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All-Phononic circuits Enabled by Opto-mechanics

Periodic Reporting for period 2 - PHENOMEN (All-Phononic circuits Enabled by Opto-mechanics)

Reporting period: 2017-09-01 to 2020-02-29

"The PHENOMEN project aims to combine phononics, photonics and radio frequency (RF) electronic signals in a single platform and lay the foundations of a new information technology operating at atmospheric conditions of temperature and pressure (see Figure attached). The project seeks to prove two concepts. One is the efficient generation of GHz to tens of GHz coherent phonons, coupling them efficiently into a waveguide, engineering their propagation with low losses and detecting them. The other is to demonstrate synchronisation of two or more OM “phonon lasers"" to spatially extend and strengthen the coherent phonon clock signals.
The concept of the project PHENOMEN relies on designing a circuit made out of optomchanical, electromechanical and purely phononic components tailored at the nanoscale. The continuous photon stream generated by a telecom diode laser carried by an optical waveguide and evanescently coupled to an OM nanobeam cavity, the ""phonon laser"", which has been carefully designed and realised in a silicon technology compatible platform to exhibit phenomena termed back-action and/or self-pulsing, which are characteristic of silicon OM systems for coherent phonon generation using optical radiation pressure as a driving force.
The generated coherent phonon signal is purposely designed to leak out part of its energy from the OM nanobeam cavity to a low loss phonon waveguide, in a manner akin to the output coupler mirror of a conventional laser. Embedded within the phonon waveguides the project devises different active and passive functionalities that will enroute, store and/or modulate the phonon coherent signal.
After leaving the previous hybrid components, phononic signals are detected by phononic detectors, which are intended to be OM nanobeam cavities as well, able to behave as an interface from phononic to photonic and/or RF electrical signals.
By taking advantage of the optimization and integration of the building blocks required for demonstrating the operations stated above, a step forward is to connect two or more “phonon lasers” through those circuits and demonstrate their spontaneous synchronization. This feature will provide a reduction in the phase noise of the coherent phonon signal that will scale down with the number of synchronized “phonon lasers”, while at the same time spreading the chip area reached by the coherent signal. Such low noise coherent signals extended in space will enhance the overall quality of the “clock” reference signals fed to electronic, photonic or phononic elements of current information processing chips for time-keeping purposes. In this context, synchronization phenomena among networks of self-sustained mechanical oscillators could be exploited to extend further the spatial range of a common coherent signal.
Photonics and RF electronics are readily integrated in current information processing devices. The outcomes of the project will have a wider impact than that explored in the context of PHENOMEN, namely in a variety of sensing applications in science and technology or metrology. The consortium is made up by 3 research institutes, 3 universities and a SME."
In its initial period, the consortium has focused in building the first practical, optically-driven phonon sources and detectors, including the engineering of phonon lasers to deliver coherent phonons to the rest of the chip, pumped by a continuous wave optical source. The project has concentrated in the design, fabrication and optimisation of main building blocks of the eventual circuit, namely sources, waveguides and detectors.
Many efforts have been dedicated to implementing new simulation methods that can address the problem of coupling the different physics coupling electrons, photons and phonons, which involve very different time scales. The main achievement was the demonstration that the coupling of closed phonon cavities can be used to route, control and modulate phononic cavity modes over large distances between cavities
Regarding the experimental demonstration of isolated components, the consortium has produced coherent sources operating at room temperature at 5 GHz (using optomechanical back‐action) and 0.3 GHz (using self-pulsing). Concerning detectors, following the demonstration of phonon detection and subsequent design of more complex geometries (ongoing), we have proposed an alternative route to characterise them experimentally without the need of having optimized OM sources. It is based on a new scheme in which the phonons can be generated and detected by the excitation of surface acoustic waves (SAW) by means of a piezoelectric material. The structure is composed of Interdigital transducers (IDT) on a piezoelectric AlN layer on top of a Si film/SiO2 layer/Si substrate.
The consortium has also demonstrated regions of coherent state bi- and tri-stability present in the OM phonon sources, which will be exploited for memory purposes. In this context, fast switching (in the few MHz range) between dynamical states has been demonstrated with an external laser heating source.
Concerning integration, the consortium has demonstrated photonic waveguide could be used to couple light to and from the OM cavities, and that it was possible to use the SAW launcher platform to coherently drive the mechanical cavity, then detected thanks to the optomechanical interaction.
There has been a significant number of training (schools, PhD students and senior researchers exchanges among partners, etc) and dissemination activities.
"A new, more suitable, material platform has been successfully demonstrated. A Silicon wafer is first thermally oxidised after which an amorphous silicon layer is deposited by Low Pressure Chemical Vapour Deposition and annealed to tune the stress of the forming nanocrystalline‐Si (nc-Si) film. This platform could replace standard monocristallyne SOI wafers, much more expensive and constrained to specific top Si layer thickness. In this new platform, the consortium has demonstrated “phonon lasing” and other typical features of the self-pulsing coupled to optomechanics system. Self-pulsing frequencies overcome those achieved by the crystalline silicon OM nanobeam cavities, reaching record frequencies of 0.3 GHz so far. Indeed, mechanical amplification at 2.7 GHz has been recently demonstrated in a localized mode.
Our consortium has proposed the first couples of OM cavities in which the interaction is purely mechanical, in which phonon detection and spontaneous synchronisation of the two ""phonon lasers"" were demonstrated, which can have application in pattern recognition and neuromorphic computing.
The consortium has also unveiled a complex dynamics, including chaos in the OM nanobeam cavities, controlled with the parameters of the excitation laser. This discovery might allow the codification of information by introducing chaos to mask an eventual signal in need to be secured.
The potential of optomechanical metasurfaces, for light polarimetry for instance, was an unexpected but highly competitive outcome of the project.
Beyond the broad scientific impact linked to microwave engineering, the outcomes can also potentially extend to provide relevant advances for light and ultrasound detection for Health applications. On a longer term, the research started in the project opens possibilities towards reducing the energy consumption of internet usage by using phonons for the low-speed-end of information processing."
Scheme of the PHENOMEN final concept