High-Performance Ultrathin Active Chiral Metamaterials

Ultrathin active chiral metamaterials with dynamically tunable and responsive optical chirality enable new optical sensors, modulators, and switches. Herein, we develop ultrathin active chiral metamaterials of highly tunable chiroptical responses by inducing tunable near-field coupling in the metamaterials and exploit the metamaterials as ultrasensitive sensors to detect trace amounts of solvent impurities. To demonstrate the active chiral metamaterials mediated by tunable near-field coupling, we design moiré chiral metamaterials (MCMs) as model metamaterials, which consist of two layers of identical Au nanohole arrays stacked upon one another in moiré patterns with a dielectric spacer layer between the Au layers. Our simulations, analytical fittings, and experiments reveal that spacer-dependent near-field coupling exists in the MCMs, which significantly enhances the spectral shift and line shape change of the circular dichroism (CD) spectra of the MCMs. Furthermore, we use a silk fibroin thin film as the spacer layer in the MCM. With the solvent-controllable swelling of the silk fibroin thin films, we demonstrate actively tunable near-field coupling and chiroptical responses of the silk-MCMs. Impressively, we have achieved the spectral shift over a wavelength range that is more than one full width at half-maximum and the sign inversion of the CD spectra in a single ultrathin (1/5 of wavelength in thickness) MCM. Finally, we apply the silk-MCMs as ultrasensitive sensors to detect trace amounts of solvent impurities down to 200 ppm, corresponding to an ultrahigh sensitivity of >10⁵ nm/refractive index unit (RIU) and a figure of merit of 10⁵/RIU

Photoswitchable Rabi Splitting in Hybrid Plasmon–Waveguide Modes

Rabi splitting that arises from strong plasmon–molecule coupling has attracted tremendous interests. However, it has remained elusive to integrate Rabi splitting into the hybrid plasmon–waveguide modes (HPWMs), which have advantages of both subwavelength light confinement of surface plasmons and long-range propagation of guided modes in dielectric waveguides. Herein, we explore a new type of HPWMs based on hybrid systems of Al nanodisk arrays covered by PMMA thin films that are doped with photochromic molecules and demonstrate the photoswitchable Rabi splitting with a maximum splitting energy of 572 meV in the HPWMs by controlling the photoisomerization of the molecules. Through our experimental measurements combined with finite-difference time-domain (FDTD) simulations, we reveal that the photoswitchable Rabi splitting arises from the switchable coupling between the HPWMs and molecular excitons. By harnessing the photoswitchable Rabi splitting, we develop all-optical light modulators and rewritable waveguides. The demonstration of Rabi splitting in the HPWMs will further advance scientific research and device applications of hybrid plasmon–molecule systems

Seedless Growth of Palladium Nanocrystals with Tunable Structures: From Tetrahedra to Nanosheets

Despite the great success that has been accomplished on the controlled synthesis of Pd nanocrystals with various sizes and morphologies, an efficient approach to systematic production of well-defined Pd nanocrystals without seed-mediated approaches remains a significant challenge. In this work, we have developed an efficient synthetic method to directly produce Pd nanocrystals with a highly controllable feature. Three distinct Pd nanocrystals, namely, Pd nanosheets, Pd concave tetrahedra, and Pd tetrahedra, have been selectively prepared by simply introducing a small amount of ascorbic acid (AA) and/or water without the other synthesis conditions changed. We found that the combined use of AA and water is of importance for the successful production of the unique Pd nanosheets. Detailed catalytic investigations showed that all the obtained Pd nanocrystals exhibit higher activity in the formic acid electrooxidation and styrene hydrogenation with respect to the Pd black, and their activities are highly shape-dependent with Pd nanosheets demonstrating a higher activity than both the Pd concave tetrahedra and Pd tetrahedra, which is likely due to the simple yet important feature of ultrathin thickness of Pd nanosheets. The present work highlights the importance of structures in tuning the related properties of metallic nanocrystals

Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10–100 mW/μm²) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm². Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery

Molecular-Fluorescence Enhancement via Blue-Shifted Plasmon-Induced Resonance Energy Transfer

We report molecular-fluorescence enhancement via the blue-shifted plasmon-induced resonance energy transfer (PIRET) from single Au nanorods (AuNRs) to merocyanine (MC) dye molecules. The blue-shifted PIRET occurs when there is a proper spectral overlap between the scattering of AuNRs and the absorption of MC molecules. Along with the quenching of scattering from AuNRs, the blue-shifted PIRET enhances the fluorescence of nearby molecules. On the basis of the fluorescence enhancement, we conclude that AuNRs can be used as donors with clear advantages to excite the fluorescence of molecules as acceptors in AuNR–molecule hybrids. On the one hand, compared to conventional molecular donors in Förster resonance energy transfer (FRET), AuNRs have much larger absorption cross sections at the plasmon resonance frequencies. On the other hand, energy-transfer efficiency of PIRET decreases at a lower rate than that of FRET when the donor–acceptor distance is increased. Besides, the blue-shifted PIRET allows excitation with incident light of lower energy than the acceptor’s absorption, which is difficult to achieve in FRET because of the Stokes shift. With the capability of enhancing molecular fluorescence with excitation light of low intensity and long wavelength, the blue-shifted PIRET will expand the applications of nanoparticle–molecule hybrids in biosensing and bioimaging by increasing signal-to-noise ratio and by reducing photodamage to biological cells and organelles at the targeted areas

Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10–100 mW/μm²) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm². Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery

Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10–100 mW/μm²) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm². Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery

Light-Directed Reversible Assembly of Plasmonic Nanoparticles Using Plasmon-Enhanced Thermophoresis

Reversible assembly of plasmonic nanoparticles can be used to modulate their structural, electrical, and optical properties. Common and versatile tools in nanoparticle manipulation and assembly are optical tweezers, but these require tightly focused and high-power (10–100 mW/μm²) laser beams with precise optical alignment, which significantly hinders their applications. Here we present light-directed reversible assembly of plasmonic nanoparticles with a power intensity below 0.1 mW/μm². Our experiments and simulations reveal that such a low-power assembly is enabled by thermophoretic migration of nanoparticles due to the plasmon-enhanced photothermal effect and the associated enhanced local electric field over a plasmonic substrate. With software-controlled laser beams, we demonstrate parallel and dynamic manipulation of multiple nanoparticle assemblies. Interestingly, the assemblies formed over plasmonic substrates can be subsequently transported to nonplasmonic substrates. As an example application, we selected surface-enhanced Raman scattering spectroscopy, with tunable sensitivity. The advantages provided by plasmonic assembly of nanoparticles are the following: (1) low-power, reversible nanoparticle assembly, (2) applicability to nanoparticles with arbitrary morphology, and (3) use of simple optics. Our plasmon-enhanced thermophoretic technique will facilitate further development and application of dynamic nanoparticle assemblies, including biomolecular analyses in their native environment and smart drug delivery