Overview

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:

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)


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)


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: Bückle et al., in preparation


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


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)


Parametric effects and nonlinear dynamics of strongly coupled modes:

Subject to a joint parametric drive tone, two strongly coupled modes exhibit signatures which are reminiscent of 2nd and 3rd order optical nonlinearities, such as non-degenerate parametric oscillation (signal-idler generation), degenerate parametric oscillation (injection locking), injection pulling or degenerate four wave mixing (Enforcing discipline on nanomechanical vibration). Different locking regimes can be identified and discriminated by the underlying parametric pump amplitude. While the non-degenerate parametric oscillation process produces an oscillator which is tunable over a broadband frequency range, the degenerate parametric oscillation leads to an ultra narrow linewidth oscillator that is highly stable against perturbation.
See: Seitner et al., Phys. Rev. Lett. 118, 254301 (2017)


Inverted conical GaAs nanowires as nanomechanical resonators:

Top-down fabricated, singly clamped nanowires oriented vertically on the chip not only represent ultraflexible nanoresonators, but also offer straightforward integration into large arrays. We have analyzed the elastic properties of such inverted conical “nanopillars”, establishing spring constants down to a 10-5 N/m. These extremely soft nanowires are employed as force sensors for cell-induced forces in a collaboration with Prof. Doris Heinrich (Fraunhofer ISC) (Dynamic force field mapping of living cells in a macro-scale transducer array based on flexible semiconductor nanopillars).
See: 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)


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).


Finanzierung

DFG via the Collaborative Research Center SFB 767
https://www.sfb767.uni-konstanz.de/sfb-767/
https://www.sfb767.uni-konstanz.de/projects/project-area-a/project-a07-weig/


EU H2020 FET Proactive “Hybrid Optomechanical Technologies (HOT)”
http://cordis.europa.eu/project/rcn/206201_en.html


EU H2020 ITN “Optomechanical Technologies (OMT)”
http://omt-etn.net


VolkswagenStiftung
https://www.volkswagenstiftung.de/foerderung/schluesselergebnisse/herausforderung/komponenten/bewilligungen-2010.html


Center of Applied Photonics (CAP)
https://cms.uni-konstanz.de/physik/cap-center-for-applied-photonics-konstanz/