80 research outputs found
Control of vortex orientation of ultrashort optical pulses using spatial chirp
Introducing a spatial chirp into a pulse with a longitudinal vortex, such as
a standard pulsed Laguerre-Gauss beam, results in a vortex pulse with an
arbitrary orientation of the line phase singularity between longitudinal and
transverse, depending on the amount of chirp. Analytical expressions are given
for such pulses with arbitrary topological charge valid at any propagation
distance.Comment: 5 pages, 6 figures, submitted for peer revie
Transverse orbital angular momentum imparted upon focusing spatiotemporally coupled ultrashort pulses
A simple focusing system such as a lens or a spherical mirror imparts
intrinsic transverse orbital angular momentum (OAM) to spatiotemporal (ST)
coupled fields with ST covariance, e.g., a tilted pulse. This extremely
simplifies the experimental setups used to date to impart transverse OAM. The
focused fields with transverse OAM include canonical, elliptical ST vortices
and rotating pulses without any ST phase singularity such as the "lighthouse"
pulse, whose OAM content has gone unnoticed. Closed-form expressions for these
fields in full space and time are provided.Comment: 5 pages, 4 figures, submitted for peer revie
Impact of feedback time-distribution on laser dynamics
Time-distributed optical feedback in semiconductor lasers has gained
attention for its ability to produce high-quality chaos and effectively
suppress the time-delay signature. However, the fundamental impact of the
distribution of feedback in time on laser dynamics remains unexplored. In this
paper, we investigate this topic by using fiber Bragg grating (FBG) feedback.
We theoretically study the laser response using FBGs of different lengths but
similar reflectivity, effectively stretching the impulse response over a longer
period while maintaining its overall shape. We observe that above a critical
value corresponding to a grating length of approximately \,cm, fluctuations
in laser stability emerge. We attribute this phenomenon to the damping of
relaxation oscillations when the zeros of the FBG reflectivity spectrum align
with the laser side lobes around the relaxation oscillation frequency. We also
uncover an asymmetrical dynamical behavior of the laser for positive and
negative frequency detuning. We deduce that this asymmetry is a characteristic
feature of FBG feedback and delve into the specificities that trigger such
behavior.Comment: 11 pages, 13 figures, submitte
Spatiotemporal Modeling of Direct Acceleration with High-Field Terahertz Pulses
We present an improved model for electron acceleration in vacuum with high-energy THz pulses that includes spatiotemporal effects. In our calculations, we examined the acceleration with 300 GHz and 3.0 THz central frequency THz pulses with properties corresponding to common sources, and compared the Gaussian and Poisson spectral amplitudes and the associated time profiles of the electric fields. Our calculation takes into account both the longitudinal field and the spatio-spectral evolution around the focus. These aspects of the model are necessary due to the tight focusing and the duration towards a single-cycle of the THz pulses, respectively. The carrier-to-envelope phase (CEP) and the tilting angle of the coincident few- or single-cycle THz pulses must be tuned in all cases in order to optimize the acceleration scheme. We reveal additionally that electron beams with different final energies and different divergences can be generated based on simulated THz pulses having different Porras factors, describing the frequency dependence of the spatiotemporal amplitude profile, which may depend strongly on the method used to generate the single-cycle THz pulses.info:eu-repo/semantics/publishe
Spectral Phase Control of Interfering Chirped Pulses for High-Energy Narrowband Terahertz Generation
Highly-efficient optical generation of narrowband terahertz (THz) radiation
enables unexplored technologies and sciences from compact electron acceleration
to charge manipulation in solids. State-of-the-art conversion efficiencies are
currently achieved using difference-frequency generation (DFG) driven by
temporal beating of chirped pulses but remain, however, far lower than desired
or predicted. Here we show that high-order spectral phase fundamentally limits
the efficiency of narrowband DFG using chirped-pulse beating and resolve this
limitation by introducing a novel technique based on tuning the relative
spectral phase of the pulses. For optical terahertz generation, we demonstrate
a 13-fold enhancement in conversion efficiency for 1%-bandwidth, 0.361 THz
pulses, yielding a record energy of 0.6 mJ and exceeding previous
optically-generated energies by over an order of magnitude. Our results prove
the feasibility of millijoule-scale applications like terahertz-based electron
accelerators and light sources and solve the long-standing problem of temporal
irregularities in the pulse trains generated by interfering chirped pulses.Comment: 25 pages, 5 figures, updated to the state before review at Nature
Communications (updated the affiliations, title, some content, methods, etc.
Influence of contouring the lithium metal/solid electrolyte interface on the critical current for dendrites
Contouring or structuring of the lithium/ceramic electrolyte interface and therefore increasing its surface area has been considered as a possible strategy to increase the charging current in solid-state batteries without lithium dendrite formation and short-circuit. By coupling together lithium deposition kinetics and the me chanics of lithium creep within calculations of the current distribution at the interface, and leveraging a model for lithium dendrite growth, we show that efforts to avoid dendrites on charging by increasing the interfacial surface area come with significant limitations associated with the topography of rough surfaces. These limitations are sufficiently severe such that it is very unlikely contouring could increase charging currents while avoiding dendrites and short-circuit to the levels required. For example, we show a sinusoidal surface topography can only raise the charging current before dendrites occur by approx. 50% over a flat interface
Roadmap on spatiotemporal light fields
Spatiotemporal sculpturing of light pulse with ultimately sophisticated
structures represents the holy grail of the human everlasting pursue of
ultrafast information transmission and processing as well as ultra-intense
energy concentration and extraction. It also holds the key to unlock new
extraordinary fundamental physical effects. Traditionally, spatiotemporal light
pulses are always treated as spatiotemporally separable wave packet as solution
of the Maxwell's equations. In the past decade, however, more generalized forms
of spatiotemporally nonseparable solution started to emerge with growing
importance for their striking physical effects. This roadmap intends to
highlight the recent advances in the creation and control of increasingly
complex spatiotemporally sculptured pulses, from spatiotemporally separable to
complex nonseparable states, with diverse geometric and topological structures,
presenting a bird's eye viewpoint on the zoology of spatiotemporal light fields
and the outlook of future trends and open challenges.Comment: This is the version of the article before peer review or editing, as
submitted by an author to Journal of Optics. IOP Publishing Ltd is not
responsible for any errors or omissions in this version of the manuscript or
any version derived from i
Magnetorheology in an aging, yield stress matrix fluid
Field-induced static and dynamic yield stresses are explored for magnetorheological (MR) suspensions in an aging, yield stress matrix fluid composed of an aqueous dispersion of Laponite® clay. Using a custom-built magnetorheometry fixture, the MR response is studied for magnetic field strengths up to 1 T and magnetic particle concentrations up to 30 v%. The yield stress of the matrix fluid, which serves to inhibit sedimentation of dispersed carbonyl iron magnetic microparticles, is found to have a negligible effect on the field-induced static yield stress for sufficient applied fields, and good agreement is observed between field-induced static and dynamic yield stresses for all but the lowest field strengths and particle concentrations. These results, which generally imply a dominance of inter-particle dipolar interactions over the matrix fluid yield stress, are analyzed by considering a dimensionless magnetic yield parameter that quantifies the balance of stresses on particles. By characterizing the applied magnetic field in terms of the average particle magnetization, a rheological master curve is generated for the field-induced static yield stress that indicates a concentration–magnetization superposition. The results presented herein will provide guidance to formulators of MR fluids and designers of MR devices who require a field-induced static yield stress and a dispersion that is essentially indefinitely stable to sedimentation.Petroleum Research Fund (ACS-PRF Grant No. 49956-ND9)American Chemical Society (ACS-PRF Grant No. 49956-ND9
Hard-Object Feeding in Sooty Mangabeys (Cercocebus atys) and Interpretation of Early Hominin Feeding Ecology
Morphology of the dentofacial complex of early hominins has figured prominently in the inference of their dietary adaptations. Recent theoretical analysis of craniofacial morphology of Australopithecus africanus proposes that skull form in this taxon represents adaptation to feeding on large, hard objects. A modern analog for this specific dietary specialization is provided by the West African sooty mangabey, Cercocebus atys. This species habitually feeds on the large, exceptionally hard nuts of Sacoglottis gabonensis, stereotypically crushing the seed casings using their premolars and molars. This type of behavior has been inferred for A. africanus based on mathematical stress analysis and aspects of dental wear and morphology. While postcanine megadontia, premolar enlargement and thick molar enamel characterize both A. africanus and C. atys, these features are not universally associated with durophagy among living anthropoids. Occlusal microwear analysis reveals complex microwear textures in C. atys unlike those observed in A. africanus, but more closely resembling textures observed in Paranthropus robustus. Since sooty mangabeys process hard objects in a manner similar to that proposed for A. africanus, yet do so without the craniofacial buttressing characteristic of this hominin, it follows that derived features of the australopith skull are sufficient but not necessary for the consumption of large, hard objects. The adaptive significance of australopith craniofacial morphology may instead be related to the toughness, rather than the hardness, of ingested foods
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