Terahertz time-domain spectroscopy (THz-TDS) has been broadly explored for applications covering all science areas. Polarization-resolved THz-TDS is considered much more powerful as it enables the extraction of polarization information, giving important insights into the anisotropic properties of the sample (for example, birefringence). This allows a complete understanding of the sample with higher accuracy. However, polarization-resolved THz-TDS is not routinely accessible yet, due to the lack of technological progress towards broadband, fast and precise THz sensing. 

In our previous work, semiconductor nanowires have been found to possess an intrinsically short photoconductivity lifetime while conserving high photocarrier mobility, particularly suitable for broadband low-noise photoconductive THz detection in THz-TDS. Additionally, owing to the 1D nature of nanowires, nanowire-based THz detectors exhibit high sensitivity to the THz polarisation, which is favored in the design of multi-contact THz detector architecture for fast and accurate measurements of the full polarisation state of THz radiation. While the performance of nanowire THz detectors has not met industrial needs, the key issue is how to efficiently scale up nanowire detector systems, e.g. to produce parallel nanowire detectors for improved signal-to-noise ratio and detector robustness or to allow large-scale multi-pixel arrays for imaging applications.  

We recently demonstrated the use of wafer-scalable horizontally-grown InAs nanowires to develop photoconductive THz detectors. Conventional device fabrication is based on vertically-grown, free-standing nanowires, requiring a time-consuming transfer and alignment procedure by nano-manipulation or transfer printing to create a horizontal device architecture. In contrast, our nanowires were grown horizontally on the as-grown substrate via selective-area epitaxy that can be straightforwardly processed into device by directly depositing the metal electrodes on the substrate, significantly reducing device fabrication complexity. Moreover, this approach is inherently wafer scalable, the parallel nanowire arrays or multi-pixel nanowire arrays can be practically achieved, promising a feasible solution towards nanowire-based polarization-sensitive focal-plane-array THz cameras that can be scaled up to thousands of pixels. We also demonstrated that the faceting and cross-sectional shape of the nanowires can be engineered as a function of growth parameters and pattern geometry. This provides additional tools to tune surface recombination dynamics, allowing the detection of terahertz radiation via both direct and integrating sampling modes, opening new routes for further device performance optimization.