Terahertz nano-opto-mechanical systems

The terahertz electromagnetic spectrum comprehends the electromagnetic radiations whose wavelength ranges from 0.1 mm to 1 mm, therefore also known as submillimiter band. These radiations fall into the so-called terahertz gap, owing to the fact that there is a lack of generation and detection technologies for these frequencies that are between the radio-waves and the mid-infrared, i.e. between the spectral regions optimized by electronics and optics respectively. Historically, terahertz spectroscopy technologies have been exploited mainly in the field of astronomy, imaging and security, but recent technological improvements are gradually closing the gap and making this band available for more and more applications for sensing and communication. The devices studied during this internship consist in nanobeam mechanical oscillators, with typical resonance frequencies in the MHz range. The mechanical movement modulates a 930 nm laser beam impinging the structure and the signal is read out by a balanced detector. The fluctuation dissipation theorem allows us to calibrate the measurement to obtain the displacement in pm from the amplitude of the electric signal. A further value of this device is given by the possibility of driving the mechanical oscillations with a dielectric gradient force generated by RF electrodes. This optomechanical arrangement is combined with a metamaterial resonator, which acts as an antenna for the THz radiation which then induces a mechanical force on the nanobeam: the transduction principle of the devices studied during the internship is a thermally induced strain, which modifies the mechanical properties of the oscillator. The THz radiation is produced by a Quantum Cascade Laser which is modulated in the MHz range, to match the oscillation modes of the system. I will present the experimental study of these opto-mechanical nanoresonators, focusing the attention on their strongly non-linear regime: measurements combining optics and electronics have been performed to characterize the Duffing response under one single drive and the transition towards chaotic dynamics in particular conditions of quasi-periodic excitation. The latter has been obtained with two distinct periodic drive signals programmed by RF generators: systematic analysis of the output signal in the frequency and temporal domain have been brought on to characterize the effects of their voltage and of their frequency on the physical state of the system. At a later stage, we have replaced one of the RF drives with a photothermal force produced by the THz wave, in order to explore new possibilities of detection: two distinct mechanisms have been studied, based on either the linear response to the radiation or to the perturbation of the strong non-linear dynamics of the system caused by the THz absorption.