R. Natali

Structural, electrical, electronic and optical properties of melanin films

We present thick, uniform and rather flat melanin films obtained using spray deposition. The morphology of the films was investigated using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Temperature-dependent electrical resistance of melanin thin films evidenced a semiconductor-like character and a hysteretic behavior linked to an irreversible process of water molecule desorption from the melanin film. X-ray Photoelectron Spectroscopy (XPS) was carried out to analyze the role of the functional groups in the primary and secondary structure of the macromolecule, showing that the contribution of the 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunit to the molecule is about 35%. Comparison of the optical absorption of the thick (800nm) and thin (80nm) films showed a spectral change when the thickness increases. From in vacuum photoconductivity (PC) measured at controlled temperatures, we suggest that the melanin films exhibit a possible charge transport mechanism by means of delocalized

Optomechanically induced transparency in a membrane-in-the-middle setup at room temperature

\pi

Probing deformed commutators with macroscopic harmonic oscillators.

states along the stacked planar secondary structure.

Optomechanical sideband cooling of a thin membrane within a cavity

Summary form only given. In cavity optomechanics one can manipulate the dynamics of a nanomechanical resonator with light, and at the same time one can control light by tayloring its interaction with one (or more) mechanical resonances. A notable example of this kind of light beam control is provided by the optomechanical analogue of electromagnetically induced transparency (EIT), the so called optomechanically induced transparency (OMIT), which has been recently demonstrated [1-3]. In OMIT, the internal resonance of the medium is replaced by a dipole-like interaction of optical and mechanical degrees of freedom which occurs when the pump is tuned to the lower motional sideband of the cavity resonance. OMIT may offer various advantages with respect to standard atomic EIT: i) it does not rely on naturally occurring resonances and could therefore be applied to previously inaccessible wavelength regions; ii) a single optomechanical element can already achieve unity contrast, which in the atomic case is only possible within the setting of cavity quantum electrodynamics; iii) one can achieve significant optical delay times, since they are limited only by the mechanical resonance lifetime 1/γm. Previous OMIT demonstrations have been carried out in a cryogenic setup [1,2]; here we show OMIT in a room temperature optomechanical setup consisting of a thin semitransparent membrane within a high-finesse optical Fabry-Perot cavity [3]. Fig. 1 (left upper panel) shows the phase shift acquired by the probe beam during its transmission through the optomechanical cavity. The derivative of such a phase shift gives the group advance due to causality-preserving superluminal effects which a probe pulse spectrally contained within the transparency window would accumulate in its transmission through the cavity. From the fitting curve we infer a maximum signal time advance τT ≈ -108 ms, which is very close to the theoretical achievable maximum τTmax = -2C/[γm(1 +C)], which is -109 ms in our case where the optomechanical cooperativity is C = 160. The reflected field is instead delayed, and from the corresponding expression for the maximum time delay τRmax = 2/[γm(1 +C)], we can also infer a group delay of the reflected probe field τR ≈ 670 μs [3]. In the left lower panel the transparency frequency window in which the probe is completely reflected by the interference associated with the optomechanical interaction is evident. The width of the transparency window is related to the effective mechanical dampingγeffm ≈ γm(1 +C). Therefore both delay and width can be tuned by changing C which in our case is achieved by shifting the membrane along the cavity axis. This is illustrated in the right panel, where the modulus of the beat amplitude vs Δ is plotted for different positions shifts z0 of the membrane from a field node (see caption).

Quantum dynamics of an optical cavity coupled to a thin semitransparent membrane: Effect of membrane absorption

A minimal observable length is a common feature of theories that aim to merge quantum physics and gravity. Quantum mechanically, this concept is associated with a nonzero minimal uncertainty in position measurements, which is encoded in deformed commutation relations. In spite of increasing theoretical interest, the subject suffers from the complete lack of dedicated experiments and bounds to the deformation parameters have just been extrapolated from indirect measurements. As recently proposed, low-energy mechanical oscillators could allow to reveal the effect of a modified commutator. Here we analyze the free evolution of high-quality factor micro- and nano-oscillators, spanning a wide range of masses around the Planck mass mP (≈22 μg). The direct check against a model of deformed dynamics substantially lowers the previous limits on the parameters quantifying the commutator deformation.

Tunable linear and quadratic optomechanical coupling for a tilted membrane within an optical cavity: theory and experiment

We present an experimental study of dynamical back-action cooling of the fundamental vibrational mode of a thin semitransparent membrane placed within a high-finesse optical cavity. We study how the radiation?pressure interaction modifies the mechanical response of the vibrational mode, and the experimental results are in agreement with a Langevin equation description of the coupled dynamics. The experiments are carried out in the resolved sideband regime, and we have observed cooling by a factor of ?350. We have also observed the mechanical frequency shift associated with the quadratic term in the expansion of the cavity mode frequency versus the effective membrane position, which is typically negligible in other cavity optomechanical devices.

Microfabrication of large-area circular high-stress silicon nitride membranes for optomechanical applications

We study the quantum dynamics of the cavity optomechanical system formed by a Fabry-Perot cavity with a thin vibrating membrane at its center. We first derive the general multimode Hamiltonian describing the radiation pressure interaction between the cavity modes and the vibrational modes of the membrane. We then restrict the analysis to the standard case of a single cavity mode interacting with a single mechanical resonator and we determine to what extent optical absorption by the membrane hinder reaching a quantum regime for the cavity-membrane system. We show that membrane absorption does not pose serious limitations and that one can simultaneously achieve ground state cooling of a vibrational mode of the membrane and stationary optomechanical entanglement with state-of-the-art apparatuses.

Normal-Mode Splitting in a Weakly Coupled Optomechanical System

We present an experimental study of an optomechanical system formed by a vibrating thin semi-transparent membrane within a high-finesse optical cavity. We show that the coupling between the optical cavity modes and the vibrational modes of the membrane can be tuned by varying the membrane position and orientation. In particular, we demonstrate a large quadratic dispersive optomechanical coupling in correspondence with avoided crossings between optical cavity modes weakly coupled by scattering at the membrane surface. The experimental results are well explained by a first order perturbation treatment of the cavity eigenmodes.

Quantum dynamics of a vibrational mode of a membrane within an optical cavity

In view of the integration of membrane resonators with more complex MEMS structures, we developed a general fabrication procedure for circular shape SiNx membranes using Deep Reactive Ion Etching (DRIE). Large area and high-stress SiNx membranes were fabricated and used as optomechanical resonators in a Michelson interferometer, where Q values up to 1.3 × 106 were measured at cryogenic temperatures, and in a Fabry-Perot cavity, where an optical finesse up to 50000 has been observed.

Critical current in granular superconductor C-Si-W with peak-type re-entrant superconductivity

Normal-mode splitting is the most evident signature of strong coupling between two interacting subsystems. It occurs when two subsystems exchange energy between themselves faster than they dissipate it to the environment. Here we experimentally show that a weakly coupled optomechanical system at room temperature can manifest normal-mode splitting when the pump field fluctuations are antisquashed by a phase-sensitive feedback loop operating close to its instability threshold. Under these conditions the optical cavity exhibits an effectively reduced decay rate, so that the system is effectively promoted to the strong coupling regime.