We observed the optically forbidden energy transfer between cubic CuCl quantum dots coupled via an optical near-field interaction using time-resolved near-field photoluminescence (PL) spectroscopy. The energy transfer time and exciton lifetime were estimated from the rise and decay times of the PL pump-probe signal, respectively. We found that the exciton lifetime increased as the energy transfer time fell. This result strongly supports the notion that near-field interaction between QD makes the anti-parallel dipole coupling. Namely, a quantum-dots pair coupled by an optical near field has a long exciton lifetime which indicates the anti-parallel coupling of QDs forming a weakly radiative quadrupole state.

This paper presents a relationship between the near-field shielding effectiveness (SE) and the far-field SE of an infinite planar shield for dipole fields. The penetration fields through the shield and the near-field SE are deduced analytically from an explicit integral expression based on certain assumptions. They further give us approximate formulas for the near-field SE. The near-field SE depends on not only wavelength and material used, but also on the distance r from a source to an observation point through the shield, the source type (magnetic dipole or electric dipole) and the orientation (vertical or horizontal to the shield face) in general. The results we obtained are as follows. The near-field SE for magnetic dipole fields vertical to the shield face is the same as that horizontal to the shield face, and their absolute values equal that of the far-field SE multiplied by k0r/3 (k0 is the wave number). The near-field SE for electric dipole fields vertical to the shield face doubles that horizontal to the shield face, and the absolute value of the latter equals that of the far-fields SE divided by k0r. The validity of the assumptions used to obtain the approximate formulas are examined. The range of r (an application range), over which the difference between the approximate value and the true value is under 1 dB, is determined, where the former value is calculated by the approximate formula of the SE and the latter value is etsimated by direct integration of the related integral expression. For instance, an application range of the approximate formula for magnetic dipole fields vertical to the shield face is from larger one of 50δ and 33µrδ to 0. 11λ0, where µr is specific permeability, δ is skin depth of the shielding material used and λ0 is wavelength in the free space.