Subwavelength terahertz imaging via virtual superlensing in the radiating near field

Abstract: Paradoxically, imaging with resolution much below the wavelength $\lambda$ - now common place in the visible spectrum - remains challenging at lower frequencies, where arguably it is needed most due to the large wavelengths used. Techniques to break the diffraction limit in microscopy have led to many breakthroughs across sciences, but remain largely confined to the optical spectrum, where near-field coupled fluorophores operate. At lower frequencies, exponentially decaying evanescent waves must be measured directly, requiring a tip or antenna to be brought into very close vicinity to the object. This is often difficult, and can be problematic as the probe can perturb the near-field distribution itself. Here we show the information encoded in evanescent waves can be probed further than previously thought possible, and a truthful image of the near-field reconstructed through selective amplification of evanescent waves - akin to a virtual superlens reversing the evanescent decay. We quantify the trade-off between noise and measurement distance, and experimentally demonstrate reconstruction of complex images with subwavelength features, down to a resolution of $\lambda/7$ and amplitude signal-to-noise ratios below 25dB between 0.18-1.5THz. Our procedure can be implemented with any near field probe far from the reactive near field region, greatly relaxes experimental requirements for subwavelength imaging in particular at sub-optical frequencies, and opens the door to non-perturbing near-field scanning.