  Electromagnetic coupling via either exponentially decaying evanescent field
or radiative wave is a primary characteristic of light, allowing optical
signal/power transfer but limiting integration density in a photonic circuit. A
leaky mode combines both evanescent field and radiative wave, causing stronger
crosstalk and thus not ideal for dense integration. Here we show that a leaky
mode with anisotropic perturbation rather can achieve completely zero crosstalk
realized by subwavelength grating (SWG) metamaterials. The oscillating fields
in the SWGs enable coupling coefficients in each direction to counteract each
other, resulting in completely zero crosstalk. We experimentally demonstrate
such an extraordinarily low coupling between closely spaced identical leaky SWG
waveguides, suppressing the crosstalk by $\approx$40 dB compared to
conventional strip waveguides, corresponding to $\approx$100 times longer
coupling length. This leaky-SWG suppresses the crosstalk of transverse-magnetic
(TM) mode, which is challenging due to its low confinement, and marks a novel
approach in electromagnetic coupling applicable to other spectral regimes and
generic devices.
