Resolution enhancement of terahertz (THz) imaging is one of central concerns in the THz science and technology research. Because of the used long wavelength (1 THz = 300 um), THz imaging’s resolution is generally in the scale of millimetre. This constitutes a major obstacle for the application of THz imaging in bio-medical diagnosis and semiconductor device inspection [1-3]. Here, we report on a novel sub-wavelength THz imaging method via the femtosecond laser filament in air, which refers to the plasma channel created by a femtosecond laser pulse .
A two-color (1 kHz, 50 fs, 1 mJ/pulse, 800 nm + 400 nm) laser filament was created at the focus of the lens (f = 30 cm). The generated THz pulse was detected by a standard electric-optic sampling (EOS) setup . Using the generated THz beam as the probe, near field THz imaging can be obtained. As compared with the image of the sample under optical microscope (resolution: 5 um), no significant blurring effect could be noticed the corresponding scanning THz image .
Furthermore, the knife-edge method  was applied to measure the THz beam diameter at different positions z. The obtained results indicates taht the diameter of the THz beam varies from 20 um to 50 um. This phenomenon makes a novel sub-wavelength THz imaging technique feasible as we have demonstrated .
Further study has revealed that due to the inhomogeneous refractive index distribution inside the femtosecond laser filament, a THz waveguide is created, confining the THz wave energy strongly within a μm scale space. Rely on this new phenomenon, subwavelength resolution THz imaging and high THz wave amplitude could be achieved [7, 8].
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J. Zhao, et al., " Strong Spatial Confinement of Terahertz Wave inside Femtosecond Laser Filamen"t, ACS Photonics, 2338–2343, 3 (2016)