arXiv subjects

Denis Gagnon

Publications and source records attributed to Denis Gagnon.

18 recordsLinked to original sources

arXiv2018

Pulse shaping in the terahertz frequency range for the control of photo-excited carriers in graphene

The shape of a few-cycle terahertz (THz) laser pulse can be optimized to provide control over conduction band populations in graphene. To demonstrate this control in a theoretical way, a spectral parametrization of the driving pulse using $B$-splines is used in order to obtain experimentally realistic pulses of bandwidth $\sim$ 30 THz. Optimization of the spectral shape is performed via differential evolution, using the $B$-splines expansion coefficients as decision variables. Numerical results show the possibility of changing the carrier density in graphene by a factor of 4 for a fixed pulse energy. In addition, we show that it is possible to selectively suppress or enhance multi-photon absorption features by optimizing over narrow windows in reciprocal space. The application of pulse shaping to the control of scattering mechanisms in graphene is also discussed.

cond-mat.mes-hall↗

arXiv2017

Suppression of multiphoton resonances in driven quantum systems via pulse shape optimization

This Letter demonstrates control over multiphoton absorption processes in driven two-level systems, which include for example superconducting qubits or laser-irradiated graphene, through spectral shaping of the driving pulse. Starting from calculations based on Floquet theory, we use differential evolution, a general purpose optimization algorithm, to find the Fourier coefficients of the driving function that suppress a given multiphoton resonance in the strong field-regime. We show that the suppression of the transition probability is due to the coherent superposition of high-order Fourier harmonics which closes the dynamical gap between the Floquet states of the two-level system.

quant-ph↗

arXiv2017

Pulse shape optimization for electron-positron production in rotating fields

We optimize the pulse shape and polarization of time-dependent electric fields to maximize the production of electron-positron pairs via strong field quantum electrodynamics processes. The pulse is parametrized in Fourier space by a B-spline polynomial basis, which results in a relatively low-dimensional parameter space while still allowing for a large number of electric field modes. The optimization is performed by using a parallel implementation of the differential evolution, one of the most efficient metaheuristic algorithms. The computational performance of the numerical method and the results on pair production are compared with a local multistart optimization algorithm. These techniques allow us to determine the pulse shape and field polarization that maximize the number of produced pairs in computationally accessible regimes.

hep-ph↗

arXiv2017

Efficiently parallelized modeling of tightly focused, large bandwidth laser pulses

The Stratton-Chu integral representation of electromagnetic fields is used to study the spatio-temporal properties of large bandwidth laser pulses focused by high numerical aperture mirrors. We review the formal aspects of the derivation of diffraction integrals from the Stratton-Chu representation and discuss the use of the Hadamard finite part in the derivation of the physical optics approximation. By analyzing the formulation we show that, for the specific case of a parabolic mirror, the integrands involved in the description of the reflected field near the focal spot do not possess the strong oscillations characteristic of diffraction integrals. Consequently, the integrals can be evaluated with simple and efficient quadrature methods rather than with specialized, more costly approaches. We report on the development of an efficiently parallelized algorithm that evaluates the Stratton-Chu diffraction integrals for incident fields of arbitrary temporal and spatial dependence. This method has the advantage that its input is the unfocused field coming from the laser chain, which is experimentally known with high accuracy. We use our method to show that the reflection of a linearly polarized Gaussian beam of femtosecond duration off a high numerical aperture parabolic mirror induces ellipticity in the dominant field components and generates strong longitudinal components. We also estimate that future high-power laser facilities may reach intensities of $10^{24}\,\text{W/cm}^2$.

physics.comp-ph↗

arXiv2016

Coherent destruction of tunneling in graphene irradiated by elliptically polarized lasers

Photo-induced transition probabilities in graphene are studied theoretically from the viewpoint of Floquet theory. Conduction band populations are computed for a strongly, periodically driven graphene sheet under linear, circular, and elliptic polarization. Features of the momentum spectrum of excited quasi-particles can be directly related to the avoided crossing of the Floquet quasi-energy levels. In particular, the impact of the ellipticity and the strength of the laser excitation on the avoided crossing structure -- and on the resulting transition probabilities -- is studied. It is shown that the ellipticity provides an additional control parameter over the phenomenon of coherent destruction of tunneling in graphene, allowing one to selectively suppress multiphoton resonances.

cond-mat.mes-hall↗

arXiv2016

Driven quantum tunneling and pair creation with graphene Landau levels

Driven tunneling between graphene Landau levels is theoretically linked to the process of pair creation from vacuum, a prediction of quantum electrodynamics (QED). Landau levels are created by the presence of a strong, constant, quantizing magnetic field perpendicular to a graphene mono-layer. Following the formal analogy between QED and the description of low-energy excitations in graphene, solutions of the fully interacting Dirac equation are used to compute electron-hole pair creation driven by a circularly or linearly polarized field. This is achieved via the coupled channel method, a numerical scheme for the solution of the time-dependent Dirac equation in the presence of bound states. The case of a monochromatic driving field is first considered, followed by the more realistic case of a pulsed excitation. We show that the pulse duration yields an experimental control parameter over the maximal pair yield. Orders of magnitude of the pair yield are given for experimentally achievable magnetic fields and laser intensities weak enough to preserve the Landau level structure.

cond-mat.mes-hall↗

arXiv2015

Lorenz-Mie theory for 2D scattering and resonance calculations

This PhD tutorial is concerned with a description of the two-dimensional generalized Lorenz-Mie theory (2D-GLMT), a well-established numerical method used to compute the interaction of light with arrays of cylindrical scatterers. This theory is based on the method of separation of variables and the application of an addition theorem for cylindrical functions. The purpose of this tutorial is to assemble the practical tools necessary to implement the 2D-GLMT method for the computation of scattering by passive scatterers or of resonances in optically active media. The first part contains a derivation of the vector and scalar Helmholtz equations for 2D geometries, starting from Maxwell's equations. Optically active media are included in 2D-GLMT using a recent stationary formulation of the Maxwell-Bloch equations called steady-state ab initio laser theory (SALT), which introduces new classes of solutions useful for resonance computations. Following these preliminaries, a detailed description of 2D-GLMT is presented. The emphasis is placed on the derivation of beam-shape coefficients for scattering computations, as well as the computation of resonant modes using a combination of 2D-GLMT and SALT. The final section contains several numerical examples illustrating the full potential of 2D-GLMT for scattering and resonance computations. These examples, drawn from the literature, include the design of integrated polarization filters and the computation of optical modes of photonic crystal cavities and random lasers.

physics.optics↗

arXiv2015

Constructive feedback for the growth of laser-induced periodic surface structures

We study the formation and growth of laser-induced periodic surface structures (LIPSSs) with the finite-difference time-domain (FDTD) method. We use a recently proposed inter-pulse feedback method to account for the evolution of the surface morphology between each laser pulse sent to the surface of the processed material. This method has been used with an ablation-like mechanism, by removing material exposed to a light intensity higher than a given threshold. We propose an inverse mechanism, an expansion-like mechanism, able to grow structures that the ablation-like process cannot. This allows us to introduce the notions of constructive and destructive feedback and explains a strong contradiction between the standard Sipe-Drude theory and the experimental observations, i. e. the formation on metals of structures usually linked to wide band gap dielectrics.

physics.comp-ph↗

arXiv2015

Toward the formation of crossed laser-induced periodic surface structures

The formation of a new type of laser-induced periodic surface structures using a femtosecond pulsed laser is studied on the basis of the Sipe-Drude theory solved with a FDTD scheme. Our numerical results indicate the possibility of coexisting structures parallel and perpendicular to the polarization of the incident light for low reduced collision frequency ($γ/ω\lesssim 1/4$, where $ω$ is the laser frequency). Moreover, these structures have a periodicity of $Λ\sim λ$ in both orientations. To explain this behavior, light-matter interaction around a single surface inhomogeneity is also studied and confirms the simultaneous presence of surface plasmon polaritons and radiation remnants in orthogonal orientations at low $γ/ω$ values.

physics.optics↗

arXiv2014

Optimization of integrated polarization filters

This study reports on the design of small footprint, integrated polarization filters based on engineered photonic lattices. Using a rods-in-air lattice as a basis for a TE filter and a holes-in-slab lattice for the analogous TM filter, we are able to maximize the degree of polarization of the output beams up to 98 % with a transmission efficiency greater than 75 %. The proposed designs allow not only for logical polarization filtering, but can also be tailored to output an arbitrary transverse beam profile. The lattice configurations are found using a recently proposed parallel tabu search algorithm for combinatorial optimization problems in integrated photonics.

physics.optics↗

arXiv2014

Ab initio investigation of lasing thresholds in photonic molecules

We investigate lasing thresholds in a representative photonic molecule composed of two coupled active cylinders of slightly different radii. Specifically, we use the recently formulated steady-state ab initio laser theory (SALT) to assess the effect of the underlying gain transition on lasing frequencies and thresholds. We find that the order in which modes lase can be modified by choosing suitable combinations of the gain center frequency and linewidth, a result that cannot be obtained using the conventional approach of quasi-bound modes. The impact of the gain transition center on the lasing frequencies, the frequency pulling effect, is also quantified.

physics.optics↗

arXiv2014

Adding SALT to Coupled Microcavities: the making of active photonic molecule lasers

A large body of work has accumulated over the years in the study of the optical properties of single and coupled microcavities for a variety of applications, ranging from filters to sensors and lasers. The focus has been mostly on the geometry of individual resonators and/or on their combination in arrangements often referred to as photonic molecules (PMs). Our primary concern will be the lasing properties of PMs as ideal candidates for the fabrication of integrated microlasers, photonic molecule lasers. Whereas most calculations on PM lasers have been based on cold-cavity (passive) modes, i.e. quasi-bound states, a recently formulated steady-state ab initio laser theory (SALT) offers the possibility to take into account the spectral properties of the underlying gain transition, its position and linewidth, as well as incorporating an arbitrary pump profile. We will combine two theoretical approaches to characterize the lasing properties of PM lasers: for two-dimensional systems, the generalized Lorenz-Mie theory will obtain the resonant modes of the coupled molecules in an active medium described by SALT. Not only is then the theoretical description more complete, the use of an active medium provides additional parameters to control, engineer and harness the lasing properties of PM lasers for ultra-low threshold and directional single-mode emission.

physics.optics↗

arXiv2013

Multiobjective optimization in integrated photonics design

We propose the use of the parallel tabu search algorithm (PTS) to solve combinatorial inverse design problems in integrated photonics. To assess the potential of this algorithm, we consider the problem of beam shaping using a two-dimensional arrangement of dielectric scatterers. The performance of PTS is compared to one of the most widely used optimization algorithms in photonics design, the genetic algorithm (GA). We find that PTS can produce comparable or better solutions than the GA, while requiring less computation time and fewer adjustable parameters. For the coherent beam shaping problem as a case study, we demonstrate how PTS can tackle multiobjective optimization problems and represent a robust and efficient alternative to GA.

physics.optics↗

arXiv2013

S and Q Matrices Reloaded: applications to open, inhomogeneous, and complex cavities

We present a versatile numerical algorithm for computing resonances of open dielectric cavities. The emphasis is on the generality of the system's configuration, i.e. the geometry of the (main) cavity (and possible inclusions) and the internal and external dielectric media (homogeneous and inhomogeneous). The method is based on a scattering formalism to obtain the position and width of the (quasi)-eigenmodes. The core of the method lies in the scattering S-matrix and its associated delay Q-matrix which contain all the relevant information of the corresponding scattering experiment. For instance, the electromagnetic near- and far-fields are readily extracted. The flexibility of the propagation method is displayed for a selected system.

physics.optics↗

arXiv2013

Coherent beam shaping using two-dimensional photonic crystals

Optical devices based on photonic crystals such as waveguides, lenses and beam-shapers, have received considerable theoretical and experimental attention in recent years. The production of these devices has been facilitated by the wide availability of silicon-on-insulator fabrication techniques. In this theoretical work, we show the possibility to design a coherent PhC-based beam-shaper. The basic photonic geometry used is a 2D square lattice of air holes in a high-index dielectric core. We formulate the beam shaping problem in terms of objective functions related to the amplitude and phase profile of the generated beam. We then use a parallel tabu search algorithm to minimize the two objectives simultaneously. Our results show that optimization of several attributes in integrated photonics design is well within reach of current algorithms.

physics.optics↗

arXiv2012

Beam shaping using genetically optimized two-dimensional photonic crystals

We propose the use of two-dimensional photonic crystals with engineered defects for the generation of an arbitrary-profile beam from a focused input beam. The cylindrical harmonics expansion of complex-source beams is derived and used to compute the scattered wavefunction of a 2D photonic crystal via the multiple scattering method. The beam shaping problem is then solved using a genetic algorithm. We illustrate our procedure by generating different orders of Hermite-Gauss profiles, while maintaining reasonable losses and tolerance to variations in the input beam and the slab refractive index.

physics.optics↗

arXiv2010

Phase Space Engineering in Optical Microcavities II: Controlling the far-field

Optical microcavities support Whispering Gallery Modes (WGMs) with a very high quality factor Q. However, WGMs typically display a far-field isotropic emission profile and modifying this far-field profile without spoiling the associated high Q remains a challenge. Using a 2D annular cavity, we present a procedure capable to achieve these two apparently conflicting goals. With the correspondence between the classical and the wave picture, properties of the classical phase space shed some light on the characteristics of the wave dynamics. Specifically, the annular cavity has a well separated mixed phase space, a characteristic that proves to be of crucial importance in the emission properties of WGMs. While the onset of directionality in the far-field may be achieved through parametric deformation of the distance cavity-hole centers, this contribution presents a method to control the emission profile via a second parameter, the hole radius. The influence of the classical dynamics to control and predict the field emission will be demonstrated.

physics.optics↗

arXiv2010

Phase Space Engineering in Optical Microcavities I: Preserving near-field uniformity while inducing far-field directionality

Optical microcavities have received much attention over the last decade from different research fields ranging from fundamental issues of cavity QED to specific applications such as microlasers and bio-sensors. A major issue in the latter applications is the difficulty to obtain directional emission of light in the far-field while keeping high energy densities inside the cavity (i.e. high quality factor). To improve our understanding of these systems, we have studied the annular cavity (a dielectric disk with a circular hole), where the distance cavity-hole centers, d, is used as a parameter to alter the properties of cavity resonances. We present results showing how one can affect the directionality of the far-field while preserving the uniformity (hence the quality factor) of the near-field simply by increasing the value of d. Interestingly, the transition between a uniform near- and far-field to a uniform near- and directional far-field is rather abrupt. We can explain this behavior quite nicely with a simple model, supported by full numerical calculations, and we predict that the effect will also be found in a large class of eigenmodes of the cavity.

physics.optics↗