Biomimetic Photonics and Nano-Lithography
February 4, 2014, Photonics West Conference, Nano/BioPhotonics, San Francisco—Min Gu from Swinburne University presented his efforts in direct laser writing (DLW) to create materials that mimic biological materials. This construction can supply new characteristics to existing materials.
In the biological world, many organisms have color, but the colors are not pigmentation. The colors are due to micro-and nano-structures that involve chirality of gyroid networks to generate photonics. For example, the green butterfly uses its color to camouflage itself and to communicate with others of its species.
The butterfly wings are gyroid networks composed of a 3-D minimal surface with a net curve of zero. This crystal structure has a lattice spacing of 335nm and an 18 percent fill. When viewing the butterfly wings, speckles from shorter wavelengths like blue are due to irregularities and iridescence. The wings have 550nm peak, which matches spring grass, and reflect light in both right and left circular polarization.
Gyroid shapes
http://mathworld.wolfram.com/Gyroid.html
Characteristics of gyroid networks are 3-D periodic surface that is the simplest cubic chiral ( has left- or right-handedness) network. It is fully interconnected and mechanically stable. The intertwining networks are suitable for post-processing; such as metallization, and are functional in many areas including fluidics.
It is possible to construct a gyroid using a 3-D printer, but the process is very slow, due to the physical complexity. The 2-D layers have voxelizatoin issues as well as excessive surface roughness. Direct laser write using 2-photon polymerization is a better choice for such 3-D surfaces. The laser(s) are able to follow the trajectory of the structure.
One example 3-D model had a lattice period of 3 microns. It needed a sacrificial border for clean-up and post processing. When they processed this model, they found that the laser beam is not spherical, but ellipsoidal, causing the patterns to have more thickness than desired. by dithering the laser beam at 0.5 to 2kHz, they formed a spherical area, which is preferable to an ellipsoid.
Applications for this material include beam splitters for circularly polarized beams. One test device had 768k rods and showed that their theories and physical implementations matched. The material separated the right- and left-hand beams. It is possible to tune the chiral materials by adding an achiral network to extend the bandwidth.
Their tests showed that the geometric and optical properties do not correlate. A chalcographic glass showed a refractive index of 2.5 with adaptive optics and acts as a spatial modulator after the aberrations. Connections show good wide-band results.
The butterfly lattice constant is 300nm, but the diffraction law predicts limits in the far field. Therefore they could only achieve a spot size of 0.61 ?/n.a., much larger than desired. They worked on a 2-beam system with one beam for photo induction and the other as a photo inhibitor. The resulting super-resolution spot is due to the combination of a dark-field and light-field illumination. Now the work is to improve sensitivity and mechanical strength.
Theory predicts that they should be able to achieve a spot size of 0.03 ?/ 30 for very high resolution. Using such a laser beam should allow for a free-standing 3-D structure. Tests have gotten parallel lines with 9 nm pitch and the 2-line resolution is able to resolve 50 nm. the depth is down to 100 nm from the previous depth of over 300nm. These changes are necessary to get mechanical strength and low surface roughness. Without the inhibition beam, the best they could achieve was a 1-micron optical limit for resolution.
For a small sample, 12 x 12 x 3 microns, the setup takes about 3-hours processing time. For a “real” size sample, this might take over 6,000 hours. Work is proceeding on parallel laser beams for fabrication, but has issues with the Weyl point in 2-D systems for spin coupling.
2-D systems have shown a Dirac point in graphene. The 3-D gyroid photonic studies have a Weyl point that creates pseudo-spin when the symmetry is broken. They measured the Weyl point in a material with broken symmetry and discovered new material properties. Three are many other 3-D hierarchical nanostructures in nature. The tree shrew has retinal cones that are gyroid. Someone needs to investigate if these are for UV filtering or aberration-less waveguides.
Their work shows lots of potential for circular polarization evaluation and processing. Photonic crystals can be constructed with meta-materials with appropriate scaling. The structure and optical chirality can be exploited to get non-linear chirality with a high-refractive index. In theory, even a negative index is possible. Applications include solar cells, energy storage, and photo-catalysts.
