Nanophononic & Nanophotonics
The Phononic and Photonic Nanostructures group (P2N) is carrying out research at the cross roads between nanotechnology and dispersion relation engineering. P2N group with its group leader has contributed to setup a community on phononics in Europe promoting and participating in many initiatives and projects. The P2N group has had an important role in establishing the field with contributions in the experimental part. These contributions span over the phonon spectrum from phonons in the hypersound to thermal phonons, studying the effects of size reduction, periodicity and disorder, for the manipulation and control of phonons and heat management. The P2N group has also been successful in merging its expertise in optical processes in nanostructres, fabrication and phononic and photonic engineering to implement hybrid opto-mechanical systems. This research is anchored in pioneering nanofabrication performed in our group and collaborators, as well developing novel methods to produce nanostructures pushing towards the limits of nanofabrication to enable cutting-edge research.
Hypersonic elastic waves
Following our previous work on confined elastic waves in ultra-thin membranes or Lamb waves, we have studied two-dimensional (2D) phononic crystal (PnC) silicon membranes by combining different strategies, such us periodic modulation of the elastic constants, and local resonances, in order to change the dispersion of acoustic phonons. We have shown direct experimental evidence of the modification of the dispersion of GHz phonons propagating in the considered PnCs by employing contactless and nondestructive Brillouin light scattering (BLS) spectroscopy. The advantage of these unsupported structures is that they ensure the confinement and guiding of the elastic waves, although they add complexity in the fabrication process and limit the integration of phononic elements with photonics and radio-frequency electronics. .
Nano-scale heat transport
The PnCs structures with characteristic sizes reported to show coherent effects in the hypersound have been found to exhibit reduced in-plane thermal conductivity, κ, values. Moreover, it has been seen that, for a given membrane thickness, the temperature evolution, κ(T), from room temperature to about 1000 K can be effectively tuned and approaching to a regime where κ is almost insensitive to T by changing the neck distance in between holes. The latter reflects the increasing role of surface scattering on k(T) by limiting the phonon mean free path at expenses of the phonon-phonon scattering. Control and manipulation of heat transport requires devices with analogous functionalities as diodes and transistors in electronics, therefore thermal circuits could be devised but, also used in thermal management and thermoelectric energy generation. This work benefits from European collaborations and membership of the European CRS network on Thermal Nanoscience and Nanoengineering. During this period we will study the tuning of k(T) in “holey” membranes as a mechanism to introduce heat transfer directionality towards efficient and direction controlled heat dissipation and thermal diode and transistor concepts.
2D atomic crystals have become leading topic in different research areas and a strategic research subject at ICN2 level. In the P2N group, we have already started to work in the heat transport properties of some of these materials mainly in the framework of the Severo Ochoa internal program. Our purpose is to expand the present research to the study of the mechanical properties of 2D materials and investigate, both, the thermal and mechanical properties of heterostructures made by assembling different 2D crystals, foreseeing the possibility of obtaining properties in the stacked assembly distinct from its component materials.
The group is developing experimental methods for the characterization of thermal properties in fluids. These techniques will allow us the establishment of new research on the modification of the thermal properties of a base fluid upon the incorporation of nanoparticles.
In the last five years, the group has developed an intense activity on optomechanics, the overlapping field of research between phononics and photonics. We demonstrated the potential of a new type of nanobeams to explore the coupling between the electromagnetic field in the 100s of THz frequency range and the mechanical vibrations in the GHz range. In this new type of nanostructures, we make profit of optical nonlinearities of the material used to fabricate these structures, silicon. The interaction between these optical nonlinearities and the optomechanical coupling in the MHz frequency range reveals a rich dynamical phase-space that can be used to control the complex dynamics of the structure, with expected impact in phononic circuits and secure data transmission. These promising results encourage us to face our current challenges. Our goals for the near future are basically to explore fabrication alternatives in order to push these complex dynamics to higher frequencies (GHz) and to analyse the robustness of these structure again fabrication imperfections. In addition, we are exploring other types of planar geometries to integrate more complex optomechanical architectures.
Cavity optomechanics is a relatively new field that has developed at an extremely rapid pace during the last decade, generating several breakthrough works every year. In our group we are mostly interested in room temperature operation of cavity optomechanic devices. Among the great variety of optomechanical cavities, we are focusing our research on Si-based optomechanical crystals (OMC), which are probably one of the most interesting proposals in terms of technological implementation and optomechanic performances.
The ongoing research can be roughly summarized in three:
High-frequency phonon sources. Those will certainly be one of the building-blocks required by a phononic-based information technology. We have recently demonstrated a novel pumping mechanism, based on the triggering of a self-pulsing limit-cycle within an OMC, that enables reaching the lasing regime with much lower requirements for the optical and mechanical modes involved than those needed by the standard methods, i.e., dynamical back-action or phononic-molecule lasers. This ability to efficiently achieve self-sustained oscillation with no need for feedback electronics makes our OMC compelling for on-chip applications such as microwave clocks, in which CW light is fuelling the oscillation. Moreover, the dynamics of the non-linear system composed by the self-pulsing and the mechanical oscillators is incredibly rich, leading to more complex features, namely hysteresis, period doubling and chaos.
High resolution thermal maps. Optomechanical crystals can display highly localized mechanical modes whose eigenfrequencies are sensible to the local environment. A pump and probe technique using two infrared tunable lasers has been implemented to locally heat (pump) and test the local temperature (probe) by quantifying the mechanical eigenfrequencies displacement.
Disordered optomechanical crystals. Our structures are based on unitary cells displaying full-bandgaps both in the optical and in the mechanical dispersion bands. This feature enables the comparison of highly localized modes (optical and mechanical) close to the band edges when geometrical disorder is introduced on the optomechanical cystal (Anderson localization). Enhanced optomechanical coupling rates are expected in comparison with standard optomechanical crystal cavities.
The group is exploring making a contribution to the emerging field topological phononics, as part of topological matter, from an experimental perspective to test the promise of creating robust phonon states to ensure robustness to defects of the energy carried by a phononic model. This is seen as a long-term strategy towards (hybrid) phonon states as information carriers in the limit of information as energy and energy as entropy within the scope of Information and Communication Technologies for the Connected Society. It belongs to one of the new strategic research lines of the ICN2. The long-term scientific question boils down to: Can topological phononic physis and its physical implementation deliver on information transmission by phonons and or hybrid phonon states?
P2N group membres have pioneered novel nanofabrication approcahes to deliver hierarchical micro/nano paterns over large areas and at high manufacturing rates. Advanced nano-manufacturing techniques based on imprint technologies have been embloyed to deliver sub-100 nm structures in various materials and substrates (ridgid and flexible). Depending on the targeted application a combination of top down and bottom up techniques are applied to produced components, devices and/or surfaces with the objective to tailor the physical properties of bulk and membrane materials. Substantinal efforts have been placed to increase the TRL level of the demonstrated nanostructured surfaces and components with the target to compile all nanofabrication approaches and materials to industrial scale manufacturing capacities (>TRL7). The P2N group has carried out innovative research in the engineering of surfaces by nanopatterning extending the limits of hydrophobicity control by combining nanoimorint and electroplating technologies using processes developed in-house. This research line has a proven resonance with technology transfer and research with increasing Technology Relevance Levels leading to the group participation in EU H2020 Pilot line project FLEXPOL and the Research international staff exchange D-SPA project focusing on nanofabrication for optical devices based on diamond technologies. This work is intrinsically linked to nanometrology for which we continue to improve our in-house-developed non-invasive and i(o)nline techniques for industrial deployment. The short and medium term engineering questions or technological challenges refer to the understanding of adhesion on patterned substrates for smart surface applications as in anti-microbial ones. Key to our efforts is maintaining the scalability, accompanied by dimensional metrology, of our processes to ease technology transfer.