Experimental Physics V

Nano-Optics and Biophotonics

Latest publications

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)


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


Cavity-assisted ultrafast long-range periodic energy transfer between plasmonic nanoantennas

Radiationless energy transfer is at the core of diverse phenomena, such as light harvesting in photosynthesis, energy-transfer-based microspectroscopies, nanoscale quantum entanglement and photonic-mode hybridization. Typically, the transfer is efficient only for separations that are much shorter than the diffraction limit. This hampers its application in optical communication and quantum information processing, which require spatially selective addressing. Here, we demonstrate highly efficient radiationless coherent energy transfer over a distance of twice the excitation wavelength by combining localized and delocalized plasmonic modes. Analogous to the Tavis–Cummings model, two whispering-gallery-mode antennas placed in the foci of an elliptical plasmonic cavity fabricated from single-crystal gold plates act as a pair of oscillators coupled to a common cavity mode. Time-resolved two-photon photoemission electron microscopy (TR 2P-PEEM) reveals an ultrafast long-range periodic energy transfer in accordance with the simulations. Our observations open perspectives for the optimization and tailoring of mesoscopic energy transfer and long-range quantum emitter coupling.

Martin Aeschlimann, Tobias Brixner, Mirko Cinchetti, Benjamin Frisch, Bert Hecht, Matthias Hensen, Bernhard Huber, Christian Kramer, Enno Krauss, Thomas H Loeber, Walter Pfeiffer, Martin Piecuch & Philip Thielen
Light: Science & Applications 6, lsa2017111


On-Chip Single-Plasmon Nanocircuit Driven by a Self-Assembled Quantum Dot

Quantum photonics holds great promise for future technologies such as secure communication, quantum computation, quantum simulation, and quantum metrology. An outstanding challenge for quantum photonics is to develop scalable miniature circuits that integrate single-photon sources, linear optical components, and detectors on a chip. Plasmonic nanocircuits will play essential roles in such developments. However, for quantum plasmonic circuits, integration of stable, bright, and narrow-band single photon sources in the structure has so far not been reported. Here we present a plasmonic nanocircuit driven by a self-assembled GaAs quantum dot. Through a planar dielectricplasmonic hybrid waveguide, the quantum dot efficiently excites narrow-band single plasmons that are guided in a two-wire transmission line until they are converted into single photons by an optical antenna. Our work demonstrates the feasibility of fully on-chip plasmonic nanocircuits for quantum optical applications.

X.Wu, P. Jiang, G. Razinskas, Y. Huo, H. Zhang, M. Kamp, A. Rastelli, O. G. Schmidt, B. Hecht, K. Lindfors & M. Lippitz
 Nano Lett., 17, 4291 (2017)


Transmission of plasmons through a nanowire


Exact quantitative understanding of plasmon propagation along nanowires is mandatory for designing and creating functional devices. Here we investigate plasmon transmission through top-down fabricated monocrystalline gold nanowires on a glass substrate. We show that the transmission through finite-length nanowires can be described by Fabry-Pérot oscillations that beat with free- space propagating light launched at the incoupling end. Using an extended Fabry-Pérot model, experimental and simulated length dependent transmission signals agree quantitatively with a fully analytical model.

P. Geisler, E. Krauss, G. Razinskas & B. Hecht
ACS Photonics 4, 1615 (2017)


Polarization dependence of plasmonic near-field enhanced photoemission from cross antennas

"Polarization dependence of plasmonic near-field enhanced photoemission from cross antennas"

The field enhancement of individual cross-shaped nanoantennas for normal incident light has been measured by the relative photoemission yield using a photoemission electron microscope. We not only measured the electron yield in dependence on the intensity of infrared light (800 nm, 100 fs), but also the polarization dependence. In the normal incidence geometry, the electrical field vector of the illuminating light lies in the surface plane of the sample, independent of the polarization state. Strong yield variations due to an out-of-plane field component as well as changes in the polarization state described by the Fresnel laws are avoided. The electron yield is related to the near-field enhancement as a function of the polarization state of the incident light. The polarization dependence is well explained by numerical simulations.

P. Klaer, G. Razinskas, M. Lehr, X. Wu, B. Hecht, F. Schertz, H.-J. Butt, G. Schönhense, H. J. Elmers
Appl. Phys. B 122:136 (2016)

Investigation of the nonlinear refractive index of single-crystalline thin gold films and plasmonic nanostructures

"Investigation of the nonlinear refractive index of single-crystalline thin gold films and plasmonic nanostructures"

The nonlinear refractive index of plasmonic materials may be used to obtain nonlinear functionality, e.g., power-dependent switching. Here, we investigate the nonlinear refractive index of single-crystalline gold in thin layers and nanostructures on dielectric substrates. In a first step, we implement a z-scan setup to investigate ~100-µm-sized thin-film samples. We determine the nonlinear refractive index of fused silica, n2(SiO2) = 2.9 × 10−20 m2/W, in agreement with literature values. Subsequent z-scan measurements of single-crystalline gold films reveal a damage threshold of 0.22 TW/cm2 and approximate upper limits of the real and imaginary parts of the nonlinear refractive index, |n 2′(Au)| < 1.2 × 10−16 m2/W and |n 2″(Au)| < 0.6 × 10−16 m2/W, respectively. To further determine possible effects of a nonlinear refractive index in plasmonic circuitry, interferometry is proposed as a phase-sensitive probe. In corresponding nanostructures, relative phase changes between two propagating near-field modes are converted to amplitude changes by mode interference. Power-dependent experiments using sub-10-fs near-infrared pulses and diffraction-limited resolution (NA = 1.4) reveal linear behavior up to the damage threshold (0.23 times relative to that of a solid single-crystalline gold film). An upper limit for the nonlinear power-dependent phase change between two propagating near-field modes is determined to Δφ < 0.07 rad.

S. Goetz, G. Razinskas, E. Krauss, C. Dreher, M. Wurdack, P. Geisler, M. Pawłowska, B. Hecht & T. Brixner
Appl. Phys. B 122:94 (2016)

Electromechanically Tunable Suspended Optical Nano-antenna

"Electromechanically Tunable Suspended Optical Nano-antenna"

Coupling mechanical degrees of freedom with plasmonic resonances has potential applications in optomechanics, sensing, and active plasmonics. Here we demonstrate a suspended two-wire plasmonic nano-antenna acting like a nano-electrometer. The antenna wires are supported and electrically connected via a thin leads without disturbing the antenna resonance. As a voltage is applied, equal charges are induced on both antenna wires. The resulting equilibrium between the repulsive Coulomb force and the restoring elastic bending force enables us to precisely control the gap size. As a result the resonance wavelength and the field enhancement of the suspended optical nano-antenna (SONA) can be reversibly tuned. Our experiments highlight the potential to realize large bandwidth optical nanoelectromechanical systems (NEMS).

K. Chen, G. Razinskas, T. Feichtner, S. Grossmann, S. Christiansen & B. Hecht
Nano Letters 16, 2680 (2016)

See also Research Highlight in Nature Photonics


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