Terahertz (THz) imaging devices have attracted more attention and been intensively explored during the last two decades. The imaging can be realized in pixel-based array imaging (e.g. linear array and focal plane array (FPA)).

The standard planar hybrid architecture, commonly used in near and mid-infrared FPA, is also expected extend to THz region. The a-Si and VOx micro-bolometer commercial THz imaging cameras as well as the FET (field effect transistor)-based THz direct detection have achieved great success to realize uncooled THz imaging [1]. Unfortunately, both the micro-bolometers and FET-based THz detector are not broadband and fast enough. In contrast, Gallium Arsenide (GaAs) -based photon-type FPA (quantum well photodetector (QWP)-FPA) may be a promising candidate for high detection sensitivity, fast response speed, wide response range as well as high damage threshold. However, stable hybrid connection between the GaAs-based FPA and Si-ROIC is still a great challenge.

Pixelless imaging based on the semiconductor up-conversion technique has made great progress at near-infrared and mid-infrared region in past 20 years. This technique makes use of an entire large size of device cell to image directly. There is no separate pixel element at all in optical receiving terminal part, which simplifies the fabrication process significantly. The entire image is transmitted in the detector, then is restored by light emitting diodes (LED) and is finally ‘seen’ by a Si charge coupled device (CCD). The greatest advantage of such pixelless imaging is that no ROIC is required. Therefore, it solves the failure of Si-ROIC at low temperature naturally, which is a critical problem for many THz FPA imaging devices. THz pixelless imaging based on the integrated QWP and LED is recently demonstrated to exhibit a promising application potential by Z. L. Fu et al [2]. However, owing to the polarization selection rule of the intersubband transitions, the QWP-based pixelless imaging device requires extra optical coupling design. Therefore, such QWP-LED pixelless imaging device is far from optimization.

Earlier on, our group investigated the MIR-QWPs, THz-QWPs, FIR-HIWIPs as well as up-conversion devices. Different type up-conversion technology for different wavelength were realized [3, 4]. The previous experience on the IR&THz detectors and the up-conversion devices support us to attempt explore more. In this work, a broadband THz up-conversion device is realized, which is based on the integrated p-type GaAs homojunction interfacial workfunction internal photoemission (HIWIP) detector and LED for pixelless imaging [5]. The choice of a HIWIP detector allows normal incidence excitation thus bypassing the need for a grating coupler required for n-QWIP-LED device. Additionally, the HIWIP-LED up-converter shows a broadband photoresponse (4.2 THz-20 THz) in contrast to the QWP-LED, which makes it general enough to be applied in more situations. Photon-type fast response, broadband, normal incidence excitation and high responsivity, all these advantages make the HIWIP-LED up-conversion device potential high performance for THz imaging.

More details can be found on the journal article of Peng Bai, Yueheng Zhang et al.. https://www.nature.com/articles/s41467-019-11465-6

References:

[1] Simoens, F. et al. Uncooled Terahertz real-time imaging 2D arrays developed at LETI: present status and perspectives. Micro-and Nanotechnology Sensors, Systems, and Applications IX. International Society for Optics and Photonics, 10194-101942N. (2017).

[2] Fu, Z. L. et al. Frequency up-conversion photon-type terahertz imager. Sci. Rep. 6, 25383 (2016).

[3] Bai, P., Zhang, Y. H., Shen, W. Z. Infrared single photon detector based on optical up-converter at 1550 nm. Sci. Rep. 7(1), 15341 (2017).

[4] Yang, Y., Zhang, Y. H., Shen, W. Z. et al. Semiconductor infrared up-conversion devices. Prog. Quant. Electron. 35(4), 77-108 (2011).

[5] Perera, A. G. U. et al. Homojunction internal photoemission farinfrared detectors: Photoresponse performance analysis. J. Appl. Phys. 77, 915-924 (1995).