Cavity Optomechanics with Anderson-Localized Optical Modes.

Confining photons in cavities enhances the interaction between light and matter. In cavity optomechanics, this enables a wealth of phenomena ranging from optomechanically induced transparency to macroscopic objects cooled to their motional ground state. Previous work in cavity optomechanics employed devices where ubiquitous structural disorder played no role beyond perturbing resonance frequencies and quality factors. More generally, the interplay between disorder, which must be described by statistical physics, and optomechanical effects has thus far been unexplored. Here, we demonstrate how sidewall roughness in air-slot photonic-crystal waveguides can induce sufficiently strong backscattering of slot-guided light to create Anderson-localized modes with quality factors as high as half a million and mode volumes estimated to be below the diffraction limit. We observe how the interaction between these disorder-induced optical modes and in-plane mechanical modes of the slotted membrane is governed by a distribution of coupling rates, which can exceed g_{o}/2π∼200 kHz, leading to mechanical amplification up to self sustained oscillations via optomechanical backaction. Our Letter constitutes the first steps towards understanding optomechanics in the multiple-scattering regime and opens new perspectives for exploring complex systems with a multitude of mutually coupled degrees of freedom.

Simulations of micro-sphere/shell 2D silica photonic crystals for radiative cooling.

Passive daytime radiative cooling has recently become an attractive approach to address the global energy demand associated with modern refrigeration technologies. One technique to increase the radiative cooling performance is to engineer the surface of a polar dielectric material to enhance its emittance at wavelengths in the atmospheric infrared transparency window (8-13 µm) by outcoupling surface-phonon polaritons (SPhPs) into free-space. Here we present a theoretical investigation of new surface morphologies based upon self-assembled silica photonic crystals (PCs) using an in-house built rigorous coupled-wave analysis (RCWA) code. Simulations predict that silica micro-sphere PCs can reach up to 73 K below ambient temperature, when solar absorption and conductive/convective losses can be neglected. Micro-shell structures are studied to explore the direct outcoupling of the SPhP, resulting in near-unity emittance between 8 and 10 µm. Additionally, the effect of material composition is explored by simulating soda-lime glass micro-shells, which, in turn, exhibit a temperature reduction of 61 K below ambient temperature. The RCWA code was compared to FTIR measurements of silica micro-spheres, self-assembled on microscope slides.

Ferromagnetic Resonance Assisted Optomechanical Magnetometer.

The resonant enhancement of mechanical and optical interaction in optomechanical cavities enables their use as extremely sensitive displacement and force detectors. In this Letter, we demonstrate a hybrid magnetometer that exploits the coupling between the resonant excitation of spin waves in a ferromagnetic insulator and the resonant excitation of the breathing mechanical modes of a glass microsphere deposited on top. The interaction is mediated by magnetostriction in the ferromagnetic material and the consequent mechanical driving of the microsphere. The magnetometer response thus relies on the spectral overlap between the ferromagnetic resonance and the mechanical modes of the sphere, leading to a peak sensitivity of 850 pT Hz^{-1/2} at 206 MHz when the overlap is maximized. By externally tuning the ferromagnetic resonance frequency with a static magnetic field, we demonstrate sensitivity values at resonance around a few nT Hz^{-1/2} up to the gigahertz range. Our results show that our hybrid system can be used to build a high-speed sensor of oscillating magnetic fields.

Anderson Photon-Phonon Colocalization in Certain Random Superlattices.

Fundamental observations in physics ranging from gravitational wave detection to laser cooling of a nanomechanical oscillator into its quantum ground state rely on the interaction between the optical and the mechanical degrees of freedom. A key parameter to engineer this interaction is the spatial overlap between the two fields, optimized in carefully designed resonators on a case-by-case basis. Disorder is an alternative strategy to confine light and sound at the nanoscale. However, it lacks an a priori mechanism guaranteeing a high degree of colocalization due to the inherently complex nature of the underlying interference processes. Here, we propose a way to address this challenge by using GaAs/AlAs vertical distributed Bragg reflectors with embedded geometrical disorder. Because of a remarkable coincidence in the physical parameters governing light and motion propagation in these two materials, the equations for both longitudinal acoustic waves and normal-incidence light become practically equivalent for excitations of the same wavelength. This guarantees spatial overlap between the electromagnetic and displacement fields of specific photon-phonon pairs, leading to strong light-matter interaction. In particular, a statistical enhancement in the vacuum optomechanical coupling rate, g_{o}, is found, making this system a promising candidate to explore Anderson localization of high frequency (∼20 GHz) phonons enabled by cavity optomechanics. The colocalization effect shown here unlocks the access to unexplored localization phenomena and the engineering of light-matter interactions mediated by Anderson-localized states.

Synchronization of Optomechanical Nanobeams by Mechanical Interaction.

The synchronization of coupled oscillators is a phenomenon found throughout nature. Mechanical oscillators are paradigmatic examples, but synchronizing their nanoscaled versions is challenging. We report synchronization of the mechanical dynamics of a pair of optomechanical crystal cavities that, in contrast to previous works performed in similar objects, are intercoupled with a mechanical link and support independent optical modes. In this regime they oscillate in antiphase, which is in agreement with the predictions of our numerical model that considers reactive coupling. We also show how to temporarily disable synchronization of the coupled system by actuating one of the cavities with a heating laser, so that both cavities oscillate independently. Our results can be upscaled to more than two cavities and pave the way towards realizing integrated networks of synchronized mechanical oscillators.

Nanocrystalline silicon optomechanical cavities.

Silicon on insulator photonics has offered a versatile platform for the recent development of integrated optomechanical circuits. However, there are some constraints such as the high cost of the wafers and limitation to a single physical device level. In the present work we investigate nanocrystalline silicon as an alternative material for optomechanical devices. In particular, we demonstrate that optomechanical crystal cavities fabricated of nanocrystalline silicon have optical and mechanical properties enabling non-linear dynamical behaviour and effects such as thermo-optic/free-carrier-dispersion self-pulsing, phonon lasing and chaos, all at low input laser power and with typical frequencies as high as 0.3 GHz.

Elastic Properties of Few Nanometers Thick Polycrystalline MoS2 Membranes: A Nondestructive Study.

The performance gain-oriented nanostructurization has opened a new pathway for tuning mechanical features of solid matter vital for application and maintained performance. Simultaneously, the mechanical evaluation has been pushed down to dimensions way below 1 µm. To date, the most standard technique to study the mechanical properties of suspended 2D materials is based on nanoindentation experiments. In this work, by means of micro-Brillouin light scattering we determine the mechanical properties, that is, Young modulus and residual stress, of polycrystalline few nanometers thick MoS2 membranes in a simple, contact-less, nondestructive manner. The results show huge elastic softening compared to bulk MoS2, which is correlated with the sample morphology and the residual stress.

Thermal transport in epitaxial Si1-x Ge x alloy nanowires with varying composition and morphology.

We report on structural, compositional, and thermal characterization of self-assembled in-plane epitaxial Si1-x Ge x alloy nanowires grown by molecular beam epitaxy on Si (001) substrates. The thermal properties were studied by means of scanning thermal microscopy (SThM), while the microstructural characteristics, the spatial distribution of the elemental composition of the alloy nanowires and the sample surface were investigated by transmission electron microscopy and energy dispersive x-ray microanalysis. We provide new insights regarding the morphology of the in-plane nanostructures, their size-dependent gradient chemical composition, and the formation of a 5 nm thick wetting layer on the Si substrate surface. In addition, we directly probe heat transfer between a heated scanning probe sensor and Si1-x Ge x alloy nanowires of different morphological characteristics and we quantify their thermal resistance variations. We correlate the variations of the thermal signal to the dependence of the heat spreading with the cross-sectional geometry of the nanowires using finite element method simulations. With this method we determine the thermal conductivity of the nanowires with values in the range of 2-3 W m-1 K-1. These results provide valuable information in growth processes and show the great capability of the SThM technique in ambient environment for nanoscale thermal studies, otherwise not possible using conventional techniques.

Nanoscale imaging of InN segregation and polymorphism in single vertically aligned InGaN/GaN multi quantum well nanorods by tip-enhanced Raman scattering.

Vertically aligned GaN nanorod arrays with nonpolar InGaN/GaN multi quantum wells (MQW) were grown by MOVPE on c-plane GaN-on-sapphire templates. The chemical and structural properties of single nanorods are optically investigated with a spatial resolution beyond the diffraction limit using tip-enhanced Raman spectroscopy (TERS). This enables the local mapping of variations in the chemical composition, charge distribution, and strain in the MQW region of the nanorods. Nanoscale fluctuations of the In content in the InGaN layer of a few percent can be identified and visualized with a lateral resolution below 35 nm. We obtain evidence for the presence of indium clustering and the formation of cubic inclusions in the wurtzite matrix near the QW layers. These results are directly confirmed by high-resolution TEM images, revealing the presence of stacking faults and different polymorphs close to the surface near the MQW region. The combination of TERS and HRTEM demonstrates the potential of this nanoscale near-field imaging technique, establishing TERS as a very potent, comprehensive, and nondestructive tool for the characterization and optimization of technologically relevant semiconductor nanostructures.

Lifetimes of confined acoustic phonons in ultrathin silicon membranes.

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultrathin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from ~4.7 ns to 5 ps with decreasing membrane thickness from ~194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures.

Engineering nanoscale hypersonic phonon transport.

Controlling vibrations in solids is crucial to tailor their elastic properties and interaction with light. Thermal vibrations represent a source of noise and dephasing for many physical processes at the quantum level. One strategy to avoid these vibrations is to structure a solid such that it possesses a phononic stop band, that is, a frequency range over which there are no available elastic waves. Here we demonstrate the complete absence of thermal vibrations in a nanostructured silicon membrane at room temperature over a broad spectral window, with a 5.3-GHz-wide bandgap centred at 8.4 GHz. By constructing a line-defect waveguide, we directly measure gigahertz guided modes without any external excitation using Brillouin light scattering spectroscopy. Our experimental results show that the shamrock crystal geometry can be used as an efficient platform for phonon manipulation with possible applications in optomechanics and signal processing transduction.

Optomechanical crystals for spatial sensing of submicron sized particles.

Optomechanical crystal cavities (OMC) have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, bacteria and viruses. In this work we demonstrate the working principle of OMCs operating under ambient conditions as a sensor of submicrometer particles by optically monitoring the frequency shift of thermally activated mechanical modes. The resonator has been specifically designed so that the cavity region supports a particular family of low modal-volume mechanical modes, commonly known as -pinch modes-. These involve the oscillation of only a couple of adjacent cavity cells that are relatively insensitive to perturbations in other parts of the resonator. The eigenfrequency of these modes decreases as the deformation is localized closer to the centre of the resonator. Thus, by identifying specific modes that undergo a frequency shift that amply exceeds the mechanical linewidth, it is possible to infer if there are particles deposited on the resonator, how many are there and their approximate position within the cavity region. OMCs have rich perspectives for detecting and indirectly analysing biological particles, such as proteins, viruses and bacteria.

Enhanced thermoelectric properties of lightly Nb doped SrTiO3 thin films.

Novel thermoelectric materials developed for operation at room temperature must have similar or better performance along with being as ecofriendly as those commercially used, e.g., Bi2Te3, in terms of their toxicity and cost. In this work, we present an in-depth study of the thermoelectric properties of epitaxial Nb-doped strontium titanate (SrTi1-x Nb x O3) thin films as a function of (i) doping concentration, (ii) film thickness and (iii) substrate type. The excellent crystal quality was confirmed by high resolution transmission electron microscopy and X-ray diffraction analysis. The thermoelectric properties were measured by the three-omega method (thermal conductivity) and van der Pauw method (electrical resistivity), complemented by Seebeck coefficient measurements. A maximum power factor of 8.9 × 10-3 W m-1 K-2 and a thermoelectric figure of merit of 0.49 were measured at room temperature in 50 nm-thick films grown on lanthanum strontium aluminate. The mechanisms behind this high figure of merit are discussed in terms of a possible two-dimensional electron gas, increase of the effective mass of the electrons, electron filtering and change in strain due to different substrates. The overall enhancement of the thermoelectric properties suggests that SrTi1-x Nb x O3 is a very promising n-type candidate for room- to high-temperature applications.

Modification of spontaneous emission of (CdSe)ZnS nanocrystals embedded in nanoimprinted photonic crystals.

Highly luminescent (CdSe)ZnS nanocrystals, with band edge emission in the red region of the visible spectrum, were successfully synthesized and incorporated in a resist, namely mr-NIL 6000. The nanocomposite material was imprinted by using conventional nanoimprint lithography (NIL) process. We report on the fabrication and characterization of nanoimprinted photonic crystals in this new functional material. Experiments showed good imprint properties of the NC/polymer based material and that the surface nanostructuration improves the light extraction efficiency by over 2 compared to a nanoimprinted unpatterned surface.

Mechanisms behind the enhancement of thermal properties of graphene nanofluids.

While the dispersion of nanomaterials is known to be effective in enhancing the thermal conductivity and specific heat capacity of fluids, the mechanisms behind this enhancement remain to be elucidated. Herein, we report on highly stable, surfactant-free graphene nanofluids, based on N,N-dimethylacetamide (DMAc) and N,N-dimethylformamide (DMF), with enhanced thermal properties. An increase of up to 48% in thermal conductivity and 18% in specific heat capacity was measured. The blue shift of several Raman bands with increasing graphene concentration in DMF indicates that there is a modification in the vibrational energy of the bonds associated with these modes, affecting all the molecules in the liquid. This result indicates that graphene has the ability to affect solvent molecules at long-range, in terms of vibrational energy. Density functional theory and molecular dynamics simulations were used to gather data on the interaction between graphene and solvent, and to investigate a possible order induced by graphene on the solvent. The simulations showed a parallel orientation of DMF towards graphene, favoring π-π stacking. Furthermore, a local order of DMF molecules around graphene was observed suggesting that both this special kind of interaction and the induced local order may contribute to the enhancement of the fluid's thermal properties.

Photoluminescence enhancement in nanoimprinted photonic crystals and coupled surface plasmons.

A method to enhance the photoluminescence of dye chromophores-loaded by coupling the emission to surface plasmons in nanoimprinted photonic crystals is reported. A 9-fold enhancement in the spontaneous emission intensity of a rhodamine-doped polymer film is achieved on a silver layer due to surface plasmon excitation. By changing the surface plasmon frequency, this enhancement can be suppressed. When the polymer film is patterned by nanoimprint lithography with a twodimensional photonic crystal the photoluminescence intensity increases up to 27 times compared to unpatterned samples on a quartz substrate.

Unveiled electric profiles within hydrogen bonds suggest DNA base pairs with similar bond strengths.

Electrical forces are the background of all the interactions occurring in biochemical systems. From here and by using a combination of ab-initio and ad-hoc models, we introduce the first description of electric field profiles with intrabond resolution to support a characterization of single bond forces attending to its electrical origin. This fundamental issue has eluded a physical description so far. Our method is applied to describe hydrogen bonds (HB) in DNA base pairs. Numerical results reveal that base pairs in DNA could be equivalent considering HB strength contributions, which challenges previous interpretations of thermodynamic properties of DNA based on the assumption that Adenine/Thymine pairs are weaker than Guanine/Cytosine pairs due to the sole difference in the number of HB. Thus, our methodology provides solid foundations to support the development of extended models intended to go deeper into the molecular mechanisms of DNA functioning.

Thermal conductivity and air-mediated losses in periodic porous silicon membranes at high temperatures.

Heat conduction in silicon can be effectively engineered by means of sub-micrometre porous thin free-standing membranes. Tunable thermal properties make these structures good candidates for integrated heat management units such as waste heat recovery, rectification or efficient heat dissipation. However, possible applications require detailed thermal characterisation at high temperatures which, up to now, has been an experimental challenge. In this work we use the contactless two-laser Raman thermometry to study heat dissipation in periodic porous membranes at high temperatures via lattice conduction and air-mediated losses. We find the reduction of the thermal conductivity and its temperature dependence closely correlated with the structure feature size. On the basis of two-phonon Raman spectra, we attribute this behaviour to diffuse (incoherent) phonon-boundary scattering. Furthermore, we investigate and quantify the heat dissipation via natural air-mediated cooling, which can be tuned by engineering the porosity.Nanostructuring of silicon allows acoustic phonon engineering, but the mechanism of related thermal transport in these structures is not fully understood. Here, the authors study the heat dissipation in silicon membranes with periodic nanoholes and show the importance of incoherent scattering.

Directional elastic wave propagation in high-aspect-ratio photoresist gratings: liquid infiltration and aging.

Determination of the mechanical properties of nanostructured soft materials and their composites in a quantitative manner is of great importance to improve the fidelity in their fabrication and to enable the subsequent reliable utility. Here, we report on the characterization of the elastic and photoelastic parameters of a periodic array of nanowalls (grating) by the non-invasive Brillouin light scattering technique and finite element calculations. The resolved elastic vibrational modes in high and low aspect ratio nanowalls reveal quantitative and qualitative differences related to the two-beam interference lithography fabrication and subsequent aging under ambient conditions. The phononic properties, namely the dispersion relations, can be drastically altered by changing the surrounding material of the nanowalls. Here we demonstrate that liquid infiltration turns the phononic function from a single-direction phonon-guiding to an anisotropic propagation along the two orthogonal directions. The susceptibility of the phononic behavior to the infiltrating liquid can be of unusual benefits, such as sensing and alteration of the materials under confinement.

Orthotropic Piezoelectricity in 2D Nanocellulose.

The control of electromechanical responses within bonding regions is essential to face frontier challenges in nanotechnologies, such as molecular electronics and biotechnology. Here, we present Iß-nanocellulose as a potentially new orthotropic 2D piezoelectric crystal. The predicted in-layer piezoelectricity is originated on a sui-generis hydrogen bonds pattern. Upon this fact and by using a combination of ab-initio and ad-hoc models, we introduce a description of electrical profiles along chemical bonds. Such developments lead to obtain a rationale for modelling the extended piezoelectric effect originated within bond scales. The order of magnitude estimated for the 2D Iß-nanocellulose piezoelectric response, ~pm V-1, ranks this material at the level of currently used piezoelectric energy generators and new artificial 2D designs. Such finding would be crucial for developing alternative materials to drive emerging nanotechnologies.