Identification of the formation phases of filamentary damage induced by nanosecond laser pulses in bulk fused silica

International audienceEmploying a pump-probe polarization-based two-frame shadowgraphy setup, the formation of filamentary damage induced in bulk fused silica by a nanosecond pulse at 1064 nm is investigated with a picosecond probe. Three different phases are exhibited in the damage experiments. The first phase is the formation of a micrometric plasma channel along the laser direction during the beginning of the pulse likely caused by multi-photon ionization. This channel exhibits growth during similar to 400 ps, and the newly grown plasma is discrete. Then, during the end of the pulse, this channel evolves into a tadpole-like morphology showing an elliptical head upstream the laser flux followed by a thin tail. This observed asymmetry is attributed to shielding effects caused by both the plasma and hot modified silica. Once the damage shows its almost final morphology, a last phase consists in the launch of a pressure wave enlarging it after the laser pulse. The physical mechanisms that might be involved in the formation of plasma channels are discussed. The experimental data are first confronted to the moving breakdown model which overestimates the filamentary damage length. Finally, taking into account the temporal shape of the laser pulses, the coupling between Kerr-induced self-focusing and stimulated Brillouin scattering is discussed to interpret the observations. (C) 2015 AIP Publishing LLC

Transient Loss‐Induced Non‐Hermitian Degeneracies for Ultrafast Terahertz Metadevices

Abstract Non‐Hermitian degeneracies, also known as exceptional points (EPs), have attracted considerable attention due to their unique physical properties. In particular, metasurfaces related to EPs can open the way to unprecedented devices with functionalities such as unidirectional transmission and ultra‐sensitive sensing. Herein, an active non‐Hermitian metasurface with a loss‐induced parity‐time symmetry phase transition for ultrafast terahertz metadevices is demonstrated. Specifically, the eigenvalues of the non‐Hermitian transmission matrix undergo a phase transition under optical excitation and are degenerate at EPs in parameter space, which is accompanied by the collapse of chiral transmission. Ultrafast EP modulation on a picosecond time scale can be realized through variations in the transient loss at a non‐Hermitian metasurface pumped by pulsed excitation. Furthermore, by exploiting the physical characteristics of chiral transmission EPs, a switchable quarter‐wave plate based on the photoactive metasurface is designed and experimentally verified and realized the corresponding function of polarization manipulation. This work opens promising possibilities for designing functional terahertz metadevices and fuses EP physics with active metasurfaces

dynamics.mp4

This video illustrates the modulation of the depletion region induced by ultrafast photo-carrier dynamics