|
Imaging with terahertz (THz) waves has great potential for applications, such as in nondestructive testing, tumor detection, and biodetection. However, due to its millimeter-scale wavelength, using conventional imaging method, one cannot get THz images within spatial resolution better than sub-millimeter, which hinders from resolving smaller objects. Sensing evanescent waves in near field (<<λ) is a feasible path to realize sub-diffraction imaging. Raster scanning object’s surface pixel by pixel via a sub-wavelength-sized metallic aperture or an extremely tiny probe tip have been used to realize THz sub-diffraction imaging, while both mechanical-scan methods have disadvantages, such as invasive effects on sample, inflexible setups, and low source energy efficiency. Another technique called electrooptic (EO) near-field microscope driven by intense THz pulsed field achieves THz sub-diffraction images using a thin EO crystal (LiNbO3). It not only needs strong THz pulsed field but also suffers from thickness of the EO crystal. More recently, a novel scheme based on single-pixel imaging (SPI), which reconstructs the image by sequentially measuring correlations between the object and a set of prearranged masks, has been demonstrated in THz regime in far field and near field based on dynamic spatial THz wave modulator.
In this work, we proposed and experimentally demonstrated a novel THz wave near-field ghost imaging with spatial resolution of ~4 μm (over λ0/100 at 0.5 THz) using single-pixel compressive sensing enabled by femtosecond-laser (fs-laser) driven spintronic nonlinear near-field THz wave emitter. By fs-laser exciting a few-nm-thick metallic ferromagnetic/nonmagnetic (FM/NM) heterostructural (Pt/Fe/W) thinfilm with a couple of digital micromirror devices, we generate spatially encoded array of near-field THz wave emitter. With single-pixel Hadamard detection of the emitted THz waves, we reconstructed the THz wave near-field ghost image of an illumined object at near-field from a serial of encoded sequential measurements, yielding improved signal-to-noise ratio by one-order magnitude over raster scanning technique. Further, we demonstrate the acquisition time was compressed by a factor of over four with 90% fidelity using total variation minimization algorithm. The proposed terahertz wave near-field ghost imaging technique enabled by nonlinear spintronic terahertz emitter inspires new and challenging applications, such as cellular imaging.
|
