57 research outputs found
Damping of metallized bilayer nanomechanical resonators at room temperature
We investigate the influence of gold thin-films subsequently deposited on a
set of initially bare, doubly clamped, high-stress silicon nitride string
resonators at room temperature. Analytical expressions for resonance frequency,
quality factor and damping for both in- and out-of-plane flexural modes of the
bilayer system are derived, which allows for the determination of effective
elastic parameters of the composite structure from our experimental data. We
find the inverse quality factor to scale linearly with the gold film thickness,
indicating that the overall damping is governed by losses in the metal.
Correspondingly, the mechanical linewidth increases by more than one order of
magnitude compared to the bare silicon nitride string resonator. Furthermore,
we extract mechanical quality factors of the gold film for both flexural modes
and show that they can be enhanced by complete deposition of the metal in a
single step, suggesting that surface and interface losses play a vital role in
metal thin-films
Finite time St\"uckelberg interferometry with nanomechanical modes
St\"uckelberg interferometry describes the interference of two strongly
coupled modes during a double passage through an avoided energy level crossing.
In this work, we experimentally investigate finite time effects in
St\"uckelberg interference and provide an exact analytical solution of the
St\"uckelberg problem. Approximating this solution in distinct limits reveals
uncharted parameter regimes of St\"uckelberg interferometry. Experimentally, we
study these regimes using a purely classical, strongly coupled nanomechanical
two-mode system of high quality factor. The classical two-mode system consists
of the in-plane and out-of-plane fundamental flexural mode of a high stress
silicon nitride string resonator, coupled via electric gradient fields. The
dielectric control and microwave cavity enhanced universal transduction of the
nanoelectromechanical system allows for the experimental access to all
theoretically predicted St\"uckelberg parameter regimes. We exploit our
experimental and theoretical findings by studying the onset of St\"uckelberg
interference in dependence of the characteristic system control parameters and
obtain characteristic excitation oscillations between the two modes even
without the explicit need of traversing the avoided crossing. The presented
theory is not limited to classical mechanical two-mode systems but can be
applied to every strongly coupled (quantum) two-level system, for example a
spin-1/2 system or superconducting qubit
Signatures of two-level defects in the temperature-dependent damping of nanomechanical silicon nitride resonators
The damping rates of high quality factor nanomechanical resonators are well
beyond intrinsic limits. Here, we explore the underlying microscopic loss
mechanisms by investigating the temperature-dependent damping of the
fundamental and third harmonic transverse flexural mode of a doubly clamped
silicon nitride string. It exhibits characteristic maxima reminiscent of
two-level defects typical for amorphous materials. Coupling to those defects
relaxes the momentum selection rules, allowing energy transfer from discrete
long wavelength resonator modes to the high frequency phonon environment
Coherent control of a nanomechanical two-level system
The Bloch sphere is a generic picture describing a coupled two-level system
and the coherent dynamics of its superposition states under control of
electromagnetic fields. It is commonly employed to visualise a broad variety of
phenomena ranging from spin ensembles and atoms to quantum dots and
superconducting circuits. The underlying Bloch equations describe the state
evolution of the two-level system and allow characterising both energy and
phase relaxation processes in a simple yet powerful manner.
Here we demonstrate the realisation of a nanomechanical two-level system
which is driven by radio frequency signals. It allows to extend the above Bloch
sphere formalism to nanoelectromechanical systems. Our realisation is based on
the two orthogonal fundamental flexural modes of a high quality factor
nanostring resonator which are strongly coupled by a dielectric gradient field.
Full Bloch sphere control is demonstrated via Rabi, Ramsey and Hahn echo
experiments. This allows manipulating the classical superposition state of the
coupled modes in amplitude and phase and enables deep insight into the
decoherence mechanisms of nanomechanical systems. We have determined the energy
relaxation time T1 and phase relaxation times T2 and T2*, and find them all to
be equal. This not only indicates that energy relaxation is the dominating
source of decoherence, but also demonstrates that reversible dephasing
processes are negligible in such collective mechanical modes. We thus conclude
that not only T1 but also T2 can be increased by engineering larger mechanical
quality factors. After a series of ground-breaking experiments on ground state
cooling and non-classical signatures of nanomechanical resonators in recent
years, this is of particular interest in the context of quantum information
processing
A universal platform for magnetostriction measurements in thin films
We present a universal nanomechanical sensing platform for the investigation
of magnetostriction in thin films. It is based on a doubly-clamped silicon
nitride nanobeam resonator covered with a thin magnetostrictive film. Changing
the magnetization direction within the film plane by an applied magnetic field
generates a magnetostrictive stress and thus changes the resonance frequency of
the nanobeam. A measurement of the resulting resonance frequency shift, e.g. by
optical interferometry, allows to quantitatively determine the magnetostriction
constants of the thin film. We use this method to determine the
magnetostriction constants of a 10nm thick polycrystalline cobalt film, showing
very good agreement with literature values. The presented technique can be
useful in particular for the precise measurement of magnetostriction in a
variety of (conducting and insulating) thin films, which can be deposited by
e.g. electron beam deposition, thermal evaporation or sputtering
Non-adiabatic dynamics of two strongly coupled nanomechanical resonator modes
The Landau-Zener transition is a fundamental concept for dynamical quantum
systems and has been studied in numerous fields of physics. Here we present a
classical mechanical model system exhibiting analogous behaviour using two
inversely tuneable, strongly coupled modes of the same nanomechanical beam
resonator. In the adiabatic limit, the anticrossing between the two modes is
observed and the coupling strength extracted. Sweeping an initialized mode
across the coupling region allows mapping of the progression from diabatic to
adiabatic transitions as a function of the sweep rate
Energy losses of nanomechanical resonators induced by atomic force microscopy- controlled mechanical impedance mismatching
Clamping losses are a widely discussed damping mechanism in nanoelectromechanical systems, limiting the performance of these devices. Here we present a method to investigate this dissipation channel. Using an atomic force microscope tip as a local perturbation in the clamping region of a nanoelectromechanical resonator, we increase the energy loss of its flexural modes by at least one order of magnitude. We explain this by a transfer of vibrational energy into the cantilever, which is theoretically described by a reduced mechanical impedance mismatch between the resonator and its environment. A theoretical model for this mismatch, in conjunction with finite element simulations of the evanescent strain field of the mechanical modes in the clamping region, allows us to quantitatively analyse data on position and force dependence of the tip-induced damping. Our experiments yield insights into the damping of nanoelectromechanical systems with the prospect of engineering the energy exchange in resonator networks
Frequency and Q-factor control of nanomechanical resonators
We present an integrated scheme for dielectric drive and read-out of high-Q
nanomechanical resonators which enables tuning of both the resonance frequency
and quality factor with an applied DC voltage. A simple model for altering
these quantities is derived, incorporating the resonator's complex electric
polarizability and position in an inhomogeneous electric field, which agrees
very well with the experimental findings as well as FEM simulations. By
comparing two sample geometries we are able to show that careful electrode
design can determine the direction of frequency tuning of flexural in- and
out-of-plane modes of a string resonator. Furthermore we demonstrate that the
mechanical quality factor can be voltage reduced more than fivefold
Electrical tuning of elastic wave propagation in nanomechanical lattices at MHz frequencies
Nanoelectromechanical systems (NEMS) that operate in the megahertz (MHz) regime allow energy transducibility between different physical domains. For example, they convert optical or electrical signals into mechanical motions and vice versa. This coupling of different physical quantities leads to frequency-tunable NEMS resonators via electromechanical non-linearities. NEMS platforms with single- or low-degrees of freedom have been employed to demonstrate quantum-like effects, such as mode cooling, mechanically induced transparency, Rabi oscillation, two-mode squeezing and phonon lasing. Periodic arrays of NEMS resonators with architected unit cells enable fundamental studies of lattice-based solid-state phenomena, such as bandgaps, energy transport, non-linear dynamics and localization, and topological properties, directly transferrable to on-chip devices. Here we describe one-dimensional, non-linear, nanoelectromechanical lattices (NEML) with active control of the frequency band dispersion in the radio-frequency domain (10–30 MHz). The design of our systems is inspired by NEMS-based phonon waveguides and includes the voltage-induced frequency tuning of the individual resonators. Our NEMLs consist of a periodic arrangement of mechanically coupled, free-standing nanomembranes with circular clamped boundaries. This design forms a flexural phononic crystal with a well-defined bandgap, 1.8 MHz wide. The application of a d.c. gate voltage creates voltage-dependent on-site potentials, which can significantly shift the frequency bands of the device. Additionally, a dynamic modulation of the voltage triggers non-linear effects, which induce the formation of a phononic bandgap in the acoustic branch, analogous to Peierls transition in condensed matter. The gating approach employed here makes the devices more compact than recently proposed systems, whose tunability mostly relies on materials’ compliance and mechanical non-linearities
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