Local Modification Of Refractive Index Research Articles

Refractive index patterning of infrared glass ceramics through laser-induced vitrification [Invited

Refractive index modification in glass or crystalline materials typically involves conversion of state (amorphous to crystalline or crystalline to amorphous) through a homogeneous, external stimulus such as laser- or current-induced heating, melting, or localized (resonant) bond modification. With the exception of traditional phase change materials that exploit reversibility, usually at high speeds and over multiple cycles, localized patterning of the refractive index is most frequently employed to induce a complete change of phase to enable the creation of embedded or surface optical structures. The present effort employs a novel, laser-induced vitrification (LIV) process developed to spatially modify the refractive index in a fully homogeneous glass ceramic material. Such processing leads to a local re-vitrification of the pre-existing nanocrystalline microstructure within the material to realize spatially-defined, refractive index profiles. Post-processing refractive index modification on the order of ∆n ~-0.062 was realized in a partially crystallized, multi-component chalcogenide glass ceramic nanocomposite, subjected to bandgap laser exposure. Spatially-varied phase modification in the lateral and axial directions within a bulk glass ceramic is quantified and the optical function of the resulting structure is demonstrated in the formation of an infrared grating. The underlying mechanism associated with the resulting local refractive index modification is explained through quantification of the multi-phase material attributes including parent glass properties, crystal phase identity and phase fraction as determined through micro-XRD and electron microscopic analysis. This correlation validates the proposed mechanism associated with the modification. A threshold power density for LIV in the starting glass ceramic has been determined based on exposure conditions and material attributes.

Tuning the refractive index in 3D direct laser writing lithography: towards GRIN microoptics

AbstractWe report a study of the determination of polymer cross‐linking, namely the degree of conversion and refractive index of the microstructures created by two‐photon polymerization (TPP). The influence of TPP processing parameters such as laser intensity and scanning velocity is investigated. The degree of conversion is analyzed via Raman microspectroscopy and the refractive index is measured with the interferometric technique employing a Michelson interferometer. Moreover, the relationship between these two properties is revealed and details are discussed. The largest refractive index change that we have obtained is of the order of 10−2. Finally, we propose and demonstrate experimentally the realization of the gradient‐index (GRIN) structure, resulting from a laser‐induced local refractive index modification due to monomer cross‐linking, i.e. degree of conversion. This work implies that the TPP technique is a valuable tool for the fabrication of GRIN microoptics for (in)homogeneous molding of light flow at the micrometer scale. image

Refractive index modification in fluoro-borate glasses containing WO3 induced by femtosecond laser

New glasses belonging to the NaF–Na2B4O7–WO3 system have been studied in order to inscribe optical waveguides within their volume through local refractive index modification after irradiation by ultrafast laser pulses. Appropriate addition of NaF to glasses belonging to the pseudo-binary vitreous system Na2B4O7–WO3 has allowed to obtain striae-free bulk glasses of large volume with high content of WO3 (35mol%). Their physical and structural properties have been investigated through UV–visible spectroscopy, differential scanning calorimetry, densitometry and Raman spectroscopy. The glass of composition (NaF)15–(Na2B4O7)50–(WO3)35 has been then exposed to 800nm femtosecond pulses of 70fs width at a repetition rate of 100kHz from a Ti-sapphire regenerative amplifier system. By varying the input pulse energy and scanning speed, homogeneous optical waveguides were inscribed and a positive refractive index change of the order of 5×10−3 has been measured.

Origin of the refractive index modification of femtosecond laser processed doped phosphate glass

The origin of the local refractive index modification in femtosecond laser inscribed structures has been investigated with confocal microfluorescence imaging. We have identified the origin of both, positive and negative refractive index changes in a commercial Er-Yb codoped phosphate glass upon irradiation in the low repetition rate regime (1 kHz). Consistent relations among the photoluminescence behavior of the dopants (erbium and ytterbium ions), the local sign of the density change and the local modification of the refractive index by means of characteristic emission features such as the intensity and the spectral peak position have been established. Blue or redshift in the photoluminescence emission have been observed and related to a local perturbation in the crystal field caused by a modification of the mean distance among the dopant ions, and thus of the local matrix density. These conclusions are additionally supported by the spatial distribution of photoluminescence emission intensities, which have been interpreted in terms of energy transfer mechanisms underlying the overall erbium-ytterbium emission process.

Molecular beam epitaxy growth of lattice-matched [formula omitted] multiple quantum well distributed feedback laser structures with gratings defined by implantation enhanced intermixing

AlGaInAs GaInAs multiple quantum wells (MQWs) are particularly suited for planar device integration using local refractive index modification by masked implantation enhanced intermixing (MIEI). Suitable molecular beam epitaxy (MBE) growth conditions are elaborated to achieve 1.3 μm AlGaInAs GaInAs MQW distributed feedback (DFB) lasers taking advantage of the capability of the MIEI process to define the gratings. High-quality MBE overgrowth on AlGaInAs necessary for device completion after H ∗ radical treatment is demonstrated.