Directed Self-Assembly of Disordered Media Towards Optical Applications
Open Access
- Author:
- Miller, Jennifer Renee
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 10, 2021
- Committee Members:
- Zhiwen Liu, Outside Unit & Field Member
Christine Keating, Chair & Dissertation Advisor
Raymond Schaak, Major Field Member
Lauren Zarzar, Major Field Member
Philip Bevilacqua, Program Head/Chair - Keywords:
- Particle assembly
Lensless imaging
Random laser
Scattering
Electric fields
Particle synthesis
Directed self-assembly
Electrodeposition
Dielectrophoresis
Electro-orientation
Polarization
Disordered optics
Hybrid particles
Quantum dots
Tunable materials
Switchable functionality - Abstract:
- Electric-field directed assembly of particles provides a broadly applicable means of controlling particles orientation, migration, and local density in a dynamically reconfigurable way. The specific field response is manipulated by changing particle material, shape, and size, suspending medium, electrode geometry, and applied field conditions. As the particles behavior is tuned, their interaction with light, and the resulting optical effects are also modulated. Controlling light scattering is not a new idea as it has been used by nature extensively for plant and animal coloration, and in scientific explorations in areas such as light transmission in waveguides. However, random or disordered light scattering, though observed in nature, has only recently begun to be studied as this type of light scattering was considered to be deleterious in many traditional optical applications due to increased noise or other unwanted effects. When this disordered or random light scattering is exploited though, it unveils a wealth of fascinating optical phenomena such as that seen in random lasers and lensless imaging. Much of the research on disordered optics has been conducted on static systems, so achieving systems with tunable functionality is a relatively nascent area, but one that is quite necessary to expand understanding and applications in this field. This dissertation uses rational design of reconfigurable particle assemblies to exploit and modulate disorder, and subsequently optical functionality, in random laser and lensless imaging systems. Gold nanowires were synthesized through a templated electrodeposition method, and glass coated using a sol-gel method to induce electrostatic repulsion between particles and electrode surfaces. Planar electrodes were lithographically patterned and fabricated. Custom electrodes for three-dimensional assemblies were constructed using indium tin oxide coated coverslips. Alternating current electric fields were used to direct particle behavior. Optical microscopy was used to observe and characterize particle assemblies. Hybrid optical set ups were built and optimized to investigate lensless imaging and random lasing behavior. Gold nanowires were assembled between planar quadrupole electrodes and used as scattering masks for lensless imaging in Chapter 2. Other assembly motifs, including vertical assembly of gold nanowires and silicon dioxide spheres, were also investigated for their propensity to dynamically tune light scattering and enable high-resolution image reconstruction. We found that assembling the wires between the quadrupole electrodes produced large, bundled chains of nanowires whose directionality could be tuned by changing the applied field direction. Cycling between the unassembled and assembled states generated sufficient scattering data to enable multishot reconstruction which enhanced image quality and improved signal-to-noise ratios by up to 10- fold. The unique chaining structure of the wires resulted in a complex wavelet structural similarity as low as 0.36, and their submicrometer thickness resulted in a large optical memory effect yielding an angular field of view of ±45°. The assembly of titanium dioxide nanowires dispersed in rhodamine B fluorescent dye solution as a means of confining and amplifying light scattering for random lasing is investigated in Chapter 3. The orientation of the wires with respect to incident excitation was dynamically tuned, and the resultant effects on lasing emission was demonstrated. Orienting the wires such that their long axis was perpendicular to incident excitation resulted in a nearly 20-fold increase in lasing intensity compared to randomly oriented wires. This followed theoretical predictions which indicated a change of up to 22% in the scattering coefficient could be achieved for the experimentally determined nanowire length distribution upon alignment. The generality of this platform was also demonstrated by assembling and exciting metallic wires, hybrid metallic/silica wires, and vanadium oxide wires in dye solution, all of which generated random lasing emission. Chapter 4 built upon the platform demonstrated in Chapter 3 by investigating the effects of chaining gold nanowires dispersed in rhodamine B fluorescent dye solution between planar quadrupole electrodes on random lasing. Chaining the nanowires not only increased their physical scattering cross-section, and subsequently improved their ability to confine and amplify light resulting in increased lasing emission, but also induced polarization-dependent lasing effects. When the wires were randomly oriented, no lasing emission was observed, only fluorescence. However, when the wires were chained, then excited with linearly polarized light parallel to their directionality, the threshold for random lasing emission was lowered and the resultant emission was higher in intensity and frequency than when the wires were chained perpendicular to the incident polarization. This demonstrated the effects of tuning physical anisotropy as well as exploiting the optically anisotropic plasmonic effects in metallic wires, and provides a platform for embedding additional optical functionality such as nonlinearity or unique gain/loss profiles. Synthetic design of hybrid quantum dot/carrier particles is demonstrated in Chapter 5. Quantum dots were ligand exchanged to transfer them from being hydrophobic to hydrophilic. The hydrophilic quantum dots were then embedded in a silica shell coated on larger carrier particles of silicon dioxide, titanium dioxide, and gold, forming a particle “tool kit” for mixing and matching optical and assembly properties. The properties of the particles were characterized using optical and confocal fluorescence microscopy, scanning electron microscopy, and fluorimetry, and their assembly behavior was investigated. Preliminary optical investigations were also conducted exploring the propensity of these particles to be used in random laser studies. Overall conclusions and future directions are discussed in Chapter 6. Investigating optical functionality such as polarization and gain/loss profiles in random lasing is discussed, and suggestions of pertinent particles for these applications are offered. Using the quantum dot hybrid particles for assembly and optical applications is also described, and proposals of methods for enhanced optical tunability such as color tunability in random lasers is suggested. The work presented herein provides meaningful insight into tuning and eliciting optical response from disordered particle assemblies which has broad application in investigations of devices and materials with switchable optical functionality. Reconfigurable optical behavior is still a nascent field, and this dissertation provides methods of achieving optical modulation that is defect tolerant and robust, both of which are necessary for practical implantation of this technology.