Laser Biomimetic Fabrication Based On Multi-beam Interference

As a result of the evolution process of billions of years, the objects in the nature have adopted the best designed structures from nanoscale to macroscale for performing various fascinating functions. Since the prehistoric times man has considered the nature as its best teacher and have learnt many skills inspired from the phenomena found in biological world.’Biomimetics’ is an emerging field of science and engineering in which new materials and processes are produced by adopting nature as a model and mentor for the source of inspiration. By using the art of biomimetics many multiscale and multifunctional natural materials have been produced artificially. The natural surfaces with integrated structures and functions are often organized in a hierarchical manner. The self-cleaning surface of lotus leaf, the reduced drag surface of a shark skin, the anisotropic wetting surface of a rice leaf, the highly adhesive and superhydrophobic surface of rose petal, the water repellent legs of a water strider, the anti-fogging compound eyes of a mosquito, the reversible adhesion of a Gecko’s foot and structural colors in wings of moths and butterflies, all these multifunctional structures are a result of hierarchical arrangement of simple patterns.In recent past, various techniques and methods have been employed to produce the mimics of natural multifunctional materials in the lab. But to find a simple, cost effective, rapid and mask free technique to produce the biomimetic structures and functions have always been a challenge. Multi beam Laser interference lithography is an emerging technology that has the capability to produce1D,2D and3D structures with different geometries on a variety of substrates. It is a mask free technique that provides the flexibility to tune the geometries of fabricated structures by controlling the process parameter like laser wavelength, period, polarization, pulse energy, exposure time and development time etc. In this thesis, we have explored the potential capabilities of various configurations of multi beam Laser interference lithography and have utilized it for the fabrication of large area SERS substrates and biomimetic multiscale hierarchical patterns with different properties.Firstly, in this thesis we have investigated the potential applications of multi beam interference technology by using it in different configurations. A variety of structures using single and multiple exposures of two and four beam interference setups have been studied. Also we explored the effect of different process parameters on the geometries of fabricated patterns. Very uniform1D Bragg’s grating structures are fabricated using the single exposure of dual beam interference. The period of the resulting pattern can be easily controlled by altering the laser wavelength and incident angle of two beams. It is found out that for a positive tone photoresist by increasing the exposure dose a there is a gradual decrease in the duty cycle. Using the multiple exposure of two beams interference more complex geometrical structures can be achieved. A double exposure at a rotation angle of0°and90°form a cubic structure whereas a three time exposure at angles of-60°,0°and60°produces a hexagonally packed2D pattern. By using different periods during the multiple exposures the hierarchical patters can be fabricated. This versatile technique of period-modulated multiple exposure of two beam was employed to fabricate a large variety of hierarchical structures.Four beams interference configuration have the capability to produce2D pattern with only a single exposure. It was investigated that in addition to laser wavelength and incident angle of the interfering beams the geometry of the resulting pattern could also be tuned by exposure dose ratio of beam pairs in four beam interference. By using different intensity ratios of the beam pairs anisotropic patterns of2D pillars array were produced. We also illustrated by experimental results that by using the multiple exposures of four beams with the displacement introduced in the sample smaller than the period, a second pattern can be printed within the period of the first pattern. By gradually tuning the displacement introduced during the two exposures of four beams we achieved the optimum value for which the period of the resulting pattern is reduced by exactly half. The MATLAB simulation results based on the mathematical model of multi beam interference were found in close comparison with most of our fabricated structures.Secondly, in this study we applied the Laser interference lithography for the fabrication of large area SERS substrates. The enhancement of the Raman signals is achieved when the molecules of an analyte are adsorbed on a metal surface with nano scale roughness. The localized surface plasmons are produced when Raman excitation laser interacts with metallic surface structures to produce the coherent electronic oscillations. The amplification of the Raman signals occurs when these surface plasmons interact with the analyte molecules which are placed at a very close proximity to the area of generation of local electromagnetic field. To fabricate the large area substrates two beams interference setup with the substrate fixed on a XY translational stage was employed. Using the graphical programming environment of LABVIEW software the XY translational stage and optical shutter were interfaced with the computer. The program was designed to scan the substrate in a zigzag profile in two different scanning modes. The stepwise scanning mode provided an easy optimization of exposure dose but a large processing time, whereas in continuous scanning mode rapid patterning of large area was possible with the exposure dose dependent on the scan speed. By using the continuous scanning mode four different periods (300,400,515and600nm) very fabricated uniformly over a large of2.5x0.5cm2to be used as SERS substrates. These large area substrates were later coated with a layer of silver nano islands by vacuum evaporation process. The thin layer (～15nm) of silver resulted in the formation of metal nano islands with nano-gaps which provided a high density of ’hot spots’ to enhance the Raman signals.The SERS spectra measured for Rhodamine6G (R6G) molecules revealed the tuning effect in the Raman signals by varying period of the gratings structure. It was observed from the measured spectra that the highest amplification of SERS signal is achieved for a period of515nm. This can be attributed due to the fact that Surface plasmons from the silver island gratings at515nm are matched with the Raman excitation line. This gives rise to very strong Raman signals for the analyte molecule adsorbed on the SERS substrate. The MATLAB simulation based on FDTD method for the distribution of electric field component (Ex) on silver islands coated gratings substrate showed similarity with our experimental findings. The stability and reproducibility of the large area SERS substrate with silver nano islands coating was also investigated by measuring the SERS spectra at different sites on each of the large area substrate. Almost constant SERS spectra intensity of R6G was detected from different randomly selected spots on each of the large area substrates with different periods. The large area SERS substrates fabricated by laser interference lithography are expected to find applications in array sensors applications.Thirdly, in this thesis we employed the period modulated multi exposure of two beam interference to fabricate biomimetic hierarchical structures. As compared with other technologies used to mimic the biological structures our methodology have the advantage that it is maskless and offers the flexibility to change the geometrical arrangement and axial symmetry of both micro and nano scaled surface patterns effortlessly. Another salient feature of our technique is that within a time of few seconds significantly large area (-9x9mm2) can be processed which enables the possibility of mass production rapidly. To change the period during multiple exposures an experimental setup was designed with two beam paths with the flexibility to change the beam paths easily in order to change the incident angle. The two beam paths were designed so that one period in the micro scale range i.e. primary pattern, and other period in the nano scale range i.e. secondary pattern could be obtained. During the multiple exposures the substrate fixed on a rotating table could also be rotated at different angles to fabricate the structures with different geometrical symmetries.A variety of biomimetic hierarchical structures were fabricated by selecting different combinations of primary and secondary patterns.1D gratings,2D cubic pillars and2D hexagonal pillars were fabricated at both micro and nano scale levels in a hierarchical structure by single, double and three times exposure, respectively. The two level multiscale surfaces with300nm structures superimposed on3μm patterns were successfully fabricated. To add more diversity to the fabricated structures we also introduced the phase shift between the axis of primary and secondary patterns. This was simply accomplished by varying the position of the initial exposure angle while fabricating the two patterns at different scales. We further elaborated the capability of our technique by fabricating three level hierarchical structures using three different scale periods i.e.4μm,1μm and300nm during the multiple exposures on the same substrate.We also studied the nature-like properties exhibited by our fabricated hierarchical structures. It was found that our lab made structures possessed the biomimetic functions of super hydrophobicity, high adhesion, structural colors and optical properties of directionality and polarization. Similar to the natural plant leaves super hydrophobic surface with contact angles more than153°were characterized. Because of nano and micro scales roughness features the water droplets were found to follow the Cassie-Baxter model of wetting surfaces. It was also found that surface with1D nano scale features exhibited more adhesion to the droplets of water by filling in some roughness features and hence possessing an intermediate state between Cassie-Baxter and Wenzel.Due to diffraction from the regular patterns of multiscale gratings patterns beautiful colors of white light were seen from the naked eye. The iridescence colors from violet to red were observed which looked very similar to the structural colors shown by various insects and flowers in the biological world. The regular diffractive surface microstructures were also associated with the directional and polarization effects exhibited by several biological species. The special case of the moth Trichoplusia orichalcea was studied and the arrangement of multiscale structures on the wings of the moth was found very similar to our lab made hierarchical structure. The optical characteristics exhibited by our biomimetic wing structure were found to be in close comparison with the natural moth wing. The dependence of diffraction Intensity on the orientation angle in both directions showed that the biomimetic wing structure can act as a nonreversible polarizing element. Also for all the incident angles it was found that S polarized light has significantly more diffracted power than the P polarized. So under the illumination of un-polarized natural light the structure can act as a linear polarizer mimicking its analogous natural wing of the moth. The biomimetic hierarchical structures fabricated by period modulation of dual beam interference can find potential applications in disciplines like microfluidics, antifouling, decorative elements, colored jewelry, micro/nano optics, polarizing filters and so forth.