Subcycle Extreme Nonlinearities in GaP Induced by an Ultrastrong Terahertz Field

We report on the experimental observation of extreme laser spectral broadening and a change in optical transmission in gallium phosphite induced by 25 MV/cm terahertz (THz) single-cycle internal field. Such intense THz radiation leads to twofold transient modifications of the optical properties in the electro-optical crystal. First, the electric field provokes extensive cross-phase modulation via the chi((2)) and chi((3)) nonlinearities on a copropagating 50 fs near infrared laser pulse which turns into 500% spectral broadening. Second, we observe an instantaneous change of the optical transmission occurring at the THz field which is alleged to interband Zener tunneling and charge carrier density modification by impact ionization turning the semiconductor in a metal-like transient state. The presented scheme displays a pathway to coherently control the optical properties of semiconductors on an ultrafast time scale by a strong THz field.

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

Industrial laser precision machining is predominately based on pulsed Nd:YAG lasers operated at a wavelength of 1.06 µm in a free running pulse regime. The objective of this thesis work has been to investigate second and third harmonic generation for such "long pulse" Nd:YAG lasers. Visible and UV radiation can be better focused. The absorption coefficient at these wavelengths is higher in most materials. Processing at the doubled or tripled frequency increases machining precision and allows processing of new classes of materials such as, e.g., high reflective metals or ceramics. Frequency doubling and tripling of Q-switched, modelocked, or CW operated Nd:YAG lasers has been the subject of numerous publications. However, the particular problems associated with the free running pulse regime have not yet been addressed. The generation of harmonics requires a laser source that can deliver radiation with high brightness. A system based on a slab crystal with a beam quality factor of M2 ≈ 1.6 and a power of up to 1.5 kW has been used in the experiments. An external electro-optical switch has been implemented in order to precisely control the laser pulse shape and duration. The non-linear coefficient and the angular acceptance have been the main criteria for selecting LBO (LiB3O5), KTP (KTiOPO4), and MgO:LiNbO3 for the frequency conversion experiments. The conversion efficiency has been measured for these crystals as a function of the intensity at the fundamental wavelength. In contrast to simple model predictions, the efficiency does not increase linearly with intensity under long pulse conditions. An intensity dependant phase mismatch is at the origin of the deviations. A heuristic model for the efficiency prediction has been developed. It is based on a linear increase of phase mismatch with intensity and it allows describing the experimental findings with sufficient accuracy. The conversion efficiency for frequency doubling is limited to about 20% at pulse durations of 100 µsec. It decreases with increasing pulse duration. Catastrophic thermal damage is at the origin of the limitation in this pulse regime. Our experiments indicate that the damage is initiated by the frequency doubled light. Temporary color centers are created in the presence of the second harmonic light. The dynamic of the creation and annihilation of these centers has been investigated. It could be shown that finally the absorption of the fundamental harmonic (infrared) radiation at these centers results in irreversible thermal damages. A power of 140 W in the second harmonic (178 MW/cm2 on a waist of 5 µm) and 12 W in the third harmonic (61 MW/cm2 in a waist of 2.5 µm) was obtained with pulses of 200 µs. This allowed to demonstrate the feasibility of material processing with very high reproducibility and definition, which for instance allowed the design and the manufacturing of micro-grippers and several other prototypes in the field of robotics and micro-engineering.

EPFL2001

Comparison of Thermal Detector Arrays for Off-Axis THz Holography and Real-Time THz Imaging

Christoph Peter Hauri, Mostafa Shalaby

In terahertz (THz) materials science, imaging by scanning prevails when low power THz sources are used. However, the application of array detectors operating with high power THz sources is increasingly reported. We compare the imaging properties of four different array detectors that are able to record THz radiation directly. Two micro-bolometer arrays are designed for infrared imaging in the 8–14 μm wavelength range, but are based on different absorber materials (i) vanadium oxide; (ii) amorphous silicon; (iii) a micro-bolometer array optimized for recording THz radiation based on silicon nitride; and (iv) a pyroelectric array detector for THz beam profile measurements. THz wavelengths of 96.5 μm, 118.8 μm, and 393.6 μm from a powerful far infrared laser were used to assess the technical performance in terms of signal to noise ratio, detector response and detectivity. The usefulness of the detectors for beam profiling and digital holography is assessed. Finally, the potential and limitation for real-time digital holography are discussed.

2016

Génération de deuxième et troisième harmonique avec un laser Nd:YAG en régime pulsé libre

Sébastian Favre

EPFL2001