Microwave field of a two-wire antenna on diamond: analytic model, design maps and finite-element validation for NV layers

An analytic model of the microwave field produced by two parallel round wires lying on a diamond surface, its projection onto the four NV-center orientations, design maps of excitation uniformity and orientation balance over the wire geometry and the crystal orientation, and the finite-element runs that validate the model. The field that two parallel wires on a diamond surface produce in a near-surface NV layer depends on the wire radius, the gap, the depth of the layer and the orientation of the crystal. The surface cut and the in-plane angle of the diamond are treated as design parameters on the same footing as the wire geometry: they are chosen when a sample is prepared, not changed during a measurement. By scale invariance the whole design space reduces to two dimensionless variables, gap/R and depth/R, and all results are normalized, so they apply to any wire size and drive power. Two field models are provided. The magnetostatic model (line currents on the wire axes) agrees with a 2D finite-element solution to within 0.5 %, the level of the mesh error. At microwave frequencies, however, the proximity effect turns out to be the dominant feature of the field rather than a correction: it changes the field under the wires by up to 40 % for a 25 µm gap, and by 8 to 14 % for a 100 µm gap. An exact image-current model of perfectly conducting wires reproduces frequency-domain finite-element solutions at 2.87 GHz to better than 1 %, and is the default model. The deposit contains the Python model and standalone tools (an interactive manipulator, design-map plots, leakage curves for scaled mock-ups), twelve design-map tables for the (100), (110) and (111) cuts, both current configurations and both field models, the COMSOL model, its exports and the point-by-point comparison with the analytic model.