Ionizing terahertz waves with 260 MV/cm from scalable optical rectification.

Terahertz (THz) waves, known as non-ionizing radiation owing to their low photon energies, can actually ionize atoms and molecules when a sufficiently large number of THz photons are concentrated in time and space. Here, we demonstrate the generation of ionizing, multicycle, 15-THz waves emitted from large-area lithium niobate crystals via phase-matched optical rectification of 150-terawatt laser pulses. A complete characterization of the generated THz waves in energy, pulse duration, and focal spot size shows that the field strength can reach up to 260 megavolts per centimeter. In particular, a single-shot THz interferometer is employed to measure the THz pulse duration and spectrum with complementary numerical simulations. Such intense THz pulses are irradiated onto various solid targets to demonstrate THz-induced tunneling ionization and plasma formation. This study also discusses the potential of nonperturbative THz-driven ionization in gases, which will open up new opportunities, including nonlinear and relativistic THz physics in plasma.

Plasmonic Double-Walled Nanoframes with Face-to-Face Nanogaps for Strong SERS Activity.

A synthesis method for plasmonic double-walled nanoframes was developed, where single-walled truncated octahedral nanoframes with (111) open facets and (100) solid flat planes are nested in a core-shell manner. By applying multiple chemical toolkits to Au cuboctahedrons as a starting template, Au double-walled nanoframes with controllable face-to-face nanogaps were successfully synthesized in high homogeneity in size and shape. Importantly, when the gap distance between inner and outer flat walled frames became closer, augmentation of electromagnetic near-field focusing was achieved, leading to generation of hot-zones, which was verified by surface-enhanced Raman spectroscopy. The unique optical property of Au double-walled nanoframes with high structural intricacy was carefully investigated and the SERS substrates comprising Au double-walled nanoframes with the narrowest nanogaps exhibited much improved near-field enhancement toward strongly and/or weakly adsorbing analytes, allowing for gas phase detection in chemical warfare agents, which is a huge challenge in early warning systems.

Three-dimensional nanoframes with dual rims as nanoprobes for biosensing.

Three-dimensional (3D) nanoframe structures are very appealing because their inner voids and ridges interact efficiently with light and analytes, allowing for effective optical-based sensing. However, the realization of complex nanoframe architecture with high yield is challenging because the systematic design of such a complicated nanostructure lacks an appropriate synthesis protocol. Here, we show the synthesis method for complex 3D nanoframes wherein two-dimensional (2D) dual-rim nanostructures are engraved on each facet of octahedral nanoframes. The synthetic scheme proceeds through multiple executable on-demand steps. With Au octahedral nanoparticles as a sacrificial template, sequential processes of edge-selective Pt deposition and inner Au etching lead to Pt octahedral mono-rim nanoframes. Then, adlayers of Au are grown on Pt skeletons via the Frank-van der Merwe mode, forming sharp and well-developed edges. Next, Pt selective deposition on both the inner and outer boundaries leads to tunable geometric patterning on Au. Finally, after the selective etching of Au, Pt octahedral dual-rim nanoframes with highly homogeneous size and shape are achieved. In order to endow plasmonic features, Au is coated around Pt frames while retaining their geometric shape. The resultant plasmonic dual-rim engraved nanoframes possess strong light entrapping capability verified by single-particle surface-enhanced Raman scattering (SERS) and show the potential of nanoprobes for biosensing through SERS-based immunoassay.

Nesting of multiple polyhedral plasmonic nanoframes into a single entity.

The development of plasmonic nanostructures with intricate nanoframe morphologies has attracted considerable interest for improving catalytic and optical properties. However, arranging multiple nanoframes in one nanostructure especially, in a solution phase remains a great challenge. Herein, we show complex nanoparticles by embedding various shapes of three-dimensional polyhedral nanoframes within a single entity through rationally designed synthetic pathways. This synthetic strategy is based on the selective deposition of platinum atoms on high surface energy facets and subsequent growth into solid platonic nanoparticles, followed by the etching of inner Au domains, leaving complex nanoframes. Our synthetic routes are rationally designed and executable on-demand with a high structural controllability. Diverse Au solid nanostructures (octahedra, truncated octahedra, cuboctahedra, and cubes) evolved into complex multi-layered nanoframes with different numbers/shapes/sizes of internal nanoframes. After coating the surface of the nanoframes with plasmonically active metal (like Ag), the materials exhibited highly enhanced electromagnetic near-field focusing embedded within the internal complicated rim architecture.

All-Hot-Spot Bulk Surface-Enhanced Raman Scattering (SERS) Substrates: Attomolar Detection of Adsorbates with Designer Plasmonic Nanoparticles.

Here we report a synthetic pathway toward Au truncated octahedral dual-rim nanoframes wherein two functional facets are formed including (1) eight hot nanogaps formed by hexagonal nanoframes embracing core circular nanorings for near-field focusing and (2) six flat squares that facilitate the formation of well-ordered arrays of nanoframes through self-assembly. The existence of intra-nanogaps in a single entity enables strong electromagnetic near-field focusing, allowing single-particle surface-enhanced Raman spectroscopy. Then, we built "all-hot-spot bulk SERS substrates" with those entities, wherein the presence of truncated terraces with high homogeneity in size and shape facilitate spontaneous self-assembly into a highly ordered and uniform superlattice, exhibiting a limit of detection of attomolar concentrations toward 2-naphthalenethiol, which is 6 orders lower than that of monorim counterparts. The observed low limit of detection originates from the combined synergic effect from both inter- and intraparticle coupling in a superlattice, which we dubbed "all-hot-spot bulk SERS substrates".

Plasmonic All-Frame-Faceted Octahedral Nanoframes with Eight Engraved Y-Shaped Hot Zones.

We report the synthesis of all-frame-faceted octahedral nanoframes containing eight Y-shaped hot zones in a single entity where electromagnetic near-field focusing can be maximized. To realize such state-of-the-art complex nanoframes, a series of multiple stepwise bottom-up processes were executed by exploiting Au octahedral nanoparticles as the initial template. By rationally controlling the chemical reactivity of different surface facets (i.e., vertexes, edges, and terraces), the Au octahedral nanoparticles went through controlled shape transformations, leading to Au-engraved nanoparticles wherein 24 edges wrap the octahedral Au nanoparticle core. Those edges were then selectively decorated with Pt, leading to the formation of eight Pt tripods in a single entity. After etching the central Au, 3D Pt tripod frame-faceted octahedral nanoframes were achieved with high integrity. By harnessing the obtained Pt nanoframes as a scaffold, AuAg alloy-based plasmonic all-frame-faceted nanoframes were obtained after the co-reduction of Ag and Au, which generated multiple hot zones within multiple surface intra-nanogaps, creating a single-particle, surface-enhanced Raman spectroscopy enhancer platform.

Exploring the dynamic nature of divalent metal ions involved in DNA cleavage by CRISPR-Cas12a.

CRISPR-Cas12a has been widely used in genome editing and nucleic acid detection. In both of these applications, Cas12a cleaves target DNA in a divalent metal ion-dependent manner. However, when and how metal ions contribute to the cleavage reaction is unclear. Here, using a single-molecule FRET assay, we reveal that these metal ions are necessary for stabilising cleavage-competent conformations and that they are easily exchangeable, suggesting that they are dynamically coordinated.

Single-Molecule FRET Detection of Sub-Nanometer Distance Changes in the Range below a 3-Nanometer Scale.

Single-molecule fluorescence energy transfer (FRET) detection has become a key technique to monitor intra- and intermolecular distance changes in biological processes. As the sensitive detection range of conventional FRET pairs is limited to 3-8 nm, complement probes are necessary for extending this typical working range. Here, we realized a single-molecule FRET assay for a short distance range of below 3 nm by using a Cy2-Cy7 pair having extremely small spectral overlap. Using two DNA duplexes with a small difference in the labeling position, we demonstrated that our assay can observe subtle changes at a short distance range. High sensitivity in the range of 1-3 nm and compatibility with the conventional FRET assay make this approach useful for understanding dynamics at a short distance.

Influence of Size and Shape Anisotropy on Optical Properties of CdSe Quantum Dots.

We used low-temperature reactions to synthesize different-sized CdSe quantum dots (QDs) capped with fatty-acid and phosphine ligands. From the correlation of high-resolution transmission electron microscopy and X-ray diffraction (XRD) analyses of the synthesized QDs, we observed size-dependent shape anisotropy. In addition, the recorded XRD patterns revealed mixed crystal facets with zinc blende and wurtzite structures in small-sized QDs. Furthermore, from differential absorption (DA) spectra, we extracted the electronic transition energies for different-sized QDs, which were found to be similar to the calculated values of the quantum size levels associated with band mixing of CdSe QDs with a moderate bandgap. We found that the excitonic absorption peaks are increasingly "hidden" with decreasing QD size because of the crystal structure and crystalline quality. The results show good agreement with the obtained diffraction patterns and the estimation errors obtained from the DA spectra.

Trilayer hybrid structures for highly efficient THz modulation.

We demonstrate a novel technique to achieve a highly efficient terahertz (THz) modulation based on hybrid structures of organic layers (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) fabricated on both sides of a silicon (Si) substrate. The organic layer generating an optically induced electron (or hole) transfer is deposited on the back (or front) side of the Si substrate. The spatial charge separation improved owing to the transferred photo-excited electrons or holes at both interfaces of PCBM/Si and TIPS-pentacene/Si, enables a highly efficient THz wave modulation. The photoexcitation on the hole-transfer organic layer (TIPS-pentacene/Si) further improves the modulation efficiency, as the diffusion of electrons through the Si substrate is faster than that of photo-excited holes.

High refractive index metamaterials using corrugated metallic slots.

We report on a method for realizing high refractive index metamaterials using corrugated metallic slot structures at terahertz frequencies. The effective refractive index and peak index frequency can be controlled by varying the width of the air gap in the corrugated slot arrays. The phenomenon occurs because of the secondary resonance effect due to the fundamental inductive-capacitive resonance, which generates a red-shift of the fundamental resonance determined by twice the length of the corrugated metallic slots. In addition, multiple gaps in the corrugated slots act as plasmonic hotspots which have the properties of three-dimensional subwavelength confinement due to extremely strong enhancement of the terahertz waves. The versatile characteristics of the structures may have many potential applications in designing compact optical devices incorporating various functionalities and in developing highly sensitive spectroscopic/imaging systems.

Monopole resonators in planar plasmonic metamaterials.

We first present a new phenomenon: the quarter-wavelength resonance of an electromagnetic field in planar plasmonic metamaterials consisting of asymmetrically coupled air-slot arrays, which is essential for a monopole resonator. The anti-nodal electric field intensity of the quarter-wavelength fundamental mode is formed by strong charge concentrations at the sharp metallic edges of the crossing position of the air-slots, and the nodal point of the electric field intensity naturally occurs at the other end of the air-slot. By tuning the structural asymmetry, the quarter-wavelength resonances were successfully split from the half-wavelength resonance, experimentally and numerically.

Three-dimensional subwavelength confinement of terahertz electromagnetic surface modes in a coupled slit structure.

We report on the three-dimensional subwavelength confinement of the electromagnetic waves at a coupled metallic slit structure beyond diffraction limit in terahertz region. Lateral confinement behavior, leading to the three-dimensional confinement, is caused by a strong funneling effect of the light which occurs at the intersection of slits with a sharp metal geometry. Tunability of the resonant frequency and the position of the light confinement is achieved by controlling the slit length and the position of the intersection of slits, respectively.