Past colloquia and seminars


DNA-Templated Optical Nanoantennas for Single-Molecule Emission Modulation

Academic or specialist Thesis defense

Optical antennas can modulate light-matter interaction by concentrating free propagating light into a localized environment or by promoting energy transmission from localized emitters to free space. Many different optical antennas have been designed and fabricated to manipulate excitation and emission processes of emitters. However, most of the works were performed at the ensemble level or in a diluted bulk solution of emitters due to a lack of a well-controlled technique for positioning and manipulating the number of emitters in the proximity to optical antennas. The bottom-up self-assembly technique of DNA origami has allowed for precise control over the positioning of single emitters, as well as the specific structure of antennas, with nanoscale precision and stoichiometry. Single emitters are considered as one of the most excellent candidates for single photon sources. However, they exhibit some drawbacks, such as low emission efficiency resulting from size mismatches between electron confinement and emission wavelength, wide emission solid angle arising from electric dipole[1]like behavior, and broad emission spectra due to the existence of multiple vibronic levels. The presence of an optical antenna can address these issues and facilitate applications based on the emission of single emitters. In this thesis, we focus on the synthesis of antenna assemblies using a DNA origami template to tune the angular and energy distribution of the emitted photons. The first chapter introduces fundamental theories of optical processes to control the final intensity, spectrum, polarization, radiation pattern, and lifetime of general dipole emitters. Basic scattering theory of nanoparticles is presented to describe the interaction of optical antennas with incident light. An energy transfer theory is introduced to understand the change of the decay channels in the presence of antennas. The second chapter discusses the total enhancement factor of antennas to understand the parameters for increasing the detected signal of single emitters. A general mechanism for directional emission is introduced next to boost the signal from the perspective of detection efficiency. Then, directional antennas are reviewed based on the number of elements, including those fabricated from different shapes, materials, and excitation configurations. In the third chapter, parameters for simulating antenna performance are presented, followed by an introduction to DNA structures and DNA origami design. We discuss general antenna synthesis, purification, and sampling methods, and describe the custom-built scanning confocal microscope setup utilized for measurements of back focal plane images and fluorescence spectra of single molecules. The main research outcomes are presented in chapter four in the form of published papers. To assess the feasibility of achieving an ultracompact design, the directional emission of one side-by-side nanorod dimer antenna is evaluated from the full-wave electromagnetic simulation using the CST software. The design exhibits robust unidirectional emission performance originating from the anti-phase hybridized plasmon mode against some unavoidable synthesis imperfections, as expected from the two[1]dipole model analysis. Subsequently, we designed a T-shaped DNA origami template to assemble the nanorod dimer in a side[1]by-side configuration and attached a single ATTO 594 molecule at a specific position in close proximity to the antenna. The radiation pattern of the dimer antennas revealed a single lobe on the back focal plane in the experiments. Moreover, the measured fluorescence pattern showed a maximum forward to backward ratio of about 10 dB. Using the same DNA origami framework, we modulated the spectra of the fluorophore to show a peak at different spectral positions as a result of coupling to AuNRs of different sizes. As anticipated from the simulations, the enhanced radiative decay rate influenced the emission spectra. Furthermore, we discovered that the modulation of the spectra for single emitters depended on the position of the emitter relative to the nanorod. Surprisingly, we observed that both the primary and secondary peaks of the fluorescence were significantly enhanced simultaneously. Finally, chapter five includes a discussion on the limitations and promising outlooks of our approach, as well as the conclusion. The DNA origami technique not only provides immense convenience in the fabrication of nanodevices but also enables research in nanophotonics at the single-molecule level. The study of directional emission and spectral reshaping effects of antennas paves the way for a greater degree of freedom in single-photon sources and their applications in biosensing, optical wireless links, and quantum circuits.

When? 13.10.2023 10:30
Where? PER 08 2.73
Chemin du Musée 3
1700 Fribourg
speaker Fangjia Zhu
Groupe Prof. Acuna
Contact Département de Physique
Prof. Acuna