Due to the electronic structure of the 4f-shell, rare-earth ions (REIs) have intrinsically long optical lifetimes and coherence times, which makes them good materials as gain media in lasers, and now, as optically addressable qubits in quantum technologies. Using molecular complexes as host material for REIs provides a new possibility to chemically engineer a favorable environment by tailoring the surrounding ligand fields of the REIs. This can tune the frequencies and branching ratios of transitions that are favorable for quantum technologies whilst still maintaining good coherence times. Furthermore, multiple ions can be embedded into one molecule, enabling controllable coupling between the ions, which is essential for multi-qubit operation.
In this thesis, an Eu3+-embedded molecular complex in the form of crystalline platelets is integrated into a fiber-based Fabry-Pérot microcavity in order to demonstrate cavity-enhanced spectroscopy. Using a recrystallization method, the crystalline platelets with thicknesses as low as 2 \(\mu\)m are grown on a planar cavity mirror. A number of crystals feature an RMS surface roughness of 0.4 nm on an area of 5 \(\mu\)m \(\times\) 5 \(\mu\)m, corresponding to scattering loss of 100 ppm. Room temperature spectroscopy proves that the branching ratios, linewidths and optical lifetime are preserved during the recrystallization process.
With a laser-machined optical fiber, with controllable movement in the \(x\)-, \(y\)-, and $z$-dimensions through the use of piezos, the cavity can be scanned laterally in order to locate the crystals. A high-finesse cavity can be formed on most of the crystals, with a maximum finesse of 4,000. Cavity-enhanced fluorescence spectroscopy can be performed using an off-resonant excitation scheme. In the so-called bad emitter regime, where the effective Q-factor is determined via the Q-factor of the emitter, a Purcell factor of 0.09 is expected. The optical lifetime in the cavity is measured by off-resonant pulsed excitation, and is then compared with the free-space lifetime. At the present time, no conclusion can be made on the lifetime shortening in the cavity, because the lifetime difference lies within the statistic uncertainty. However, some individual cavity lifetimes show a reduction of about 10% compared to the free-space lifetime of 540 \(\mu\)s.
This research project was supervised by Prof. David Hunger in Karlsruhe Institute of Technology
For more details, please refer to my master thesis.