THz dynamics in MUltiFerroIc Nanostructures and Superlattices – THz-MUFINS

To achieve the THz acoustic domain, one must confine the light / acoustic wave interaction to nanometric scales. Thus, in the project (WP1) a targeted design will make it possible to produce nanostructures such as superlattices of multiferroics or thin films whose ferroelectric domains will be nanostructured. These periodic nanostructures will be based on the BiFeO3 material, a room temperature small gap multiferroic system with the best properties to achieve our objectives. The growth will enable to control in the superlattices not only the chemical/ structural periodicities but also the ferroelectric and magneto-electric orders. These specificities, unique to multiferroics, are crucial in revealing prominent acousto-optical and electro-mechanical responses, and absent in traditional semiconductor-based superlattices that have been used until now for THz opto-acoustic and acousto-optic technologies. To achieve these objectives, the opto-acoustic and acousto-optical properties will be evaluated by pump-probe measurements (WP2) : we will study the generation of THz acoustic waves by resonant optical excitation (energy of the pump photon greater than the BiFeO3 gap ) or by ultra-short electrical pulse (energy of the pump photon less than the BiFeO3 gap) thanks to intense THz light sources (10-500 kV/cm). This complete range of the electromagnetic spectrum is an original point of this project. Finally, to validate our application ideas, an electrically reconfigurable acousto-optic transducer/modulator device is proposed (TRL 3). This device is based on a control of the nanostructuring of the ferroelectric domains (orientation, size) by reversible cycling of the ferroelectric polarization. With the distribution of the domains thus controlled, it will be possible to control the spectrum of the acoustic waves emitted and the nature of the acousto-optical interaction at the picosecond scale.

under progress

These artificially-architectured multiferroic nanostructures have the potential to become key ingredients in devices such as:
1) Transducers: The nanostructured devices will be a source for photo-controlled THz acoustic waves (photo-transducers). THz acoustic waves (with longitudinal-LA and transverse-TA acoustic components) will be either controlled with visible (eV) or THz (meV) radiation. Such THz acoustic sources are envisioned for optical, acoustic and acousto-optic nano-metrology.
2) Modulators: The device-oriented nanostructures will be a platform to investigate the acousto-optic coupling i.e., light manipulation with acoustic waves, including the light polarization rotation in the NIR-VIS-NUV range and the modulation of the emission of acousto-electric-driven THz radiation.

under progress

This project aims to open up new avenues for acousto-optical sensors, actuators and modulators involving THz acoustic waves and the electromagnetic domain that covers the near-infrared to near ultra-violet (NIR-VIS-UV) as well as the terahertz (THz) range. These two frequency ranges are the current key issues of a rich physics for high frequency photonic and optoelectronic technologies (GHz-THz). To address these challenges, nanostructures based on multiferroic materials appear as extremely promising tools, for which we must first establish a complete description of the ultrafast opto-acoustic and acousto-optic processes involved in these systems. The opto-acoustic process consists in generating acoustic waves with ultra-short light pulses (femtosecond laser) while the acousto-optical process corresponds to the modulation of the properties of light by acoustic waves (photoelastic effect).
To reach the THz acoustic, it is necessary to confine the light-acoustic wave interaction to nanometric scales. Thus, in the project, a targeted design will allow the realization of nanostructures based on the multiferroic model BiFeO3 (BFO) such as (i) superlattices where the chemical/structural periodicity is imposed by the alternation of thin layers, as well as (ii) nanostructures in the plane of a layer where the period is imposed by the intrinsic properties of the multiferroic (ferroelectric and magneto-electric domains). These specificities of multiferroics are crucial to reveal original acousto-optical and electro-mechanical responses which are notably absent in the traditional semiconductor-based superlattices that have been, until now, used for opto-acoustics and THz acousto-optics.
Also once architected (WP1), the opto-acoustic and acousto-optic properties of these nanostructures will be evaluated by pump-probe measurements (WP2). We will study the generation of THz acoustic waves by resonant optical excitation (pump photon energy higher than the BFO gap) or by ultra-short electrical excitation (pump photon energy lower than the BFO gap) using intense THz light sources (10-500 kV/cm). This extended range of the electromagnetic spectrum is also one of the original points of this project.
Finally, as a proof of concept (TRL 3), an electrically reconfigurable acousto-optic transducer/modulator device is proposed (WP3). This device is based on an in situ and reversible control of the ferroelectric domains in-plane nanostructures (orientation, size) by the application of an electrical voltage on the material. This external control of the nanostructure will allow to control the spectrum of acoustic waves emitted on demand.