Direct and integrating sampling in terahertz receivers from wafer-scalable InAs nanowires

Abstract

Terahertz (THz) radiation will play a pivotal role in wireless communications, sensing, spectroscopy and imaging technologies in the decades to come. THz emitters and receivers should thus be simplified in their design and miniaturized to become a commodity. In this work we demonstrate scalable photoconductive THz receivers based on horizontally-grown InAs nanowires (NWs) embedded in a bow-tie antenna that work at room temperature. The NWs provide a short photoconductivity lifetime while conserving high electron mobility. The large surface-to-volume ratio also ensures low dark current and thus low thermal noise, compared to narrow-bandgap bulk devices. By engineering the NW morphology, the NWs exhibit greatly different photoconductivity lifetimes, enabling the receivers to detect THz photons via both direct and integrating sampling modes. The broadband NW receivers are compatible with gating lasers across the entire range of telecom wavelengths (1.2-1.6 μm) and thus are ideal for inexpensive all-optical fibre-based THz time-domain spectroscopy and imaging systems. The devices are deterministically positioned by lithography and thus scalable to the wafer scale, opening the path for a new generation of commercial THz receivers.

Fig. 1. Nanowire device geometry.

Schematic diagram of a horizontal InAs (a) multi-faceted NW and (b) single-faceted NW grown on the top of a GaAs nanoridge. c, d HAADF-STEM micrographs of the NW cross sections, corresponding to (a) and (b), respectively, showing InAs NW (bright grey) and GaAs nanoridge (dark grey). SEM images of the fabricated InAs NW receivers: e 1NW receiver; f 5NW receiver; (g) 20NW receiver.

Fig. 2. Direct vs. integrating sampling.

a Schematic diagram of a photoconductive NW THz receiver in operation, whose THz response for a detection material with (case 1) short carrier lifetime (<<1 ps) and with (case 2) long carrier lifetime (>>1 ps) illustrated on the right. b–d Raw data of THz responses obtained from a multi-faceted 1NW receiver, a single-faceted 5NW receiver and a single-faceted 20NW receiver, respectively. All above receivers have an identical antenna gap size of 1 µm and were optically gated with 1.5-µm laser pulses of 1.66 mJ/cm² fluence. The red dash-dotted line represents raw data of THz response obtained from an ion-implanted InP bulk receiver (as reference), which was optically gated with 800-nm laser pulses of 1.5 mJ/cm² fluence.

Fig. 3. THz characterisation.

a Comparison of raw data of THz responses obtained from a 5NW and a 1NW receiver under varying excitation wavelengths of 0.98 mJ/cm² fluence, respectively. b Comparison of raw data of THz responses obtained from a 5NW (left) and a 1NW (right) receiver under varying excitation fluences. The 5NW is optically gated by a 1.2 μm-wavelength laser and the 1NW receiver is optically gated by a 1.55 μm-wavelength laser. c Comparison of raw data of THz responses obtained from 1NW receivers made of a 6.75-µm-long NW (identified as multi-faceted) and a 14-µm-long NW (identified as undeterminable-faceted). d Raw data of THz response obtained from a multi-faceted 1NW receiver and its corresponding THz spectral response as shown in (e). f Raw data of THz response from a single-faceted 5NW THz receiver and its recovered THz transient as shown in (g) by differentiation of data in (f), which was further converted into THz spectral response as shown in (h). Both receivers were optically gated by a 1.5-μm-wavelength laser under fluence of 0.98 mJ/cm².