Nanomechanical systems are freely suspended, vibrating nanostructures.  Examples include doubly clamped beams or strings, singly clamped nanopillars, or membranes with nanoscale thickness. Their flexural eigenmodes, typically in the megahertz range, are excited by resonant actuation, parametric pumping, or even by thermal noise. The vibrational properties of these tiny objects resemble those of a macroscopic guitar string. However, their response fundamentally differs from their macroscopic counterparts: Nanomechanical resonators can exhibit remarkably high mechanical quality factors, such that the system performs several 100,000 free oscillations before decaying. The dissipation increases with shrinking dimensions, while strong anharmonicities provide a rich nonlinear mechanical response. Nanomechanical resonators are highly sensitive to changes in their environment, and coupling to external degrees of freedom can give rise to strong backaction effects.

In our lab, we are conducting experimental research on nanomechanical systems, with an emphasis on the dissipation, nonlinear dynamics, coupling and coherent control. We employ state of the art cleanroom fabrication technology to process nanoresonators based on strongly pre-stressed silicon nitride, crystalline semiconductor materials such as indium gallium phosphide or gallium arsenide, as well as carbon nanotubes and atomically thin two-dimensional materials. We have pioneered an integrated dielectric transduction scheme to coherently control high Q nanomechanical systems and continuously enhance the functionality of this versatile nano-electromechanical platform, but also explore cavity nano-optomechanical systems, nanoresonator arrays, or nanomechanical charge transport.

Our current research focuses on the following aspects:

Nonlinear dynamics of High Q nanomechanical resonators

We demonstrate a conceptually new and radically simple technique to probe squeezed fluctuations. So far, squeezing has been conventionally quantified in phase-sensitive measurements tracking the fluctuations in phase space. By contrast, we characterize the squeezed state through a spectral measurement. To this end, we employ a nanomechanical resonator of extremely high quality operated in the classical regime. Its thermal fluctuations are squeezed by driving it into a nonlinear regime. The measured power spectrum exhibits two clearly resolved satellite peaks around the drive frequency. Theoretical analysis shows that the peaks’ heights encode the squeezing parameter, which can hence be directly extracted from the power spectrum.
Our results demonstrate that, in driven systems, squeezing can be revealed and characterized in a single shot measurement of the power spectrum. The concept is generic and is not limited to the presented case of a classical resonator, but fully applies in the quantum regime as well. It provides a new perspective on the squeezing of fluctuations and thus should further extend its important applications, including high-resolution sensing and signal processing.
See: Huber et al.,Phys. Rev. X 10, 021066 (2020)

Further research on nonlinear high Q nanoresonators addresses the multiphoton amplification and squeezing by a nonlinear nanomechanical mode (J.S. Ochs et al., arXiv:2007.15382), effects arising from resonantly induced friction (M.I. Dykman et al., Phys. Rev. Lett. 122, 254301 (2019)), as well as the dispersive coupling and parametric effects in multimode resonator systems (K. Gajo et al., Phys. Rev. B 101, 075420 (2020) und M. Seitner et al., Phys. Rev. Lett. 118, 254301 (2017)).

Crystalline InGaP nanostrings

Crystalline nanostrings have the potential to outperform SiN strings, provided they exhibit a comparable tensile stress. One possible material system to realize crystalline, yet pre-stressed strings are InGaP heterostructures. We demonstrate high Q InGaP string resonators, and characterize their anisotropic elastic properties arising from the crystalline structure of the underlying InGaP crystal.
See: M. Bückle et al., Appl. Phys. Lett. 113, 201903 (2018)

Inverted conical GaAs nanopillars as nanomechanical resonators

Semiconductur nano- and micropillars represent a promising platform for hybrid nanodevices. Their ability to couple to a broad variety of nanomechanical, acoustic, charge, spin, excitonic, polaritonic, or electromagnetic excitations is utilized in fields as diverse as force sensing or optoelectronics. In order to fully exploit the potential of these versatile systems e.g. for metamaterials, synchronization or topologically protected devices an intrinsic coupling mechanism between individual pillars needs to be established. This can be accomplished by taking advantage of the strain field induced by the flexural modes of the pillars.
We explore top-down fabricated GaAs nanopillars and demonstrate strain-induced, strong coupling between two adjacent nanomechanical pillar resonators. Both mode hybridization and the formation of an avoided level crossing in the response of the nanopillar pair are experimentally observed. The described coupling mechanism is readily scalable, enabling hybrid nanomechanical resonator networks for the investigation of a broad range of collective dynamical phenomena. See: J. Doster et al., Nature Comm. 10, 5246 (2019)
Further research on nanopillar arrays targets their use as biosensors (Paulitschke et al., Appl. Phys. Lett. 103, 261901 (2013)).

Cavity nano-optomechanics

Cavity optomechanics experiments with nanoscale resonators are challenging because of their subwavelengh dimensions, but offer interesting insights into dispersive as well as dissipative coupling and dynamical backaction. We employ a resonator-in-the-middle approach using a fiber-based Fabry-Perot micro cavity as a high finesse and low mode volume probe of the nanoresonator. Systems under investigation include silicon nitride string resonators, carbon nanotubes and atomically thin two dimensional materials.
See: Stapfner et al., Appl. Phys. Lett. 102, 151910 (2013)

Dielectric transduction of nanomechanical systems


Any polarizable object exposed to an inhomogeneous field will experience a force. We employ this simple concept as an innovative and highly efficient scheme to actuate, frequency tune and couple the eigenmodes of nanomechanical resonators by a dielectric gradient force generated by electrodes located in the vicinity of the resonator. Refined electrode geometries are currently being explored to increase control over the string’s eigenmodes, and novel concepts to improve microwave cavity assisted heterodyne displacement detection are developed.
See:  Unterreithmeier et al., Nature 458, 1001 (2009), Rieger et al., Appl. Phys. Lett. 101 103110 (2012), T. Faust et al., Nat. Commun. 3,728 (2012)

Strongly coupled nanomechanical modes


When tuned on resonance, two strongly coupled modes exhibit an avoided crossing with a splitting exceeding the linewidth of the two modes. The underlying coupling mechanisms can be manifold, and include dielectrically induced coupling, or coupling mediated by strain in a joint substrate or clamping point. We are striving to control the coupling strength both within a single and between neighboring resonators, with the goal to realize nanomechanical resonator networks.
See: Faust et al., Phys. Rev. Lett. 109, 037205 (2012), Gajo et al., arXiv:1707.02926, Doster et al., in preparation

Dissipation in strongly pre-stressed silicon nitride nanoresonators


In recent years it has been shown that the dissipation in strongly pre-stressed SiN strings is limited by defects in the amorphous material. For the case of a metallized SiN-Au bilayer system, we have carefully analyzed the evolution of both the eigenfrequency and the dissipation as a function of the metallization thickness.
See: Faust et al., Phys. Rev. B 89, 100102(R) (2014), Seitner et al., Appl. Phys. Lett. 105, 213101 (2014)

Coherent control of a nanomechanical two-mode system


The nanomechanical two-mode system realized in the avoided crossing of two strongly coupled nanomechanical modes is a remarkable testbed to study Landau-Zener dynamics of single or multiple passages (Stückelberg interference) through the avoided crossing. Even more it entails  analogies with two level systems, and allows for state manipulation on a classical Bloch sphere by means of radio frequency pulses, and to explore the underlying decoherence processes via Rabi, Ramsey and Hahn echo measurements.
See: Faust et al., Phys. Rev. Lett. 109, 037205 (2012), Faust, Nature Physics 9, 485 (2013), Seitner et al., Phys. Rev. B 94, 245406 (2016), Seitner et al., New. J. Phys. 19, 033011 (2017)

Electromechanical charge shuttle


The quest of counting electrons is one of the key challenges in metrology. It relates to the attempt of linking the electrical units directly with fundamental constants, as it is the case for voltage and resistance using the Josephson effect and the Quantum Hall effect, respectively. So far, a similar definition of current is yet to be achieved. The creation, measurement and control of current at the single electron level represents a natural limit of precision, and although realization of this ultimate current standard has yet to be achieved, mechanical electron shuttles provide a promising approach.
The shuttle is realized by a gold island hosted in the center of a doubly-clamped silicon nitride nanostring which is situated in a gap between the source and drain electrode. Oscillation of the beam brings the island into contact with the electrodes, and in the presence of a DC bias the repetitive charging of the island results in a current mediated by  the moving island.
See: König et al., Nature Nano 3, 482 (2008), König et al., Appl. Phys. Lett. 101, 213111 (2012).


QuaSeRT - NanoKOM:
Optomechanische Quantensensoren bei Raumtemperatur (QuaSeRT)
Teilvorhaben: Nanomechanische Plattform für kohärente Messprotokolle (NanoKoM)

EU H2020 FET Proactive “Hybrid Optomechanical Technologies (HOT)”

EU H2020 ITN “Optomechanical Technologies (OMT)”