Nitric oxide (NO) sensors are of great interest because of the role NO plays in regulating chemical and biological processes, e.g. relaxation of smooth muscle tissue, neurotransmission, and immune response. A healthy human exhales between 4 and 200 ppb NO. A person suffering from an inflammatory desease, like cancer or asthma can easily exhale a ten times higher concentration. NO can therefore serve as a marker for the monitoring of a desease.
Atoms excited to Rydberg states can be ionized very easily. In hot vapors the amount of ions is large enough to create a current in the nanoampere regime. This current signal shows by far better signal to noise properties than a pure optical detection. We want to use this measurement method to realize a new kind of gas sensor based on Rydberg excitations.
We are optically exciting Rydberg states of NO with lasers tuned specifically on transitions in NO. Since the electron of the Rydberg excited NO molecules is only weakly bound, already low energy collisions at room temperature with other atoms are sufficient to ionize a Rydberg-excited NO molecule. The emerging charges can then be measured as a current. Because the lasers are ideally narrowband is the occurrence of a current a clear indication of the presence of the molecule in question.
In a very first experiment we demonstrated the detection of 10 ppm NO in a buffer gas. This sensor can be used at room-temperature and even at normal atmospheric pressures. With the achieved detection efficiency an detection limit of 5 ppb at a realisitic medical examination seems to be in reach.
For this very first proof-of-concept study we were using two pulsed dye-lasers, which had the disadvantage though of being very broadband in the frequency domain. Our new approach will be to employ very narrowband solid state cw lasers to excite Rydberg states in NO.
Popular article on Scilight: https://aip.scitation.org/doi/10.1063/1.5046926
J. Schmidt, M. Fiedler, R. Albrecht, D. Djekic, P. Schalberger, H. Baur, R. Löw, N. Fruehauf, T. Pfau, J. Anders, E. R. Grant and H. Kübler, "Proof of concept for an optogalvanic gas sensor for NO based on Rydberg excitations", Appl. Phys. Lett., 113, 011113 (2018)
or on arXiv: https://arxiv.org/abs/1801.09453