Tailorable Dynamics in Nonlinear Optical Metasurfaces

Plasmonic Ultrafast All-Optical Switching with a Superior On-Off Ratio

Plasmonic ultrafast all-optical switching holds great promise for advancing next-generation optical communication and optical computing technologies. However, achieving subpicosecond all-optical switching with a high on-off ratio remains challenging due to the slow dynamics of electron-phonon scattering in plasmonic materials. Here, we report an innovative method that utilizes the negative signal induced by plasmonic excited hot electrons in the transient spectrum and the positive signal caused by hot electrons excited by off-resonant pumping, both designed at the same wavelength to effectively offset slow dynamics. This approach enables plasmonic ultrafast all-optical switching with a 500 fs response time and a superior on-off ratio exceeding 20 within 1 ps. The strategy offers a promising path for high-performance all-optical modulation and can be widely applied across various plasmonic materials.

Ultrafast chirality-dependent dynamics from helicity-resolved transient absorption spectroscopy

Chirality, a pervasive phenomenon in nature, is widely studied across diverse fields including the origins of life, chemical catalysis, drug discovery, and physical optoelectronics. The investigations of natural chiral materials have been constrained by their intrinsically weak chiral effects. Recently, significant progress has been made in the fabrication and assembly of low-dimensional micro and nanoscale chiral materials and their architectures, leading to the discovery of novel optoelectronic phenomena such as circularly polarized light emission, spin and charge flip, advocating great potential for applications in quantum information, quantum computing, and biosensing. Despite these advancements, the fundamental mechanisms underlying the generation, propagation, and amplification of chirality in low-dimensional chiral materials and architectures remain largely unexplored. To tackle these challenges, we focus on employing ultrafast spectroscopy to investigate the dynamics of chirality evolution, with the aim of attaining a more profound understanding of the microscopic mechanisms governing chirality generation and amplification. This review thus provides a comprehensive overview of the chiral micro-/nano-materials, including two-dimensional transition metal dichalcogenides (TMDs), chiral halide perovskites, and chiral metasurfaces, with a particular emphasis on the physical mechanism. This review further explores the advancements made by ultrafast chiral spectroscopy research, thereby paving the way for innovative devices in chiral photonics and optoelectronics.

A Sensitive Probe of Meso-Cyanophenyl Substituted BODIPY Derivative as Fluorescent Chemosensor for the Detection of Multiple Heavy Metal Ions

A fluorescent turn-on chemosensor (BA) was constructed by attaching bis(pyridin-2-ylmethyl)-amine (DPA) unit to the BODIPY scaffold. It can give a prominent green/yellow fluorescent response selectivity with each of Zn2+/Hg2+/Cd2+/Ca2+/Mn2+/Pb2+/Al3+. The 1:1 stoichiometry of BA and metal ions was drawn from the analysis of Job's plot. The limit detection of BA in recognition of Zn2+/Hg2+/Cd2+/Ca2+/Mn2+/Pb2+/Al3+ is ranged in 50.8-146.6 nM. There exists a linear relationship between the fluorescence intensity and concentration of metal ions (Zn2+: 4-15 µM). The mechanism of fluorescence signal "turn-on" is based on the photo induced transfer (PET) in the excited state of BA. The coordinated metal ions significantly weakened the electron-donating ability nitrogen atom in DPA, thus recovering the emission character of BODIPY. The substituted group at the phenyl ring in meso-position of BODIPY scaffold determines the recognizable list of metal ions.

Investigating the Properties of Triangle Terthiophene and Triphenylamine Configured Propeller-like Photochromic Dye with Ethyne Bridge

Synthesis-oriented design led us to the construction of a propeller-like dye, containing the triangle terthiophene and triphenylamine units. It reveals typical photochromic properties with alternated UV (390 nm) and visible light (˃ 440 nm) irradiation and the dye solution (in THF) color was also toggled between yellow-green and colorless. A new absorption band was observed in visible region (415-600 nm). Additionally, the photochromic dye was highly emissive with the absolute quantum yield being 0.27. After UV light irradiation, the emission was quenched significantly (Φ = 0.08) at photo-stationary state, and thus establishing a switchable emission "on-off" system by alternated UV/visible light irradiation cycle. Detailed structural analysis was carried out based on the optimized dye structure. Both the antiparallel conformation and the distance of reactive carbon atoms (< 4.2 Å) led to the smoothly photochromic behavior.

Control of Chirality and Directionality of Nonlinear Metasurface Light Source via Moiré Engineering

Metasurfaces have revolutionized nonlinear and quantum light manipulation in the past decade, enabling the design of materials that can tune polarization, frequency, and direction of light simultaneously. However, tuning of metasurfaces is traditionally achieved by changing their microscopic structure, which does not allow in situ tuning and dynamic optimization of the metasurfaces. In this Letter, we explore the concept of twisted bilayer and tetralayer nonlinear metasurfaces, which offer rich tunability in its effective nonlinear susceptibilities. Using gold-based metasurfaces, we demonstrate that a number of different singularities of nonlinear susceptibilities can exist in the parameter space of twist angle and interlayer gap between different twisted layers. At the singularities, reflected or transmitted light from the nonlinear process (such as second-harmonic generation) can either become circularly polarized (for C points), or entirely vanish (for V points). By further breaking symmetries of the system, we can independently tune all aspects of the reflected and transmitted nonlinear emission, achieving unidirectional emission with full-Poincaré polarization tunability, a dark state (V-V point), or any other bidirectional output. Our work paves the way for multidimensional control of polarization and directionality in nonlinear light sources, opening new avenues in ultrafast optics, optical communication, sensing, and quantum optics.

Voltage-controlled nonlinear optical properties in gold nanofilms via electrothermal effect

Dynamic control of the optical properties of gold nanostructures is crucial for advancing photonics technologies spanning optical signal processing, on-chip light sources and optical computing. Despite recent advances in tunable plasmons in gold nanostructures, most studies are limited to the linear or static regime, leaving the dynamic manipulation of nonlinear optical properties unexplored. This study demonstrates the voltage-controlled Kerr nonlinear optical response of gold nanofilms via the electrothermal effect. By applying relatively low voltages (~10 V), the nonlinear absorption coefficient and refractive index are reduced by 40.4% and 33.1%, respectively, due to the increased damping coefficient of gold nanofilm. Furthermore, a voltage-controlled all-fiber gold nanofilm saturable absorber is fabricated and used in mode-locked fiber lasers, enabling reversible wavelength-tuning and operation regimes switching (e.g., mode-locking-Q-switched mode-locking). These findings advance the understanding of electrically controlled nonlinear optical responses in gold nanofilms and offer a flexible approach for controlling fiber laser operations.

Strong enhancement of third harmonic generation from a Tamm plasmon multilayer structure with WS2

Improving the conversion efficiency is particularly important for the generation and applications of harmonic waves in optical microstructures. Herein, we propose to enhance the efficiency of third harmonic generation by integrating a monolayer WS2 with the metal/dielectric/photonic crystal multilayer structure. The numerical simulations show that the multilayer structure enables to generate the Tamm plasmon mode between the metal film and photonic crystal around the telecommunication wavelength, which is consistent with the experimental result. By measuring with a self-built nonlinear optical micro-spectroscopy system, we find that the third harmonic signal can be reinforced by 16-fold through inserting the monolayer WS2 in the dielectric spacer. This work will provide a new way for improving nonlinear optical response, especially THG in multilayer photonic microstructures.

High Q lithium niobate metasurfaces with transparent electrodes for efficient amplitude and phase modulation

Lithium niobate (LN)-based metasurfaces have demonstrated remarkable potential in integrated electro-optically adjustable metadevices with the maturation of thin film LN on insulator (LNOI) technology. Here, we proposed a type of high Q factor tunable metasurface with etchless LN, which is electrically driven in the vertical direction by using transparent conductive film. A transmission amplitude modulation of over 60 dB at a voltage of 20 V is realized through guided mode resonances created at the LN layer with a Q factor of 1320. Meanwhile, phase modulation is also realized with a reflective design by adding a gold layer at the bottom of the metasurface. With a gate voltage of 80 V, about 1.75π phase modulation is achieved while keeping reflection over 92%. Our proposed device achieves effective modulation of optical amplitude and phase in the near-infrared band, which lays a good foundation for the development of high performance LN-based active nanophotonic devices.

Engineering the temporal dynamics of all-optical switching with fast and slow materials

All-optical switches control the amplitude, phase, and polarization of light using optical control pulses. They can operate at ultrafast timescales - essential for technology-driven applications like optical computing, and fundamental studies like time-reflection. Conventional all-optical switches have a fixed switching time, but this work demonstrates that the response-time can be controlled by selectively controlling the light-matter-interaction in so-called fast and slow materials. The bi-material switch has a nanosecond response when the probe interacts strongly with titanium nitride near its epsilon-near-zero (ENZ) wavelength. The response-time speeds up over two orders of magnitude with increasing probe-wavelength, as light's interaction with the faster Aluminum-doped zinc oxide (AZO) increases, eventually reaching the picosecond-scale near AZO's ENZ-regime. This scheme provides several additional degrees of freedom for switching time control, such as probe-polarization and incident angle, and the pump-wavelength. This approach could lead to new functionalities within key applications in multiband transmission, optical computing, and nonlinear optics.

Electrically and all-optically switchable nonlocal nonlinear metasurfaces

Nonlocal effects on metasurfaces play an important role to achieve high-Q spectral selectivity, beneficial for development of multifunctional, multispectral integrated optics. In addition, they enhance the optical interaction and promote a variety of nonlinear effects, including frequency conversion and stimulated scattering. Active tuning of nonlocal nonlinearity is highly desirable for sensing and signal processing but was hardly explored until now. Here, we show drastic electric and all-optical tunability of nonlocal second-harmonic generation (SHG) from nonlinear metasurface, functionalized with a twisted nematic liquid-crystal (LC) layer. The addition of LC results in the emergence of strong nonlocal SHG, due to a surface lattice resonance of the system. We demonstrate a notable enhancement of SHG on resonance, more than 25 dB electrical switching amplitude, and all-optically induced phase transition imprinted on SHG. Our results on dynamic nonlocal effects introduce a very promising route for active nonlinear optical metadevices at the nanoscale.

Investigating the Properties of Double Triangle Terthiophene Configured Dumbbell-Like Photochromic Dye with Ethyne and 1,3-Butadiene Bridge

Dumbbell-like photochromic dyes were constructed by incorporation of double triangle terthiophene with ethyne or 1,3-butadiene bridge. Regular photochromic behavior was investigated with alternated UV (365 nm) and Visible light (˃ 400 nm) irradiation. However, the different bridge group leads to distinct difference in their photochromic wavelength. For the ethyne bridged triangle terthiophene (DT1), the photochromic wavelength was observed around 500-700 nm (peak value: 605 nm) and the solution turned to red with 365 nm light irradiation. However, the photochromic wavelength was blue shift to 418-550 nm and the solution was turned to light yellow for 1,3-butadiene bridged dye (DT2). Both of the colored solution can be bleached via visible light irradiation. Additionally, the two dyes in THF were emissive with absolute quantum yield (QY) of 0.36/0.40. Along with the photo-induced photocyclization process, the emissive solution can be effectively quenched at photo-stationary sate (Φ = 0.05/0.04). And emission "on-off" cycle could be established based on the UV/visible light irradiation cycle.

Transition metalides based on facially polarized all-cis-1,2,3,4,5,6-hexafluorocyclohexane - a new class of high performance second order nonlinear optical materials

Continuous attempts are being made to discover new approaches to design materials with extraordinary nonlinear optical responses. Herein, for the first time, we report the geometric, electronic, and nonlinear optical properties of novel Janus transition metalides AM-J-TM (where AM = Li, Na and K, and TM = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) containing alkali metals as a source of excess electrons for transition metals to generate metalides. The Janus organic complexant used for the study is all cis 1,2,3,4,5,6-hexafluorocyclohexane F₆C₆H₆ (J). These complexes contain the unique involvement of alkali metals (AM = Li, Na and K) as a source of excess electrons, which significantly affects the hyperpolarizability values of the resulting transition metalides. The NBO analysis reveals the charge transfer from alkali metals to the transition metals, thereby confirming the metalide behavior of the complexes. Moreover, the metalide nature of these complexes is validated through frontier molecular orbital (FMO) analysis. The values of interaction energies, vertical ionization potential (VIP) and vertical electron affinity (VEA) illustrate the stability of the metalide complexes. Ultimately, the hyperpolarizability values confirm the excellent nonlinear optical response of the designed transition metalides. The remarkable static first hyperpolarizability (β₀) response up to 4 × 10⁸ a.u. is observed for complexes of vanadium. Similarly, the complexes of AM-J-Mn and Li/Na-J-Sc show significantly high NLO response. These compounds besides providing a new entry into excess electron compounds will also pave the way for the design and synthesis of further novel NLO materials.

Magnetic and Luminescent Dual Responses of Photochromic Hexaazamacrocyclic Lanthanide Complexes

Herein, hexaazamacrocyclic ligand L^N6 was employed to construct a series of photochromic rare-earth complexes, [Ln(L^N6)(NO₃)₂](BPh₄) [1-Ln, Ln = Dy, Tb, Eu, Gd, Y; L^N6 = (3E,5E,10E,12E)-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane-3,5,10,12-tetraene]. The behavior of photogenerated radicals of hexaazamacrocyclic ligands was revealed for the first time. Upon 365 nm light irradiation, complexes 1-Ln exhibit photochromic behavior induced by photogenerated radicals according to EPR and UV-vis analyses. Static and dynamic magnetic studies of 1-Dy and irradiated product 1-Dy* indicate weak ferromagnetic interactions among Dy^III ions and photogenerated L^N6 radicals, as well as slow magnetization relaxation behavior under a 2 kOe applied field. Further fitting analyses show that the magnetization relaxation in 1-Dy* is markedly different from 1-Dy. Time-dependent fluorescence measurements reveal the characteristic luminescence quenching dynamics of lanthanide in the photochromic process. Especially for irradiated product 1-Eu*, the luminescence is almost completely quenched within 5 min with a quenching efficiency of 98.4%. The results reported here provide a prospect for the design of radical-induced photochromic lanthanide single-molecule magnets and will promote the further development of multiresponsive photomagnetic materials.

Wideband and high-efficiency spin-locked achromatic meta-device

Achromatic devices present unique capabilities in efficient manipulation of waves and have wide applications in imaging and communication systems. However, the research of achromatic devices is limited by the narrow bandwidth, low efficiency as well as large configurations. In this paper, we propose a general strategy to design spin-locked achromatic metasurface with broadband and high efficiency properties in microwave region. A multi-resonant model is used to control the dispersion within a wide bandwidth by tuning its resonant intensity, resonance numbers as well as resonant frequency. As a proof of the concept, two achromatic meta-devices with ultra-thin profile at microwave frequency are experimentally investigated. The achromatic deflector can reflect the normal incident waves to the same angle within 9.5 to 11.5 GHz, while the other achromatic lens can focus the excitations at the same focal points. The experimentally working efficiency of the meta-devices fluctuates around 71-82% and 57-65% within the target working bandwidth, respectively. Moreover, our meta-devices can preserve the charity of the excitations. The scheme of this research shows great advances in the design of broadband and high-efficiency achromatic devices which can also be applied to other frequency ranges and inspires the realization of ultrabroadband and high-efficiency metadevices.

Focusing Characteristics and Widefield Imaging Performance of the Silicon Metalens in the Visible Range

Conventional optical high numerical aperture lenses are essential for high-resolution imaging, but bulky and expensive. In comparison, metalens-based optical components are the subjects of intensive investigation for their flexible manipulation of light. Methods of detecting and characterizing focal spots and scanning imaging produced by metalenses are well established. However, widefield imaging by metalenses is experimentally challenging. This study demonstrates the design and realization of silicon-based metalenses with numerical apertures of 0.447 and 0.204 in the broadband spectrum of 580–780 nm for microscopic widefield imaging. The optimized aspect ratio of the single nanorod is 5.1:1, which reduces the fabrication difficulty compared to other, more complicated designs and fabrication. Furthermore, we successfully demonstrate widefield imaging by the designed metalens and compare the simulated and the experimentally extracted modulation transfer function curves of the metalens.

Optical metasurfaces towards multifunctionality and tunability

Optical metasurfaces is a rapidly developing research field driven by its exceptional applications for creating easy-to-integrate ultrathin planar optical devices. The tight confinement of the local electromagnetic fields in resonant photonic nanostructures can boost many optical effects and offer novel opportunities for the nanoscale control of light-matter interactions. However, once the structure-only metasurfaces are fabricated, their functions will be fixed, which limits it to make breakthroughs in practical applications. Recently, persistent efforts have led to functional multiplexing. Besides, dynamic light manipulation based on metasurfaces has been demonstrated, providing a footing ground for arbitrary light control in full space-time dimensions. Here, we review the latest research progress in multifunctional and tunable metasurfaces. Firstly, we introduce the evolution of metasurfaces and then present the concepts, the basic principles, and the design methods of multifunctional metasurface. Then with more details, we discuss how to realize metasurfaces with both multifunctionality and tunability. Finally, we also foresee various future research directions and applications of metasurfaces including innovative design methods, new material platforms, and tunable metasurfaces based metadevices.

Recent advances of wide-angle metalenses: principle, design, and applications

Optical imaging systems, like microscopes, cameras, and telescopes, continue to expand the scope of human observation of the world. As one of the key indicators of imaging systems, the field-of-view (FOV) is often limited by coma aberration. Expanding it generally relies on a combination of complex lenses, leading to a bulky and cumbersome system. Recently, the emergency of meta-optics provides an alternative to constructing compact and lightweight large-FOV metalens through elaborated phase modulation within a flat surface, showing great potential in surveillance, unmanned vehicles, onboard planes or satellites, medical science, and other new applications. In this article, we review recent advances of wide-angle metalenses, including operation principles, design strategies, and application demos. Firstly, basic principles of wide-angle imaging using a single metalens are interpreted. Secondly, some advanced methods for designing subwavelength structures with high angle robustness and high efficiency are discussed. Thirdly, some representative functional devices and applications are surveyed. Finally, we conclude with an outlook on future potentials and challenges that need to be overcome.

Ultrafast, All Optically Reconfigurable, Nonlinear Nanoantenna

The enhancement of nonlinear optical effects via nanoscale engineering is a hot topic of research. Optical nanoantennas increase light-matter interaction and provide, simultaneously, a high throughput of the generated harmonics in the scattered light. However, nanoscale nonlinear optics has dealt so far with static or quasi-static configurations, whereas advanced applications would strongly benefit from high-speed reconfigurable nonlinear nanophotonic devices. Here we propose and experimentally demonstrate ultrafast all-optical modulation of the second harmonic (SH) from a single nanoantenna. Our design is based on a subwavelength AlGaAs nanopillar driven by a control femtosecond light pulse in the visible range. The control pulse photoinjects free carriers in the nanostructure, which in turn induce dramatic permittivity changes at the band edge of the semiconductor. This results in an efficient modulation of the SH signal generated at 775 nm by a second femtosecond pulse at the 1.55 μm telecommunications (telecom) wavelength. Our results can lead to the development of ultrafast, all optically reconfigurable, nonlinear nanophotonic devices for a broad class of telecom and sensing applications.

All-Optically Reconfigurable Plasmonic Metagrating for Ultrafast Diffraction Management

Hot-electron dynamics taking place in nanostructured materials upon irradiation with fs-laser pulses has been the subject of intensive research, leading to the emerging field of ultrafast nanophotonics. However, the most common description of nonlinear interaction with ultrashort laser pulses assumes a homogeneous spatial distribution for the photogenerated carriers. Here we theoretically show that the inhomogeneous evolution of the hot carriers at the nanoscale can disclose unprecedented opportunities for ultrafast diffraction management. In particular, we design a highly symmetric plasmonic metagrating capable of a transient symmetry breaking driven by hot electrons. The subsequent power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (∼2 mJ/cm²) laser fluence. Our theoretical investigation also indicates that the recovery time of the symmetric configuration can be controlled by tuning the geometry of the metaatom, and can be as fast as 2 ps for electrically connected configurations.