Multi-wavelength lock-in spectroscopy for extracting perturbed spectral responses: molecular signatures in nanocavities.

Detecting small changes in spectral fingerprints at multiple wavelength bands simultaneously is challenging for many spectroscopic techniques. Because power variations, drift, and thermal fluctuations can affect such measurements on different timescales, high speed lock-in detection is the preferred method, however this is typically a single channel (wavelength) technique. Here, a way to achieve multichannel (multi-wavelength) lock-in vibrational spectroscopy is reported, using acousto-optic modulators to convert nanosecond periodic temporal perturbations into spatially distinct spectra. This simultaneously resolves perturbed and reference spectra, by projecting them onto different locations of the spectrometer image. As an example, we apply this multichannel time-resolved methodology to detect molecular frequency upconversion in plasmonic nanocavities from the perturbed Raman scattering at different wavelengths. Our phase-sensitive detection scheme can be applied to any spectroscopy throughout the visible and near-infrared wavelength ranges. Extracting perturbed spectra for measurements on nanosecond timescales allows for capturing many processes, such as semiconductor optoelectronics, high-speed spectro-electrochemistry, catalysis, redox chemistry, molecular electronics, or atomic diffusion across materials.

Atomistic Simulations of the Crystalline-to-Amorphous Transformation of γ-Al2O3 Nanoparticles: Delicate Interplay between Lattice Distortions, Stresses, and Space Charges.

The size-dependent phase stability of γ-Al2O3 was studied by large-scale molecular dynamics simulations over a wide temperature range from 300 to 900 K. For the γ-Al2O3 crystal, a bulk transformation to α-Al2O3 by an FCC-to-HCP transition of the O sublattice is still kinetically hindered at 900 K. However, local distortions of the FCC O-sublattice by the formation of quasi-octahedral Al local coordination spheres become thermally activated, as driven by the partial covalency of the Al-O bond. On the contrary, spherical γ-Al2O3 nanoparticles (NPs) (with sizes of 6 and 10 nm) undergo a crystalline-to-amorphous transformation at 900 K, which starts at the reconstructed surface and propagates into the core through collective displacements of anions and cations, resulting in the formation of 7- and 8-fold local coordination spheres of Al. In parallel, the reconstructed Al-enriched surface is separated from the stoichiometric core by a diffuse Al-depleted transition region. This compositional heterogeneity creates an imbalance of charges inside the NP, which induces a net attractive Coulombic force that is strong enough to reverse the initial stress state in the NP core from compressive to tensile. These findings disclose the delicate interplay between lattice distortions, stresses, and space-charge regions in oxide nanosystems. A fundamental explanation for the reported expansion of metal-oxide NPs with decreasing size is provided, which has significant implications for, e.g., heterogeneous catalysis, NP sintering, and additive manufacturing of NP-reinforced metal matrix composites.

Interfering Plasmons in Coupled Nanoresonators to Boost Light Localization and SERS.

Plasmonic self-assembled nanocavities are ideal platforms for extreme light localization as they deliver mode volumes of <50 nm3. Here we show that high-order plasmonic modes within additional micrometer-scale resonators surrounding each nanocavity can boost light localization to intensity enhancements >105. Plasmon interference in these hybrid microresonator nanocavities produces surface-enhanced Raman scattering (SERS) signals many-fold larger than in the bare plasmonic constructs. These now allow remote access to molecules inside the ultrathin gaps, avoiding direct irradiation and thus preventing molecular damage. Combining subnanometer gaps with micrometer-scale resonators places a high computational demand on simulations, so a generalized boundary element method (BEM) solver is developed which requires 100-fold less computational resources to characterize these systems. Our results on extreme near-field enhancement open new potential for single-molecule photonic circuits, mid-infrared detectors, and remote spectroscopy.

Phase stabilization of a coherent fiber network by single-photon counting.

Coherent optical fiber networks are extremely sensitive to thermal, mechanical, and acoustic noise, which requires elaborate schemes of phase stabilization with dedicated auxiliary lasers, multiplexers, and photodetectors. This is particularly demanding in quantum networks operating at the single-photon level. Here, we propose a simple method of phase stabilization based on single-photon counting and apply it to quantum fiber networks implementing single-photon interference on a lossless beamsplitter and coherent perfect absorption on a metamaterial absorber. As a proof of principle, we show dissipative single-photon switching with visibility close to 80%. This method can be employed in quantum networks of greater complexity without classical stabilization rigs, potentially increasing efficiency of the quantum channels.

Nanostructure and Optical Property Tailoring of Zinc Tin Nitride Thin Films through Phenomenological Decoupling: A Pathway to Enhanced Control.

This work addresses the need for precise control of thin film sputtering processes to enable thin film material tailoring on the example of zinc tin nitride (ZTN) thin films deposited via microwave plasma-assisted high power reactive magnetron sputtering (MAR-HiPIMS). The applied in situ diagnostic techniques (Langmuir probe and energy-resolved time-of-flight mass spectrometry) supported monitoring changes in the deposition environment with respect to microwave (MW) power. During MAR-HiPIMS, the presence of nitride ions in the gas phase (ZnN+, ZnN2+, SnN+, SnN2+) was detected. This indicates that the MW plasma facilitated their production, as opposed to pure R-HiPIMS. Additionally, MW plasma caused post-ionisation of sputtered atoms and reduced the overall energy-per-charge range of incoming charged species. By varying the MW power and substrate biasing, films with comparable chemical compositions (approximately Zn0.92Sn1.08N2) but different structures, ranging from polycrystalline to preferentially textured, were successfully produced. The application of density functional theory (DFT) further enabled the relationship between the lattice parameters and the optical properties of ZTN to be explored, where the material's optical anisotropy nature was determined. It was found that despite considerable differences in crystallinity, the changes induced in the lattice parameters were subangstrom, causing only minor changes in the final optical properties of ZTN.

Accurate Transfer of Individual Nanoparticles onto Single Photonic Nanostructures.

Controlled integration of metallic nanoparticles (NPs) onto photonic nanostructures enables the realization of complex devices for extreme light confinement and enhanced light-matter interaction. For instance, such NPs could be massively integrated on metal plates to build nanoparticle-on-mirror (NPoM) nanocavities or photonic integrated waveguides (WGs) to build WG-driven nanoantennas. However, metallic NPs are usually deposited via drop-casting, which prevents their accurate positioning. Here, we present a methodology for precise transfer and positioning of individual NPs onto different photonic nanostructures. Our method is based on soft lithography printing that employs elastomeric stamp-assisted transfer of individual NPs onto a single nanostructure. It can also parallel imprint many individual NPs with high throughput and accuracy in a single step. Raman spectroscopy confirms enhanced light-matter interactions in the resulting NPoM-based nanophotonic devices. Our method mixes top-down and bottom-up nanofabrication techniques and shows the potential of building complex photonic nanodevices for multiple applications ranging from enhanced sensing and spectroscopy to signal processing.

Mid-infrared-perturbed molecular vibrational signatures in plasmonic nanocavities.

Recent developments in surface-enhanced Raman scattering (SERS) enable observation of single-bond vibrations in real time at room temperature. By contrast, mid-infrared (MIR) vibrational spectroscopy is limited to inefficient slow detection. Here we develop a new method for MIR sensing using SERS. This method utilizes nanoparticle-on-foil (NPoF) nanocavities supporting both visible and MIR plasmonic hotspots in the same nanogap formed by a monolayer of molecules. Molecular SERS signals from individual NPoF nanocavities are modulated in the presence of MIR photons. The strength of this modulation depends on the MIR wavelength, and is maximized at the 6-12 µm absorption bands of SiO2 or polystyrene placed under the foil. Using a single-photon lock-in detection scheme we time-resolve the rise and decay of the signal in a few 100 ns. Our observations reveal that the phonon resonances of SiO2 can trap intense MIR surface plasmons within the Reststrahlen band, tuning the visible-wavelength localized plasmons by reversibly perturbing the localized few-nm-thick water shell trapped in the nanostructure crevices. This suggests new ways to couple nanoscale bond vibrations for optomechanics, with potential to push detection limits down to single-photon and single-molecule regimes.

Giant mid-IR resonant coupling to molecular vibrations in sub-nm gaps of plasmonic multilayer metafilms.

Nanomaterials capable of confining light are desirable for enhancing spectroscopies such as Raman scattering, infrared absorption, and nonlinear optical processes. Plasmonic superlattices have shown the ability to host collective resonances in the mid-infrared, but require stringent fabrication processes to create well-ordered structures. Here, we demonstrate how short-range-ordered Au nanoparticle multilayers on a mirror, self-assembled by a sub-nm molecular spacer, support collective plasmon-polariton resonances in the visible and infrared, continuously tunable beyond 11 µm by simply varying the nanoparticle size and number of layers. The resulting molecule-plasmon system approaches vibrational strong coupling, and displays giant Fano dip strengths, SEIRA enhancement factors ~ 106, light-matter coupling strengths g ~ 100 cm-1, Purcell factors ~ 106, and mode volume compression factors ~ 108. The collective plasmon-polariton mode is highly robust to nanoparticle vacancy disorder and is sustained by the consistent gap size defined by the molecular spacer. Structural disorder efficiently couples light into the gaps between the multilayers and mirror, enabling Raman and infrared sensing of sub-picolitre sample volumes.

Detecting mid-infrared light by molecular frequency upconversion in dual-wavelength nanoantennas.

Coherent interconversion of signals between optical and mechanical domains is enabled by optomechanical interactions. Extreme light-matter coupling produced by confining light to nanoscale mode volumes can then access single mid-infrared (MIR) photon sensitivity. Here, we used the infrared absorption and Raman activity of molecular vibrations in plasmonic nanocavities to demonstrate frequency upconversion. We converted approximately 10-micrometer-wavelength incoming light to visible light by surface-enhanced Raman scattering (SERS) in doubly resonant antennas that enhanced upconversion by more than 1010. We showed 140% amplification of the SERS anti-Stokes emission when an MIR pump was tuned to a molecular vibrational frequency, obtaining lowest detectable powers of 1 to 10 microwatts per square micrometer at room temperature. These results have potential for low-cost and large-scale infrared detectors and spectroscopic techniques.

Energy-resolved plasmonic chemistry in individual nanoreactors.

Plasmonic resonances can concentrate light into exceptionally small volumes, which approach the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances spanning the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules following surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows.

Controlling Optically Driven Atomic Migration Using Crystal-Facet Control in Plasmonic Nanocavities.

Plasmonic nanoconstructs are widely exploited to confine light for applications ranging from quantum emitters to medical imaging and biosensing. However, accessing extreme near-field confinement using the surfaces of metallic nanoparticles often induces permanent structural changes from light, even at low intensities. Here, we report a robust and simple technique to exploit crystal facets and their atomic boundaries to prevent the hopping of atoms along and between facet planes. Avoiding X-ray or electron microscopy techniques that perturb these atomic restructurings, we use elastic and inelastic light scattering to resolve the influence of crystal habit. A clear increase in stability is found for {100} facets with steep inter-facet angles, compared to multiple atomic steps and shallow facet curvature on spherical nanoparticles. Avoiding atomic hopping allows Raman scattering on molecules with low Raman cross-section while circumventing effects of charging and adatom binding, even over long measurement times. These nanoconstructs allow the optical probing of dynamic reconstruction in nanoscale surface science, photocatalysis, and molecular electronics.

Split-cube-resonator-based metamaterials for polarization-selective asymmetric perfect absorption.

A split-cube-resonator-based metamaterial structure that can act as a polarization- and direction-selective perfect absorber for the infrared region is theoretically and experimentally demonstrated. The structure, fabricated by direct laser writing and electroless silver plating, is comprised of four layers of conductively-coupled split-cube magnetic resonators, appropriately rotated to each other to bestow the desired electromagnetic properties. We show narrowband polarization-selective perfect absorption when the structure is illuminated from one side; the situation is reversed when illuminating from the other side, with the orthogonal linear polarization being absorbed. The absorption peak can be tuned in a wide frequency range by a sparser or denser arrangement of the split cube resonators, allowing to cover the entire atmospheric transparency window. The proposed metamaterial structure can find applications in polarization-selective thermal emission at the IR atmospheric transparency window for radiative cooling, in cost-effective infrared sensing devices, and in narrowband filters and linear polarizers in reflection mode.

Fibre-optic metadevice for all-optical signal modulation based on coherent absorption.

Recently, coherent control of the optical response of thin films in standing waves has attracted considerable attention, ranging from applications in excitation-selective spectroscopy and nonlinear optics to all-optical image processing. Here, we show that integration of metamaterial and optical fibre technologies allows the use of coherently controlled absorption in a fully fiberized and packaged switching metadevice. With this metadevice, which controls light with light in a nanoscale plasmonic metamaterial film on an optical fibre tip, we provide proof-of-principle demonstrations of logical functions XOR, NOT and AND that are performed within a coherent fibre network at wavelengths between 1530 and 1565 nm. The metadevice has been tested at up to 40 gigabits per second and sub-milliwatt power levels. Since coherent absorption can operate at the single-photon level and with 100 THz bandwidth, we argue that the demonstrated all-optical switch concept has potential applications in coherent and quantum information networks.