Atmospheric nitrogen can be ionized by high-intensity femtosecond pulses. The produced molecular nitrogen ions emerge as a potential quantum optical platform for studying light-matter interactions and quantum coherence effects, which has attracted broad interest for quantum optics, ultra-fast optics, and strong-field laser physics. Molecular nitrogen ions exhibit rich energy levels for light-matter interactions and abundant nitrogen molecules in the atmosphere provide possibilities to realize coherent quantum control of photons in the remote atmosphere in the near future. Hence, a research collaboration team from SIOM, SJTU, NUDT, PKU, and TAMU proposed and demonstrated for the first time the photon retention in coherently excited nitrogen gas.

We found after the nitrogen gas is ionized by a strong 800 nm femtosecond laser, photons are retained through the quantum coherence. As shown in Fig. 1, a time-delayed 1580 nm femtosecond pulse is used to trigger two-photon resonant absorption with the retained photon in "N" _"2" ^"+" , resulting in a strong ultraviolet (UV) radiation, as an optical readout of the retained photon. We unveil the nature of photon retention in "N" _"2" ^"+"  both theoretically and experimentally. As shown in Fig. 2, both the non-resonant FWM signal and 330.8 nm radiation disappear due to the temporal separation of two pulses, but the 329.3 nm radiation remains strong. Its evolution dynamics is different from the non-resonant FWM, remaining over several picoseconds. Numerical simulations using Maxwell-Bloch equations convincingly prove the photon retention effect in generating the 329.3 nm radiation and reveal a pivotal role of the excited-state population for the optical readout. Such photon retention in "N" _"2" ^"+"  points to potential applications of optical information processing and coherent optical information storage at room temperature in the remote atmosphere and triggers new interest in studying quantum optics and coherent quantum control of photons in the remote atmosphere. This new finding facilitates further exploration of fundamental interactions in the quantum optical platform with strong-field ionized molecules.

Fig. 1. Schematic illustration of the photon retention with "N" _"2" ^"+"  ions. (a) Processes of photon retention and optical readout and (b) corresponding energy diagrams.

Fig. 2. (a) Measured spectra of UV radiations and (b) evolution dynamics of 329.3 nm radiation and non-resonant FWM with 1-ps time delay. (c) Simulated signal intensity versus delay for different excited-state populations.

This research was published with the title “Photon retention in coherently excited nitrogen ions” in Science Bulletin 66, 1511 (2021).

Link: https://doi.org/10.1016/j.scib.2021.04.001