600-GHz Fourier imaging based on heterodyne detection at the 2nd sub-harmonic

Abstract

Fourier imaging is an indirect imaging method which records the diffraction pattern of the object scene coherently in the focal plane of the imaging system and reconstructs the image using computational resources. The spatial resolution, which can be reached, depends on one hand on the wavelength of the radiation, but also on the capability to measure – in the focal plane – Fourier components with high spatial wave-vectors. This leads to a conflicting situation at THz frequencies, because choosing a shorter wavelength for better resolution usually comes at the cost of less radiation power, concomitant with a loss of dynamic range, which limits the detection of higher Fourier components. Here, aiming at maintaining a high dynamic range and limiting the system costs, we adopt heterodyne detection at the 2nd sub-harmonic, working with continuous-wave (CW) radiation for object illumination at 600 GHz and local-oscillator (LO) radiation at 300 GHz. The detector is a single-pixel broad-band Si CMOS TeraFET equipped with substrate lenses on both the front- and backside for separate in-coupling of the waves. The entire scene is illuminated by the object wave, and the Fourier spectrum is recorded by raster scanning of the single-detector unit through the focal plane. With only 56 µW of power of the 600-GHz radiation, a dynamic range of 60 dB is reached, sufficient to detect the entire accessible Fourier space spectrum in the test measurements. We present a detailed comparison between plane-to-plane imaging and Fourier imaging, and show that, with both, a lateral spatial resolution of better than 0.5 mm, at the diffraction limit, is reached.

Superstrate-lens integration using paraffin wax on top of semiconductor-based THz detector chips

“…The IF readout signal is heavily dependent on A LO *𝑛 . In our imaging work of [11,16], we employed signal radiation at 600 GHz and LO radiation at 300 GHz for 2nd subharmonic heterodyne detection, hence 𝑛 = 2. As a result, the improvement in sensitivity by the enhancement of the effective antenna area is squared in relation to the enhancement of the coupling efficiency of LO radiation.…”

“…The effect of the paraffin-wax superstrate lens on the THz detection efficiency was explored with the Fourier imaging system, where the dynamic range of detection plays a crucial role for the image quality [16]. We show the imaging results for the object consisting of a two-dimensional planar metal grid with 2.5-mm-wide stripes and 5-mmwide square openings (as shown in Fig.…”

“…1(a), a Si substrate lens is attached to the backside of a Si carrier wafer onto which a detector chip is mounted. We have recently used such an arrangement for subharmonic heterodyne detection, with backsidecoupled signal radiation at 600 GHz and front-sidecoupled LO radiation at 300 GHz [11,16]. The right side of Fig.…”