Direct Laser Writing of Magneto-Photonic Sub-Microstructures for Prospective Applications in Biomedical Engineering

Abstract

We report on the fabrication of desired magneto-photonic devices by a low one-photon absorption (LOPA) direct laser writing (DLW) technique on a photocurable nanocomposite consisting of magnetite ( Fe 3 O 4 ) nanoparticles and a commercial SU-8 photoresist. The magnetic nanocomposite was synthesized by mixing Fe 3 O 4 nanoparticles with different kinds of SU-8 photoresists. We demonstrated that the degree of dispersion of Fe 3 O 4 nanoparticles in the nanocomposite depended on the concentration of Fe 3 O 4 nanoparticles, the viscosity of SU-8 resist, and the mixing time. By tuning these parameters, the most homogeneous magnetic nanocomposite was obtained with a concentration of about 2 wt % of Fe 3 O 4 nanoparticles in SU-8 2005 photoresist for the mixing time of 20 days. The LOPA-based DLW technique was employed to fabricate on demand various magneto-photonic submicrometer structures, which are similar to those obtained without Fe 3 O 4 nanoparticles. The magneto-photonic 2D and 3D structures with sizes as small as 150 nm were created. We demonstrated the strong magnetic field responses of the magneto-photonic nanostructures and their use as micro-actuators when immersed in a liquid solution.

Keywords: magnetic nanocomposite; magneto-photonic microstructures; one-photon absorption direct laser writing.

Figure A1

(a) TEM image of magnetite nanoparticles (Fe${}_3$O${}_4$) in ethanol; (b) statistical results of Fe${}_3$O${}_4$ size distribution.

Figure A2

Magnetic hysteresis loop M(H) of Fe${}_3$O${}_4$ nanoparticles taken at 300 K. The solid line shows the best fit of the M(H) data using the Langevin function.

Figure A3

The dispersion of Fe${}_3$O${}_4$ nanoparticles in SU-8 2000.5, SU-8 2002, and SU-8 2005. (a) images of three bottles of nanocomposites solutions, taken just after mixing; (b) after a few hours, the sedimentation occurred in the nanocomposite solution having a low viscosity of 2.49 cSt and 7.5 cSt; zoom in image of the bottle containing the Fe${}_3$O${}_4$/SU-8 2005 nanocomposite, after (c) one day and (d) 20 days of the preparation.

Figure 1

(a) experimental setup of a LOPA-based DLW technique for realization of submicrometer magneto-photonic structures; (b) a comparison of absorption spectra of pure SU-8 2005 and Fe${}_3$O${}_4$/SU-8 2005 nanocomposite.

Figure 2

Dependence of the size of magneto-photonic pillars on exposure doses (laser power and scanning velocity). (a) the SEM image of a periodic pillars array with different diameters realized by different scanning velocities (the laser power was fixed at 38 mW). Images on the right show side and top views of a single pillar; (b) plot of pillar diameters as a function of scanning speed for three different exposure powers.

Figure 3

Influence of the dose accumulation effect on pillars sizes. SEM images of periodic pillars arrays realized with a period of 1.5 $\mathsf{\mu }$m (a) and with a period of 0.6 $\mathsf{\mu }$m (b); (c) pillar diameters as a function of the distance between pillars (the period of the array). The laser power and the writing speed were fixed at 36 mW and 4 $\mathsf{\mu }$m/s, respectively, for all pillars; (d) schematic illustration of the dose accumulation effect due to the short distance between two pillars, which creates a nano-connection between pillars.

Figure 4

SEM images of various magneto-photonic submicrometer structures fabricated by the LOPA-based DLW technique. (a) a 2D hexagonal structure with a period of 1 $\mathsf{\mu }$m; (b) an arbitrary “LPQM” letter; (c) a 3D woodpile structure.

Figure 5

(a) illustration of the fabrication process and (b) series of screenshots illustrating the movement of magnetic submicropillars (the diameter is about 300 nm) towards the magnetic tip (see videos S.2 in the Supplementary Materials).