Absence of narrow-band and frequency-tunable laser sources in the ultraviolet (UV) part of the spectrum limits the application of the laser cooling and trapping methods for atoms of organic chemistry such as hydrogen, carbon, oxygen, and nitrogen, as well as for technology atoms such as chromium, indium, silver, and aluminium. There are numerous applications of such atoms in astrophysics, precision measurements and technology, precision experiments with anti-atoms such as anti-hydrogen.
A new approach to laser cooling and trapping in the UV spectral range can be based on conventional pulsed laser radiation sources which could be converted into UV radiation with a high efficiency. The conversion efficiency of pulses to the UV spectral range is strongly depends on the peak laser intensity. Because of these pulsed lasers with a femtosecond pulse duration seems more preferable for future applications in the UV spectral range. Trapping with femtosecond laser radiation was theoretically considered in [1,2]. The first experimental implementation of atom localization using a sequence of ultrashort pulses was carried out by the E. Riis et. al with picosecond pulses [3].
We demonstrated [4] the first experimental optical dipole trapping using the femtosecond laser radiation. We used Rb atoms for the investigating of atom trapping parameters. The loading of the dipole trap was carried out from a magneto – optical trap (MOT). The atom’s temperature reached 40 μK with the usage of a sub-Doppler cooling scheme. The Ti:sapphire laser with 50 fs pulse duration and 80 MHz repetition rate was used to form the dipole trap. The central wavelength and the spectrum width of the pulsed laser were 825 nm and 13 nm respectively. The laser beam was focused (16 μm spot size) into the MOT region. Any resonant with the Rb atoms laser spectral components were removed with Rb spectral filter [5]. The scheme of the experimental setup is shown on Fig.1. We show that properties of the atom localization in the trap depend on the heating of atoms caused by the momentum diffusion due to dipole force fluctuations because of high peak intensities of pulsed laser radiation.
We also investigated experimentally the spectral properties of trapped atoms [6]. Under the small average intensities of localizing field, the spectral properties of atoms trapped by pulsed laser field are similar to the properties of atoms trapped by the cw radiation. Theoretical calculations (Fig.2) show that it is possible to localize atoms at a zero shift in the frequency of the D2 transition line of the Rb atom caused by the ac Stark effect in case of increasing of average intensity of localizing field. It can be done experimentally using 8 ps pulse duration to compensate the high heating rate of trapped atoms. This configuration can be used to construct optical frequency standards without the need of the “magic” wavelength of the trapping laser radiation.
References
[1] V. Balykin, “Motion of an Atom under the Effect of Femtosecond Laser Pulses: From Chaos to spatial Localisation”, JETP Letters, 81, 209 (2005)
[2] D. N. Yanyshev, et al., “Dynamics of atoms in a femtosecond optical dipole trap”, Physical Review A, 87, 033411 (2013)
[3] R. B. M. Clarke, T. Graf, E. Riis, “Dipole traps with mode-locked lasers”, Applied Physics B 70, 695 (2000)
[4] A.E. Afanasiev, et al., “Atom femto trap: experimental realization”, Applied Physics B, 126, 26 (2020)
[5] A.M. Mashko, et al., “Atom femtosecond optical trap based on spectrally filtered laser radiation”, Quantum Electronics, 50, 530 (2020)
[6] A.E. Afanasiev, et al., “Spectroscopy of Rubidium Atoms in a Femtosecond Pulsed Optical Dipole Trap”, JETP Letters, 111, 608 (2020)