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

Abstract

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.

Tight control of light beams in photonic crystals with spatially-variant lattice orientation

“…The material was used as received (74 wt-% solids) when preparing thick films (> 100 μm), and it was diluted with cyclopentanone (CAS 120-92-3) when preparing thin films. Microscope slides were cut to 25 mm × 25 mm sections and cleaned [27]. A 5 μm thick adhesion layer of SU-8 was then applied by spin-coating diluted SU-8 (52 wt-%) onto the substrate (6000 rpm, 30 s), prebaking the sample on a hotplate (65 °C for 1 min.…”

“…3. In single-beam DLW, horizontal scanning of the focal spot produces resolution-limited rods with sub-micron axial and transverse widths typically in the ratio of ~2.5:1 [27]. With this in mind, the unit cell was designed to be simple cubic, having edge-length a, and a basis consisting of three orthogonal intersecting rods with ellipsoidal cross-section, as illustrated in Figs.…”

“…The SVPCs were fabricated by DLW in 160 μm thick films of spin-coated SU-8. The method of DLW has been described elsewhere [31] and the specifics of our implementation are reported in [27]. A computer controlled DLW system executed the write-file, translating the sample relative to the focused laser beam at 50 μm/s with synchronous control of an optomechanical shutter to expose along the line-segments that define the pedestal and the SVPC.…”

Two-photon reduction: a cost-effective method for fabrication of functional metallic nanostructures

“…[5,[20][21][22][23][24][25][26][27] Applying an ultrafast laser in DLW method, we are able to focus the ultrafast laser beam tightly into a tiny spot, in which the chemical reactions of materials are controlled via the multiphoton absorption process that enables the fabrication of micro/nanostructures with complex geometries. [25][26][27][28][29][30][31][32][33]Due to the 3D confinement of the nonlinear multiphoton absorption only in the focal region and the material threshold response, high spatial resolution surpassing the diffraction limit can be achieved. [34]In addition, the detrimental thermal effect that degrades the spatial resolution can be minimized by controlling the pulse width and the repetition rate of the ultrafast laser.…”

“…Compared to the two-step templating methods, the single-step photoreduction method can directly fabricate 3D metallic micro/nanostructures from solution with a great simplicity using the multiphoton reduction mechanism. [23][24][25][26][27][28][29][30][31][32]However, due to the structure formation mechanism, which is based on the nucleation of small nanoparticles (NPs), the current challenges in this method lie in the realization of high-resolution metallic structures with small surface roughness and high electrical conductivity towards functional plasmonic nanostructures with resonances in the optical regime.…”