Tailoring a sub-diffraction optical focus via a straightforward interferometric approach

Abstract

An approach for yielding light focuses below Abbe´s diffraction limit in Gaussian beams is presented. The method uses only standard passive optical elements as lenses, filters and mirrors and consists of a Michelson interferometric setup, where one of the light branches is modified in amplitude and/or phase. The focus narrowing process is carried out at the focal plane of a spherical lens by the interference of altered and unaltered light branches. The main focus features, namely, the focus intensity and size as well as the sidelobe intensity, are adjusted by varying two external parameters in a controllable manner under the conditions of pure destructive interference. Narrowing of the diffraction limit close to 40% with reduced intensity sidelobes (10%) is achieved. Due to the use of only lenses and mirrors, the approach does work with laser beams within a broad optical bandwidth ranging from infrared to ultraviolet in continuum regime as well as in ultra-short pulse regime. The method can also be implemented for high-power lasers and temporal domains. The focus-narrowing process emerges as a natural mechanism to the light interference, bringing a fresh perspective to applications from a few controllable degrees of freedom. The good performance of the sub-diffraction optical focus and the simplicity of the experimental setup promote new opportunities in fields ranging from optical manipulation of particles at sub-wavelength scale to optical writing and super-resolution microscopy.