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Sound-Light Manipulation in the Terahertz

Final Report Summary - SOULMAN (Sound-Light Manipulation in the Terahertz)

The interaction between mechanical vibrations and photons has been the object of intensive applied and theoretical studies since the beginning of the last century. The rapid technological advancements of the last years have allowed for a rapid miniaturization of these so-called optomechanical systems, reaching sizes comparable with the electromagnetic wavelength itself. A wealth of configurations have been devised, and, by cooling down the mechanical motion, these still “macroscopic” systems have fully entered the quantum realm, where new and exciting physics can be explored.
In this general context, SouLMan has tackled the investigation of optomechanical interactions within active electromagnetic systems, such as lasers and oscillators.
The focus has been mostly on quantum cascade lasers (a unipolar class of semiconductor lasers) operating at THz frequencies (wavelength ranging around hundred microns). The peculiarity of THz radiation, in between microwaves and infrared light, allows to use concepts and techniques which belong to both photonics and electronics. For instance, some resonant structures in the THz range can either be described by using the picture of an optical cavity or by considering them as LC (inductance-capacitance) circuits. We have conceived that the “inductance” can be made with a suspended, mechanically compliant bridge. By oscillating, the bridge will change the resonator inductive element and, with that, the resonant frequency itself, realizing optomechanical coupling. In SouLMan we have produced the first devices inspired by this idea. We have indeed been able to get a clear lasing action in a LC-like cavity with a suspended gold bridge whose mechanical resonances are in the tens of MHz range. The structure operates as a dipolar antenna collimating the THz emission in the vertical direction with high-emitted powers even in continuous wave.
Alternatively, we have also considered optically-pumped microsphere lasers and demonstrated that the thermally activated “breathing” vibrations of the microsphere resonantly produce oscillations directly in the laser emission, up to frequencies of 80 MHz.
A completely different configuration for active optomechanical systems exploits a reflecting oscillating membrane externally coupled to the laser. For the purpose, we have developed several new approaches to THz lasers with collimated emission by employing special photonic structures. We have demonstrated the first THz lasers based on quasi-periodic 2D photonic crystals as well as optimized distributed feedback lasers based on metallic gratings. With the latter, and using an external movable mirror, we have shown state-of-the-art tuning of the emission wavelength. In parallel, we have realized micro-mirrors on suspended silicon nitride membranes, a technology ensuring very high mechanical quality factors. The membrane motion has then been directly monitored by exploiting an interferometric effect called external optical feedback or self-mixing. Despite its simplicity, this approach is very sensitive and just by studying the time evolution of the laser emission amplitude we have been able to easily detect the thermal motion of the membrane, as well as drive it with the light intensity in the cavity, realizing the first truly active opto-mechanical laser.
Further types of membrane have been realized, even in graphene (a single-atom layer of Carbon) by developing a novel technique to control the strain and tension of the membrane, or containing photonic crystals. In this latter case we have developed chiral membranes which, when coupled to the laser, have allowed high-speed optomechanical modulation of the polarization of the emitted light.