C. M. Sotomayor-Torres

Tuning thermal transport in ultrathin silicon membranes by surface nanoscale engineering

A detailed understanding of the connections of fabrication and processing to structural and thermal properties of low-dimensional nanostructures is essential to design materials and devices for phononics, nanoscale thermal management, and thermoelectric applications. Silicon provides an ideal platform to study the relations between structure and heat transport since its thermal conductivity can be tuned over 2 orders of magnitude by nanostructuring. Combining realistic atomistic modeling and experiments, we unravel the origin of the thermal conductivity reduction in ultrathin suspended silicon membranes, down to a thickness of 4 nm. Heat transport is mostly controlled by surface scattering: rough layers of native oxide at surfaces limit the mean free path of thermal phonons below 100 nm. Removing the oxide layers by chemical processing allows us to tune the thermal conductivity over 1 order of magnitude. Our results guide materials design for future phononic applications, setting the length scale at which nanostructuring affects thermal phonons most effectively.

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.

Self-sustained coherent phonon generation in optomechanical cavities

Optical forces can set tiny objects in states of mechanical self-sustained oscillation, spontaneously generating periodic signals by extracting power from steady sources. Miniaturized self-sustained coherent phonon sources are interesting for applications such as mass-force sensing, intra-chip metrology and intra-chip time-keeping among others. In this paper, we review several mechanisms and techniques that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical cavities, namely stimulated emission, dynamical back-action, forward stimulated Brillouin scattering and self-pulsing.

In-line metrology for roll-to-roll UV assisted nanoimprint lithography using diffractometry

We describe and discuss the optical design of a diffractometer to carry out in-line quality control during roll-to-roll nanoimprinting. The tool measures diffractograms in reflection geometry, through an aspheric lens to gain fast, non-invasive information of any changes to the critical dimensions of target grating structures. A stepwise tapered linear grating with constant period was fabricated in order to detect the variation in grating linewidth through diffractometry. The minimum feature change detected was ∼40 nm to a precision of 10 nm. The diffractometer was then integrated with a roll-to-roll UV assisted nanoimprint lithography machine to gain dynamic measurements in situ.

Optical modulation of coherent phonon emission in optomechanical cavities

Optomechanical (OM) structures are well suited to study photon-phonon interactions, and they also turn out to be potential building blocks for phononic circuits and quantum computing. In phononic circuits, in which information is carried and processed by phonons, OM structures could be used as interfaces to photons and electrons thanks to their excellent coupling efficiency. Among the components required for phononic circuits, such structures could be used to create coherent phonon sources and detectors, but more complex functions remain challenging. Here, we propose and demonstrate a way to modulate the coherent phonon emission from OM crystals by a photothermal effect induced by an external laser, effectively creating a phonon switch working at ambient conditions of pressure and temperature and the working speed of which is only limited by the build-up time of the mechanical motion of the OM structure. We additionally demonstrate two other modulation schemes: modulation of harmonics in which the mechanical mode remains active but different harmonics of the optical force are used, and modulation to and from a chaotic regime. Furthermore, due to the local nature of the photothermal effect used here, we expect this method to allow us to selectively modulate the emission of any single cavity on a chip without affecting its surroundings in the absence of mechanical coupling between the structures, which is an important step toward freely controllable networks of OM phonon emitters.Optomechanical (OM) structures are well suited to study photon-phonon interactions, and they also turn out to be potential building blocks for phononic circuits and quantum computing. In phononic circuits, in which information is carried and processed by phonons, OM structures could be used as interfaces to photons and electrons thanks to their excellent coupling efficiency. Among the components required for phononic circuits, such structures could be used to create coherent phonon sources and detectors, but more complex functions remain challenging. Here, we propose and demonstrate a way to modulate the coherent phonon emission from OM crystals by a photothermal effect induced by an external laser, effectively creating a phonon switch working at ambient conditions of pressure and temperature and the working speed of which is only limited by the build-up time of the mechanical motion of the OM structure. We additionally demonstrate two other modulation schemes: modulation of harmonics in which the mechani...

Mechanical oscillations in lasing microspheres

We investigate the feasibility of activating coherent mechanical oscillations in lasing microspheres by modulating the laser emission at a mechanical eigenfrequency. To this aim, 1.5% Nd3+:Barium-Titanium-Silicate microspheres with diameters around 50 {\mu}m were used as high quality factor (Q>10^6) whispering gallery mode lasing cavities. We have implemented a pump-and-probe technique in which the pump laser used to excite the Nd3+ ions is focused on a single microsphere with a microscope objective and a probe laser excites a specific optical mode with the evanescent field of a tapered fibre. The studied microspheres show monomode and multi-mode lasing action, which can be modulated in the best case up to 10 MHz. We have optically transduced thermally-activated mechanical eigenmodes appearing in the 50-70 MHz range, the frequency of which decreases with increasing the size of the microspheres. In a pump-and-probe configuration we observed modulation of the probe signal up to the maximum pump modulation frequency of our experimental setup, i.e., 20 MHz. This modulation decreases with frequency and is unrelated to lasing emission, pump scattering or thermal effects. We associate this effect to free-carrier-dispersion induced by multiphoton pump light absorption. On the other hand, we conclude that, in our current experimental conditions, it was not possible to resonantly excite the mechanical modes. Finally, we discuss on how to overcome these limitations by increasing the modulation frequency of the lasing emission and decreasing the frequency of the mechanical eigenmodes displaying a strong degree of optomechanical coupling.

Self-pulsing and phonon lasing in optomechanical crystals

We report on a novel and efficient strategy that can drive a mechanical mode into the lasing regime by exploiting the radiation pressure force in optomechanical (OM) cavities. The pumping mechanism is based on a self-pulsing limit-cycle, which is a spontaneous process that modulates the intracavity radiation pressure force in resonance with a mechanical eigenmode of the OM cavity.

Nanoscale thermal transport and phonon dynamics in ultra-thin Si based nanostructures

We study the dynamics of acoustic phonons in ultra-thin free-standing silicon membranes both experimentally and theoretically. We discuss the impact of the lifetimes of the acoustic phonons on the thermal transport properties of the membranes with thicknesses ranging from 8 nm to 1.5 μm. The phonon lifetimes are determined by measuring the dynamic variation of the reflectivity using ultra-fast pump-probe spectroscopy. This is achieved by asynchronous optical sampling (ASOPS) of two actively coupled femto-second laser oscillators. The coherent acoustic phonon lifetime is obtained from the dynamical variations of the reflectivity with a sensitivity of 10-5 and a time resolution of about 50 fs. The experimental results are compared to theoretical calculations considering both intrinsic and extrinsic relaxation scattering processes.