By integrating wave-type analysis and fluctuation-dissipation theorem, the enhancement of photon tunneling distance in near field thermal radiation through metallic nanopatterns with/without dielectric structures is theoretically studied. When metallic patterns are in the immediate proximity of the conductive emitter, substantial thermal electric enhancement at surface plasmon frequency is observed between the metallic patterns and the emitter when the periodicity of the thermal electric field along the emitter surface is around integer times of the period of the metallic patterns. The mechanism of field amplification is similar to Fabry-Perot type resonance between two reflecting surfaces. The strong thermal electric field from resonance allows long-distance photon tunneling observed in near field radiation at a ~5 µm separation distance when the same metallic patterns are placed on the collector surfaces. This value is nearly 50 times longer than that with bared emitter surfaces. This long-distance photon tunneling can also happen at a broader range of parallel wavenumbers at the surface plasmon frequency when the periodic metallic patterns' sizes are different each period. However, increasing the range of parallel wavenumbers in long-distance photon tunneling with this approach can reduce the strength of photon tunneling. The reduced tunneling strength can be brought up by attaching high refractive index dielectric resonators on top of the metallic patterns through additional Mie-type resonance when displacement current is induced at the interface between the metallic patterns and the high refractive index dielectric.
A high numerical aperture (NA) scanning digital oil immersion lithography scheme is proposed and demonstrated in this study. To successfully conduct the scanning digital oil immersion lithography, immersion oil should be removed from the photoresist layer before the development process. Also, uniformity of the projected light patterns becomes crucial in the quality of this high NA photolithography. To solve these issues, we developed a cleaning procedure for the immersion oil and an intensity calibration scheme to achieve a highly uniform intensity distribution of the projected patterns. With these preparations, we were able to achieve 400 nm resolution large area patterning with the developed scanning digital oil immersion lithography system and a better than 200 nm resolution in the single line patterning. Also, with a double layered photoresist scheme and our lab-prepared photoresist, we successfully achieved large area lift-off patterns of 400 nm metallic dot arrays through the scanning digital oil immersion lithography system.
By placing high-index dielectric resonators on surfaces supporting surface plasmons in the near field, strong magnetic resonance can be observed in the high-index dielectric resonators with appropriate heights around the surface plasmon resonance frequencies. The strong magnetic resonance allows strong thermal photon tunneling across a 1 μm gap, which is one order longer than the previous demonstrations of near field radiation with surface plasmons. The thermal photon tunneling happens when the horizontal wavenumber is kx∼4πw with w is the width of the high-index resonators. The height of the high-index dielectric resonators should provide enough retardation of the electric field between the top and bottom of the resonator to form a displacement current loop. Therefore, similar magnetic field resonance occurs in the resonator when we triple rather than double the height of the high-index dielectric resonators. The usage of dielectric resonators to amplify the thermal electric field in the near field domain can be a potential method to increase the quasi-monochromatic radiation distance of an emission domain by one order or more at the frequencies of the surface waves.
Based on the microscale 3D point cloud projection with a digital micromirror device (DMD) and a microlens array (MLA) developed recently, we explore the capabilities of this specific type of 3D projection in 3D lithography with femtosecond light in this study. Unlike 3D point cloud projection with UV continuous light demonstrated before, high accuracy positioning between the DMD and the MLA is required to have rays simultaneously arrive at the designed voxel positions to induce two-photon absorption with femtosecond light. Because of this additional requirement, a new positioning method through direct microscope inspection of the relative positions of the DMD and the MLA is developed in this study. Because of the usage of a rectangular MLA, around four rays can arrive at each projecting voxel at the same time. Thus, to the best of our knowledge, a new algorithm for determining the pixel map on the DMD to the 3D point cloud projection with a femtosecond laser is also developed. It is observed that a very long exposure time is required to generate 3D patterns with the new 3D projection scheme because of the very limited number of rays used for projecting each voxel with the new algorithm. It is also found that 3D structures with desired shapes should be projected far away from the MLA ( ∼ 15 f to 30 f , with f being the focal distance of the MLA) in the 3D lithography with this femtosecond 3D point cloud projection. For patterns projected closer than 10 f , shapes are distorted because of unwanted voxels cured with the 3D projection technique using a DMD and MLA.
Through years of development, we have successfully demonstrated 3D light field lithography with UV continuous light. We recently combined this approach with femtosecond laser sources as two-photon femtosecond 3D light lithography. It is found that consistent results can happen under limited conditions with this direct combination. Our theoretical analysis reported last year shows that the experimental difficulty can be attributed to digital micro-mirror devices (DMD) and microlens arrays (MLA) used in the current 3D light field projection. Though they can control the propagation directions and interact at designed 3D locations, rays from such a system diverge with respect to the propagation distance. As a result, 3D voxel intensity in the 3D projection changes as a function of the separation distance with respect to the MLA in the 3D projection. To solve this problem, we replace the combination of DMD and MLA with a phase-controlled spatial light modulator. With a lab-developed algorithm, a single femtosecond laser pulse can generate up to a million sub-rays through the phase-controlled spatial light modulator. These sub-rays with a precisely controlled propagation direction can intersect at designed 3D locations as voxels for 3D virtual object constructions. Moreover, these sub-rays have minimum divergence angles to ensure that the voxel intensities are maintained consistently at each 3D location. We also demonstrated that versatile 3D patterns could be generated with two-photon femtosecond 3D light field lithography based on this innovative approach.
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