Experimental Physics V


Nano-Optics and Biophotonics

Latest publications

Single quantum emitter Dicke enhancement

Coupling N identical emitters to the same field mode is a well-established method to enhance light-matter interaction. However, the resulting N boost of the coupling strength comes at the cost of a “linearized” (effectively semiclassical) dynamics. Here, we instead demonstrate a new approach for enhancing the coupling constant of a single quantum emitter, while retaining the nonlinear character of the light-matter interaction. We consider a single quantum emitter with N nearly degenerate transitions that are collectively coupled to the same field mode. We show that in such conditions an effective Jaynes-Cummings model emerges with a boosted coupling constant of order N. The validity and consequences of our general conclusions are analytically demonstrated for the instructive case N=2. We further observe that our system can closely match the spectral line shapes and photon autocorrelation functions typical of Jaynes-Cummings physics, proving that quantum optical nonlinearities are retained. Our findings match up very well with recent broadband plasmonic nanoresonator strong-coupling experiments and will, therefore, facilitate the control and detection of single-photon nonlinearities at ambient conditions.

T. Tufarelli, D. Friedrich, H. Groß, J. Hamm, O. Hess, B. Hecht
Phys. Rev. Research 3, 033103 (2021)

Near-infrared nanospectroscopy using a low-noisesupercontinuum source

Unlocking the true potential of optical spectroscopy on the nanoscale requires development of stable and low-noise laser sources. Here,we have developed a low-noise supercontinuum (SC) source based on an all-normal dispersion fiber pumped by a femtosecond fiber laserand demonstrate high resolution, spectrally resolved near-field measurements in the near-infrared (NIR) region. Specifically, we explore thereduced-noise requirements for aperture-less scattering-type scanning near-field optical microscopy (s-SNOM), including inherent pulse-to-pulse fluctuation of the SC. We use our SC light source to demonstrate the first NIR, spectrally resolved s-SNOM measurement, a situationwhere state-of-the-art commercial SC sources are too noisy to be useful. We map the propagation of surface plasmon polariton (SPP) waveson monocrystalline gold platelets in the wavelength region of 1.34–1.75μm in a single measurement, thereby characterizing experimentallythe dispersion curve of the SPP in the NIR. Our results represent a technological breakthrough that has the potential to enable a wide rangeof new applications of low-noise SC sources in near-field studies.

K. J. Kaltenecker, Shreesha Rao D. S., M. Rasmussen, H. B. Lassen, E. J. R. Kelleher, E. Krauss, B. Hecht, N. A. Mortensen, L. Grüner-Nielsen, C. Markos, O. Bang, N. Stenger, P. U. Jepsen
APL Photonics 6, 066106 (2021)

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

Driving plasmonic nanoantennas at perfect impedance matching using generalized coherent perfect absorption

Coherent perfect absorption (CPA) describes the absence of all outgoing modes from a lossy resonator, driven by lossless incoming modes. Here, we show that for nanoresonators that also exhibit radiative losses, e.g., plasmonic nanoantennas, a generalized version of CPA (gCPA) can be applied. In gCPA outgoing modes are suppressed only for a subset of (guided plasmonic) modes while other (radiative) modes are treated as additional loss channels - a situation typically referred to as perfect impedance matching. Here we make use of gCPA to show how to achieve perfect impedance matching between a single nanowire plasmonic waveguide and a plasmonic nanoantenna. Antennas with both radiant and subradiant characteristics are considered. We further demonstrate potential applications in background-free sensing.

P. Grimm, G. Razinskas, J-S. Huang, B. Hecht

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)


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