Exciton-Enabled Meta-Optics in Two-Dimensional Transition Metal Dichalcogenides

Enabling Waveguide Optics in Rhombohedral-Stacked Transition Metal Dichalcogenides with Laser-Patterned Grating Couplers

Waveguides play a key role in the implementation of on-chip optical elements and, therefore, lie at the heart of integrated photonics. To add the functionalities of layered materials to existing technologies, dedicated fabrication protocols are required. Here, we build on laser writing to pattern grating structures into bulk noncentrosymmetric transition metal dichalcogenides with grooves as sharp as 250 nm. Using thin flakes of 3R-MoS2 that act as waveguides for near-infrared light, we demonstrate the functionality of the grating couplers with two complementary experiments: first, nano-optical imaging is used to visualize transverse electric and magnetic modes, whose directional outcoupling is captured by finite element simulations. Second, waveguide second-harmonic generation is demonstrated by grating-coupling femtosecond pulses into the slabs in which the radiation partially undergoes frequency doubling throughout the propagation. Our work provides a straightforward strategy for laser patterning of van der Waals crystals, demonstrates the feasibility of compact frequency converters, and examines the tuning knobs that enable optimized coupling into layered waveguides.

An entropy-controlled objective chip for reflective confocal microscopy with subdiffraction-limit resolution

Planar diffractive lenses (PDLs) with optimized but disordered structures can focus light beyond the diffraction limit. However, these disordered structures have inevitably destroyed wide-field imaging capability, limiting their applications in microscopy. Here, we introduce information entropy S to evaluate the disorder of an objective chip by using the probability of its structural deviation from standard Fresnel zone plates. Inspired by the theory of entropy change, we predict an equilibrium point [Formula: see text] to balance wide-field imaging (theoretically evaluated by the Strehl ratio) and subdiffraction-limit focusing. To verify this, a [Formula: see text] objective chip with a record-long focal length of 1 mm is designed with [Formula: see text], which is the nearest to the equilibrium point among all reported PDLs. Consequently, our fabricated chip can focus light with subdiffraction-limit size of 0.44 λ and image fine details with spatial frequencies up to 4000 lp/mm experimentally. These unprecedented performances enable ultracompact reflective confocal microscopy for superresolution imaging.

Greatly Enhanced Resonant Exciton-Trion Conversion in Electrically Modulated Atomically Thin WS2 at Room Temperature

Excitonic resonance in atomically thin semiconductors offers a favorite platform to study 2D nanophotonics in both classical and quantum regimes and promises potentials for highly tunable and ultra-compact optical devices. The understanding of charge density dependent exciton-trion conversion is the key for revealing the underlaying physics of optical tunability. Nevertheless, the insufficient and inefficient light-matter interactions hinder the observation of trionic phenomenon and the development of excitonic devices for dynamic power-efficient electro-optical applications. Here, by engaging an optical cavity with atomically thin transition metal dichalcogenides (TMDCs), greatly enhanced exciton-trion conversion is demonstrated at room temperature (RT) and achieve electrical modulation of reflectivity of ≈40% at exciton and 7% at trion state, which correspondingly enables a broadband large phase tuning in monolayer tungsten disulfide. Besides the absorptive conversion, ≈100% photoluminescence conversion from excitons to trions is observed at RT, illustrating a clear physical mechanism of an efficient exciton-trion conversion for extraordinary optical performance. The results indicate that both excitons and trions can play significant roles in electrical modulation of the optical parameters of TMDCs at RT. The work shows the real possibility for realizing electrical tunable and multi-functional ultra-thin optical devices using 2D materials.

Ultra-thin light-weight laser-induced-graphene (LIG) diffractive optics

The realization of hybrid optics could be one of the best ways to fulfill the technological requirements of compact, light-weight, and multi-functional optical systems for modern industries. Planar diffractive lens (PDL) such as diffractive lenses, photonsieves, and metasurfaces can be patterned on ultra-thin flexible and stretchable substrates and be conformally attached on top of arbitrarily shaped surfaces. In this review, we introduce recent research works addressed to the design and manufacturing of ultra-thin graphene optics, which will open new markets in compact and light-weight optics for next-generation endoscopic brain imaging, space internet, real-time surface profilometry, and multi-functional mobile phones. To provide higher design flexibility, lower process complexity, and chemical-free process with reasonable investment cost, direct laser writing (DLW) of laser-induced-graphene (LIG) is actively being applied to the patterning of PDL. For realizing the best optical performances in DLW, photon-material interactions have been studied in detail with respect to different laser parameters; the resulting optical characteristics have been evaluated in terms of amplitude and phase. A series of exemplary laser-written 1D and 2D PDL structures have been actively demonstrated with different base materials, and then, the cases are being expanded to plasmonic and holographic structures. The combination of these ultra-thin and light-weight PDL with conventional bulk refractive or reflective optical elements could bring together the advantages of each optical element. By integrating these suggestions, we suggest a way to realize the hybrid PDL to be used in the future micro-electronics surface inspection, biomedical, outer space, and extended reality (XR) industries.

Engineering a strong and stable ultraviolet chiroptical effect in a large-area chiral plasmonic shell

Ultraviolet chiral metamaterials (UCM) are highly desired for their strong interaction with the intrinsic resonance of molecules and ability in manipulating the polarization state of high energy photons, but rarely reported to date due to their small feature size and complex geometry. Herein, we design and fabricate a kind of novel ultraviolet chiral plasmonic shell (UCPS) by combing the stepwise Al deposition and colloid-sphere assembled techniques. The cancellation effect originated from the disorder lattices of micro-domains in the colloid monolayer has been successfully overcome by optimizing the deposition parameters, and a strong CD signal of larger than 1 deg in the UV region is demonstrated both in simulation and experiment. This strong ultraviolet chiroptical resonances mainly come from the surface chiral lattice resonance mode, the whispering gallery mode and also the interaction between neighbor shells, and can be effectively tuned by changing structural parameters, for example, the sphere diameter, or even slightly increasing the deposition temperature in experiment. To improve the stability, the fabricated UCPSs are protected by N₂ in the deposition chamber and then passivated by UV-ozone immediately after each deposition step. The formed UCPS show an excellent stability when exposing in the atmospheric environment. The computer-aided geometrical model, electromagnetic modes, and the tunable chiroptical resonance modes have been systematically investigated.

Engineering van der Waals Materials for Advanced Metaphotonics

The outstanding chemical and physical properties of 2D materials, together with their atomically thin nature, make them ideal candidates for metaphotonic device integration and construction, which requires deep subwavelength light-matter interaction to achieve optical functionalities beyond conventional optical phenomena observed in naturally available materials. In addition to their intrinsic properties, the possibility to further manipulate the properties of 2D materials via chemical or physical engineering dramatically enhances their capability, evoking new science on light-matter interaction, leading to leaped performance of existing functional devices and giving birth to new metaphotonic devices that were unattainable previously. Comprehensive understanding of the intrinsic properties of 2D materials, approaches and capabilities for chemical and physical engineering methods, the resulting property modifications and novel functionalities, and applications of metaphotonic devices are provided in this review. Through reviewing the detailed progress in each aspect and the state-of-the-art achievement, insightful analyses of the outstanding challenges and future directions are elucidated in this cross-disciplinary comprehensive review with the aim to provide an overall development picture in the field of 2D material metaphotonics and promote rapid progress in this fast emerging and prosperous field.

Beam steering at the nanosecond time scale with an atomically thin reflector

Techniques to mold the flow of light on subwavelength scales enable fundamentally new optical systems and device applications. The realization of programmable, active optical systems with fast, tunable components is among the outstanding challenges in the field. Here, we experimentally demonstrate a few-pixel beam steering device based on electrostatic gate control of excitons in an atomically thin semiconductor with strong light-matter interactions. By combining the high reflectivity of a MoSe₂ monolayer with a graphene split-gate geometry, we shape the wavefront phase profile to achieve continuously tunable beam deflection with a range of 10°, two-dimensional beam steering, and switching times down to 1.6 nanoseconds. Our approach opens the door for a new class of atomically thin optical systems, such as rapidly switchable beam arrays and quantum metasurfaces operating at their fundamental thickness limit.

Andersen et al. have demonstrated a new type of beam steering device based on the excitonic response of an atomically thin semiconductor. Using electrostatic gates, the authors achieved tunable steering with switching times on the nanosecond scale.

2D Functional Minerals as Sustainable Materials for Magneto-Optics

Liquid crystal devices using organic molecules are nowadays widely used to modulate transmitted light, but this technology still suffers from relatively weak response, high cost, toxicity and environmental concerns, and cannot fully meet the demand of future sustainable society. Here, an alternative approach to color-tunable optical devices, which is based on sustainable inorganic liquid crystals derived from 2D mineral materials abundant in nature, is described. The prototypical 2D mineral of vermiculite is massively produced by a green method, possessing size-to-thickness aspect ratios of >10³ , in-plane magnetization of >10 emu g^-1 , and an optical bandgap of >3 eV. These characteristics endow 2D vermiculite with sensitive magneto-birefringence response, been several orders of magnitude larger than organic counterparts, as well as capability of broad-spectrum modulation. The finding consequently permits the fabrication of various magnetochromic or mechanochromic devices with low or even zero-energy consumption during operation. This work creates opportunities for the application of sustainable materials in advanced optics.

Enhanced light-matter interaction in two-dimensional transition metal dichalcogenides

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS₂, WS₂, MoSe₂, and WSe₂, have received extensive attention in the past decade due to their extraordinary electronic, optical and thermal properties. They evolve from indirect bandgap semiconductors to direct bandgap semiconductors while their layer number is reduced from a few layers to a monolayer limit. Consequently, there is strong photoluminescence in a monolayer (1L) TMDC due to the large quantum yield. Moreover, such monolayer semiconductors have two other exciting properties: large binding energy of excitons and valley polarization. These properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research.