Optical methods for nanoparticle detection and imaging

Thesis (Ph. D.)--University of Rochester. Institute of Optics, 2014.As the size of particulate matter decreases, individual particles are both more difficult to detect and show unique scattering properties. This thesis presents methods for optically detecting and imaging single nanoparticles and demonstrates the directional scattering properties of semiconductor nanoparticles. The first section describes a real-time interferometric method using multiple wavelengths to both detect and classify single nanoparticles in solution. A mixture of colloidal 60nm diameter Ag and 80nm diameter Au nanoparticles are identified within a 1 ms detection window. Detection sensitivity is limited by phase variations in the signal and two phase sensitive interferometric measurement techniques are proposed to improve the performance. In the second part of the thesis, GaAs nanoparticles are fabricated on a fused silica substrate using epitaxial lift-o and e-beam lithography. Recent theoretical work has predicted a suppression in the backscattering intensity in high refractive index semiconductor nanoparticles caused by interference between electric and magnetic dipole Mie resonances. Measurements of the fabricated GaAs particles show a suppressed backscattering at λ = 725nm agreeing with the modeled scattering properties and providing the first experimental verification of dipole interference in the visible spectrum. Potential applications for these highly directional scatterers include structured anti-reflection coatings and low-loss optical antennas for enhancing magnetic dipole interactions. Finally, a point-scanning confocal microscopy system is developed using a diamond turned ellipsoidal mirror, with a numerical aperture of ~1, as an illumination and collection objective. The focal properties are modeled using the Richards-Wolf integrals and alignment challenges are discussed. The imaging system wavefront performance is measured in an interferometer and the expected aberrated focal spot is simulated. A 40nm fluorescent nanoparticle is raster scanned through the focus to verify the measured performance. The tested system is the first confocal microscope implementing an ellipsoidal mirror as an imaging objective and provides high resolution, chromatic aberration free imagery of nanoparticles and nanostructures on opaque substrates. An additional spherical reference surface is proposed to monitor and simplify the system alignment