ARC Centre of Excellence for Transformative Meta-Optical Systems

Electron spins coupled with optical transitions in solids stand out as a promising platform for developing spin-based quantum technologies. Recently, hexagonal boron nitride has emerged as a promising host for optically addressable spin systems. However, controlled generation of isolated single-photon emitters with predetermined spin transitions has remained elusive. Here we report on a single-step thermal processing of hexagonal boron nitride flakes that produces high-density, narrowband quantum emitters with optically active spin transitions, with over 25% of the emitters exhibiting a clear signature of an optical spin read-out at room temperature. The generated spin defect complexes exhibit both S = 1 and S = 1/2 transitions, which are explained by charge transfer from strongly to weakly coupled spin pairs. Our work advances the understanding of spin complexes in hexagonal boron nitride and paves the way for single spin-photon interfaces in layered materials with applications in quantum sensing and information processing.

ABSTRACT Biphotons from spontaneous parametric down conversion with broad bandwidth are highly wanted in many quantum technologies. However, achieving broad bandwidth in both frequency and momentum while keeping a high rate remains a challenge for both conventional nonlinear crystals and recently emerging nonlinear metasurfaces. Here, we address this challenge by introducing a periodically poled Brillouin zone folding (PP‐BZF) metasurface incorporating a periodically perturbed silica grating atop a periodically poled lithium niobate film. PP‐BZF metasurfaces support high‐Q guided mode resonances at pump, signal and idler wavelengths while enabling quasi‐phase matching between three guided modes in a broad frequency/momentum range. The predicted biphoton rate is over 100 MHz/mW with a frequency bandwidth of 20.5 THz (1472.5–1637.8 nm) and a momentum bandwidth of , improving the state‐of‐the‐art for metasurface biphoton sources by over three orders of magnitude in rate and one order of magnitude in bandwidths. This ultrathin broadband bright biphoton source could stimulate system‐level miniaturization of various free‐space quantum photonic technologies.

ABSTRACT Integrated quantum photonics is rapidly emerging as a transformative platform for realizing scalable, chip‐based quantum technologies, from secure communication to advanced sensing and computing. At the heart of this revolution are solid‐state quantum emitters: atom‐like light sources embedded in a variety of host materials that enable single‐photon generation, quantum interference, and spin–photon coupling. We begin by discussing quantum emitters across bulk crystals, van der Waals materials, and chemically synthesized nanostructures, highlighting their performance trade‐offs and integration potential. We then examine nanophotonic cavities that enhance the light‐matter interaction, from photonic crystals to metasurfaces and Moiré resonators. Next, we discuss fabrication techniques with sub‐10 nm precision, including material‐specific etching strategies and hybrid integration schemes. Finally, we address emerging approaches for deterministic placement, cavity tuning, and on‐chip scalability. By bringing together insights from materials science, nanofabrication, and quantum optics, this Review provides a comprehensive perspective on the design and integration of high‐performance quantum emitters for scalable photonic platforms.

Abstract Time-reversal and space-inversion symmetries are fundamental properties of crystals and play a role in underlying phenomena such as magnetism, topology and non-trivial spin textures. Transition metal dichalcogenides (TMDs) represent an excellent tunable model system to explore the interplay between these symmetries as they can be engineered on demand by tuning the number of layers and via all-optical bandgap modulation. In this work, we modulate and study time-reversal symmetry in mono- and bilayer TMDs with all-optical methods using third-harmonic Faraday rotation. We excite the samples using elliptically polarized light, achieve spin-selective bandgap modulation and consequent breaking of time-reversal symmetry. The reduced symmetry modifies the nonlinear susceptibility tensor, causing a rotation of the emitted third-harmonic polarization. With this method, we probe broken time-reversal symmetry in both non-centrosymmetric (monolayer) and centrosymmetric (bilayer) WS 2 crystals. Furthermore, we discuss how the detected third-harmonic rotation angle directly links to spin-valley locking in monolayer TMDs and spin-valley-layer locking in bilayer TMDs. Our results show a powerful approach to study broken time-reversal symmetry in crystals regardless of space-inversion symmetry, and shed light on the spin, valley and layer coupling of atomically thin semiconductors.

Abstract III–V semiconductors, particularly InP, have demonstrated significant potential for seamless integration into nanophotonic systems. However, conventional zincblende InP crystals exhibit inherently weak second‐harmonic generation (SHG) due to limited second‐order nonlinear susceptibility, presenting a major challenge for developing effective nonlinear optical components in integrated photonic circuits. Here, it is demonstrated that large‐area biphasic wurtzite‐zincblende (WZ–ZB) interfaces can remarkably enhance SHG in InP nanomembranes, yielding up to a 200‐fold intensity increase in the short‐wave infrared range. This enhancement stems from the type‐II band alignment at the polytypic interface, which spatially separates photogenerated carriers, prolonging carrier lifetimes and sustaining a strong nonlinear polarization source. The abrupt phase boundary and the nanomembrane's large surface‐to‐volume ratio introduce additional symmetry‐breaking at the interface, which further amplifies the SHG signal. The polarization dependence of SHG and the resulting SHG‐induced photoluminescence exhibit anisotropic features. The results establish crystalline phase engineering in WZ–ZB InP homostructures as a highly effective strategy for tailoring nonlinear optical processes, offering new pathways for high‐efficiency wavelength conversion and integrated on‐chip nonlinear photonic circuits.

This paper describes the analysis and comparison of the electron current collection of a cylindrical tether for different variables like plasma density and temperature, plasma ion species, tether material, and applied bias. The plasma is generated by ionizing hydrogen, oxygen, and ambient air in a 50 cm × 40 cm plasma chamber. Multiple plasma densities were generated in the range 1 0 14 - 1 0 15 particles per m 3 with corresponding plasma temperatures 2.5 - 4.3 eV. EDT materials — aluminum, tungsten, and aracon were used in the experiment to examine the material performance in the generated plasma environment and evaluate the impact on the current collected. Several observations have been made from the data collected. The I-V curves vary due to each ionized plasma, indicating the impact of ion species on the electron current. The OML model overestimates the current for each experiment run analyzed. Aracon sample exhibited superior performance when placed in oxygen and air plasma, whereas aluminium demonstrated higher I-V curves in hydrogen plasma. Plasma bubbles and secondary plasma were observed during the experiment for both positively and negatively biased tether samples. In the positively biased regime, anomalous electron current occurred for certain plasma densities. In the negatively biased regime, the ion current for hydrogen plasma was recorded to be higher than the oxygen and air plasma. It was concluded that the OML does not accurately estimate the collected current, as more complex interactions are involved. • Electron current collection varies with plasma ion species and tether material. • OML theory overestimates EDT current collected. • Neutral particle density strongly influences EDT current collection efficiency. • OML deviations arise from secondary plasma effects. • Aracon material shows superior performance in oxygen and air plasma conditions.

Enhancing light-matter interactions with photonic structures is critical in classical and quantum nanophotonics. Recently, Moiré twisted bilayer optical materials have been proposed as a promising means toward a tunable platform for nanophotonic devices. However, the realization of Moiré photonic crystal (PhC) cavities has been challenging, due to a lack of advanced nanofabrication techniques and availability of stand-alone transparent membranes. Here, we leverage the properties of the van der Waals material hexagonal boron nitride to realize Moiré bilayer PhC cavities. We design and fabricate a range of devices with controllable twist angles, with flatband modes in the visible spectral range (∼450 nm). Optical characterization confirms the presence of spatially periodic cavity modes originating from the engineered dispersion relation (flatband). Our findings present a major step toward harnessing a two-dimensional van der Waals material for the next generation of on chip, twisted nanophotonic systems.

Abstract There is a need for high performance and compact mid‐infrared hyperspectral imaging and spectroscopy systems for the reliable optical detection of chemical contaminants in environmental science, agriculture, heavy industry, and defense contexts. These applications are traditionally serviced by Fourier transform infrared (FTIR) spectrometers, but such systems are large, heavy, power hungry and of high cost. Tunable bandpass filters present a means to achieve mid‐infrared spectrometers that are very compact, but previous approaches based on microelectromechanical systems are hindered by complex fabrication methods. Herein, how the thermo‐optic effect can be exploited in zero‐contrast grating (ZCG) based filters to create tunable filters for optical sensing applications that are solid state and narrowband is shown. Thermo‐optically tunable mid‐infrared bandpass filters are fabricated with center wavelengths around 3.45 µm based on one and two‐dimensional ZCGs. It is showed that the center wavelength of the passband can be tuned up to 141 nm by varying the temperature (by 780 K), which exceeds the full‐width‐at‐half‐maximum of the passband (29 nm). It is demonstrated that the application of these devices to mid‐infrared spectral reconstruction of the transmittance of several IR‐transparent samples.

Bound states in the continuum (BICs) in photonics have drawn wide interest due to the infinite quality factors (Q factors) and the topological features of the BICs. BICs can be realized by leveraging the coupling of electromagnetic modes, referred to as Friedrich-Wintgen BICs (FW-BICs). However, the impact of coupling coefficients has not been profoundly explored. In this study, we numerically and experimentally demonstrate that a metasurface composed of two concentric split-ring resonators (SRRs) supports multiple FW-BICs. This can be observed by tuning the intra-action of two meta-atoms of SRRs. We observed that the resonant frequency, radiative line width, and coupling of resonance modes can be tuned by varying the width of an outer SRR or that of the inner SRR. Eigenmode analysis indicates that all coupling-assisted FW-BICs were in the center of the far-field polarization vortex with a certain topological charge. We observed that when a FW-BIC was induced by the coupling of modes with different bandwidths, the narrower mode would evolve into FW-BIC. In the coupled-mode theory (CMT) framework, the radiative damping rate determines at which mode BIC occurs. Our results reveal the branch selection character of FW-BIC, which can facilitate the design of metasurfaces with high-Q factors.