Plasmonic nanostructures and nanoparticle assemblies with spatial symmetry breaking interact with right- and left-handed circularly polarized (RCP and LCP, respectively) lights differently. The difference between absorption of RCP and LCP by these nanostructures is called circular dichroism (CD). Compared to their molecular counterparts, broken symmetry nanostructures demonstrate orders of magnitude higher CDs, making them suitable for promising applications in photonic devices, such as circular polarizers and devices with negative refractive index, as well as in pharmaceutical sciences for enantioselectivity enhancement of chiral molecules. Although CD is useful information in characterization of these devices, since it is a far-field quantity, it cannot retrieve the underlying near-field interactions. Understanding the near-field interactions, however, is necessary in designing novel photonic devices with different applications. In this thesis, we investigate the near-field circular dichroic forces on plasmonic nanoparticle assemblies at nanoscale using photoinduced force microscopy (PiFM). We first introduce a PiFM technique based on gold nanoparticles (AuNPs) and silicon tip. Conventionally, gold-coated silicon tips are used in PiFM, which causes some anisotropy-induced distortions in the images because of the irregularities of gold grains on the silicon tip. This is especially important in CD studies as these asymmetries can create fake signals. Using AuNPs and silicon tip, we demonstrate much more accurate and distortion-free images, as well as an order of magnitude enhancement in signal-to-noise ratio. We then use this PiFM technique to study the near-field circular dichroic forces on different AuNP assemblies. We show that while two different mirror-symmetric (structurally achiral) assemblies can have zero CD, their near-field circular dichroic force maps can be different depending on their rotational symmetry. We finally demonstrate a metasurface, made of broken symmetry ramp-shaped nanostructures, which exhibits giant CD at the visible frequencies using a simple fabrication technique based on gradient focused-ion beam milling, addressing complicated fabrication challenges to create 3-D asymmetric nanostructures.

Dissecting the regulation of gene expression processes is fundamental to understand how cells function. Techniques used to obtain our current view of gene expressions rely on isolating mRNAs from large numbers of cells, which often associates with a loss or damage of the material, as well as the loss of spatial information. Also, individual cells within a population are unlikely to behave all in the same way, and current standard techniques are unable to detect cell-to-cell differences that can result from genetic variation, biological noise and different characteristics of genes within a population.

To solve these limitations, we developed a sensitive and non-destructive method and apparatus for tracking gene expressions on single cell level within a population of cells, either on a collagen cell culture gel or in a microfluidic environment. The developed bench-top instrument utilizes a modified AFM (Atomic Force Microscopy) probes for in situ extraction of mRNA molecules from single cells. The modified coaxial AFM probe serves as nanotweezer, and wherein the application of an alternating potential between the inner and outer electrodes of the co-axial cable creates a dielectrophoretic force for attracting target molecules toward the tip-end. For adherent cell lines, the samples are cultured on collagen gel for select-and-probe analysis. An integrated microfluidic/nanoprobe platform is also developed for gene expression analysis of suspension cells on single cell level.

This system directly targets and samples down to a few molecules within a single living cell, without the need for purification and averaging typically of conventional technologies. By providing reliable and exceptional sensitivity to identify differences between individual cells in a seemingly homogeneous population, the system creates possibilities for assaying the genomic analysis in living cells and tracking them in response to external stimuli. This technique has potential impacts on understanding the heterogeneity of transcriptional responses as well as its implications for cell function and disease. It is going to have broad application areas ranging from systems biology to cancer research.

ABSTRACT OF THE THESIS

Numerical Simulation of Photoinduced Force Microscopy

By

Mohammad Almajhadi

Master of Science in Electrical and Computer Engineering

University of California, Irvine, 2016

Professor H. Kumar Wickramasinghe, Chair

In this thesis we introduce a method of imaging: photoinduced force microscopy (PiFM). In this method, the images are accomplished by detecting tip-sample force at the near field which is optically excited. The origin of this force is related to the dipole momentum of the tip and its image in the sample. The dipole momentum strength is function of the polarizability of the sample under test; the polarizability is a wavelength dependent. Therefore, driving sample at resonance will give higher force than off resonance. This polarizability contains the spectroscopic information about the sample. By tuning the excited laser over broad range of frequency spectra, sample can by identified by comparing the resulting force measurement to a well-known absorption curve. PiFM Spatial resolution is well below 10nm and it is limited only by the tip geometry. This is because the field is spatially confined and the gradient of tip-sample force is proportional to z-4 where z is the distance away from the tip. Finite Element Method (FEM) is utilized to calculate the force gradient, and the tip-sample force.

The probe is attached to the end of a cantilever vibrating at one or two mechanical frequencies. Simultaneously, topographic and optical imaging process can be done by detecting them at different mechanical frequencies. In fact, the force will shift the mechanical frequencies, and this shift will be detected through quad-photodetector and the extracted signal will be translated into an image.