News

Sep 2025

Inter-resonator coupling in topological insulator dimers

Surface plasmon polaritons in topological insulators hold promise for confining and manipulating light at THz frequencies. However, it is still challenging to detect these excitations, and therefore their properties are barely known.

In a recently published Article in Light: Science and Applications, we experimentally demonstrate that coupling between neighbouring Bi2Se3 dipole antennas (forming a dimer) is sufficiently strong to affect the dispersion of surface plasmons. The inter-resonator coupling leads to an increase of the plasmon-polariton momentum by over 20%, resulting in values more than one order of magnitude higher than the free-space photon momentum.

We investigated surface plasmons by launching and detecting them simultaneously with a scattering-type scanning THz near-field microscopy probe. This near-field microscopy technique revealed surface plasmon-polariton resonances in single and coupled Bi2Se3 antennas.

Article by L. Viti et al. in Light: Science and Applications

Graphics: Illustration of THz surface plasmon-polaritons launched on the surface of a TI antenna by a metallic AFM tip.

July 2025

Carrier-Envelope Phase Control in Terahertz Pulse Generation

Generation of terahertz (THz) pulses with variable carrier-envelope phase can enable the tailoring of THz beams, opening the door to advanced structured THz wavefront synthesis.

In our recently published Article in ACS Photonics, we demonstrate metasurfaces composed of InAs nanoscale ribbon arrays capable of generating THz pulses with variable carrier-envelope phase when photoexcited by femtosecond optical pulses. We achieved this phase control solely through the ribbon geometry that allows activating different THz generation mechanisms.

The InAs ribbon geometry provides a simple approach to controlling the effect of THz pulse generation via two distinct mechanisms, the lateral photocurrents and shift currents, while supporting some of the most efficient non-linear THz generation processes.

Article by S. Norman et al. in ACS Photonics

Graphics: Illustration of THz pulse generation with differen carrier-envelope phases using InAs nanoscale ribbon arrays (graphics by S. Norman)

Jun 2025

Inaugural Lecture - UCL - Unveiling terahertz electromagnetic fields

Terahertz (THz) research and technology development have been experiencing a rapid growth in recent years. THz technologies are now viewed as an integral part of future photonics and electronics. They promise applications in wide-bandwidth wireless communications, spectroscopy and sensing, industrial and biomedical imaging, and in fundamental scientific research.

In this Lecture, I will overview the development of THz near-field microscopy, the technique that has helped unveiling THz electromagnetic fields and related phenomena, playing a key role in scientific investigations and in the development of THz photonics.

Lecture recording (June 19, 2025)

Lecture Info - UCL Discovery

Dec 2024

InAs Terahertz Metalens Emitter

December issue of Advanced Photonics Research features a non-linear metalens terahertz (THz) emitter developed by UCL and Sandia National Laboratories in the United States. A binary-phase Fresnel lens comprising InAs nanoscale optical resonators can generate and focus short THz pulses when photoexcited by femtosecond optical pulses.

This metalens combines two essential functionalities, generation of THz pulses and their spatial structuring, in a compact flat-optics device, demonstrating the potential of nonlinear metasurfaces for novel THz applications, including imaging, spectroscopy and future wireless communications.

The InAs metasurface provides one of the most efficient non-linear THz generation platforms.

Article by H. Jung et al. in Advanced Photonics Research

Back Cover in Advanced Photonics Research

Graphics: Illustration of the THz emitter lens consisting of nanoscale InAs resonators (graphics by H. Jung)

June 2024

Non-contact imaging of terahertz surface currents with aperture-type near-field microscopy

Plasmonic resonators enabled engineering of electromagnetic properties of metasurfaces at Terahertz (THz) frequencies, whereas surface plasmon currents in these resonators define the underlying physics. Direct detection of these surface plasmon currents remains a challenge.

In our recent publication in Optics Express, we demonstrate non-contact imaging and local spectroscopy of THz surface currents in subwavelength resonators using aperture-type scanning near-field microscopy.

Using near-field mapping of an asymmetric split-ring resonator, we show the direct correlation between the detected near-field signal and the THz surface currents, highlighting the technique's potential for understanding the response of THz devices and exploring fundamental light-matter interactions.

Article by S. Norman et al. in Optics Express

Graphics: Excitation of asymmetric surface currents in a metallic THz resonator. The insets show experimental maps of two modes detected by the aperture-type THz near-field microscopy.

Apr 2024

Emission from single GaAs Quantum Dots Embedded in Huygens' Metasurface

Single-photon emitters are essential for quantum communications and quantum computing. Semiconductor quantum dots represent a promising technology as a source of single photons.

In our recent article published in Nano Letters we demonstrate a new approach to enhance the performance of semiconductor quantum dots as single-photon sources. When the material around a quantum dot is nanostructured as a Huygens' metasurface, for example as an array of dielectric cubes, the efficiency of photon outcoupling from the quantum dots embedded in the metasurface is enhanced by over one order of magnitude.

We demonstrate this outcoupling enhancement experimentally using GaAs QDs embedded in an AlGaAs Huygens' metasurface. Our approach is compatible with standard semiconductor fabrication processing.

Article by P. Iyer et al. in Nano Letters

Graphics: Illustration of a quantum dot emitter embedded in a Huygens' metasurface.

Feb 2024

LT GaAs metasurface harnesses two-step photon absorption in Mie resonators enabling terahertz detection using telecom lasers

Terahertz (THz) spectroscopy has a wide range of scientific and practical applications, from probing atomic excitations in rare-earth doped materials to atmospheric sensing and biomedical imaging. Continuous wave (CW) THz spectroscopy can achieve very high spectral resolution by photo-mixing telecommunication-band lasers. Traditional THz CW detectors compatible with telecom lasers however face limitations due to low resistivity leading to relatively high detector noise.

Photoconductive metasurfaces opened new possibilities for improving THz detection. In our recent publication in Optics Express (highlighted as Editor's Choice), we developed and demonstrated ultrafast low-temperature grown GaAs (LT-GaAs) photoconductive detector which can operate with 1550 nm telecommunication lasers.

The LT-GaAs metasurface harnesses two-step photon absorption enhanced in nano-structured Mie resonators. Our results demonstrate enhanced light absorption at 1550 nm and improved detector noise performance, highlighting the potential of LT-GaAs metasurfaces for compact, low-noise THz detection in next-generation spectroscopy systems.

Article in Optics Express

Graphics: Photoexcitation in the LT GaAs metasurface region between electrodes in the THz photoconductive detector.

Oct 2023

Review of Near-field imaging and spectroscopy of terahertz resonators and metasurfaces

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, exploiting advanced modes, 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.

In our Review article in Optical Materials Express, we discuss a series of case studies which highlight recently developed THz near-field microscopy capabilities for research on THz metasurfaces and subwavelength THz resonators.

Review Article in Optical Materials Express

Graphics: Illustration of THz metasurface composed of dipolar antennae with regions of resonantly enhanced fields glowing blue.