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

Trusting the Process: Current Fashions in History of Political Thought

Some very recent “states of the field” of the history of political thought are rich in valuable information and raise many considerations. As often happens, though, rather than dwelling on what is shared, perhaps it could be more fruitful to reason in dialogue with the authors on how their reflection surprises or perplexes. In order to avoid unconscious ideological trends, the main issues to debate seem to be the idea of a role for the history of political thinking, the global perspective, the meaning of context, the relationship between the history of political thought and political theory and, mainly, the concept of scholar’s interest

Resonant driving of a single photon emitter embedded in a mechanical oscillator

Coupling a microscopic mechanical resonator to a nanoscale quantum system enables control of the mechanical resonator via the quantum system and vice-versa. The coupling is usually achieved through functionalization of the mechanical resonator, but this results in additional mass and dissipation channels. An alternative is an intrinsic coupling based on strain. Here we employ a monolithic semiconductor system: the nanoscale quantum system is a semiconductor quantum dot (QD) located inside a nanowire. We demonstrate the resonant optical driving of the QD transition in such a structure. The noise spectrum of the resonance fluorescence signal, recorded in the single-photon counting regime, reveals a coupling to mechanical modes of different types. We measure a sensitivity to displacement of 65 fm/root Hz limited by charge noise in the device. Finally, we use thermal excitation of the different modes to determine the location of the QD within the trumpet, and calculate the contribution of the Brownian motion to the dephasing of the emitter

Nicotine but not saline self-administering or yoked control conditions produces sustained neuroadaptations in the accumbens shell

IntroductionUsing yoked animals as the control when monitoring operant drug-self-administration is considered the golden standard. However, instrumental learning per se recruits several neurocircuits that may produce distinct or overlapping neuroadaptations with drugs of abuse. The aim of this project was to assess if contingent responding for nicotine or saline in the presence of a light stimulus as a conditioned reinforcer is associated with sustained neurophysiological adaptations in the nucleus accumbens shell (nAcS), a brain region repeatedly associated with reward related behaviors.MethodsTo this end, nicotine-or saline-administrating rats and yoked-saline stimulus-unpaired training conditions were assessed in operant boxes over four consecutive weeks. After four additional weeks of home cage forced abstinence and subsequent cue reinforced responding under extinction conditions, ex vivo electrophysiology was performed in the nAcS medium spiny neurons (MSNs).ResultsWhole cell recordings conducted in voltage and current-clamp mode showed that excitatory synapses in the nAcS were altered after prolonged forced abstinence from nicotine self-administration. We observed an increase in sEPSC amplitude in animals with a history of contingent nicotine SA potentially indicating higher excitability of accumbal MSNs, which was further supported by current clamp recordings. Interestingly no sustained neuroadaptations were elicited in saline exposed rats from nicotine associated visual cues compared to the yoked controls.ConclusionThe data presented here indicate that nicotine self-administration produces sustained neuroadaptations in the nAcS while operant responding driven by nicotine visual stimuli has no long-term effects on MSNs in nAcS

A fiber-coupled quantum-dot on a photonic tip

International audienc

Un romanzo in un endecasillabo?

Durante il corso di Letteratura italiana (anno 2017/18), dedicato alla Commedia di Dante, gli studenti sono stati invitati a cimentarsi nell’impresa di concentrare un romanzo nello spazio conchiuso di un endecasillabo o più terzine dantesche – o, in alternativa, nella misura di un tweet: si trattava (in un certo senso) di emulare la capacità del sommo poeta di distillare un’intera esistenza (Paolo e Francesca, Ulisse, Conte Ugolino…) nel volgere di poche terzine. Una giuria di ‘addetti ai lavori’ ha selezionato i migliori componimenti – sorvolando, di tanto in tanto, su alcune infrazioni creative rispetto alle misure canoniche