Exciton dynamics in layered magnetic semiconductors provide a sensitive probe of the interplay between spin order and light-matter interaction. Here, we study thin CrSBr layers using time-resolved photoluminescence spectroscopy in an external magnetic field, revealing a step-like reduction in the exciton lifetime from 11 to 7 ps, during the magnetization flip from the antiferromagnetic to the ferromagnetic phase. The reduction of the exciton lifetime in the ferromagnetic phase persists below the Néel temperature, as evidenced by its strong magnetic-field dependence that disappears in the paramagnetic phase. Ab initio calculations reveal a one-dimensional nature of free excitons accompanied by a pronounced change in the oscillator strength across the magnetic phase transition predicting a shorter radiative lifetime of free excitons in the antiferromagnetic phase of CrSBr contradicting the experimental observations. This discrepancy is explained by strong localization of excitons at low tempature. We show both experimentally and theoretically that the observed magnetic switching of the exciton lifetime is attributed to a larger exciton localization volume leading to a larger oscillator strength in the ferromagnetic phase. The results show that disorder-induced localization effects play a key role in exciton dynamics in CrSBr.

Two-dimensional (2D) semiconductors are emerging as a versatile platform for nanophotonics, offering unprecedented tunability in optical properties through exciton resonance engineering, van der Waals heterostructuring, and external field control. These materials enable active optical modulation, single-photon emission, quantum photonics, and valleytronic functionalities, paving the way for next-generation optoelectronic and quantum photonic devices. However, key challenges remain in achieving large-area integration, maintaining excitonic coherence, and optimizing amplitude-phase modulation for efficient light manipulation. Advances in fabrication, strain engineering, and computational modeling will be crucial to overcoming these limitations. This Perspective highlights recent progress in 2D semiconductor-based nanophotonics, emphasizing opportunities for scalable integration into photonics.

Efficient photoluminescence (PL) of layered semiconductors is crucial for advancing next-generation photonic devices. However, thermal effect-induced destruction typically hinders the practical applications, such as biosensing and imaging. Here, the upconversion PL of multilayer GaSe is reported, which circumvents thermal damage. A high-order multiphoton (up to 8-photon) PL is first reported in multilayer GaSe. Both experimental and theoretical results reveal a power-dependent redshift of the PL peak (approximate to 40 meV, equivalent to 2% of the bandgap) and PL spectral broadening (full width at half maximum increased by approximate to 2 times), attributed to the hot electron-hole plasma. Time-resolved PL resolves the multistage of carrier relaxation, revealing an ultrafast transition (approximate to 58 ps) from electron-hole plasma to excitonic states, which establishes hot electron-hole plasma engineering as a critical mechanism for manipulating PL processes in Group-III-VI chalcogenides. Furthermore, wavelength-dependent two- and three-photon PL spectra are explored. These results establish a microscopic framework connecting hot electron-hole plasma dynamics with macroscopic optoelectronic phenomena, providing critical insights for designing ultrafast photonic modulators and nonlinear optical devices based on 2D layered semiconductors.

Deterministic and versatile approaches to sample preparation on nanoscopic scales are important in many fields including photonics, electronics, biology and material science. However, challenges exist in meeting many nanostructuring demands–particularly in emerging optical materials and component architectures. Here, we report a nanofabrication workflow that overcomes long-standing challenges in deterministic and top-down sample preparation procedures. The salient feature is a carbon mask with a low sputter yield that can be readily shaped using high resolution electron beam processing techniques. When combined with focused ion beam processing, the masking technique yields structures with ultra-smooth, near-vertical side walls. We target different material platforms to showcase the broad utility of the technique. As a first test case, we prepared nanometric gaps in evaporated Au. Gap widths of 7 plus/minus 2 nm, aspect ratios of 17, and line edge roughness values of 3sigma = 2.04 nm are achieved. Furthermore, the gap widths represent an order of magnitude improvement on system resolution limits. As a second test case, we designed and fabricated dielectric resonators in the ternary compounds MnPSe3 and NiPS3; a class of van der Waals material resistant to chemical etch approaches. Nanoantenna arrays with incrementally increasing diameter were fabricated in crystalline, exfoliated flakes. The optical response was measured by dark field spectroscopy and is in agreement with simulations. The workflow reported here leverages established techniques in material processing without the need for custom or specialized hardware. It is broadly applicable to functional materials and devices, and extends high speed focused ion beam milling to true sub-10 nm length scales.

Coherent broadband light generation has attracted massive attention due to its numerous applications ranging from metrology, sensing, and imaging to communication. In general, spectral broadening is realized via third-order and higher-order nonlinear optical processes (e.g., self-phase modulation, Raman transition, four-wave mixing, multiwave mixing), which are typically weak and thus require a long interaction length and the phase matching condition to enhance the efficient nonlinear light-matter interaction for broad-spectrum generation. Here, for the first time, we report octave-spanning coherent light generation at the nanometer scale enabled by a phase-matching-free frequency down-conversion process. Up to octave-spanning coherent light generation with a −40dB spectral width covering from 565 to 1906 nm is demonstrated in discreate manner via difference-frequency generation, a second-order nonlinear process in gallium selenide and niobium oxide diiodide crystals at the 100-nanometer scale. Compared with conventional coherent broadband light sources based on bulk materials, our demonstration is 5 orders of magnitude thinner and requires 3 orders of magnitude lower excitation power. Our results open a new way to possibly create compact, versatile and integrated ultra-broadband light sources. The article introduces a new method for nanoscale octave-spanning coherent light generation via phase-matching-free down-conversion, offering high-efficiency and applications in metrology, spectroscopy, and telecommunications.

Polaritons in nanophotonic structures have attracted long-standing interest owing to their fundamental importance and potential for applications in nonlinear and quantum optics. Nanoantennas (NAs) made from high refractive index dielectrics offer a suitable platform for polariton physics thanks to the strongly confined optical Mie resonances and low optical losses in contrast to metallic NAs. However, Mie modes are mainly confined within the NA, making inefficient their coupling with excitons in materials deposited externally. Here, we overcome this limitation by using a high-refractive index van der Waals material WS_2, which allows straightforward fabrication of NAs on gold. The combination of a 27 nm tall WS_2 NA and a gold substrate enables strong modification of the Mie mode distribution and field enhancement inside and in the vicinity of the NA. This allows observation of room-temperature Mie-polaritons (with a Rabi splitting above 80 meV) arising from the strong coupling between Mie modes and the exciton in a monolayer WSe_2 placed on WS_2/gold NAs. We demonstrate strong nonlinearity of Mie-polaritons, one order of magnitude higher than for excitons in monolayer WSe_2 on gold. Our results highlight applicability of van der Waals materials for the realisation of hybrid dielectric-metallic nanophotonics for the study of the strong light-matter interaction.

The field of nanophotonics requires high-quality materials for the fabrication of resonant structures that can confine light down to the nanoscale. Metallic nanostructures often used for this purpose exhibit high optical losses, so high-refractive-index dielectrics such as silicon (Si) and III–V semiconductors are widely used instead. Recently, layered materials, often referred to as ‘van der Waals materials’ for the forces holding atomic planes together in bulk crystals, have been introduced as alternative dielectric building blocks for nanophotonics. Compared to traditional semiconductors, these materials exhibit higher refractive indices and transparency in the visible and near-infrared favourable for compact waveguides; strong birefringence and large nonlinear optical coefficients attractive for nonlinear optics; and out-of-plane van der Waals adhesive forces enabling novel tuning techniques and heterointegration approaches for the realization of previously inaccessible photonic structures. Recently, these properties of quasi-bulk van der Waals materials (as opposed to their widely studied monolayers) have been applied in a variety of photonic structures and devices, which will be discussed here. We report on recent progress in utilizing layered materials in waveguiding, wavefront shaping, Purcell enhancement, quantum nanophotonics, lasing, nonlinear optics, and strong light–matter coupling, as well as offer a snapshot of future developments in hybrid and tunable nanophotonics, three-dimensional photonic structures, optical trapping, polariton devices and van der Waals integrated nanophotonic circuits. This Review reports the recent progress in utilizing van der Waals layered materials in various nanophotonics applications and provides an overview of their future developments in hybrid and tunable nanophotonics, 3D photonic structures, optical trapping, polariton devices and van der Waals integrated nanophotonic circuits.

Deterministic and versatile approaches to sample preparation on nanoscopic scales are important in many fields including photonics, electronics, biology and material science. However, challenges exist in meeting many nanostructuring demands─particularly in emerging optical materials and component architectures. Here, we report a nanofabrication workflow that overcomes long-standing challenges in deterministic and top down sample preparation procedures. The salient feature is a carbon mask with a low sputter yield that can be readily shaped using high resolution electron beam processing techniques. When combined with focused ion beam processing, the masking technique yields structures with ultrasmooth, near-vertical sidewalls. We target different material platforms to showcase the broad utility of the technique. As a first test case, we prepared nanometric gaps in evaporated Au. Gap widths of 7 ± 2 nm, aspect ratios of 17, and line edge roughness values of 3σ = 2.04 nm are achieved. Furthermore, the gap widths amount to an order of magnitude improvement on system resolution limits. As a second test case, we designed and fabricated dielectric resonators in the ternary compounds MnPSe3 and NiPS3; a class of van der Waals material resistant to chemical etch approaches. Nanoantenna arrays with incrementally increasing diameter were fabricated in crystalline, exfoliated flakes. The optical response was measured by dark field spectroscopy and is in agreement with simulations. The workflow reported here leverages established techniques in material processing without the need for custom or specialized hardware. It is broadly applicable to functional materials and devices, and extends high speed focused ion beam milling to true sub-10 nm length scales.

Dielectric nanoresonators have been shown to circumvent the heavy optical losses associated with plasmonic devices; however, they suffer from less confined resonances. By constructing a hybrid system of both dielectric and metallic materials, one can retain low losses, while achieving stronger mode confinement. Here, we use a high refractive index multilayer transition-metal dichalcogenide WS2 exfoliated on gold to fabricate and optically characterize a hybrid nanoantenna-on-gold system. We experimentally observe a hybridization of Mie resonances, Fabry-Perot modes, and surface plasmon-polaritons launched from the nanoantennas into the substrate. We measure the experimental quality factors of hybridized Mie-plasmonic (MP) modes to be up to 33 times that of standard Mie resonances in the nanoantennas on silica. We then tune the nanoantenna geometries to observe signatures of a supercavity mode with a further increased Q factor of over 260 in experiment. We show that this quasi-bound state in the continuum results from strong coupling between a Mie resonance and Fabry-Perot-plasmonic mode in the vicinity of the higher-order anapole condition. We further simulate WS2 nanoantennas on gold with a 5 nm thick hBN spacer in between. By placing a dipole within this spacer, we calculate the overall light extraction enhancement of over 107, resulting from the strong, subwavelength confinement of the incident light, a Purcell factor of over 700, and high directivity of the emitted light of up to 50%. We thus show that multilayer TMDs can be used to realize simple-to-fabricate, hybrid dielectric-on-metal nanophotonic devices granting access to high-Q, strongly confined, MP resonances, along with a large enhancement for emitters in the TMD-gold gap.

Coherent broadband light generation has attracted massive attention due to its numerous applications ranging from metrology, sensing, and imaging to communication. In general, spectral broadening is realized via third-order and higher-order nonlinear optical processes (e.g., self-phase modulation, Raman transition, four-wave mixing, multiwave mixing), which are typically weak and thus require a long interaction length and the phase matching condition to enhance the efficient nonlinear light-matter interaction for broad-spectrum generation. Here, for the first time, we report octave-spanning coherent light generation at the nanometer scale enabled by a phase-matching-free frequency down-conversion process. Up to octave-spanning coherent light generation with a -40dB spectral width covering from ~ 565 to 1906 nm is demonstrated via difference-frequency generation, a second-order nonlinear process in gallium selenide and niobium oxide diiodide crystals at the 100-nanometer scale. Compared with conventional coherent broadband light sources based on bulk materials, our demonstration is ~5 orders of magnitude thinner and requires ~ 3 orders of magnitude lower excitation power. Our results open a new way to possibly create compact, versatile and integrated ultra-broadband light sources.

Dielectric nanoresonators have been shown to circumvent the heavy optical losses associated with plasmonic devices, however they suffer from less confined resonances. By constructing a hybrid system of both dielectric and metallic materials, one can retain the low losses of dielectric resonances, whilst gaining additional control over the tuning of the modes with the metal, and achieving stronger mode confinement. In particular, multi-layered van der Waals materials are emerging as promising candidates for integration with metals owing to their weak attractive forces, which enable deposition onto such substrates without the requirement of lattice matching. Here we use layered, high refractive index WS_2 exfoliated on gold, to fabricate and optically characterize a hybrid nanoantenna-on-gold system. We experimentally observe a hybridization of Mie resonances, Fabry-Pérot modes, and surface plasmon-polaritons launched from the nanoantennas into the substrate. We achieve experimental quality factors of Mie-plasmonic modes of up to 20 times that of Mie resonances in nanoantennas on silica, and observe signatures of a supercavity mode with a Q factor of 263 ± 28, resulting from strong mode coupling between a higher-order anapole and Fabry-Pérot-plasmonic mode. We further simulate WS_2 nanoantennas on gold with an hBN spacer, resulting in calculated electric field enhancements exceeding 2600, and a Purcell factor of 713. Our results demonstrate dramatic changes in the optical response of dielectric nanophotonic structures placed on gold, opening new possibilities for nanophotonics and sensing with simple-to-fabricate devices.

The photonics-based approach has recently become a strong candidate for realising a large-scale, practical quantum processor. Particularly in recent years, two-dimensional (2D) materials have become a strong candidate for developing an ideal integrated light source owing to their several unique advantages such as convenient on-chip integration. In this work, we study the effect of strain on the emission wavelength and carrier lifetime. We first show that the geometry of stressors can adjust the amount of strain and emission wavelength. Using this strain engineering technique, we demonstrate that the emission wavelength can be significantly shifted by ~10 nm while the carrier lifetime can also be engineered by ~30 %.

Second-order nonlinear χ 2 processes hold the key to realizing various promising classical and quantum applications. Only conventional non-centrosymmetric materials like aluminium nitride (AlN) and lithium niobate (LN) exhibit a strong second-order nonlinearity. While germanium (Ge) has the advantage of mature foundry processing due to its complementary metal-oxide-semiconductor (CMOS) compatibility compared to conventional χ 2 materials, it has a weak second-order nonlinearity owning to its inversion symmetry. It is predicted that exploiting micro-scale strain gradients induced by silicon nitride (SiN) stressors can break the inversion symmetry of Ge and enable more efficient second harmonic generation (SHG) in mid-infrared (MIR). We herein demonstrate SHG in visible and telecom S-band ranges by pumping our germanium-on-insulator (GOI) sample with a tunable femtosecond pulse laser. The signature quadratic power dependence curve of SHG has also been experimentally observed. In this report, we also propose a novel approach to breaking the inversion symmetry of Ge, which can amplify its weak second-order nonlinearity. We designed a Ge micro-bridge with a periodically changing nano-scale strain gradient that is orders of magnitude higher than the previously proposed structures utilizing SiN stressors. This results in a 3-orders of magnitude enhancement in χ 2 according to both a classic anharmonic model and density potential theory. Our work paves the way toward a CMOS compatible and high χ 2 nonlinear material for integrated photonic applications.

We report broadband ultrafast photoluminescence from graphene ranging from visible to telecom wavelengths. We show strongly modified emission spectra owing to the presence of the cavity effect and demonstrate improved thermal stability enabled by hBN.

High-harmonic generation (HHG), an extreme nonlinear optical phenomenon beyond the perturbation regime, is of great significance for various potential applications, such as high-energy ultrashort pulse generation with outstanding spatiotemporal coherence. However, efficient active control of HHG is still challenging due to the weak light-matter interaction displayed by currently known materials. Here, we demonstrate optically controlled HHG in monolayer semiconductors via the engineering of interband polarization. We find that HHG can be efficiently controlled in the excitonic spectral region with modulation depths up to 95% and ultrafast response speeds of several picoseconds. Quantitative time-domain theory of the nonlinear optical susceptibilities in monolayer semiconductors further corroborates these experimental observations. Our demonstration not only offers an in-depth understanding of HHG but also provides an effective approach toward active optical devices for strong-field physics and extreme nonlinear optics.

The second-order χ(2) process underpins many important nonlinear optical applications in the field of classical and quantum optics. Generally, the χ(2) process manifests itself only in a non-centrosymmetric dielectric medium via an anharmonic electron oscillation when driven by an intense optical field. Due to inversion symmetry, group-IV semiconductors, such as silicon (Si) and germanium (Ge), are traditionally not considered as ideal candidates for second-order nonlinear optics applications. Here, we report the experimental observation of the second-harmonic generation (SHG) in a Ge-on-insulator (GOI) sample under femtosecond optical pumping. We especially, report the measurement of the SHG signal from a GOI sample in the telecom S-band by pumping at ∼3000 nm.

The ever-growing demand for faster and more efficient data transfer and processing has brought optical computation strategies to the forefront of research in next-generation computing. Here, we report a universal computing approach with the chirality degree of freedom. By exploiting the crystal symmetry-enabled well-known chiral selection rules, we demonstrate the viability of the concept in bulk silica crystals and atomically thin semiconductors and create ultrafast (<100-fs) all-optical chirality logic gates (XNOR, NOR, AND, XOR, OR, and NAND) and a half adder. We also validate the unique advantages of chirality gates by realizing multiple gates with simultaneous operation in a single device and electrical control. Our first demonstrations of logic gates using chiral selection rules suggest that optical chirality could provide a powerful degree of freedom for future optical computing.

Light modulation is of paramount importance for photonics and optoelectronics. Here we report all-optical coherent modulation of third-harmonic generation (THG) with chiral light via the symmetry enabled polarization selectivity. The concept is experimentally validated in monolayer materials (MoS 2 ) with modulation depth approaching ~100%, ultra-fast modulation speed (<~130 fs), and wavelength-independence features. Moreover, the power and polarization of the incident optical beams can be used to tune the output chirality and modulation performance. Major performance of our demonstration reaches the fundamental limits of optical modulation: near-unity modulation depth, instantaneous speed (ultra-fast coherent interaction), compact footprint (atomic thickness), and unlimited operation bandwidth, which hold an ideal optical modulation solution for emerging and future nonlinear optical applications (e.g., interconnection, imaging, computing, and quantum technologies).