Producing Nanoscale Laser Spot and Its Applications
Driven by the exponential growth in the field of nanotechnology in the last few decades, there has been a huge impetus in the design and production of subwavelength nanoscale laser spot which has found wide range of applications in different fields such as nanofabrication and data storage, among others. Limited by the diffraction limit of light when using conventional optics, generally metallic nano-apertures and nano-antennas are used for producing sub-100 nm spots. Design of such types of nano structures typically involves the use of surface plasmons to effectively collect and concentrate light below the diffraction limit. This work discusses the design and performance of several types of nanostructures to produce a nanoscale hotspot and their applications in different fields are also studied both experimentally and numerically. First we discuss the bowtie aperture and its light focusing performance and enhancement in the near field. Experiments were conducted to validate its application in near field optical lithography. Using a massive array of bowtie apertures, we have performed scanning optical lithography experiments with high precision gap control mechanism with the help of an Interferometric Spatial Phase Imaging (ISPI) system. We successfully demonstrated simultaneous writing of more than one thousand patterns with resolution less than 50 nm. Further, a novel type of cross sectional ridge waveguide nanoscale aperture is introduced and designed. Rather than using a sequential fabrication technique, layer-by layer fabrication method is used to make these nanostructures which ensures a very fine feature capable of producing a very tiny hot spot. We illustrated the performance of these apertures by scattering near field scanning optical microscope (s-NSOM) which show good near field localization characteristics. Next, we look at another emerging application of these nanoscale hotspots, in the field of data storage, where heat assisted magnetic recording (HAMR) is widely thought to be one of the next generation technologies to achieve high density data storage. We studied the optical performance of several types of apertures and antenna, also called near field transducers (NFT) in the HAMR terminology, including the bowtie aperture, E antenna, triangular antenna, C-aperture and the lollipop antenna in the presence of the recording medium. Subsequent thermal performance of the recording medium and the NFTs are calculated and several thermal figures of merit are established. Some design strategies and simple modifications of the NFTs are discussed which aim at improving the performance of the NFT like introducing a taper in its geometry. Also, it was found that changing the working wavelength of these NFTs from the typical 800 nm to longer wavelengths can increase the thermal performance of the HAMR system. Other nanostructure designs capable of generating similar hotspots are explored further such as a split-ring resonator (SRR) type of nanostructure. In addition to plasmonic resonance peaks, SRR has been shown to possess LC-circuit type of resonance in the infrared and optical frequency range which can help in generating hot spots in different wavelength range. These nanostructures are fabricated and characterized with the help of s-NSOM. Apart from generating sub wavelength focused spots using nanostructures, it has been found that arrayed nanostructures can also be used to enhance the force at the nanoscale and one can achieve a larger electromagnetic force acting on the metallic sample than is possible without the nanostructure. In the final part of the work, we experimentally verify and measure the enhanced force on a metallic surface due to presence of resonant slots on the surface. Experiments are performed to measure the deflection of a thin membrane under the effect of an incident laser both with and without the slots and results are compared and it is found that depending on the dimension and geometry of the slots, enhanced pushing force as well as pulling force can be observed.
Xu, Purdue University.
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