In nonlinear nanophotonics, scalable sequential signal transformation of second-harmonic generation (SHG) in pure plasmonic waveguides remains challenging. Precise phase matching becomes instrumental to achieve efficient SHG and enable sequential generation of nonlinear signals. We experimentally demonstrate the phase matching in SHG between two orthogonal modes through dispersion engineering. Accurate tuning of plasmonic two-wire transmission-line (TWTL) design parameters results in SHG from the antisymmetric excitation mode being approximately 15 times stronger than that from the symmetric excitation mode, greatly raising the conversion efficiency to . Simultaneously, the measured devices show that the phase matching extends our TWTL operational length at least up to . Based on the improved efficiency and operational length, we demonstrate a multi-stage device capable of performing polarization-dependent switching between the linear propagation of SHG and frequency up-conversion in the later stages. We further realize a nonlinear OR logic operation using a single long waveguide. These results underscore the potential of phase-matched plasmonic TWTLs for compact, efficient, and scalable nonlinear optical circuitry.

Spatiotemporal vortices are polychromatic modes that intertwine orbital angular momentum in space and time. Here, we introduce a new class of such vortices, "spatiotemporal plasmonic vortices," carrying nontrivial topological spin textures. They are generated by chronotopic interference of temporally delayed plasmonic eigenvortices, where a π-phase dislocation in the space-frequency domain maps into a 2π spiraling phase in space-time, with the resulting focus-defocus dynamics emulating U(1) gauge transitions. Using interferometric time-resolved photoemission electron microscopy, we directly image their nanometer-attosecond evolution and control vortex number and position. Quantum-path analysis of coherent two-photon photoemission processes reveals the nonlinear plasmonic polarization fields and angular-momentum conservation, establishing spatiotemporal plasmonic vortices as a platform for probing spatiotemporally structured quantum matter.

A planar chiral plasmonic nanoantenna exhibiting an excitation-chirality-dependent hot spot in a nanogap is numerically investigated. Additionally, the underlying design principles are examined, providing a broadly applicable framework for engineering chiral nanoantennas through controlled geometrical or modal asymmetry. The hot spot can be turned on and off by changing the handedness of the exciting circularly polarized light (CPL). This effect stems from the rationally designed interference of plasmonic modes excited by the linearly polarized orthogonal components of CPL. The hot spot exhibits maximal near-field dissymmetry factor (approximate to -2) at a wavelength of 842 nm. The intensity at the hot spot can also be continuously modulated by varying the excitation ellipticity and handedness, approaching a modulation depth of 100%. These attributes enable chirality- and ellipticity-dependent switching and dynamic modulation of the plasmonic near field. Moreover, placing an achiral quantum emitter in the nanogap generates almost perfectly circularly polarized emission, offering a simple yet effective avenue to realize nanoscale circularly polarized single-photon sources.

Antimony trisulfide (Sb_2S_3), as an emerging material for integrated photonic devices, has attracted significant attention due to its high index, low loss, and phase-changing property in the optical regime. However, conventional lithography-based fabrication methods involve complex, time-consuming, multistep processes, rendering the photonic application of Sb_2S_3 challenging. Here, we demonstrate that positive-tone fabrication of Sb_2S_3 nanostructures using wet-etch femtosecond laser processing, a straightforward technique for the engraving of micro- and nanoscale structures, can address major fabrication challenges. The patterning mechanism and factors influencing resolution of Sb_2S_3 thin film structures deposited on quartz (transmissive) and gold (reflective) substrates are experimentally investigated and supported by theoretical modelling. Using this approach, the smallest linewidth fabricated is measured at 178 nm. Consequently, multiple test patterns are demonstrated showing versatile functionalities. Functional Fresnel Zone Plates (FZPs) with varying focal length are fabricated and characterized. This study provides a significantly simplified approach for realizing Sb_2S_3 based integrated photonic devices.

Photon-recoil-based actuation enables maneuvering of micro- and nanoscale objects without beam steering or tight focusing, mitigating system complexity and photodamage. Recent light-driven microdrones achieved full control in two dimensions using multiple laser fields; however, for many applications, sacrificing degrees of freedom allows substantial miniaturization and improved propulsion efficiency. Here, we demonstrate sub-micrometer nanorobots actuated by a plasmonic directional antenna that simultaneously provides propulsion force and orientation control. The nanorobots reach propulsion speeds up to 50 μm/s, with their motion direction intrinsically locked perpendicular to the linear polarization axis. Circularly polarized light pulses lift the resulting twofold orientational degeneracy through spin-momentum transfer. Using opto-thermophoretic forces, nanorobots efficiently capture, transport, reversibly assemble, and release bacteria. By sequencing linear and circular polarization states, they execute complex, high-precision trajectories to systematically sweep defined regions, functioning as light-driven robotic cleaners. This work expands the capabilities of nanorobots for biological manipulation and high-speed, localized sensing.

3D plasmonic structures can provide giant optical chirality (C) in the near field, enabling strong interactions with enantiomers for chiral sensing applications. However, these structures face several limitations, including narrow operational bandwidth constrained by resonance, flipping handedness of C around the resonant frequency, spatially confined distribution of C, and difficulty in trapping enantiomers. Our numerical predictions reveal that a 3D plasmonic Archimedean spiral (AS) exhibits spectrally broadband, spatially extended, and single-handed near-field C in the visible-to-near-infrared range. However, realizing highly deterministic 3D AS remains challenging. We develop an effective fabrication strategy that combines focused ion beam milling and a template-stripping method to realize high-definition 3D AS. Furthermore, we experimentally characterize the fabricated structure's far-field chiroptical behavior to confirm the predicted optical response. Owing to its conical hole-like geometry, 3D AS can potentially function as a sieve for trapping enantiomer-functionalized nanoparticles in the hot zone with enhanced C for sensitive broadband chiroptical detection.

In nonlinear nanophotonics, cascaded second-harmonic generation (SHG) in pure plasmonic waveguides for sequential signal transformation and complex on-chip functionality remains a long-standing challenge. Precise phase matching becomes instrumental to achieve efficient SHG and enable true cascading of nonlinear processes. We experimentally demonstrate phase matching in SHG is achievable in a plasmonic system between two orthogonal modes. Accurate tuning of plasmonic two-wire transmission-line (TWTL) design parameters result in SHG from the antisymmetric excitation mode being approximately 15 times stronger than that from the symmetric excitation mode, greatly raising the conversion efficiency to 0.021

Recently light-driven microdrones have been demonstrated, making use of plasmonic nanomotors based on directional resonant chiral light scattering. These nanomotors can be addressed individually, without requiring the tracking of a focused laser, leading to exceptional 2D maneuverability which renders microdrones a versatile robotic platform in aqueous environments. Here, we incorporate a light-operated manipulator, a plasmonic nano-tweezer, into the microdrone platform, rendering it a microrobot by enabling precise, all-optical transport and delivery of single nanoparticles suspended in solution. The plasmonic nano-tweezer consists of a resonant cross-antenna nanostructure exhibiting a central near-field hot spot, extending the ability of traditional optical tweezers based on focused laser beams to the trapping of nanoparticles. However, most of plasmonic nano-tweezers are fixed to the substrates and lack mobility. Our plasmonic microrobot utilizes circularly polarized light to control both motors and for stable trapping of a 70-nanometer fluorescent nanodiamond in the cross-antenna center. Complex sequences of microrobot operations, including trap-transport-release-trap-transport actions, demonstrate the microrobot's versatility and precision in picking up and releasing nanoparticles. Our microrobot design opens potential avenues in advancing nanotechnology and life sciences, with applications in targeted drug delivery, single-cell manipulation, and by providing an advanced quantum sensing platform, facilitating interdisciplinary research at the nanoscale.

We report on the experimental investigation, by means of spatially-resolved cathodoluminescence spectroscopy, of rectangular all-dielectric Ge nanoantennas sustaining Fabry-Perot resonances. The combination of spatial and spectral resolution allows us to directly image the standing-wave pattern of the local density of optical states inside the nanoantennas, which is the fingerprint of the resonant Purcell contribution to the overall emission enhancement previously reported in the literature for the same structures. Our results confirm that the emission properties of Ge nanostructures can be effectively tuned by engineering the local density of optical states and that cathodoluminescence provides valuable information to experimentally address such modulation in their emission properties.

The sensitivity of molecular spectroscopy based on surface-enhanced coherent anti-Stokes Raman scattering (SECARS) is limited by the spectrally overlapping background from nonresonant four-wave mixing (FWM). While the SECARS signal is mediated by the molecular vibrational eigenstates and exhibits long lifetime (a few picoseconds), the FWM background stems from the instantaneous electronic polarization and decays rapidly with the vanishment of excitation. Therefore, CARS and FWM can be separated by time-resolved CARS (trCARS) using ultrashort pulsed lasers. The broad spectral bandwidth also enables broadband CARS (BCARS) for simultaneous identification of multiple vibrational signatures. This work combines trCARS and BCARS with an optimized plasmonic system to demonstrate time-resolved surface-enhanced BCARS) and show its capability in obtaining background-suppressed broadband vibrational spectra from a monolayer of 4-Aminothiophenol (4-ATP) molecules self-assembled on a gold grating. The vibrational dephasing time of the ring-breathing mode for the 4-ATP monolayer on the gold grating (approximate to 1.00 +/- 0.17 ps) is significantly shortened compared to that for 4-ATP powder on a glass substrate (2.10 +/- 0.05 ps). This work presents an effective method to suppress FWM background and investigate the vibrational dynamics of molecules in the vicinity of plasmonic nanostructures. Time-resolved surface-enhanced broadband coherent anti-Stokes Raman scattering allows temporally separating nonresonant four-wave mixing background and molecular vibrational CARS signal by introducing a time delay (Delta t) between the narrowband and the broadband laser pulse. This enhances the signal-to-noise ratio in SECARS and enables the tracing of vibrational dynamics of molecules near plasmonic nanostructures. image

Plasmonic nanoparticle arrays with a specific lattice arrangement can support surface lattice resonances (SLRs). SLR exhibits a sharp spectral peak and finds many applications including optical sensing and plasmonic lasers. To optimize SLR for application, a robust method that allows the mass production of plasmonic nanoparticle arrays with refined particle morphology and well-defined lattice arrangement is required. In this work, we combine nanosphere lithography (NSL) with thermal annealing or nanosecond-pulsed laser treatment to refine plasmonic nanoparticles in a honeycomb lattice. We comparatively study the effects of the two treatment methods on the particle morphology and lattice arrangement of mono (Ag and Pd) and bi-metallic (Ag-Pd) nanoparticle lattices. In general, thermal annealing preserves the lattice arrangement but fairly changes the particle roundness, while laser treatment produces particles with varying morphologies and spatial distribution. We also theoretically and experimentally investigate the optical responses of Ag nanoparticle lattices produced by different treatment methods. The observed difference in spectra can be attributed to the varying particle morphology, which shifts the localized surface plasmon resonance differently, resulting in a significant change in SLR. These findings provide valuable insights for optimizing plasmonic nanoparticle arrays for various applications.

We report the observation of hotspot-induced emitters and photoluminescence enhancement of up to 42-fold from DNA origami-assisted plasmonic dimer nanoantennas upon excess polarized laser illumination. The presence of DNA and laser polarization alignment along the dimer axis are critical for the generation of bright emitters responsible for the observed PL increase. The emission spectrum reveals characteristic Raman peaks of amorphous carbon, suggesting the formation of carbon-based emitters in the nanoantenna due to the plasmonic hotspots at the longitudinal antenna resonance.

Plasmonic enhancement of nonlinear light-matter interaction can be achieved via dedicated optimization of resonant plasmonic modes that are spectrally matched to the different wavelengths involved in the particular nonlinear optical process. In this work, we investigate the generation and enhancement of broadband four-wave mixing (FWM) in a plasmonic azimuthally chirped grating (ACG). The azimuthally varying grating periodicity in an ACG offers a well-defined channel to mediate the near field and the far field over a broad range of wavelengths. However, the particular mechanism responsible for field enhancement in such a platform depends on the interplay between the effects manifested by both the groove geometry and the grating's periodicity. This work delineates the collective contribution of groove geometry-dependent localized surface plasmon resonance (LSPR) and periodicity-dependent plasmonic surface lattice resonance (PSLR) over a broad range of wavelengths to bring into effect the enhancement of broadband FWM in an ACG.

Optical quantum emitters near nanostructures have access to additional relaxation channels and thus exhibit structure-dependent emission properties, including quantum yield and emission directionality. A well-engineered quantum emitter-plasmonic nanostructure hybrid can be considered as an optical meta-emitter consisting of a transmitting nanoantenna driven by an optical-frequency generator. In this work, the DNA origami fabrication method is used to construct ultracompact unidirectional meta-emitters composed of a plasmonic trimer nanoantenna driven by a single dye molecule. The origami is designed to bring the dye to the gap to simultaneously excite the electric and magnetic dipole modes of the trimer nanoantenna. The interference of these modes fulfills the Kerker condition at the fluorophore's emission band, enabling unidirectional emission. We report unidirectional emission from a single molecule with a front-to-back ratio of up to 10.7 dB accompanied by a maximum emission enhancement of 23-fold.

The control of nonlinear optical signals in nanostructured systems is pivotal to develop functional devices suitable for integration in optical platforms. A possible control mechanism is exploiting coherent interactions between different nonlinear optical processes. Here, this concept is implemented by taking advantage of the strong field enhancement and high optical nonlinearity provided by plasmonic nanostructures. Two beams, one at the angular frequency ω, corresponding to the telecom wavelength λ = 1551 nm, and the other at 2ω, are combined to generate a sum‐frequency signal at 3ω from single asymmetric gold nanoantennas. This nonlinear signal interferes with the third‐harmonic radiation generated by the beam at ω, resulting in a modulation up to 50% of the total signal at 3ω depending on the relative phase between the beams. Such a large intensity modulation of the nonlinear signal is accompanied by a rotation of its polarization axis, due to the lack of central symmetry of the nanostructure. The demonstration that the nonlinear emission can be coherently controlled through the phase difference of the two‐color illumination represents a promising route toward all‐optical logic operations at the nanoscale through nonlinear optical signal manipulation.

Plasmonic gratings are simple and effective platforms for nonlinear signal generation since they provide a well-defined momentum for photon-plasmon coupling and local hot spots for frequency conversion. Here, a plasmonic azimuthally chirped grating (ACG), which provides spatially resolved broadband momentum for photon-plasmon coupling, was exploited to investigate the plasmonic enhancement effect in two nonlinear optical processes, namely two-photon photoluminescence (TPPL) and second harmonic generation (SHG). The spatial distributions of the nonlinear signals were determined experimentally by hyperspectral mapping with ultrashort pulsed excitation. The experimental spatial distributions of nonlinear signals agree very well with the analytical prediction based on photon-plasmon coupling with the momentum of the ACG, revealing the "antenna" function of the grating in plasmonic nonlinear signal generation. This work highlights the importance of the antenna effect of the gratings for nonlinear signal generation and provides insight into the enhancement mechanism of plasmonic gratings in addition to local hot spot engineering.

Nanoparticle-on-mirror plasmonic nanocavities, capable of extreme optical confinement and enhancement, have triggered state-of-the-art progress in nanophotonics and development of applications in enhanced spectroscopies. However, the optical quality factor and thus performance of these nanoconstructs are undermined by the granular polycrystalline metal films (especially when they are optically thin) used as a mirror. Here, we report an atomically smooth single-crystalline platform for low-loss nanocavities using chemically synthesized gold microflakes as a mirror. Nanocavities constructed using gold nanorods on such microflakes exhibit a rich structure of plasmonic modes, which are highly sensitive to the thickness of optically thin (down to ∼15 nm) microflakes. The microflakes endow nanocavities with significantly improved quality factor (∼2 times) and scattering intensity (∼3 times) compared with their counterparts based on deposited films. The developed low-loss nanocavities further allow for the integration with a mature platform of fiber optics, opening opportunities for realizing nanocavity-based miniaturized photonic devices for practical applications.

The ongoing drive towards the miniaturization of nonlinear optics is motivated by the number of functionalities it could bring about in integrated devices, such as frequency conversion, information processing, and holography. The main hindrance to the technological deployment of nonlinear optics is the perturbative character of nonlinear interactions, whose intrinsic weakness is compounded at the nanoscale by the small volume of matter involved. The exploitation of the resonant electromagnetic modes supported by nanostructures in order to increase the strength of light–matter coupling is therefore a thriving area of research. In this work, we investigate gold dimers, where a V-antenna tuned to resonate at a telecom frequency ω (corresponding to a 1550 nm wavelength) is electromagnetically coupled to a rod resonating at 2ω. The structures were patterned lithographically out of a 40 nm-thick monocrystalline gold flake with a focused-ion (Ga) beam. In our experiment, a pump pulse at ω is mixed with its frequency-doubled replica, resulting in SFG at frequency ω+2ω = 3ω. A thorough experimental characterization discloses a rich phenomenology as the SFG is ruled by the resonant response. Such sensitive dependence on the geometry of the system and the excitation is unraveled through a systematic comparison to full-vectorial numerical simulations. In our dimers, on the one hand the doubly resonant design boosts both pumps of the SFG; on the other hand, the non-centrosymmetric shape allows interference of SFG with third-harmonic generation (THG), which also occurs at ω+ω+ω = 3ω. Delay traces of the signal at 3ω exhibit strong interference fringes (power modulation above 400%) with a 2ω periodicity. The small dephasing between pumps required to achieve such sizable modulation can be imparted by either mechanical or electro-optic means; along with the instantaneous (electronic) character of the materials response, this suggests enticing perspectives for ultrafast modulation and coherent control.

When photons interact with matter, forces and torques occur due to the transfer of linear and angular momentum, respectively. The resulting accelerations are small for macroscopic objects but become substantial for microscopic objects with small masses and moments of inertia, rendering photon recoil very attractive to propel micro- and nano-objects. However, until now, using light to control object motion in two or three dimensions in all three or six degrees of freedom has remained an unsolved challenge. Here we demonstrate light-driven microdrones (size roughly 2 μm and mass roughly 2 pg) in an aqueous environment that can be manoeuvred in two dimensions in all three independent degrees of freedom (two translational and one rotational) using two overlapping unfocused light fields of 830 and 980 nm wavelength. To actuate the microdrones independent of their orientation, we use up to four individually addressable chiral plasmonic nanoantennas acting as nanomotors that resonantly scatter the circular polarization components of the driving light into well-defined directions. The microdrones are manoeuvred by only adjusting the optical power for each motor (the power of each circular polarization component of each wavelength). The actuation concept is therefore similar to that of macroscopic multirotor drones. As a result, we demonstrate manual steering of the microdrones along complex paths. Since all degrees of freedom can be addressed independently and directly, feedback control loops may be used to counteract Brownian motion. We posit that the microdrones can find applications in transport and release of cargos, nanomanipulation, and local probing and sensing of nano and mesoscale objects.