High precision dynamic alignment and gap control for optical near-field nanolithography

The authors demonstrate the use of interferometric-spatial-phase-imaging (ISPI) to control a gap distance of the order of nanometers for parallel optical near-field nanolithography. In optical near-field nanolithography, the distance between the optical mask and the substrate needs to be controlled within tens of nanometers or less. The ISPI technique creates interference fringes from checkerboard gratings fabricated on the optical mask, which are used to determine the gap distance between the mask and the substrate surfaces. The sensitive of this gapping technique can reach 0.15 nm. With the use of ISPI and a dynamic feedback control system, the authors can precisely align the mask and the substrate and keep variation of the gap distance below 6 nm to realize parallel nanolithography. (C) 2013 American Vacuum Society

Optical nanolithography with λ/15 resolution using bowtie aperture array

We report optical parallel nanolithography using bowtie apertures with the help of the interferometric-spatial-phase-imaging (ISPI) technique. The ISPI system can detect and control the distance between the bowtie aperture, and photoresist with a resolution of sub-nanometer level. It overcomes the difficulties brought by the light divergence of bowtie apertures. Parallel nanolithography with feature size of 22 ± 5 nm is achieved. This technique combines high resolution, parallel throughput, and low cost, which is promising for practical applications.United States. Defense Advanced Research Projects Agency (Grant N66001-08-1-2037)National Science Foundation (U.S.) (Grant CMMI-1120577

Efficient subdiffraction focusing of light using zone plate and bowtie aperture

This work proposes an approach to produce high efficient subdiffraction spot especially for selective laser chemical vapor deposition (LCVD) and lithography applications. Our study indicates that Fresnel zone plate (FZP) operating in near field is different from that in the far field. The scalar methods accurately calculate the FZPs\u27 diffraction in far field, but do not capture the polarization-dependent behaviors in the near field. We present the vectorial diffraction theory to correctly predict the focus spot of the FZPs in the near field. In addition, plasmonic zone plates were studied and optimized for subdiffraction focusing. Surface plasmon polaritons (SPP)-related phenomena were explained using analytical solutions and numerical methods. Optimizations of bowtie apertures for highest transmission were discussed. Additional concentric grating grooves surrounding the bowtie aperture on the incident side boost its transmission and on the exit side improve the confinement. Nevertheless, the enhancement from the bowtie alone was insufficient for the LCVD application. A phase FZP was integrated in the mask to provide adequate power at the bowtie output. The fabrication process for a mask containing FZP and bowtie were given. FZP is fabricated by electron beam lithography and requires extensive characterization. Bowtie aperture can be made for a smaller optical spot with inverted fabrication. Finally, a demonstration of this near-field mask requires nanometer-level alignment method. Interferometric-spatial-phase-imaging (ISPI) was implemented to both the LCVD and the lithography setup. The ISPI has detection sensitivity better than 1 nm. Gap alignment between mask and substrate can be well maintained within 20 nm range for the current system noise of 15 nm

Ridge aperture antenna array as a high efficiency coupler for photovoltaic applications

Weak absorption of light near the absorption band edge of a photovoltaic material is one limiting factor on the efficiency of photovoltaics. This is particularly true for silicon thin-film solar cells because of the short optical path lengths and limited options for texturing the front and back surfaces. Directing light laterally is one way to increase the optical path length and absorption. We investigate the use of a periodic array of apertures originated from bowtie aperture antennas to couple incident light into guided modes supported within a thin silicon film. We show the presence of the aperture array can increase the efficiency of a solar cell by as much as 39%. (C) 2011 Society of Photo-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3644613

Improving near-field confinement of a bowtie aperture using surface plasmon polaritons

Bowtie aperture is known to produce subdiffraction-limited optical spot with high intensity. In this work, we investigate integrating a bowtie aperture with circular grooves to reduce the divergence of the near-field produced by the bowtie aperture. Numerical results indicate that surface waves reflected from circular grooves improve the field confinement of a bowtie aperture along the polarization axis. These circular grooves with period near half the wavelength of surface plasmon polaritons reduce the spot size by as much as 40% at distances between 20 and 100 nm from the surface and create a more symmetrical optical spot. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3595412

Optimization of modified volume Fresnel zone plates

Modified volume Fresnel zone plates (MVFZPs) fabricated with laser direct writing were optimized for higher diffraction efficiencies. The Fresnel radii in each layer of a volume zone plate were iteratively adjusted by a simulation-based direct search optimization. The results show that optimization is effective but depends strongly on the starting diffraction efficiencies determined by the MVFZP parameters. The simulations indicate that the optimized MVFZP can achieve 93% diffraction efficiency

Beam propagation modeling of modified volume Fresnel zone plates fabricated by femtosecond laser direct writing

Light diffraction by volume Fresnel zone plates (VFZPs) is simulated by the Hankel transform beam propagation method (Hankel BPM). The method utilizes circularly symmetric geometry and small step propagation to calculate the diffracted wave fields by VFZP layers. It is shown that fast and accurate diffraction results can be obtained with the Hankel BPM. The results show an excellent agreement with the scalar diffraction theory and the experimental results. The numerical method allows more comprehensive studies of the VFZP parameters to achieve higher diffraction efficiency

Extraordinary transmission from high-gain nanoaperture antennas

This letter describes a bowtie nanoaperture antenna for coupling light to a subdiffraction limited near-field spot (\u3cλ/8). The gain of the antenna is increased using a concentric grating structure to coherently diffract normally incident light toward the aperture. We experimentally demonstrate that the addition of the grating structure enhances the far-field transmission through the aperture by 6.9 times while the intensity at the near-field is increased more than 15 times. The nanoantenna is useful for applications including nanolithography and data storage

Real-Time Quality Monitoring of Laser Cladding Process on Rail Steel by an Infrared Camera

Laser cladding is considered to be a highly complex process to set up and control because it involves several parameters, such as laser power, laser scanning speed, powder flow rate, powder size, etc. It has been widely studied for metal-part coating and repair due to its advantage in controllable deposited materials on a small target substrate with low heat-affected distortion. In this experiment, laser cladding of U75V and U20Mn rail steels with Inconel 625 powder was captured by an infrared camera with image analysis software to monitor the laser cladding process in order to determine the quality of the cladded substrates. The cladding temperature, thermal gradient, spot profile, and cooling rate were determined from infrared imaging of the molten pool. The results showed that cladding temperature and molten pool’s spot closely related to the laser cladding process condition. Infrared imaging provided the cooling rate from a temperature gradient which was used to correctly predict the microhardness and microstructure of the HAZ region. This approach was able to effectively detect disturbance and identify geometry and microstructure of the cladded substrate