Rydberg Gas Sensors
A Rydberg atom is an excited atom with one or more electrons that have a very high principal quantum number, n. Rydberg atoms have a number of peculiar properties including an exaggerated response to electric and magnetic fields, long decay periods and electron wavefunctions that approximate, under some conditions, classical orbits of electrons about the nuclei. 
Rydberg excitation and subsequent ionization are promising to be used as a new method to develop trace gas sensors at ppb sensitivities and yet highly selective. The flexibility offered by such a Rydberg gas sensor will benefit current small-sized and integrable systems that are now limited to a few gases of interest. In the medical context of breath analysis, one molecule of interest is nitric oxide (NO), which is an indicator for several severe conditions.
Objective: in macQsimal, partners with the lead of the University of Stuttgart (STUTT) work closely together to design and develop a microfabricated Rydberg-based Gas Sensor demonstrator focusing on nitric oxide (NO) detection.
Related macQsimal publications and presentations
An optogalvanic gas sensor based on Rydberg excitations
Schmidt, J., Münzenmaier, Y., Kaspar, P., Schalberger, P., Baur, H., Löw, R., Fruehauf, N., Pfau, T., Kübler, H. (2020) Journal of Physics B: Atomic, Molecular and Optical Physics, 53(9), 1-5.
The method we use at macQsimal can be explained in a was that is illustrated with the figure on the right. First, a gas mixture of an unknown concentration of NO enters the gas cell. In a second step NO is excited to a Rydberg using the excitation scheme on the left. The laser systems are involved. The ionization of the molecule is a result of collisions with other particles and since we apply a small voltage to electrodes we can then guide these free particles to our electrical circuit where a TIA finally produces the desired signal.
Within the macQsimal project a prototype of through-flow gas cells has beed developed by the research team. An unknown mixture of NO with some background enters the cell from left to right. Using the laser excitation scheme involving three wavelengths, we excite NO to a Rydberg state which subsequently ionizes. Then free electrons can be detected. For that, we use sputtered electrodes on glass (top) where we apply a small voltage too. The glass is provided by the Institute for Large Area Microelectronics here at the University of Stuttgart. A trans-impedance amplifier (TIA) bonded to glass allows amplifying the resulting current. The TIA is designed at the Institute of Smart Sensors. The attached Flex PCB configures the desired detection range.
The green standard PCB is used to provide the supply voltage as well as for reading out the signal.
To keep our laser linewidth of each transition as narrow as possible we use a locking setup which provides us with a feedback, we can than apply to our laser systems. The aim is to have a master laser (780nm) locked to an ultra-low expansion cavity, rather than several transfer cavities locked to the signal of that master laser and finally our lasers locked to these transfer cavities.
Electro-optical modulators additionally allow us to lock the wavelength we do need for a particular measurement. Our laser light is guided to this table via optical fibers. Due to protective reasons, an optical table is hidden in special boxes filling the space of the labolatory.