Olivier Allegre

Dynamic modulation of spatially structured polarization fields for real-time control of ultrafast laser-material interactions

The polarization state of an ultrafast laser is dynamically controlled using two Spatial Light Modulators and additional waveplates. Consequently, four states of polarization, linear horizontal and vertical, radial and azimuthal, all with a ring intensity distribution, were dynamically switched at a frequency ν = 12.5 Hz while synchronized with a motion control system. This technique, demonstrated here for the first time, enables a remarkable level of real-time control of the properties of light waves and applied to real-time surface patterning, shows that highly controlled nanostructuring is possible. Laser ablation of Induced Periodic Surface Structures is used to directly verify the state of polarization at the focal plane.

Complete wavefront and polarization control for ultrashort-pulse laser microprocessing

We report on new developments in wavefront and polarization control for ultrashort-pulse laser microprocessing. We use two Spatial Light Modulators in combination to structure the optical fields of a picosecond-pulse laser beam, producing vortex wavefronts and radial or azimuthal polarization states. We also carry out the first demonstration of multiple first-order beams with vortex wavefronts and radial or azimuthal polarization states, produced using Computer Generated Holograms. The beams produced are used to nano-structure a highly polished metal surface. Laser Induced Periodic Surface Structures are observed and used to directly verify the state of polarization in the focal plane and help to characterize the optical properties of the setup.

Tailored optical vector fields for ultrashort-pulse laser induced complex surface plasmon structuring

Precise tailoring of optical vector beams is demonstrated, shaping their focal electric fields and used to create complex laser micro-patterning on a metal surface. A Spatial Light Modulator (SLM) and a micro-structured S-waveplate were integrated with a picosecond laser system and employed to structure the vector fields into radial and azimuthal polarizations with and without a vortex phase wavefront as well as superposition states. Imprinting Laser Induced Periodic Surface Structures (LIPSS) elucidates the detailed vector fields around the focal region. In addition to clear azimuthal and radial plasmon surface structures, unique, variable logarithmic spiral micro-structures with a pitch Λ ∼1μm, not observed previously, were imprinted on the surface, confirming unambiguously the complex 2D focal electric fields. We show clearly also how the Orbital Angular Momentum(OAM) associated with a helical wavefront induces rotation of vector fields along the optic axis of a focusing lens and confirmed by the observed surface micro-structures.

Real-time control of polarisation in ultra-short-pulse laser micro-machining

The use of a fast-response, transmissive, ferroelectric liquid-crystal device for real-time control of the polarisation direction of a femtosecond laser beam, and the benefits for various aspects of ultra-short pulse micro-machining, are discussed. Several configurations have been used to drive the polarisation in real-time. Our microscopic investigations of the resulting features revealed significant improvements in process efficiency and quality, compared to static linear and circular polarisations. Following our successful micro-machining tests, real-time polarisation control could emerge as a powerful tool in laser engineering.

NUV femtosecond laser inscription of volume Bragg gratings in poly(methyl)methacrylate with linear and circular polarizations

Large, high efficiency, volume Bragg gratings with dimensions of 5 mm × 5 mm and thickness between 1 and 7 mm with 20 μm pitch have been inscribed in poly(methyl)methacrylate (PMMA) with 180 fs, 387 nm parallel beams using both linear and circular polarizations. Linear polarization (perpendicular to the scan direction) produced the highest refractive index contrast, while circular polarization produced the lowest. The measured first-order diffraction efficiency with grating thickness L agrees well with theoretical expectations, and reached a maximum of 94% near L = 4 mm, the highest yet observed in pure PMMA. The source of the variation in refractive index contrast was investigated, and it was found to be due to the polarization-dependent nonlinear filamentation, the first such observation in a pure polymer.

Real-time control of polarization in ultra-short pulse laser micro-processing

The use of a fast-response, transmissive, ferroelectric liquid-crystal device for real-time control of the polarisation direction of a femtosecond laser beam, and the benefits for various aspects of ultra-short pulse micro-machining, are discussed. Several configurations have been used to drive the polarisation in real-time. Our microscopic investigations of the resulting features revealed significant improvements in process efficiency and quality, compared to static linear and circular polarisations. Following our successful micro-machining tests, real-time polarisation control could emerge as a powerful tool in laser engineering.