Low temperature confocal spectroscopy in bright nanodiamonds

While colour centres in diamond have numerous exciting room-temperature applications, it remains important to be able to study and control their properties at cryogenic temperatures where crystal phonon activity is suppressed. A cryogenic confocal microscope has been developed using galvanometer beam scanning and a closed-cycle helium cold-finger cryostat. This has required novel software control, and a new set of confocal scanning modules have been developed for the Qudi experimental control software suite. Testing has shown remarkable galvanometer repeatability and stability over many hours. The high scanning speed and low thermal drift are outstanding. Combined with the low level of vibrations and high temperature stability of the cryostat, this allows precise measurements over periods of many days. A series of group grown nanodiamond samples were characterised at close to 10 K with photoluminescence measurements. Inhomogeneous broadening is characterised and thermal broadening of the spectra as temperature varies allowed for estimates of the nanodiamond temperatures. Active investigation of more home grown samples is ongoing, including looking for germanium vacancy sites, examining variation between nanodiamond samples and looking at the cooling performance of nanodiamond samples in this cryostat.