Broadband sub-wavelength terahertz subsurface imaging using a solid-immersion lens

•Broadband sub-wavelength terahertz subsurface imaging is demonstrated by combining a solid-immersion lens (SIL) with a THz time-domain spectrometer (TDS) for the first time.•The beam size reduction of more than a factor of nSIL is observed, and the result is supported by numerical simulations.•An analytical calculation is employed to obtain insights into the beam size reduction mechanism and estimate the beam size in a given experimental condition.•The increased reflection from the waveguide near the cutoff frequency, inaccessible with other THz-TDS-based or CW source-based THz imaging systems, is experimentally observed.•Sub-wavelength details of a credit card covered with a plastic cover are revealed with the imaging system. We demonstrate a broadband sub-wavelength terahertz (THz) imaging system by combining a solid-immersion lens (SIL) with a THz time-domain spectrometer (TDS). The imaging resolution of the system is enhanced in a wide spectral range (from 0.3 to 2.0 THz) with the aid of the SIL. The beam size is within 120–280 μm (0.28–0.8 λ) in the measured frequency range. Experimentally achieved imaging resolution enhancement results are consistent with the numerical simulation results. Unlike the previous reports on imaging systems using SIL, the beam size reduction by the SIL is more than a factor of nSIL (3.42) for all frequencies near the focal spot. An analytical calculation is employed to obtain insights into the beam size reduction mechanism and estimate the beam size in a given experimental condition. THz image of the WR1.9 waveguide is measured to highlight the broadband sub-wavelength imaging capability. The increased reflection from the waveguide near the cutoff frequency for TE10 and TE01 modes, inaccessible with other THz-TDS-based or continuous wave (CW) source-based THz imaging systems, is experimentally observed. In addition, we show the multispectral sub-wavelength images of subsurface structures by selecting the appropriate measurement time window of the temporal THz signal. THz images of a credit card show the sub-wavelength details of the card, including an IC chip, invisible due to the plastic cover. The experimental results highlight the potential applications of the imaging system in non-contact, non-destructive testing of the sub-wavelength scale structures in various fields.