Color-Switchable Subwavelength Organic Light-Emitting Antennas

Future photonic devices require efficient, multifunctional, electrically driven light sources with directional emission properties and subwavelength dimensions. Electrically driven plasmonic nanoantennas have been demonstrated as enabling technology. Here, we present the concept of a nanoscale organic light-emitting antenna (OLEA) as a color- and directionality-switchable point source. The device consists of
laterally arranged electrically contacted gold nanoantennas with their gap filled by the organic semiconductor zinc phthalocyanine (ZnPc). Since ZnPc shows preferred hole conduction in combination with gold, the recombination zone relocates depending on the polarity of the applied voltage and couples selectively to either of the two antennas. Thereby, the emission characteristics of the device also depend on polarity. Contrary to large-area OLEDs where recombination at metal contacts significantly contributes to losses, our ultracompact OLEA structures facilitate efficient radiation into the far-field rendering transparent electrodes obsolete. We envision OLEA structures to serve as wavelength-scale pixels with tunable color and directionality for advanced display applications.

P. Grimm, S. Zeißner, M. Rödel, S. Wiegand, S. Hammer, M. Emmerling, E. Schatz,
R. Kullock, J. Pflaum & B. Hecht
Nano Lett. 22, XXX-XXX (2022).

Nanoscale Electrical Excitation of Distinct Modes in Plasmonic Waveguides

The electrical excitation of guided plasmonic modes at the nanoscale enables integration of optical nanocircuitry into nanoelectronics. In this context, exciting plasmons with a distinct modal field profile constitutes a key advantage over conventional single-mode integrated photonics. Here, we demonstrate the selective electrical excitation of the lowest-order symmetric and antisymmetric plasmonic modes in a two-wire transmission line. We achieve mode selectivity by precisely positioning nanoscale excitation sources, i.e., junctions for inelastic electron tunneling, within the respective modal field distribution. By using advanced fabrication that combines focused He-ion beam milling and dielectrophoresis, we control the location of tunnel junctions with sub-10 nm accuracy. At the far end of the two-wire transmission line, the guided plasmonic modes are converted into far-field radiation at separate spatial positions showing two distinct orthogonal polarizations. Hence, the resulting device represents the smallest electrically driven light source with directly switchable polarization states with possible applications in display technology.

M. Ochs, L. Zurak, E. Krauss, J. Meier, M. Emmerling, R. Kullock & B. Hecht
Nano Lett. 21, 4225-4230 (2021).

Electrically-driven Yagi-Uda antennas for light

Yagi-Uda antennas are a key technology for efficiently transmitting information from point to point using radio waves. Since higher frequencies allow higher bandwidths and smaller footprints, a strong incentive exists to shrink Yagi-Uda antennas down to the optical regime. Here we demonstrate electrically-driven Yagi-Uda antennas for light with wavelength-scale footprints that exhibit large directionalities with forward-to-backward ratios of up to 9.1 dB. Light generation is achieved via antenna-enhanced inelastic tunneling of electrons over the antenna feed gap. We obtain reproducible tunnel gaps by means of feedback-controlled dielectrophoresis, which precisely places single surface-passivated gold nanoparticles in the antenna gap. The resulting antennas perform equivalent to radio-frequency antennas and combined with waveguiding layers even outperform RF designs. This work paves the way for optical on-chip data communication that is not restricted by Joule heating but also for advanced light management in nanoscale sensing and metrology as well as light emitting devices.

R. Kullock, M. Ochs, P. Grimm & B. Hecht
Nature Comm. 11, 115 (2020)

Gold Nanoparticles (nanopartz)

Nonclassical optical properties of mesoscopic gold

Gold nanostructures have important applications in nanoelectronics, nano-optics as well as in precision metrology due to their intriguing opto-electronic properties. These properties are governed by the bulk band structure but to some extend are tunable via geometrical resonances. Here we show that the band structure of gold itself exhibits significant size-dependent changes already for mesoscopic critical dimensions below 30 nm. To suppress the effects of geometrical resonances and grain boundaries, we prepared atomically flat ultrathin films of various thicknesses by utilizing large chemically grown single-crystalline gold platelets. We experimentally probe thickness-dependent changes of the band structure by means of two-photon photoluminescence and observe a surprising 100-fold increase of the nonlinear signal when the gold film thickness is reduced below 30 nm allowing us to optically resolve single-unit-cell steps. The effect is well explained by density functional calculations of the thickness-dependent 2D band structure of gold.

S. Großmann, D. Friedrich, M. Karolak, R. Kullock, E. Krauss, M. Emmerling, G. Sangiovanni & B. Hecht
Phys. Rev. Lett., 122, 246802 (2019)
arXiv:1905.09942 (2019)

Controlled Growth of High-Aspect-Ratio Single-Crystalline Gold Platelets

We describe the wet-chemical synthesis of high-aspect-ratio single-crystalline gold platelets with thicknesses down to 20 nm and edge lengths up to 0.2 mm. By employing statistical analysis of a large number of platelets, we investigate the effect of temperature on the growth velocities of the top and side facets for constant concentrations of the three common ingredients: ethylene glycol, chloroauric acid, and water. We further show that by varying the chemical environment during growth, the ratio between the growth velocities can be adjusted, and thus thickness and lateral size can be tuned independently. Very large but ultrathin single-crystalline gold platelets represent an important starting material for top-down nanofabrication and may also find applications as transparent conducting substrates as well as substrates for high-end scanning probe and electron microscopy.

E. Krauss, R. Kullock, X. Wu, P. Geisler, N. Lundt, M. Kamp & B. Hecht
Cryst. Growth Des., 18 (3), 1297-1302 (2018)

Directed emission by electrically-driven optical antennas

Antennas play a key role in today’s wireless communication networks and it would be hugely beneficial to extent their use into the optical regime. However, classical signal generators do not work at those frequencies and therefore new concepts are needed. Here, we demonstrate how to electrically drive an optical nanoantenna using an atomic-scale feed gap provided by a gold-particle pushed into a precisely tailored interstice between two antenna arms. Upon applying a voltage, inelastic electron tunneling leads to current fluctuations in the optical regime and, hence, light emission. We show how the antennas spectrally shape the emission, how the exact particle position influences these properties and how to increase the directivity via Yagi-Uda arrangements or plasmonic waveguides structures in order to make electricallydriven optical nanoantennas more suitable for on-chip data communicatio.

R. Kullock, P. Grimm, M. Ochs & B. Hecht
Proc. SPIE 10540, Quantum Sensing and Nano Electronics and Photonics XV 1054012 (2018)

Grazing-incidence optical magnetic recording with super-resolution

Heat-assisted magnetic recording (HAMR) is often considered the next major step in the storage industry: it is predicted to increase the storage capacity, the read/write speed and the data lifetime of future hard disk drives. However, despite more than a decade of development work, the reliability is still a prime concern. Featuring an inherently fragile surface-plasmon resonator as a highly localized heat source, as part of a near-field transducer (NFT), the current industry concepts still fail to deliver drives with sufficient lifetime. This study presents a method to aid conventional NFT-designs by additional grazing-incidence laser illumination, which may open an alternative route to high-durability HAMR. Magnetic switching is demonstrated on consumer-grade CoCrPt perpendicular magnetic recording media using a green and a near-infrared diode laser. Sub-500 nm magnetic features are written in the absence of a NFT in a moderate bias field of only μ0H = 0.3 T with individual laser pulses of 40 mW power and 50 ns duration with a laser spot size of 3 μm (short axis) at the sample surface – six times larger than the magnetic features. Herein, the presence of a nanoscopic object, i.e., the tip of an atomic force microscope in the focus of the laser at the sample surface, has no impact on the recorded magnetic features – thus suggesting full compatibility with NFT-HAMR.

Scheunert, Gunther; Cohen, Sidney R.; Kullock, René; McCarron, Ryan; Rechev, Katya; Kaplan-Ashiri, Ifat; Bitton, Ora; Dawson, Paul; Hecht, Bert; Oron, Dan
Beilstein J. Nanotechnol., 8(1), 28-37 (2017).

Silica–gold bilayer-based transfer of focused ion beam-fabricated nanostructures

The demand for using nanostructures fabricated by focused ion beam (FIB) on delicate substrates or as building blocks for complex devices motivates the development of protocols that allow FIB-fabricated nanostructures to be transferred from the original substrate to the desired target. However, transfer of FIB-fabricated nanostructures is severely hindered by FIB-induced welding of structure and substrate. Here we present two (ex and in situ) transfer methods for FIB-fabricated nanostructures based on a silica–gold bilayer evaporated onto a bulk substrate. Utilizing the poor adhesion between silica and gold, the nanostructures can be mechanically separated from the bulk substrate. For the ex situ transfer, a spin-coated poly(methyl methacrylate) film is used to carry the nanostructures so that the bilayer can be etched away after being peeled off. For the in situ transfer, using a micro-manipulator inside the FIB machine, a cut-out piece of silica on which a nanostructure has been fabricated is peeled off from the bulk substrate and thus carries the nanostructure to a target substrate. We demonstrate the performance of both methods by transferring plasmonic nano-antennas fabricated from single-crystalline gold flakes by FIB milling to a silicon wafer and to a scanning probe tip.

Wu, Xiaofei; Geisler, Peter; Krauss, Enno; Kullock, René; Hecht, Bert in Nanoscale, 7(39), 16427-16433 (2015).

Electrically driven optical antennas

Unlike radiowave antennas, so far optical nanoantennas cannot be fed by electrical generators. Instead, they are driven by light or indirectly via excited discrete states in active materials in their vicinity. Here we demonstrate the direct electrical driving of an in-plane optical antenna by the broadband quantum-shot noise of electrons tunnelling across its feed gap. The spectrum of the emitted photons is determined by the antenna geometry and can be tuned via the applied voltage. Moreover, the direction and polarization of the light emission are controlled by the antenna resonance, which also improves the external quantum efficiency by up to two orders of magnitude. The one-material planar design offers facile integration of electrical and optical circuits and thus represents a new paradigm for interfacing electrons and photons at the nanometre scale, for example for on-chip wireless communication and highly configurable electrically driven subwavelength photon sources.

Kern, Johannes; Kullock, René; Prangsma, Jord; Emmerling, Monika; Kamp, Martin; Hecht, Bert in
Nature Photon, 9(9), 582-586 (2015).
arxiv: 1502.04935

Die Welt (in german), (in german), Physik in unserer Zeit (in german)

Single-crystalline gold microplates grown on substrates by solution-phase synthesis

Chemically synthesized single-crystalline gold microplates have been attracting increasing interest because of their potential as high-quality gold films for nanotechnology. We present the growth of tens of nanometers thick and tens of micrometers large single-crystalline gold plates directly on solid substrates by solution-phase synthesis. Compared to microplates deposited on substrates from dispersion phase, substrate-grown plates exhibit significantly higher quality by avoiding severe small-particle contamination and aggregation. Substrate-grown gold plates also open new perspectives to study the growth mechanism via step-growth and observation cycles of a large number of individual plates. Growth models are proposed to interpret the evolution of thickness, area and shape of the plates. It is found that the plate surface remains smooth after regrowth, implying the applicability of regrowth for producing giant plates as well as unique single-crystalline nano-structures.

Wu, Xiaofei; Kullock, René; Krauss, Enno; Hecht, Bert in
Cryst. Res. Technol., 50(8), 595-602 (2015).

Utilizing Dog-Boning to Build up High-Aspect-Ratio Nanofences

The material built-up at sharp corners of a workpiece during electroplating, so-called dog-boning, was utilized for bottom-up creation of nanofences – free standing lines of high aspect ratio nanowires – on silicon oxide substrates. This was realized by electroplating gold into nanoporous aluminum oxide templates with underlying micropatterned gold pads, serving as working electrodes. Using the right parameter set, initial preferred nucleation at the gold pad rim site and the unique nanomatrix geometry led to the formation of lines of nanowires at the rim site separated by an adjacent depletion area from an extended bulk nanorod array region. It was found that the underlying mechanisms of this growth process are the increased electric field densities at the electrode edges and a field screening effect due to already grown neighboring wires, both identified by finite element electric field simulations, as well as the restricted diffusion dynamics of gold ions in the presence of nanopores. The obtained high aspect ratio nanofences might be applicable for enhanced catalysis, dielectric index sensing or in optical filters.

G. Scheunert, V. Hoffmann, R. Kullock, J. R. Whyte, R. Kirchner, S. Grafström, W.-J. Fischer, und L. M. Eng,
J. Electrochem. Soc., 161(1) D26–D30 (2014).

Local photochemical plasmon mode tuning in metal nanoparticle arrays

We report on the local modification of gold nanoparticle arrays by photochemical deposition of gold from solution. Our method allows to alter the localized surface plasmon resonance (LSPR) in a restricted area by exposure of gold salt (HAuCl4) to light, whereas the expansion of such sections depends on the illumination optics. The geometry parameters of the individual nanoparticles in the modified regions are characterized by SEM and AFM, while the optical properties of distinct array sections are analyzed by means of optical spectroscopy. A blueshift of the surface plasmon resonance wavelength is observed upon the deposition process. An explanation for the blueshift is found by performing calculations using an analytical dipolar interaction model (DIM), which allows us to distinguish the individual contributions of the particle geometry on the one hand and the changes in particle interaction on the other hand. The resulting simulated scattering spectra verify the blueshift of the LSPR, which can be attributed to an increase in aspect ratio of the particles during growth. Since plasmonically active nanoparticle arrays are known to be candidates for sensing applications, this method and the gained understanding can be exploited to fabricate large sensor substrates with local LSPR variations.

S. Derenko, R. Kullock, Z. Wu, A. Sarangan, C. Schuster, L. M. Eng, und T. Härtling
Opt. Mat. Express, 3(6), 794(2013).

Metallic Nanorod Arrays:
Linear Optical Properties and Beyond

Arrays of free-standing metallic nanorods are promising candidates for sensors, switches and spectroscopy. They have structure sizes much smaller than the wavelength of visible light, feature a long-axis surface plasmonic resonance (LSPR) and show metamaterial-like properties. This thesis provides a detailed investigation of their linear optical properties and highlights some nonlinear optical aspects. By means of graded structures having a tunable LSPR and three different theoretical models — a numerical multiple-multipole method (MMP) model, a semi-analytic collective surface plasmon (CSP) model and an analytic dipolar interaction model (DIM) — the optical properties were analyzed. Using the DIM, the experimentally observed blueshift of the LSPR in comparison to a single nanorod is confirmed and a physical explanation is provided. The LSPR strongly depends on the angle of incidence and the rod diameter. However, for a varying length the changes are small with the long-axis mode showing a lower energy limit. The detailed arrangement of the nanorods and the azimuthal angle of the incoming light plays only a minor role for small nanorod separations. Similarly, the dependence on the metal is the same as for single particles, whereas the sensitivity to the surrounding dielectric is much stronger than in the single-particle case. For longer nanorods made of silver, angle-dependent higher-order modes are observed and reproduced using MMP. The CSP model is applied and Fabry-Pérot-like oscillations of the CSPs are found. The propagating nature of these modes leads to the discovery that the p component of the transmitted light experiences a phase jump and to the observation of polarization conversion inside the structures. Negative refraction is found in nanorod arrays; it is revealed that a negative energy flux occurs only within a bandwidth given by the LSPR of a single nanorod and the array resonance. For smaller wavelengths, the in-plane component of the Poynting vector reverses, leading to an (extraordinary) positive flux. At the LSPR itself, the flux parallel to the surface is found to be zero. The negative refraction is also exploited to mimic a nanolens with structure parameters that are infact technical realizable. In the visible regime the nanolens shows a NA of 1.06 and superlens-like features such as identical rotation and linear translation of image and object. The nonlinear measurements on graded structures are conducted using femtosecond pump-probe spectroscopy resulting in kinetics showing either an increased transmission or absorption with signal changes of up to 40%. By converting them to transient spectra and by comparison with the literature, electron distribution changes at the Fermi edge and hot electrons/phonons are identified as the main reasons for the changes. Probing at the inflection points of the LSPR reveals ultrafast signals. Using transient spectra they are traced back to a short blueshift of the LSPR.

René Kullock (2011). Metallic Nanorod Arrays: Linear Optical Properties and Beyond. [Doctoral dissertation, Technische Universität Dresden].
Qucosa – the Saxon Document and Publication Server

SHG simulations of plasmonic nanoparticles using curved elements

We demonstrate that simulating plasmonic nanostructures by means of curved elements (CEs) significantly increases the accuracy and computation speed not only in the linear but also in the nonlinear regime. We implemented CEs within the discontinuous Galerkin (DG) method and, as an example of a nonlinear effect, investigated second-harmonic generation (SHG) at a silver nanoparticle. The second-harmonic response of the material is simulated by an extended Lorentz model (ELM). In the linear regime the CEs are ≈ 9 times faster than ordinary elements for the same accuracy, provide a much better convergence and show fewer unphysical field artifacts. For DG-SHG calculations CEs are almost indispensable to obtain physically reasonable results at all. Additionally, their boundary approximation has to be of the same order as their polynomial degree to achieve artifact-free field distributions. In return, the use of such CEs with the DG method pays off more than evidently, since the additional computation time is only 1%.

R. Kullock, A. Hille, A. Haußmann, S. Grafström, und L. M. Eng
Opt. Express 19, 14426–14436 (2011).

Three-dimensional, arbitrary orientation of focal polarization

We demonstrate a simple setup for generating a three-dimensional arbitrary orientation of the polarization vector in a laser focus. The key component is the superposition of a linearly and a radially polarized laser beam, which both can be controlled individually in intensity and relative phase. We exemplify the usefulness of this setup by determining the spatial orientation of a single silver nanorod in three-dimensional space by recording the angle-variable backscattered light intensity.

P. Olk, T. Härtling, R. Kullock, und L. M. Eng
Applied Optics 49, 4479–4482 (2010).

Metallic nanorod arrays: negative refraction and optical properties explained by retarded dipolar interactions

We show that the optical properties of arrays of parallel-aligned metallic nanorods can be understood by means of a retarded dipolar interaction model. Exemplarily, arrays of gold nanorods having various lengths and diameters are investigated experimentally. A strong diameter dependence of the long-axis surface plasmon resonance (LSPR) as well as a lower energy limit of this mode for varying length was found. The model also shows that, for small nanorod distances (d<λ/2), the optical properties are independent of the azimuthal angle of the incoming plane wave and of the detailed arrangement of the nanorods. Furthermore, the model was used to explain the dependence of the LSPR on the angle of incidence and to find the conditions for which negative and extraordinary positive refractions occur in these structures.

R. Kullock, S. Grafström, P. R. Evans, R. J. Pollard, und L. M. Eng
J. Opt. Soc. Am. B 27, 1819–1827 (2010).

Improving Nano-Optical Simulations Through Curved Elements Implemented within the Discontinuous Galerkin Method

We report on implementing curved elements (CEs) into the discontinuous Galerkin (DG) method in order to improve the nano-optical simulation of realistic nanostructures. Compared to straight meshes, CEs as realized here by triangles with one bent side allow for a faster, more accurate and robust computation. Clearly, CEs much better match real physical nanostructures, where surface energies, tension and adhesion give rise to rounded geometries rather than planar surfaces. The novel code was tested by calculating the spatial field distributions and scattering cross sections for two-dimensional (2D) nano-objects, namely a nanosphere and a V-groove. When using a fixed mesh size, we found that CEs much more accurately describe the curved geometries than ordinary linear elements, leading to significantly smaller errors. Moreover, CEs turned out to be between 2.5 and 37 times faster for the same error margin and to be more robust against unphysical behavior, such as hot spots at element vertices or unmotivated spectral features. Integrating CEs into the DG algorithm thus constitutes an excellent choice for numerical modeling of complex problems–especially in the context of plasmonics.

A. Hille, R. Kullock, S. Grafström, und L. M. Eng
J. Comput. Theor. Nanosci. 7, 1581–1586 (2010).

Polarization conversion through collective surface plasmons in metallic nanorod arrays

For two-dimensional (2D) arrays of metallic nanorods arranged perpendicular to a substrate several methods have been proposed to determine the electromagnetic near-field distribution and the surface plasmon resonances, but an analytical approach to explain all optical features on the nanometer length scale has been missing to date. To fill this gap, we demonstrate here that the field distribution in such arrays can be understood on the basis of surface plasmon polaritons (SPPs) that propagate along the nanorods and form standing waves. Notably, SPPs couple laterally through their optical near fields, giving rise to collective surface plasmon (CSP) effects. Using the dispersion relation of such CSPs, we deduce the condition of standing-wave formation, which enables us to successfully predict several features, such as eigenmodes and resonances. As one such property and potential application, we show both theoretically and in an experiment that CSP propagation allows for polarization conversion and optical filtering in 2D nanorod arrays. Hence, these arrays are promising candidates for manipulating the light polarization on the nanometer length scale.

R. Kullock, W. R. Hendren, A. Hille, S. Grafström, P. R. Evans, R. J. Pollard, R. Atkinson, und L. M. Eng
Opt. Express 16, 21671 (2008).

Photochemical Tuning of Plasmon Resonances in Single Gold Nanoparticles

We report on the spectrally controlled photochemical tuning of the size, shape, and localized surface plasmon resonances of individual gold nanoparticles. Single spheres, extracted from a colloidal solution, and elongated nanodiscs, fabricated by electron beam lithography, were exposed to a gold salt solution while being illuminated one by one by a focused 532-nm laser beam. The photochemical reduction of tetrachloroaureate complexes, followed by the subsequent agglomeration of gold atoms at the particle surface, lead to a well-controlled single-particle growth. This fully in situ monitored method allows us to tune the radius of single spheres as well as the aspect ratio of single ellipsoidal particles, enabling spectral control of their respective localized surface plasmon resonances.
We report on the spectrally controlled photochemical tuning of the size, shape, and localized surface plasmon resonances of individual gold nanoparticles. Single spheres, extracted from a colloidal solution, and elongated nanodiscs, fabricated by electron beam lithography, were exposed to a gold salt solution while being illuminated one by one by a focused 532-nm laser beam. The photochemical reduction of tetrachloroaureate complexes, followed by the subsequent agglomeration of gold atoms at the particle surface, lead to a well-controlled single-particle growth. This fully in situ monitored method allows us to tune the radius of single spheres as well as the aspect ratio of single ellipsoidal particles, enabling spectral control of their respective localized surface plasmon resonances.

T. Härtling, Y. Alaverdyan, M. T. Wenzel, R. Kullock, M. Kall, und L. M. Eng
J. Phys. Chem. C 112, 4920–4924 (2008).

Improving the light extraction efficiency of polymeric light emitting diodes using two-dimensional photonic crystals

We have fabricated light emitting diodes based on a conjugated polymer, in which a planarized two-dimensional photonic crystal (PC) was inserted between the glass substrate and the ITO anode. Planarized PCs were fabricated into a high-index layer via interference lithography, followed by dry-etching and the spin-casting of a low-index glass. We characterize the electroluminescence (EL) emission from devices containing a PC, and compare this with photoluminescence (PL) generated from within the same structure. We show that LEDs incorporating the PC have an increased EL external quantum efficiency of (2.3 ± 1.0) times compared to a standard non-patterned control. This efficiency increase is in excellent agreement with PL measurements on similar structures, which also demonstrate relative increases in external emission intensity of 2.3 times.

A. M. Adawi, R. Kullock, J. L. Turner, C. Vasilev, D. G. Lidzey, A. Tahraoui, P. W. Fry, D. Gibson, E. Smith, C. Foden, M. Roberts, F. Qureshi, und N. Athanassopoulou
Organic Electronics 7, 222–228 (2006).