Scalar Pulses Research Articles

Conditions for Scalar and Electromagnetic Wave Pulses to Be “Strange” or Not

Vector-valued electromagnetic waves for which the integral of the electric field over time is zero at every location in space were characterized as “usual” by Bessonov several decades ago. Otherwise, they were called “strange”. Recently, Popov and Vinogradov studied conditions leading to usual waves using a spectral representation. Their main result is that pulses of finite energy in free space are usual and, consequently, bipolar. However, they do not exclude the possibility of the existence of finite-energy strange pulses, although quite exotic, in a vacuum. Our emphasis in this article is to examine what the relevant necessary and sufficient conditions are for usual and strange waves, particularly for scalar pulses. Illustrative examples are provided, including spherical symmetric collapsing pulses, propagation-invariant, and the so-called almost undistorted spatiotemporally localized waves. Finally, source-generated strange electromagnetic fields are reported.

Direct generation of mid-infrared pulsed optical vortices at ∼ 2.7 µm

We present the first, to the best of our knowledge, direct generation of pulsed optical vortices in the 2.7-µ m spectral range by employing polycrystalline Fe:ZnSe as a saturable absorber (SA). A modified theoretical model taking into account the propagation features of the reshaped annular pump beam is elaborated to accurately determine the excitation conditions of the Laguerre–Gaussian (LG0,l) modes, yielding a lasing efficiency comparable to the fundamental TEM00 mode in continuous-wave (CW) regime. Nanosecond scalar optical vortices with well-defined handedness are successfully produced by taking advantages of designated mode-matching, high polarization extinction ratio (PER), and the "spatial filter" effect of the SA on other transverse modes. Such scalar vortex laser pulses in the mid-infrared region will enable new applications such as frequency down conversion to produce optical vortices at longer (far-infrared) wavelengths, structuring organic materials, novel molecular spectroscopy, etc.

Microfiber-Knot-Resonator-Induced Energy Transferring From Vector Noise-Like Pulse to Scalar Soliton Rains in an Erbium-Doped Fiber Laser

We report on scalar soliton rains (SRs) shedding from vector square-wave noise-like pulse (NLP) by introducing a microfiber-knot-resonator (MKR) into an erbium-doped fiber (EDF) laser. The EDF laser has an ultralong cavity of ∼3.07 km and a large anomalous cavity-dispersion, facilitating the formation of NLP. Once an MKR was further incorporated, comb-like filtering effect could be clearly observed on the emission spectrum. Apart from that, SRs shedding from the NLP were observed, which resulted from the MKR-induced energy transferring effect from the NLP to SRs. It was further found that the SRs existed only along a particular polarization direction, whereas the NLPs could be found in any pair of orthogonal polarization directions, despite some differences in pulse peak power. These suggest that the SRs possess scalar feature, whereas the co-circulating NLP is a package of vector pulses. Our results represent the first observation of MKR-induced pulse energy transferring effect and also the first observation of the co-existence of scalar and vector pulses in a single fiber system. Besides enriching the dynamics and interactions of optical pulses in fiber lasers, our results extend the applications of MKR to the area of pulse manipulations through spectral tailoring and local nonlinearity engineering.

Colliding scalar pulses in the Einstein-Gauss-Bonnet gravity

We numerically investigated how the nonlinear dynamics depends on the dimensionality and on the higher-order curvature corrections in the form of Gauss-Bonnet (GB) terms, with a model of colliding scalar pulses in plane-symmetric space-time. We observed that a collision of large scalar pulses will produce a large-curvature region, of which the magnitude depends on αGB. The normal corrections (αGB > 0) work for avoiding the appearance of singularity, although it is inevitable.

Nonlinear dynamics in the Einstein-Gauss-Bonnet gravity

We numerically investigated how the nonlinear dynamics depends on the dimensionality and on the higher-order curvature corrections in the form of Gauss-Bonnet (GB) terms. We especially monitored the processes of appearances of a singularity (or black hole) in two models: (i) a perturbed wormhole throat in spherically symmetric space-time, and (ii) colliding scalar pulses in plane-symmetric space-time. We used a dual-null formulation for evolving the field equations, which enables us to locate the trapping horizons directly, and also enables us to follow close to the large-curvature region due to its causal integrating scheme. We observed that the fate of a perturbed wormhole is either a black hole or an expanding throat depending on the total energy of the structure, and its threshold depends on the coupling constant of the GB terms ($\alpha_{\rm GB}$). We also observed that a collision of large scalar pulses will produce a large-curvature region, of which the magnitude also depends on $\alpha_{\rm GB}$. For both models, the normal corrections ($\alpha_{\rm GB}>0$) work for avoiding the appearance of singularity, although it is inevitable. We also found that in the critical situation for forming a black hole, the existence of the trapped region in the Einstein-GB gravity does not directly indicate the formation of a black hole.

All-Polarization Maintaining Passively Mode-Locked Fiber Laser Using Evanescent Field Interaction With Single-Walled Carbon Nanotube Saturable Absorber

We implemented a single-walled carbon nanotube (SWCNT)-based saturable absorber (SA) that evanescently interacts with light on a side-polished polarization-maintaining fiber (PMF) platform. A high-quality SWCNT/polymer composite was spin-coated onto a side-polished PMF embedded in a quartz block, where the slow optical birefringence axis of the PMF was perpendicular to the polished surface. The fabricated in-line SA had an insertion loss of -1.5 dB with a modulation depth of 0.5% for the polarization direction associated with the transvers magnetic mode. An all-PMF laser was built using the fabricated in-line SA to demonstrate self-starting, turn-key operation of Er-doped, all-fiber lasers delivering stable scalar soliton pulses. The measured 3-dB spectral bandwidth and pulse duration of the laser output were 5.1 nm and 510 fs, respectively, resulting in a time-bandwidth product of 0.320. We examined the polarization state of the soliton fiber laser output and found that the laser stably generated linearly polarized ultrashort pulses with a polarization extinction ratio of -18.4 dB.

Universal form of the carrier frequency of scalar and vector paraxialXwaves with orbital angular momentum and arbitrary frequency spectrum

We report on the dependence of the carrier frequency of a nondiffracting optical pulse on the amount of orbital angular momentum it carries. We provide a unified universal form of such a dependence for the cases of both scalar and vector pulses with arbitrary frequency spectra. For the case of paraxial optical pulses we consider two different examples, namely, pulses with exponentially decaying spectra and Gaussian spectra.

Scalar Nanosecond Pulse Generation in a Nanotube Mode-Locked Environmentally Stable Fiber Laser

We report an environmentally stable nanotube mode-locked fibre laser producing linearly-polarized, nanosecond pulses. A simple all-polarization-maintaining fibre ring cavity is used, including 300 m of highly nonlinear fibre to elongate the cavity and increase intracavity dispersion and nonlinearity. The laser generates scalar pulses with a duration of 1.23 ns at a centre wavelength of 1042 nm, with 1.3-nm bandwidth and at 641-kHz repetition rate. Despite the long cavity, the output characteristics show no significant variation when the cavity is perturbed, and the degree of polarization remains at 97%.

The role of light-induced nanostructures in femtosecond laser micromachining with vector and scalar pulses

In this article we compare the results of micromachining of fused silica and silicon with tightly focused scalar (viz., circularly and linearly polarized) and vector (viz., azimuthally and radially polarized) femtosecond laser pulses. We show that drilling with radially polarized pulses produces holes with smoother and better-delineated walls compared with the other polarizations used, whereas linearly polarized pulses can machine 20-nm wide single grooves in fused silica when the electric field of the pulse is aligned perpendicular to the cutting direction. The observed polarization-controlled micromachining is due to the formation of sub-diffraction-limited nanostructures that are optically produced in the multi-pulse irradiation regime.

Scalar soliton generation in all-polarization-maintaining, graphene mode-locked fiber laser

All-polarization-maintaining, self-starting Erbium-doped fiber laser based on graphene-saturable absorber is presented. Scalar soliton pulses with 570 fs duration and 114.1 MHz repetition rate were achieved at a 1557 nm center wavelength and with 6 nm bandwidth. The graphene-saturable absorber was formed by mechanical exfoliation of pure graphite. The laser was environmentally stable and could operate for long periods of time with linearly polarized output and degree of polarization at the level of 98%.

Fractional Fourier transform in temporal ghost imaging with classical light

We investigate temporal, second-order classical ghost imaging with long, incoherent, scalar plane-wave pulses. We prove that in rather general conditions, the intensity correlation function at the output of the setup is given by the fractional Fourier transform of the temporal object. In special cases, the correlation function is shown to reduce to the ordinary Fourier transform and the temporal image of the object. Effects influencing the visibility and the resolution are considered. This work extends certain known results on spatial ghost imaging into the time domain and could find applications in temporal tomography of pulses.

Responses of the Brans–Dicke field due to gravitational collapses

We study responses of the Brans–Dicke field due to gravitational collapses of scalar field pulses using numerical simulations. Double-null formalism is employed to implement the numerical simulations. If we supply a scalar field pulse, it will asymptotically form a black hole via dynamical interactions of the Brans–Dicke field. Hence, we can observe the responses of the Brans–Dicke field by two different regions. First, we observe the late time behaviors after the gravitational collapse, which include formations of a singularity and an apparent horizon. Second, we observe the fully dynamical behaviors during the gravitational collapse and view the energy–momentum tensor components. For the late time behaviors, if the Brans–Dicke coupling is greater (or smaller) than −1.5, the Brans–Dicke field decreases (or increases) during the gravitational collapse. Since the Brans–Dicke field should be relaxed to the asymptotic value with the elapse of time, the final apparent horizon becomes time-like (or space-like). For the dynamical behaviors, we observed the energy–momentum tensors around ω ∼ −1.5. If the Brans–Dicke coupling is greater than −1.5, the Tuu component can be negative at the outside of the black hole. This can allow an instantaneous inflating region during the gravitational collapse. If the Brans–Dicke coupling is less than −1.5, the oscillation of the Tvv component allows the apparent horizon to shrink. This allows a combination that violates weak cosmic censorship. Finally, we discuss the implications of the violation of the null energy condition and weak cosmic censorship.

Polarized optical pulses in a medium with third- and fifth-order nonlinearities

The dynamics of polarized solitary optical waves in a medium characterized by nonlinear susceptibilities of the third and fifth orders (χ3 > 0, χ5 < 0) is studied by numerical methods. The interaction of scalar pulses with the maximum amplitude leads to the creation of a coupled state of two solitary waves which oscillates during the propagation into the depth of the nonlinear medium. As a result of the inelastic collision of two circularly polarized steady-state pulses, a series of secondary pulses of different degrees of ellipticity appears.

Diffraction of Scalar Pulses at Plane Apertures: A Different Approach

We generalize to scalar pulses with finite duration a previous work [1] in which a new approach to diffraction at plane apertures is developed for scalar harmonic waves. A particular attention is given to rectangular pulse modulated signals for which an exact solution to the diffraction problem is obtained. As an example, the diffraction of a truncated harmonic pulse is investigated and the numerical problems to be solved are discussed with an important simplification when one is only interested in the diffraction pattern far from the aperture. More works are needed for apertures with no simple geometrical form.

Diffraction of Scalar Pulses At Plane Apertures: a Different Approach - Abstract

We generalize to scalar pulses with finite duration a previous work [1] in which a new approach to diffraction at plane apertures is developed for scalar harmonic waves. A particular attention is given to rectangular pulse modulated signals for which an exact solution to the diffraction problem is obtained. As an example, the diffraction of a truncated harmonic pulse is investigated and the numerical problems to be solved are discussed with an important simplification when one is only interested in the diffraction pattern far from the aperture. More works are needed for apertures with no simple geometrical form.

Dynamics of scalar fields in the background of rotating black holes. II. A note on superradiance

We analyze the amplification due to so-called superradiance from the scattering of pulses off rotating black holes as a numerical time evolution problem. We consider the "worst possible case" of scalar field pulses for which superradiance effects yield amplifications $< 1%$. We show that this small effect can be isolated by numerically evolving quasi-monochromatic, modulated pulses with a recently developed Teukolsky code. The results show that it is possible to study superradiance in the time domain, but only if the initial data is carefully tuned. This illustrates the intrinsic difficulties of detecting superradiance in more general evolution scenarios.

On the gravitational interaction of plane symmetric clouds of null dust

Plane symmetric solutions of the Einstein field equations are considered, solutions that represent the collision of oppositely moving clouds of initially unidentified massless particles—clouds of ‘‘null dust.’’ In terms of specific examples it is shown that the corresponding Cauchy problem remains ambiguous even when the Riemann tensor is free of any kind of discontinuity. By solving the corresponding Einstein–Klein–Gordon and Einstein–Maxwell–Weyl field equations the ambiguity is resolved and space-time models are constructed representing, among others, various types of collisions between a pair of scalar wave pulses as well as of a pulse of electromagnetic waves with a cloud of neutrinos.