Live electrooptic imaging (LEI) in the microscopic range has been successfully demonstrated for the first time. The finest resolution achieved in the present study is 2.7 μm, which is finer than the previous record by more than an order of magnitude. This drastic improvement in the resolution record has been achieved through comprehensive improvement of the limiting factors of the conventional LEI system. Residual limiting factors in the improved system have been systematically analyzed and ideas for even finer resolution have also been presented.It should be highly attractive if the space-domain behaviors of radio wave, electrical signals and/or noises in and around circuits, as well as conditions of electromagnetic interferences, could be grasped agilely. Prompt visualization of invisible electric fields in terms of both their spatial distributions and dynamics would be the most effective. For this purpose, in addition to various numerical approaches, there is a technique named live electrooptic imaging (LEI) 1-3 schematically shown in Fig. 1(a), which is unique due to its experimental agility; high frequency electric fields are visualized in real time in the phase-evolving video formats. The LEI technique, whose operation principle is briefly described in the Methods section, is based on two excellent benefits of photonics 4 , ultra-parallel 5, 6 and ultra-fast 7,8 properties, which are merged in an electrooptic (EO) sensor 9, 10 shaped in the form of a thin plate 11,12 . Although further applications are expected in various areas in the future, the following basics have been demonstrated so far: wave vector mapping 13 , respectively. In addition to these figures, the highest spatial resolution has been reported as 30 to 40 μm in our recent paper 21 , which was an improvement from the conventional in the sub-millimeter range achieved primarily by reducing the thickness of the ZnTe EO layer on the same glass platform as shown in Fig. 2 to 10 μm and having its proximity contact to circuit patterns. This improvement is reasonable since it is generally supposed that a thinner EO sensor plate leads to a higher spatial resolution at expense of sensitivity, if a fringe-shaped electric field distribution from a circuit pattern is dealt with. The resolution is almost fine enough for patterns on printed circuit boards (PCBs), but is insufficient for microscopic circuit patterns on semiconductor substrates. For this reason, the remaining factors limiting the resolution have been discussed and concluded to be the following: three optics originating factors (restricted diffraction limit, deviated focus, and inadequately large pixel size on EO images) and one EO interaction property (room to thin the ZnTe layer) 21 . For a finer spatial resolution in a new LEI system, each of these factors has been improved as follows. A microscope objective lens ( Fig. 1(a)) was introduced together with an infinity correction optical system ( Fig. 1(b)) with fine focus adjusting mechanisms. The magnification power of the optics was tun...
