Amplification Of Modes Research Articles

Computational Investigation of Step Excrescence Sensitivity in a Swept-Wing Boundary Layer

The interaction between an existing stationary crossflow instability and a two-dimensional step excrescence placed into a swept boundary layer on a lifting surface has been investigated computationally. Backward-facing steps were found to not amplify the stationary modes. The existence of an additional local traveling instability was found in the recirculation region following the step. Further time-resolved calculations were recommended to confirm this mode as the cause of breakdown. Forward-facing steps were found to affect the transition location only once a critical step height was exceeded. This height was associated with sudden amplification of stationary crossflow modes and transition moving forward to the step. A physics-based correlation based on linear stability theory and associated with the height of the core of the stationary crossflow vortex was proposed for that critical height, which agreed well with results for the geometry and Reynolds numbers tested experimentally. Critical forward-facing step heights were in the majority of cases higher than those associated with backward-facing steps for the same conditions. Moreover, the physics associated with two-dimensional excrescences in swept-wing flow was fundamentally different from that associated with other types of roughness, as well as all roughnesses in two-dimensional flowfields. Upon further validation, the proposed correlation for forward-facing steps would provide a reasonable manufacturing criteria available to airframe designers that would prove less restrictive and more accurate than existing empirical criteria.

Demonstration of a homogeneous Yb-doped core fully aperiodic large-pitch fiber laser.

The first demonstration of a 40μm core homogeneously ytterbium-doped fully aperiodic large-pitch fiber laser, to the best of our knowledge, is reported here. In this concept, the amplification of unwanted high-order modes is prevented by means of an aperiodic inner-cladding structure, while the core and inner-cladding material has a higher refractive index than pure silica. In a laser configuration, up to 252W of extracted power, together with an optical-to-optical efficiency of 63% with respect to the incident pump power, have been achieved. While an average M2 of 1.4 was measured, the emitted power becomes temporally unstable when exceeding 95W, owing to the occurrence of modal instabilities.

Boundary-layer disturbances subjected to free-stream turbulence and simulation on bypass transition

The phenomena associated with the entrainment of free-stream turbulence (FST) into boundary-layer flows are relevant for a number of subjects. It has been believed that the continuous spectra of the Orr-Sommerfeld (O-S)/Squire equations describe the entrainment process, and thus they are used to specify the inlet condition in simulation of bypass transition. However, Dong and Wu (Dong, M. and Wu, X. On continuous spectra of the Orr-Sommerfeld/Squire equations and entrainment of free-stream vortical disturbances. Journal of Fluid Mechanics, 732, 616–659 (2013)) pointed out that continuous spectra exhibit several non-physical features due to neglecting the non-parallelism. They further proposed a large-Reynolds-number asymptotic approach, and showed that the non-parallelism is a leading-order effect even for the short-wavelength disturbance, for which the response concentrates in the edge layer. In this paper, the asymptotic solution is verified numerically by studying its evolution in incompressible boundary layers. It is found that the numerical results can be accurately predicted by the asymptotic solution, implying that the latter is adequate for moderate Reynolds numbers. By introducing a series of such solutions as the inflow perturbations, the bypass transition is investigated via the direct numerical simulation (DNS). The transition processes, including the evolution of streaks, the amplification of secondary-instability modes, and the emergence of turbulent spots, agree with the experimental observations.

Evidence of partial temporal coherence effects in the linear autocorrelation of extreme ultraviolet laser pulses.

We study how the degree of temporal coherence of plasma-based extreme ultraviolet lasers operated in the amplification of the spontaneous emission mode is encoded in the shape of the linear autocorrelation function, which is obtained from the variation of the fringe visibility while varying the delay in a variable path-difference interferometer. We discuss the implications of this effect when the technique is used to infer the spectral properties of the source. Our numerical simulations, based on a partial coherence model developed by other authors for x-ray free electron lasers, are in good agreement with previously reported sets of measurements, illustrating similar statistical properties for both sources.

Scattering of gravity waves in subcritical flows over an obstacle

We numerically study the scattering coefficients of linear water waves on stationary flows above a localized obstacle. We compare the scattering on trans- and subcritical flows, and then focus on the latter which have been used in recent analog gravity experiments. The main difference concerns the magnitude of the mode amplification: whereas transcritical flows display a large amplification (which is generally in good agreement with the Hawking prediction), this effect is heavily suppressed in subcritical flows. This is due to the transmission across the obstacle for frequencies less than some critical value. As a result, subcritical flows display high- and low-frequency behaviors separated by a narrow band around the critical frequency. In the low-frequency regime, transmission of long wavelengths is accompanied by non-adiabatic scattering into short wavelengths, whose spectrum is approximately linear in frequency. By contrast, in the high-frequency regime, no simple description seems to exist. In particular, for obstacles similar to those recently used, we observe that the upstream slope still affects the scattering on the downstream side because of some residual transmission.

Linear instability analysis of low- incompressible flow over a long rectangular finite-span open cavity

TriGlobal linear instability analysis and direct numerical simulations have been performed to unravel the mechanisms ultimately responsible for transition of steady laminar flow over a long rectangular finite-span open cavity with dimensions $L$ : $D$ : $W$$=$ 6 : 1 : 2 to unsteadiness. The steady laminar three-dimensional flow loses stability at $\mathit{Re}_{D,cr}\approx 1080$ as a consequence of linear amplification of a travelling eigenmode that is qualitatively analogous to the shear-layer mode known from analyses of flow in spanwise-periodic cavities, but has a three-dimensional structure which is strongly influenced by the cavity lateral walls. Differences in the eigenspectrum of the present and the spanwise homogeneous flow configuration are documented. Topological changes exerted on the steady laminar flow by linear amplification of the unstable shear-layer mode are reminiscent of observations in experiments at an order of magnitude higher Reynolds number.

Dynamical strong coupling and parametric amplification of mechanical modes of graphene drums.

Mechanical resonators are ubiquitous in modern information technology. With the possibility of coupling them to electromagnetic and plasmonic modes, they hold promise as the key building blocks in future quantum information technology. Graphene-based resonators are of interest for technological applications due to their high resonant frequencies, multiple mechanical modes and low mass. The tension-mediated nonlinear coupling between various modes of the resonator can be excited in a controllable manner. Here we engineer a graphene resonator with large frequency tunability at low temperatures, resulting in a large intermodal coupling strength. We observe the emergence of new eigenmodes and amplification of the coupled modes using red and blue parametric excitation, respectively. We demonstrate that the dynamical intermodal coupling is tunable. A cooperativity of 60 between two resonant modes of ∼100 MHz is achieved in the strong coupling regime. The ability to dynamically control the coupling between the high-frequency eigenmodes of a mechanical system opens up the possibility of quantum mechanical experiments at low temperatures.

Experimental study of diode pumped rubidium amplifier for single higher-order Laguerre-Gaussian modes.

In this paper, we have set up a diode laser pumped rubidium amplifier for higher-order Laguerre-Gauss (LG) modes. We experimentally realized amplification of higher-order LG modes including helical and sinusoidal LG03, LG13, LG23, and LG33 modes with their high purity held. This novel scheme of generating high-purity higher-order LG beams at high laser power is preferred to the second-generation gravitational wave interferometers. To the best of our knowledge, it is the first time this scheme is formulated.

Identification of disbond and high density core region in a honeycomb composite sandwich structure using ultrasonic guided waves

The aim of this study is to identify disbond and high-density (HD) core region in a honeycomb composite sandwich structure (HCSS) using ultrasonic guided waves (GWs) and surface-bonded piezoelectric wafer transducers (PWTs). Towards this, an organized theoretical, numerical and experimental study has been carried out in order to understand the characteristics of GW propagation in a HCSS. A global matrix method based efficient and fast two dimensional (2D) semi-analytical model is used to study transient response and dispersion characteristics of the healthy HCSS under PWT excitations. Numerical simulation of GW propagation in HCSS with disbond and HD-core is then carried out using finite element package, ABAQUS. Laboratory experiments are then carried out to validate the theoretical and numerical results. A good agreement is observed between the theoretical, numerical and experimental results in all cases studied. It is found that the presence of HD-core region leads to decrease in amplitude of the propagating GW modes and the presence of disbond leads to substantial amplification of the primary anti-symmetric mode. Finally, based on these changes in modal behaviors, the location and size of unknown disbond and HD-core region within the PWT array are experimentally determined using a probability based damage detection algorithm.

Mode-selective amplification in a large mode area Yb-doped fiber using a photonic lantern.

We demonstrate selective spatial mode amplification in a few mode, double-clad Yb-doped large mode area (LMA) fiber, utilizing an all-fiber photonic lantern. Amplification to multi-watt output power is achieved while preserving high spatial mode selectivity. We observe gain values of over 12 dB for all modes: LP01, LP11a, and LP11b, when amplified individually. Additionally, we investigate the simultaneous amplification of LP01+LP11a and LP11a+LP11b, and the resultant mode competition. The proposed architecture allows for the reconfigurable excitation of spatial modes in the LMA fiber amplifiers, and represents a promising method that could enable dynamic spatial mode control in high power fiber lasers.

Sensitive SERS detection of DNA methyltransferase by target triggering primer generation-based multiple signal amplification strategy

A novel and sensitive surface-enhanced Raman scattering (SERS) method is proposed for the assay of DNA methyltransferase (MTase) activity and evaluation of inhibitors by developing a target triggering primer generation-based multiple signal amplification strategy. By using of a duplex substrate for Dam MTase, two hairpin templates and a Raman probe, multiple signal amplification mode is achieved. Once recognized by Dam MTase, the duplex substrate can be cleaved by Dpn I endonuclease and two primers are released for triggering the multiple signal amplification reaction. Consequently, a wide dynamic range and remarkably high sensitivity are obtained under isothermal conditions. The detection limit is 2.57×10−4UmL−1. This assay exhibits an excellent selectivity and is successfully applied in the screening of inhibitors for Dam MTase. In addition, this novel sensing system is potentially universal as the recognition element can be conveniently designed for other target analytes by changing the substrate of DNA MTase.

Interaction of the recent 50 year SST trend and La Niña 2010: amplification of the Southern Annular Mode and Australian springtime rainfall

Australia experienced record high rainfall in austral spring 2010, which has previously been attributed to the concurrence of a strong La Nina event and a strong positive excursion of the Southern Annular Mode (SAM). In this study, we examine the role of the sea surface temperature (SST) trend over the recent 50 years, which has large warming over the tropical Indian, western Pacific and North Atlantic Oceans, in driving the extraordinary climate conditions of spring 2010, using the Australian Bureau of Meteorology coupled model seasonal forecast system. Four forecast sensitivity experiments were designed by using randomly chosen atmospheric initial conditions but with: (1) observed ocean initial conditions for 1 September 2010; (2) the same ocean initial conditions except the linear temperature trend over the period 1960–2010 was removed; (3) ocean initial conditions in which the trend was added to the climatological ocean state for 1 September; and (4) climatological ocean conditions only. A synergistic response to the La Nina SST anomalies and the SST trend was detected: the tropical rainfall anomalies were amplified over the western side of the Indo-Pacific warm-pool, which led to a significant increase of tropical upper tropospheric warming and a resultant increase of meridional temperature gradient in the Southern Hemisphere (SH) extratropics. Consequently, the SH eddy-driven jet was shifted poleward (i.e. positive phase of the SAM), which induced rainfall over subtropical Australia. Our findings highlight that the interaction of interannual anomalies and the trend may play an important role in the amplification of extreme events.

Amplification of the Zeroth Order Mode in Ultra-thin Layers

We acknowledge the financial support through projects TEC2015-64189-C3-2-R and PCIN-2013-179 (MINECO/FEDER), AIC-B-2011-0806 (MINECO), S2013/MAE-2780 (Community of Madrid) and by the European Space Agency contract No 4000112049/14/NL/MV under the GSP/ARIADNA program.

Ultrasensitive Detection of MicroRNA in Tumor Cells and Tissues via Continuous Assembly of DNA Probe.

Nucleic acids have been engineered to participate in a wide variety of tasks. Among them, the enzyme-free amplification modes, enzyme-free DNA circuits (EFDCs), and hybridization chain reactions (HCRs) have been widely applied in a series of studies of bioanalysis. We demonstrated here an ultrasensitive hairpin probe-based circulation for continuous assemble of DNA probe. This strategy improved the analyte stability-dependent amplification efficiency of EFDC and signal enhancement without being limited by the analyte's initial concentration, and it was used to produce a novel microRNA (miRNA) trace analysis assay with ultrasensitive amplification properties. Through the detection of standard miRNA substances, 1 amol-level sensitivity and satisfactory specificity were achieved. Compared with EFDCs and HCRs, the sensitivity of ultrasensitive hairpin probe-based circulation was higher by 3 or 4 orders of magnitude. Furthermore, the excellent performance of this platform was also demonstrated in the detection of miRNAs in tumor cells. The sensitivities for the detection of miRNAs in HepG2, A549 and MCF-7 tumor cells were 10, 10, and 100 cells, respectively. In addition, a high detection rate of 83% was achieved for tumor tissues. Thus, this ultrasensitive hairpin probe-based circulation possesses the potential to be a technological innovation in the field of tumor diagnosis.

Selection and amplification of a single optical frequency comb mode for laser cooling of the strontium atoms in an optical clock

In this paper, we report on the active filtering and amplification of a single mode from an optical femtosecond laser comb with mode spacing of 250 MHz by optical injection of two external-cavity diode lasers operating in cascade to build a narrow linewidth laser for laser cooling of the strontium atoms in an optical lattice clock. Despite the low injection of individual comb mode of approximately 50 nW, a single comb line at 689 nm could be filtered and amplified to reach as high as 10 mW with 37 dB side mode suppression and a linewidth of 240 Hz. This method could be applied over a broad spectral band to build narrow linewidth lasers for various applications.

Parity-time anti-symmetric parametric amplifier.

We describe the process of parametric amplification in a directional coupler of quadratically nonlinear and lossy waveguides, which belongs to a class of optical systems with spatial parity-time (PT) symmetry in the linear regime. We identify a distinct spectral PT anti-symmetry associated with optical parametric interaction, and show that pump-controlled symmetry breaking can facilitate spectrally selective mode amplification in analogy with PT lasers. We also establish a connection between the breaking of spectral and spatial mode symmetries, revealing the potential to implement unconventional regimes of spatial light switching through ultrafast control of PT breaking by pump pulses.

Accurate γ and MeV-electron track reconstruction with an ultra-low diffusion Xenon/TMA TPC at 10 atm

We report the performance of a 10atm Xenon/trimethylamine time projection chamber (TPC) for the detection of X-rays (30keV) and γ-rays (0.511–1.275MeV) in conjunction with the accurate tracking of the associated electrons. When operated at such a high pressure and in ~1%-admixtures, trimethylamine (TMA) endows Xenon with an extremely low electron diffusion (1.3±0.13mm-σ (longitudinal), 0.95±0.20mm-σ (transverse) along 1m drift) besides forming a convenient ‘Penning-Fluorescent’ mixture. The TPC, that houses 1.1kg of gas in its fiducial volume, operated continuously for 100 live-days in charge amplification mode. The readout was performed through the recently introduced microbulk Micromegas technology and the AFTER chip, providing a 3D voxelization of 8mm×8mm×1.2mm for approximately 10cm/MeV-long electron tracks. Resolution in energy (ε) at full width half maximum (R) inside the fiducial volume ranged from R=14.6% (30keV) to R=4.6%(1.275MeV).This work was developed as part of the R&D program of the NEXT collaboration for future detector upgrades in the search of the neutrino-less double beta decay (ββ0ν) in 136Xe, specifically those based on novel gas mixtures. Therefore we ultimately focus on the calorimetric and topological properties of the reconstructed MeV-electron tracks. In particular, the obtained energy resolution has been decomposed in its various contributions and improvements towards achieving the R=1.4%1MeV/ε levels obtained in small sensors are discussed.

Control of Crossflow Transition at High Reynolds Numbers Using Discrete Roughness Elements

Transition analysis is performed for a swept wing at a Mach number of 0.75 and chord Reynolds number of approximately , with a focus on roughness-based crossflow-transition control at high Reynolds numbers relevant to subsonic flight. The roughness-based transition control involves controlled seeding of suitable, subdominant crossflow modes to weaken the growth of naturally occurring, linearly more unstable instability modes via a nonlinear modification of the mean boundary-layer profiles. Therefore, a synthesis of receptivity, linear and nonlinear growth of crossflow disturbances, and high-frequency secondary instabilities becomes desirable to model this form of control. Because experimental data are currently unavailable for passive crossflow-transition control on high-Reynolds-number configurations, a holistic computational approach is used to assess the feasibility of roughness-based-control methodology. The potential challenges inherent to this control application, as well as the associated difficulties in modeling this form of control in a computational setting, are highlighted. At high Reynolds numbers, a broad spectrum of stationary-crossflow disturbances have large-enough linear amplification to cause transition, and, while it may be possible to control a specific target mode using discrete roughness elements, the nonlinear interaction between the control and target modes may yield strong amplification of the difference mode and, hence, produce an adverse impact on the transition delay using spanwise periodic roughness elements.

Asymmetric signal amplification for simultaneous SERS detection of multiple cancer markers with significantly different levels.

Simultaneous detection of cancer biomarkers holds great promise for the early diagnosis of different cancers. However, in the presence of high-concentration biomarkers, the signals of lower-expression biomarkers are overlapped. Existing techniques are not suitable for simultaneously detecting multiple biomarkers at concentrations with significantly different orders of magnitude. Here, we propose an asymmetric signal amplification method for simultaneously detecting multiple biomarkers with significantly different levels. Using the bifunctional probe, a linear amplification mode responds to high-concentration markers, and quadratic amplification mode responds to low-concentration markers. With the combined biobarcode probe and hybridization chain reaction (HCR) amplification method, the detection limits of microRNA (miRNA) and ATP via surface-enhanced Raman scattering (SERS) detection are 0.15 fM and 20 nM, respectively, with a breakthrough of detection concentration difference over 11 orders of magnitude. Furthermore, successful determination of miRNA and ATP in cancer cells supports the practicability of the assay. This methodology promises to open an exciting new avenue for the detection of various types of biomolecules.

Parallel amplification in electronic systems with distributed interaction

The possibility of parallel amplification of various wave modes in the case of their synchronism with a single electron beam is analyzed theoretically. The specific features and the basic characteristics of this process are studied with the use of an elementary model of a relativistic Cherenkov traveling-wave tube in which two waves with counterclockwise and clockwise rotation of the transverse structure interact with an electron beam. It is demonstrated that, in the case of sequential electrodynamic coupling of operating waves, this approach can be used to achieve a high gain in a relativistic traveling-wave tube and retains typical values of other parameters (electronic efficiency, amplification band, etc.).