Room-Temperature Silicon Platform for GHz-Frequency Nanoelectro-Opto-Mechanical Systems.

Nanoelectro-opto-mechanical systems enable the synergistic coexistence of electrical, mechanical, and optical signals on a chip to realize new functions. Most of the technology platforms proposed for the fabrication of these systems so far are not fully compatible with the mainstream CMOS technology, thus, hindering the mass-scale utilization. We have developed a CMOS technology platform for nanoelectro-opto-mechanical systems that includes piezoelectric interdigitated transducers for electronic driving of mechanical signals and nanocrystalline silicon nanobeams for an enhanced optomechanical interaction. Room-temperature operation of devices at 2 GHz and with peak sensitivity down to 2.6 cavity phonons is demonstrated. Our proof-of-principle technology platform can be integrated and interfaced with silicon photonics, electronics, and MEMS devices and may enable multiple functions for coherent signal processing in the classical and quantum domains.

Record Low Thermal Conductivity of Polycrystalline MoS2 Films: Tuning the Thermal Conductivity by Grain Orientation.

We report a record low thermal conductivity in polycrystalline MoS2 obtained for ultrathin films with varying grain sizes and orientations. By optimizing the sulfurization parameters of nanometer-thick Mo layers, five MoS2 films containing a combination of horizontally and vertically oriented grains, with respect to the bulk (001) monocrystal, were grown. From transmission electron microscopy, the average grain size, typically below 10 nm, and proportion of differently oriented grains were extracted. The thermal conductivity of the suspended samples was extracted from a Raman laser-power-dependent study, and the lowest value of thermal conductivity of 0.27 W m-1 K-1, which reaches a similar value as that of Teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains in similar proportion. Analysis by means of molecular dynamics and finite element method simulations confirm that such a grain arrangement leads to lower grain boundary conductance. We discuss the possible use of these thermal insulating films in the context of electronics and thermoelectricity.

Nonlinear dynamics and chaos in an optomechanical beam.

Optical nonlinearities, such as thermo-optic mechanisms and free-carrier dispersion, are often considered unwelcome effects in silicon-based resonators and, more specifically, optomechanical cavities, since they affect, for instance, the relative detuning between an optical resonance and the excitation laser. Here, we exploit these nonlinearities and their intercoupling with the mechanical degrees of freedom of a silicon optomechanical nanobeam to unveil a rich set of fundamentally different complex dynamics. By smoothly changing the parameters of the excitation laser we demonstrate accurate control to activate two- and four-dimensional limit cycles, a period-doubling route and a six-dimensional chaos. In addition, by scanning the laser parameters in opposite senses we demonstrate bistability and hysteresis between two- and four-dimensional limit cycles, between different coherent mechanical states and between four-dimensional limit cycles and chaos. Our findings open new routes towards exploiting silicon-based optomechanical photonic crystals as a versatile building block to be used in neurocomputational networks and for chaos-based applications.

Thermal noise and optomechanical features in the emission of a membrane-coupled compound cavity laser diode.

We demonstrate the use of a compound optical cavity as linear displacement detector, by measuring the thermal motion of a silicon nitride suspended membrane acting as the external mirror of a near-infrared Littrow laser diode. Fluctuations in the laser optical power induced by the membrane vibrations are collected by a photodiode integrated within the laser, and then measured with a spectrum analyzer. The dynamics of the membrane driven by a piezoelectric actuator is investigated as a function of air pressure and actuator displacement in a homodyne configuration. The high Q-factor (~3.4 · 10(4) at 8.3 · 10(-3) mbar) of the fundamental mechanical mode at ~73 kHz guarantees a detection sensitivity high enough for direct measurement of thermal motion at room temperature (~87 pm RMS). The compound cavity system here introduced can be employed as a table-top, cost-effective linear displacement detector for cavity optomechanics. Furthermore, thanks to the strong optical nonlinearities of the laser compound cavity, these systems open new perspectives in the study of non-Markovian quantum properties at the mesoscale.

High Q light-emitting Si-rich Si3N4 microdisks.

We report on the optical properties of active silicon (Si)-rich Si3N4 microdisk cavities in the visible range. We have studied the correlation between the quality (Q) factor of the cavities and the active material deposition parameters. Microphotoluminescence measurements revealed subangstrom whispering galley modes resonances and a maximum Q of 104 around 760 nm. These values improve significantly the best results reported so far for Si-based light-emitting circular resonators in the visible range. In contrast to what is reported for Si-rich SiO2-based microcavities, we demonstrate the absence of a spectral widening at high pump fluxes associated to carrier absorption mechanisms, which allows high emitted power without degrading the Q of the cavity. These results open the route toward the monolithic integration of those structures into more complex circuits including Si photodetectors.

Reduction of the amplified spontaneous emission threshold in semiconducting polymer waveguides on porous silica.

Hybrid organic-inorganic monomode waveguides of conjugated polymers on porous silicon (PS) substrates have been fabricated. Different low refractive index PS substrates, varying from 1.46 down to 1.18 have been studied. Amplified spontaneous emission (ASE) has been observed for all the samples and the ASE threshold has been monitored as a function of the PS refractive index. A decrease in the ASE threshold is detected when the PS refractive index decreases. These results have been analysed in the frame of a four level waveguide amplifier model and the theoretical predictions are in agreement with the experimental data.

Whispering-gallery modes and light emission from a Si-nanocrystal-based single microdisk resonator.

We report on visible light emission from Si-nanocrystal based optically active microdisk resonators. The room temperature photoluminescence (PL) from single microdisks shows the characteristic modal structure of whispering-gallery modes. The emission is both TE and TM-polarized in 300 nm thick microdisks, while thinner ones (135 nm) support only TE-like modes. Thinner disks have the advantage to filter out higher order radial mode families, allowing for measuring only the most intense first order modal structure. We reveal subnanometer linewidths and corresponding quality factors as high as 2800, limited by the spectral resolution of the experimental setup. Moreover, we observe a modification of mode linewidth by a factor 13 as a function of pump power. The origin of this effect is attributed to an excited carrier absorption loss mechanism.

Porous silicon-based rugate filters.

We report an experimental study of porous silicon-based rugate filters. We performed filter apodization, following a half-apodization approach, which successfully attenuated the sidelobes at both sides of the photonic stop band. We achieved successful reduction of interference ripples through the insertion of index-matching layers on the first and last interfaces. An apodized dielectric mirror and a rugate filter are compared: Appreciable differences in the harmonic presence and stop-band performance were observed and are commented on. Bandwidth control when index contrast is modified is also demonstrated. Finally, the possibility of combining different rugate filter designs to attain more complex responses is demonstrated by the achievement of a multi-stop-band filter. Numerical calculations for design optimization and comparison with experimental data are reported too.