Near-field imaging techniques at terahertz (THz) frequencies are severely restricted by diffraction. To date, different detection schemes have been developed, based either on sub-wavelength metallic apertures or on sharp metallic tips. However high-resolution THz imaging, so far, has been relying predominantly on detection techniques that require either an ultrafast laser or a cryogenically-cooled THz detector, at the expenses of a lack of sensitivity when high resolution levels are needed. Here, we demonstrate two novel near-field THz imaging techniques able to combine strongly sub-wavelength spatial resolution with highly sensitive amplitude and phase detection capability. The first technique exploits an interferometric optical setup based on a THz quantum cascade laser (QCL) and on a near-field probe nanodetector, operating at room temperature. By performing phase-sensitive imaging of THz intensity patterns we demonstrate the potential of our novel architecture for coherent imaging with sub-wavelength spatial resolution improved up to 17 μm. The second technique is a detector-less s-SNOM system, exploiting a THz QCL as source and detector simultaneously. This approach enables amplitude- and phase-sensitive imaging by self-mixing interferometry with spatial resolution of 60-70 nm.
Terahertz spectroscopy plays a key role in understanding ultrafast carrier dynamics in nanomaterials. Diffraction, however, limits time-resolved terahertz spectroscopy to ensemble measurements. By combining time-resolved terahertz spectroscopy in the multi-terahertz range with scattering-type near-field scanning optical microscopy, we show that we can directly trace ultrafast local carrier dynamics in single nanoparticles with sub-cycle temporal resolution (10 fs). Our microscope provides both 10 nm lateral resolution and tomographic sensitivity, allowing us to observe the ultrafast build-up of a local surface depletion layer in an InAs nanowire.
We report on the development of a novel class of nanowire-based THz detectors in which the field effect transistor (FET) is integrated in a narrow-band antenna. When the THz field is applied between the gate and the source terminals of the FET, a constant source-to-drain photovoltage appears as a result of the non-linear transfer characteristic of the transistor. In order to achieve attoFarad-order capacitance we fabricate lateral gate FET with gate widths smaller than 100 nm. Our devices show a maximum responsivity of 110 V/W without amplification, with noise equivalent power levels ≤ 1 nW/√Hz at room temperature. The 0.3 THz resonant antenna has bandwidth of ~ 10 GHz and opens a path to novel applications of our technology including metrology, spectroscopy, homeland security, biomedical and pharmaceutical applications. Moreover the possibility to extend this approach to relatively large multi-pixel arrays coupled with THz sources makes it highly appealing for a future generation of THz detectors.
Semiconductor nanowires (NWs) represent an ideal building block for implementing rectifying diodes or plasma wave detectors that could operate well into the THz, thanks to the typical attofarad-order capacitance. Despite the strong effort in developing these nanostructures for a new generation of complementary metal-oxide semi conductors (CMOS), memory and photonic devices, their potential as radiation sensors into the Terahertz is just starting to be explored. We report on the development of NW-based field effect transistors operating as high sensitivity THz detectors in the 0.3 - 2.8 THz range. By feeding the radiation field of either an electronic THz
source or a quantum cascade laser (QCL) at the gate-source electrodes by means of a wide band dipole antenna, we measured a photovoltage signal corresponding to responsivity values up to 100 V IW, with impressive noise equivalent power levels < 6 x 10-11W/Hz at room temperature and a > 300kHz modulation bandwidth. The potential scalability to even higher frequencies and the technological feasibility of realizing multi-pixel arrays coupled with QCL sources make the proposed technology highly competitive for a future generation of THz detection systems.
Self-assembled nanowires represent a new interesting technology to be explored in order to increase the cut-off
frequency of electronic THz detectors. They can be developed in field effect transistor (FET) and diode geometries
exploiting non-linearities of either the transconductance or the current-voltage characteristic as detection mechanism. In
this work we demonstrate that semiconductor nanowires can be used as building blocks for the realization of highsensitivity
terahertz one-dimensional FET detectors. In order to take advantage of the low effective mass and high
mobilities achievable in III-V compounds, we have used InAs nanowires, grown by vapor-phase epitaxy, and properly
doped with selenium to control the charge density and to optimize source-drain and contact resistance. The detection
mechanism exploits the non-linearity of the transconductance: the THz radiation field is fed at the gate-source electrodes
with wide band antennas, and the rectified signal is then read at the drain output in the form of a DC voltage.
Responsivity values as large as 1 V/W at 0.3 THz have been obtained, with noise equivalent powers (NEP) < 2 × 10-9
W/√Hz at room temperature. The large existing margins for technology improvements, the scalability to higher
frequencies, and the possibility of realizing multi-pixel arrays, make these devices highly competitive as a future
solution for THz detection.