Light scattering from closely-spaced atom pairs

While light scattering is a fundamental phenomenon that has been covered in dozens of references, it has recently become a topic of intense investigation because of its relevance to new technologies — for instance, harnessing Dicke super- and subradiance for photon storage and readout or using atom arrays as ideal, switchable mirrors. Of particular interest is the regime where scatterers are densely packed compared to the resonant wavelength. In this limit, atoms no longer scatter independently, but cooperatively, leading to uniquely collective behavior. Furthermore, the dipole-dipole interactions induced by the illuminating light play a major role in the light scattering behavior in this regime. Due to experimental challenges, however, these effects have not been studied in the near-field, where the interatomic potential scales with the cube root of distance.

We explore collective scattering in the near-field regime by using 10,000 pairs of ultracold atoms, each of which occupy a single harmonic oscillator potential. This realizes scatterers with a spacing of 50 nm, smaller than the reduced resonant wavelength of 100 nm. Using coherence spectroscopy and time-of-flight measurements we report three main results. In the limit of ultra-short light pulses, the dipolar interactions are highly suppressed, and we observe a factor of 2 increase in the light scattered per atom due to Dicke superradiance. Without suppressing the dipolar interactions, however, the repulsive interaction between the atoms leads to the ejection of atom pairs from the trap with a kinetic energy of thousands of recoil energies. Finally, we observe how the s-wave scattering length modifies the ground state two-atom wavefunction and consequently alters the scattered incoherent light. Our results can be explained in terms of spatial overlap with the resonant region and varying Debye-Waller factor and has implications for future investigations of few-body light scattering and their associated applications.