Ultrasensitive nanosensors based on localized surface plasmon resonances:From theory to applications

The subwavelength confinement feature of localized surface plasmon resonance（LSPR） allows plasmonic nanostructures to be functionalized as powerful platforms for detecting various molecular analytes as well as weak processes with nanoscale spatial resolution. One of the main goals of this field of research is to lower the absolute limit-of-detection（LOD）of LSPR-based sensors. This involves the improvement of（i） the figure-of-merit associated with structural parameters such as the size, shape and interparticle arrangement and,（ii） the spectral resolution. The latter involves advanced target identification and noise reduction techniques. By highlighting the strategies for improving the LOD, this review introduces the fundamental principles and recent progress of LSPR sensing based on different schemes including 1） refractometric sensing realized by observing target-induced refractive index changes, 2） plasmon rulers based on target-induced relative displacement of coupled plasmonic structures, 3） other relevant LSPR-based sensing schemes including chiral plasmonics,nanoparticle growth, and optomechanics. The ultimate LOD and the future trends of these LSPR-based sensing are also discussed.

纳腔量子等离激元及其应用

表面等离激元是一种由电磁波与固体中的自由电子集体振荡耦合形成的元激发,可以将光场束缚在金属与电介质界面亚波长尺度范围内,导致电磁场强度极大增强,这是等离激元光子学领域诸多物理效应和器件应用的基础.由两个或以上紧邻的金属纳米结构构成的纳米间隙是最典型的等离激元纳腔,具有最优异的光场束缚能力,可以将光场压缩到纳米尺度甚至是原子尺度.这种结构在亚纳米级的空间尺度上,隧穿、空间非局域等量子效应极其显著,导致纳腔等离激元具有许多新奇的量子特性,为新型光电器件的设计与制造提供了新的平台.尤其是最近10年来,纳腔量子等离激元在基础和应用方面都迎来了蓬勃的发展.本文首先简述了等离激元的量子化处理,回顾了亚纳米级间隙纳腔中的量子效应,重点介绍了近年在基于等离激元纳腔的量子隧穿发光、光与物质相互作用、量子光源以及精密传感等应用方向的进展,并展望了纳腔量子等离激元未来的应用方向和发展前景.

Probing the limits of plasmonic enhancement using a two-dimensional atomic crystal probe

Achieving larger electromagnetic enhancement using a nanogap between neighboring metallic nanostructures has been long pursued for boosting light–matter interactions.However,the quantitative probing of this enhancement is hindered by the lack of a reliable experimental method for measuring the local fields within a subnanometer gap.Here,we use layered MoS2 as a two-dimensional atomic crystal probe in nanoparticle-on-mirror nanoantennas to measure the plasmonic enhancement in the gap by quantitative surface-enhanced Raman scattering.Our designs ensure that the probe filled in the gap has a well-defined lattice orientation and thickness,enabling independent extraction of the anisotropic field enhancements.We find that the field enhancement can be safely described by pure classical electromagnetic theory when the gap distance is no<1.24 nm.For a 0.62 nm gap,the probable emergence of quantum mechanical effects renders an average electric field enhancement of 114-fold,38.4%lower than classical predictions.

Controllable defects implantation in MoS2 grown by chemical vapor deposition for photoluminescence enhancement

Photoluminescence （PL） of transition metal dichalcogenides （TMDs） can be engineered by controlling the density of defects, which provide active sites for electron-hole recombination, either radiatively or non-radiatively. However, the implantation of defects by external stimulation, such as uniaxial tension and irradiation, tends to introduce local damages or structural non-homogeneity, which greatly degrades their luminescence properties and impede their applicability in constructing optoelectronic devices. In this paper, we present a strategy to introduce a controllable level of defects into the MoS2 monolayers by adding a hydrogen flow during the chemical vapor deposition, without sacrificing their luminescence characteristics. The density of the defect is controlled directly by the concentration of hydrogen. For an appropriate hydrogen flux, the monolayer MoS2 sheets have three times stronger PL emission at the excitonic transitions, compared with those samples with nearly perfect crystalline structure. The defect-bounded exciton transitions at lower energies arising in the defective samples and are maximized when the total PL is the strongest. However, the B exciton, exhibits a monotonic decline as the defect density increases. The Raman spectra of the defective MoS2 reveal a redshift （blueshift） of the in-plane （out-of-plane） vibration modes as the hydrogen flux increases. All the evidence indicates that the generated defects are in the form of sulfur vacancies. This study renders the high-throughput synthesis of defective MoS2 possible for catalysis or light emitting applications.

Merging bound states in the continuum by harnessing higher-order topological charges

Bound states in the continuum(BICs)can confine light with a theoretically infinite Q factor.However,in practical on-chip resonators,scattering loss caused by inevitable fabrication imperfection leads to finite Q factors due to the coupling of BICs with nearby radiative states.Merging multiple BICs can improve the robustness of BICs against fabrication imperfection by improving the Q factors of nearby states over a broad wavevector range.To date,the studies of merging BICs have been limited to fundamental BICs with topological charges±1.Here we show the unique advantages of higher-order BICs(those with higher-order topological charges)in constructing merging BICs.Merging multiple BICs with a higher-order BIC can further improve the Q factors compared with those involving only fundamental BICs.In addition,higher-order BICs offer great flexibility in realizing steerable off-T merging BICs.A higher-order BIC at F can split into a few off-T fundamental BICs by reducing the system symmetry.The split BICs can then be tuned to merge with another BIC,e.g.,an accidental BIC,at an off-Γpoint.When the in-plane mirror symmetry is further broken,merging BICs become steerable in the reciprocal space.Merging BICs provide a paradigm to achieve robust ultrahigh-Q resonances,which are important in enhancing nonlinear and quantum effects and improving the performance of optoelectronic devices.

Band alignment and interlayer hybridization in monolayer organic/WSe_(2) heterojunction

Semiconducting heterojunctions(HJs),comprised of atomically thin transition metal dichalcogenides(TMDs),have shown great potentials in electronic and optoelectronic applications.Organic/TMD hybrid bilayers hold enhanced pumping efficiency of interfacial excitons,tunable electronic structures and optical properties,and other superior advantages to these inorganic HJs.Here,we report a direct probe of the interfacial electronic structures of a crystalline monolayer(ML)perylene-3,4,9,10-tetracarboxylic-dianhydride(PTCDA)/ML-WSe_(2) HJ using scanning tunneling microscopy,photoluminescence,and first-principle calculations.Strong PTCDAAA/Se_(2) interfacial interactions lead to appreciable hybridization of the WSe_(2) conduction band with PTCDA unoccupied states,accompanying with a significant amount of PTCDA-to-WSe_(2) charge transfer(by 0.06 e/PTCDA).A type-ll band alignment was directly determined with a valence band offset of-1.69 eV,and an apparent conduction band offset of-1.57 eV.Moreover,we found that the local stacking geometry at the HJ interface differentiates the hybridized interfacial states.