Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2

The density-driven transition of an exciton gas into an electron–hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron–hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron–hole pairs within a WSe2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron–hole plasma above 7 × 1012 cm–2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials

Efficiency Enhancement of Scattering Near-Field Probes

We measure, for the first time, the scattering efficiency of resonant terahertz (THz) probes for scattering-type THz near-field microscopy. We fabricate the probes by placing an indium 'antenna' directly on the tine of a quartz tuning fork (QTF), which we use as an atomic force microscope (AFM) probe in tapping mode. THz time-domain spectroscopy (TDS) of the THz field scattered from the probe shows that the scattering efficiency of the indium antenna exhibits resonant enhancement determined by the antenna length. These resonant scattering probes can enable THz near-field imaging applications where THz contrast is weak, such as 2D materials or biological systems.</p

Terahertz nano-spectroscopy with resonant scattering probes

We propose and demonstrate tunable resonant scattering probes for terahertz (THz) near-field microscopy, using sharp indium tips fabricated to the tine of a quartz tuning fork. We find the antenna resonance of the indium tips can be tuned by altering the tip length, which we support with numerical models. We also demonstrate the indium tips can provide nanoscale field confinement at the tip apex, with spatial resolution better than 100 nm.</p

Resonant scattering probes for terahertz near-field microscopy

We propose and demonstrate a scattering-type near-field probe, designed to increase the sensitivity of high-resolution scattering probe microscopy at terahertz (THz) frequencies. For efficient scattering of THz radiation, the probe, fabricated from indium, is designed to resonate like a dipole antenna. Efficient excitation is achieved by integrating the probe with a radially-polarized THz source. Using time-domain spectroscopy (TDS), we observe resonant enhancement of the scattered fields, and using aperture-type near-field microscopy, we see high field confinement at the scattering probe apex.</p

Resonant scattering probes for terahertz near-field microscopy

We propose and demonstrate a scattering-type near-field probe, designed to increase the sensitivity of high-resolution scattering probe microscopy at terahertz (THz) frequencies. For efficient scattering of THz radiation, the probe, fabricated from indium, is designed to resonate like a dipole antenna. Efficient excitation is achieved by integrating the probe with a radially-polarized THz source. Using time-domain spectroscopy (TDS), we observe resonant enhancement of the scattered fields, and using aperture-type near-field microscopy, we see high field confinement at the scattering probe apex.</p

Nanostructured photoconductive terahertz detector for near-field microscopy

We demonstrate that a nanostructured terahertz (THz) photoconductive detector enables sampling of THz fields localized on micrometer scale. The nanostructure, consisting of an optical nanoantenna array and a distributed Bragg reflector, acts as a hybrid cavity, which traps optical gate pulses within an optically thin photoconductive layer of the THz detector. This allows us to detect highly confined (&lt;1 μm) evanescent THz fields. By monolithically integrating this THz detector with apertures ranging from 2 μm to 5 μm we achieve higher spatial resolution and higher sensitivity in aperture-type THz near-field microscopy and THz time-domain spectroscopy.</p

Tunable Fully Absorbing Metasurfaces for Efficient THz Detection

Terahertz photoconductive antennas with a nanostructured active region have been actively investigated recently with a goal to achieve high efficiency THz detectors and emitters. Here we provide a novel design of perfectly-absorbing photoconductive region without plasmonic elements using a metasurface, and provide a systematic method by which the metasurface can be designed to work optimally for varying optical gate frequencies across the GaAs band-gap. This paves the way to using metasurface devices for THz detection and other applications in a wide range of laser systems operating at different wavelengths or with different photoconductive materials.</p

Probe-sample interaction in aperture-type THz near-field microscopy of complementary resonators

Subwavelength complementary metallic resonators operating in the terahertz (THz) regime are investigated with aperture near-field microscopy and spectroscopy. In contrast to far-field methods, the spectra of individual isolated resonators can be retrieved. We find that we can experimentally gain spectral information without modifying the spectral properties of the resonator with the aperture-type near-field probe by operating it at a separation distance greater than 10 μ {m}.</p

Efficient Terahertz Detection with Perfectly-Absorbing Metasurface

We demonstrate a unique photoconductive design for terahertz (THz) detection based on a perfectly absorbing, all-dielectric metasurface. Our design exploits Mie resonances in electrically connected cubic resonators fabricated in low-temperature grown (LT) GaAs. Experimentally, the detector achieves very high contrast between ON/OFF conductivity states (107) whilst also requiring extremely low optical power for optimal operation (100 muW). We find that the Mie resonances dissipate sufficiently fast and maintain the detection bandwidth up to 3 THz.</p