Surface-enhanced infrared spectroscopy using metal oxide plasmonic antenna arrays

Surface-enhanced optical-mid-infrared photothermal microscopy using shortened colloidal silver nanowires: a noble approach for mid-infrared surface sensing

We propose surface-enhanced optical-mid-infrared photothermal (MIP) microscopy using highly crystalline silver nanowires, acting as a Fabry-Perot resonator, and demonstrate its applicability to enhanced mid-infrared surface sensing of thin polymer layers as thin as 20 nm.

Optical nanoimaging of laser-switched phase-change plasmonic infrared antennas

We investigate the plasmonic properties of laser-printed chalcogenide phase-change material In3SeTb2 (IST) antennas through near-field nanoimaging. Antennas of varying lengths were fabricated by laser switching an amorphous IST film into its crystalline metallic state. Near-field imaging elucidates the pronounced field confinement and enhancement at the antenna extremities along with the emergence of different ordered plasmonic modes with increasing length. Compared to gold antennas, the PCM antennas exhibit slightly lower but still substantial near-field enhancement with greater compactness. The interplay between antenna length, illumination angle, and excitation frequency enables versatile control over the resonant near-field distribution. Our work provides deeper understanding and tunable functionalities of laser-printed PCM nanoantennas for potential applications in compact, dynamically reconfigurable nanophotonic devices.

Dipole-multipole plasmonic coupling between gold nanorods and titanium nitride nanoparticles for enhanced photothermal conversion

The plasmonic photothermal conversion efficiency can be enhanced by coupling among plasmonic atoms or plasmonic molecules due to the amplified local electric field and extinction cross-section. Recently, it has been theoretically proved that hybridization between dipolar modes and higher order modes can provide higher enhancement than that among dipolar modes in terms of both near- and far-field, which may lead to a higher photothermal conversion rate. In this work, we systematically investigated the photothermal conversion enhancement of plasmonic coupling between a dipolar mode of a titanium nitride nanoparticle (TiN NP) and a higher order mode of a gold nanorod (Au NR), which was compared to that of coupling among TiN NPs' dipolar modes. We evaluated the photothermal conversion efficiency of dipole-dipole coupling and dipole-multipole coupling in the nanocluster under the illumination of a monochromatic laser of 808 nm wavelength and simulated solar light, respectively. Both experimental tests and numerical simulations suggested that the plasmonic dipole-multipole coupling exhibited higher enhancement in photothermal conversion than dipole-dipole plasmonic coupling.

Mechanistic insights into C-C coupling in electrochemical CO reduction using gold superlattices

Developing in situ/operando spectroscopic techniques with high sensitivity and reproducibility is of great importance for mechanistic investigations of surface-mediated electrochemical reactions. Herein, we report the fabrication of highly ordered rhombic gold nanocube superlattices (GNSs) as substrates for surface-enhanced infrared absorption spectroscopy (SEIRAS) with significantly enhanced SEIRA effect, which can be controlled by manipulating the randomness of GNSs. Finite difference time domain simulations reveal that the electromagnetic effect accounts for the significantly improved spectroscopic vibrations on the GNSs. In situ SEIRAS results show that the vibrations of CO on the Cu2O surfaces have been enhanced by 2.4 ± 0.5 and 18.0 ± 1.3 times using GNSs as substrates compared to those on traditional chemically deposited gold films in acidic and neutral electrolytes, respectively. Combined with isotopic labeling experiments, the reaction mechanisms for C-C coupling of CO electroreduction on Cu-based catalysts are revealed using the GNSs substrates.

A super asymmetric cross antenna structure with tunable dual-frequency resonances

The detection performance of traditional infrared spectroscopy can be very limited in the case of molecular vibrational modes with low absorption cross-sections. On account of its electric field enhancement, plasmonic antenna can be combined with infrared spectroscopy to realize surface enhanced infrared detection and characterization of molecules. In this work, a super asymmetric cross antenna structure with tunable dual-frequency resonance and a high enhancement factor is designed. By systematically studying the transmission spectrum and charge distribution of this super asymmetric cross antenna structure, the physical origin of the dual-frequency resonance and its tunability are characterized in detail. In addition, in order to target desired molecular ensembles, the relationship between the resonance frequency and electric-field intensity of the two resonance modes and the parameters of structure and incident light are examined, yielding an enhancement factor close to 100 in the desired frequency region. Finally, the experimental results show that the proposed super asymmetric cross antenna structure can indeed generate dual-frequency resonances, agreeing reasonably with the theoretical results. It is believed that the super asymmetric cross antenna structure can be widely used to sensitively detect trace molecules, and in monolayered chemistry and bio-molecules, allowing their structures and dynamics to be studied using nonlinear infrared spectroscopy.

Hierarchically Doped Plasmonic Nanocrystal Metamaterials

Assembling plasmonic nanocrystals in regular superlattices can produce effective optical properties not found in homogeneous materials. However, the range of these metamaterial properties is limited when a single nanocrystal composition is selected for the constituent meta-atoms. Here, we show how continuously varying doping at two length scales, the atomic and nanocrystal scales, enables tuning of both the frequency and bandwidth of the collective plasmon resonance in nanocrystal-based metasurfaces, while these features are inextricably linked in single-component superlattices. Varying the mixing ratio of indium tin oxide nanocrystals with different dopant concentrations, we use large-scale simulations to predict the emergence of a broad infrared spectral region with near-zero permittivity. Experimentally, tunable reflectance and absorption bands are observed, owing to in- and out-of-plane collective resonances. These spectral features and the predicted strong near-field enhancement establish this multiscale doping strategy as a powerful new approach to designing metamaterials for optical applications.

Metasurface-Enhanced Infrared Spectroscopy: An Abundance of Materials and Functionalities

Infrared spectroscopy provides unique information on the composition and dynamics of biochemical systems by resolving the characteristic absorption fingerprints of their constituent molecules. Based on this inherent chemical specificity and the capability for label-free, noninvasive, and real-time detection, infrared spectroscopy approaches have unlocked a plethora of breakthrough applications for fields ranging from environmental monitoring and defense to chemical analysis and medical diagnostics. Nanophotonics has played a crucial role for pushing the sensitivity limits of traditional far-field spectroscopy by using resonant nanostructures to focus the incident light into nanoscale hot-spots of the electromagnetic field, greatly enhancing light-matter interaction. Metasurfaces composed of regular arrangements of such resonators further increase the design space for tailoring this nanoscale light control both spectrally and spatially, which has established them as an invaluable toolkit for surface-enhanced spectroscopy. Starting from the fundamental concepts of metasurface-enhanced infrared spectroscopy, a broad palette of resonator geometries, materials, and arrangements for realizing highly sensitive metadevices is showcased, with a special focus on emerging systems such as phononic and 2D van der Waals materials, and integration with waveguides for lab-on-a-chip devices. Furthermore, advanced sensor functionalities of metasurface-based infrared spectroscopy, including multiresonance, tunability, dielectrophoresis, live cell sensing, and machine-learning-aided analysis are highlighted.

Optical Modeling of Plasmonic Nanoparticles with Electronically Depleted Layers

Doped metal oxide (MO) nanocrystals (NCs) are well-known for the localized surface plasmon resonance in the infrared range generated by free electrons in the conduction band of the material. Owing to the intimate connection between plasmonic features and the NC's carrier density profile, proper modeling can unveil the underlying electronic structure. The carrier density profile in MO NCs is characterized by the presence of an electronically depleted layer as a result of the Fermi level pinning at the surface of the NC. Moreover, the carrier profile can be spatially engineered by tuning the dopant concentrations in core-shell architectures, generating a rich plethora of plasmonic features. In this work, we systematically studied the influence of the simulation parameters used for optical modeling of representative experimental absorption spectra by implementing multilayer models. We highlight in particular the importance of minimizing the fit parameters by support of experimental results and the importance of interparameter relationships. We show that, in all cases investigated, the depletion layer is fundamental to correctly describe the continuous spectra evolution. We foresee that this multilayer model can be used to design the optoelectronic properties of core-shell systems in the framework of energy band and depletion layer engineering.

Molecular Plasmonics with Metamaterials

Molecular plasmonics, the area which deals with the interactions between surface plasmons and molecules, has received enormous interest in fundamental research and found numerous technological applications. Plasmonic metamaterials, which offer rich opportunities to control the light intensity, field polarization, and local density of electromagnetic states on subwavelength scales, provide a versatile platform to enhance and tune light-molecule interactions. A variety of applications, including spontaneous emission enhancement, optical modulation, optical sensing, and photoactuated nanochemistry, have been reported by exploiting molecular interactions with plasmonic metamaterials. In this paper, we provide a comprehensive overview of the developments of molecular plasmonics with metamaterials. After a brief introduction to the optical properties of plasmonic metamaterials and relevant fabrication approaches, we discuss light-molecule interactions in plasmonic metamaterials in both weak and strong coupling regimes. We then highlight the exploitation of molecules in metamaterials for applications ranging from emission control and optical modulation to optical sensing. The role of hot carriers generated in metamaterials for nanochemistry is also discussed. Perspectives on the future development of molecular plasmonics with metamaterials conclude the review. The use of molecules in combination with designer metamaterials provides a rich playground both to actively control metamaterials using molecular interactions and, in turn, to use metamaterials to control molecular processes.

Elevating Surface-Enhanced Infrared Absorption with Quantum Mechanical Effects of Plasmonic Nanocavities

Plasmonic nanocavities, with the ability to localize and concentrate light into nanometer-scale dimensions, have been widely used for ultrasensitive spectroscopy, biosensing, and photodetection. However, as the nanocavity gap approaches the subnanometer length scale, plasmonic enhancement, together with plasmonic enhanced optical processes, turns to quenching because of quantum mechanical effects. Here, instead of quenching, we show that quantum mechanical effects of plasmonic nanocavities can elevate surface-enhanced infrared absorption (SEIRA) of molecular moieties. The plasmonic nanocavities, nanojunctions of gold and cadmium oxide nanoparticles, support prominent mid-infrared plasmonic resonances and enable SEIRA of an alkanethiol monolayer (CH₃(CH₂)_n-1SH, n = 3-16). With a subnanometer cavity gap (n < 6), plasmonic resonances turn to blue shift and the SEIRA signal starts a pronounced increase, benefiting from the quantum tunneling effect across the plasmonic nanocavities. Our findings demonstrate the new possibility of optimizing the field enhancement and SEIRA sensitivity of mid-infrared plasmonic nanocavities.

Properties of Free Charge Carriers Govern Exciton Polarization in Plasmonic Semiconductor Nanocrystals

Interaction between a plasmon, as a collective property of charge carriers, and electronic or spin states in complex nanostructures has emerged as one of the fascinating topics that intertwines the fields of photonics, optoelectronics, and spintronics. Here, we investigate the magneto-optical properties of plasmonic InN and Cu_2-xSe nanocrystals and show that the complete exciton polarization induced by cyclotron motion of free carriers is a universal phenomenon in semiconductor nanocrystals. The selective exciton polarization is governed by the angular momentum transfer from the carriers following cyclotron orbits to the excited electronic band states and can be controlled by carrier type (electrons or holes), mass, and velocity. The results of this work demonstrate the free-carrier-induced control of the states around the Fermi level and the exciton polarization in technologically important III-V nanocrystals, allowing for new ways of tailoring quantum states for spintronic and optoelectronic applications.

Selective broadband absorption by mode splitting for radiative cooling

A plasmonic-photonic structure based on colloidal lithography was designed for a scalable radiative cooling system and its absorption properties were theoretically investigated. The structure comprises a SiO₂ core, which is on top of an Au reflector and partially covered by an indium tin oxide (ITO) shell. This simple and scalable structure possesses a strong selective absorption in the primary atmospheric transparency window (8-13 µm). The strong selective absorption is attributed to a mode splitting of the localized surface plasmon (LSP) of the ITO shell. To understand the mechanisms of the mode splitting, a quantitative analysis was conducted using a coupled-oscillator model and a coupled-dipole method. The analysis revealed that the mode splitting is induced by a strong coupling between the LSP of the ITO shell and a magnetic dipole Mie resonance of the SiO₂ core.

Control of electronic band profiles through depletion layer engineering in core-shell nanocrystals

Fermi level pinning in doped metal oxide (MO) nanocrystals (NCs) results in the formation of depletion layers, which affect their optical and electronic properties, and ultimately their application in smart optoelectronics, photocatalysis, or energy storage. For a precise control over functionality, it is important to understand and control their electronic bands at the nanoscale. Here, we show that depletion layer engineering allows designing the energetic band profiles and predicting the optoelectronic properties of MO NCs. This is achieved by shell thickness tuning of core-shell Sn:In₂O₃-In₂O₃ NCs, resulting in multiple band bending and multi-modal plasmonic response. We identify the modification of the band profiles after the light-induced accumulation of extra electrons as the main mechanism of photodoping and enhance the charge storage capability up to hundreds of electrons per NC through depletion layer engineering. Our experimental results are supported by theoretical models and are transferable to other core-multishell systems as well.

Recent Progress in Near-Infrared Organic Electroluminescent Materials

Near-infrared (NIR) refers to the section of the spectrum from 650 to 2500 nm. NIR luminescent materials are widely employed in organic light-emitting diodes (OLEDs), fiber optic communication, sensing, biological detection, and medical imaging. This paper reviews organic NIR electroluminescent materials, including organic NIR electrofluorescent materials and organic NIR electrophosphorescent materials that have been investigated in the past 6 years. Small-molecule, polymer NIR fluorescent materials and platinum(II) and iridium(III) complex NIR phosphorescent materials are described, and the limitations of the development of NIR luminescent materials and future prospects are discussed.

Active spoof plasmonics: from design to applications

Spoof plasmonic metamaterials enable the transmission of electromagnetic energies with strong field confinement, opening new pathways to the miniaturization of devices for modern communications. The design of active, reconfigurable, and nonlinear devices for the efficient generation and guidance, dynamic modulation, and accurate detection of spoof surface plasmonic signals has become one of the major research directions in the field of spoof plasmonic metamaterials. In this article, we review recent progress in the studies on spoof surface plasmons with a special focus on the active spoof surface plasmonic devices and systems. Different design schemes are introduced, and the related applications including reconfigurable filters, high-resolution sensors for chemical and biological sensing, graphene-based attenuators, programmable and multi-functional devices, nonlinear devices, splitters, leaky-wave antennas and multi-scheme digital modulators are discussed. The presence of active SSPPs based on different design schemes makes it possible to dynamically control electromagnetic waves in real time. The promising future of active spoof plasmonic metamaterials in the communication systems is also speculated.

Fingerprint detection in the mid-infrared region based on guided-mode resonance and phonon-polariton coupling of analyte

Mid-infrared absorption spectroscopy is an effective method for detecting analyte fingerprints without labeling, but the inherent loss of metals in current methods is a main issue. Here, a sensing scheme was proposed that uses an all-dielectric grating metasurface and angular scanning of polarized light, and then it was verified by numerical simulation. The proposed fingerprint detection scheme could effectively couple a guided-mode resonance spectrum peak with the characteristic peak of the analyte's phonon-polariton in the mid-infrared region, significantly enhancing the interaction between light and the analyte. The novel scheme would realize broadband enhancement to detect a variety of substances, and facilitate mid-infrared sensing and analysis of trace substances.

Monitoring the effects of chemical stimuli on live cells with metasurface-enhanced infrared reflection spectroscopy

Infrared spectroscopy has found wide applications in the analysis of biological materials. A more recent development is the use of engineered nanostructures - plasmonic metasurfaces - as substrates for metasurface-enhanced infrared reflection spectroscopy (MEIRS). Here, we demonstrate that strong field enhancement from plasmonic metasurfaces enables the use of MEIRS as a highly informative analytic technique for real-time monitoring of cells. By exposing live cells cultured on a plasmonic metasurface to chemical compounds, we show that MEIRS can be used as a label-free phenotypic assay for detecting multiple cellular responses to external stimuli: changes in cell morphology, adhesion, and lipid composition of the cellular membrane, as well as intracellular signaling. Using a focal plane array detection system, we show that MEIRS also enables spectro-chemical imaging at the single-cell level. The described metasurface-based all-optical sensor opens the way to a scalable, high-throughput spectroscopic assay for live cells.

Infrared Plasmonic Biosensor with Tetrahedral DNA Nanostructure as Carriers for Label-Free and Ultrasensitive Detection of miR-155

MicroRNAs play an important role in early development, cell proliferation, apoptosis, and cell death, and are aberrantly expressed in many types of cancers. To understand their function and diagnose cancer at an early stage, it is crucial to quantitatively detect microRNA without invasive labels. Here, a plasmonic biosensor based on surface-enhanced infrared absorption (SEIRA) for rapid, label-free, and ultrasensitive detection of miR-155 is reported. This technology leverages metamaterial perfect absorbers stimulating the SEIRA effect to provide up to 1000-fold near-field intensity enhancement over the microRNA fingerprint spectral bands. Additionally, it is discovered that the limit of detection (LOD) of the biosensor can be greatly improved by using tetrahedral DNA nanostructure (TDN) as carriers. By using near-field enhancement of SEIRA and specific binding of TDN, the biosensor achieves label-free detection of miR-155 with a high sensitivity of 1.162% pm^-1 and an excellent LOD of 100 × 10^-15 m. The LOD is about 5000 times lower than that using DNA single strand as probes and about 100 times lower than that of the fluorescence detection method. This work can not only provide a powerful diagnosis tool for the microRNAs detection but also gain new insights into the field of label-free and ultrasensitive SEIRA-based biosensing.

Surface Plasmon Resonance from Gallium-Doped Zinc Oxide Nanoparticles and Their Electromagnetic Enhancement Contribution to Surface-Enhanced Raman Scattering

In recent years, semiconductor-based surface-enhanced Raman scattering (SERS) substrates have received considerable attention and led to a forefront in the SERS field. However, the lack of electromagnetic (EM) enhancement contributions highly precludes the development of semiconductive-substrate-based SERS. In this study, Ga-doped ZnO nanoparticles (NPs) were fabricated and employed as novel SERS substrates based on the EM enhancement contribution of surface plasmon resonance (SPR). The obtained Ga-doped ZnO NPs exhibited obvious SPR absorptions in the visible and near- and mid-infrared regions. SPR absorption can be readily tuned by changing the doping ratios of Ga³⁺ ions. The SERS spectra of Ga-doped ZnO/4-mercaptopyridine (MPy) were investigated at different excitation wavelengths of 488, 532, 633, and 785 nm. The spectral enhancement of Ga-doped ZnO substrates depended on the doping ratios, excitation wavelengths, and nearby SPR absorption. Ga-doped ZnO NPs with the highest free charge carrier density and the doping ratio of 5% showed the strongest SERS spectra. For the fixed doping ratio of 5%, the better is the match between excitation wavelengths and SPR absorption, the higher is the SERS spectral enhancement. This study showed the feasibility of EM contributions to SERS by using semiconductive substrates and can contribute to the development of the semiconductor-based EM mechanism.

Measuring Molecular Diffusion Through Thin Polymer Films with Dual-Band Plasmonic Antennas

A general and quantitative method to characterize molecular transport in polymers with good temporal and high spatial resolution, in complex environments, is an important need of the pharmaceutical, textile, and food and beverage packaging industries, and of general interest to the polymer science community. Here we show how the amplified infrared (IR) absorbance sensitivity provided by plasmonic nanoantenna-based surface enhanced infrared absorption (SEIRA) provides such a method. SEIRA enhances infrared (IR) absorbances primarily within 50 nm of the nanoantennas, enabling localized quantitative detection of even trace quantities of analytes and diffusion measurements in even thin polymer films. Relative to a commercial attenuated total internal reflection (ATR) system, the limit of detection is enhanced at least 13-fold, and as is important for measuring diffusion, the detection volume is about 15 times thinner. Via this approach, the diffusion coefficient and solubility of specific molecules, including l-ascorbic acid (vitamin C), ethanol, various sugars, and water, in both simple and complex mixtures (e.g., beer and a cola soda), were determined in poly(methyl methacrylate), high density polyethylene (HDPE)-based, and polypropylene-based polyolefin films as thin as 250 nm.

Probing subwavelength in-plane anisotropy with antenna-assisted infrared nano-spectroscopy

Infrared nano-spectroscopy based on scattering-type scanning near-field optical microscopy (s-SNOM) is commonly employed to probe the vibrational fingerprints of materials at the nanometer length scale. However, due to the elongated and axisymmetric tip shank, s-SNOM is less sensitive to the in-plane sample anisotropy in general. In this article, we report an easy-to-implement method to probe the in-plane dielectric responses of materials with the assistance of a metallic disk micro-antenna. As a proof-of-concept demonstration, we investigate here the in-plane phonon responses of two prototypical samples, i.e. in (100) sapphire and x-cut lithium niobate (LiNbO₃). In particular, the sapphire in-plane vibrations between 350 cm^-1 to 800 cm^-1 that correspond to LO phonon modes along the crystal b- and c-axis are determined with a spatial resolution of < λ/10, without needing any fitting parameters. In LiNbO₃, we identify the in-plane orientation of its optical axis via the phonon modes, demonstrating that our method can be applied without prior knowledge of the crystal orientation. Our method can be elegantly adapted to retrieve the in-plane anisotropic response of a broad range of materials, i.e. subwavelength microcrystals, van-der-Waals materials, or topological insulators.

Synthesis and defect engineering of molybdenum oxides and their SERS applications

Surface-enhanced Raman scattering (SERS) spectroscopy has been developed into a cross-disciplinary analytical technology through exploring various materials' Raman vibrational modes with ultra-high sensitivity and specificity. Although conventional noble-metal based SERS substrates have achieved great success, oxide-semiconductor-based SERS substrates are attracting researchers' intensive interest due to their merits of facile fabrication, high uniformity and tunable SERS characteristics. Among all the SERS active oxide semiconductors, molybdenum oxides (MoO_x) possess exceptional advantages of high Raman enhancement factor, environmental stability, recyclable detection, etc. More interestingly, the SERS effect of the MoO_x SERS substrates may involve both the electromagnetic enhancement mechanism and the chemical enhancement mechanism, which is determined by the stoichiometry and morphology of the material. Therefore, the focus of this review will be on two critical points: (1) synthesis and material engineering methods of the functional MoO_x material and (2) MoO_x SERS mechanism and performance evaluation. First, we review recent works on the MoO_x preparation and material property tuning approaches. Second, the SERS mechanism and performance of various MoO_x substrates are surveyed. In particular, the performance uniformity, enhancement factor and recyclability are evaluated. In the end, we discuss several challenges and open questions related to further promoting the MoO_x as the SERS substrate for monitoring extremely low trace molecules and the theory for better understanding of the SERS enhancement mechanism.

Electron Beam Infrared Nano-Ellipsometry of Individual Indium Tin Oxide Nanocrystals

Leveraging recent advances in electron energy monochromation and aberration correction, we record the spatially resolved infrared plasmon spectrum of individual tin-doped indium oxide nanocrystals using electron energy-loss spectroscopy (EELS). Both surface and bulk plasmon responses are measured as a function of tin doping concentration from 1-10 atomic percent. These results are compared to theoretical models, which elucidate the spectral detuning of the same surface plasmon resonance feature when measured from aloof and penetrating probe geometries. We additionally demonstrate a unique approach to retrieving the fundamental dielectric parameters of individual semiconductor nanocrystals via EELS. This method, devoid from ensemble averaging, illustrates the potential for electron-beam ellipsometry measurements on materials that cannot be prepared in bulk form or as thin films.

Gold nanonails for surface-enhanced infrared absorption

Surface-enhanced infrared absorption (SEIRA) can dramatically enhance the vibrational signals of analyte molecules owing to the interaction between plasmons and molecular vibrations. It has huge potential for applications in various detection and diagnostic fields. High-aspect-ratio rod-like metal nanostructures have been the most widely studied nanomaterials for SEIRA. However, nearly all of the rod-like nanostructures reported previously are fabricated using physical methods. They suffer from damping and low areal number densities. In this work, high-aspect-ratio Au nanorods are synthesized, and Au nanonails are prepared through Au overgrowth on the as-prepared Au nanorods. The aspect ratios of the Au nanorods and nanonails can be varied in the range of ∼10 to ∼60, and their longitudinal dipolar plasmon resonance wavelengths can be correspondingly tailored from ∼1.6 to ∼8.3 μm. The Au nanonails exhibit superior SEIRA performance with 4-aminothiophenol used as the probe molecules. They are further used to detect the common biomolecule l-cysteine. Numerical simulations are further performed to understand the experimental results. They match well with the experimental observations, revealing the mechanism of the SEIRA enhancement. Our study demonstrates that colloidal high-aspect-ratio Au nanonails and nanorods can function as SEIRA nanoantennas for highly sensitive molecular detection in various situations.

Supercritical CO2 synthesis of Co-doped MoO3-x nanocrystals for multifunctional light utilization

Here, we demonstrate, for the first time, that Co-MoO_3-x nanocrystals (NCs) have been synthesized with the assistance of supercritical CO₂. Their unique structural features of transition-metal doping and high oxygen vacancy concentrations, lead to synchronous outstanding surface enhanced Raman scattering (SERS) detection and photothermal conversion performances.

Embedded Metal Oxide Plasmonics Using Local Plasma Oxidation of AZO for Planar Metasurfaces

New methods for achieving high-quality conducting oxide metasurfaces are of great importance for a range of emerging applications from infrared thermal control coatings to epsilon-near-zero nonlinear optics. This work demonstrates the viability of plasma patterning as a technique to selectively and locally modulate the carrier density in planar Al-doped ZnO (AZO) metasurfaces without any associated topographical surface profile. This technique stands in strong contrast to conventional physical patterning which results in nonplanar textured surfaces. The approach can open up a new route to form novel photonic devices with planar metasurfaces, for example, antireflective coatings and multi-layer devices. To demonstrate the performance of the carrier-modulated AZO metasurfaces, two types of devices are realized using the demonstrated plasma patterning. A metasurface optical solar reflector is shown to produce infrared emissivity equivalent to a conventional etched design. Second, a multiband metasurface is achieved by integrating a Au visible-range metasurface on top of the planar AZO infrared metasurface. Independent control of spectral bands without significant cross-talk between infrared and visible functionalities is achieved. Local carrier tuning of conducting oxide films offers a conceptually new approach for oxide-based photonics and nanoelectronics and opens up new routes for integrated planar metasurfaces in optical technology.

Spectral emissivity design using aluminum-based hybrid gratings

We propose a strategy to design infrared emitters with predefined spectral response using aluminum gratings as building blocks. We begin by identifying 3 target spectra with resonances in the 7-15 µm wavelength range. Next, we use FDTD simulations and interpolation to create a reference library of gratings relating their structural parameters to attributes of their infrared spectra. By using a search algorithm based on minimization of errors in spectral attributes, we identify gratings from this library corresponding to peaks in the target spectra. Finally, we discuss an approach for designing hybrid structures from these gratings to generate each of the 3 target spectra. This strategy can be extended to design structures with complex spectral responses.

The IR plasmonic properties of sub-wavelength ITO rod arrays predicted by anisotropic effective medium theory

Simple three-layer Fresnel equations combined with Maxwell-Garnett approximation were applied to study the IR plasmonic properties of indium-tin-oxide (ITO) nanorods. By treating the anisotropic nanorod layer as a layer with an effective dielectric constant, and using anisotropic effective medium theory, we were able to accurately predict the surface plasmon resonance behavior of ITO nanorods with different nanorod length, spacing, and tilt angle. This model allows a fast and computationally inexpensive calculation to predict the plasmonic properties of arrayed nanorods.

Interfacial Defect-Mediated Near-Infrared Silicon Photodetection with Metal Oxides

Due to recent breakthroughs in silicon photonics, sub-band-gap photodetection in silicon (Si) has become vital to the development of next-generation integrated photonic devices for telecommunication systems. In particular, photodetection in Si using complementary metal-oxide semiconductor (CMOS) compatible materials is in high demand for cost-effective integration. Here, we achieve broad-band near-infrared photodetection in Si/metal-oxide Schottky junctions where the photocurrent is generated from interface defects induced by aluminum-doped zinc oxide (AZO) films deposited on a Si substrate. The combination of photoexcited carrier generation from both interface defect states and intrinsic Si bulk defect states contributes to a photoresponse of 1 mA/W at 1325 nm and 0.22 mA/W at 1550 nm with zero-biasing. From a fit to the Fowler equation for photoemission, we quantitatively determine the individual contributions from these effects. Finally, using this analysis, we demonstrate a gold-nanoparticle-coated photodiode that has three distinct photocurrent responses resulting from hot carriers in the gold, interface defects from the AZO, and bulk defects within the Si. The hot carrier response is found to dominate near the band gap of Si, while the interface defects dominate for longer wavelengths.

Quantitative Analysis of Extinction Coefficients of Tin-Doped Indium Oxide Nanocrystal Ensembles

The optical extinction coefficients of localized surface plasmon resonance (LSPR) in doped semiconductor nanocrystals (NCs) have intensities determined by the density and damping mechanisms of free charge carriers. We investigate the dependence of the extinction coefficient of tin-doped indium oxide (ITO) NCs on size and dopant concentration and find extinction coefficients as high as 56.6 μm^-1 in the near-infrared for 20 nm diameter ITO NCs with 7.5 atomic% Sn. We find ITO NCs to be more efficient infrared light absorbers than metal nanoparticles or molecular dyes. We also find the intensive, volume-normalized extinction coefficient increases significantly with NC doping and NC diameter, but only up to the point of saturation in both cases. We qualitatively analyze trends in LSPR peak position and width to explain the effect of doping and size on extinction.

Faceting-Controlled Zeeman Splitting in Plasmonic TiO2 Nanocrystals

Dynamic manipulation of discrete states in nanostructured materials is critical for emerging quantum technologies. However, this process often requires a correlation of mutually competing degrees of freedom. Here we report the control of magnetic-field-induced excitonic splitting in colloidal TiO₂ nanocrystals by control of their faceting. By changing nanocrystal morphology via reaction conditions, we control the concentration and location of oxygen vacancies, which can generate localized surface plasmon resonance and foster the reduction of lattice cations leading to the emergence of individual or exchange-coupled Ti(III) centers with high net-spin states. These species can all couple with the nanocrystal lattice under different conditions resulting in distinctly patterned excitonic Zeeman splitting and selective control of conduction band states in an external magnetic field. This work demonstrates the concept of using nanocrystal morphology to control carrier polarization in individual nanocrystals using both intrinsic and collective electronic properties, representing a unique approach to multifunctionality in reduced dimensions.

Metasurface-Based Molecular Biosensing Aided by Artificial Intelligence

Molecular spectroscopy provides unique information on the internal structure of biological materials by detecting the characteristic vibrational signatures of their constituent chemical bonds at infrared frequencies. Nanophotonic antennas and metasurfaces have driven this concept towards few-molecule sensitivity by confining incident light into intense hot spots of the electromagnetic fields, providing strongly enhanced light-matter interaction. In this Minireview, recently developed molecular biosensing approaches based on the combination of dielectric metasurfaces and imaging detection are highlighted in comparison to traditional plasmonic geometries, and the unique potential of artificial intelligence techniques for nanophotonic sensor design and data analysis is emphasized. Because of their spectrometer-less operation principle, such imaging-based approaches hold great promise for miniaturized biosensors in practical point-of-care or field-deployable applications.

Indium⁻Tin⁻Oxide Nanostructures for Plasmon-Enhanced Infrared Spectroscopy: A Numerical Study

Plasmonic nanoantennas can significantly enhance the light–matter interactions at the nanoscale, and as a result have been used in a variety of applications such as sensing molecular vibrations in the infrared range. Indium–tin–oxide (ITO) shows metallic behavior in the infrared range, and can be used for alternative plasmonic materials. In this work, we numerically studied the optical properties of hexagonal ITO nanodisk and nanohole arrays in the mid-infrared. Field enhancement up to 10 times is observed in the simulated ITO nanostructures. Furthermore, we demonstrated the sensing of the surface phonon polariton from a 2-nm thick SiO₂ layer under the ITO disk arrays. Such periodic arrays can be readily fabricated by colloidal lithography and dry etching techniques; thus, the results shown here can help design efficient ITO nanostructures for plasmonic infrared applications.

Tuning infrared plasmon resonances in doped metal-oxide nanocrystals through cation-exchange reactions

Metal-oxide nanocrystals doped with aliovalent atoms can exhibit tunable infrared localized surface plasmon resonances (LSPRs). Yet, the range of dopant types and concentrations remains limited for many metal-oxide hosts, largely because of the difficulty in establishing reaction kinetics that favors dopant incorporation by using the co-thermolysis method. Here we develop cation-exchange reactions to introduce p-type dopants (Cu⁺, Ag⁺, etc.) into n-type metal-oxide nanocrystals, producing programmable LSPR redshifts due to dopant compensation. We further demonstrate that enhanced n-type doping can be realized via sequential cation-exchange reactions mediated by the Cu⁺ ions. Cation-exchange transformations add a new dimension to the design of plasmonic nanocrystals, allowing preformed nanocrystals to be used as templates to create compositionally diverse nanocrystals with well-defined LSPR characteristics. The ability to tailor the doping profile postsynthetically opens the door to a multitude of opportunities to deepen our understanding of the relationship between local structure and LSPR properties.

Controlling the Mechanism of Excitonic Splitting in In2O3 Nanocrystals by Carrier Delocalization

Degenerately doped metal oxide nanocrystals have emerged as infrared plasmonic materials with promising applications in optoelectronics, surface-enhanced infrared spectroscopies, and sensing. They also have potential for technological applications in electronics and photonics owing to the possibility of coupling between plasmon and exciton in the absence of a heterojunction. Here, we demonstrate the control of excitonic splitting in In₂O₃ nanocrystals upon excitation with circularly polarized light in an external magnetic field by simultaneous control of the electronic structure of donor defects and the nanocrystal host lattice. Using variable-temperature-variable-field magnetic circular dichroism spectroscopy, we show that the nanocrystal band splitting has two distinct contributions in plasmonic In₂O₃ nanocrystals. Temperature-independent splitting arises from the cyclotron magnetoplasmonic modes, which impart angular momentum to the conduction band excited states near the Fermi level, and increases with the intensity of the corresponding plasmon resonance. Temperature-dependent splitting is associated with the localized electron spins trapped in defect states. The ratio of the two components can be controlled by the formation of oxygen vacancies or introduction of aliovalent dopants. Using these experimental results in conjunction with the density functional theory modeling, relative contribution of the two mechanisms is discussed in the context of the perturbation theory taking into account energy separation between the nanocrystal excited states and the localized defect states. The results of this work demonstrate the ability to control carrier polarization in nonmagnetic metal oxide nanocrystals using both individual and collective electronic properties and allow for their application as an emerging class of multifunctional materials with strongly interacting degrees of freedom.

Impacts of surface depletion on the plasmonic properties of doped semiconductor nanocrystals

Degenerately doped semiconductor nanocrystals (NCs) exhibit a localized surface plasmon resonance (LSPR) in the infrared range of the electromagnetic spectrum. Unlike metals, semiconductor NCs offer tunable LSPR characteristics enabled by doping, or via electrochemical or photochemical charging. Tuning plasmonic properties through carrier density modulation suggests potential applications in smart optoelectronics, catalysis and sensing. Here, we elucidate fundamental aspects of LSPR modulation through dynamic carrier density tuning in Sn-doped In₂O₃ (Sn:In₂O₃) NCs. Monodisperse Sn:In₂O₃ NCs with various doping levels and sizes were synthesized and assembled in uniform films. NC films were then charged in an in situ electrochemical cell and the LSPR modulation spectra were monitored. Based on spectral shifts and intensity modulation of the LSPR, combined with optical modelling, it was found that often-neglected semiconductor properties, specifically band structure modification due to doping and surface states, strongly affect LSPR modulation. Fermi level pinning by surface defect states creates a surface depletion layer that alters the LSPR properties; it determines the extent of LSPR frequency modulation, diminishes the expected near-field enhancement, and strongly reduces sensitivity of the LSPR to the surroundings.

Radiative Enhancement of Linear and Third-Order Vibrational Excitations by an Array of Infrared Plasmonic Antennas

Infrared gold antennas localize enhanced near fields close to the metal surface, when excited at the frequency of their plasmon resonance, and amplify vibrational signals from the nearby molecules. We study the dependence of the signal enhancement on the thickness of a polymer film containing vibrational chromophores, deposited on the antenna array, using linear (FTIR) and third-order femtosecond vibrational spectroscopy (transient absorption and 2DIR). Our results show that for a film thickness beyond only a few nanometers the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano line shape, suggesting a radiative origin of the molecule-plasmon interaction. The mutual radiative damping of plasmonic and molecular transitions leads to the spectroscopic signal of a molecular vibrational excitation to be enhanced by up to a factor of 50 in the case of linear spectroscopy and over 2000 in the case of third-order spectroscopy. A qualitative explanation for the observed effect is given by the extended coupled oscillators model, which takes into account both near-field and radiative interactions between the plasmonic and molecular transitions.

Perfect-absorption graphene metamaterials for surface-enhanced molecular fingerprint spectroscopy

Graphene plasmon with extremely strong light confinement and tunable resonance frequency represents a promising surface-enhanced infrared absorption (SEIRA) sensing platform. However, plasmonic absorption is relatively weak (approximately 1%-9%) in monolayer graphene nanostructures, which would limit its sensitivity. Here, we theoretically propose a hybrid plasmon-metamaterial structure that can realize perfect absorption in graphene with a low carrier mobility of 1000 cm² V^-1 s^-1. This structure combines a gold reflector and a gold grating to the graphene plasmon structures, which introduce interference effect and the lightning-rod effect, respectively, and largely enhance the coupling of light to graphene. The vibration signal of trace molecules can be enhanced up to 2000-fold at the hotspot of the perfect-absorption structure, enabling the SEIRA sensing to reach the molecular level. This hybrid metal-graphene structure provides a novel path to generate high sensitivity in nanoscale molecular recognition for numerous applications.

Nanomaterial-Based Plasmon-Enhanced Infrared Spectroscopy

Surface-enhanced infrared absorption (SEIRA) has attracted increasing attention due to the potential of infrared spectroscopy in applications such as molecular trace sensing of solids, polymers, and proteins, specifically fueled by recent substantial developments in infrared plasmonic materials and engineered nanostructures. Here, the significant progress achieved in the past decades is reviewed, along with the current state of the art of SEIRA. In particular, the plasmonic properties of a variety of nanomaterials are discussed (e.g., metals, semiconductors, and graphene) along with their use in the design of efficient SEIRA configurations. To conclude, perspectives on potential applications, including single-molecule detection and in vivo bioassays, are presented.

Localized Surface Plasmon Coupling between Mid-IR-Resonant ITO Nanocrystals

Sn-doped indium oxide (ITO) nanocrystals (NC) provide tunable localized surface plasmon resonance in the mid-infrared. To evaluate the applicability of these n-doped plasmonic semiconductors in field-enhanced spectroscopies, it is necessary to assess how the low, free-electron density affects the E-field localization and plasmon coupling in NC films when compared to metal nanoparticles (NP). In this article, we investigate plasmon coupling between approximate 6 nm diameter ITO NC on the collective resonance and quantify the effect of the electromagnetic field enhancement on the absorbance signal of surface-attached ligands in NC films and monolayers with different ratios of doped and undoped indium oxide NC.

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control. In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs. Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials. We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures and crystal structures that predominate and quantum effects that emerge at nonclassical sizes. We will also emphasize the impact of dopant hybridization with bands of the host lattice as well as the interplay of shape and crystal structure in determining the LSPR characteristics of semiconductor NCs. To illustrate the discussion regarding both physical and synthetic aspects of LSPR-active NCs, we will focus on metal oxides with substantial consideration also of copper chalcogenide NCs, with select examples drawn from the literature on other doped semiconductor materials. Furthermore, we will discuss the promise that LSPR in doped semiconductor NCs holds for a wide range of applications such as infrared spectroscopy, energy-saving technologies like smart windows and waste heat management, biomedical applications including therapy and imaging, and optical applications like two photon upconversion, enhanced luminesence, and infrared metasurfaces.

Higher order Fano graphene metamaterials for nanoscale optical sensing

Plasmonic Fano metamaterials provide a unique platform for optical sensing applications due to their sharp spectral response and the ability to confine light to nanoscale regions that make them a strong prospect for refractive-index sensing. Higher order Fano resonance modes in noble metal plasmonic structures can further improve the sensitivity, but their applications are heavily limited by crosstalk between different modes due to the large damping rates and broadband spectral responses of the metal plasmon modes. Here, we create pure higher order Fano modes by designing asymmetric metamaterials comprised of a split-ring resonator and disk with a low-loss graphene plasmon. These higher order modes are highly sensitive to the nanoscale analyte (8 nm thick) both in refractive-index and in infrared vibrational fingerprint sensing, as demonstrated by the numerical calculation. The frequency sensitivity and figure-of-merit of the hexacontatetrapolar mode can reach 289 cm^-1 per RIU and 29, respectively, and it can probe the weak infrared vibrational modes of the analyte with more than 400 times enhancement. The enhanced sensitivity and tunability of higher order Fano graphene metamaterials promise a high-performance nanoscale optical sensor.

Cavity-based aluminum nanohole arrays with tunable infrared resonances

This work details the successful computational design, fabrication, and characterization of a cavity-based aluminum nanohole array. The designs incorporate arrays of aluminum nanoholes that are patterned on a dielectric-coated (SiO₂ or ZnSe) aluminum base mirror plane. This architecture provided a means of exploring the coupling of the localized resonances, exhibited by the aluminum nanohole array, with the cavity resonance that is generated within the dielectric spacer layer, which resides between the base plane mirror and the nanohole array. Rigorous coupled wave analysis (RCWA) was first used to computationally design the structures. Next, a range of lithographic techniques, including photolithography, E-beam lithography, and nanosphere lithography, were used to fabricate the structures. Finally, infrared spectroscopy and scanning electron microscopy (SEM) were used to characterize the spectral and structural properties of the multilayered devices, respectively. The overall goal of this study was to demonstrate our ability to design and fabricate aluminum-based structures with tunable resonances throughout the infrared region, i.e. from the short-wave through longwave infrared regions of the electromagnetic spectrum (1.5 -12 µm).

Highly improved, non-localized field enhancement enabled by hybrid plasmon of crescent resonator/graphene in infrared wavelength

The development of surface enhanced infrared absorption has been constrained by the limited field enhancement and narrow-band resonance of commonly used metal resonators. In this theoretical work, the design of a crescent resonator (CR) combined with graphene-enabled plasmon tuning is proposed to settle the drawbacks. The CR is similar to a split ring resonator (SRR), but exhibits a much improved field enhancement. The influence of graphene on the field enhancement of the CR has been systematically investigated. Coupling from localized plasmon of CR to propagating plasmon of graphene has been observed, and the constructive interference of the plasmon wave has led to not only better enhancement inside the gap but also usable enhancements all over the graphene film, which go beyond the localized nature of metal plasmons.

Nanogapped Au Antennas for Ultrasensitive Surface-Enhanced Infrared Absorption Spectroscopy

Surface-enhanced infrared absorption (SEIRA) spectroscopy has outstanding potential in chemical detection as a complement to surface-enhanced Raman spectroscopy (SERS), yet it has historically lagged well behind SERS in detection sensitivity. Here we report a new ultrasensitive infrared antenna designed to bring SEIRA spectroscopy into the few-molecule detection range. Our antenna consists of a bowtie-shaped Au structure with a sub-3 nm gap, positioned to create a cavity above a reflective substrate. This three-dimensional geometry tightly confines incident mid-infrared radiation into its ultrasmall junction, yielding a hot spot with a theoretical SEIRA enhancement factor of more than 10⁷, which can be designed to span the range of frequencies useful for SEIRA. We quantitatively evaluated the IR detection limit of this antenna design using mixed monolayers of 4-nitrothiophenol (4-NTP) and 4-methoxythiolphenol (4-MTP). The optimized antenna structure allows the detection of as few as ∼500 molecules of 4-NTP and ∼600 molecules of 4-MTP with a standard commercial FTIR spectrometer. This strategy offers a new platform for analyzing the IR vibrations of minute quantities of molecules and lends insight into the ultimate limit of single-molecule SEIRA detection.