Plasmons in strongly coupled metallic nanostructures.

When light interacts with a metal nanoparticle (NP), its conduction electrons can be driven by the incident electric field in collective oscillations known as localized surface plasmon resonances (LSPRs). These give rise to a drastic alteration of the incident radiation pattern and to striking effects such as the subwavelength localization of electromagnetic (EM) energy, the formation of high intensity hot spots at the NP surface, or the directional scattering of light out of the structure. LSPRs can also couple to the EM fields emitted by molecules, atoms, or quantum dots placed in the vicinity of the NP, leading in turn to a strong modification of the radiative and nonradiative properties of the emitter. Since LSPRs enable an efficient transfer of EM energy from the near to the far-field of metal NPs and vice versa, we can consider plasmonic nanostructures as nanoantennas, because they operate in a similar way to radio antennas but at higher frequencies. Typically, plasmonic nanoantennas at optical frequencies are made of gold and silver due to their goodmetallic properties and low absorption. Controlling and guiding light has been one of science’s most influential achievements. It affects everyday life in many ways, such as the development of telescopes, microscopes, spectrometers, and optical fibers, to name but a few. These examples exploit the wave nature of light and are based on the reflection, refraction and diffraction of light by optical elements such as mirrors, lenses or gratings. However, the wave nature of light limits the resolution to which an object can be imaged, as well as the size of the transverse cross section of efficient guiding structures to the wavelength dimension. Plasmonic resonances in nanoantennas overcome these constraints, allowing unprecedented control of light-matter interactions within subwavelength volumes (i.e., within the nanoscale at optical frequencies). Such properties have attracted much interest lately, due to the implications they have both in fundamental research and in technological applications. Metal NPs have been used since antiquity. Due to their strong scattering properties in the visible range, they show attractive colors. One of their first applications, dating back to the Roman Empire more than 2000 years ago, was as a colorant for clothing. In art, they were used to stain window glass and ceramics. Obviously, it was not known then that the colorants being used contained metal NPs or that the spectacular colors were due to the excitation of LSPRs. The first reported intentional production of metal NPs dates from 1857, when Faraday synthesized gold colloids. However, at the time there was not much interest in understanding the physics behind the optical properties of colloids due to the impossibility of synthesizing NPs with well-controlled shapes and sizes, as well as the lack of accurate detection techniques. The first theoretical work on the scattering of light by particles smaller than the incident wavelength was carried out by Lord Rayleigh at the end of the 19th century. He analyzed the diffusion of light by diluted gases, and his theory explained physical phenomena such as the blueness of the sky, the redness of the sunset, or the yellow color of the sun. Mie took the next step forward by deriving an analytical solution to Maxwell’s equations to describe the interaction of light with spheres of arbitrary radius and composition. Subsequently, based on the results of Rayleigh and Mie, Gans considered elliptical geometries. He demonstrated that the optical response of metal NPs is

Plasmonic nanoantennas: fundamentals and their use in controlling the radiative properties of nanoemitters.

Nanoantennas for visible and infrared radiation can strongly enhance the interaction of light with nanoscale matter by their ability to efficiently link propagating and spatially localized optical fields. This ability unlocks an enormous potential for applications ranging from nanoscale optical microscopy and spectroscopy over solar energy conversion, integrated optical nanocircuitry, opto-electronics and density-of-states engineering to ultra-sensing as well as enhancement of optical nonlinearities. Here we review the current understanding of metallic optical antennas based on the background of both well-developed radiowave antenna engineering and plasmonics. In particular, we discuss the role of plasmonic resonances on the performance of nanoantennas and address the influence of geometrical parameters imposed by nanofabrication. Finally, we give a brief account of the current status of the field and the major established and emerging lines of investigation in this vivid area of research.

https://iopscience.iop.org/article/10.1088/0034-4885/75/2/024402/pdf

Nanoantennas for visible and infrared radiation.

Materials whose optical properties can be reconfigured are crucial for photonic applications such as optical memories. Phase-change materials offer such utility and here recent progress is reviewed. Phase-change materials (PCMs) provide a unique combination of properties. On transformation from the amorphous to crystalline state, their optical properties change drastically. Short optical or electrical pulses can be utilized to switch between these states, making PCMs attractive for photonic applications. We review recent developments in PCMs and evaluate the potential for all-photonic memories. Towards this goal, the progress and existing challenges to realize waveguides with stepwise adjustable transmission are presented. Colour-rendering and nanopixel displays form another interesting application. Finally, nanophotonic applications based on plasmonic nanostructures are introduced. They provide reconfigurable, non-volatile functionality enabling manipulation and control of light. Requirements and perspectives to successfully implement PCMs in emerging areas of photonics are discussed.

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Phase-change materials for non-volatile photonic applications

Nonlinear optical phenomena are crucial for a broad range of applications, such as microscopy, all-optical data processing, and quantum information. However, materials usually exhibit a weak optical nonlinearity even under intense coherent illumination. We report that indium tin oxide can acquire an ultrafast and large intensity-dependent refractive index in the region of the spectrum where the real part of its permittivity vanishes. We observe a change in the real part of the refractive index of 0.72 ± 0.025, corresponding to 170% of the linear refractive index. This change in refractive index is reversible with a recovery time of about 360 femtoseconds. Our results offer the possibility of designing material structures with large ultrafast nonlinearity for applications in nanophotonics.

https://science.sciencemag.org/content/sci/352/6287/795.full.pdf

Large optical nonlinearity of indium tin oxide in its epsilon-near-zero region

We demonstrate experimentally picosecond all-optical control of a single plasmonic nanoantenna embedded in indium tin oxide (ITO). We identify a picosecond response of the antenna–ITO hybrid system, which is distinctly different from transient bleaching observed for gold antennas on a nonconducting SiO2 substrate. Our experimental results can be explained by the large free-carrier nonlinearity of ITO, which is enhanced by plasmon-induced hot-electron injection from the gold nanoantenna into the conductive oxide. The combination of tunable antenna–ITO hybrids with nanoscale plasmonic energy transfer mechanisms, as demonstrated here, opens a path for new ultrafast devices to produce nanoplasmonic switching and control.

/pdf/all-optical-control-of-a-single-plasmonic-nanoantenna-ito-3n5hgbw9qn.pdf

All-optical control of a single plasmonic nanoantenna-ITO hybrid.

We propose and explore theoretically a new concept of ultrafast optical switches based on nonlinear plasmonic nanoantennas. The antenna nanoswitch operates on the transition from the capacitive to conductive coupling regimes between two closely spaced metal nanorods. By filling the antenna gap with amorphous silicon, progressive antenna-gap loading is achieved due to variations in the free-carrier density in the semiconductor. Strong modification of the antenna response is observed both in the far-field response and in the local near-field intensity. The large modulation depth, low switching threshold, and potentially ultrafast time response of antenna switches holds promise for applications ranging from integrated nanophotonic circuits to quantum information devices.

/pdf/photoconductively-loaded-plasmonic-nanoantenna-as-building-4p0wp68588.pdf

Photoconductively loaded plasmonic nanoantenna as building block for ultracompact optical switches.

We successfully demonstrate surface-enhanced infrared spectroscopy using arrays of indium tin oxide (ITO) plasmonic nanoantennas. The ITO antennas show a strongly reduced plasmon wavelength, which holds promise for ultracompact antenna arrays and extremely subwavelength metamaterials. The strong plasmon confinement and reduced antenna cross section allows ITO antennas to be integrated at extremely high densities with no loss in performance due to long-range transverse interactions. By further reducing the spacing of antennas in the arrays, we access the regime of plasmonic near field coupling where the response is enhanced for both Au and ITO devices. Ultracompact ITO antennas with high spatial and spectral selectivity in spectroscopic applications offer a viable new platform for infrared plasmonics, which may be combined with other functionalities of these versatile materials in devices.

/pdf/surface-enhanced-infrared-spectroscopy-using-metal-oxide-o2exq2q3dt.pdf

Surface-Enhanced Infrared Spectroscopy Using Metal Oxide Plasmonic Antenna Arrays

The dielectric response of transparent conductive oxides near the bulk plasmon frequency is characterized by a refractive index less than vacuum. In analogy with x-ray optics, it is shown that this regime results in total external reflection and air-guiding of light. In addition, the strong reduction of the wavevector in the indium-tin oxide below that of free space enables a new surface plasmon polariton mode which can be excited without requiring a prism or grating coupler. Ultrafast control of the surface plasmon polariton mode is achieved with a modulation amplitude reaching 20%.

/pdf/ultrafast-plasmonics-using-transparent-conductive-oxide-1z4195rw9l.pdf

Ultrafast plasmonics using transparent conductive oxide hybrids in the epsilon-near-zero regime

We demonstrate milling of partial antenna gaps and narrow conducting bridges with nanometer precision using a helium ion beam microscope. Single particle spectroscopy shows large shifts in the plasmonic mode spectrum of the milled antennas, associated with the transition from capacitive to conductive gap loading. A conducting bridge of nanometer height is found sufficient to shift the antenna from the capacitive to the conductive coupling regime, in agreement with circuit theory. Picosecond pump–probe spectroscopy reveals an enhanced nonlinear response for partially milled antennas, reaching an optimum value for an intermediate bridge height. Our results show that manipulation of the antenna load can be used to increase the nonlinear response of plasmonic antennas.

/pdf/ultrafast-nonlinear-control-of-progressively-loaded-single-4n3l6c0e69.pdf

Martina Abb

Papers

All-optical control of a single plasmonic nanoantenna-ITO hybrid.

Photoconductively loaded plasmonic nanoantenna as building block for ultracompact optical switches.

Surface-Enhanced Infrared Spectroscopy Using Metal Oxide Plasmonic Antenna Arrays

Ultrafast plasmonics using transparent conductive oxide hybrids in the epsilon-near-zero regime

Ultrafast nonlinear control of progressively loaded, single plasmonic nanoantennas fabricated using helium ion milling.