Diamond: a gem for micro-optics
A lot of research in photonics is being directed towards optics miniaturization and light field control at the microscale. The concept of metasurfaces has proved to be highly efficient to engineer arbitrary light fields. Nevertheless, the performance of optical components still highly depends on their material properties, thus various compositions are being investigated to reduce loses and assure stable performance over a broad range of operation conditions. Low absorption, high refractive index, high threshold of radiation damage and thermal stability are greatly desired features. All these properties are well combined in diamond. Being extraordinarily hard and chemically inert diamond withstands harsh environments and aging. The large band gap makes it appear crystal clear and highly transparent from the UV to the far infrared bands. In addition, high refractive index at visible wavelengths and ultimate thermal conductivity prove diamond to be the material of choice. However, its hardness makes fabrication of micro-optical elements challenging.
Combination of electron beam lithography (EBL) and oxygen plasma etching is a possible way to pattern diamond. By using these techniques, we have successfully developed diamond micro-optical elements for the visible domain. As EBL patterning is in principle lateral, the desired optical functionalities are encoded by creating form birefringence domains. The domains consist of subwavelength gratings that act as waveplates for light field manipulation. As an example, vortex beam micro-generators of arbitrary topological charges were made by creating azimuthally varying subwavelength grating structure in diamond. The design of this structure benefits from the spin-orbit interaction of light, which converts a circularly polarized Gaussian beam into a vortex beam carrying an orbital angular momentum defined by the structure of micro-generator. Such elements provide high purity broadband optical vortex beams in the visible domain for a variety of applications in optical manipulation, microscopy and optical communications.