Two-dimensional Dirac plasmon-polaritons in graphene, 3D topological insulator and hybrid systems

Collective oscillations of massless particles in two-dimensional (2D) Dirac materials offer an innovative route toward implementing atomically thin devices based on low-energy quasiparticle interactions. Strong confinement of near-field distribution on the 2D surface is essential to demonstrate extraordinary optoelectronic functions, providing means to shape the spectral response at the mid-infrared (IR) wavelength. Although the dynamic polarization from the linear response theory has successfully accounted for a range of experimental observations, a unified perspective was still elusive, connecting the state-of-the-art developments based on the 2D Dirac plasmon-polaritons. Here, we review recent works on graphene and three-dimensional (3D) topological insulator (TI) plasmon-polariton, where the mid-IR and terahertz (THz) radiation experiences prominent confinement into a deep-subwavelength scale in a novel optoelectronic structure. After presenting general light-matter interactions between 2D Dirac plasmon and subwavelength quasiparticle excitations, we introduce various experimental techniques to couple the plasmon-polaritons with electromagnetic radiations. Electrical and optical controls over the plasmonic excitations reveal the hybridized plasmon modes in graphene and 3D TI, demonstrating an intense near-field interaction of 2D Dirac plasmon within the highly-compressed volume. These findings can further be applied to invent optoelectronic bio-molecular sensors, atomically thin photodetectors, and laser-driven light sources

Femtosecond pump-probe studies of carrier transport and gain dynamics in quantum cascade lasers

Time-resolved pump-probe differential transmission measurements have been performed on various sets of quantum cascade lasers below and above threshold. Bias-, temperature-, and polarization dependent studies enable a comprehensive understanding of the gain dynamics to be possible in terms of tunneling, transport, and coupling with the intra-cavity photon density. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57927/1/225_ftp.pd

Time-Resolved Investigations of Electronic Transport Dynamics in Quantum Cascade Lasers Based on Diagonal Lasing Transition

In this study, the nature of electronic transport in quantum cascade lasers (QCLs) has been extensively investigated using an ultrafast time-resolved, degenerate, pump-probe optical technique. Our investigations enable a comprehensive understanding of the gain recovery dynamics in terms of a coupling of the electronic transport to the oscillating intracavity laser intensity. In QCLs that have a lasing transition diagonal in real space, studies of the near-threshold reveal that the transport of electrons changes bias region from phonon-limited relaxation (tens of picoseconds) below threshold to photon-driven transport via stimulated emission (a few picoseconds) above threshold. The gain recovery dynamics in the photon-driven regime is compared with conventional four-level lasers such as atomic, molecular, and semiconductor interband lasers. The depopulation dynamics out of the lower lasing state is explained using a tight-binding tunneling model and phonon-limited relaxation. For the superlattice relaxation, it is possible to explain the characteristic picosecond transport via dielectric relaxation; Monte Carlo simulations with a simple resistor model are developed, and the Esaki–Tsu model is applied. Subpicosecond dynamics due to carrier heating in the upper subband are isolated and appear to be at most about 10% of the gain compression compared with the contribution of stimulated emission. Finally, the polarization anisotropy in the active waveguide is experimentally shown to be negligible on our pump-probe data, supporting our interpretation of data in terms of gain recovery and transport.Engineering and Applied Science

Evaluation of automated calibration and quality control processes using the Aptio total laboratory automation system

Background The objective of this study was to determine whether manually performed calibration and quality control (QC) processes could be replaced with an automated laboratory system when installed analyzers fail to provide automated calibration and QC functions. Methods Alanine aminotransferase (ALT), total cholesterol (TC), creatinine (Cr), direct bilirubin (DB), and lipase (Lip) items were used as analytes. We prepared pooled serum samples at 10 levels for each test item and divided them into two groups; five for the analytical measurement range (AMR) group and five for the medical decision point (MDP) group. Calibration and QC processes were performed for five consecutive days, and ALT, TC, Cr, DB, and Lip levels were measured in the two groups using automated and manual methods. Precision and the mean difference between the calibration and QC methods were evaluated using the reported values of the test items in each group. Results Repeatability and within-laboratory coefficients of variation (CVs) between the automated system and the conventional manual system in the AMR group were similar. However, the mean reported values for test items were significantly different between the two systems. In the MDP group, repeatability and within-laboratory CVs were better with the automation system. All calibration and QC processes were successfully implemented with the Aptio total laboratory automation system. Conclusion The Aptio total laboratory automation system could be applied to routine practice to improve precision and efficiency

Controllable synthesis of molybdenum tungsten disulfide alloy for vertically composition-controlled multilayer

The effective synthesis of two-dimensional transition metal dichalcogenides alloy is essential for successful application in electronic and optical devices based on a tunable band gap. Here we show a synthesis process for Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> alloy using sulfurization of super-cycle atomic layer deposition Mo<inf>1-x</inf>W<inf>x</inf>O<inf>y</inf>. Various spectroscopic and microscopic results indicate that the synthesized Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> alloys have complete mixing of Mo and Watoms and tunable band gap by systematically controlled composition and layer number. Based on this, we synthesize a vertically composition-controlled (VCC) Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer using five continuous super-cycles with different cycle ratios for each super-cycle. Angle-resolved X-ray photoemission spectroscopy, Raman and ultraviolet-visible spectrophotometer results reveal that a VCC Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer has different vertical composition and broadband light absorption with strong interlayer coupling within a VCC Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer. Further, we demonstrate that a VCC Mo<inf>1-x</inf>W<inf>x</inf>S<inf>2</inf> multilayer photodetector generates three to four times greater photocurrent than MoS<inf>2</inf>-and WS<inf>2</inf>-based devices, owing to the broadband light absorption. © 2015 Macmillan Publishers Limitedopen1

Exciton dynamics in atomically thin MoS2: Interexcitonic interaction and broadening kinetics

We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS2. Spectrally and temporally resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons (A and B) and their associated broadening kinetics. The two excitons show strongly correlated interexcitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the B-exciton linewidth in turn lowers the peak spectral amplitude of the A exciton. Resonant excitation at the B-exciton energy reveals that interexcitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.Keywords: Relaxation, Monolayer MoS[subscript 2], Blue shift, Semiconductor, Transient absorption spectra, Layer, Femtosecond, Photoexcited carriers, Regenerative amplifier, Quantum well structur