At the 37th European Time and Frequency Forum in Neuchâtel, Mona Kempkes won the Student Best Poster Competition, for the presentation of her poster on “Testing Novel High-Reflectivity Mirror Technologies from Room-Temperature to 4 K” coauthored by T. Legero, U. Sterr, and D. Nicolodi. Mona Kempkes is currently pursuing her PhD research within the Cluster of Excellence Quantum Frontiers at PTB on novel highly reflecting mirror coatings for improving the frequency stability of cavity stabilized lasers.
Thermal noise of traditional Ta2O5/SiO2 high-reflectivity mirror coatings constrains the stability of state-of-the-art optical resonators and limits the performance of many precision laser experiments. Crystalline AlGaAs/GaAs multilayer coatings offer significant thermal noise reduction, but these coatings exhibit significant birefringence which can be modified by illumination with infrared or visible light. Moreover, two experiments at 4 K - 16 K and 124 K revealed spontaneous fluctuations of the coating’s birefringence and an additional noise source with hitherto unknown origin limiting the frequency stability above the expected thermal noise flicker floor at cryogenic temperatures.
At PTB a closed-cycle, low-vibration cryostat was set up for characterizing high-reflectivity mirror coatings from room-temperature to 4 K. To investigate AlGaAs/GaAs coatings, a dual frequency modulation technique was further developed to track the birefringent line splitting as a function of temperature and investigate its noise. The results are expected to help understanding the underlying birefringent effects in crystalline coatings, gain further understanding in the source of the observed excess noise and possibly find strategies to suppress or mitigate it.
