By creating defects via oxygen plasma treatment, we demonstrate optical properties variation of single-layer MoS 2 . We found that, with increasing plasma exposure time, the photoluminescence (PL) evolves from very high intensity to complete quenching, accompanied by gradual reduction and broadening of MoS 2 Raman modes, indicative of distortion of the MoS 2 lattice after oxygen bombardment. X-ray photoelectron spectroscopy study shows the appearance of Mo 6+ peak, suggesting the creation of MoO 3 disordered regions in the MoS 2 flake. Finally, using band structure calculations, we demonstrate that the creation of MoO 3 disordered domains upon exposure to oxygen plasma leads to a direct to indirect bandgap transition in single-layer MoS 2 , which explains the observed PL quenching. KEYWORDS 2D materials, defect engineering, optical properties, bandgap tuning, molybdenum trioxide INTRODUCTIONThe ability to controllably tailor the properties of a material is a key factor in the development of many novel applications. In the case of bulk semiconductors, creating and manipulating defects constitutes an essential element in controlling the electrical, magnetic, and optical properties of the host material. 1 Although the role of defects is well understood in bulk semiconductors, it has received little attention in emerging two-dimensional (2D) layered semiconductors, preventing their full exploitation for tailored 2D nanoelectronic and photonic devices. Graphene and graphene oxide are examples of the impact that defects can have on 2D materials. Pristine graphene, which contains no intrinsic defect, is well known for its extraordinary high mobility, and is of great importance for high frequency device applications. 2, 3 However, its inherent lack of bandgap and low absorption of solar photons greatly limit its use in electronic and photonic devices. On the other hand, its solution processed counterparts, graphene oxide and reduced graphene oxide, have a large amount of defects, which lead to formation of a bandgap and open the way to many other applications in photodetectors, sensors, catalysis, and solar cell. [4][5][6][7][8] Recently, layered transition metal dichalcogenides (TMDs) have emerged as important materials for 2D device engineering. 9-11 Molybdenum disulfide (MoS 2 ), composed of weak van der Waals bonded S-Mo-S units, offers a large intrinsic bandgap that is strongly dependent on the number of layers, with an indirect bandgap (1.2 eV) in bulk MoS 2 transitioning to a direct

We demonstrate that the electrical property of a single layer molybdenum disulfide (MoS 2 ) can be significantly tuned from semiconducting to insulating regime via controlled exposure to oxygen plasma. The mobility, on-current and resistance of single layer MoS 2 devices were varied up to four orders of magnitude by controlling the plasma exposure time. Raman spectroscopy, X-ray photoelectron spectroscopy and density functional theory studies suggest that the significant variation of electronic properties is caused by the creation of insulating MoO 3 -rich disordered domains in the MoS 2 sheet upon oxygen plasma exposure, leading to an exponential variation of resistance and mobility as a function of plasma exposure time. The resistance variation calculated using an effective medium model is in excellent agreement with the measurements. The simple approach described here can be used for the fabrication of tunable two dimensional nanodevices on MoS 2 and other transition metal dichalcogenides.

An engineered plasmonic gold surface, specifically designed to couple with 980 nm radiation, is shown to enhance near-infrared-to-visible upconversion luminescence from a monolayer of β-NaYF4: 17%Yb, 3%Er nanocrystals in poly(methyl methacrylate) on that gold surface. Confocal imaging of upconversion luminescence from the surface is used to characterize the nature of the enhancement. It is shown that the luminescence data were acquired below the so-called “high power limit” for excitation, but some saturation was evident, as the observed power dependence was less than quadratic. Over the range of excitation power densities used, the intrinsic enhancement factor for upconversion from the patterned surface was greater than a factor of 3 but decreased slowly with increasing excitation power. The red and green upconversion were enhanced by similar factors, which would support the intensification of the excitation field by the plasmonic surface as being the mechanism of enhancement. In the absence of other enhancement or quenching mechanisms, the data imply an approximate 2-fold magnification of the excitation field intensity relative to smooth gold.

The imaging performance of an optical microscope can be degraded by sample-induced aberrations. A general strategy to undo the effect of these aberrations is to apply wavefront correction with a deformable mirror (DM). In most cases the DM is placed conjugate to the microscope pupil, called pupil adaptive optics (AO). When the aberrations are spatially variant an alternative configuration involves placing the DM conjugate to the main source of aberrations, called conjugate AO. We provide a theoretical and experimental comparison of both configurations for the simplified case where spatially variant aberrations are produced by a well defined phase screen. We pay particular attention to the resulting correction field of view (FOV). Conjugate AO is found to provide a significant FOV advantage. While this result is well known in the astronomy community, our goal here is to recast it specifically for the optical microscopy community.

A surface plasmon (SP) is a fundamental excitation state that exists in metal nanostructures. Over the past several years, the performance of optoelectronic devices has been improved greatly via the SP enhancement effect. In our previous work, the responsivity of GaN ultraviolet detectors was increased by over 30 times when using Ag nanoparticles. However, the physics of the SP enhancement effect has not been established definitely because of the lack of experimental evidence. To reveal the physical origin of this enhancement, Kelvin probe force microscopy (KPFM) was used to observe the SP-induced surface potential reduction in the vicinity of Ag nanoparticles on a GaN epilayer. Under ultraviolet illumination, the localized field enhancement induced by the SP forces the photogenerated electrons to drift close to the Ag nanoparticles, leading to a reduction of the surface potential around the Ag nanoparticles on the GaN epilayer. For an isolated Ag nanoparticle with a diameter of ~200 nm, the distribution of the SP localized field is located within 60 nm of the boundary of the Ag nanoparticle. For a dimer of Ag nanoparticles, the localized field enhancement between the nanoparticles was the strongest. The results presented here provide direct experimental proof of the localized field enhancement. These results not only explain the high performance of GaN detectors observed with the use of Ag nanoparticles but also reveal the physical mechanism of SP enhancement in optoelectronic devices, which will help us further understand and improve the performance of SP-based optoelectronic devices in the future.

We demonstrate adaptive optics system based on nonlinear feedback from 3- and 4-photon fluorescence. The system is based on femtosecond pulses created by soliton self-frequency shift of a 1550-nm fiber-based femtosecond laser together with micro-electro-mechanical system (MEMS) phase spatial light modulator (SLM). We perturb the 1020-segment SLM using an orthogonal Walsh sequence basis set with a modified version of three-point phase shifting interferometry. We show the improvement after aberrations correction in 3-photon signal from fluorescent beads. In addition, we compare the improvement obtained in the same adaptive optical system for 2-, 3- and 4-photon fluorescence using dye pool. We show that signal improvement resulting from aberration correction grows exponentially as a function of the order of nonlinearity.

Adaptive optics is a strategy to compensate for sample-induced aberrations in microscopy applications. Generally, it requires the presence of "guide stars" in the sample to serve as localized reference targets. We describe an implementation of conjugate adaptive optics that is amenable to widefield (i.e. non-scanning) microscopy, and can provide aberration corrections over potentially large fields of view without the use of guide stars. A unique feature of our implementation is that it is based on wavefront sensing with a single-shot partitioned-aperture sensor that provides large dynamic range compatible with extended samples. Combined information provided by this sensor and the imaging camera enable robust image de-blurring based on a rapid estimation of sample and aberrations obtained by closed-loop feedback. We present the theoretical principle of our technique and proof of concept experimental demonstrations. ...

Spectroscopic imaging and time-resolved spectroscopy are used to study the surface plasmon polariton (SPP) enhanced infrared to visible upconversion luminescence from NaYF 4 :Tm:Yb nanoparticles embedded in polymethyl methylacrylate (PMMA) supported on Au nanopillar arrays. The arrays have a lattice resonance associated with the SPP near 980 nm, near-resonant with the peak absorption of the Yb 3+ ion, while a local surface plasmon resonance (LSPR) associated with the individual pillars is seen to enhance the near-infrared emission of Tm 3+ ions near 780 nm. The two combined channels of enhancement result in a significantly higher enhancement of the near-infrared emission when compared to the visible upconversion lines of the Tm 3+ ion, consistent with the interpretation of sequential surface plasmon assisted absorption and emission at two separate and disparate energies. The presence of SPP and LSPR was confirmed by spectrally resolved reflectivity, and the mechanisms for luminescence enhancement were further confirmed by time-resolved measurements of the upconversion luminescence.