The THz spectral range is characterized by the low-energy excitations of condensed matter — phonons, polarons, excitons, electron-phonon coupling — while, from the technological point of view, it bridges the domains of electronics and photonics. In the first part of this talk, I will introduce terahertz time-domain spectroscopy (THz-TDS), in which the coherent detection of few-cycle THz pulses gives direct access to both the amplitude and phase of the transmitted field, and show how it can serve the field of iontronics, from fundamental studies of ion-gated materials to applicative aspects such as modulators and transistors [1].

Despite their scientific and technological appeal, THz waves have a major drawback coming from the absence of optoelectronic techniques and devices that can manipulate such light waves. To solve this issue, one of the most promising approaches relies on the use of metasurfaces, engineered composites whose optical properties can be specifically tailored to interact with THz waves. Research is now focused on designing reconfigurable metamaterials, also known as metadevices, able to modulate the amplitude, phase, and frequency of THz pulses. In our work, we explore for the first time the functioning and the effectiveness of an organic semiconductor-driven metadevice based on a matrix of metal Split-Ring Resonators, which can modulate the amplitude of a THz pulse passing through them. The modulation capabilities come from the electrically driven change of charge carrier density in the organic semiconductor, which enables the metadevice to act as an optical transistor, varying THz transmission around a specific frequency (~0.7 THz) with modulation depths of approximately 65%. These performances, which are comparable with current state-of-the-art technologies but with a way lower driving voltage (<1 V), result from the unique 3-dimensional charge modulation properties of a class of organic mixed ion-electron conductors based on conjugated polymers with glycolated sidechains, such as p(g2T-TT). We also show that it is possible to shift towards mass-scalable and cost-effective manufacturing techniques, exploiting high-throughput deposition methods for both the metamaterial matrix and the organic semiconductor, on either rigid or flexible plastic substrates, without losing modulation efficiency [2].

In the second part of the talk, I will show how iontronics can also control the ultrafast optical response of two-dimensional crystals, probed using mid-infrared pulses. Ionic liquid gating enables the accumulation of large charge carrier density upon applying moderate gating fields. Here, it is used to tune the Fermi level EF of single-layer graphene to hundreds of meV, demonstrating the strong tunability of its linear and nonlinear optical response in the mid-infrared range, where absorption is governed by the interplay between interband and intraband transitions. A femtosecond pump pulse photoexcites graphene, creating a charge carrier distribution at an elevated electronic temperature, and the cooling dynamics is tracked by monitoring the changes to graphene transmission with mid-infrared probe pulses of tunable wavelength between 3 and 7 microns. Ionic gating enables EF to cross the Pauli blocking threshold for interband absorption and switch the sign of the transient optical response - from a photobleaching to a photoinduced absorption - while higher doping levels suppress hot electron cooling via optical phonon emission. Interestingly, the femtosecond time resolution reveals a sub-picosecond, ultrafast sign reversal in transient transmission near the Pauli blocking threshold. While this phenomenon is of fundamental interest for understanding the properties of Dirac fermions, the electrostatic control of recovery dynamics and transmission opens exciting prospects for graphene-based ultrafast optical modulators and tunable saturable absorbers.