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Real-time, high-sensitivity, and label-free detection to single nanoparticles has been achieved via visualizing the interaction between surface plasmon polaritons (SPPs) and nanoparticles, which is widely applied to chemistry and biology. In this work, aiming to enhance the detection sensitivity to nanoparticles, we explore the interaction of SPP standing waves with single nanoparticles. Compared with SPPs, the inhomogeneous fields of SPP standing waves modulate charge distributions around the particle and excite different electric dipole modes that tailor localized enhancements. For nanoparticles situating at electric antinodes of SPP standing waves, a vertical electric dipole is excited and high-density charges are stimulated around nanoparticle-film nanocavities, leading to further increased localized enhancement. The localized enhancement experiences more increase with smaller particle size, lower dielectric constant of surrounding medium, and lower particle refractive index. Via tailoring the localized enhancement by SPP standing waves, the sensitivity of SPP microscopy can be improved, which would broaden its applications on nanotechnology, biomedicine, and environmental monitoring.

By eliminating the photodamage and photobleaching induced by high intensity laser and fluorescent molecular, the label-free laser scanning microscopy shows powerful capability for imaging and dynamic tracing to biological tissues and cells. In this review, three types of label-free laser scanning microscopies: laser scanning coherent Raman scattering microscopy, second harmonic generation microscopy and scanning localized surface plasmon microscopy are discussed with their fundamentals, features and recent progress. The applications of label-free biological imaging of these laser scanning microscopies are also introduced. Finally, the performance of the microscopies is compared and the limitation and perspectives are summarized.

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.

We presented a method to manipulate the surface plasmon propagation by using the total reflection of single hollow nanoparticle. We used plasmonic imaging to observe the manipulation by single hollow nanoparticle in different surrounding medium.

The synergistic combination of electric double-layer capacitance (EDLC) and pseudocapacitance is one of the most effective approaches to realize high-performance supercapacitor energy storage. Herein high specific energy and power supercapacitor is realized through the surface-dominant pseudocapacitive charge storage. To demonstrate that, a hybrid 1T-MoS2/Ti3C2Tx porous aerogel is rationally fabricated via bidirectional freeze-casting. The conductive Ti3C2Tx electrically connects the 1T-MoS2 nanosheets, which significantly improves the electron transfer and ion transport, leading to the surface-dominant (up to 86.9%) pseudocapacitive energy storage. As a result, the hybrid aerogel exhibits an outstanding capacitance of 392 F g−1 at 5 mV s−1, greatly higher than the conventional 1T-MoS2/Ti3C2Tx film and 1T-MoS2 film. Meanwhile, the Ti3C2Tx-connected 1T-MoS2 architecture achieves high capacitance retention (∼ 38%) at 1000 mV s−1, which is about 2.9 and 4.7 times higher than that of conventional 1T-MoS2/Ti3C2Tx film and 1T-MoS2 film, respectively. Moreover, the asymmetric supercapacitor delivers both high specific energy (45.3 Wh kg–1 at 924 W kg–1) and specific power (76.4 kW kg–1 at 18.9 Wh kg–1), among the best records of supercapacitors. This work opens new opportunities to develop next-generation high specific energy and power supercapacitors via synergistic effects of EDLC and pseudocapacitance.

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.

We study the physical mechanism of surface plasmon polariton in-plane scattering with single nanoparticle, which is vital to understand the electromagnetic interaction with nanoparticle. Also, the polarized bright spot is linear with the nanoparticle's size.

We measured terahertz (THz) radiation from GaAs (1 0 0) surface coated with rough Au film in the thickness ranging from 5 to 21 nm under the incident angle from 0° to 50°. Anomalous THz emission was observed with inhomogeneous crack structures at normal incidence, which originates dominantly from the lateral photo-Dember current. Meanwhile, enhanced THz radiation from Au/GaAs was investigated with the variation of the Au morphology, which confirmed that localized surface plasmons play an important role in the THz radiation. The results indicate the prospect of harnessing surface plasmons for efficient THz emission with controllable morphology of Au on semiconductors.

Terahertz (THz) emission from octadecanthiol (ODT) passivated (1 0 0) surface of the semi-insulating GaAs was measured, and compared with those from the native oxidized and the fresh surfaces. It was shown that the self-assembled ODT monolayer can stabilize the GaAs (1 0 0) surface, and maintain a THz surface emission 1.4 times as efficient as the native oxidized surface under equal conditions. Surface passivation can reduce the built-in electric field in the depletion region of the GaAs (1 0 0), resulting in the suppression of the THz radiation to a different extent. Oxidation of GaAs surface reduces the THz amplitude mainly in the low-frequency region. These results indicate that GaAs can be made a more effective THz source by choosing molecular passivation technique. Conversely, the THz emission features such as polarity, amplitude, and phase from molecule-passivated surfaces may be used to characterize the attached molecules.

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.

A review of localized plasmonic properties of 2-D terahertz (THz) metallic geometries is presented. The high electric conductivity of metals in the THz regime enables fascinating plasmonic behaviors observed in metallic structures of subwavelength dimensions. Strong electric field enhancement was demonstrated in arrays of various subwavelength metallic particles including patches, rings, and ring-patch coaxials. We also show that periodic arrays of integrated coaxials comprised of patches and subwavelength holes not only resulted in a factor-of-eight increase in normalized transmittance compared to that of the hole-only counterpart, but also tailored polarization dependent transmission discrepancy encountered in arrays of rectangular holes. The localized plasmonic properties and the resulted electric field enhancement will find fascinating applications in THz sensing, spectroscopy, and integrated components.

Firstly employing coupled oscillator model to quantitatively analyze the Terahertz enhanced transmission through sub-wavelength hole arrays, we clarify the surface plasmons resonance is excited by the localized resonance via coupling instead of the external E-field.

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.

The fascinating properties of plasmonic structures have had significant impact on the development of next generation ultracompact photonic and optoelectronic components. We study two-dimensional plasmonic structures functioning at terahertz frequencies. Resonant terahertz response due to surface plasmons and dipole localized surface plasmons were investigated by the state-of-the-art terahertz time domain spectroscopy (THz-TDS) using both transmission and reflection configurations. Extraordinary terahertz transmission was demonstrated through the subwavelength metallic hole arrays made from good conducting metals as well as poor metals. Metallic arrays made from Pb, generally a poor metal, and having optically thin thicknesses less than one-third of a skin depth also contributed in enhanced THz transmission. A direct transition of a surface plasmon resonance from a photonic crystal minimum was observed in a photo-doped semiconductor array. Electrical controls of the surface plasmon resonances by hybridization of the Schottky diode between the metallic grating and the semiconductor substrate are investigated as a function of the applied reverse bias. In addition, we have demonstrated photo-induced creation and annihilation of surface plasmons with appropriate semiconductors at room temperature. According to the Fano model, the transmission properties are characterized by two essential contributions: resonant excitation of surface plasmons and nonresonant direct transmission. Such plasmonic structures may find fascinating applications in terahertz imaging, biomedical sensing, subwavelength terahertz spectroscopy, tunable filters, and integrated terahertz devices.

We studied a metasurface constituted of a periodic array of semiconductor split-ring resonators. The resonance frequencies were found to be continuously tunable in the terahertz regime through an external magnetostatic field or temperature control.

We demonstrate ultrafast optical control of surface plasmon assisted terahertz (THz) transmission in a subwavelength metallic hole array on a semiconductor substrate. The transient photoconductivity of the substrate allows modulation of the THz resonance amplitude via optical excitation with a time scale of ∼10 ps.

We experimentally demonstrate a chiral metamaterial exhibiting negative refractive index at terahertz frequencies. The presence of strong chirality in the terahertz metamaterial lifts the degeneracy for the two circularly polarized waves and allows for the achievement of negative refractive index without requiring simultaneously negative permittivity and negative permeability. The realization of terahertz chiral negative index metamaterials offers opportunities for investigation of their novel electromagnetic properties, such as negative refraction and negative reflection, as well as important terahertz device applications.

We have investigated the broadband terahertz (THz) optical properties of nanoporous silicon samples with different porosities and the ultrafast carrier dynamics of photogenerated charge carriers in these materials. Following photoexcitation, we observe a fast carrier recovery time consisting of two dominant recombination processes with decay constants below ~10 ps. All samples exhibit initially low THz absorption that increases at higher frequencies, and is likely due to contributions from phonon bands and oxidation of the porous surface. The refractive index depends on porosity but shows little frequency dependence. These properties indicate that nanoporous silicon is a useful material for fast, ultrabroadband THz applications (e.g. intensity modulation).

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.

We report the coupling between surface plasmons and dipole-localized surface plasmons in a composite hole-patch structure at terahertz frequencies. The coupling is found to be changed by increasing the inner patch size, which causes prominent resonance frequency shift in the enhanced transmission. The experimental results show good agreement with numerical simulation. The study clarifies well the nature of coupling between surface plasmons and dipole-localized surface plasmons and is thus of help to identify the role of surface plasmons in the enhanced transmission observed in subwavelength metallic structures.

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.

We demonstrate optical control of surface plasmon enhanced resonant terahertz transmission in two-dimensional subwavelength metallic hole arrays fabricated on gallium arsenide based substrates. Optically pumping the arrays creates a conductive layer in the substrate, reducing the terahertz transmission amplitude of both the resonant mode and the direct transmission. Under low optical fluence, the terahertz transmission is more greatly affected by resonance damping than by propagation loss in the substrate. An ErAs:GaAs nanoisland superlattice substrate is shown to allow ultrafast control with a switching recovery time of ∼10 ps.

We study terahertz properties of periodic arrays of subwavelength metallic ring and coaxial ring-disk structures. Terahertz time-domain spectroscopy and numerical simulations have revealed unique resonance behaviors at terahertz frequencies. A high-frequency antisymmetric resonance in the rings concurs simultaneously with the symmetric mode. In the coaxial structures, the strong and weak resonance interactions are characterized by the plasmon hybridization theory, while the anomalous interaction results in a degenerate resonance which has not yet been observed at visible frequencies. The frequency tunability and strong electric field enhancement in such subwavelength structures are promising in terahertz sensing, spectroscopy, and integrated components.

We present reflection properties of terahertz pulses in periodic arrays of metallic rectangles of subwavelength dimensions. Resonant reflection is characterized by terahertz time-domain spectroscopy and is attributed to the contributions from the dipole localized surface plasmons of isolated metallic rectangles, their interactions between the rectangles, and the nonresonant direct reflection. A number of factors, including the polarization of the terahertz electric field, the shape of the metal structures, the dielectric function of the substrate, and the lattice constant of the arrays are found to influence terahertz reflection properties. The measured resonant reflectance is well fit by numerical simulations.