Ionization of Rydberg Atoms in a Standing-Wave Laser Field
Research by U-M Physics Ph.D. Candidate Sarah Anderson and her advisor Professor Georg Raithel has been featured in the latest issue of Nature Communications. Anderson and Raithel have employed a standing-wave light field as a spatially sensitive probe to prove that the photoionization of Rydberg atoms only occurs near the nucleus, and not where the Rydberg electrons spend most of their time. The work verifies important aspects of the theory of light-matter interactions. Their research findings are paramount to harnessing Rydberg atoms in field-sensing applications, such as in detection devices for small static electric fields and for Terahertz radiation. Field sensors of this type are of interest for terrestrial surveillance and for astronomy. Further, the present research is important for quantum information work elsewhere, in which optically trapped Rydberg atoms are employed. There, the use of optical Rydberg atom traps necessitates a complete understanding of collateral (unintended) photoionization, because photoionization could negatively affect performance. Anderson’s and Raithel’s work is described in the December 16 issue of Nature Communications, Vol 4 , Article number 2967 (2013).
Rydberg atoms are micrometer-sized atoms (enormous by atomic standards) with a highly excited and isolated valence electron far outside the inner electrons of the atom. Because of the resulting tenuous binding of the outermost electron, these atoms are highly sensitive to external electric and magnetic fields, as well as to other nearby Rydberg atoms. This sensitivity, combined with advancing abilities to manipulate, cool and trap such atoms with laser fields, has driven Rydberg-atom research to branch out into the development of ultra-sensitive measurement devices for electromagnetic fields and for measuring fundamental constants of nature. Laser-trapped Rydberg atoms are also important building blocks in several quantum computation efforts around the globe. In these applications, photoionization of the Rydberg atoms by the optical trapping field can either represent a (often undesired) mechanism through which the Rydberg atoms are lost, or be exploited as a mechanism to detect them. A complete understanding of photoionization in Rydberg-atom laser traps is therefore important.
When electromagnetic radiation induces transitions between atomic states, the size of the atom is usually much smaller than the wavelength of the radiation, allowing the spatial variation of the radiation field's phase to be neglected in the description of transition rates. This approximation is known as the electric dipole approximation. In the photoionization of micrometer-sized Rydberg atoms in laser fields with wavelengths of about the same size as the Rydberg atoms, the small-size argument cannot be used in order to justify the electric-dipole approximation. Yet, the approximation still holds in this case.
Using a standing-wave light field as a spatially sensitive probe, Anderson and Raithel have measured that the photoionization of Rydberg atoms occurs near the nucleus, within a volume that is small with respect to the wavelength of the light. The localization of photoionization to a small volume validates the electric dipole approximation, which consequently applies despite the fact that the atoms are about as large as the wavelength of the light. The work verifies the theory of light-matter interactions, which are ubiquitous in nature, in an interesting limiting case.
In their experiments, Anderson and Raithel have investigated photoionization as a function of position within the volume of a Rydberg atom using a one-dimensional standing wave of light, which serves as a spatially sensitive probe of photoionization with sub-atomic resolution. The atom size approximately equals the standing-wave period, which results in maximal variation of the light field within the volume of the atom (see Figure). The light-field maxima are placed either near the atom's center (upper atom) or within the main lobes of the electronic probability distribution (lower atom). The photoionization rates measured under these contrasting conditions indicate whether it is the light-field intensity near the center of the atom or within the main lobes of the electronic probability distribution that matters in the photoionization process. While intuition might suggest that photoionization occurs where the probability of finding the electron is greatest (i.e. within the lobes of the electronic probability distribution), they have demonstrated that the process in fact happens near the nucleus.
Photoionization of Rydberg atoms by light fields can either represent a mechanism through which Rydberg atoms are lost, or be exploited as a detection method. Its complete understanding is important in applications of optical Rydberg-atom traps, which include the realization of exotic phases of matter, field-sensing applications, quantum information processing, and high-precision measurements of fundamental constants. Advanced research in these areas has potential impacts on everyday life through new technology that might result, such as new surveillance instruments, new astronomical detectors, and quantum computers. High-precision measurements of fundamental constants are important because they test the science community’s understanding of nature, which feeds back into research and development of advanced technologies, such as the GPS and atomic clocks.
Figure Above: Two Rydberg atoms (green) with respect to the standing-wave light-field intensity (red). The atom is more likely to be photoionized by the light field when its nucleus is near the intensity maximum, even though the main lobes of the electronic probability distribution in this case are located near intensity minima.