Reconstruction Filters Improving the Spatial Resolution and Signal-to-Noise Ratio of Surface Plasmon Resonance Microscopy.

Benefitting from high sensitivity, real-time, and label-free imaging, surface plasmon resonance microscopy (SPRM) has become a powerful tool for dynamic detection of nanoparticles. However, the evanescent propagation of surface plasmon polaritons (SPPs) induces interference between scattered and launched SPPs, which deteriorates the spatial resolution and signal-to-noise ratio (SNR). Due to the simplicity and fast processing, image reconstruction based on deconvolution has shown the feasibility of improving the spatial resolution of SPRM imaging. Retrieving the particle scattering from SPRM interference imaging by filters is crucial for reconstruction. In this work, we illustrate the effect of filters extracting SPP scattering of nanoparticles with different sizes and shapes for reconstruction. The results indicate that the optimum filters are determined by the material of nanoparticles instead of particle sizes. The reconstruction of single Au and PS nanospheres as well as Ag nanowires with optimum filters is achieved. The reconstructed spatial resolution is improved to 254 nm, and the SNR is increased by 8.1 times. Our research improves the quality of SPRM imaging and provides a reliable method for fast detection of particles with diverse sizes and shapes.

Directional surface plasmon polariton scattering using single magnetic nanoparticles.

Directional surface plasmon polaritons (SPPs) are expected to promote the energy efficiency of plasmonic devices, via limiting the energy in a given spatial domain. The directional scattering of dielectric nanoparticles induced by the interference between electric and magnetic responses presents a potential candidate for directional SPPs. Magnetic nanoparticles can introduce permeability as an extra manipulation, whose directional scattered SPPs have not been investigated yet. In this work, we demonstrated the directional scattered SPPs by using single magnetic nanoparticles via simulation and experiment. By increasing the permeability and particle size, the high-order TEM modes are excited inside the particle and induce more forward directional SPPs. It indicated that the particle size manifests larger tuning range compared with the permeability. Experimentally, the maximum forward-to-backward (F-to-B) SPP scattering intensity ratio of 118.52:1 is visualized by using a single 1 µm Fe3O4 magnetic nanoparticle. The directional scattered SPPs of magnetic nanoparticles are hopeful to improve the efficiency of plasmonic devices and pave the way for plasmonic circuits on-chip.

Curved and Annular Diaphragm Coupled Piezoelectric Micromachined Ultrasonic Transducers for High Transmit Biomedical Applications.

In this paper, we present a novel three-dimensional (3D) coupled configuration of piezoelectric micromachined ultrasound transducers (pMUTs) by combing a curved and an annular diaphragm for transmit performance optimization in biomedical applications. An analytical equivalent circuit model (EQC) is developed with varied excitation methods to incorporate the acoustic-structure coupling of the curved and annular diaphragm-coupled pMUTs (CAC-pMUTs). The model-derived results align well with the reference simulated by the finite element method (FEM). Using this EQC model, we optimize the key design parameters of the CAC-pMUTs in order to improve the output sound pressure, including the width of the annular membrane, the thickness of the passive layer, and the phase difference of the driving voltage. In the anti-phase mode, the designed CAC-pMUTs demonstrate a transmit efficiency 285 times higher than that of single annular pMUTs. This substantial improvement underscores the potential of CAC-pMUTs for large array applications.

Effects of nanoparticle sizes, shapes, and permittivity on plasmonic imaging.

Plasmonic imaging has exhibited superiority in label-free and fast detection to single nanoparticles due to its high sensitivity and high temporal resolution, which plays an important role in environmental monitoring and biomedical research. As containing plenty of information associated with particle features, plasmonic imaging has been used for identifying the particle sizes, shapes, and permittivity. Yet, the effects of the nanoparticle features on plasmonic imaging are not investigated, which hinders the in-depth understanding to plasmonic imaging and its applications in particle identification. In this work, we analyzed five types of nanoparticles, including polystyrene (PS), Au, silicon nanospheres as well as PS and Ag nanowires. We illustrated the effects of nanoparticle sizes, shapes, and permittivity on spatial resolution, imaging contrast, and interference fringes. We found that nanoparticle sizes and permittivity influenced the imaging contrast. Via introducing size parameter relevant to interference fringes, the connection between particle shape and reduction rate of size parameter is built, and the effects of particle shapes on the interference patterns are revealed. Our research provides a basis for improving the plasmonic imaging and presents guidance for applications on particle identification in nano-detection, biosensor, and environmental monitoring.

Locally excited surface plasmon resonance for refractive index sensing with high sensitivity and high resolution.

An angle-interrogated surface plasmon resonance (SPR) sensor based on a prism-coupled configuration has been extensively applied in biomedicine, environment monitoring, and food safety. Yet, the low sensitivity and low spatial resolution impede its further development. In this Letter, we investigated objective-coupled locally excited SPR for refractive index (RI) sensing with high sensitivity and high resolution. Through theoretical analysis, the SPR angle was retrieved from back focal plane imaging, which was highly correlated to the RI of the surrounding medium. Experimentally, a RI sensitivity of 77.41° refractive index unit (RIU)-1 was achieved with a detection range of 0.068 RIU when using glucose solutions for the demonstration. Furthermore, we acquired the spatial resolution of the configuration being 290 nm, and the RI measurement to a polydimethylsiloxane droplet with high spatial resolution was implemented. As a result, compared with the classical prism-coupled configuration, the locally excited SPR provides a method to achieve RI sensing with high sensitivity and high resolution.

Detecting the morphology of single graphene sheets by dual channel sampling plasmonic imaging.

Due to their excellent physical and chemical properties, graphene sheets are widely used in industry, which makes detection important to guarantee their performance. Atomic force microscopy, scanning electron microscopy, and Raman spectroscopy are the most common detection methods, which is either time-consuming or easily destructive. In this work, we presented a fast and nondestructive method to detect single graphene sheets by using plasmonic imaging. Dual channel sampling plasmonic imaging combining the image processing algorithm is used to improve the deterioration from propagation length of surface plasmon polaritons and reconstruct the complete morphology of single graphene sheets. The fast and nondestructive detection method paves the way to applications of graphene, and can be extended to the detections of two-dimensional materials, single biological molecule, viruses, and nanomaterials.

Detecting a single nanoparticle by imaging the localized enhancement and interference of surface plasmon polaritons: erratum.

In this erratum, the function ${\lambda _{{\rm SPP}}}$λSPP in the third page of Opt. Lett.44, 5707 (2019)OPLEDP0146-959210.1364/OL.44.005707 has been corrected.

Detecting a single nanoparticle by imaging the localized enhancement and interference of surface plasmon polaritons.

Label-free single-nanoparticle detection is crucial for the fast detection of nanoparticles and viruses in environmental monitoring and biological sciences. In this Letter, benefiting from the leakage radiation that transforms the near-field surface plasmon polariton (SPP) distribution along the interface to the far field, we demonstrated the plasmonic imaging of single polystyrene nanoparticles with a particle size down to 39 nm. The imaging is composed of the localized enhancement and interference of SPPs. The localized enhancement is the result of the accumulation of charges around the nanoparticle, and it is connected to the size and refractive index of nanoparticles. The interference is induced by the coupling between the incident SPPs and the scattered SPPs, verified by extracting the interference fringe periodicity to be half of the SPP wavelength. Our study provides an in-depth physical understanding of plasmonic imaging of single nanoparticles, which paves the way for a fast identification of nanomaterials.

Nanophotonic Enhanced Chiral Sensing and Its Biomedical Applications.

Chiral sensing is crucial in the fields of biology and the pharmaceutical industry. Many naturally occurring biomolecules, i.e., amino acids, sugars, and nucleotides, are inherently chiral. Their enantiomers are strongly associated with the pharmacological effects of chiral drugs. Owing to the extremely weak chiral light-matter interactions, chiral sensing at an optical frequency is challenging, especially when trace amounts of molecules are involved. The nanophotonic platform allows for a stronger interaction between the chiral molecules and light to enhance chiral sensing. Here, we review the recent progress in nanophotonic-enhanced chiral sensing, with a focus on the superchiral near-field and enhanced circular dichroism (CD) spectroscopy generated in both the dielectric and in plasmonic structures. In addition, the recent applications of chiral sensing in biomedical fields are discussed, including the detection and treatment of difficult diseases, i.e., Alzheimer's disease, diabetes, and cancer.

Sensing self-assembled alkanethiols by differential transmission interrogation with terahertz metamaterials.

Surface-enhanced electromagnetic response in the resonant regions of split-ring resonators offers a sensitive way to probe the surface dipoles formed by alkanethiol molecules with a terahertz wave by a differential transmission (DT) interrogation method. The DT signal mainly comes from the interaction between alkanethiols and metamaterials by electron transfer and/or the variation of the dielectric constant. The Lorentz model is used to demonstrate the principle of DT interrogation theoretically, which suggests the variation of both frequency and damping of resonance can be captured cooperatively. This method has been employed to experimentally demonstrate the sensing feasibility for the chain length dependence of the alkanethiol molecules. Numerical simulations confirm that the enhancement is large at the gap and corner regions of this kind of metamaterials.

Far-infrared properties of hybrid plasmonic geometries.

Plasmonic structures made of periodically arranged metallic rings integrated into subwavelength holes are investigated at the far-infrared terahertz frequencies. The emergence and the interplay of various resonances sustained by such plasmonic samples are elucidated. To reveal a coherent physical picture, relevant dimensions of the samples are modified and their impact on the resonance properties is analyzed. The experimental work is fully supported by numerical simulations. The understanding of the interplay of various resonances will foster applications which require plasmonic substrates to exhibit simultaneously resonances at well-defined frequencies and line widths.

Large dynamic resonance transition between surface plasmon and localized surface plasmon modes.

We present resonant terahertz transmission in a composite plasmonic film comprised of an array of subwavelength metallic patches and semiconductor holes. A large dynamic transition between a dipolar localized surface plasmon mode and a surface plasmon resonance near 0.8 THz is observed under near infrared optical excitation. The reversal in transmission amplitude from a stop-band to a pass-band and up to pi/2 phase shift achieved in the composite plasmonic film make it promising in large dynamic phase modulation, optical changeover switching, and active terahertz plasmonics.

Transmission field enhancement of terahertz pulses in plasmonic, rectangular coaxial geometries.

We report on anomalous field enhancement of terahertz transmission in metallic, rectangular coaxial geometries. The integration of a particle in the hole not only results in a factor of eight increase in normalized transmittance compared to that of the hole-only counterpart, but it also tailors polarization-dependent transmission discrepancy encountered in arrays of rectangular holes. Our experimental and numerical studies on the impact of geometrical dimensions of the integrated particles indicate that dipolar localized surface plasmons of the particles contribute substantially to the terahertz field enhancement through coupling with surface plasmons and localized surface plasmons of the holes.

Label-Free Imaging of Single Nanoparticles Using Total Internal Reflection-Based Leakage Radiation Microscopy.

Label-free, fast, and single nanoparticle detection is demanded for the in situ monitoring of nano-pollutants in the environment, which have potential toxic effects on human health. We present the label-free imaging of single nanoparticles by using total internal reflection (TIR)-based leakage radiation microscopy. We illustrate the imaging of both single polystyrene (PS) and Au nanospheres with diameters as low as 100 and 30 nm, respectively. As both far-field imaging and simulated near-field electric field intensity distribution at the interface showed the same characteristics, i.e., the localized enhancement and interference of TIR evanescent waves, we confirmed the leakage radiation, transforming the near-field distribution to far-field for fast imaging. The localized enhancement of single PS and Au nanospheres were compared. We also illustrate the TIR-based leakage radiation imaging of single polystyrene nanospheres with different incident polarizations. The TIR-based leakage radiation microscopy method is a competitive alternative for the fast, in situ, label-free imaging of nano-pollutants.

Broadband resonant terahertz transmission in a composite metal-dielectric structure.

We present a systematic numerical study of a metal-dielectric-metal sandwich plasmonic structure for broadband resonant transmission at terahertz frequencies. The proposed structure consists of periodic slotted metallic arrays on both sides of a thin dielectric substrate and is demonstrated to exhibit a broad passband transmission response. Various design considerations have been investigated to exploit their influence on the transmission passband width and the center resonance frequency. The structure ensures a broadband transmission over a wide range of incident angles.

A close-ring pair terahertz metamaterial resonating at normal incidence.

We present a systematic study of a close-ring pair based freestanding metamaterial fabricated by double-layer, self-aligned photolithography. Terahertz time-domain spectroscopy transmission measurements and numerical simulations have revealed negative index of refraction in the frequency range of 0.66-0.90 THz under normal wave incidence. The observed resonance behaviors can be well explained by a theoretical circuit model. The electromagnetic properties and the figure of merit of such close-ring metamaterials are also explored in terms of geometrical parameters of the unit cell with a goal of providing optimized design for three-dimensional metamaterials and potential device applications.

A robust indium-porphyrin framework for CO2 capture and chemical transformation.

An indium based metal-porphyrinic framework, denoted NUPF-3, was prepared based on a new amido-decorated porphyrin ligand. NUPF-3 possesses a rarely seen 4-fold interpenetrated pts framework with segmented pores and dense metalloporphyrin central sites. The structure can retain its crystallinity in commonly used solvents, as well as acidic/alkaline solutions with pH ranging from 1 to 12 for 48 h, exhibiting high chemical stability. Meanwhile, thermal analysis reveals that NUPF-3 possesses relatively high thermal stability. Owing to the presence of amido groups, structural interpenetration and a charged framework, NUPF-3 exhibits relatively high CO2 uptake. Moreover, NUPF-3 could be used as a good heterogeneous catalyst for cycloaddition of CO2 and epoxides, under relatively mild conditions, with good recyclability.

Spoof surface plasmon polaritons in terahertz transmission through subwavelength hole arrays analyzed by coupled oscillator model.

Both the localized resonance and excitation of spoof surface plasmon polaritons are observed in the terahertz transmission spectra of periodic subwavelength hole arrays. Analyzing with the coupled oscillator model, we find that the terahertz transmission is actually facilitated by three successive processes: the incident terahertz field first initiates the localized oscillation around each hole, and then the spoof surface plasmon polaritons are excited by the localized resonance, and finally the two resonances couple and contribute to the transmission. Tailoring the localized resonance by hole size, the coupling strength between spoof surface plasmon polaritons and localized resonances is quantitatively extracted. The hole size dependent transmittance and the coupling mechanism are further confirmed by fitting the measured spectra to a modified multi-order Fano model.

Alkanethiol-functionalized terahertz metamaterial as label-free, highly-sensitive and specific biosensor.

Specific biorecognition is essential for many biological processes, for which highly sensitive and label-free biosensors are strongly demanded. The recently developed metamaterials are a potential choice for biosensing due to their exotic properties. In the current work, a label-free and specific sensor for streptavidin-agarose (SA) was fabricated based on terahertz metamaterial functionlized by octadecanthiols and biotins. Both low and high frequency resonant modes from the metamaterials are found applicable for the detection of SA, and a redshift up to 6.76 GHz for the high frequency mode was measured in the undiluted commercial solution. The low frequency mode is attributed to inductor-capacitor (LC) oscillation, while the high frequency mode originates from the plasmonic dipole oscillator, both of which are highly sensitive to the micro-environment change. Adsorption of SA of different concentrations causes different redshifts, and the replacement of high refractive-index substrate with low refractive-index substrate can efficiently promote the sensitivity, well agreeing with the numerical simulation. Moreover, for a particular biomolecule, the sensitivity can be further improved by optimizing the metamaterial design. This method might be very helpful for desirable biorecognition in biology, medicine, and drug industry.

Magnetic and magnetothermal tunabilities of subwavelength-hole arrays in a semiconductor sheet.

In the low-terahertz regime, the resonance frequency of an array of subwavelength holes in a semiconductor sheet can be doubled or more by isothermally increasing the magnitude of a dc magnetic field, by increasing the temperature in the presence of a constant dc magnetic field, and by increasing both the temperature and the dc magnetic field magnitude.