Active infrared tuning of metal-insulator-metal resonances by VO2 thin film.

VO2 is a promising phase change material offering a large contrast of electric, thermal, and optical properties when transitioning from semiconductor to metallic phase. Here we show that a hybrid metamaterial obtained by proper combination of a VO2 layer and a nanodisk gold array provides a tunable plasmonic gap resonance in the infrared range. Specifically, we have designed and fabricated a metal-insulator-metal gap resonance by inserting sub-wavelength VO2 film between a flat gold layer and a gold nanodisk resonator array. The resonance of the hybrid metamaterial is centered in the useful 3-5 µm range when VO2 is in its semiconductor state. The experimental study highlights a monotonical spectral tuning of the resonance when increasing temperature up to 50 °C above the room temperature, providing a continuous resonance shift of almost 1 µm in the mid-infrared range. Wavelength range and intensity tunability can be further optimized by modifying the thicknesses of the layers and metamaterial parameters.

A universal route to efficient non-linear response via Thomson scattering in linear solids.

Non-linear materials are cornerstones of modern optics and electronics. Strong dependence on the intrinsic properties of particular materials, however, inhibits the at-will extension of demanding non-linear effects, especially those second-order ones, to widely adopted centrosymmetric materials (for example, silicon) and technologically important burgeoning spectral domains (for example, terahertz frequencies). Here we introduce a universal route to efficient non-linear responses enabled by exciting non-linear Thomson scattering, a fundamental process in electrodynamics that was known to occur only in relativistic electrons in metamaterial composed of linear materials. Such a mechanism modulates the trajectory of charges, either intrinsically or extrinsically provided in solids, at twice the driving frequency, allowing second-harmonic generation at terahertz frequencies on crystalline silicon with extremely large non-linear susceptibility in our proof-of-concept experiments. By offering a substantially material- and frequency-independent platform, our approach opens new possibilities in the fields of on-demand non-linear optics, terahertz sources, strong field light-solid interactions and integrated photonic circuits.

Enhancement of Scattering and Near Field of TiO2-Au Nanohybrids Using a Silver Resonator for Efficient Plasmonic Photocatalysis.

Recently, localized surface plasmon resonances (SPRs) of metallic nanoparticles (NPs) have been widely used to construct plasmonic nanohybrids for heterogeneous photocatalysis. For example, the combination of plasmonic Au NPs and TiO2 provides pure TiO2 visible-light activity. The SPR effect induces an electric field and consequently enhances light scattering and absorption, favoring the transfer of photon energy to hot carriers for catalytic reactions. Numerous approaches have been dedicated to the improvement of SPR absorption in photocatalysts. Here, we have designed a core@shell-satellite nanohybrid catalyst whereby an Ag NP core, as a plasmonic resonator featuring unique dual functions of strong scattering and near-field enhancement, is encapsulated by SiO2 and TiO2 layers in sequence, with Au NPs on the outer surface, Ag@SiO2@TiO2-Au, for efficient plasmonic photocatalysis. By varying the size and number of Ag NP cores, the Au SPR can be tailored over the visible and near-infrared spectral region to reabsorb the scattered photons. In the presence of the Ag core, the incident light is efficiently confined in the reaction suspension by undergoing multiple scattering, thus leading to an increase of the optical path to the photocatalysis. Moreover, using numerical analysis and experimental verifications, we demonstrate that the Ag core also induces a strong near-field enhancement at the Au-TiO2 interface via SPR coupling with Au. Consequently, the activity of the TiO2-Au plasmonic photocatalyst is significantly enhanced, resulting in a high H2 production rate under visible light. Thus, the design of a single structural unit with strong scattering and field enhancement, induced by a plasmonic resonator, is a highly effective strategy to boost photocatalytic activity.

Effect of Extrinsic Disorder on the Magnetoresistance Response of Gated Single-Layer Graphene Devices.

Analogous to the case of classical metal oxide semiconductor field-effect transistors, transport properties of graphene-based devices are determined by scattering from adventitious charged impurities that are invariably present. The presence of charged impurities renders experimental graphene samples "extrinsic" in that their electrical performances also depend on the environment in which graphene operates. While the role of such an extrinsic disorder component has been studied for conventional charge transport in graphene, its impact on the magnetotransport remains unexplored. Here, we show that single-layer graphene transistors with a low density of extrinsic disorder feature a larger magnetoresistance (MR) than those with a high density. Importantly, in gated single-layer devices with a low density of charged impurities, we find that MR peaks at gate voltage values far from the charge neutrality point not only at a low temperature but also at room temperature; in particular, MR approaches 800% at room temperature and 1400% at 50 K in such devices. In addition, dynamic measurements of MR on devices with a low degree of extrinsic disorder lead to stable and reliable single-layer graphene magnetosensors endowed with an ultralow power consumption of 2.5 nW. Our work indicates that the initial value of the minimum conductivity σ0 at room temperature along with carrier mobility must be looked at to select the most promising devices for magnetosensing.

Analysis of External and Internal Disorder to Understand Band-Like Transport in n-Type Organic Semiconductors.

Charge transport in organic semiconductors is notoriously extremely sensitive to the presence of disorder, both internal and external (i.e., related to interactions with the dielectric layer), especially for n-type materials. Internal dynamic disorder stems from large thermal fluctuations both in intermolecular transfer integrals and (molecular) site energies in weakly interacting van der Waals solids and sources transient localization of the charge carriers. The molecular vibrations that drive transient localization typically operate at low-frequency (

Homodyne Solid-State Biased Coherent Detection of Ultra-Broadband Terahertz Pulses with Static Electric Fields.

We present an innovative implementation of the solid-state-biased coherent detection (SSBCD) technique, which we have recently introduced for the reconstruction of both amplitude and phase of ultra-broadband terahertz pulses. In our previous works, the SSBCD method has been operated via a heterodyne scheme, which involves demanding square-wave voltage amplifiers, phase-locked to the THz pulse train, as well as an electronic circuit for the demodulation of the readout signal. Here, we demonstrate that the SSBCD technique can be operated via a very simple homodyne scheme, exploiting plain static bias voltages. We show that the homodyne SSBCD signal turns into a bipolar transient when the static field overcomes the THz field strength, without the requirement of an additional demodulating circuit. Moreover, we introduce a differential configuration, which extends the applicability of the homodyne scheme to higher THz field strengths, also leading a two-fold improvement of the dynamic range compared to the heterodyne counterpart. Finally, we demonstrate that, by reversing the sign of the static voltage, it is possible to directly retrieve the absolute THz pulse polarity. The homodyne configuration makes the SSBCD technique of much easier access, leading to a vast range of field-resolved applications.

A Microorganism Bred TiO2/Au/TiO2 Heterostructure for Whispering Gallery Mode Resonance Assisted Plasmonic Photocatalysis.

The TiO2/Au nanostructure has been acknowledged as one of the most classic visible-light active photocatalysts due to the surface plasmon resonance (SPR) of Au nanoparticles. In many cases, the SPR effect only features weak visible light absorption in conventional TiO2/Au nanostructures. Here, we demonstrate a design of TiO2/Au/TiO2 with a combination of whispering gallery mode (WGM) resonances and SPR for efficient visible-light-driven photocatalysis. Escherichia coli (E. coli) were used as natural reactants as well as a template to construct an E. coli-like TiO2/Au/TiO2 nanostructure. Using numerical simulations, we show that the E. coli-like TiO2 capsule acts as the WGM resonator to interplay with the SPR effect of the Au NPs on TiO2 surface, which leads to a significant increase of visible light absorption and the local field enhancement at the Au-TiO2 interface. Accordingly, with the synergistic effect of WGM and SPR, the E. coli-like TiO2/Au/TiO2 nanostructure exhibits enhanced photocatalytic activity in the visible range. Our work reveals a promising bioapproach to a design highly visible light active plasmonic photocatalyst.

Terahertz three-dimensional monitoring of nanoparticle-assisted laser tissue soldering.

In view of minimally-invasive clinical interventions, laser tissue soldering assisted by plasmonic nanoparticles is emerging as an appealing concept in surgical medicine, holding the promise of surgeries without sutures. Rigorous monitoring of the plasmonically-heated solder and the underlying tissue is crucial for optimizing the soldering bonding strength and minimizing the photothermal damage. To this end, we propose a non-invasive, non-contact, and non-ionizing modality for monitoring nanoparticle-assisted laser-tissue interaction and visualizing the localized photothermal damage, by taking advantage of the unique sensitivity of terahertz radiation to the hydration level of biological tissue. We demonstrate that terahertz radiation can be employed as a versatile tool to reveal the thermally-affected evolution in tissue, and to quantitatively characterize the photothermal damage induced by nanoparticle-assisted laser tissue soldering in three dimensions. Our approach can be easily extended and applied across a broad range of clinical applications involving laser-tissue interaction, such as laser ablation and photothermal therapies.

Extremely broadband terahertz generation via pulse compression of an Ytterbium laser amplifier.

We present a system for extremely broadband terahertz (THz) generation based on an Ytterbium (Yb) amplified laser emitting 170-fs-long pulses centered at 1030 nm. The pulses are first spectrally broadened in an Ar-filled hollow-core capillary fiber (HCF) and then recompressed down to ∼18 fs with a chirped-mirror pair. Extreme broadband THz pulses of bandwidths up to 60 THz and peak electric field as high as 55 kV/cm are obtained via two-color plasma generation. The combination of high-power Yb laser systems with gas-filled HCF opens the path towards the realization of the next generation high-repetition-rate, extremely broadband, and intense-field THz time-domain spectroscopy systems.

Room-Temperature and Selective Triggering of Supramolecular DNA Assembly/Disassembly by Nonionizing Radiation.

Recent observations have suggested that nonionizing radiation in the microwave and terahertz (THz; far-infrared) regimes could have an effect on double-stranded DNA (dsDNA). These observations are of significance owing to the omnipresence of microwave emitters in our daily lives (e.g., food preparation, telecommunication, and wireless Internet) and the increasing prevalence of THz emitters for imaging (e.g., concealed weapon detection in airports, skin cancer screenings) and communication technologies. By examining multiple DNA nanostructures as well as two plasmid DNAs, microwaves were shown to promote the repair and assembly of DNA nanostructures and single-stranded regions of plasmid DNA, while intense THz pulses had the opposite effect (in particular, for short dsDNA). Both effects occurred at room temperature within minutes, showed a DNA length dependence, and did not affect the chemical integrity of the DNA. Intriguingly, the function of six proteins (enzymes and antibodies) was not affected by exposure to either form of radiation under the conditions examined. This particular detail was exploited to assemble a fully functional hybrid DNA-protein nanostructure in a bottom-up manner. This study therefore provides entirely new perspectives for the effects, on the molecular level, of nonionizing radiation on biomolecules. Moreover, the proposed structure-activity relationships could be exploited in the field of DNA nanotechnology, which paves the way for designing a new range of functional DNA nanomaterials that are currently inaccessible to state-of-the-art assembly protocols.

Molecular gases for pulse compression in hollow core fibers.

We introduce hydrofluorocarbon molecules as an alternative medium to noble gases with low ionization potential like krypton or xenon to compress ultrashort pulses of relatively low energy in a conventional hollow core fiber with subsequent dispersion compensation. Spectral broadening of pulses from two different laser systems exceeded those achieved with argon and krypton. Initially 40 fs, 800 nm, 120 µJ pulses were compressed to few optical cycles duration. With the same approach a compression factor of more than 10 was demonstrated for an ytterbium-based laser (1030 nm, 170 fs, 200 µJ) leading to 15.6 fs.

Direct compression of 170-fs 50-cycle pulses down to 1.5 cycles with 70% transmission.

We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.

Plasmonic Au-Loaded Hierarchical Hollow Porous TiO2 Spheres: Synergistic Catalysts for Nitroaromatic Reduction.

Plasmonic Au nanoparticle (NP)-loaded hierarchical hollow porous TiO2 spheres are designed and synthesized with the purpose of enhancing the overall catalytic activity by introducing the Au plasmonic effect into the system, where Au NPs themselves are catalytically active. The constructed nanohybrid exhibits both high activity in 4-nitrophenol reduction, compared to all of the previously reported Au-based catalysts, and high selectivity. The synergy of the inherent catalytic property of Au NPs and the plasmonic effect (mainly via hot electron transfer) under irradiation is confirmed by a series of control experiments. The specifically designed, porous hollow structure also greatly contributes to the good catalytic activity because it provides a large surface area, facilitates reactant adsorption, and hinders charge recombination. In addition, theoretical calculations reveal that such a structure also leads to an increase in light absorption of about 21% in the range of 400-800 nm with respect to a uniform water-TiO2 background featuring the same filling factor. This work provides insight into the rational design of plasmon-enhanced catalysts that will show their versatility in various electro-/photocatalysis.

Reshaping the phonon energy landscape of nanocrystals inside a terahertz plasmonic nanocavity.

Phonons (quanta of collective vibrations) are a major source of energy dissipation and drive some of the most relevant properties of materials. In nanotechnology, phonons severely affect light emission and charge transport of nanodevices. While the phonon response is conventionally considered an inherent property of a nanomaterial, here we show that the dipole-active phonon resonance of semiconducting (CdS) nanocrystals can be drastically reshaped inside a terahertz plasmonic nanocavity, via the phonon strong coupling with the cavity vacuum electric field. Such quantum zero-point field can indeed reach extreme values in a plasmonic nanocavity, thanks to a mode volume well below λ3/107. Through Raman measurements, we find that the nanocrystals within a nanocavity exhibit two new "hybridized" phonon peaks, whose spectral separation increases with the number of nanocrystals. Our findings open exciting perspectives for engineering the optical phonon response of functional nanomaterials and for implementing a novel platform for nanoscale quantum optomechanics.

Generation of high-field terahertz pulses in an HMQ-TMS organic crystal pumped by an ytterbium laser at 1030 nm.

We present the generation of high-peak-electric-field terahertz pulses via collinear optical rectification in a 2-(4-hydroxy-3-methoxystyryl)-1-methilquinolinium-2,4,6-trimethylbenzenesulfonate (HMQ-TMS) organic crystal. The crystal is pumped by an amplified ytterbium laser system, emitting 170-fs-long pulses centered at 1030 nm. A terahertz peak electric field greater than 200 kV/cm is obtained for 420 µJ of optical pump energy, with an energy conversion efficiency of 0.26% - about two orders of magnitude higher than in common inorganic crystals collinearly pumped by amplified femtosecond lasers. An open-aperture Z-scan measurement performed on an n-doped InGaAs thin film using such terahertz source shows a nonlinear increase in the terahertz transmission of about 2.2 times. Our findings demonstrate the potential of this terahertz generation scheme, based on ytterbium laser technology, as a simple and efficient alternative to the existing intense table-top terahertz sources. In particular, we show that it can be readily used to explore nonlinear effects at terahertz frequencies.

Engineering the Absorption and Field Enhancement Properties of Au-TiO2 Nanohybrids via Whispering Gallery Mode Resonances for Photocatalytic Water Splitting.

Recently, surface plasmon resonance (SPR) effects have been widely used to construct photocatalysts which are active in the visible spectral region. Such plasmonic photocatalysts usually comprise a semiconductor material transparent in the visible range (such as TiO2) and plasmonic nano-objects (e.g., Au nanoparticles (Au NPs)). Specific SPRs, though, only partially cover the visible spectrum and feature weak light absorption. Here, we explore the unique role played by whispering gallery mode (WGM) resonances in the expression of the photocatalytic activity of plasmonic photocatalysts. Using numerical simulations, we demonstrate that, by solely exploiting a proper geometrical arrangement and WGM resonances in a TiO2 sphere, the plasmonic absorption can be extended over the entire visible range and can be increased by more than 40 times. Furthermore, the local electric field at the Au-TiO2 interface is also considerably enhanced. These results are experimentally corroborated, by means of absorption spectroscopy and Raman measurements. Accordingly, such WGM-assisted plasmonic photocatalysts, when employed in water splitting experiments, exhibit enhanced activity in the visible range. Our findings show a promising and straightforward way to design full solar spectrum photocatalysts.

Laser-assisted guiding of electric discharges around objects.

Electric breakdown in air occurs for electric fields exceeding 34 kV/cm and results in a large current surge that propagates along unpredictable trajectories. Guiding such currents across specific paths in a controllable manner could allow protection against lightning strikes and high-voltage capacitor discharges. Such capabilities can be used for delivering charge to specific targets, for electronic jamming, or for applications associated with electric welding and machining. We show that judiciously shaped laser radiation can be effectively used to manipulate the discharge along a complex path and to produce electric discharges that unfold along a predefined trajectory. Remarkably, such laser-induced arcing can even circumvent an object that completely occludes the line of sight.

Squeezing terahertz light into nanovolumes: nanoantenna enhanced terahertz spectroscopy (NETS) of semiconductor quantum dots.

Terahertz spectroscopy has vast potentialities in sensing a broad range of elementary excitations (e.g., collective vibrations of molecules, phonons, excitons, etc.). However, the large wavelength associated with terahertz radiation (about 300 µm at 1 THz) severely hinders its interaction with nano-objects, such as nanoparticles, nanorods, nanotubes, and large molecules of biological relevance, practically limiting terahertz studies to macroscopic ensembles of these compounds, in the form of thick pellets of crystallized molecules or highly concentrated solutions of nanomaterials. Here we show that chains of terahertz dipole nanoantennas spaced by nanogaps of 20 nm allow retrieving the spectroscopic signature of a monolayer of cadmium selenide quantum dots, a significant portion of the signal arising from the dots located within the antenna nanocavities. A Fano-like interference between the fundamental antenna mode and the phonon resonance of the quantum dots is observed, accompanied by an absorption enhancement factor greater than one million. NETS can find immediate applications in terahertz spectroscopic studies of nanocrystals and molecules at extremely low concentrations. Furthermore, it shows a practicable route toward the characterization of individual nano-objects at these frequencies.

Active terahertz two-wire waveguides.

We demonstrate, by generating a THz electric field directly within the guiding structure, an active two-wire waveguide operating in the terahertz (THz) range of wavelengths. We compare the energy throughput of the active configuration with that of a radiatively coupled semi-large photoconductive antenna, in which the radiation is generated outside the waveguide, reporting a 60 times higher energy throughput for the same illumination power and applied voltage. This novel, active waveguide design allows to have efficient coupling of the THz radiation in a dispersion-less waveguide without the need of involved radiative coupling geometries.

Integrated frequency comb source of heralded single photons.

We report an integrated photon pair source based on a CMOS-compatible microring resonator that generates multiple, simultaneous, and independent photon pairs at different wavelengths in a frequency comb compatible with fiber communication wavelength division multiplexing channels (200 GHz channel separation) and with a linewidth that is compatible with quantum memories (110 MHz). It operates in a self-locked pump configuration, avoiding the need for active stabilization, making it extremely robust even at very low power levels.