Super-resolution Fourier microscopy for Mid-IR

Abstract

We propose a novel scheme for subwavelength-resolved imaging in the mid-IR. Our approach relies on scattering from an acoustic grating and allows far-field detection of high spatial frequency Fourier components of the object under study.Wavelengths between 5 and 20 μm form a so-called "fingerprint region," where a number of important organic substances can be identified. Consequently, the mid-infrared spectral band is extremely important in chemical analysis. Spatially-resolved mid-IR spectroscopy can reveal microscopic chemical morphology of surfaces, thin films, and biological tissues. This non-destructive technique is of great use in many branches of materials science and biology.The conventional λ/2 diffraction limit, however, constrains the spatial resolution of IR microscopy. In particular, the scale of the smallest resolved features in biological samples is comparable to the size of cells (5-30 μm). To perform imaging of subcellular chemistry, as well as other nanoscale processes (e.g. photoresist reflow), submicron resolution is required.To attain chemical contrast on a submicron scale several techniques have been developed, including coherent anti-Stokes Raman spectroscopy (CARS) [1], as well as various scanning probe methods [3,2]. While successful in resolving subwavelength structures, these approaches possess several drawbacks. CARS is a complicated nonlinear technique which requires expensive tunable sources. Scanning probe setups, in turn, suffer from inefficient coupling of IR light into the near-field taper[2], low throughput, and the necessity for substantial post-processing of collected data. Scanning probe microscopy is, furthermore, poorly suited for observing dynamic processes that span different parts of the imaged object. An ideal mid-IR microscopy setup would combine a low-cost tunable source (e.g. a quantum cascade laser) with a rapid-acquisition subwavelength imaging system.In recent years, several devices have been proposed for obtaining a direct optical far field image that includes subwavelength features. For instance, a "superlens" based on materials with negative refraction [4] was theoretically shown to amplify the evanescent waves, as well as focus the propagating waves, resulting in a potential for resolution far below the diffraction limit. Practical implementation of such a device, however, has proved challenging due to the difficulties involved in fabricating low-loss negative index materials [5]. More recently, schemes have been proposed that rely on metamaterial-based or nanopatterned devices to convert near-field evanescent field spectrum into propagating waves, which can be processed with conventional optics. In one such approach, subwavelength features are magnified by a metamaterial crystal to the point that they are no longer diffraction limited [6,7]. However, fabrication of such systems presents a significant technological challenge, which so far has not been overcome.In the present paper, we propose an alternative approach to far-field imaging and spectroscopy of sub...