Supplement 1: Spatial beam intensity shaping using phase masks on single-mode optical fibers fabricated by femtosecond direct laser writing

Supplemental Document Originally published in Optica on 20 April 2016 (optica-3-4-448

Wavelength-Dependent Third-Harmonic Generation in Plasmonic Gold Nanoantennas: Quantitative Determination of the d‑Band Influence

Plasmonic gold nanoantennas are highly efficient nanoscale nonlinear light converters. The nanoantennas provide large resonant light interaction cross sections as well as strongly enhanced local fields. The actual frequency conversion, however, takes places inside the gold volume and is thus ultimately determined by the microscopic gold nonlinearity, which has been found to significantly surpass common bulk nonlinear materials. While the influence of the nanoantenna geometry and hence the plasmonic resonance has been studied in great detail, only little attention has been paid to the microscopic material nonlinearity. Here we show that the microscopic third-order nonlinearity of gold is in fact a resonant one by virtue of interband transitions between the d- and sp-bands. Utilizing a large set of resonant nanoantennas and a fiber-feedback optical parametric oscillator as a broadband-tunable light source, we show that the radiated third-harmonic signals significantly increase at the onset of interband transitions, namely, as soon as the third harmonic becomes resonant with allowed interband transitions. With the help of an anharmonic oscillator model and independent reference measurements on a gold film we can unambiguously demonstrate that the observed third-harmonic increase is related to a strongly wavelength-dependent microscopic third-order gold nonlinearity, which is additionally underlined by quantitative agreement between simulation and measurement. This additional tuning parameter allows further manipulation and optimization of nonlinear nanoscale systems and thus renders the investigation of other plasmonic materials, especially with interband transitions located in the ultraviolet range, highly intriguing

Ultrafast Spectroscopy of Quantum Confined States in a Single CdSe Nanowire

We measure for the first time transient absorption spectra of individual CdSe nanowires with about 10 nm diameter. Confinement of the carrier wave functions leads to discrete states which can be described by a six-band effective mass model. Combining transient absorption and luminescence spectroscopy allows us to track the excitation dynamics in the visible and near-infrared spectral range. About 10% of all absorbed photons lead to an excitation of the lowest energy state. Of these excitations, less than 1% lead to a photon in the optical far-field. Almost all emission is reabsorbed by other parts of the nanowire. These findings might explain the low overall quantum efficiency of CdSe nanowires

Hybrid Organic-Plasmonic Nanoantennas with Enhanced Third-Harmonic Generation

Resonantly excited plasmonic gold nanoantennas are strong sources of third-harmonic (TH) radiation. It has been shown that the response originates from large microscopic nonlinearity of the gold itself, which is enhanced by the near-field of the plasmonic nanoantenna. To further enhance this response, one can incorporate nonlinear media into the near-fields of the nanoantenna, as an additional TH source. To obtain a significant contribution from the added medium, its nonlinear susceptibility should be comparable to that of the antenna material. Many organic materials offer the necessary nonlinear susceptibility and their incorporation is possible with simple spin-coating. Furthermore, organic materials are often susceptible to photodegradation. This degradation can be used to investigate the influence of organic materials on the hybrid system. Our investigated hybrid organic plasmonic nanoantenna system consists of a gold nanorod array and poly­(methyl methacrylate) as the nonlinear dielectric medium. The experiments clearly reveal two contributions to the generated TH radiation, one from the nanoantenna itself and one from the polymer. The nonlinear response of the hybrid material exceeds the response of both individual constituents and opens the path to more efficient nanoscale nonlinear light generation

Refractory Plasmonics without Refractory Materials

Refractory plasmonics deals with metallic nanostructures that can withstand high temperatures and intense laser pulses. The common belief was that refractory materials such as TiN are necessary for this purpose. Here we show that refractory plasmonics is possible without refractory materials. We demonstrate that gold nanostructures which are overcoated with 4 and 40 nm Al₂O₃ (alumina) by an atomic layer deposition process or by thick IC1-200 resist can withstand temperatures of over 800 °C at ambient atmospheric conditions. Furthermore, the alumina-coated structures can withstand intense laser radiation of over 10 GW/cm² at ambient conditions without damage. Thus, it is possible to combine the excellent linear and nonlinear plasmonic properties of gold with material properties that were believed to be only possible with the lossier and less nonlinear refractory materials

Interpreting Chiral Nanophotonic Spectra: The Plasmonic Born–Kuhn Model

One of the most intuitive ways to classically understand the generation of natural optical activity in chiral media is provided by the coupled oscillator model of Born and Kuhn consisting of two identical, vertically displaced, coupled oscillators. We experimentally realize and discuss its exact plasmonic analog in a system of corner-stacked gold nanorods. In particular, we analyze the arising circular dichroism and optical rotatory spectra in terms of hybridized electromagnetic modes and retardation. Specifically, we demonstrate how tuning the vertical distance between the nanorods can lead to a selective excitation of the occurring bonding and antibonding chiral plasmonic modes

Topology of Surface Plasmon Polaritons with Integer and Fractional Orbital Angular Momentum

The topology of surface plasmon polariton fields (SPPs) with orbital angular momentum (OAM) is characterized by the winding numbers of the phase singularities in the field, also known as topological charges. Using theoretical expressions for the surface plasmon fields, we identify the phase singularities as points where the field is zero and investigate their properties for both integer and noninteger, or fractional, orbital angular momentum. The phase singularities act as vortex centers for the rotating fields. We analyze the behavior of the vortex–antivortex pairs and the breakup of the central vortex and discuss their influence on the measured topology as the orbital angular momentum changes from one integer value l to the next l +1 via the fractional states. Our work highlights the fact that measures of the topological charges do not always equate with the orbital angular momentum and shows how the topology can change discontinuously, even though all of the parameters controlling the fields change smoothly

Topology of Surface Plasmon Polaritons with Integer and Fractional Orbital Angular Momentum

The topology of surface plasmon polariton fields (SPPs) with orbital angular momentum (OAM) is characterized by the winding numbers of the phase singularities in the field, also known as topological charges. Using theoretical expressions for the surface plasmon fields, we identify the phase singularities as points where the field is zero and investigate their properties for both integer and noninteger, or fractional, orbital angular momentum. The phase singularities act as vortex centers for the rotating fields. We analyze the behavior of the vortex–antivortex pairs and the breakup of the central vortex and discuss their influence on the measured topology as the orbital angular momentum changes from one integer value l to the next l +1 via the fractional states. Our work highlights the fact that measures of the topological charges do not always equate with the orbital angular momentum and shows how the topology can change discontinuously, even though all of the parameters controlling the fields change smoothly

Quantitative Modeling of the Third Harmonic Emission Spectrum of Plasmonic Nanoantennas

Plasmonic dimer nanoantennas are characterized by a strong enhancement of the optical field, leading to large nonlinear effects. The third harmonic emission spectrum thus depends strongly on the antenna shape and size as well as on its gap size. Despite the complex shape of the nanostructure, we find that for a large range of different geometries the nonlinear spectral properties are fully determined by the linear response of the antenna. We find excellent agreement between the measured spectra and predictions from a simple nonlinear oscillator model. We extract the oscillator parameters from the linear spectrum and use the amplitude of the nonlinear perturbation only as scaling parameter of the third harmonic spectra. Deviations from the model only occur for gap sizes below 20 nm, indicating that only for these small distances the antenna hot spot contributes noticeable to the third harmonic generation. Because of its simplicity and intuitiveness, our model allows for the rational design of efficient plasmonic nonlinear light sources and is thus crucial for the design of future plasmonic devices that give substantial enhancement of nonlinear processes such as higher harmonics generation as well as difference frequency mixing for plasmonically enhanced terahertz generation

Comprehensive Study of Plasmonic Materials in the Visible and Near-Infrared: Linear, Refractory, and Nonlinear Optical Properties

Plasmonic nanostructures are used today for a variety of applications. Choosing the best suited plasmonic material for a specific application depends on several criteria, such as chemical and thermal stability, bulk plasma frequency, nonlinear response, and fabrication constraints. To provide a comprehensive summary, we compare these properties for eight different plasmonic materials, namely, Ag, Al, Au, Cu, Mg, Ni, Pd, and Pt. All these materials can be fabricated with electron beam lithography and subsequent evaporation of the desired material. First, we heated rod-antenna-type nanostructures made from these materials up to 1100 °C in air and investigated their linear optical response. Most structures lose their plasmonic properties at temperatures far below the melting point of the respective material. Gold, silver, and platinum structurally deform, whereas the other materials appear to chemically degrade. Second, to improve the thermal stability, structures with a 4 nm thin Al₂O₃ capping layer are fabricated. The thermal stability is significantly increased with the capping layer for all materials except for copper and magnesium. Lastly, the laser damage threshold is investigated for silver, aluminum, gold, and copper, which exhibit high nonlinear optical susceptibilities and are therefore particularly interesting for nonlinear optical applications