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

Abstract

We 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 gradientindex (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.

Optically Clear and Resilient Free-Form <em>&micro;</em>-Optics 3D-Printed via Ultrafast Laser Lithography

“…This hints that without photoinitiator the crosslinking process is not as efficient and provides a final polymer matrix that considerably weaker. This result coincides well with other works showing that degree of crosslinking during 3DLL is essential for mechanical and optical properties of finished structures [22,23]. However, despite this, if fabrication parameters are within the fabrication window even advanced microoptical elements, like suspended microlenses on a tip of a optical fiber, can be fabricated out of pure material [ Figure 2 (b)].…”

Synthesis, Characterization and 3D Micro-Structuring via 2-Photon Polymerization of Poly(glycerol sebacate)-Methacrylate–An Elastomeric Degradable Polymer

“…This is in line with previous reports using Raman spectroscopy to determine the degree of crosslinking. At the reported laser intensities the irradiation converts the functionalised prepolymers into a crosslinked polymer, while higher pulse energies are needed for further chemical conversion of the polymer and carbonisation by the laser [68,69].…”

Nanoscale Precision of 3D Polymerization via Polarization Control

“…[8][9][10] Pure optical light delivery at tight focusing cannot explain the fi nal size of the structure, due to the threshold effect of laser modifi cation via polymerization/ crosslinking, thermal effects within the 3D focal volume, and wet chemistry development, which all affect the fi nal shape and size of the 3D structure. [11][12][13][14][15] Heat accumulation at the focus, defi ned by pulse repetition rate and scan speed, is used to increase the productivity of 3D polymerization and makes thermal issues very important and actively debated. [16][17][18] Polarization effects in laser fabrication in 2D and 3D geometries are now explored in polymerization by DLW.…”