A. Baum

Photochemistry of refractive index structures in poly(methyl methacrylate) by femtosecond laser irradiation

Femtosecond, subablation threshold photomodification of poly(methyl methacrylate) (PMMA) at 387 nm is explored to enable fabrication of optical components. Volatile fragment analysis (thermal desorption gas chromatography-mass spectrometry) and molecular weight distribution monitoring (size exclusion chromatography) suggest photochemical modification, involving direct cleavage of the polymer backbone and propagation via chain unzipping under formation of monomers, similar to the pyrolytic degradation of PMMA. Waveguides were produced in undoped, clinical-grade PMMA, showing an increased refractive index in the laser focal region (Dnmax=4x10(-3)).

Pulse-duration dependency of femtosecond laser refractive index modification in poly(methyl methacrylate)

Refractive index modification of pure poly(methyl methacrylate) (PMMA) is investigated as a function of pulse duration using femtosecond lasers at 800 and 387 nm wavelength. It is observed that at 800 nm, the refractive index is modified more efficiently as the pulse duration decreases below 100 fs, whereas at 387 nm, efficient index modification is accomplished with longer, 180 fs pulses. Results suggest that three- and two-photon absorption is responsible for modification of pure PMMA at 800 nm and 387 nm, respectively. Repeated irradiation with short pulses of low laser fluence allows control of the photomodification via incubation, thus reducing bulk damage.

Mechanisms of femtosecond laser-induced refractive index modification of poly(methyl methacrylate)

The mechanisms of refractive index change in poly(methyl methacrylate) by frequency doubled femtosecond laser pulses are investigated. It is demonstrated that positive refractive index modificaton can be caused by a combination of depolymerization and crosslinking.

Femtosecond laser micro-structuring of materials in the nir and UV regime

Ultrafast laser micro-structuring of ceramic alumina, titanium and stainless steel are presented confirming that precision, melt free micro-structuring is indeed possible with intense femtosecond optical pulses. While metals are best machined at a fluence a few times ablation threshold, ceramic alumina, on the other hand, shows excellent structuring with no evidence of melt at high fluence F ∼ 21Jcm−2. In PMMA, at 387nm with pulselength ∼ 200fs, we demonstrate refractive index modification within the bulk induced by two photon absorption to generate phase gratings. By using a holographic technique combined with 387nm femtosecond radiation and 0.5NA UV objective, sub-micron pitch periodic structures were ablated on stainless steel, titanium and silicon. With a lower (0.15NA) objective and focussing the periodic optical field distribution within bulk PMMA, micron pitch phase gratings can be generated.Ultrafast laser micro-structuring of ceramic alumina, titanium and stainless steel are presented confirming that precision, melt free micro-structuring is indeed possible with intense femtosecond optical pulses. While metals are best machined at a fluence a few times ablation threshold, ceramic alumina, on the other hand, shows excellent structuring with no evidence of melt at high fluence F ∼ 21Jcm−2. In PMMA, at 387nm with pulselength ∼ 200fs, we demonstrate refractive index modification within the bulk induced by two photon absorption to generate phase gratings. By using a holographic technique combined with 387nm femtosecond radiation and 0.5NA UV objective, sub-micron pitch periodic structures were ablated on stainless steel, titanium and silicon. With a lower (0.15NA) objective and focussing the periodic optical field distribution within bulk PMMA, micron pitch phase gratings can be generated.

Refractive Index Structures in Polymers

Refractive index structuring of poly(methyl methacrylate) (PMMA) by femtosecond (fs) laser irradiation is discussed, including writing conditions defined by wavelength, pulse duration, and associated photochemistry. The aim is to determine optimal conditions for refractive index modification, Δn without doping for photosensitivity. The work presented here forms a generic methodology for other polymers. Nanostructuring using holographic optics and precise control of beam parameters has versatile application for three-dimensional (3D) photonic devices. Self-focusing and filamentation at various depths below the surface of bulk PMMA are discussed together with parallel processing using a spatial light modulator. Applications of refractive index structures in polymers include microfluidics, lab-on-a-chip, organic optoelectronic devices, and gratings in polymer optical fibres.

Femtosecond laser modification of poly(methyl methacrylate): Photochemistry

Femtosecond, sub-ablation threshold UV photo-modification of poly(methyl methacrylate) is explored. Volatile fragment analysis (thermal desorption gas chromatography - mass spectrometry) and molecular weight distribution monitoring (size exclusion chromatography) suggest direct backbone cleavage and monomer formation.

Optical characterization of phase gratings written by a UV femtosecond laser in PMMA

We report on the fabrication of optical Bragg type phase gratings in polymethyl methacrylate substrates irradiated by a femtosecond Ti: Sapphire laser. In order to investigate the distribution of the refractive index change produced by the femtosecond laser irradiation, we performed a two-dimensional visualization and spatially resolved optical analysis of the induced refractive index profile by using a digital holographic technique and an adaptive-iterative algorithm for wavefront reconstruction. The technique gives a direct and quantitative two-dimensional profile of the index of refraction in irradiated samples, providing information how the fabrication process depends on the laser irradiation.

Femtosecond laser modification of poly(methyl methacrylate) at 387 nm wavelength

Permanent refractive index modifications (Deltanmax = 2times10-3) were induced in clinical grade poly(methyl methacrylate) by UV femtosecond laser irradiation. Holographic writing realised 0.8 mum feature size, enabling gratings in polymer optical fibre and 3-D bulk structuring.