THz Optomechanical CMOS-Compatible Detector with Integrated Germanium Nanolasers – TIGER

Our device benefits from 3 different physical domains: (1) metamaterial, (2) optomechanics, and (3) laser physics. In our structure, a metamaterial resonator receives THz radiation and confines a strong electromagnetic field into an ultra-sub-wavelength scale, which leads to a mechanical vibration within a pre-stressed germanium photonic crystal nanobeam laser. The mechanical action induces strong changes in the output characteristics of germanium laser (e.g. peak wavelength position, amplitudes), which, in turn, provide detailed information on the incoming THz radiation. The output of germanium laser can also be fed into integrated photodetectors that give rise to electrical signals. As the mechanical element is typically of nanometer dimensions, its frequency response is in the MHz range, thus much faster than any existing THz detectors operating at room temperature. Furthermore, the germanium devices are fully compatible with silicon-based technology; our THz detectors can be naturally integrated in CMOS platforms, leading to compact THz sensing devices.

The first step of the project was to validate and optimize Ge-based mechanical nanoresonators, that a crucial part of the full THz detectors. For that purpose NTU fabricated nanobeam structures that were characterized at LPENS by Brownian motion spectroscopy. The feedback from this measurements were used to optimize the geometry and fabrication procedure of the samples. After several iterations we obtained Ge-nanoresonators that had typical resonant frequencies in the 1 – 10 MHz range, and had much higher quality factors, up to Q~20 000 that what was previously observed with GaAs beams of similar geometry.
In parallel with the optimization of the Ge-nanobeams, LPENS developed a device geometry where semiconductor nano-beam is driven by side RF electrodes. It was shown that the nanobeam oscillations can be very efficiently excited by moderate RF voltage (up to 10V). Furthermore, the nanombeam can also be driven into highly nonlinear regime (Duffin regime), which opened new possibilities for THz detection that were not previously anticipated by the project:
-We build an experimental set-up which allowed to measure the two quadratures of the forced mechanical oscillator. When the oscillator is driven into the Duffin regime, the noise in one quadrature is reduced at the expense of the other (phenomenon very similar to the “squeezing” in quantum optics). This phenomena will be explored for increasing the sensitivity in our future THz detectors.
- By injecting small RF harmonic signal on the top of the driving RF voltage we observed a formation of frequency combs around the main mechanical oscillation. This method provides an active way to control the mechanical spectrum which can also be useful for the THz detector.

Our silicon-compatible THz optomechanical detector integrated with a pre-stressed germanium laser would enable the creation of a mass-producible, ultra-compact, monolithically-integrated lab-on-a-chip using traditional CMOS processes, and the chip can function as wearable bio sensors and toxic chemical sensors. One can fabricate this miniaturised THz chip in the same way that one fabricates the state-of-the-art electronic chips using matured silicon technology. Owing to its ability to mass-produce, its cost can be dramatically reduced while the use of silicon process may enable the THz technology to be available to a wide range of end users through MPW services in the near future.

Optomechanical temporal sampling of THz signals at room temperature
Baptiste Chomet et al., under preparation (2020)

The Terahertz (THz) frequency domain has a myriad of anticipated applications such as wireless THz communications, security screening, and bio-chemical sensing. More specifically, bio-chemical sensors operating in the THz spectral region (1-20 THz) are becoming increasingly valuable, as the vibrational transitions of molecules are two to three orders of magnitude stronger in the THz spectral domain than in the visible counterpart, leaving distinctive spectral fingerprints that are very convenient for sensing. The paramount issue to be solved in order to enable these applications is the lack of a THz detector that is altogether fast, sensitive, compact and operating at room temperature.

The proposal TIGER aims at creating a new generation of THz optomechanical CMOS-compatible detectors with integrated germanium nanolasers. The substantial originality of our proposal lies in the convergence of the two significant research fields, THz optomechanical metamaterial system and mechanically-engineered silicon photonics, that has never envisioned before for the technologically important THz frequency range. Our device benefits from 3 different physical domains: (1) metamaterial, (2) optomechanics, and (3) laser physics. In our structure, a metamaterial resonator receives THz radiation and confines a strong electromagnetic field into an ultra-sub-wavelength scale, which leads to a mechanical vibration within a pre-stressed germanium photonic crystal nanobeam laser. The mechanical action induces strong changes in the output characteristics of germanium laser (e.g. peak wavelength position, amplitudes), which, in turn, provide detailed information on the incoming THz radiation. The output of germanium laser can also be fed into integrated photodetectors that give rise to electrical signals. As the mechanical element is typically of nanometer dimensions, its frequency response is in the MHz range, thus much faster than any existing THz detectors operating at room temperature. Furthermore, the germanium devices are fully compatible with silicon-based technology; our THz detectors can be naturally integrated in CMOS platforms, leading to compact THz sensing devices.