Figure 1. Reversible assembly of plasmonic nanoparticles with software lasers. Courtesy of Yuebing Zheng.
Nanoparticles, a bridge between bulk materials and atomic or molecular structures, are used in a wide variety of physical and scientific research. Nanoparticle research provides new applications both for industry and everyday life. A team at the University of Texas (UT) at Austin is doing interdisciplinary research to innovate optical nanotechnologies in health, energy, manufacturing and national security. According to Dr. Yuebing Zheng, Assistant Professor, Department of Mechanical Engineering, Materials Science and Engineering Program, University of Texas at Austin, “We make use of metal nanoparticles with high-efficiency optical heating to create a light-controlled temperature field, which can trap and manipulate colloidal particles, biological cells, and biomolecules in a low-power, versatile, and noninvasive manner. We also utilize the strong light absorption of plasmonic nanoparticles to obtain nanopatches with near-unity absorption, which can benefit the light harvesting in solar energy conversion.”
The team does research on the reversible assembly of plasmonic nanoparticles that can be used to modulate their structural, electrical, and optical properties. Common tools in nanoparticle manipulation and assembly are optical tweezers which are similar to regular tweezers because they can physically hold and move microscopic dielectric objects by using a highly focused laser beam to provide an attractive or repulsive force. However, optical tweezers require tightly focused and high-power (10–100 mW/μm2) laser beams with precise optical alignment. As described in Nature Photonics, the TACC team uses low-power optical tweezing to manipulate metal nanoparticles of a wide range of materials, sizes and shapes down to single-particle resolution.
As shown in figure 1, the team’s experiments use lower power software-controlled laser beams and turn the lasers on and off to assemble nanoparticles. To enable their research, the team performed computer simulations that revealed that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated local thermoelectric field over the plasmonic substrate.
While the team has their own 8-core work station, they prefer using the Texas Advanced Computing Center (TACC) Stampede and Stampede2 HPC Systems because simulations that would take up to a week on their workstation can be done in several hours on one of the supercomputers. “For example, a simple simulation to estimate the optothermal effect of the plasmonic substrate may take several months to complete the simulation part on our work station but we can finish it in two weeks using Stampede or Stampede2,” states Zheng.
Zilong Wu (PhD student in Zheng’s group) indicates, “The HPC system can also help us do ‘virtual experiments’, which provide very useful guidance to our experimental designs for plasmonic nanopatches and moiré metasurfaces for different applications including solar energy harvesting and molecule sensing. We recently used Stampede2 for some new work including plasmon-enhanced moiré chiral metamaterials which benefit the applications such as ultrasensitive sensing of chiral drug molecules and solvent impurities. Chiral molecules are building blocks of life. Analysis and separation of the chiral molecules with regard to their handedness are critical for applications in pharmaceuticals and space life detection. Using Stampede2 enables simulations with complex moiré structures, which require a huge amount of simulation resources due to their quasiperiodic patterns.”
Radiative Enhancement of Plasmonic Nanopatch Antennas
As described in the Radiative Enhancement of Plasmonic Nanopatch Antennas paper, the team is doing work on creating plasmonic nanopatch antennas (PNAs), which consist of metal nanoparticles and metal films with thin layers of dielectric materials sandwiched between them as spacers. They perform numerical analysis for four types of PNAs consisting of commonly used silver (Ag) nanoparticles (i.e., nanosquare, nanotriangle, nanodisk, and nanorod) on Ag thin films with dielectric spacers of refractive index n=1.4.
Figure 2 shows the simulated scattering spectra of four types of PNAs where the nanoparticles have the same lateral and vertical dimensions (i.e., 80 and 30 nm, respectively). The intensity of the spectra is normalized. The inset graphic shows a representative cross-sectional view of the electric field amplitude distribution in the spacer of a PNA.
Figure 2. Scattering spectra of four plasmonic nanopatch antennas. Courtesy of Yuebing Zheng.
“We comparatively analyze the effects of the nanoparticle geometry on the far- and near-field properties of PNAs and the radiative properties of dipole emitters at the PNAs. Based on our analysis, we suggest the preferred PNAs for different applications. Our study paves the way towards establishing the design rules for the PNAs for the optimal performance in the targeted application,” states Zheng.
Tunable Multiband Metasurfaces by Moiré Nanosphere Lithography Research
There is great interest in metasurfaces for flat optics, where the designed two-dimensional arrangements of plasmonic nanostructures exhibit intriguing light-manipulation capabilities beyond the individual nanostructures. With light-coupled coherent oscillation of the electron clouds in metals, plasmonic nanostructures have enabled numerous applications, including sensors, plasmonic lenses, light emitting diodes, and super absorbers. However, most of nanoparticle self-assembly methods can only fabricate simple periodic patterns of plasmonic nanostructures with a single type of shape and size, which limits the bandwidth, light-manipulation capability, and applications of the metasurfaces.
The team’s work using moiré nanosphere lithography (MNSL), which utilizes two stacked layers of polystyrene (PS) nanospheres as etching and metal deposition masks, provides a cost-effective, high-throughput method to create metasurfaces with moiré patterns. Moiré metasurfaces that feature a large number of component sets and high rotational symmetry are promising candidates to achieve broadband optical responses.
Hardware used in the TACC Nanoparticle Research
Stampede and its successor Stampede2 at TACC are some of the most powerful supercomputers in the U.S. for open science research. Stampede2 entered full production in fall 2017 as an 18 petaflop system that builds on the successes of the original Stampede cluster it replaces. Stampede2 features 4,200 Intel Xeon Phi nodes, the second generation of processors based on Intel’s Many Integrated Core (MIC) architecture, and 1,736 Intel Xeon Scalable processor nodes.
Stampede2 Specifications:
- Strategic national resource designed to serve thousands of researchers across the nation
- 18 petaflops at peak performance
- 4,200 Intel Xeon Phi nodes, each with 68 cores, 96GB of DDR RAM, and 16GB of high speed MCDRAM
- 1,736 Intel Xeon Scalable processor nodes, each with 48 cores and 192GB of RAM.
- 100 Gb/sec Intel Omni-Path network with a fat tree topology employing six core switches
- Two dedicated high performance Lustre file systems with a storage capacity of 31PB
- TACC’s Stockyard-hosted Global Shared File System provides additional Lustre storage
Nanoparticle Results on Work Station and Stampede HPC Systems
According to Dr. Linhan Lin (postdoctoral associate in Dr. Yuebing Zheng’ group), “We used the HPC systems for various simulations,” including:
1. studying the photothermal effect of the plasmonic materials for materials optimization
2. simulating the thermal convection and thermophoretic flow in the opto-thermofluidics to understand the working principle
3. design of the optothermal potential for versatile optothermal manipulation
Figure 3 shows the performance of plasmonic nanopatch simulation on three systems, using their own work station in their lab (8-core desktop), Stampede supercomputer (full usage of Development mode), and Stampede2 supercomputer (full usage of Development mode). The time required to run simulations drops drastically, especially when running on the Stampede2 HPC system, allowing the team more time to focus on their nanoparticle research.
Figure 3. Processing time required for TACC nanoparticle research on three computer systems. Courtesy of Zilong Wu, UT Austin.
Challenges for Future Nanoparticle Research
The challenges the team faces in their research includes:
1. how to design the opto-thermofluidics to achieve thermophilic migration of colloidal particles or biological cells, such as, migration from the cold to the hot regions.
2. how to design the substrate and optics to obtain the temperature gradient for versatile manipulation.
3. how to design the plasmonic nanopatches and metasurfaces with broadband or multiband absorption to match the required wavelengths of different applications including solar energy harvesting and biomolecular sensing.
According to Zheng, “Our future work will focus on the diverse applications of the plasmon-enhanced thermophoretic tweezers. For example, we will use the thermophoretic tweezers to assemble different colloidal optical devices for optical sensing and lasing. We will make good use of HPC systems to simulate the optical performance of our devices, to study the mechanism, and to optimize the configuration”
Linda Barney is the founder and owner of Barney and Associates, a technical/marketing writing, training and web design firm in Beaverton, OR.
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