Nanomechanics and scanning probe microscopy with nanowires

Nanowires (NWs) have drawn growing interest in the last years due to the incredible versatility and
consequent variety of suitable applications. Due to favorable geometry and low mass, NWs present
themselves as excellent force transducers. Their almost perfect structure free of defects significantly
increases the quality of mechanical resonators based on them. The possibility to control geometrical
parameters like diameter and length makes it possible to move the resonance frequency of these objects
to higher frequencies so it is easier to decouple them from noise. However, such slender beams are more
susceptible to enter non-linear regimes of motion, where the analysis becomes more complex but
sometimes difficult to avoid. For this reason, we studied the nonlinear regime of motion of GaAs NWs. Our
results indicate that although nonlinear motion can be non-negligible for NWs, the nonlinearity can also be
turned into an advantage using simple measurement schemes.
We observed how the fundamental mode of our NWs is always split in two orthogonal modes due to a small
asymmetry in the cross section and we studied how these modes interact through a non-linear coupling.
When driving one mode to high enough amplitudes its displacement affects the motion of the other mode
shifting its frequency. Such mode coupling could have several applications, including tuning the resonance
frequency and quality factor of one mode through driving of the other mode, and implementing quantum
non-demolition measurements of mechanical excitation. Another prospective use of the two orthogonal
modes in the nanowires lies in bidimensional sensing.
Nonetheless, we are not limited to bidimensional surface interactions: the versatility in the growth of NWs
that can be grown with different structures and materials, opens the gate to a vast range of possible probes
suitable to different environments. NWs can also be host of quantum objects such as QDs. Semiconductor
NWs make excellent waveguiding platforms. As a consequence, an optically active QD embedded in a
tailored GaAs NW results in a high fidelity single photon source. Moreover, the QD emission results
intrinsically coupled to the nanoresonator through strain, creating effectively a monolithic hybrid system
with potential for various quantum applications. By exploiting this coupling, we read the resonance
fluorescence signal of the QD to detect sub-picometer displacements of the mechanical modes at cryogenic
temperature by measuring the fluctuations in the single photon count rate. As an application of the sensing
capabilities of our device, we used the thermal excitation of a series of mechanical modes to determine the
location of the QD within the nanowire. Finally we discuss the impact of the strain coupling on the coherence
of the single photon emitter, introducing extra noise and dephasing in the QD emission, and, as a
consequence, reduces the indistinguishability of the photons.
To this end, we developed a new device, a quantum-fiber pigtail, where we directly couple a QD to an
optical fiber eliminating the need for complex optical setup and at the same time reducing the strain coupling
that leads to dephasing of the QD emission. The prototype of our device resulted to be a robust and compact
single photon source but with relative low collection efficiency. In this direction, with the support of numerical
simulations, we demonstrated how the efficiency of our device can be increased by one order of magnitude
with no subversive changes of the wire. Last, due to the favorable geometry of our device, we demonstrated
how it can be implemented as sensor for local electric fields. As a first proof of-principle, we map the vertical
component of the electric field produced by two parallel gold electrodes. This device has proven to be an
excellent starting point for sensing electric fields and with the help of FEM simulations we found the optimal
geometry for the optimal scanning PW.