Direct Hot-Electron Transfer at the Au Nanoparticle/Monolayer Transition-Metal Dichalcogenide Interface Observed with Ultrahigh Spatiotemporal Resolution.

Plasmon-induced hot-electron transfer at the metallic nanoparticle/semiconductor interface is the basis of plasmon-enhanced photocatalysis and energy harvesting. However, limited by the nanoscale size of hot spots and femtosecond time scale of hot-electron transfer, direct observation is still challenging. Herein, by using spatiotemporal-resolved photoemission electron microscopy with a two-color pump-probe beamline, we directly observed such a process with a concise system, the Au nanoparticle/monolayer transition-metal dichalcogenide (TMD) interface. The ultrafast hot-electron transfer from Au nanoparticles to monolayer TMDs and the plasmon-enhanced transfer process were directly measured and verified through an in situ comparison with the Au film/TMD interface and free TMDs. The lifetime at the Au nanoparticle/MoSe2 interface decreased from 410 to 42 fs, while the photoemission intensities exhibited a 27-fold increase compared to free MoSe2. We also measured the evolution of hot electrons in the energy distributions, indicating the hot-electron injection and decay happened in an ultrafast time scale of ∼50 fs without observable electron cooling.

Bright and Dark Quadrupolar Excitons in the WSe_{2}/MoSe_{2}/WSe_{2} Heterotrilayer.

Transition metal dichalcogenide heterostructures have been extensively studied as a platform for investigating exciton physics. While heterobilayers such as WSe_{2}/MoSe_{2} have received significant attention, there has been comparatively less research on heterotrilayers, which may offer new excitonic species and phases, as well as unique physical properties. In this Letter, we present theoretical and experimental investigations on the emission properties of quadrupolar excitons (QXs), a newly predicted type of exciton, in a WSe_{2}/MoSe_{2}/WSe_{2} heterotrilayer device. Our findings reveal that the optical brightness or darkness of QXs is determined by horizontal mirror symmetry and valley and spin selection rules. Additionally, the emission intensity and energy of both bright and dark QXs can be adjusted by applying an out-of-plane electric field, due to changes in hole distribution and the Stark effect. These results not only provide experimental evidence for the existence of QXs in heterotrilayers but also uncover their novel properties, which have the potential to drive the development of new exciton-based applications.

Active fluorescent modulation for low-noise super-resolution microscopy.

Extracting the position of individual molecular probes with high precision is the basis and core of super-resolution microscopy. However, with the expectation of low-light conditions in life science research, the signal-to-noise ratio (SNR) decreases and signal extraction faces a great challenge. Here, based on temporally modulating the fluorescence emission at certain periodical patterns, we achieved super-resolution imaging with high sensitivity by largely suppressing the background noise. We propose simple bright-dim (BD) fluorescent modulation and delicate control by phase-modulated excitation. We demonstrate that the strategy can effectively enhance signal extraction in both sparsely and densely labeled biological samples, and thus improve the efficiency and precision of super-resolution imaging. This active modulation technique is generally applicable to various fluorescent labels, super-resolution techniques, and advanced algorithms, allowing a wide range of bioimaging applications.

Enhancing Weak-Signal Extraction for Single-Molecule Localization Microscopy.

Single-molecule localization microscopy (SMLM) has been widely used in biological imaging due to its ultrahigh spatial resolution. However, due to the strategy of reducing photodamage to living cells, the fluorescence signals of emitters are usually weak and the detector noises become non-negligible, which leads to localization misalignments and signal losses, thus deteriorating the imaging capability of SMLM. Here, we propose an active modulation method to control the fluorescence of the probe emitters. It actually marks the emitters with artificial blinking character, which directly distinguishes weak signals from multiple detector noises. We demonstrated from simulations and experiments that this method improves the signal-to-noise ratio by about 10 dB over the non-modulated method and boosts the sensitivity of single-molecule localization down to -4 dB, which significantly reduces localization misalignments and signal losses in SMLM. This signal-noise decoupling strategy is generally applicable to the super-resolution system with versatile labeled probes to improve their imaging capability. We also showed its application to the densely labeled sample, showing its flexibility in super-resolution nanoscopy.

Single-Mode Lasing in Polymer Circular Gratings.

In recent years, conjugated polymers have become the materials of choice to fabricate optoelectronic devices, owing to their properties of high absorbance, high quantum efficiency, and wide luminescence tuning ranges. The efficient feedback mechanism in the concentric ring resonator and its circularly symmetric periodic geometry combined with the broadband photoluminescence spectrum of the conjugated polymer can generate a highly coherent output beam. Here, the detailed design of the ultralow-threshold single-mode circular distributed feedback polymer laser is presented with combined fabrication processes such as electron beam lithography and the spin-coating technique. We observe from the extinction spectra of the circular gratings that the transverse electric mode shows no change with the increase of incident beam angle. The strong enhancement of the conjugated polymer photoluminescence spectra with the circular periodic resonator can reduce the lasing threshold about 19 µJ/cm2. A very thin polymer film of about 110 nm is achieved with the spin-coating technique. The thickness of the gain medium can support only the zero-order transverse electric lasing mode. We expect that such a low threshold lasing device can find application in optoelectronic devices.

Information transfer from the optical phase to a plasmonic digital encoder composed of a gold nanorod pair.

Small all-optical devices are central to the optical computing. Plasmonic digital encoders (PDEs) with a featured dimension of ∼1µm hold the key for transferring information from far field to photonic processing systems. Here we propose a PDE design composed of two gold nanorods (AuNRs), whose pattern represents 2-bit digital information. We implanted information into the spectral phase of a femtosecond pulse by pulse shaping and controlled the two-photon photoluminescence pattern of an AuNR pair. The high contrast ratios were achieved with 13.01 and 6.02 dB for binary codes "1-0" and "0-1", respectively.

Configurable Integration of On-Chip Quantum Dot Lasers and Subwavelength Plasmonic Waveguides.

The integration of on-chip dielectric lasers and subwavelength plasmonic waveguides has attracted enormous attention because of the combination of both the advantages of the high performances of the small dielectric lasers and the subwavelength plasmonic waveguides. However, the configurable integration is still a challenge owing to the complexity of the hybrid structures and the damageability of the gain media in the multistep micro/nanofabrications. By employing the dark-field optical imaging technique with a position uncertainty of about 21 nm and combining the high-resolution electron beam lithography, the small colloidal quantum dot (CQD) lasers without any damages are accurately aligned with the silver nanowires. As a result, the integration of the CQD lasers and the silver nanowires can be flexibly configured on chips. In the experiment, the tangential coupling, radial coupling, and complex coupling between the high-performance CQD lasers and the subwavelength silver nanowires are demonstrated. Because of the subwavelength field confinements of the silver nanowires, the deep-subwavelength coherent sources (multimode, one-color single-mode, or two-color single-mode) with a mode area of only 0.008λ2 are output from these hybrid structures. This configurable on-chip integration with high flexibility and controllability will greatly facilitate the developments of the complex functional hybrid photonic-plasmonic circuits.

Continuously tunable distributed feedback polymer laser.

A fanshaped structure is proposed to achieve a continuously tunable polymer laser. The structure with gradual periods is fabricated by electron beam lithography, which acts as a distributed feedback cavity for the polymer laser. A light-emitting polymer is spin-coated on the cavity to form an active layer. The pump beam is focused by a cylindrical lens to a narrow stripe on the sample surface. When the position of the pump stripe on the fanshaped cavity is changed from long period (370 nm) to short period (340 nm) and vice versa, the output wavelength of the laser is continuously tuned from 584 nm to 552 nm. Tuning behavior can be interpreted by the Bragg condition. These results can be used to explore compact laser sources.

Tuning the photo-response in monolayer MoS2 by plasmonic nano-antenna.

Monolayer molybdenum disulfide (MoS2) has recently attracted intense interests due to its remarkable optical properties of valley-selected optical response, strong nonlinear wave mixing and photocurrent/photovoltaic generation and many corresponding potential applications. However, the nature of atomic-thin thickness of monolayer MoS2 leads to inefficient light-matter interactions and thereby hinders its optoelectronic applications. Here we report on the enhanced and controllable photo-response in MoS2 by utilizing surface plasmonic resonance based on metallic nano-antenna with characteristic lateral size of 40 × 80 nm. Our nano-antenna is designed to have one plasmonic resonance in the visible range and can enhance the MoS2 photoluminescence intensity up to 10 folds. The intensity enhancement can be effectively tuned simply by the manipulation of incident light polarization. In addition, we can also control the oscillator strength ratio between exciton and trion states by controlling polarization dependent hot carrier doping in MoS2. Our results demonstrate the possibility in controlling the photo-response in broad two-dimensional materials by well-designed nano-antenna and facilitate its coming optoelectronic applications.

Improved light absorption and charge transport for perovskite solar cells with rough interfaces by sequential deposition.

Recently, highly efficient solar cells based on organic-inorganic perovskites have been intensively reported for developing fabricating methods and device structures. Additional power conversion efficiency should be gained without increasing the thickness and the complexity of the devices to accord with practical applications. In this paper, a rough interface between perovskite and HTM was fabricated in perovskite solar cells to enhance the light scattering effect and improve the charge transport. The parameters related to the morphology have been systematically investigated by sequential deposition. Simultaneous enhancements of short-circuit current and power conversion efficiency were observed in both CH3NH3PbI3 and CH3NH3PbI3-xClx devices containing the rough interface, with power conversion efficiencies of 10.2% and 10.8%, respectively. Our finding provides an efficient and universal way to control the morphology and further optimize perovskite solar cells for devices by sequential deposition with various structures.

A highly efficient mesoscopic solar cell based on CH3NH3PbI(3-x)Cl(x) fabricated via sequential solution deposition.

A mixed halide perovskite of CH3NH3PbI(3-x)Cl(x) is synthesized via two-step sequential solution deposition by using a mixture of PbCl2 and PbI2 as the precursor to overcome the low solubility of pure PbCl2 with easy morphology control. 11.7% power conversion efficiency is achieved for the mesoscopic cell, much higher than the cell constructed via a spin-coating process.

Ultrafast excited-state dynamics associated with the photoisomerization of a cyanine dye.

The ultrafast excited-state dynamics of a cyanine dye, 3,3'-bis(3-sulfopropyl)-5,5'-dimethoxy-thiacyanine triethylaminium salt, was investigated by using conventional time-resolved fluorescence up-conversion technique. The fluorescence decay can be well described as tri-exponential kinetics, which indicates the excited-state population decays through the bond-twist, vibrational and radiative relaxation channels. Further analysis shows that the contributions of the three relaxation channels to the fluorescence decay demonstrate very different change with increasing the fluorescence wavelength, through which the detailed dynamics at different regions in the excited-state potential energy surface can be retrieved.

[All wavelength picosecond fluorescent anisotropy of aligned MEH-PPV/PVP electrospun fibers].

Electrospinning is a simple and effect technology which can produce continuous nanofibers. We get aligned electrospun nanofibers successfully by using parallel electrodes. We report our studies on transient fluorescence of aligned electrospun fibers. The fibers are excited and their fluorescences are observed both at axial and radial polarization. Steady-state PL spectra shows radial emission blue-shift more than axial! emission, due to weakened aggregation of molecular chains in radial direction. At all emission wavelength, radial emission excitons migrate faster than axial emission excitons.

Study on the third and second-order nonlinear optical properties of GeS(2)-Ga(2)S3-AgCl chalcohalide glasses.

Third-order optical nonlinearities, chi((3)) of GeS(2)-Ga(2)S(3)-AgCl chalcohalide glasses have been studied systematically utilizing the femtosecond time-resolved optical Kerr effect (OKE) technique at 820nm, showing that the value of chi((3)) enhances with increasing atomic ratio of (S+Cl/2)/(Ge+Ga). From the compositional dependence of glass structure by Raman spectra, a strong dependence of chi;(3) upon glass structure has been found, i.e. compared with [Cl(x)S(3-x)Ge(Ga)-Ge(Ga)S(3-x)Cl(x)] ethane-like s.u. as the structural defectiveness, [Ge(Ga)S(4-x)Cl(x)] mixed tetrahedra make greater contribution to the enhancement of chi((3)). The maximum chi(3) among the present glasses is as large as 5.26x10(-13)esu (A1 (80GeS(2)-10Ga(2)S(3-) 10AgCl)), and the nonlinear refractive index (n2) of A1 glass is also up to 4.60x10(-15) cm(2)/W. In addition, using Maker fringe technique, SHG was observed in the representative A1 glass poled by electron beam (25 kV, 25 nA, 15 min), and the second-order optical nonlinear susceptibility is estimated to be greater than 6.1 pm/V. There was no evident structural change detected in the as-prepared and after irradiated A1 glass by the Raman spectra, and maybe only electronic transition and distortion of electron cloud occurred in the glasses. The large third/second-order optical nonlinearities have made these GeS(2)-Ga(2)S(3)-AgCl chalcohalide glasses as promising materials applied in photoelectric fields.