Electric and magnetic optical fields carry the same amount of energy. Nevertheless, the efficiency with which matter interacts with electric optical fields is commonly accepted to be at least 4 orders of magnitude higher than with magnetic optical fields. Here, we experimentally demonstrate that properly designed photonic nanoantennas can selectively manipulate the magnetic versus electric emission of luminescent nanocrystals. In particular, we show selective enhancement of magnetic emission from trivalent europium-doped nanoparticles in the vicinity of a nanoantenna tailored to exhibit a magnetic resonance. Specifically, by controlling the spatial coupling between emitters and an individual nanoresonator located at the edge of a near field optical scanning tip, we record with nanoscale precision local distributions of both magnetic and electric radiative local densities of states (LDOS). The map of the radiative LDOS reveals the modification of both the magnetic and electric quantum environments induced by the presence of the nanoantenna. This manipulation and enhancement of magnetic light-matter interaction by means of nanoantennas opens up new possibilities for the research fields of opto-electronics, chiral optics, nonlinear&nano-optics, spintronics and metamaterials, amongst others.

The development of luminescent mechanochromic materials depends mainly on the possibility to rationally design them with the desired properties. Molecular copper iodide clusters constitute an unprecedented family of compounds exhibiting great changes of their luminescence properties upon mechanical stress. From previous studies, the mechanochromic properties of cubane [Cu₄I₄L₄] (L = organic ligand) clusters have been attributed to modifications of cuprophilic interactions induced by mechanical solicitation. In this study, we ascertain our hypothesis by choosing to study the luminescence mechanochromism of a [Cu₄I₄(PPh₃)₄] cluster which presents two crystalline polymorphs exhibiting strikingly different Cu–Cu bond lengths. As forecasted, only one of these two polymorphs exhibits mechanochromic properties. Structural and optical characterization methods are reported along with structural characterization under controlled pressure allowing a precise analysis of the structural changes occurring under mechanical stress. In addition to confirming our mechanism based on enhancement of cuprophilic interactions under pressure, this study demonstrates the possibility of prediction of mechanochromic properties in the family of copper iodide compounds that constitutes a step further toward the rational design of stimuli-responsive materials.