This research focuses on the development and use of laser spectroscopies for the study of light-matter interactions, gases, solid and liquid surfaces, as well as (bio-)organic materials and nanomaterials. Jointly, LLS researchers are members of the ILEE, NISM, NARILIS and naXys.

On the experimental front, LLS's particularity lies in its unique expertise in the development of original optical instruments developed by the laboratory's researchers. This equipment, like the commercial instruments with which the laboratory is equipped, is overwhelmingly integrated into the LOS technology platform. Alongside the design of these experimental tools, the LLS also develops theoretical tools, to analyze, understand and predict classical and quantum optical phenomena, as well as molecular interaction processes in the gas phase, to which the instruments give access.

The LLS regularly collaborates with other teams from the Departments and Faculties of the University of Namur. A solid international reputation has been built up over the years, thanks to the originality and excellence of the research carried out in very specific niches, in collaboration with numerous external laboratories, in Belgium and abroad.

The cutting-edge research carried out at the LLS falls into three main themes:

• Study of molecules in very low concentration ("pollutants") in the gas phase (Muriel Lepère)

Understanding atmospheric physico-chemical processes is a growing concern in our industrialized society, which is confronted with global climatic phenomena such as global warming or urban pollution, as well as health problems such as the rising incidence of diseases like asthma or lung cancer. Progress in atmospheric science requires precise knowledge of the concentrations of numerous pollutants and their spatial and temporal evolution, which in turn requires very precise knowledge of the spectroscopic properties of gases.

Generally speaking, high-resolution molecular spectroscopy is a powerful tool for the quantitative study of gas-phase mixtures, both for fundamental understanding of intra- and inter-molecular interactions and for environmental and health studies. It is in this context that the experimental and theoretical research carried out at LLS is developing. Measurements are carried out using laser-based spectroscopies (Quantum Cascade Laser, Dual Comb Spectroscopy ...) with a wealth of "home-made" equipment, while the modeling of observed phenomena is carried out using semi-classical or quantum theories. Parameters such as individual line intensities, pressure-induced broadening and collisional displacements, spectral profiles and line interference are studied for many atmospheric pollutants. The evolution of these phenomena with temperature is also studied, since it proves essential both from a fundamental and applied point of view (studies of "cold" or "hot" planets, combustion residual gases, fumes...).

• Nonlinear optical spectroscopies of molecular layers, surfaces, interfaces and nanostructures

Nonlinear optical phenomena are mainly observed at the high light intensities produced by pulsed lasers. Thanks to the non-linear properties of matter, light beams are able to interact with each other or change frequency, contrary to the usual experiment. Non-linear phenomena are at the heart of modern optical technologies. The LLS exploits them to produce frequency-tunable laser beams and investigate light-matter interactions by means of spectroscopies specifically sensitive to material surfaces, interfaces and nanostructures. In this way, the selection of appropriate laser beam frequencies enables the vibrational and electronic excitations of matter to be probed through frequency-sum generation (SFG) and second harmonic generation (SHG) spectroscopies.

The experimental and theoretical research themes pursued by the unit include determining the structure of molecular films and the orientation of molecules within self-assembled layers, the behavior of water on the surface of (bio-)materials, understanding the physico-chemical properties of biofilms and bio-matter/nanomaterial interactions, exalting nonlinear signals exploiting surface plasmon resonances, and, in general, modeling spectroscopies and nonlinear light interactions at interfaces.

• Quantum optics and weak quantum measurements

At very low light intensities, the quantum properties of light become preponderant. Discrete exchanges of light energy with matter correspond to the emission and absorption of a photon, or even several photons in the case of the quantum description of non-linear optical phenomena. In order to distinguish between the classical properties of light (electromagnetic waves obeying Maxwell's equations) and its quantum properties, the LLS measures the correlations of light intensity after the propagation of pairs of entangled photons within optical set-ups, such as interferometers. Detection coincidences are counted using four photon detectors. The LLS has techniques, such as quantum tomography and the measurement of Bell inequality violation, to study entanglement within quantum optical systems, particularly as a function of their interactions with matter.

The success of quantum information, due to its potential or proven applications such as quantum computing and cryptography, is sparking renewed interest in questions concerning the foundations of quantum mechanics. In this context, the LLS is developing research into weak quantum measurements. A standard quantum measurement typically involves a significant and irreversible perturbation of the state of the quantum system. One of the aims of a weak measurement is to make this perturbation negligible. In this case, experimental observations depend on a complex quantity, the so-called weak value. The LLS studies the interpretation and modeling of weak measurements and weak values. On the one hand, research focuses on questions that shed light on the foundations of quantum mechanics, including certain paradoxical aspects such as the three-box paradox and the quantum Cheshire cat paradox. On the other hand, these notions are a source of high-precision measurement techniques, particularly in optics. For example, the LLS studies the minimal deviations of the propagation of light beams from the laws of propagation of isolated light rays (geometrical optics), one manifestation of which is the Goos-Hänchen shift.

• Prof. Muriel Lepère, Director, Ordinary Professor and Honorary Qualified Researcher of the F.R.S.-FNRS
• Prof. Francesca Cecchet, F.R.S.-FNRS Qualified Researcher
• Prof. Yves Caudano, F.R.S.-FNRS Qualified Researcher