arXiv subjects

Bruno Macke

Publications and source records attributed to Bruno Macke.

At least 19 recordsLinked to original sources

arXiv2020

Material slow and fast light in a zero-dispersion configuration

We study the propagation of light pulses in an absorbing medium when the frequency of their carrier coincides with a zero of the refractive index dispersion. Although slow light and, a fortiori, fast light are not expected in such conditions, we show that both can be obtained by selecting particular phase-components of the transmitted field. Analytical expressions of the resulting signals are obtained by a procedure of periodic continuation of the incident pulse and a proof of principle of the predicted phenomena is performed by means of a very simple electrical network, the transfer function of which mimics that of the medium.

physics.optics↗

arXiv2019

Comment on "Gain-assisted superluminal propagation and rotary drag of photon and surface polaritons"

In their study of superluminal propagation, rotary drag and surface polaritons [Phys. Rev. A 96, 013848 and 049906(E) (2017)], Khan et al. consider a four-level atomic arrangement with transitions in the optical domain. In fact, the values they give to the parameters lead to a probe wavelength lying in the decimeter band and we point out that, in such conditions, all their results are irrelevant.

physics.optics↗

arXiv2019

Comment on ''Inverse Doppler shift and control field as coherence generators for the stability in superluminal light''

In their study of inverse Doppler shift and superluminal light [Phys. Rev. A 91, 053807 (2015)], Ghafoor et al. consider a three-level atomic arrangement with transitions in the optical domain. In fact, the values they give to the parameters lead to a probe wavelength lying in the decimeter band. We point out that the Doppler shifts are then negligible and remark that the simulations performed by Ghafoor et al. do not evidence any superluminal effect.

physics.optics↗

arXiv2018

On-resonance material fast light

We theoretically revisit the problem of the propagation of coherent light pulses through a linear medium when the carrier frequency of the pulses coincides with the minimum of a narrow dip in the medium transmission. Considering realistic contrasts between the maximum and minimum transmission of the medium and incident pulses of strictly finite duration, we combine temporal and spectral approaches to obtain analytical expressions of the transmitted pulse that reproduce the main features of the exact numerical solutions derived by fast Fourier transform. A special attention is paid to the advance of the pulse maximum over that of a pulse covering the same distance in vacuum and the ratio of this advance to the pulse duration (fractional advance) is optimized.

physics.optics↗

arXiv2017

Slow light in saturable absorbers : Progress in the resolution of a controversy

There are two opposing models in the analysis of the slow transmission of light pulses through saturable absorbers. The canonical incoherent bleaching model simply explains the slow transmission by combined effects of saturation and of non-instantaneous response of the medium resulting in absorption of the front part of the incident pulse larger than that of its rear. The second model, referred to as the coherent-population-oscillations (CPO) model, considers light beams whose intensity is slightly pulse modulated and attributes the time delay of the transmitted pulse to a reduction of the group velocity. We point out some inconsistencies in the CPO model and show that the two models lie in reality on the same hypotheses, the equations derived in the duly rectified CPO model being local expressions of the integral equations obtained in the incoherent bleaching model. For large modulation index, the CPO model, based on linearized equations, breaks down. The incoherent bleaching model then predicts that the transmitted light should vanish when the intensity of the incident light is strictly zero. This point is confirmed by the experiments that we have performed on ruby with square-wave incident pulses and we show that the whole shape of the observed pulses agrees with that derived analytically by means of the incoherent bleaching model. We also determine in this model the corresponding evolution of the fluorescence light, which seems to have been evidenced in other experiments.

physics.optics↗

arXiv2016

Simultaneous slow and fast light involving the Faraday effect

We theoretically study the linear transmission of linearly polarizedlight pulses in an ensemble of cold atoms submitted to a static magneticfield parallel to the direction of propagation. The carrier frequencyof the incident pulses coincides with a resonance frequency of theatoms. The transmitted light, the electric field of which is transversal,is examined in the polarizations parallel and perpendicular to thatof the incident pulses. We give explicit analytic expressions for thetransfer functions of the system for both polarizations and for thecorresponding group delays. We demonstrate that slow light can beobserved in a polarization, whereas fast light is simultaneously observedin the perpendicular polarization. Moreover, we point out that, dueto the polarization post selection, the system is not necessarilyminimum phase shift. Slow light can then be obtained in situationswhere an irrelevant application of the Kramers-Kronig relations could lead oneto expect fast light. When the incident light is step modulated, wefinally show that, in suitable conditions, the system enables oneto separate optical precursor and main field.

physics.optics↗

arXiv2016

Comment on "Gain-assisted superluminal light propagation through a Bose-Einstein condensate cavity system"

In a recent theoretical article [Eur. Phys. J. D 70, 1 (2016)], Kazemi et al. claim to have demonstrated superluminal light transmission in an optomechanical system where a Bose-Einstein condensate serves as the mechanical oscillator. In fact the superluminal propagation is only inferred from the existence of a minimum of transmission of the system at the probe frequency. This condition is not sufficient and we show that, in all the cases where superluminal propagation is claimed by Kazemi et al., the propagation is in reality subluminal. Moreover, we point out that the system under consideration is not minimum-phase-shift. The Kramers-Kronig relations then only fix a lower limit to the group delay and we show that these two quantities have sometimes opposite signs.

physics.optics↗

arXiv2015

Brillouin precursors in Debye media

We theoretically study the formation of Brillouin precursors in Debye media. We point out that the precursors are visible only at propagation distances such that the impulse response of the medium is essentially determined by the frequency dependence of its absorption and is practically Gaussian. By simple convolution, we then obtain explicit analytical expressions of the transmitted waves generated by reference incident waves, distinguishing precursor and main signal by a simple examination of the long-time behavior of the overall signal. These expressions are in good agreement with the signals obtained in numerical or real experiments performed on water in the radio-frequency domain and explain in particular some observed shapes of the precursor. Results are obtained for other remarkable incident waves. In addition, we show quite generally that the shape of the Brillouin precursor appearing alone at sufficiently large propagation distance and the law giving its amplitude as a function of this distance do not depend on the precise form of the incident wave but only on its integral properties. The incidence of a static conductivity of the medium is also examined and explicit analytical results are again given in the limit of weak and strong conductivities.

physics.optics↗

arXiv2013

From Sommerfeld and Brillouin forerunners to optical precursors

The Sommerfeld and Brillouin forerunners generated in a single-resonance absorbing medium by an incident step-modulated pulse are theoretically considered in the double limit where the susceptibility of the medium is weak and the resonance is narrow. Combining direct Laplace-Fourier integration and calculations by the saddle-point method, we establish an explicit analytical expression of the transmitted field valid at any time, even when the two forerunners significantly overlap. We examine how their complete overlapping, occurring for shorter propagation distances, originates the formation of the unique transient currently named resonant precursor or dynamical beat. We obtain an expression of this transient identical to that usually derived within the slowly varying envelope in spite of the initial discontinuity of the incident field envelope. The dynamical beats and 0Pi pulses generated by ultra-short incident pulses are also briefly examined

physics.optics↗

arXiv2012

Simple asymptotic forms for Sommerfeld and Brillouin precursors

This article mainly deals with the propagation of step-modulated light pulses in a dense Lorentz-medium at distances such that the medium is opaque in a broad spectral region including the carrier frequency. The transmitted field is then reduced to the celebrated precursors of Sommerfeld and Brillouin, far apart from each other. We obtain simple analytical expressions of the first (Sommerfeld) precursor whose shape only depends on the order of the initial discontinuity of the incident field and whose amplitude rapidly decreases with this order (rise-time effects). We show that, in a strictly speaking asymptotic limit, the second (Brillouin) precursor is entirely determined by the frequency-dependence of the medium attenuation and has a Gaussian or Gaussian-derivative shape. We point out that this result applies to the precursor directly observed in a Debye medium at decimetric wavelengths. When attenuation and group-delay dispersion both contribute to its formation, we establish a more general expression of the Brillouin precursor, containing the previous one (dominant-attenuation limit) and that obtained by Brillouin (dominant-dispersion limit) as particular cases. We finally study the propagation of square or Gaussian pulses and we determine the pulse parameters optimizing the Brillouin precursor. Obtained by standard Laplace-Fourier procedures, our results are explicit and contrast by their simplicity from those derived by the uniform saddle point methods, from which it is very difficult to retrieve our asymptotic forms.

physics.optics↗

arXiv2011

Optical precursors in the singular and weak dispersion limits: Reply to reply to comment

The reply by Oughstun et al. [J. Opt. Soc. Am. B 28, 468-469 (2011)] to our comment [J. Opt. Soc. Am. B 28, 450-452 (2011)] on their recently published paper [J. Opt. Soc. Am. B 27, 1664-1670 (2010)] is shown to make no response to the main points raised in our comment and even to bring additional confusion. The present reply provides the necessary clarifications.

physics.optics↗

arXiv2011

Comment on "Optical precursors in the singular and weak dispersion limits"

We point out inconsistencies in the recent paper by Oughstun et al. on Sommerfeld and Brillouin precursors [J. Opt. Soc. Am. B 27, 1664-1670 (2010)]. Their study is essentially numerical and, for the parameters used in their simulations, the difference between the two limits considered is not as clear-cut as they state. The steep rise of the Brillouin precursor obtained in the singular limit and analyzed as a distinguishing feature of this limit simply results from an unsuitable time scale. In fact, the rise of the precursor is progressive and is perfectly described by a Airy function. In the weak dispersion limit, the equivalence relation, established at great length in Section 3 of the paper, appears as an immediate result in the retarded-time picture. Last but not least, we show that, contrary to the authors claim, the precursors are catastrophically affected by the rise-time of the incident optical field, even when the latter is considerably faster than the medium relaxation time.

physics.optics↗

arXiv2010

From fast to slow light in a resonantly driven absorbing medium

We theoretically study the propagation through a resonant absorbing medium of a time-dependent perturbation modulating the amplitude of a continuous wave (cw). Modeling the medium as a system of two-level atoms and linearizing the Maxwell-Bloch equations for the perturbation, we establish an exact analytical expression of the transfer function relating the Fourier transforms of the incident and transmitted perturbations. It directly gives the gain and the phase shift undergone in the medium by a harmonic modulation. For the case of a pulse modulation, it enables us to determine the transmission time of the pulse center-of-mass (group delay), evidencing the relative contributions of the coherent and incoherent (population) relaxation. We show that the group delay has a negative value (fast light) fixed by the coherent effects when the cw intensity is small compared to the saturation intensity and becomes positive (slow light) when this intensity increases, before attaining a maximum that cannot exceed the population relaxation time. The analytical results are completed by numerical determinations of the shape of the transmitted pulses in the different regimes.

physics.optics↗

arXiv2009

Optical Precursors with Self-induced Transparency

Optical Sommerfeld-Brillouin precursors significantly ahead of a main field of comparable amplitude have been recently observed in an opaque medium with an electromagnetically induced transparency window [Wei et al., Phys. Rev. Lett. 103, 093602 (2009)]. We theoretically analyze in this paper the somewhat similar results obtained when the transparency is induced by the propagating field itself and we establish an approximate analytic expression of the time-delay of the main-field arrival, which fits fairly well the result obtained by numerically solving the Maxwell-Bloch equations.

physics.optics↗

arXiv2009

Optical precursors in transparent media

We theoretically study the linear propagation of a stepwise pulse through a dilute dispersive medium when the frequency of the optical carrier coincides with the center of a natural or electromagnetically induced transparency window of the medium (slow-light systems). We obtain fully analytical expressions of the entirety of the step response and show that, for parameters representative of real experiments, Sommerfeld-Brillouin precursors, main field and second precursors "postcursors" can be distinctly observed, all with amplitudes comparable to that of the incident step. This behavior strongly contrasts with that of the systems generally considered up to now.

physics.optics↗

arXiv2008

Slow light in saturable absorbers

In connection with the experiments recently achieved on doped crystals, biological samples, doped optical fibers and semiconductor heterostructures, we revisit the theory of the propagation of a pulse-modulated light in a saturable absorber. Explicit analytical expressions of the transmitted pulse are obtained, enabling us to determine the parameters optimizing the time-delay of the transmitted pulse with respect to the incident pulse. We finally compare the maximum fractional delay or figure of merit so attainable to those which have been actually demonstrated in the experiments.

physics.optics↗

arXiv2007

Two-pulse interference and superluminality

We examine how the interference of a coherent light-pulse with its slightly time-delayed copy may generate a pulse nearly identical to the original one and ahead of it. The simplicity of this 2-pulse system enabled us to obtain exact analytic expressions of the pulse distortion, valid for a wide class of pulse shapes. Explicit results are given for the pulses usually considered (gaussian, hyperbolic secant) but also for more realistic pulses of strictly limited duration. We finally show that the efficiency of the 2-pulse system is comparable to that of the other superluminal systems, at least for the pulse advancements actually demonstrated in the optical experiments.

physics.optics↗

arXiv2006

Comment on "Direct Observation of Optical Precursors in a Region of Anomalous Dispersion"

We show that the optical transients recently read as "Sommerfeld-Brillouin precursors" by Jeong, Dawes and Gauthier are not identifiable with optical precursors (unless one considers that any coherent transient propagating in a dilute medium at the velocity c is an optical precursor) and that they can be interpreted in very simple physical terms.

physics.optics↗