Henry E. Williams

Fabrication of three-dimensional micro-photonic structures on the tip of optical fibers using SU-8.

A method is reported for fabricating truly three-dimensional micro-photonic structures directly onto the end face of an optical fiber using the cross-linkable resist SU-8. This epoxide-based material is well suited for micro-device fabrication because it is photo-processed as a solid and the cross-linked material is mechanically robust, chemically resistant, and optically transparent. Yet, procedures commonly used to process SU-8, particularly spin-coating, are impractical when the intended fabrication substrate is the end-face of an optical fiber. A melt-reflow process was developed to prepare optical fibers having SU-8 resin deposited at controlled thickness on the fiber end-face. Multi-photon direct laser writing was then used to fabricate various refractive lenses, a compound lens system, and a woodpile photonic crystal within the resin on the end-face of the optical fiber. Data are presented that show how the refractive lenses can be used to alter the output of the optical fiber. This work opens a new path to low-profile integrated photonic devices.

Fabrication and characterization of three-dimensional copper metallodielectric photonic crystals

Three-dimensional metallodielectric photonic crystals were created by fabricating a micron-scale polymeric template using multiphoton direct laser writing (DLW) in SU-8 and conformally and selectively coating the template with copper (Cu) via nanoparticle-nucleated electroless metallization. This process deposits a uniform metal coating, even deep within a lattice, because it is not directional like sputter-coating or evaporative deposition. Infrared reflectance spectra show that upon metallization the optical behavior transitions fully from a dielectric photonic crystal to that of a metal photonic crystal (MPC). After depositing 50 nm of Cu, the MPCs exhibit a strong plasmonic stop band having reflectance greater than 80% across the measured part of the band and reaching as high as 95% at some wavelengths. Numerical simulations match remarkably well with the experimental data and predict all dominant features observed in the reflectance measurements, showing that the MPCs are structurally well formed. These data show that the Cu-based process can be used to create high performance MPCs and devices that are difficult or impossible to fabricate by other means.

Effect of refractive index mismatch on multi-photon direct laser writing

This work reports how the process of three-dimensional multi-photon direct laser writing (mpDLW) is affected when there is a small mismatch in refractive index between the material being patterned and the medium in which the focusing objective is immersed. Suspended-line microstructures were fabricated by mpDLW in the cross-linkable epoxide SU-8 as a function of focus depth and average incident power. It is found that even a small refractive index contrast of Δn = + 0.08 causes significant variation in feature width and height throughout the depth of the material. In particular, both the width and height of features can either increase or decrease with depth, depending upon how much the average incident laser power exceeds the threshold for writing. Vectorial diffraction theory is used to obtain insight into the origin of the effect and how to compensate for it. We demonstrate that varying the average focused power is a practical means for controlling the variation in feature size with focal depth.

Processing and properties of arsenic trisulfide chalcogenide glasses for direct laser writing of 3D microstructures

Arsenic trisulfide (As2S3) is a transparent material from ~620 nm to 11 μm with direct applications in sensors, photonic waveguides, and acousto-optics. As2S3 may be thermally deposited to form glassy films of molecular chalcogenide (ChG) clusters. It has been shown that linear and multi-photon exposure can be used to photo-pattern thermally deposited As2S3. Photo-exposure cross-links the film into a network solid. Treating the photo-patterned material with a polarsolvent removes the unexposed material leaving behind a structure that is a negative-tone replica of the photo-pattern. In this work, nano-structure arrays were photo-patterned in As2S3 films by multi-photon direct laser writing (DLW) and the resulting structure, morphology, and chemical composition were characterized and correlated with the conditions of the thermal deposition, patterned irradiation, and etch processing. Raman spectroscopy was used to characterize the chemical structure of the unexposed and photo-exposed material, and near infrared ellipsometry was used to measure the refractive index. Physical characterization including structure size and surface adhesion of nano-scale features is related to the processing conditions.

Order of multiphoton excitation of sulfonium photo-acid generators used in photoresists based on SU-8

Multiphoton lithography (MPL), Z-scan spectroscopy, and quantum chemical calculations were employed to investigate the order of multiphoton excitation that occurs when femtosecond laser pulses are used to excite two sulfonium photo-acid generators (PAGs) commonly used in photoresists based on the cross-linkable epoxide SU-8. The mole-fractions of the mono- and bis-sulfonium forms of these PAGs were determined for the commercially available photoresist SU-8 2075 and for the PAGs alone from a separate source. Both were found to contain similar fractions of the mono- and bis-forms, with the mono form present in the majority. Reicherts method was used to determine the solvatochromic strength of the SU-8 matrix, so that results obtained for the PAGs in SU-8 and in solution could be reliably compared. The PAGs were found to exhibit a minimal solvatochromic shift for a series of solvents that span across the solvatochromic strength of SU-8 itself. Sub-micron-sized features were fabricated in SU-8 2075 by MPL us...