Experimental Physics V

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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
arXiv:1702.03103 (2017)


Mode-matching for optical antennas

"Plasmonic cavity antenna"

The emission rate of a point dipole can be strongly increased in 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 antenna-enhanced single-emitter spectroscopy and sensing.

T. Feichtner, S. Christiansen & B.Hecht
arXiv:1611.05399 (2016), DOI:

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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)

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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)

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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)
DOI: 10.1021/acs.nanolett.6b00323

See also Research Highlight in Nature Photonics

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Plasmonic nanoantenna design and fabrication based on evolutionary optimization

"Plasmonic nanoantenna design and fabrication based on evolutionary optimization"

Nanoantennas for light enhance light-matter interaction at the nanoscale making them useful in optical communication, sensing, and spectroscopy. So far nanoantenna engineering has been largely based on rules derived from the radio frequency domain which neglect the inertia of free metal electrons at optical frequencies causing phenomena such as complete field penetration, ohmic losses and plasmon resonances. Here we introduce a general and scalable evolutionary algorithm that accounts for topological constrains of the fabrication method and therefore yields unexpected nanoantenna designs exhibiting strong light localization and enhancement which can directly be "printed" by focused-ion beam milling. The fitness ranking in a hierarchy of such antennas is validated experimentally by two-photon photoluminescence. Analysis of the best antennas' operation principle shows that it deviates fundamentally from that of classical radio wave-inspired designs. Our work sets the stage for a widespread application of evolutionary optimization to a wide range of problems in nano photonics.

T. Feichtner, O. Selig & B. Hecht
ArXive (2015), DOI:

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

"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.

X. Wu, P. Geisler, E. Krauss, R. Kullock & B. Hecht
Nanoscale 7, 16427-16433 (2015)
DOI: 10.1039/C5NR04262C

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Electrically driven optical antennas

"Electrically driven optical antennas"

Unlike radiowave antennas, so far optical nanoantennas cannot be fed by electrical generators. Instead, they are driven by light1 or indirectly via excited discrete states in active materials2, 3 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

J. Kern, R. Kullock, J. Prangsma, M. Emmerling, M. Kamp & B. Hecht
Nature Photonics 9, 582-586 (2015)
DOI: 10.1038/nphoton.2015.141

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

"Sketch of a gold flake"

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.

X. Wu, R. Kullock, E. Krauss & B. Hecht
Crystal Research and Technology 50, 595-602 (2015)
DOI: 10.1002/crat.201400429

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Remote detection of single emitters via optical waveguides

"Mode intensity and incoupling efficiency of an optical fiber with spherical end facet"

The integration of lab-on-a-chip technologies with single-molecule detection techniques may enable new applications in analytical chemistry, biotechnology, and medicine. We describe a method based on the reciprocity theorem of electromagnetic theory to determine and optimize the detection efficiency of photons emitted by single quantum emitters through truncated dielectric waveguides of arbitrary shape positioned in their proximity. We demonstrate experimentally that detection of single quantum emitters via such waveguides is possible, confirming the predicted behavior of the detection efficiency. Our findings blaze the trail towards efficient lensless single-emitter detection compatible with large-scale optofluidic integration.

P. Then, G. Razinskas, T. Feichtner, P. Haas, A. Wild, N. Bellini, R. Osellame, G. Cerullo & B. Hecht
Phys. Rev. A 89, 053801 (2014)

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