Research activities

The goal of the PHENOMEN project is to propose an implementation of an in-chip phononic circuits based on OM cavities working at ambient conditions. To be able to complement current state-of-the-art electronic and photonic circuitry by extending the frequency range of detected RF signals, the selected platform relies on fully CMOS compatible materials with a focus on silicon.

The experimental implementation of phonons as information carriers in a chip is completely novel and of a clear foundational character. To achieve this objective, the project brings complementary expertise in terms of both theory and experiments that are divided in three technical workpackages, which correspond to the three axis of research that are centred on theory and modelling, components and integration, respectively. The related research activities are detailed here.

Theory and Modelling

Theory and modelling are crucial elements of a research investigation and contribute in different ways, including in contributing to design experiments by selecting parameters of interest, in understanding the physics and in understanding and analysing experimental results. The theory and modelling activities are therefore strongly linked to the experimental studies and permeate the whole project. They are divided into three main tasks.

• Phonon propagation – This task focuses on the physics of phonons and their parameters in order to design the structures targeted in the project. In particular, modelling strongly focuses on the transmission and reflection of phonons within and at the boundaries of the optomechanical systems, while also investigating coupling between different phonons modes. This activity helps us design more efficient components after assessing loss mechanisms for instance.
• Optomechanical coupling and cooperativity design – The goal here is to characterize the relation between optics and mechanics in our systems and take into account feedback elements such as the photoelastic (material dependent) and moving boundaries (geometry dependent) mechanisms. We also address optical and mechanical quality factors, coupling rates, as well as non-linear and multiphonon processes will be addressed.
• Device parameterization – This tasks is strongly linked to WP3, in that it consists in modelling various elements integrated together, form interdigitated transducers to optomechanical systems and processing elements. This is crucial to optimize the design of the experimental sample, such as the optimized coupling distance between a waveguide and the optomechanical cavity for optimal coupling.

For all these tasks, we use a variety of modelling techniques. Focus is put on a few of them that are well adapted to the geometrical and timescales of interest. The main techniques used at the beginning of the project were the Finite elements Method and the Finite difference time domain method while during the project, the transformation optics was developed and adapted to study our optomechanical systems combining both optics and mechanics in a single simulation and taking into account the couplings from one to the other and vice versa. This technique, combined with the Transmission Line Matrix (TLM) method, is particularly useful in dealing with objects possessing very disparate geometrical or electrical scales, and phenomena with very different space-time dynamics. This is made possible thanks to a new multigrid-subgridding formulation using a mesh with differential spatial resolution.

Device fabrication

Device fabrication is a core component of the project as it determines the physical events we are then able to observe. Indeed, one of the crucial characteristic of an optomechanical system is its quality factor, both optical and mechanical, and the quality of the fabrication plays a crucial role, alongside design, in obtaining high quality factors.
We use clean-room nanofabrication to fabricate all our devices. Optomechanical nanobeams are fabricated from crystalline or nanocrystalline silicon. The structures are drawn in a special resist on the surface with electron-beam lithography and transferred to the silicon layer by reactive ion etching. The structures are then suspended by removing the underlying oxide layer with hydrofluoric acid. The electrical components, namely the interdigitated transducers and corresponding electrical contacts, are drawn by photolithography.

Device characterisation

We use two main measurements techniques during the project, one purely for the evaluation of phonon propagation whereas the other combines is tuned for optomechanical measurements.

Laser Doppler vibrometry
This technique is used in PHENOMEN to measurement the amplitude of vibration of different elements, such as a nanostring (used as a model system) when excited by the IDTs. It relies on the Doppler effect applied to a probe laser to measure extremely small displacement with amplitudes of a few picometres (10-12 metres).

Electro-Optomechanical measurements
This platform is the core of the project measurement capabilities. An optics fibre is thinned down, twisted in the shape of a loop and brought in proximity with the optomechanical structure to be measured or with a photonic waveguide. A telecom-wavelength (~1.5 μm) infrared laser is launched in the fibre and its wavelength is progressively increased. When it matches that of a resonance of the optomechanical system, the transmission or reflection at either end of the thinned fibre is modified. If the mechanical resonances of the optomechanical system are excited, the optical signal will be modulated at the frequency of these resonances, which can therefore be detected in a spectrum analyser thanks to a Fourier transform of the detected signal.

Components and integration

During the project, we investigate a large range of components, from sources to detectors, processing elements. Study of the components is grouped within workpackage 2, which deals with the fabrication and testing of all the single optomechanical components which will then be combined in the final opto-electro-mechanical circuit. To this end, we will take care of improving our optomechanical phonon sources and devise completely new ones, such as the Phonon Parametric Oscillator (PPO). The strong nonlinearities related to the schemes for phonon generation will be exploited for RF photonics and secure communications applications. Using the same optomechanical devices as detectors, we will use them to disentangle light and mechanics dynamics, allowing to reach an unprecedented clarity in understanding complex nonlinear systems. Along with the development of sources and detectors, we will investigate phonon and photon functionalities enabled by optomechanics: phonon switching, memories, filters, polarization manipulation and mechanical phase synchronization are some of the features obtained within the WP lines of investigation.
Once these feasibility of these components is proven, we set to integrate them in a single chip, combining them in a fully CMOS compatible platform that involves electrical driving with interdigitated transducers. Integration involves the use of photonic and mechanical waveguides, optomechanical elements and electrical driving and poses challenges in terms of design, fabrication and experimental measurements.