Experimental Physics V


Nano-Optics and Biophotonics

Latest publications

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)


Mono-crystalline Gold Platelets: A High Quality Platform for Surface Plasmon Polaritons

We use mono-crystalline gold platelets with ultra-smooth surfaces and superior plasmonic properties to investigate the formation of interference patterns caused by surface plasmon polaritons (SPPs) with scattering-type scanning near-field microscopy (s-SNOM) at 521 nm and 633 nm. By applying a Fourier analysis approach, we can identify and separate several signal channels related to SPPs launched and scattered by the AFM tip and the edges of the platelet. Especially at the excitation wavelength of 633 nm, we can isolate a region in the center of the platelets where we find only contributions of SPPs which are launched by the tip and reflected at the edges. These signatures are used to determine the SPP wavelength of lambda_SPP = 606 nm good agreement with theoretical predictions. Furthermore, we were still able to measure SPP signals after 20  μm propagation, which demonstrates impressively the superior plasmonic quality of these mono-crystalline gold platelets.

K.J. Kaltenecker, E. Krauss, L. Casses, M. Geisler, B. Hecht, N.A. Mortensen, P.U. Jepsen & N. Stenger
 Nanophotonics 0362 (2019)  arxiv:1909.08321 (2019)


Driving optical nanoantennas using coherent perfect absorption

Driving nanoantennas with perfect impedance matching is challenging. Here, we exploit nanoscale coherent perfect absorption (CPA) of the propagating plasmon on a gold nanowire to perfectly drive a single gold nanorod. By carefully tuning the rod length and gap width, CPA can be achieved at a fixed frequency to perfectly drive a nanorod antenna in its sub-radiant or super-radiant resonance as an efficient heat source or a perfectly matched transmitting nanoantenna, respectively. We also demonstrate superior sensing capability of the proposed CPA scheme. Resulting applications encompass perfect driving of optical nanoantennas, background-free sensing, and coherent control of optical nanoantennas, opening new perspectives for the exploitation of CPA in plasmonic nanocircuitry.

P. Grimm, G. Razinskas, J.-S. Huang & B. Hecht
arxiv:1907.08174v2 (2020)


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)


Spatial variations in femtosecond field dynamics within a plasmonic nanoresonator mode

Plasmonic resonators can be designed to support spectrally well-separated discrete modes. The associated characteristic spatial patterns of intense electromagnetic hot-spots can be exploited to enhance light–matter interaction. Here, we study the local field dynamics of individual hot-spots within a nanoslit resonator by detecting characteristic changes of the photoelectron emission signal on a scale of ~12 nm using time-resolved photoemission electron microscopy (TR-PEEM) and by excitation with the output from a 20 fs, 1 MHz noncollinear optical parametric amplifier (NOPA). Surprisingly, we detect apparent spatial variations of the Q-factor and resonance frequency that are commonly considered to be global properties for a single mode. By using the concept of quasinormal modes we explain these local differences by crosstalk of adjacent resonator modes. Our findings are important in view of time-domain studies of plasmon-mediated strong light–matter coupling at ambient conditions.

M. Hensen, B. Huber, D. Friedrich, E. Krauss, S. Pres, P. Grimm, D. Fersch, J. Lüttig, V. Lisinetskii, B. Hecht & T. Brixner
Nano Letters, 19, 7, 4651-4658 (2019)


Reversible Mapping and Sorting the Spin of Photons on the Nanoscale: A Spin-Optical Nanodevice

The photon spin is an important resource forquantum information processing as is the electron spin inspintronics. However, for subwavelength confined optical excita-tions, polarization as a global property of a mode cannot be defined.Here, we show that any polarization state of a plane-wave photoncan reversibly be mapped to a pseudospin embodied by the twofundamental modes of a subwavelength plasmonic two-wiretransmission line. We design a device in which this pseudospinevolves in a well-defined fashion throughout the device reminiscentof the evolution of photon polarization in a birefringent medium andthe behavior of electron spins in the channel of a spinfield-effecttransistor. The significance of this pseudospin is enriched by the factthat it is subject to spin−orbit locking. Combined with optically active materials to exert external control over the pseudospinprecession, ourfindings could enable spin-optical transistors, that is, the routing and processing of quantum information withlight on a subwavelength scale.

E. Krauss, G. Razinskas, D. Köck, S. Grossmann & B. Hecht
Nano Letters, 19, 5, 3364-3369 (2019)
arxiv:1812.03721 (2018)


High-Q, low-mode-volume and multiresonant plasmonic nanoslit cavities fabricated by helium ion milling

Helium ion milling of chemically-synthesized micron-sized gold flakes is performed to fabricate ultra-narrow nanoslit cavities with a varying length and width down to 5 nm. Their plasmon resonances are characterized by one-photon photoluminescence spectroscopy. The combination of fabrication based on single-crystalline gold and resonant modes with low radiative losses leads to remarkably high quality factors of up to 24. Multiple Fabry–Pérot-type resonances in the visible/near infrared spectral range are observed due to the achieved narrow slit widths and the resulting short effective wavelengths of nanoslit plasmons. These features make nanoslit cavities attractive for a range of applications such as surface-enhanced spectroscopy, ultrafast nano-optics and strong light–matter coupling.

K. Chen, G. Razinskas, H. Vieker, H. Gross, X. Wu, A. Beyer, A. Gölzhäuser & B. Hecht
Nanoscale. (2018), doi:10.1039/C8NR02160K


Parallel mapping of optical near-field interactions by molecular motor-driven quantum dots

In the vicinity of metallic nanostructures, absorption and emission rates of optical emitters can be modulated by several orders of magnitude. Control of such near-field light–matter interaction is essential for applications in biosensing, light harvesting and quantum communication and requires precise mapping of optical near-field interactions, for which single-emitter probes are promising candidates. However, currently available techniques are limited in terms of throughput, resolution and/or non-invasiveness. Here, we present an approach for the parallel mapping of optical near-field interactions with a resolution of <5 nm using surface-bound motor proteins to transport microtubules carrying single emitters (quantum dots). The deterministic motion of the quantum dots allows for the interpolation of their tracked positions, resulting in an increased spatial resolution and a suppression of localization artefacts. We apply this method to map the near-field distribution of nanoslits engraved into gold layers and find an excellent agreement with finite-difference time-domain simulations. Our technique can be readily applied to a variety of surfaces for scalable, nanometre-resolved and artefact-free near-field mapping using conventional wide-field microscopes.

H. Groß, H. S. Heil, J. Ehrig, F. W. Schwarz, B. Hecht & S. Diez
Nature Nanotech. (2018), doi:10.1038/s41565-018-0123-1


Evidence for cascaded third harmonic generation in non-centrosymmetric gold nanoantennas

The optimization of nonlinear optical processes at the nanoscale is a crucial step for the development of nanoscale photon sources for quantum-optical networks. The development of innovative plasmonic nanoantenna designs and hybrid nanostructures to enhance optical nonlinearities in very small volumes represents one of the most promising routes. In such systems, the upconversion of photons can be achieved with high efficiencies via third-order processes, such as third harmonic generation (THG), thanks to the resonantly-enhanced volume currents. Conversely, second-order processes, such as second harmonic generation (SHG), are often inhibited by the symmetry of metal lattices and of common nanoantenna geometries. SHG and THG processes in plasmonic nanostructures are generally treated independently, since they both represent a small perturbation in the light-matter interaction mechanisms. In this work, we demonstrate that this paradigm does not hold in general, by providing evidence of a cascaded process in THG, which is fueled by SHG and sizably contributes to the overall yield. We address this mechanism by unveiling an anomalous fingerprint in the polarization state of the nonlinear emission from non-centrosymmetric gold nanoantennas and point out that such cascaded processes may also appear for structures that exhibit only moderate SHG yields - signifying its general relevance in plasmon-enhanced nonlinear optics. The presence of this peculiar mechanism in THG from plasmonic nanoantennas at telecommunication wavelengths allows gaining further insight on the physics of plasmon-enhanced nonlinear optical processes. This could be crucial in the realization of nanoscale elements for photon conversion and manipulation operating at room-temperature.

M. Celebrano, A. Locatelli, L. Ghirardini, G. Pellegrini, P. Biagioni, X. Wu, S. Grossmann, L. Carletti, C. D. Angelis, L. Duò, B. Hecht & M. Finazzi
arXiv:1803.03617 (2018)


Near-field strong coupling of single quantum dots

Strong coupling and the resultant mixing of light and matter states is an important asset for future quantum technologies. We demonstrate deterministic room temperature strong coupling of a mesoscopic colloidal quantum dot to a plasmonic nanoresonator at the apex of a scanning probe. Enormous Rabi splittings of up to 110 meV are accomplished by nanometer-precise positioning of the quantum dot with respect to the nanoresonator probe. We find that, in addition to a small mode volume of the nanoresonator, collective coherent coupling of quantum dot band-edge states and near-field proximity interaction are vital ingredients for the realization of near-field strong coupling of mesoscopic quantum dots. The broadband nature of the interaction paves the road toward ultrafast coherent manipulation of the coupled quantum dot-plasmon system under ambient conditions.

H. Groß, J.M. Hamm, T. Tufarelli, O. Hess & B. Hecht
Sci. Adv., 4, 3 (2018)
BR2 - IQ Wissenschaft und Forschung - Magazin (min 19:51)


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)


Limits of Kirchhoff’s Laws in Plasmonics

The validity of Kirchhoff’s laws in plasmonic nanocircuitry is investigated by studying a junction of plasmonic two-wire transmission lines. We find that Kirchhoff’s laws are valid for sufficiently small values of a phenomenological parameter κ relating the geometrical parameters of the transmission line with the effective wavelength of the guided mode. Beyond such regime, for large values of the phenomenological parameter, increasing deviations occur and the equivalent impedance description (Kirchhoff’s laws) can only provide rough, but nevertheless useful, guidelines for the design of more complex plasmonic circuitry. As an example we investigate a system composed of a two-wire transmission line and a nanoantenna as the load. By addition of a parallel stub designed according to Kirchhoff’s laws we achieve maximum signal transfer to the nanoantenna.

G. Razinskas, P. Biagioni & B. Hecht
Sci. Rep. 8, 1921 (2018)


Mode Matching for Optical Antennas

The emission rate of a point dipole can be strongly increased in the presence of a well-designed optical antenna. Yet, optical antenna design is largely based on radio-frequency rules, ignoring, e.g., Ohmic losses and non-negligible field penetration in metals at optical frequencies. Here, we combine reciprocity and Poynting’s theorem to derive a set of optical-frequency antenna design rules for benchmarking and optimizing the performance of optical antennas driven by single quantum emitters. Based on these findings a novel plasmonic cavity antenna design is presented exhibiting a considerably improved performance compared to a reference two-wire antenna. Our work will be useful for the design of high-performance optical antennas and nanoresonators for diverse applications ranging from quantum optics to antennaenhanced single-emitter spectroscopy and sensing.

T. Feichtner, S. Christiansen, B. Hecht
Physical Review Letters 119 (21), 217401