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TeraHertz detection enabled by mOleculaR optomechanics

Periodic Reporting for period 1 - THOR (TeraHertz detection enabled by mOleculaR optomechanics)

Reporting period: 2019-03-01 to 2020-02-29

Generation, manipulation and detection of electromagnetic waves across the entire frequency spectrum is the cornerstone of modern technologies, underpinning many disciplines such as sensing, imaging, spectroscopy and telecom networks. Whilst the last century has witnessed an impressive evolution in devices operating at frequencies either below 0.1 THz (microwave and antenna technology) or above 50 THz (near-infrared – NIR - and visible – Vis - optical technology), in between the lack of suitable materials and structures for efficient electromagnetic manipulation has resulted in the so-called “THz gap”: a band of frequencies in the ~0.1 – 30 THz region for which compact and cost-effective sources and detectors do not exist – even though their application has enormous potential in medical diagnostics, remote sensing, security, astronomy, and wireless communication. While coherent detection of waves in the 0.1 – 2.5 THz domain is now widely performed in the laboratory using ultrashort laser pulses and photoconductive antennae or other non-linear materials, these techniques are far from commercial applications due to their high complexity and cost. Recent advances in III-V Schottky diodes and field-effect transistors are closing the gap up to ~1 THz with some prospects for commercialisation,3 but the available techniques for direct detection in the 1-30 THz regime (micro-bolometer, Golay cells, pyroelectrics – all based on thermal effects) are slow, noisy and bulky. Clearly, a new concept for the detection of THz waves in this range would have dramatic impacts on THz technologies.

In THOR, we will demonstrate the first nano-scale, cost-effective, fast and low-noise detectors working in the 1 – 30 THz range by developing a radically novel concept of signal up-conversion to visible/near-infrared (Vis/NIR) radiation, leveraging the latest scientific breakthroughs in the emerging field of molecular cavity optomechanics.

THOR’s impact on society and economy will be achieved indirectly via the future development of molecular OM devices that will play a key role in several of the societal challenges addressed in H2020, e.g. in (i) Health, demographic change and wellbeing; and (iii) Secure societies - protecting freedom and security of Europe and its citizens. In addition, THOR’s concept could be applied to improve Raman spectroscopy, thus playing a role in disciplines such as chemistry or biology.
Here, we summarize the work performed from the beginning of the project and main results achieved during the 1st reporting period towards the achievement of the main THOR objective.

WP1. Molecules.
In WP1, we have developed an extensive study using DFT of the vibrational properties of more than 2000 molecules with the goal to maximize simultaneously the infrared absorption and the Raman activity of the molecules in the proposed range of the detector, by means of machine learning techniques. This has led to the timely achievement of Milestone MS1. We have also concluded that porphyrin ion based metallic complexes are promising candidates for the final device due to the presence of intense and well-defined low-energy Raman peaks, coincident with the range of energies targeted in the detector.

WP2. Cavities.
In WP2 we have studied hybrid plasmonic-dielectric cavities featuring high optical quality factors (Q) as well as low effective volumes (V) in order to boost the optomechanical coupling rate. We have also implemented a numerical simulation framework for hybrid cavity geometries as well as THz-to-SERS transduction metrics, which means that Milestone MS2 has been achieved. Some of the designs are currently under fabrication and test, including THz/MIR antennas from enhanced excitation of the molecules. Building and verification of experimental set-ups is ongoing in labs of several partners, including Raman capability and QCL illumination, as required to achieve Milestone MS3.

WP3. THz-to-optics molecular converter.
The main result in WP3 is the fact that we can observe picocavities at room temperature in a regular fashion (achievement of MS4). We have conducted many SERS experiments observing new phenomena due to the strong field confinement. In many of them, we have observed Raman peaks at THz frequencies, a key step towards the final goal of the project. Experiments including THz/MIR illumination to excite the molecules via absorption are on-going.

WP4. Management.
All administrative, legal and technical issues of the project have been properly managed. We have held two face-to-face meetings (AMOLF and CSIC partners) and three videoconferences. In the first meeting, the Project Management Committee (PMC) was formed. Since then, the PMC has efficiently managed all the technical aspects of the project. The coordinator has ensured an efficient communication between partners. The coordinator has also conveniently transferred to the partners the information coming from the Project Officer. Deliverable D4.1- this document - has been delivered.

WP5. Dissemination and exploitation.
The Website was set-up ( and a logo was created, as detailed in Deliverable D5.1. The Website includes a private area where we have been uploading the documents generated by the different partners during the project. We have been updating the Website since its creation. The consortium developed a data management plan, which was explained in Deliverable D5.1. The plan for dissemination and exploitation has also been updated, as reported in Deliverable D5.3. Here, we have also informed about the composition of the Industrial Advisory Committee (IAC), the new THOR competitors (arisen after the starting of the project) and an issue about intellectual property protection that was satisfactorily handled by the PMC.
Here we briefly summarise the progress beyond the state of the art:

- We have studied the vibrational properties of more than 2000 molecules, identiyng some of them as promising candidates for THz detection based on molecular optomechanics.
- We have studied collective effects the optomechanical interaction of many molecules in plasmonic systems.
- We have provided a complete picture of the optical properties (Q factor and effective volume) of hybrid plasmonic-photonic cavities.
- We have observed the response of picocavities at room temperature.
- We have been able to observe Raman peaks at frequencies below 30 THz.
- We have demonstrated SERS in Au nanoantennas integrated with SiN waveguides.
- We have developed a whole theory of the THz-to-optics converter to be developed in THOR.

We expect to continue our advances and demonstrate the promised THz detector whoch should be disruptive in a number of scientific and technological disciplines.
Sketch of the THOR device