Terahertz and water
Il giorno Martedì 8 Aprile alle ore 11:00 in aula 0M04 del Dipartimento di Fisica,
il Dr. Fabio Novelli (Ruhr University Bochum, Germany - University of Southampton, United Kingdom, fabio.novelli@rub.de) terrà un seminario dal titolo:
"Terahertz and water"
Abstract
On the microscopic scale, water molecules in the liquid phase make hydrogen bonds (HB) with tetrahedral structures that fluctuate on the picosecond (ps) timescale. On the macroscopic scale, thermo-dynamic properties are anomalous as they scale non-continuously. It is unclear how the macroscopic behavior of water emerges from its microscopic properties. An inherently powerful tool to study water is terahertz (THz) radiation because it can reveal the sub-ps fluctuations of the water network. Radiation between about 1 and 25 THz is strongly absorbed by intermolecular collective modes of HB water molecules, which can be thought of as "small and short-lived phonons".
The talk is divided into two parts. In the first part I will describe a novel approach, dubbed "intense THz time-domain spectroscopy", to probe the subtle variations of the water modes due to the interaction of the liquid molecules with different solutes [1]. In the second part, I will discuss how to trigger and detect photo-ionization processes in pure water and salt solutions, that is, how liquid water reacts to the injection of a "photo-doped" charge carrier [2]. Optical-pump THz-probe results imply that the transient, short-lived (<300 fs) photoconductivity of the ionized liquid is as high as ~5/(Ohm m), comparable to that found in typical electronic materials like intrinsic Germanium [3]. These electronic states resemble Wannier-Mott excitons or Rydberg orbitals and could assist in the development of liquid-based optoelectronic devices. For example, previous results [4] demonstrated that a highly concentrated iodide solution (9 M NaI) can modulate the temporal profile of pulsed THz fields, thereby upshifting the transmitted spectrum from 1 to 3 THz with an efficiency of 4%.
The talk is divided into two parts. In the first part I will describe a novel approach, dubbed "intense THz time-domain spectroscopy", to probe the subtle variations of the water modes due to the interaction of the liquid molecules with different solutes [1]. In the second part, I will discuss how to trigger and detect photo-ionization processes in pure water and salt solutions, that is, how liquid water reacts to the injection of a "photo-doped" charge carrier [2]. Optical-pump THz-probe results imply that the transient, short-lived (<300 fs) photoconductivity of the ionized liquid is as high as ~5/(Ohm m), comparable to that found in typical electronic materials like intrinsic Germanium [3]. These electronic states resemble Wannier-Mott excitons or Rydberg orbitals and could assist in the development of liquid-based optoelectronic devices. For example, previous results [4] demonstrated that a highly concentrated iodide solution (9 M NaI) can modulate the temporal profile of pulsed THz fields, thereby upshifting the transmitted spectrum from 1 to 3 THz with an efficiency of 4%.
[1] Terahertz spectroscopy of thick and diluted water solutions Opt. Exp. 32 11041 (2024)
[2] The birth and evolution of solvated electrons in the water PNAS 120 e2216480120 (2023)
[3] High-Mobility Electrons in Aqueous Iodide Solutions ACS Omega 10 5097 (2025)
[4] An ultra-fast liquid switch for terahertz radiation APL Photonics 7 121302 (2022)
[2] The birth and evolution of solvated electrons in the water PNAS 120 e2216480120 (2023)
[3] High-Mobility Electrons in Aqueous Iodide Solutions ACS Omega 10 5097 (2025)
[4] An ultra-fast liquid switch for terahertz radiation APL Photonics 7 121302 (2022)
Il proponente,
Prof. Andrea Rubano
Data:
08/04/2025