arXiv subjects

M. S. Mrudul

Publications and source records attributed to M. S. Mrudul.

16 recordsLinked to original sources

arXiv2026

Symmetry-Selective Topological Magnon Engineering by Phonon Angular Momentum

Dynamical control of Berry curvature remains an outstanding challenge in the engineering of topological phases. Here, we demonstrate control of magnon band structures via coherently driven phonons, based on \textit{ab initio} spin-lattice coupling and Floquet theory. We show that this control is symmetry selective: linearly polarized phonons leave the spectrum unchanged, whereas circular and elliptical phonons carrying finite phonon angular momentum (PAM) induce chiral interactions that open and tune gaps at Dirac points, generating and reversing topological magnon phases. The gap magnitude and Chern numbers are directly governed by the PAM, enabling handedness-selective topology control. Applied to monolayer CrI$_3$, and supported by symmetry analysis, our results establish driven lattice dynamics as a general route to engineering topological bosonic excitations and a versatile platform for Floquet control of magnetism.

cond-mat.mes-hall↗

arXiv2026

A crystal-field route to THz-driven magnetization

Light carries angular momentum, but the microscopic pathways that transform it into magnetization remain elusive. Here we establish that crystal-field excitations, historically viewed primarily as equilibrium spectroscopic fingerprints of localized 4$f$ electrons, constitute an active microscopic route through which circularly-polarized terahertz (THz) light creates magnetic polarization. Using wavelength-selective ultrafast Faraday spectroscopy on the paramagnetic insulator CeF$_3$, we show that resonant excitation of localized 4$f$ crystal-field transitions generates a helicity-dependent magnetization that survives for up to about 100 ps. Most strikingly, while the optical helicity is held fixed, the THz-driven response reverses sign as the excitation wavelength is tuned across the crystal-field resonance. The resulting dispersive spectral response follows the crystal-field excitation spectrum rather than that of optical phonons, and is captured by resonant electronic theory of the inverse Faraday effect. Our results identify crystal-field excitations as a previously unrecognized dynamical reservoir for optical angular momentum and broaden the microscopic pathways through which THz light can create and manipulate magnetic states.

cond-mat.mtrl-sci↗

arXiv2026

Valley-polarized Orbital and Spin Magnetism Induced by Femtosecond Optical Pulses in Two-Dimensional Semiconductors

We theoretically investigate the ultrafast generation of spin and orbital magnetism in a two-dimensional gapped Dirac system with spin-orbit coupling. This system is representative of two-dimensional hexagonal semiconductors, such as transition-metal dichalcogenides that exhibit valley-selective optical selection rules arising from the valley-contrasting magnetic texture of their band structure. Using a time-dependent density-matrix formalism, we demonstrate that circularly polarized laser pulses generate nonequilibrium magnetization under both resonant and multiphoton resonant conditions. We show that the induced spin and orbital magnetic moments can be distinctly controlled via the photon energy and polarization of the driving field. Furthermore, spin and orbital dynamics originate from fundamentally different light-matter coupling mechanisms, leading to qualitatively dissimilar temporal behaviors. The orbital magnetic moment couples directly to the external electric field, resulting in faster dynamics and pronounced Rabi-like oscillations, whereas the spin response develops gradually through spin-orbit coupling. Consequently, orbital dynamics is significantly more sensitive to electron-hole dephasing than the spin response. Our results highlight the importance of properly accounting for orbital contributions in future technologies that utilize femtosecond control of magnetism.

cond-mat.mes-hall↗

arXiv2025

Chiral Phonons Arising From Chirality-Selective Magnon-Phonon Coupling

Chiral phonons are desirable for applications in spintronics but their generation and control remains a challenge.Here we demonstrate the emergence of truly chiral phonons from selective magnon-phonon coupling in inversion-symmetric magnetic systems. Considering bcc Fe as example, we quantitatively calculate hybridized magnon-phonon quasiparticle states across the entire Brillouin zone utilizing first-principles calculations. Our findings challenge conventional magneto-elastic interpretations and reveal finite zero-point phonon angular momentum and strong anomalous thermal Hall responses linked to finite (spin) Berry curvatures. Our results further establish that the existence of chiral phonons, particularly along high-symmetry directions, is common in many magnetic materials, offering promising avenues for novel spintronic and phononic devices.

cond-mat.mtrl-sci↗

arXiv2025

Nonlinear Optical Spectroscopy of Nodal-Line Semimetals

Intense laser-driven nonlinear optical phenomena in two-dimensional (2D) nodal-line semimetals (NLS) exhibit complex mechanisms, particularly in the NbSi$_{x}$Te$_{2}$ material systems characterized by nonsymmorphic symmetry-protected band degeneracy. Our findings reveal how nonsymmorphic symmetry-protected band degeneracy fundamentally influences the material's nonlienar optical responses. Notably, the nonsymmorphic glide-mirror symmetry leads to the exclusive generation of odd-order harmonics from inversion-symmetry-broken NLS. Moreover, harmonics are emitted parallel and perpendicular to the driving laser's polarization. We demonstrate distinct generation mechanisms arise from intrachain and interchain processes, with their relative contributions varying significantly with the polarization of the driving laser pulse. The polarization-dependence exhibits two-fold anisotropy, with each harmonic order showing characteristic angular distributions of maximum yield. Additionally, our analysis of the ellipticity-dependence reveals an intricate interplay between interband and intraband mechanisms. These insights open new possibilities for controlling harmonic generation through precise tuning parameters of the driving laser and highlight the potentials of NLS materials to fabricate lightwave-based photonics, optoelectronic and quantum devices operating on ultrafast timescales.

physics.optics↗

arXiv2025

Generation of Phonons with Angular Momentum During Ultrafast Demagnetization

A major question in the field of femtosecond laser-induced demagnetization is whereto the angular momentum lost by the electrons is transferred. Recent ultrafast electron diffraction measurements [Tauchert \textit{et al.}, Nature {\bf 602}, 73 (2022)] suggest that this angular momentum is transferred to the rotational motion of atoms on a sub-picosecond timescale, but a theory confirmation of this proposition has yet to be given. Here we investigate the coupled electron-nuclear dynamics during ultrafast demagnetization of L1$_0$ FePt, using Ehrenfest nuclear dynamics simulations combined with the time-dependent density functional theory (TDDFT) framework. We demonstrate that atomic rotations appear, i.e., the generation of phonons carrying finite angular momentum following ultrafast demagnetization. We further show that both ultrafast demagnetization and the generation of phonons with angular momentum arise from symmetry constraints imposed by the spin-orbit coupling, thus providing insight in spin-phonon interaction at ultrafast timescales.

cond-mat.mtrl-sci↗

arXiv2024

High-Harmonic Generation from Engineered Graphene for Polarization Tailoring

Strain engineering is a versatile method to boost the carrier mobility of two-dimensional materials-based electronics and optoelectronic devices. In addition, strain is ubiquitous during device fabrication via material deposition on a substrate with a different lattice structure. Here, we show that the polarization properties of the harmonics in graphene under uniaxial strain are strongly yet differently affected in the lower and higher orders. The polarization plane of the lower-order emitted harmonics is rotated -- a manifestation of Faraday rotation due to the broken symmetry planes. In contrast, we observe elliptically-polarized higher-order harmonics due to the intricate interplay of the interband and intraband electron dynamics. The implications of these findings are twofold: First, we show how the rotation of the polarization plane of the lower-order harmonics can be used as a probe to characterize the strain's nature, strength, and angle. Second, we demonstrate how strain engineering can be used to alter the polarization properties of higher-order harmonics, relevant for applications in ultrafast chiral-sensitive studies. Our research opens a promising avenue for strain-tailored polarization properties of higher-order harmonics in engineered solids.

cond-mat.mes-hall↗

arXiv2023

Ab initio investigation of laser-induced ultrafast demagnetization of L1$_0$ FePt: Intensity dependence and importance of electron coherence

We theoretically investigate the optically-induced demagnetization of ferromagnetic FePt using the time-dependent density functional theory (TDDFT). We compare the demagnetization mechanism in the perturbative and nonperturbative limits of light-matter interaction and show how the underlying mechanism of the ultrafast demagnetization depends on the driving laser intensity. Our calculations show that the femtosecond demagnetization in TDDFT is a longitudinal magnetization reduction and results from a nonlinear optomagnetic effect, akin to the inverse Faraday effect. The demagnetization scales quadratically with the electric field $E$ in the perturbative limit, i.e., $ΔM_z \propto E^{2}$. Moreover, the magnetization dynamics happens dominantly at even multiples $nω_0$, ($n = 0, 2, \cdots$) of the pump-laser frequency $ω_0$, whereas odd multiples of $ω_0$ do not contribute. We further investigate the demagnetization in conjunction to the optically-induced change of electron occupations and electron correlations. Correlations within the Kohn-Sham local-density framework are shown to have an appreciable yet distinct effect on the amount of demagnetization depending on the laser intensity. Comparing the ${ab~initio}$ computed demagnetizations with those calculated from spin occupations, we show that electronic coherence plays a dominant role in the demagnetization process, whereas interpretations based on the time-dependent occupation numbers poorly describe the ultrafast demagnetization.

cond-mat.mtrl-sci↗

arXiv2023

Generation of Circularly-Polarised High-Harmonics with Identical Helicity in Two-Dimensional Materials

Generation of circularly-polarized high-harmonics with the same helicity to all orders is indispensable for chiral-sensitive spectroscopy with attosecond temporal resolution. Solid-state samples have added a valuable asset in controlling the polarization of emitted harmonics. However, maintaining the identical helicity of the emitted harmonics to all orders is a daunting task. In this work, we demonstrate a robust recipe for efficient generation of circularly-polarized harmonics with the same helicity. For this purpose, a nontrivial tailored driving field, consisting of two co-rotating laser pulses with frequencies $ω$ and $2ω$, is utilized to generate harmonics from graphene. The Lissajous figure of the total driving pulse exhibits an absence of the rotational symmetry, which imposes no constraint on the helicity of the emitted harmonics. Our approach to generating circularly-polarized harmonics with the same helicity is robust against various perturbations in the setup, such as variation in the subcycle phase difference or the intensity ratio of the $ω$ and $2ω$ pulses, as rotational symmetry of the total driving pulse remains absent. Our approach is expected to be equally applicable to other two-dimensional materials, among others, transition-metal dichalcogenides and hexagonal boron nitride as our approach is based on absence of the rotational symmetry of the driving pulse. Our work paves the way for establishing compact solid-state chiral-XUV sources, opening a new realm for chiral light-matter interaction on its intrinsic timescale.

physics.optics↗

arXiv2022

High-harmonic spectroscopy of light-driven nonlinear anisotropic anomalous Hall effect in a Weyl semimetal

Weyl semimetals are promising quantum materials that offer unique topological properties. Lately, it has been shown that laser-driven electron dynamics have characteristic signatures in two-dimensional and three-dimensional Dirac semimetals. The transition from Dirac to Weyl semimetal requires the breaking of either inversion or time-reversal symmetry. The present work shows that the laser-driven electron dynamics in a Weyl semimetal with broken time-reversal symmetry has intriguing features in its high-harmonic spectrum. It is found that the parity and magnitude of the non-zero Berry curvature's components control the direction and strength of the anomalous current, which leads to the generation of the anomalous odd harmonics. We demonstrate that the non-trivial topology of the Berry curvature in time-reversal symmetry broken quantum materials can be probed by measuring the polarisation of the emitted anomalous odd harmonics. Our findings unequivocally illustrate that laser-driven electron dynamics leads to the generation of nonlinear anisotropic anomalous Hall effect in time-reversal symmetry broken quantum materials on an ultrafast timescale.

cond-mat.mes-hall↗

arXiv2022

High-Harmonic Spectroscopy of Coherent Lattice Dynamics in Graphene

High-harmonic spectroscopy of solids is a powerful tool, which provides access to both electronic structure and ultrafast electronic response of solids, from their band structure and density of states, to phase transitions, including the emergence of the topological edge states, to the PetaHertz electronic response. However, in spite of these successes, high harmonic spectroscopy has hardly been applied to analyse the role of coherent femtosecond lattice vibrations in the attosecond electronic response. Here we study coherent phonon excitations in monolayer graphene to show how high-harmonic spectroscopy can be used to detect the influence of coherent lattice dynamics, particularly longitudinal and transverse optical phonon modes, on the electronic response. Coherent excitation of the in-plane phonon modes results in the appearance of sidebands in the spectrum of the emitted harmonic radiation. We show that the spectral positions and the polarisation of the sideband emission offer a sensitive probe of the dynamical symmetries associated with the excited phonon modes. Our work brings the key advantage of high harmonic spectroscopy -- the combination of sub-femtosecond to tens of femtoseconds temporal resolution -- to the problem of probing phonon-driven electronic response and its dependence on the dynamical symmetries in solids.

physics.optics↗

arXiv2022

High-harmonic spectroscopy of two-dimensional materials

Recent advancements in the generation of mid-infrared and terahertz laser pulses have enabled us to observe strong-field driven non-perturbative high-harmonic generation (HHG) from semiconductors, dielectrics, and semimetals. HHG has added another dimension to time-resolved ultrafast electron dynamics in materials with unprecedented temporal resolution. Present thesis discusses how HHG is an emerging method to probe static and dynamical properties in two-dimensional materials. In this thesis, two-dimensional materials with hexagonal symmetry are studied. We have demonstrated that the high-harmonic spectrum encodes the fingerprints of electronic band structure and interband coupling between different bands. Furthermore, by analysing gapped and gapless graphene, we show how electron dynamics in a semimetal and a semiconductor are different as the harmonic spectrum depends differently on the polarisation of the driving laser. To explore the role of defects in HHG, spin-polarised vacancy defects in hexagonal boron nitride are considered. It has been found that electron-electron interaction is crucial for electron dynamics in a defected solid. In all cases, we present how different symmetries of the lattice can be extracted from the harmonic spectrum. Finally, a light-driven method is proposed for observing valley-polarisation in pristine graphene, using a tailored laser pulse. Also, a recipe is discussed to write and read valley-selective electron excitations in materials with zero bandgap and zero Berry curvature.

physics.optics↗

arXiv2021

Controlling Valley-Polarisation in Graphene via Tailored Light Pulses

Analogous to charge and spin, electrons in solids endows an additional degree of freedom: the valley pseudospin. Two-dimensional hexagonal materials such as graphene exhibit two valleys, labelled as $\mathbf{K}$ and $\mathbf{K}^{\prime}$. These two valleys have the potential to realise logical operations in two-dimensional materials. Obtaining the desired control over valley polarisation between the two valleys is a prerequisite for the logical operations. Recently, it was shown that two counter-rotating circularly polarised laser pulses can induce a significant valley-polarisation in graphene. The main focus of the present work is to optimise the valley polarisation in monolayer graphene by controlling different laser parameters, such as wavelength, intensity ratio, frequency ratio and sub-cycle phase in two counter-rotating circularly polarised laser setup. Moreover, an alternate approach, based on single or few-cycle linearly polarised laser pulse, is also explored to induce significant valley polarisation in graphene. Our work could help experimentalists to choose a suitable method with optimised parameter space to obtain the desired control over valley polarisation in monolayer graphene.

cond-mat.mes-hall↗

arXiv2021

High-Harmonic Generation from Monolayer and Bilayer Graphene

High-harmonic generation (HHG) in solids is an emerging method to probe ultrafast electron dynamics in solids at attosecond timescale. In this work, we study HHG from monolayer and bilayer graphene. Bilayer graphenes with AA and AB stacking are considered in this work. It is found that the monolayer and bilayer graphenes exhibit significantly different harmonic spectra. The difference in the spectra is attributed to the interlayer coupling between the two layers. Also, the intraband and interband contributions to the total harmonic spectrum play a significant role. Moreover, interesting polarization and ellipticity dependence are noticed in total harmonic spectrum for monolayer and bilayer graphene.

physics.optics↗

arXiv2021

Light-Induced Valleytronics in Pristine Graphene

Electrons in two-dimensional hexagonal materials have valley degree of freedom, which can be used to encode and process quantum information. The valley-selective excitations, governed by the circularly polarised light resonant with the material's band-gap, continues to be the foundation of valleytronics. It is often assumed that achieving valley selective excitation in pristine graphene with all-optical means is not possible due to the inversion symmetry of the system. Here we demonstrate that both valley-selective excitation and valley-selective high-harmonic generation can be achieved in pristine graphene by using the combination of two counter-rotating circularly polarized fields, the fundamental and its second harmonic. Controlling the relative phase between the two colours allows us to select the valleys where the electron-hole pairs and higher-order harmonics are generated. We also describe an all-optical method for measuring valley polarization in graphene with a weak probe pulse. This work offers a robust recipe to write and read valley-selective electron excitations in materials with zero bandgap and zero Berry curvature.

cond-mat.mes-hall↗

arXiv2018

Simultaneous control of harmonic yield and energy cutoff of high-harmonic generation using seeded plasmonic-enhanced fields

We study high-order harmonic generation (HHG) driven by seeded plasmonic-enhanced fields. On one hand, plasmonic-enhanced fields have shown a great potential to extend the HHG cutoff, an instrumental pre-requisite for the generation of attosecond pulses. On another hand, the use of XUV seeds appears to have a considerable potential to improve the HHG conversion efficiency, which is typically modest when a unique fundamental laser pulse is employed. By mixing these two sources, we show it is possible to, simultaneously, boost the HHG cutoff and to increase the harmonic photon flux. The combination of these features potentially enables to generate intense and spectrally broad attosecond pulse trains.

physics.atom-ph↗