ozaki, Roberto Morandotti, and Koichiro Tanaka

Fig. 1. Experimental setup. (a) THz transmission scheme using wire-grid polarizer and a LN sensor. (b) Illustration of the sample used for near-field investigations. (c) Visible image of a gold ring array structure patterned on a In0.53Ga0.47As${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ epilayer thin film.

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-43-20-4997#articleFigures

Terahertz (THz) imaging is currently based on linear effects, but there is great interest on how nonlinear effects induced by terahertz radiation could be exploited to provide extra information that is unobtainable by conventional imaging schemes. In particular, at field strengths on the order of 100  kV cm−1$100\mathrm{kV}{\mathrm{cm}}^{-1}$ to 1  MV cm−1$1\mathrm{MV}{\mathrm{cm}}^{-1}$, transmission properties inside semiconductor materials are largely affected at the picosecond time-scale, which raise the prospect of interesting nonlinear imaging applications at THz frequencies. Here, we experimentally investigate a method to map the two-dimensional nonlinear near-field distribution of an intense THz pulse passing through a thin film-doped semiconductor. By inserting a metamaterial structure between the electro-optic sensor and the doped film, the nonlinear near-field dynamics shows a different and enhanced contrast of the sample when compared to its linear counterpart.

© 2018 Optical Society of America

Abstract-Control of Spoof Localized Surface Plasmons Using Terahertz Near-field Microscope

Takashi Arikawa, Shohei Morimoto, Tomoki Hiraoka, François Blanchard, Kyosuke Sakai, Keiji Sasaki, and Koichiro Tanaka

https://www.blogger.com/blogger.g?blogID=124073320791841682#editor/target=post;postID=2704300894288992135

We performed time-resolved terahertz near-field imaging of a gold disk with sub-wavelength periodic grooves and successfully observed spoof localized surface plasmons. A selective excitation method is also demonstrated with orbital angular momentum of light.

© 2018 The Author(s)

Abstract-Coexistence of Kosmotropic and Chaotropic Impacts of Urea on Water As Revealed by Terahertz Spectroscopy

Keiichiro Shiraga , Yuichi Ogawa, Koichiro Tanaka, Takashi Arikawa, Naotaka Yoshikawa, Masahito Nakamura, Katsuhiro Ajito, Takuro Tajima,

http://pubs.acs.org/doi/10.1021/acs.jpcb.7b11839

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broad-band THz spectroscopy (0.2–12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2, 4, and above 12 THz. After eliminating the contribution of these modes, the “urea-vibration-free” complex dielectric constant was decomposed into the relaxation modes of bulk water and the oscillation modes of water. When hydration water is defined to be reorientationally retarded relative to bulk, our analysis revealed that the hydration number is 1.9 independent of urea concentrations up to 5 M, and this number is in close agreement with that of water constrained by strong acceptor hydrogen bonds of urea oxygen. Regarding the hydrogen bond structure, it was found that the tetrahedral-like water structure is mostly preserved (though the hydrogen bond lifetime is significantly shortened) but the population of non-hydrogen-bonded water molecules fragmented from the network is markedly increased, presumably due to urea’s NH2 inversion. These experimental results point to the coexistence of apparently two contradictory aspects of urea: dynamical retardation (the kosmotropic aspect) by the −CO group and slight structural disturbance (the chaotropic aspect) by the −NH2 group.

Abstract-Focusing light with orbital angular momentum by circular array antenna

Takashi Arikawa, Shohei Morimoto, and Koichiro Tanaka

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-25-12-13728&origin=search

We experimentally demonstrated focusing of light with orbital angular momentum (OAM) using an 8-element circular array of linear antennas. A spiral phase plate was used to generate a vortex beam with an OAM of ħ in the terahertz (THz) frequency region. We used THz near-field microscope to directly measure the phase vortex. A beam profile with a center dark spot and 2π phase rotation was observed in the small center gap region of the circular array antenna after the vortex beam excitation. The beam size is reduced by a factor of 3.4 ± 0.2. Half-wave resonance of the antenna element is responsible for the focusing function, indicating the scalability of this method to other frequency regions. This method will enable deep subwavelength focusing of light with OAM and eliminate the obstacle for the observation of the dipole forbidden transition with finite OAM of the vortex beam.

© 2017 Optical Society of America

Abstract-Superradiant Decay of Cyclotron Resonance of Two-Dimensional Electron Gases

Qi Zhang, Takashi Arikawa, Eiji Kato, John L. Reno, Wei Pan, John D. Watson, Michael J. Manfra, Michael A. Zudov, Mikhail Tokman, Maria Erukhimova, Alexey Belyanin, and Junichiro Kono
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.113.047601

We report on the observation of collective radiative decay, or superradiance, of cyclotron resonance (CR) in high-mobility two-dimensional electron gases in GaAs quantum wells using time-domain terahertz magnetospectroscopy. The decay rate of coherent CR oscillations increases linearly with the electron density in a wide range, which is a hallmark of superradiant damping. Our fully quantum mechanical theory provides a universal formula for the decay rate, which reproduces our experimental data without any adjustable parameter. These results firmly establish the many-body nature of CR decoherence in this system, despite the fact that the CR frequency is immune to electron-electron interactions due to Kohn’s theorem.

DOI: http://dx.doi.org/10.1103/PhysRevLett.113.047601

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• Published 21 July 2014
• Received 5 May 2014

© 2014 American Physical Society

Terahertz polarizer nears perfection

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http://www.noodls.com/viewNoodl/12927503/rice-university/1302012---terahertz-polarizer-nears-perfection
1/30/2012

Mike Williams713-348-6728mikewilliams@rice.edu

Research at Rice University leads to nanotube-based device for communication, security, sensing 

Researchers at Rice University are using carbon nanotubes as the critical component of a robust terahertz polarizer that could accelerate the development of new security and communication devices, sensors and non-invasive medical imaging systems as well as fundamental studies of low-dimensional condensed matter systems.
The polarizer developed by the Rice lab of Junichiro Kono, a professor of electrical and computer engineering and of physics and astronomy, is the most effective ever reported; it selectively allows 100 percent of a terahertz wave to pass or blocks 99.9 percent of it, depending on its polarization. The research was published in the online version of the American Chemical Society journal, Nano Letters.
The broadband polarizer handles waves from 0.5 to 2.2 terahertz, far surpassing the range of commercial polarizers that consist of fragile grids wrapped in gold or tungsten wires.
Kono said technologies that make use of the optical and electrical regions of the electromagnetic spectrum are mature and common, as in lasers and telescopes on one end and computers and microwaves on the other. But until recent years, the terahertz region in between was largely unexplored. "Over the past decade or two, people have been making impressive progress," he said, particularly in the development of such sources of radiation as the terahertz quantum cascade laser.
"We have pretty good terahertz emitters and detectors, but we need a way to manipulate light in this range," Kono said. "Our work is in this category, manipulating the polarization state -- the direction of the electric field -- of terahertz radiation."
Terahertz waves exist at the transition between infrared and microwaves and have unique qualities. They are not harmful and penetrate fabric, wood, plastic and even clouds, but not metal or water. In combination with spectroscopy, they can be used to read what Kono called "spectral fingerprints in the terahertz range"; he said they would, for instance, be useful in a security setting to identify the chemical signatures of specific explosives.
The work by Kono and lead author Lei Ren, who recently earned his doctorate at Rice, makes great use of the basic research into carbon nanotubes for which the university is famous. Co-authors Robert Hauge, a distinguished faculty fellow in chemistry, and his former graduate student Cary Pint developed a way to grow nanotube carpets and to transfer well-aligned arrays of nanotubes from a catalyst to any substrate they chose, limited only by the size of the growth platform.
While Hauge and Pint were developing their nanotube arrays, Kono and his team were thinking about terahertz. Four years ago, they came across a semiconducting material, indium antimonide, that would stop or pass terahertz waves, but only in a strong magnetic field and at very low temperatures.
At about the same time, Kono's lab began working with carbon nanotube arrays transferred onto a sapphire substrate by Pint and Hauge. Those aligned arrays -- think of a field of wheat run over by a steamroller -- turned out to be very effective at filtering terahertz waves, as Kono and his team reported in a 2009 paper. 
"When the polarization of the terahertz wave was perpendicular to the nanotubes, there was absolutely no attenuation," Kono recalled. "But when the polarization was parallel to the nanotubes, the thickness was not enough to completely kill the transmission, which was still at 30-50 percent."
The answer was clear: Make the polarizer thicker. The current polarizer has three decks of aligned nanotubes on sapphire, enough to effectively absorb all of the incident terahertz radiation. "Our method is unique, and it's simple," he said.
Kono sees use for the device beyond spectroscopy by manipulating it with an electric field, but that will only become possible when all of the nanotubes in an array are of a semiconducting type. As they're made now, batches of nanotubes are a random mix of semiconductors and metallics; recent work by Erik Hároz, a graduate student in Kono's lab, detailed the reasons that nanotubes separated through ultracentrifugation have type-dependent colors. But finding a way to grow specific types of nanotubes is the focus of a great deal of research at Rice and elsewhere.
Co-authors are former Rice postdoctoral researcher Takashi Arikawa and research associate Iwao Kawayama and Professor Masayoshi Tonouchi of the Institute of Laser Engineering at Osaka University, Japan.
The Department of Energy, the National Science Foundation and the Robert A. Welch Foundation supported the research.

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Read the abstract at http://pubs.acs.org/doi/abs/10.1021/nl203783q
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Image for download:

media.rice.edu/images/media/NewsRels/0127_FigureLeiRen.jpg
CAPTION:
A triple layer of carbon nanotube arrays on a sapphire base are the basis for a new type of terahertz polarizer invented at Rice University. The polarizer could lead to new security and communication devices, sensors and non-invasive medical imaging systems. (Credit: Lei Ren/Rice University)