Periodic Reporting for period 2 - TerAqua (Compact and powerful strong-field terahertz light source for exploring water in new regimes)
Reporting period: 2020-10-01 to 2022-03-31
Water is undoubtedly the most important chemical solution for mankind - often referred to as the “solvent of life”. However many of its microscopic details and role in biological function remains mostly a mystery. In this project, the main target is to demonstrate a unique light tool that should contribute to make advances in this area and better understand one of the biggest mysteries of nature: the microscopic details of water.
More specifically, the light tool we aim to demonstrate is an ultrafast laser-driven, few-cycle Terahertz source with unprecedented efficiency, operating with high-average power and to apply this source for THz nonlinear spectroscopy of water-based solutions. Terahertz time-domain spectroscopy is a powerful technique for studying the dynamics of many fundamental constituents of matter. Recent progress in the generation of few-cycle THz pulses with electric field strengths exceeding several MV/cm has opened up exciting new directions in the area of nonlinear THz spectroscopy. However, many fields, including the wide field of physical chemistry studying aqueous solutions, suffer from a persistent lack of table-top THz sources combining high field strength and high repetition rate (i.e. sources operating with high average power). So far, these parameters can only be achieved by costly accelerator facilities with very restrictive access. In most cases, due to this persistent lack of convenient sources, scientists either abandon research lines where high pulse energy is required or operate at low repetition rate (typically well below 1 kHz). A low repetition rate imposes severe limits on the explored parameter space in many measurements, for which minuscule average powers result in very long integration time and low signal-to-noise ratio, or simply makes the study of relevant samples impossible.
The objective of this project is to fill this performance gap by achieving table-top strong-field pulsed THz sources with watt-level average power. The key feature is to leverage existing THz generation techniques to higher efficiency by driving THz generation inside the resonator of a high-power ultrafast thin-disk oscillator, operated in the little explored mid-infrared (2 μm) spectral region.
This groundbreaking THz source will have tremendous impact in various fields and subfields of physics, physical chemistry, engineering, biology and medicine. The vision of this project is to apply this source to bring answers to one specific long-standing scientific puzzle: understanding the microscopic details of water and its role as “the solvent of life”. Although THz spectroscopy is extremely well suited to study aqueous samples, so far, most THz studies in liquid phase have been performed in the linear domain, and in a very restricted parameter space due to the inadequacy of the THz sources. Our source will enable us to realize nonlinear THz-TDS of samples in aqueous phase at high repetition rate, enabling to study samples in biological conditions and disentangle the details of the dynamics involved.
More specifically, the light tool we aim to demonstrate is an ultrafast laser-driven, few-cycle Terahertz source with unprecedented efficiency, operating with high-average power and to apply this source for THz nonlinear spectroscopy of water-based solutions. Terahertz time-domain spectroscopy is a powerful technique for studying the dynamics of many fundamental constituents of matter. Recent progress in the generation of few-cycle THz pulses with electric field strengths exceeding several MV/cm has opened up exciting new directions in the area of nonlinear THz spectroscopy. However, many fields, including the wide field of physical chemistry studying aqueous solutions, suffer from a persistent lack of table-top THz sources combining high field strength and high repetition rate (i.e. sources operating with high average power). So far, these parameters can only be achieved by costly accelerator facilities with very restrictive access. In most cases, due to this persistent lack of convenient sources, scientists either abandon research lines where high pulse energy is required or operate at low repetition rate (typically well below 1 kHz). A low repetition rate imposes severe limits on the explored parameter space in many measurements, for which minuscule average powers result in very long integration time and low signal-to-noise ratio, or simply makes the study of relevant samples impossible.
The objective of this project is to fill this performance gap by achieving table-top strong-field pulsed THz sources with watt-level average power. The key feature is to leverage existing THz generation techniques to higher efficiency by driving THz generation inside the resonator of a high-power ultrafast thin-disk oscillator, operated in the little explored mid-infrared (2 μm) spectral region.
This groundbreaking THz source will have tremendous impact in various fields and subfields of physics, physical chemistry, engineering, biology and medicine. The vision of this project is to apply this source to bring answers to one specific long-standing scientific puzzle: understanding the microscopic details of water and its role as “the solvent of life”. Although THz spectroscopy is extremely well suited to study aqueous samples, so far, most THz studies in liquid phase have been performed in the linear domain, and in a very restricted parameter space due to the inadequacy of the THz sources. Our source will enable us to realize nonlinear THz-TDS of samples in aqueous phase at high repetition rate, enabling to study samples in biological conditions and disentangle the details of the dynamics involved.
In this reporting period, we have made significant technological advances that will enable us to demonstrate our groundbreaking source in the last period of the project in line with the proposed milestones. The main achievement was the demonstration of record-holding laser sources in the targeted wavelength region of 2 µm: we demonstrated the first 100W fundamental mode thin-disk laser operating in this wavelength region, and the highest average power modelocked laser in this wavelength region with 40W of power. These results are important milestones towards the final goal of achieving efficient, powerful THz sources. Furthermore, we have performed a detailed preliminary investigation and simulation of thermal and nonlinear effects occurring in nonlinear conversion crystals at the targeted average powers and intensities, which were critical know-how to achieve the goals of the project in the second phase.
The laser source demonstrated in the 2 µm region represents the state-of-the-art of ultrafast oscillators in this wavelength region – i.e. no other oscillator so far has demonstrated a higher average power level, placing us in a unique position to achieve to full set of goals of our proposal. Furthermore, the achieved results in continuous wave (100 W single-mode operation) represent the highest power so far achieved with thin-disk lasers in this wavelength region, surpassing previous results by a factor of >4. This last aspect opens the door also to new technological breakthroughs using this technology, i.e. with amplifier-based systems for example. In the next phase, we will focus our attention on energy scaling of the laser system: the current results are achieved at 63 MHz repetition rate, leading to intracavity values of tens of µJ, the final target is to operate at 3 MHz, so we expect intracavity pulse energies of several hundreds of microjoules to be achievable with this system.
THz source: During this first period, we carried out a detailed investigation of the limitations and compromises to be realized from the nonlinear materials in order to use the final laser source once ready in the final phase of the project. We developed simulation tools to simulate nonlinear conversion at the targeted wavelength showing that the targeted THz source can be reached. We believe there are therefore no major roadblockers to achieve the levels targeted (1W at 3 MHz repletion rate) in the second phase of the project.
THz source: During this first period, we carried out a detailed investigation of the limitations and compromises to be realized from the nonlinear materials in order to use the final laser source once ready in the final phase of the project. We developed simulation tools to simulate nonlinear conversion at the targeted wavelength showing that the targeted THz source can be reached. We believe there are therefore no major roadblockers to achieve the levels targeted (1W at 3 MHz repletion rate) in the second phase of the project.