Efficient probes for terahertz near-field microscopy and spectroscopy

This thesis focuses on improving the sensitivity and spatial resolution of two near-field microscopy techniques at terahertz (THz) frequencies: direct detection in the near-field using a collection-mode aperture probe, and using the apex of a metallic tip to scatter a near-field interaction into the far-field. The first technique is limited in spatial resolution primarily due to strong attenuation of THz fields transmitted through the subwavelength aperture. By integrating a terahertz detector with an optical metasurface it is possible to make nanoscale terahertz detectors, which can efficiently detect non-propagating THz fields close to the rear of the aperture, increasing probe sensitivity and spatial resolution. The scattering technique suffers from high background signals and weak scattering from the tip apex. By designing a scattering probe to act as a resonant dipole antenna, the efficiency of the scattering process can be improved, and by efficiently coupling THz radiation to the probe using a radially polarized THz source, interference from background signals can be reduced. These improvements can enable imaging of a variety of fascinating systems, including polaritons in monolayers and heterostructures of 2D materials, biological systems and topological insulators

Near-field imaging and spectroscopy of terahertz resonators and metasurfaces [Invited]

Terahertz (THz) metasurfaces have become a key platform for engineering light-matter interaction at THz frequencies. They have evolved from simple metallic resonator arrays into tunable and programmable devices, displaying ultrafast modulation rates and incorporating emerging quantum materials. The electrodynamics which govern metasurface operation can only be directly revealed at the scale of subwavelength individual metasurface elements, through sampling their evanescent fields. It requires near-field spectroscopy and imaging techniques to overcome the diffraction limit and provide spatial resolution down to the nanoscale. Through a series of case studies, this review provides an in-depth overview of recently developed THz near-field microscopy capabilities for research on metamaterials.</p

Near-field imaging and spectroscopy of terahertz resonators and metasurfaces [Invited]

Terahertz (THz) metasurfaces have become a key platform for engineering light-matter interaction at THz frequencies. They have evolved from simple metallic resonator arrays into tunable and programmable devices, displaying ultrafast modulation rates and incorporating emerging quantum materials. The electrodynamics which govern metasurface operation can only be directly revealed at the scale of subwavelength individual metasurface elements, through sampling their evanescent fields. It requires near-field spectroscopy and imaging techniques to overcome the diffraction limit and provide spatial resolution down to the nanoscale. Through a series of case studies, this review provides an in-depth overview of recently developed THz near-field microscopy capabilities for research on metamaterials.ISSN:2159-393

Near-field imaging and spectroscopy of terahertz resonators and metasurfaces [Invited]

Terahertz (THz) metasurfaces have become a key platform for engineering light-matter interaction at THz frequencies. They have evolved from simple metallic resonator arrays into tunable and programmable devices, displaying ultrafast modulation rates and incorporating emerging quantum materials. The electrodynamics which govern metasurface operation can only be directly revealed at the scale of subwavelength individual metasurface elements, through sampling their evanescent fields. It requires near-field spectroscopy and imaging techniques to overcome the diffraction limit and provide spatial resolution down to the nanoscale. Through a series of case studies, this review provides an in-depth overview of recently developed THz near-field microscopy capabilities for research on metamaterials.</p

Resonant terahertz probes for near-field scattering microscopy

We propose and characterize a scattering probe for terahertz (THz) near-field microscopy, fabricated from indium, where the scattering efficiency is enhanced by the dipolar resonance supported by the indium probe. The scattering properties of the probe were evaluated experimentally using THz time-domain spectroscopy (TDS), and numerically using the finite-difference time-domain (FDTD) method in order to identify resonant enhancement. Numerical measurements show that the indium probes exhibit enhanced scattering across the THz frequency range due to dipolar resonance, with a fractional bandwidth of 0.65 at 1.24 THz. We experimentally observe the resonant enhancement of the scattered field with a peak at 0.3 THz. To enable practical THz microscopy applications of these resonant probes, we also demonstrate a simple excitation scheme utilizing a THz source with radial polarization, which excites a radial mode along the length of the tip. Strong field confinement at the apex of the tip, as required for THz near-field microscopy, was observed experimentally.</p

Resonant scattering probes in the terahertz range

We propose and demonstrate a novel near-field probe, resonant at terahertz (THz) frequencies. A conical metal tip, with length 50-1000 μm is fabricated directly to the surface of a semiconductor surface, acting as a radially-polarized THz source. High field confinement is seen at the apex of the probe. The scattering efficiency is frequency dependent, and is likely related to the dipolar resonance along the length of the probe.</p

Resonance-Enhanced Terahertz Nanoscopy Probes

Near-field nanoscopy at terahertz (THz) frequencies is uniquely placed to probe a multitude of physical phenomena at the nanoscale. However, at THz frequencies, the scattering efficiency of standard near-field probes is poor, limiting the sensitivity and, hence, resolution of the technique. Here, we propose and demonstrate tunable resonant scattering from metal tips that allow us to overcome this limitation. Tips supporting a λ/2 dipolar resonance in the THz range are fabricated from indium metal directly on the tine of a quartz tuning fork. We observe enhancement of the THz scattering efficiency at the resonance frequency with a Q-factor of ∼2-3. These tips enable a subwavelength spatial resolution better than 100 nm. We support the experimentally observed enhancement using a numerical model. The enhanced scattering efficiency afforded by the resonant indium tips can enable the probing of new phenomena, such as plasmons in two-dimensional materials, that have proven difficult to observe thus far.</p

Resonant terahertz probes for near-field scattering microscopy

We propose and characterize a scattering probe for terahertz (THz) near-field microscopy, fabricated from indium, where the scattering efficiency is enhanced by the dipolar resonance supported by the indium probe. The scattering properties of the probe were evaluated experimentally using THz time-domain spectroscopy (TDS), and numerically using the finite-difference time-domain (FDTD) method in order to identify resonant enhancement. Numerical measurements show that the indium probes exhibit enhanced scattering across the THz frequency range due to dipolar resonance, with a fractional bandwidth of 0.65 at 1.24 THz. We experimentally observe the resonant enhancement of the scattered field with a peak at 0.3 THz. To enable practical THz microscopy applications of these resonant probes, we also demonstrate a simple excitation scheme utilizing a THz source with radial polarization, which excites a radial mode along the length of the tip. Strong field confinement at the apex of the tip, as required for THz near-field microscopy, was observed experimentally.</p

Resonant scattering probes in the terahertz range

We propose and demonstrate a novel near-field probe, resonant at terahertz (THz) frequencies. A conical metal tip, with length 50-1000 μm is fabricated directly to the surface of a semiconductor surface, acting as a radially-polarized THz source. High field confinement is seen at the apex of the probe. The scattering efficiency is frequency dependent, and is likely related to the dipolar resonance along the length of the probe.</p