Coherent Generation Research Articles

1.7–18 µm mid-infrared supercontinuum generation in a dispersion-engineered step-index chalcogenide fiber

We demonstrate 1.7–18 µm mid-infrared coherent supercontinuum generation by means of the full transmission window of a unique dispersion-engineered step-index Ge-Se-Te fiber. Our study thus opens the entire molecular fingerprint region to future chalcogenide fiber platforms.

Understanding the coherent structures for different fluctuation frequencies at partial load condition of a Francis turbine

More and more hydropower plants are operating as regulator of renewable energy such as solar and wind energy resources. Because these renewable energy resources are highly volatile, so hydraulic turbines have to switch working condition in a wide flow range to guarantee the steady output to electric grid. It’s known that there are many fluctuating frequency components in runner passage of hydraulic turbines, especially at part-load conditions, which are induced by unsteady flow phenomena like guide vane wake, flow separation or rotor-stator interaction (RSI) and so on. These fluctuation frequency components are varied with working condition due to different dominant inducing resources. Although we can estimate the frequency component resources according to rotating speed, numbers of blades and guide vanes, we can’t know exactly coherent structure pattern of every frequency component. In present study, firstly, we will compute the internal flow of a Francis turbine at lower discharge, and verify the validation of computational results by experimental data. Secondly, Dynamic Mode Decomposition (DMD) method will be employed to extract the three-dimensional coherent structures in runner passage and draft tube for every frequency component based on computational snapshots. Finally, we will demonstrate the patterns and locations of coherent structures for every frequency component and predict the coherent structure or fluctuation component generation resources.

Formula omitted]Li intermolecular multiple-quantum coherences in liquids

We report here evidence for the generation of 7Li multiple-quantum coherences in aqueous solutions outside of regimes where conventional multiple-quantum coherences due to alignment or quadrupolar relaxation could be observed. These coherences are shown to observe nonlinear behavior as a function of concentration, and hence these effects can be identified as arising from intermolecular multiple-quantum coherences. Due to the importance of lithium ion solutions for the study of electrochemical systems, awareness of such coherences is particularly important in the interpretation of experimental results, and new applications using lithium as a probe may become possible on this basis.

Second Harmonic Generation Covering the Entire Visible Range from a 2D Material–Plasmon Hybrid Metasurface

AbstractOn‐chip coherent light source has always been fascinating and intriguing due to its various potential applications. In the past decades, there has been some progress in the development of chip coherent light (e.g., nanolaser, Bose‐Einstein condensation and nonlinear optical effects). However, these methods strictly depend on materials and extreme experimental conditions, and are usually not tunable. Here, a hybrid structure is designed which combines a chirped surface plasmon metasurface with a monolayer transition metal dichalcogenide (TMDC) to achieve a coherent second harmonic generation (SHG) covering the entire visible light spectrum. Using only finite number of metallic grooves, a continuous resonance tuning is obtained. By translating the metasurface in space, a space‐frequency locking SHG is demonstrated. Although the Q factor of the surface plasmon cavity is low, its near‐field enhancements of both fundamental and SH waves are still obvious. Significantly, the broad linewidth of plasmonic cavity leads to a large degree of overlap between adjacent localized modes, that enables the tuning of the output wavelength continuously at room temperature. Meanwhile, the exciton resonance also plays an important role. This monolithic tunable device demonstrates the potentials of 2D material‐plasmon hybrid metasurface and to construct an efficient broadband tunable on‐chip coherent light source.

Topological optical and phononic interface mode by simultaneous band inversion

Band inversion in one-dimensional superlattices is a strategy to generate topological interface modes in electronics, optics, acoustics, and nanophononics. Despite their potential for the control of topologically robust interactions, most realizations of these states have so far explored only a single kind of excitation. In this work, we design and fabricate GaAs/AlAs devices with simultaneously inverted band structures for light and phonons. We experimentally observe colocalized interface modes for 1.34 eV photons by optical reflectivity and 18 GHz phonons by coherent phonon generation and detection. Through numerical simulations, we demonstrate the ensuing robustness of the Brillouin interaction between them with respect to a specific type of disorder. Furthermore, we theoretically analyze the efficiency of time-domain Brillouin scattering in different topological designs presenting colocalized states and deduce a set of engineering rules. Potential future applications include the engineering of robust optomechanical resonators in a material system compatible with active media such as quantum wells and quantum dots.

Supercontinuum Generation in High Order Waveguide Mode with near-Visible Pumping Using Aluminum Nitride Waveguides

Optical sources emitting in the ultraviolet (UV) to near-infrared wavelength range are an enabling tools for a wide variety of applications. To achieve broadband coherent generation within visible ...

Coherent near-infrared light generation based on self-seeded parametric four-wave mixing in Rb vapor

We demonstrate 1.37 µm coherent near-infrared light generation via self-seeded parametric four-wave mixing (FWM) process in Rb atomic vapor. Two lasers (780 nm and 776 nm) in the co-propagating configuration, drive the Rb atoms from the 5S1/2 ground state to the 5D5/2 excited state. The parametric FWM around the 5P3/2–5D5/2–6P3/2–6S1/2–5P3/2 transition loop is effectively established under phase matching conditions. The generated coherent light wavelength is confirmed using an optical spectrum analyzer, and the output power is investigated with the input laser frequency, power and atomic density. Consequently, a 1.37 µm coherent light with the power of 27 µW and the beam quality of = 1.22 and = 1.25 is obtained. An additional 795 nm laser is introduced to enhance the output power by 1.5 times through velocity-selective optical hyperfine repumping.

Quantum analysis of sub harmonic generation with two-mode coherent light

In this work the statistical and squeezing properties of light-driven by sub-harmonic generation with two-mode coherent light are studied. With interaction Hamiltonian of both two-mode coherent and sub harmonic generation, we have driven master equation of system under consideration. From the master equation, the solution of the C-number Langevin equation is derived. It helps us to solve quadrature variance, quadrature squeezing, mean, and variance of photon number for light produced by sub-harmonic generation with the two-mode coherent light state. And the result shows that; the squeezing occurs in plus quadrature with the maximum squeezing of 87%. The photon statistics of the system under consideration is subpoissonian in which both mean & variance are increasing as kappa increase.

Coherence and mixedness of neutrino oscillations in a magnetic field

The radical departure from classical physics implies quantum coherence, i.e., coherent superposition of eigenstates of Hermitian operators. In resource theory, quantum coherence is a resource for quantum operations. Typically the stochastic phenomenon induces decoherence effects. However, in the present work, we prove that nonunitary evolution leads to the generation of quantum coherence in some cases. Specifically, we consider the neutrino propagation in the dissipative environment, namely in a magnetic field with a stochastic component, and focus on neutrino flavour, spin and spin-flavour oscillations. We present exact analytical results for quantum coherence in neutrino oscillations quantified in terms of the relative entropy. Starting from an initial zero coherence state, we observe persistent oscillations of coherence during the dissipative evolution of an ultra-high energy neutrino in a random interstellar magnetic field. We found that after dissipative evolution, the initial spin-polarized state entirely “thermalizes”, and in the final steady state, the spin-up/down states have the same probabilities. On the other hand, neutrino flavour states also “thermalize”, but the populations of two flavour states do not equate to each other. The initial flavour still dominates in the final steady state.

Parametric resonance of acoustic and optical phonons in GaAs/GaAsAl quantum well in the presence of laser field

In this paper, we analytically investigated the possibility of parametric resonance of acoustic and optical phonons. We obtained a general dispersion equation for parametric amplification and transformation of phonons. The dispersions of the resonant acoustic phonon modes and the threshold amplitude of the field for acoustic phonon parametric amplification are obtained. The parametric amplification for acoustic phonons in a GaAs/GaAsAl quantum well can occur under the condition that the amplitude of the external electromagnetic field is higher than the threshold amplitude. Full Text: PDF ReferencesB. A. Glavin, V. A. Kochelap, T. L. Linnik, P. Walker, A. J. Kent and M. Henini, "Monochromatic terahertz acoustic phonon emission from piezoelectric superlattices", Journal of Physics: Conference Series, Vol 92, (2007). CrossRef O. A. C. Nunes, "Piezoelectric surface acoustical phonon amplification in graphene on a GaAs substrate", Journal of Applied Physics 115, 233715 (2014). CrossRef Yu. E. Lozovik, S. P. Merkulova, I. V. Ovchinnikov, "Sasers: resonant transitions in narrow-gap semiconductors and in exciton system in coupled quantum wells", Phys. Lett. A 282, 407-414, (2001). CrossRef R.P. Beardsley, A.V. Akimov, M. Henini and A.J. Kent, "Coherent Terahertz Sound Amplification and Spectral Line Narrowing in a Stark Ladder Superlattice", PRL 104, 085501, (2010). CrossRef Pascal Ruello, Vitalyi E. Gusev, "Physical mechanisms of coherent acoustic phonons generation by ultrafast laser action", Ultrasonics 56, 21-35, (2015). CrossRef L. Esaki, in Proc. 17th Int. Conf. Phys. Semiconductors, San Francisco, CA, Aug, J.D. Chadi and W.A. Harrison, Eds, Berlin: Springer- Verlag, 473, (1984). DirectLink W. Xu, F. M. Peeters and J. T. Devreese, "Electrophonon resonances in a quasi-two-dimensional electron system", Phys. Rev. B 48, 1562 (1993). CrossRef Tran Cong Phong, Nguyen Quang Bau, "Parametric resonance of acoustic and optical phonons in a quantum well", Journal of the Korean Physical Society, Vol. 42, No. 5, 647-651, (2003). DirectLink

A coherent harmonic generation method for producing femtosecond coherent radiation in a laser plasma accelerator based light source.

The electron beam generated in laser plasma accelerators (LPAs) has two main initial weaknesses - a large beam divergence (up to a few milliradians) and a few percent level energy spread. They reduce the beam brightness and worsen the coherence of the LPA-based light source. To achieve fully coherent radiation, several methods have been proposed for generating strong microbunching on LPA beams. In these methods, a seed laser is used to induce an angular modulation into the electron beam, and the angular modulation is converted into a strong density modulation through a beamline with nonzero longitudinal position and transverse angle coupling. In this paper, an alternative method to generate microbunching into the LPA beam by using a seed laser that induces an energy modulation and transverse-longitudinal coupling beamlines that convert the energy modulation into strong density modulation is proposed. Compared with the angular modulation methods, the proposed method can use more than one order of magnitude lower seed laser power to achieve similar radiation performance. Simulations show that with the proposed method a coherent pulse of a few microjoules pulse energy and femtosecond duration can be generated with a typical LPA beam.

Design of photonic crystal fibers with flat dispersion and three zero dispersion wavelengths for coherent supercontinuum generation in both normal and anomalous regions

A photonic crystal fiber (PCF) structure with three-zero dispersion wavelengths (ZDWs) and a flat dispersion curve was designed by adjusting the structural parameters (the diameters of small air holes inserted between standard lattices in the first and third rings) and fiber core size to obtain the required shape, number of extreme points and the dynamic range of waveguide dispersion (Dw) curve. A quantitative method which was realized by fixing the positions of the ZDWs and tailoring different dispersion slopes was proposed to design another PCF with the same ZDWs and a slightly larger dispersion slope. Both of the designed PCFs were used for contrast experiments of supercontinuum (SC) generation. The simulation results show that the flat dispersion can produce the coherent SC in both normal and anomalous dispersion regions for the designed two PCFs. The degree of flatness and the wavelength range of coherent SC are determined by the great time-domain compression due to the direct soliton spectrum tunneling (DSST) resulting from the mismatch between self-phase modulation and gradually decreasing dispersion. A wide, flat, and coherent SC with a wavelength range of 1281–2200 nm and power range of −14.8 ~ −9.4 dB was obtained for the PCF with flatter dispersion by pumping in the anomalous dispersion region with 50 fs incident pulse. This study is instructive for PCF design in different application scenarios and finds a new way for the generation of wide flat coherent spectrum.

Optimal controlled islanding considering frequency‐arresting and frequency‐stabilising constraints: A graph theory‐assisted approach

The optimal controlled islanding of power systems is a practical solution to prevent system blackouts if the boundary of islands and corrective control actions in each island are carefully specified. This paper establishes a model for the controlled islanding problem by a proposed mixed integer linear program (MILP). The frequency-arresting (FA) and frequency-stabilising (FS) constraints are linearised and incorporated in our FA-FS-constrained MILP model to prevent triggering of load shedding (LS) relays. This achievement makes our model capable of handling low-inertia networks. Intentional LS and stepwise generation curtailment are corrective actions accommodated for frequency control and power mismatch considerations. These corrective actions increase the degree of freedom and broaden the feasible space of the MILP model under the envisaged tough operating conditions. Our model's computational efficiency is improved using a proposed graph theory-based network reduction technique. The basic groups of coherent generators, determined in the offline mode, are aggregated as equivalent buses using an extended Steiner tree method. A graph-path determination technique is also proposed to generate disconnection constraints (between equivalent buses of incoherent areas) and bus-allocation constraints. Simulation results on the IEEE 39-bus test system and a 76-bus case study verify the proposed network reduction technique's effectiveness and the MILP model.

Distribution and Generation of Quantum Coherence for Gaussian States in de Sitter Space

AbstractThe distribution and generation of quantum coherence for two‐mode and multi‐mode Gaussian states in de Sitter space are studied. It is found that the quantum coherence is redistributed among the mode in different open charts under the curvature effect of de Sitter space. In particular, the Gaussian coherence for the initially correlated state is found to survive in the limit of infinite curvature, while quantum entanglement vanishes in this limit. Unlike entanglement and steering, the coherence of a massive scalar field is more robust than a massless field under the influence of curvature of de Sitter space. In addition, it is shown that the curvature generates two‐mode Gaussian state and three‐mode Gaussian state quantum coherence among the open charts, even though the observers are localized in causally disconnected regions. It is worth noting that the gravity‐generated three‐mode coherence is extremely sensitive to the curvature effect for the conformal and massless scalar fields, which may be in principle employed to design an effective detector for the space curvature.

Octave-spanning coherent supercontinuum generation in a step-index tellurite fiber and towards few-cycle pulse compression at 2 [formula omitted

We experimentally demonstrate 140-THz bandwidth (at −20 dB) supercontinuum generation in a 10 cm-long all-normal dispersion step-index tellurite fiber pumped by a turn-key femtosecond fiber laser emitting at 2.11μm at a repetition rate of 19 MHz. The soliton self-frequency shifted thulium-doped fiber mode-locked laser emits initial transform-limited pulses, with 85-fs pulse duration, that are subsequently quasi-linearly chirped (over more than 50 THz) during the above nJ-level nonlinear propagation. Moreover, we numerically demonstrate the possible pulse compression down to 12 fs by means of additional linear propagation in a standard step-index fluoride fiber with anomalous dispersion.

Spatial and Spectral Separation of Radiation Generated by Powerful Laser Diodes

A study is performed of the relationship between the number (width) of channels of unphased coherent generation and the spectral separation between these channels within the line of amplification for a powerful laser diode with wide contacts. The reasons for this separation are discussed in connection with the transformation of parameters in the active medium in proportion to the operation of the laser.

The Dynamic Construal and Generation of Abstract Coherence with the Framework of Cognitive Reference Point, Target Concept Point and the Processing Mental Path

Langacker’s model of Cognitive Reference Point (CRP) and Wang Yin’s model of Remedied Cognitive Reference Point (RCRP) could be integrated to form a cognitive trinity to construe and generate the text coherence. This trinity framework is composed of cognitive reference point, target concept point and the processing mental path (CTM) within certain semantic domain. This research tested the efficacy of CTM and found it promotes the dynamic construal and generation of local and global coherence in Abstracts of scientific papers. Activating the related information schema and integrating the mental representation with the semantic information embedded in the Abstract sentences, the three-dimensional CTM establishes a cognitive referential framework for Abstract viewers. This CTM framework assists Abstract viewers in efficiently construing and constructing the local and global coherence of Abstracts.

Enhanced coherent phonon excitation in Fe3GeTe2 via resonance Raman effect

Coherent phonon excitation via femtosecond laser pulses can be used to control physical properties of matter, and enhancing coherent phonon excitation is highly relevant. Here, we report the strong enhancement of coherent phonon excitation in ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ (FGT) via the resonance Raman effect. On the basis of the femtosecond transient optical spectroscopy measurements, the ${A}_{1\mathrm{g}}$ coherent phonon excitation in FGT is obtained as a function of pump photon energy. Its excitation can be maximized by tuning the pump photon energy. The maximum coherent phonon excitation at the 1.574-eV pump photon energy corresponds to an electronic transition in FGT, and is a direct result of the resonance Raman effect. The ${A}_{1\mathrm{g}}$ coherent phonon generation follows the impulsive stimulated Raman scattering mechanism. Our work demonstrates that the resonance Raman effect can be an effective way to enhance coherent phonon excitation via electronic excitation.

Lifetime Broadening and Impulsive Generation of Vibrational Coherence Triggered by Ultrafast Electron Transfer.

The absorption band shape of chromophores in liquid solution at room temperature is usually dominated by pure electronic dephasing dynamics, which occurs on the sub-100 fs time scale. Herein, we report on a series of dyads consisting of a naphthalenediimide (NDI) electron acceptor with one or two phenyl-based donors for which photoinduced intramolecular electron transfer is fast enough to be competitive with pure electronic dephasing. As a consequence, the absorption band of the π-π* transition of these dyads is broader than that of the NDI alone to an extent that scales with the electron transfer rate. Additionally, this reaction is so fast that it leads to the impulsive excitation of a low-frequency vibrational mode of the charge-separated product. Quantum-chemical calculations suggest that this vibration involves the C-N donor-acceptor bond, which shortens considerably upon electron transfer.

Exceptional Points as Lasing Prethresholds

AbstractThe genesis of lasing, as an evolution of the laser hybrid eigenstates comprised of electromagnetic modes and atomic polarization, is considered. It is shown that the start of coherent generation at the laser threshold is preceded by the formation of a special hybrid state at the lasing prethreshold. This special state is characterized by an enhanced coupling among excited atoms and electromagnetic modes. This leads to an increase in the rate of stimulated emission in the special state and, ultimately, to lasing. At the lasing prethreshold, the transformation of hybrid eigenstates has the features of an exceptional point (EP) observed in non‐Hermitian systems. The special state is formed when eigenfrequencies of two hybrid states coalesce or come close to each other. Below the prethreshold, lifetimes of all hybrid states grow with increasing pump rate. When the pump rate crosses the prethreshold, resonance trapping occurs with the lifetime of the special state continuing to increase while the lifetimes of all other eigenstates begin to decrease. Consequently, the latter eigenstates do not participate in the lasing. Thus, above the prethreshold, a laser transitions into the single‐mode regime.