Abstract
Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. Here, we conceive a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.
Highlights
Being localized at the aperture, the evanescent fields remain undetected in most near-field imaging systems
Field-effect transistor (FET) architectures have been utilized for detection in the 0.3–3 THz range with less than 100 pW/Hz1/2 noise equivalent power (NEP)[24] and signal-to-noise ratio as high as 2000029
The 1D InAs NWs provide a clear benefit for the development of detectors with fast read-out, due to the inherently small atto-farad level capacitance[23] whereas the 2D geometry of the atomically thin black phosphorus (BP) and graphene field-effect transistor (FET) is promising for the detection of highly localized evanescent fields
Summary
Being localized at the aperture, the evanescent fields remain undetected in most near-field imaging systems. We integrate a THz nanodetector based on a semiconductor nanowire (NW) or a thin flake of crystalline black phosphorus (BP) into the evanescent field region of a sub-wavelength aperture to enable efficient detection of the transmitted wave This THz near-field probe provides sub-wavelength resolution at room temperature (RT) in a compact THz imaging system without the need for the mode-locked table-top femtosecond pulse lasers, atomic force interaction or demodulation techniques. FETs can detect THz radiation very efficiently by means of the thermoelectric effect[28] as a consequence of the temperature gradient (ΔT) along the channel, caused by the asymmetric feeding of the incident THz wave to the FET electrodes Under this condition, a steady-state thermoelectric voltage ΔuSD =ΔTSb (where Sb is the Seebeck coefficient) develops across the channel. The field-concentrating trapezoid extension, acting as the G electrode, was aligned with the S electrode at a S-G distance of 500 nm (see Methods)
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