Subwavelength focusing by optical surface transformation

We propose a novel beam-compression device to achieve sub-wavelength focusing (SF) by optical surface transformation (a new theoretical branch derived from transformation optics). When a Gaussian beam is incident on the input surface of our device, we can obtain a sub-wavelength focused spot at the output surface of the compression device. The focused mode can still propagate to the far-field and keep very good directivity as incident Gaussian beam. The size of the sub-wavelength beam can be tuned by changing the size of the output surface. We only need one kind of homogeneous anisotropic medium, the optic-null medium (ONM), to realize the proposed beam-compression device. After some simplifications, we design a layered structure of two isotropic homogeneous media to realize the device. Numerical simulations verify the performance of our beam-compression device.