Dissertation Title:
“ The physics of nanoaperture optical trapping: design, fabrication and experimentation ”
Abstract:
The last 20 years have seen an explosion in the use of nanotechnology in almost every research and industrial setting. This development has required the invention of new and revolutionary characterization tools including but not limited to zero-mode waveguides, interferometric scattering, fluorescence resonance energy transfer, super-resolution microscopy, and etc. However, the invention of a fast-detection single-particle method that can measure the physical characteristics of sub-10 nm particles (including molecules) without steric hindrance has been challenging. One such technique that has become important in this field is optical trapping, which, due to its reliance on the gradient force, cannot trap single nanoparticles at low powers. Recent progress in nano optics, spurred by progress in nanofabrication, has allowed us to overcome these challenges. We use surface plasmon polaritons to break the optical diffraction limit and squeeze the photon energy into a local hot spot. The small mode volume of a plasmonic antenna or nanoaperature significantly enhances the local field and can be designed to resonate at a desired wavelength. Therefore, the cut-off wavelength of the nanoaperture is increased and varies with the aperture geometry.
By designing, fabricating, and testing these nanoapertures we can trap single nanoparticles with significantly reduced laser power by measuring the monochromatic transmission change of a resonant aperture. A freely diffused nanoparticle, behaving like a dipole antenna, interacts with the nanoaperture when trapped and shifts the resonance of the nanoaperture. By only monitoring a single wavelength, the presence of the particle changes the transmission signal. The effect of particle-induced transmission spectrum shift is called the self-induced back-action effect. This particle-induced spectrum change increases the transmission amplitude and variance once trapped. Furthermore, the monochromatic transmission measurement is a faster detection method than the spectrum measurement. It is able to follow up the diffusion, folding or conformation change of the trapped particle.
In this work, I first optimize the plasmonic antenna which has a double nanohole (DNH) geometry in a gold film with COMSOL Multiphysics. The nanoapertures are designed to have a transmission resonance around 1064nm or 1550nm. In order to fabricate the nanoaperture with a 20nm feature in a 100nm-thick gold film, I develop a sample preparation process and use helium ion beam lithography. With the sample preparation procedure, including thermal evaporation, annealing and template stripping, the DNH can be milled inside one single grain with a clear structure outline. The template stripping technique also helps to decrease the surface roughness of the film to the atomic level. With this sample, the characterization of its spectrum after the nanohole is fabricated can be investigated. The experimental spectrum strongly agrees with the simulation result, indicating a successful fabrication process. The finished DNH nanoaperture also succeeds in trapping 5nm gold nanoaperture and 6.9nm quantum dots with a low power intensity <3.4mW/µm2 in the resonant self-induced back-action condition. This ultra-low intensity trapping method validates nano-biomolecule observation in nanoaperture traps. The demonstration also shows the different signs of amplitude change are due to the detuning between the nanoaperture and the laser wavelength. The detuning protects the particle from strong near field enhancement. And also, it proves the NAOT signal is detectable even if off resonance. Furthermore, an observation of a ‘negative polarizability’ in a trapped quantum dot is demonstrated using a nanoaperture close to the particle size. The nanoaperture-enhanced optical field excites higher order polariton modes in the quantum dot which leads to more absorption, potentially explaining this phenomenon. These results: simulation of the double nanohole nanoaperture to optimize its antenna resonance, successful nanofabrication of the double nanohole nanoaperture with feature sizes less than 20 nm in 100 nm gold film, and successful self-induced back-action trapping of a single 5 nm gold nanosphere and 6.9 nm quantum dot has created a solid foundation for the nanoaperture optical trapping of sub-10nm particles and subsequent physics and biophysics studies of nanoparticles and protein biomolecules.
Committee in Charge:
Dr. Ryan M. Gelfand, Chair
Dr. Kyu Young Han, Co-Chair
Dr. Stephen M. Kuebler
Dr. Ellen Hyeran Kang
Read More