Successful working meeting in Stuttgart: important progress in ODMR signalling

On 21 February 2025, the time had come again: another working meeting was held on site at Hahn-Schickard in Stuttgart. There was a lively discussion about microwave excitation, more specifically about the microwave generator and the microwave resonator. The current test setup was also inspected. It still consists of a green but inexpensive laser, a discrete omega resonator, a photodiode with a glued-on colour filter and microdiamond as well as a simple amplifier that converts the photocurrent of the photodiode into a voltage. The mechanical parts all come from the 3D printer.

The microwave generator used to date (ADF4351) and the microcontroller box required to control it, which will later be replaced by the senseBox, can also be seen.

Our colleagues from IIS had a demo board of the same microwave generator with them, but with slightly more complex circuitry. Apparently this makes the decisive difference, because with this we were able to elicit the subsequent ODMR signals (optically detectable magnetic resonance) of the electron spin pairs of the NVs from the microdiamond:

During the experiment, the fluorescent light of the microdiamond was detected by the photodiode with colour filter and converted into a voltage by the amplifier and recorded with an oscilloscope. The microwave frequency was continuously cycled in the range from 2.67 to 3.07 GHz within 30 seconds before the cycle started again at 2.67 GHz. A first dip in the signal can therefore be recognised at -12.5 s and a second at approx. 16 s. At -12.5 s, two closely spaced dips (= dips or drop of the fluorescence in the measured signal) can be identified, which lie around 2.87 GHz and overlap. Our physicist colleagues explain this by mechanical stresses in the synthetically produced diamond, which was manufactured under high pressure and high temperatures. These tensions lead to a so-called Zeeman splitting, so that two spin states (ms = -1 and ms = +1) were already visible without an external magnetic field.

In any case, the signals from this simple setup are impressive and we have taken another big step forward on our way to a low-cost magnetometer for teaching.