Giant Photoluminescence Enhancement of Monolayer WSe2 Using a Plasmonic Nanocavity with On-Demand Resonance.

Monolayer transition metal dichalcogenides (TMDs) are considered promising building blocks for next-generation photonic and optoelectronic devices, owing to their fascinating optical properties. However, their inherent weak light absorption and low quantum yield severely hinder their practical applications. Here, we report up to 18000-fold photoluminescence (PL) enhancement in a monolayer WSe2-coupled plasmonic nanocavity. A spectroscopy-assisted nanomanipulation technique enables the assembly of a nanocavity with customizable resonances to simultaneously enhance the excitation and emission processes. In particular, precise control over the magnetic cavity mode facilitates spectral and spatial overlap with the exciton, resulting in plasmon-exciton intermediate coupling that approaches the maximum emission rate in the hybrid system. Meanwhile, the cavity mode exhibits high radiation directivity, which overwhelmingly directs surface-normal PL emission and leads to a 17-fold increase in the collection efficiency. Our approach opens up a new avenue to enhance the PL intensity of monolayer TMDs, facilitating their implementation in highly efficient optoelectronic devices.

Precisely constructing hybrid nanogap arrays via wet-transfer of dielectric metasurfaces onto a plasmonic mirror.

We propose a new method for fabricating hybrid metasurfaces by combining Mie and plasmonic resonances. Our approach involves obtaining an ultrasmooth gold film and separately structuring monocrystalline silicon (c-Si) nanoantenna arrays, which are then wet-transferred and finally immobilized onto the gold film. The experimental and simulation analysis reveals the importance of the native oxide layer of Si and demonstrates fascinating dispersion curves with nanogap resonances and bound states in the continuum. The localized field enhancements in the nanogap cavities result from the coupling between multipolar Mie resonances and their mirror images in the gold film. This effective method improves our understanding of hybrid modes and offers opportunities for developing active metasurfaces, such as depositing c-Si nanoantenna arrays onto stretchable polydimethylsiloxane substrates or electro-optic and piezoelectric sensitive lithium niobate films for potential applications in MEMS, LiDAR, and beyond.

Greatly Enhanced Raman Scattering of Graphene on Metals by a Boron Nitride Film Covering.

Raman spectroscopy, a nondestructive fingerprinting technique, is mainly utilized to identify molecular species and phonon modes of materials. However, direct Raman characterization of two-dimensional materials typically synthesized on catalytic metal substrates is extremely challenging because of the significant electric screening and interfacial electronic couplings. Here, we demonstrate that by covering as-grown graphene with boron nitride (BN) films, the Raman intensity of graphene can be enhanced by two orders of magnitude and is also several times stronger than that of suspended graphene. This great Raman enhancement originates from the optical field amplification by Fabry-Pérot cavity in BN films and the local field plasmon near copper steps. We further demonstrate the direct characterization of the local strain and doping level of as-grown graphene and in situ monitoring of the molecule reaction process by enhanced Raman spectroscopy. Our results will broaden the optical investigations of interfacial sciences on metals, including photoinduced charge transfer dynamics and photocatalysis at metal surfaces.

Selective triggering in-plane and out-of-plane dipolar modes of hexagonal Au nanoplate with the polarization of excitation beam.

The dipolar responses of a single hexagonal Au nanoplate are investigated under the illuminations of linearly polarized beam and tightly focused radially polarized beam (RPB). It is found from the scattering spectra that the in-plane and out-of-plane electric dipole modes can be selectively triggered with a linearly polarized beam and tightly focused RPB, respectively. The features of these two dipolar modes are further confirmed in terms of electrical field and charge maps by the finite-difference time-domain simulation. Additionally, using the multipole expansion method, the existence of the out-of-plane dipole mode is further verified by the fact that thez-component of electric dipole response has a dominant contribution to the scattered power. Moreover, by combining the back focal plane imaging technique with the simulation, the appearance of in-plane and out-of-plane dipoles in the scattering pattern are clearly discerned. Our results provide an efficient method for selectively exciting the in-plane and out-of-plane dipolar modes of the nanoplate. We envision that the ease of tuning the dipolar momentum may facilitate the enhancement of the interaction between the plasmon and emitters at single-particle level.

Toward high-performance refractive index sensor using single Au nanoplate-on-mirror nanocavity.

Refractive index sensors based on the localized surface plasmon resonance (LSPR) have emerged as powerful tools in various chemosensing and biosensing applications. However, owing to their limited decay length and strong radiation damping, LSPR sensors always suffer from low sensitivity and small figure of merit (FOM). Here, we fabricate a plasmonic nanocavity sensor consisting of a hexagonal Au nanoplate positioned over an ultrasmooth Au film. The strong coupling between the nanoplate and the lower metal film allows for the formation of a plasmonic gap mode that enhances the interaction of the local field with the ambient glycerol solution to increase the sensitivity. Meanwhile, the plasmonic gap mode has a trait of an antiphase charge oscillation in the gap region, imparting a strongly reduced radiative damping and a subsequently promoted FOM. The performance of our proposed refractive index sensor is further boosted by decreasing the gap size of the nanocavity, yielding an outstanding FOM of 11.2 RIU-1 that is the highest yet reported for LSPR sensing in a single nanostructure.

Exciting Magnetic Dipole Mode of Split-Ring Plasmonic Nano-Resonator by Photonic Crystal Nanocavity.

On-chip exciting electric modes in individual plasmonic nanostructures are realized widely; nevertheless, the excitation of their magnetic counterparts is seldom reported. Here, we propose a highly efficient on-chip excitation approach of the magnetic dipole mode of an individual split-ring resonator (SRR) by integrating it onto a photonic crystal nanocavity (PCNC). A high excitation efficiency of up to 58% is realized through the resonant coupling between the modes of the SRR and PCNC. A further fine adjustment of the excited magnetic dipole mode is demonstrated by tuning the relative position and twist angle between the SRR and PCNC. Finally, a structure with a photonic crystal waveguide side-coupled with the hybrid SRR-PCNC is illustrated, which could excite the magnetic dipole mode with an in-plane coupling geometry and potentially facilitate the future device application. Our result may open a way for developing chip-integrated photonic devices employing a magnetic field component in the optical field.

Nanometric displacement sensor with a switchable measuring range using a cylindrical vector beam excited silicon nanoantenna.

We demonstrate a nanometric displacement sensor with a switchable measuring range by using a single silicon nanoantenna. It is revealed that the interference between the longitudinal and transverse dipolar scattering can be well tuned by moving the nanoantenna in the focal field of the cylindrical vector beam. As a result, a position related scattering directivity is found and is used as a displacement sensor with a 4.5 nm lateral resolution. Interestingly, the measuring range of this displacement sensor can be extended by twice through simply changing the excitation from the azimuthally polarized beam to the radially polarized beam. Our results provide a facile way to tune the measuring range of the nanometric displacement sensor and may open up an avenue to super-resolution microscopy and optical nanometrology.

Complete structural characterization of single carbon nanotubes by Rayleigh scattering circular dichroism.

Non-invasive, high-throughput spectroscopic techniques can identify chiral indices (n,m) of carbon nanotubes down to the single-tube level1-6. Yet, for complete characterization and to unlock full functionality, the handedness, the structural property associated with mirror symmetry breaking, also needs to be identified accurately and efficiently7-14. So far, optical methods fail in the handedness characterization of single nanotubes because of the extremely weak chiroptical signals (roughly 10-7) compared with the excitation light15,16. Here we demonstrate the complete structure identification of single nanotubes in terms of both chiral indices and handedness by Rayleigh scattering circular dichroism. Our method is based on the background-free feature of Rayleigh scattering collected at an oblique angle, which enhances the nanotube's chiroptical signal by three to four orders of magnitude compared with conventional absorption circular dichroism. We measured a total of 30 single-walled carbon nanotubes including both semiconducting and metallic nanotubes and found that their absolute chiroptical signals show a distinct structure dependence, which can be qualitatively understood through tight-binding calculations. Our strategy enables the exploration of handedness-related functionality of single nanotubes and provides a facile platform for chiral discrimination and chiral device exploration at the level of individual nanomaterials.

Magnetic plasmon resonances in nanostructured topological insulators for strongly enhanced light-MoS2 interactions.

Magnetic resonances not only play crucial roles in artificial magnetic materials but also offer a promising way for light control and interaction with matter. Recently, magnetic resonance effects have attracted special attention in plasmonic systems for overcoming magnetic response saturation at high frequencies and realizing high-performance optical functionalities. As novel states of matter, topological insulators (TIs) present topologically protected conducting surfaces and insulating bulks in a broad optical range, providing new building blocks for plasmonics. However, until now, high-frequency (e.g. visible range) magnetic resonances and related applications have not been demonstrated in TI systems. Herein, we report for the first time, to our knowledge, a kind of visible range magnetic plasmon resonances (MPRs) in TI structures composed of nanofabricated Sb2Te3 nanogrooves. The experimental results show that the MPR response can be tailored by adjusting the nanogroove height, width, and pitch, which agrees well with the simulations and theoretical calculations. Moreover, we innovatively integrated monolayer MoS2 onto a TI nanostructure and observed strongly reinforced light-MoS2 interactions induced by a significant MPR-induced electric field enhancement, remarkable compared with TI-based electric plasmon resonances (EPRs). The MoS2 photoluminescence can be flexibly tuned by controlling the incident light polarization. These results enrich TI optical physics and applications in highly efficient optical functionalities as well as artificial magnetic materials at high frequencies.

Optical trapping of single nano-size particles using a plasmonic nanocavity.

Trapping and manipulating micro-size particles using optical tweezers has contributed to many breakthroughs in biology, materials science, and colloidal physics. However, it remains challenging to extend this technique to a few nanometers particles owing to the diffraction limit and the considerable Brownian motion of trapped nanoparticles. In this work, a nanometric optical tweezer is proposed by using a plasmonic nanocavity composed of the closely spaced silver coated fiber tip and gold film. It is found that the radial vector mode can produce a nano-sized near field with the electric-field intensity enhancement factor over 103 through exciting the plasmon gap mode in the nanocavity. By employing the Maxwell stress tensor formalism, we theoretically demonstrate that this nano-sized near field results in a sharp quasi-harmonic potential well, capable of stably trapping 2 nm quantum dots beneath the tip apex with the laser power as low as 3.7 mW. Further analysis reveals that our nanotweezers can stably work in a wide range of particle-to-tip distances, gap sizes, and operation wavelengths. We envision that our proposed nanometric optical tweezers could be compatible with the tip-enhanced Raman spectroscopy to allow simultaneously manipulating and characterizing single nanoparticles as well as nanoparticle interactions with high sensitivity.

Double-arrow metasurface for dual-band and dual-mode polarization conversion.

We present experimentally a double-arrow metasurface for high-efficiently manipulating the polarization states of electromagnetic waves in the dual-band. The metasurface is capable of converting a linearly polarized (LP) incident wave into a circularly polarized (CP) wave or its cross-polarized LP wave at different frequencies. It is numerically shown that in the two bands from 14.08 to 15.71 GHz and from 17.63 to 19.55 GHz the metasurface can convert the LP wave into CP wave, of which the axis ratio is lower than 3 dB. Meanwhile, the proposed metasurface also can convert the LP wave into its cross-polarized LP wave at 13.39 GHz and 20.29 GHz. To validate the theoretical analysis and simulated results, a prototype is fabricated and measured. The experimental results are reasonably consistent with the theoretical and simulated results, which demonstrates that such a metasurface can successfully achieve dual-band and dual-mode polarization conversion.

Structured light beams created through a multimode fiber via virtual Fourier filtering based on digital optical phase conjugation.

Digital optical phase conjugation (DOPC) is a newly developed technique in wavefront shaping to control light propagation through complex media. Currently, DOPC has been demonstrated for the reconstruction of two- and three-dimensional targets and enabled important applications in many areas. Nevertheless, the reconstruction results are only phase conjugated to the original input targets. Herein, we demonstrate that DOPC could be further developed for creating structured light beams through a multimode fiber (MMF). By applying annular filtering in the virtual Fourier domain of the acquired speckle field, we realize the creation of the quasi-Bessel and donut beams through the MMF. In principle, arbitrary amplitude and/or phase circular symmetry filtering could be performed in the Fourier domain, thus generating the corresponding point spread functions. We expect that the reported technique can be useful for super-resolution endoscopic imaging and optical manipulation through MMFs.

Reversible optical binding force in a plasmonic heterodimer under radially polarized beam illumination.

We investigated the optical binding force in a plasmonic heterodimer structure consisting of two nano-disks. It is found that when illuminated by a tightly focused radially polarized beam (RPB), the plasmon modes of the two nano-disks are strongly hybridized, forming bonding/antibonding modes. An interesting observation of this setup is that the direction of the optical binding force can be controlled by changing the wavelength of illumination, the location of the dimer, the diameter of the nano-disks, and the dimer gap size. Further analysis yields that the inhomogeneous polarization state of RPB can be utilized to readily control the bonding type of plasmon modes and distribute the underlying local field confined in the gap (the periphery) of the dimer, leading to a positive (negative) optical binding force. Our findings provide a clear strategy to engineer optical binding forces via changes in device geometry and its illumination profile. Thus, we envision a significant role for our device in emerging nanophotonics structures.

Characterizing localized surface plasmon resonances using focused radially polarized beam.

We demonstrate a scheme to characterize the localized surface plasmon resonances (LSPRs) of an individual metallic nanorod by employing a focused radially polarized beam (RPB) illumination under normal incidence. The focused RPB has a unique three-dimensional electric field polarization distribution in the focal plane, which can effectively and selectively excite the dipole and multipole plasmon resonances in a metallic nanorod by just moving the nanorod within the focal plane. This performance can be attributed to the mode matching between the excitation electric field of the incident RPB and the LSPRs in a metallic nanorod. Emphatically, in contrast to the commonly used oblique incidence illumination with the linearly polarized light, our proposed scheme is based on the normally incident light illumination and compatible with conventional optical microscopy, which is more scalable for spectroscopic characterization of individual nanostructures.

Strong Coupling between Dark Plasmon and Anapole Modes.

Plasmonic nanocavities enable extreme light-matter interaction by pushing light down to the nanoscale. The dipolar feature of bright modes allows coupling with the external excitation from free space but results in a radiating background, whereas nonradiating dark plasmon modes can hardly be excited. Here, we report for the first time on strong coupling between dark plasmon and anapole modes in a hybrid metal-dielectric nanostructure. With the aid of vanishing dipole characteristics of the anapole and dark plasmons, the hybrid modes exhibit minimum far-field scattering and maximum near-field enhancement. The dark mode coupling in the metal-dielectric nanostructure offers a nonradiating air cavity with greatly improved field enhancement in a broadened band, thus providing a background-free experimental platform for spectroscopic applications. The proposed approach to dark plasmon excitation, i.e., via anapole, may boost practical exploitation of dark plasmons by allowing linearly polarized light illumination and scalable arrays of individual nanostructure units.

Enhanced second-harmonic generation assisted by breathing mode in a multi-resonant plasmonic trimer.

Boosting the nonlinear conversion rate in nanoscale is pivotal for practical applications such as highly sensitive biosensors, extreme ultra-violate light sources, and frequency combs. Here, we theoretically study the enhancement of second-harmonic generation (SHG) in a plasmonic trimer assisted by breathing modes. The geometry of the trimer is fine-tuned to produce strong plasmonic resonances at both the fundamental and SH wavelengths to boost SHG intensity. Moreover, it is found that breathing modes show remarkable ability to augment SHG by increasing the enhancement area. In particular, these breathing modes ensure a substantial spatial mode overlap at the fundamental and SH wavelengths, resulting in further promotion of the SHG conversation rate. We envision that our findings could enable applications in nanoscale frequency converters with high efficiency.

Unidirectional scattering exploited transverse displacement sensor with tunable measuring range.

We propose a scheme to extend the measuring range of a transverse displacement sensor by exploiting the interaction of an azimuthally polarized beam (APB) with a single metal-dielectric core-shell nanoparticle. The focused APB illumination induces a longitudinal magnetic dipole (MD) in the core-shell nanoparticle, which interferes with the induced transverse electric dipole (ED) to bring forth a transverse unidirectional scattering at a specific position within the focal plane. Emphatically, the rapidly varying electromagnetic field within the focal plane of an APB leads to a remarkable sensitivity of the far-field scattering directivity to nanoscale displacements as the nanoparticle moves away from the optical axis. Moreover, the scattering directivity of the APB illuminated core-shell nanoparticle is also a function of structure-dependent Mie scattering coefficients, rendering the measuring range of the transverse displacement sensor widely tunable. The culmination of all these features enables the continuous tuning of the displacement measuring range from several nanometers to a few micrometers. Thus, we envision the proposed scheme is of high value for modern optical nanometrology.

Radial breathing modes coupling in plasmonic molecules.

Metallic hexamer, very much the plasmonic analog of benzene molecule, provides an ideal platform to mimic modes coupling and hybridization in molecular systems. To demonstrate this, we present a detailed study on radial breathing mode (RBM) coupling in a plasmonic dual-hexamers. We excite RBMs of hexamers by symmetrically matching the polarization state of the illumination with the distribution of electric dipole moments of the dual-hexamer. It is found that the RBM coupling exhibits a nonexponential decay when the inter-hexamer separation is increased, owing to the dark mode nature of RBM. When the outer hexamer is subjected to the in-plane twisting, resonant wavelengths of two coupled RBMs as well as the coupling constant show cosine variations with the twist angle, indicating the symmetry of hexamer structure plays a critical role in the coupling of RBMs. Moreover, it is demonstrated that the coupling of RBMs is dominated by the in-plane interaction as the outer hexamer is under an out-of-plane tilting, causing convergence of resonant wavelengths of the two coupled RBMs with increasing tilt angle. Our results not only provide an insight into the plasmonic RBM coupling mechanism, but also pave the way to systematically control the spectral response of plasmonic molecules.

Measurement of complex optical susceptibility for individual carbon nanotubes by elliptically polarized light excitation.

The complex optical susceptibility is the most fundamental parameter characterizing light-matter interactions and determining optical applications in any material. In one-dimensional (1D) materials, all conventional techniques to measure the complex susceptibility become invalid. Here we report a methodology to measure the complex optical susceptibility of individual 1D materials by an elliptical-polarization-based optical homodyne detection. This method is based on the accurate manipulation of interference between incident left- (right-) handed elliptically polarized light and the scattering light, which results in the opposite (same) contribution of the real and imaginary susceptibility in two sets of spectra. We successfully demonstrate its application in determining complex susceptibility of individual chirality-defined carbon nanotubes in a broad optical spectral range (1.6-2.7 eV) and under different environments (suspended and in device). This full characterization of the complex optical responses should accelerate applications of various 1D nanomaterials in future photonic, optoelectronic, photovoltaic, and bio-imaging devices.

Reconstruction of structured laser beams through a multimode fiber based on digital optical phase conjugation.

The digital optical phase conjugation (DOPC) technique is being actively developed for optical focusing and imaging through or inside complex media. Due to its time-reversal nature, DOPC has been exploited to regenerate different intensity targets. However, whether the targets with three-dimensional information through complex media could be recovered has not been experimentally demonstrated, to the best of our knowledge. Here, we present a method to regenerate structured laser beams based on DOPC. Although only the phase of the original scattered wave is time reversed, the reconstruction of a quasi-Bessel beam and vortex beams through a multimode fiber (MMF) is demonstrated. The regenerated quasi-Bessel beam shows the features of sub-diffraction focusing and a longer depth of field with respect to a Gaussian beam. Moreover, the reconstruction of vortex beams shows the fidelity of DOPC both in amplitude and phase, which is demonstrated for the first time, to the best of our knowledge. We also prove that the reconstruction results of DOPC through the MMF are indeed phase conjugate to the original targets. We expect that these results could be useful in super-resolution imaging and optical micromanipulation through complex media, and further pave the way for achieving three-dimensional imaging based on DOPC.