arXiv subjects

Samudra Roy

Publications and source records attributed to Samudra Roy.

29 records · Page 2Linked to original sources

arXiv2019

Heat-induced soliton self-frequency redshift in the ultrafast nonlinear dynamics of active plasmonic waveguides

We investigate the ultrafast nonlinear dynamics of light emitted in an active plasmonic waveguide composed of a thin film of gold sandwiched by two silicon layers immersed in externally pumped Al$_2$O$_3$:Er$^{3+}$. We model optical propagation in such a dissipative system through a generalized cubic Ginzburg-Landau equation accounting for the amplification of the active medium and the effect of absorption and thermo-modulational nonlinearity of gold. We find that heating heavily affects the propagation of temporal dissipative solitons in such a plasmonic waveguide by producing a soliton self-frequency redshift accompanied by soliton deceleration in the time domain. By adopting a semi-analytical variational approach, we evaluate the dependence of the self-induced redshift by deriving a set of coupled differential equations for the pulse parameters. These equations provide physical insight into the complex nonlinear dynamics through simple approximate analytic expressions for temporal and frequency shifts. Such analytical predictions are found in excellent agreement with direct numerical simulations of the generalized cubic Ginzburg-Landau equation. Our results provide a general understanding of ultrafast nonlinear dynamics in gold-based active plasmonic waveguides, as in particular the spectral shaping properties of propagating optical pulses.

physics.optics↗

arXiv2018

Dissipative solitons in self-defocussing nonlinear media: The curious case of zero-nonlinearity point

We theoretically model a dissipative system which exhibits self-defocussing nonlinearity and numerically study the dynamics of optical dissipative solitons (DSs) whose evolution is governed by a complex Ginzburg-Landau equation (GLE). We show that the formation of DSs is not restricted in the domain exhibiting positive nonlinearity. Stable DSs are excited even in the regime where the nonlinearity is negative. Based on the numeric sign of dispersion and nonlinear coefficient, we classify the operational regime into four discrete domains and study the formation of DSs in those regimes. We design a realistic waveguide that exhibits strong frequency dependent nonlinearity which changes its sign across a certain frequency called zero-nonlinearity point (ZNP). We adopt a variational technique to theoretically study the overall dynamics of DSs under various perturbations by choosing Pereira-Stenflo type soliton as our ansatz since it is the natural solution of the unperturbed GLE. An extensive numerical study reveals that the ZNP plays a dominant role on the pulse dynamics and depending on its relative location with respect to input frequency, it can either suppress or enhance Raman induced frequency down-shifting. This is further supported by the variational method which quantitatively determines the location of the Raman frequency as a function of the ZNP. The dispersive radiation generated due to third-order dispersion changes drastically with the location of the ZNP. We analytically derive a phase matching equation that predicts the location of radiation frequency in presence of the ZNP.

physics.optics↗

arXiv2018

Collision-mediated radiation due to Airy-soliton interaction in nonlinear Kerr medium

We study the interaction of a co-propagating finite energy Airy pulse and a soliton in a Kerr medium under third order dispersion. It is observed that a strong radiation appears when selfaccelerating Airy pulse collides with a delayed soliton in time domain. We confirm both analytically and numerically that this radiation can only be initiated in the environment of third order dispersion (TOD). The radiation frequency is red or blue shifted depending on the numeric sign of TOD parameter. We adopt a simple Si-based waveguide to validate the normalised parameter that we use for the analysis. We develop a theory to explain the collision-mediated radiation and our theoretical results are found to be in good agreement with the direct numerical modelling of the generalised nonlinear Schrodinger equation (GNLSE).

physics.optics↗

arXiv2017

Dynamics of Airy pulse under phase modulation: Few interesting aspects

In this work we have tried to study the interesting dynamics of Airy pulse under phase modulation. We investigate the role of linear, quadratic and cubic phase modulation on Airy pulse dynamics in pure linear regime. As a specific example, we study the influence of cubic phase modulation (CPM) as a self healing effect when the propagation dynamics of a finite energy Airy pulse is perturbed under the third order dispersion (TOD). As a consequence of TOD, the pulse flips in time domain and propagates with a reverse acceleration. This unusual propagation characteristic can be restricted through CPM which counterbalance the TOD induced flipping phenomenon. With proper analytical and numerical treatment we demonstrate, how CPM not only resists the temporal flipping but helps in preserving the pulse shape under perturbation. We put special emphasis on the study of the Airy pulse at flipping point where it loses its characteristic shape and converts itself to a pure Gaussian pulse. We derive the Gaussian width analytically and demonstrate that a suitable quadratic phase modulation can focus the Airy pulse to a point and leads to a chirp-free Gaussian pulse. The present study is useful in understanding the physical insight of the unique Airy dynamic under phase modulation

physics.optics↗

arXiv2017

Perturbed Dissipative Solitons: A Variational Approach

We adopt a variational technique to study the dynamics of perturbed dissipative solitons, whose evolution is governed by a Ginzburg--Landau equation (GLE). As a specific example of such solitons, we consider a silicon-based active waveguide in which free carriers are generated through two-photon absorption. In this case, dissipative solitons are perturbed by physical processes such as third-order dispersion, intrapulse Raman scattering, self-steepening, and free-carrier generation. To solve the variational problem, we adopt the Pereira--Stenflo soliton as an ansatz since this soliton is the exact solution of the unperturbed GLE. With this ansatz, we derive a set of six coupled differential equations exhibiting the dynamics of various pulse parameters. This set of equations provides considerable physical insight in the complex behavior of perturbed dissipative solitons. Its predictions are found to be in good agreement with direct numerical simulations of the GLE. More specifically, the spectral and temporal shifts of the chirped soliton induced by free carriers and intrapulse Raman scattering are predicted quite accurately. We also provide simple analytic expressions of these shifts by making suitable approximations. Our semi-analytic treatment is useful for gaining physical insight into complex soliton-evolution processes.

physics.optics↗

arXiv2017

Dissipative Soliton Mediated Radiations in Active Silicon-Based Waveguides

The Ginzburg-Landau (GL) equation is in general not integrable by the inverse scattering method and support solitary-wave solution, called dissipative soliton (DS). We numerically demonstrate that, a DS can radiate dispersive waves (DWs) in presence of third-order dispersion (TOD). We propose a silicon-based active waveguide that excites stable DSs. Energy can be transferred from these stable DS to linear DWs when a resonance condition is achieved. The dynamics of the DS is governed by the complex GL equation which we solve numerically for different operational parameters. Numerical solution of the perturbed GL equation exhibits multiple radiations, when the stable DS is allowed to propagate through a large distance. We theoretically derive a special phase-matching relation that can predict the frequencies of these multiple radiations, which are found numerically. In our theoretical and numerical calculations we include the role of free carriers which appear inside semiconductor waveguides as a consequence of two-photon absorption (TPA). We demonstrate that apart from TOD, TPA and gain dispersion are two additional parameters that can control the radiation emitted by DS. The DS-mediated radiation is different in nature and demands an intuitive understanding. In this work we try to provide some insights of this fascinating radiation phenomenon by elaborate analytical and numerical calculations.

physics.optics↗

arXiv2015

Experimental and theoretical study of red-shifted solitonic resonant radiation in photonic crystal fibers and generation of radiation seeded Raman solitons

The red shifted solitonic resonant radiation is a fascinating phase matching phenomenon that occurs when an optical pulse, launched in the normal dispersion regime of photonic crystal fiber, radiates across the zero dispersion wavelength. The formation of such phase-matched radiation is independent of the generation of any optical soliton and mainly governed by the leading edge of input pump which forms a shock front. The radiation is generated at the anomalous dispersion regime and found to be confined both in time and frequency domain. We experimentally investigate the formation of such radiations in photonic crystal fibers with detailed theoretical analysis. Our theoretical predictions corroborate well with experimental results. Further we extend our study for long length fiber and investigate the interplay between red-shifted solitonic resonant radiation and intrapulse Raman scattering (IPRS). It is observed that series of radiation-seeded Raman solitons are generated in anomalous dispersion regime.

physics.optics↗

arXiv2014

Free-carrier driven spatio-temporal dynamics in amplifying silicon waveguides

We theoretically investigate the free-carrier induced spatio-temporal dynamics of continuous waves in silicon waveguides embedded in an amplifying medium. Optical propagation is governed by a cubic Ginzburg-Landau equation coupled with an ordinary differential equation accounting for the free-carrier dynamics. We find that, owing to free-carrier dispersion, countinuous waves are modulationally unstable in both anomalous and normal dispersion regimes and chaotically generate unstable accelerating pulses.

physics.optics↗

arXiv2013

High-energy, shock-front assisted resonant radiation in the normal dispersion regime

We present a simple yet effective theory that predicts the existence of resonant radiation bands in the deep normal group velocity dispersion region of a medium, even in absence of a zero-group velocity dispersion point. This radiation is evident when the medium is pumped with high-energy ultrashort pulses, and it is driven by the interplay between the Kerr and the shock terms in the NLSE. Accurate experiments performed in bulk silica fully support the theoretical phase-matching condition found by our theory.

physics.optics↗

arXiv2013

Self-frequency blue-shift of dissipative solitons in silicon based waveguides

We analyze the dynamics of dissipative solitons in silicon on insulator waveguides embedded in a gain medium. The optical propagation is modeled through a cubic Ginzburg-Landau equation for the field envelope coupled with an ordinary differential equation accounting for the generation of free carriers owing to two-photon absorption. Our numerical simulations clearly indicate that dissipative solitons accelerate due to the carrier-induced index change and experience a considerable blue-shift, which is mainly hampered by the gain dispersion of the active material. Numerical results are fully explained by analytical predictions based on soliton perturbation theory.

physics.optics↗

arXiv2012

Localized frequency comb and formation of embedded solitons in silicon-based slot waveguides

We explore the possibility to excite the so-called embedded solitons in specially designed slot waveguides based on silicon and silica or silicon nanocrystals. This requires the excitation of the structure with quasi-TM polarized pulses -- for which Raman effect is absent -- and at a specific infrared wavelength for which only the second- and fourth-order group velocity coefficients are non-vanishing. Pulses launched in these conditions will generate a spectrally localized continuum, associated to a frequency comb coming from the spectral interference of many embedded solitons.

physics.optics↗