Abstract:
Optical microcavities are fascinating optical micro-devices that allow to control and modify light-matter interaction and open up exciting possibilities for advanced applications in photonics, quantum technologies, and materials science. This dissertation investigates the control of light-matter interactions in optical microcavities at room and low temperatures, with an emphasis on how the photonic mode density affects the light emission of single-photon emitters and non-radiative energy transfer processes between donor and acceptor. The research includes the study of single nitrogen-vacancy (NV) centers in a diamond at cryogenic temperatures, the control of multi-color Förster resonance energy transfer (FRET), and the exploration of strong coupling regimes in coupled Fabry-Pérot (F-P) microcavities. Control of light-matter interactions in optical microcavities at low temperatures presents several challenges like maintaining stable cryogenic temperatures for single-molecule spectroscopy, detecting single molecules within microcavities when the emission intensity is weak or molecules exhibit poor photostability, and suppressing unwanted background signals interfere with emission from single molecules within the cavity. In addition, fabricating high-quality λ/2 Fabry-Pérot microcavities with precise dimensions is essential for achieving the desired optical properties and coupling strengths.
A key aspect of this work is the use of single-molecule imaging and spectroscopy at cryogenic temperatures, which is the powerful technique to study the behavior of individual molecules with high accuracy where reduction of temperature leads to minimizing the thermal vibrations of molecules and resulting in sharper spectral lines. This technique enables high-resolution spectral measurements of individual molecules. Comparable in size to single molecules, NV centers act as highly stable single-photon emitters. NV centers are point defects in the diamond lattice, consisting of a nitrogen atom adjacent to a vacant lattice site. When a single NV center is placed in a microcavity, the spectral mode density influences the spectral and radiative properties of the NV center, leading to selective emission at resonance, narrow spectral linewidth, and enhanced photon emission through the Purcell effect. The unique optical and quantum properties of NV centers, as well as their photostability, make them suitable samples to study single-photon imaging and spectroscopy.
Förster resonance energy transfer is a non-radiative process where energy can be transferred from a donor molecule to a close acceptor molecule through dipole-dipole coupling. A triple FRET process between three different dyes embedded in a single polystyrene nanosphere with excitation maxima at 505 nm, 575 nm, and 655 nm is studied here, where the single nanospheres were placed inside a Fabry−Pérot microcavity at low temperatures. To understand the impact of cavities on triple FRET, the lifetime and spectral properties of the individual nanospheres were measured as a function of the spectral optical mode density at low mode order. This allows us to control the interactions between the dyes effectively and to select enhancement of desired FRET processes while suppressing undesired interactions. The results show that the energy transfer rate between the three different dyes can be controlled by microcavity.
Combining optical strong mode coupling with polaritonic coupling in hybrid systems enhances light-matter interactions, leading to the formation of new hybrid modes and supermodes. Such a combination provides greater flexibility in manipulating both optical and electronic properties of the hybrid system. In this work a hybrid system is presented that combines optically strong mode coupling with polaritonic coupling within a λ/2 Fabry-Pérot microcavity. By integrating a thin TDBC J-aggregate film into an optical λ/2 microcavity coupled to a second microcavity, the study demonstrates the simultaneous occurrence of two strong coupling types: between purely optical modes of adjacent microcavities and between the TDBC J-aggregate molecules and the optical modes. This hybrid system's coupling strength and damping are highly sensitive to the position and concentration of the molecules in the microcavity, and the system can be modeled very effectively with coupled damped oscillators. The results highlight that the components of the coupled system cannot be treated independently; altering one parameter impacts the entire system.
Microcavity