Grigorios Boulogiannis

Investigating Nonlinear Absorption Mechanisms in Transparent and Semi-Transparent Materials for Advanced Laser Processing in Photovoltaics

Understanding and controlling ultrashort pulse (USP) laser interaction with transparent and semi-transparent materials is crucial for next-generation photovoltaic (PV) applications. These materials include transparent conductive oxides (TCOs), wide-bandgap semiconductors such as 4H-silicon carbide (SiC) and gallium nitride (GaN), transparent dielectrics like soda-lime glass, and perovskites used in state-of-the-art PV concepts.

Ultrafast laser processing offers unique opportunities for precision structuring, surface functionalization, and targeted material modification. However, achieving predictive control of these processes remains challenging, particularly in transparent and wide-bandgap materials. While analytical and semi-analytical models derived from metal processing are widely applied in practice, the transfer of this theoretical understanding to dielectrics and wide-bandgap semiconductors is still limited.

Within the framework of the RTG project, our work aims to address this gap by systematically investigating how absorbed laser intensity governs material transformation processes. The research covers the entire chain of light–matter interaction: from nonlinear optical absorption mechanisms, such as multiphoton absorption and Kerr nonlinearity, to electron–lattice energy transfer and phonon excitation, and further to heat accumulation, ablation, and ultimately plasma formation. Particular attention is given to temperature profiles and cumulative heating effects induced by laser irradiation, which dynamically modify the optical response of the material.

Experimentally, this approach combines laser-induced surface modification studies with nonlinear optical characterization. An in-house built, fully automated Z-scan setup enables the measurement of temperature-dependent nonlinear optical properties over a broad wavelength range (310–2600 nm) and at substrate temperatures up to 1500 °C. These measurements provide quantitative insight into how intrinsic material parameters and laser pulse characteristics jointly determine processing outcomes.

The overarching goal of this research is to establish analytical scaling relations that directly link laser parameters, such as wavelength, pulse duration, fluence, and repetition rate, to temperature-dependent linear and nonlinear material properties. By bridging fundamental nonlinear optics with application-oriented laser processing, our work aims to enable predictive and transferable process control across material classes critical for photovoltaic technologies.

Supervisor: Giuseppe Sansone