Ultra-fast Photonics

Article DOI: 10.1002/adpr.202400125

Terahertz (THz) waves are enabling novel applications in imaging and spectroscopy, and expanding the bandwidth for wireless communications. Generation of THz waves of desired spatial structure is often required in modern applications. Recently, advances in nonlinear THz metasurface research offered an alternative approach for generation and spatial structuring of THz beams. 

In our recent publication in Advanced Photonics Research, we demonstrate a binary-phase Fresnel lens comprising InAs nanoscale resonators, which efficiently generate and focus short THz pulses when photoexcited by femtosecond optical pulses. This metalens combines two essential photonic functionalities in a compact flat-lens THz device, illustrating the potential of nonlinear metasurfaces for THz applications.

Article DOI: 10.1364/OE.531690

Terahertz (THz) surface currents in subwavelength plasmonic resonators define the electromagnetic properties of engineered metamaterials and metasurfaces. However, direct detection of these currents remains a challenge. Terahertz aperture-type near-field microscopy offers a solution, enabling the investigation and mapping of surface plasmon currents with subwavelength resolution.

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. Through 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 DOI: 10.1364/OE.517422

Terahertz (THz) continuous wave (CW) spectroscopy can achieve very high spectral resolution by photo-mixing telecommunications-band lasers and has a wide range of scientific and practical applications, from probing atomic excitations in rare-earth doped materials to atmospheric sensing and biomedical imaging. However, traditional THz CW detectors relying on photoconductors, such as InGaAs, face limitations due to low resistivity leading to relatively high noise. Dielectric metasurfaces opened new possibilities for improving THz detection.

In our recent Editor’s Choice publication in Optics Express, we present a solution using a nano-structured low-temperature grown GaAs (LT-GaAs) metasurface. This design harnesses two-step photon absorption enhanced in Mie resonators to enable ultrafast photoconductive switching with 1550 nm telecommunications lasers. Our findings demonstrate improved noise performance and enhanced 1550 nm absorption, highlighting the potential of LT-GaAs metasurfaces for compact, low-noise THz detection in next-generation spectroscopy systems.

Article DOI: 10.1021/acs.nanolett.3c04846

Single-photon sources are essential for quantum communications and quantum computing. Currently, existing solutions are limited by their emission rates and efficiency, as well as by difficulties in single-photon source fabrication and integration with photonic devices. Over the last several years, there have been significant research efforts towards improving the performance of single-photon sources. 

Our recent article published in NanoLetters demonstrates a new approach to enhance the performance of semiconductor quantum dots as single-photon sources. We nanostructured the material around quantum dots as a Huygens’ metasurface consisting of an array of dielectric cubes. It dramatically increases the efficiency of photon outcoupling from the quantum dots embedded in the metasurface. We demonstrate over one order of magnitude enhancement in the emission of single photons, together with the robustness of the developed platform to variations in nanoscale fabrication, making our approach promising for the development of quantum dot-based quantum technologies.

Article DOI: doi.org/10.1364/OE.532840

Since the 1970s, photonic techniques have been developed to enhance the sampling step of Analog-to-Digital Converters (ADCs), leading to the emergence of photonic analog-to-digital conversion. This approach takes advantage of ultra-low jitter optical pulses generated by mode-locked lasers. However, adjusting the cavity length of an MLL during operation is challenging, which limits flexible control over the repetition rate of the generated optical pulse train—crucial for the sampling frequency of photonic ADCs. 

In response to these challenges, we propose and demonstrate a novel photonic sampling technique using an agile optical frequency comb generator that achieves flexible sampling frequencies. By carving multiple comb lines into sampling pulses and dispersing them along a single-mode optical fiber, we can adjust the comb line spacing and select the number of comb lines electronically. Successful sampling and demodulation of QAM signals up to a 17 GHz carrier frequency with a sampling rate up to 50 GSa/s have been demonstrated. A minimum 1.5% EVM at 3 GHz and a maximum 4.4 bits (ENOB) at 2.1 GHz were also measured.

Article DOI: 10.1109/JLT.2024.3452950

Increasing the operating frequency in wireless communications is essential to meet the growing demand for data traffic and higher transmission speeds. The 300 GHz band has been extensively studied for this purpose, with various technologies offering distinct advantages. Purely optoelectronic solutions are particularly appealing, as they provide photonic benefits such as ultra-wide tunability and seamless integration with existing fibre networks. However, electronic receivers, typically based on GaAs Schottky contacts, still offer better sensitivity than current optoelectronic receivers, resulting in higher data rates.

In our recent publication in the IEEE Journal of Lightwave Technology, we present a 300 GHz multi-channel link employing a novel optoelectronic receiver. The receiver is based on a low-barrier InGaAs Schottky mixer, pumped by a photonic local oscillator generated via a high-speed photodiode. This approach achieves sensitivities comparable to state-of-the-art electronic receivers while retaining the advantages of photonics. We demonstrate an aggregated data rate of up to 180 Gbps, illustrating the potential of this approach for use in 6G networks, particularly in backhaul and fronthaul applications.