Free-form Light Actuators — Fabrication and Control of Actuation in Microscopic Scale

Figure 1 shows the optical set up for laser writing. The system consists of a 780 nm fiber laser generating 130 fsec pulse at the repetition rate of 100 MHz. The laser beam is reflected into a telescope to adjust the beam profile to the optical microscope objective aperture where it is focused into the sample. On the microscope, a 3D piezo stage is installed with a 300 × 300 × 300 µm³ travelling range for sample translation with a maximum speed of 100 µm/sec at 2 nm resolution. Linearly polarized light from a red lamp illuminates the sample from the top, while the image is collected at the bottom by the same objective and reflected by a beam splitter into a CCD camera. Before the camera, another polarizer is used to obtain cross polarized illumination for enhanced contrast.

Figure 2 shows the scanning electron microscope (SEM) images of the laser written IP-L micrograting patterns (Step 1). The groove spacing is in the range of 400 – 1,200 nm, while the height of the grooves (top-to-valley) is around 700 nm. Grating patterns with different orientations can induce different LC alignments, depending on the desired actuation of the LCE element.

Figure 3 shows the LC monomer orientation induced by the IP-L grating patterns (Step 2.7). First, four kinds of micro-grating pattern with 100 × 100 µm² size each were fabricated on opposite sides of a glass cell (schematically shown in Figure 3a). Due to the surface anchoring, the infiltrated LC monomers have been oriented along with the grating lines direction, thus exhibiting 45° contrast inversion in the polarized optical microscope (POM) image (Figure 3b).

Figure 4 shows the SEM images of an LCE nano dot/line fabricated on IP-L grating networks with different orientation (Step 2.10). Within the grating network, the LCE structures become more confined, with much higher resistance to the development in toluene. A minimum width of the disconnected LCE has been measured to be ~300 nm, which is consistent with the resolution of DLW without the grating pattern. Another interesting approach for photonic application could be the realization of large scale periodic structure. Figure 4 (c, d) shows 2D LCE periodic structures within a micro-grating network. The alignments are well preserved inside these nanostructures, as shown in the inserted POM images of Figure 4 (c, d). However, light induced deformation could not be obtained in these nanostructures. This is because within the IP-L grating, the nano-LCE elements have been highly confined and adhesion prevents any visible deformation.

The micro manipulation system is based on a home-made reflected microscope and is shown schematically in Figure 5. A 10X objective is fixed on a lens tube placed on a vertically standing optical breadboard. A 730 nm IR LED light source is used for illumination through a non-polarized beam splitter. The reflected image is collected by the same objective and projected on the camera. A continuous solid state 532 nm laser is coupled into the objective by a long pass dichroic mirror (50% transmission and reflection at 567 nm) at an incidence angle of 45°. A power meter measures the transmitted beam after the dichroic mirror for real time detection of laser power. A loosely focused laser spot of ~150 µm diameter generates maximum illumination intensity of ~10 W/mm². Laser intensity is controlled by a variable neutral density filter placed in front of the laser. Below the objective, a 3D manual translation stage is used for sample translation. A heating stage installed on the translation stage is used for precise control of the sample temperature in a range from -20 to 120 °C with 0.5 °C accuracy. Two glass tips mounted on two manual translation stages have been placed on the left and right sides, near the sample position. Structure micro manipulation can be realized by carefully moving the tips with the help of the translation stages.

To demonstrate the alignment and deformation correlation, we fabricate four LCE cylindrical structures with 60 µm diameter and 20 µm height. These cylinders are written on four differently orientated IP-L grating regions (1 µm period). Under light excitation, the dyes inside the LCE absorb light energy and transfer it into the network. The LCE structures are heated up and then undergo phase transition (nematic to isotropic). Such phase transition is also helped by the trans a cis isomerization of the dye under the same light stimuli. Thus, the structures contract along the original LC alignment director and expand in the perpendicular direction⁷. Depending on different local alignments induced by the IP-L gratings, these structures deform along different directions, as shown in Figure 6 (Step 3.1).

This technique enables the creation of compound actuators, which contain more than one type of alignment in one single structure. A 400 × 40 × 20 µm³ size LCE stripe with two sections of alignment pattern was fabricated, as schematically shown in Figure 7 (a). Those alignment sections contain each a 90° twisted orientation in a different direction. The surface with parallel alignment contracts, while the one with perpendicular alignment expands under light illumination. The structure has been picked up by the micromanipulation system, and held in the air by a glass tip. Double bending was observed under light illumination (Step 3.3). A modulated laser beam (using an optical chopper) can induce cyclical deformations. LCE can respond following the laser modulation frequency (>1k Hz). However, the deformation amplitude decreases with increasing frequency¹⁴.

Figure 1: Optical Set Up for Direct Laser Writing. A 780 nm laser beam (130 fsec pulse, repetition rate of 100 MHz) is coupled into a microscope and focused by an optical microscope objective into the sample. A 3D piezo stage with 300 × 300 × 300 µm³ travel range is used for the sample translation during laser exposure. Please click here to view a larger version of this figure.

Figure 2: SEM Images of IP-L Micro-gratings. a) Unidirectional parallel line structure. b) Radial grating pattern. Scale bar: 10 µm. Please click here to view a larger version of this figure.

Figure 3: IP-L Micro-grating Induce LC Orientation. a) Schematic of the micro-grating patterns designed for the LC orientation. b) POM image of the the LC orientation induced by the micrograting patterns. The scale bar is 50 µm. The red color is due to the filter which prevents the photo-polymerization. Please click here to view a larger version of this figure.

Figure 4: SEM Images of LCE Nanostructures Embedded Inside IP-L Grating Networks. a) and b) Two micro-grating patterns were fabricated by DLW along different directions, while LCE nanodots are fabricated within the grating network. c) and d) Periodic LCE nano structures embedded within the same type of IP-L gratings. Insets are POM image of the structures. Please click here to view a larger version of this figure.

Figure 5: Schematic of the Micromanipulation Setup. A CW solid state 532 nm laser is coupled into a home-made microscope system. A 10X Objective is used for imaging and focusing the 532 nm laser for excitation. Two manual translation stages equipped with glass tip manipulators are used for sample micro-manipulation. Please click here to view a larger version of this figure.

Figure 6: Light Actuation of LCE Micro-cylinders on Four Different IP-L Micrograting Regions with Different Orientations. a) Four LCE cylindrical structures with 60 µm diameter and 20 µm height, written on four differently orientated micro-grating regions. b) LCE cylinders deform along different axes (depending on the grating induced alignments) when exposed to a 532 nm laser radiation (10 W mm^-2). Scale bar: 100 µm. Please click here to view a larger version of this figure.

Figure 7: Light-driven Deformation of LCE Microstructures with Multiple Molecular Alignments. a) Schematic of two sections of opposite 90° twisted alignments in a single LCE stripe. b) and c) Optical images of a 400 µm long LCE stripe bending in opposite directions under 532 nm laser illumination (3 W mm^-2)⁸. Please click here to view a larger version of this figure.