arXiv subjects

Gopal Dixit

Publications and source records attributed to Gopal Dixit.

At least 19 recordsLinked to original sources

arXiv2026

Helicity-engineered nonlinear optical responses in photo-excited topological semimetals

Topological materials provide a transformative arena for light-driven control of electronic motion; yet, direct manipulation of electron dynamics on sub-cycle timescale remains a significant challenge. We demonstrate that the strong-field-driven high-harmonic generation of a Weyl semimetal can be controlled and manipulated through bicircular pump-probe driving fields. While a lone probe pulse generates exclusively odd-order harmonics, the addition of a pump pulse triggers a series of sidebands arising from nonlinear frequency mixing of pump and probe photons. Our results reveal that the sideband intensities are highly sensitive to the relative helicity of the pulses and the orientation of the polarization plane. This sensitivity stems from the chiral nature of the Weyl nodes, which couples efficiently to the light's helicity only when the node-separation axis is perpendicular to the polarization plane. Furthermore, the significant suppression of sideband intensity with increasing pump-probe delay identifies these features as a potential clock for electron-hole decoherence. These findings establish frequency-mixed high-harmonic generation as a sensitive probe of chiral quantum dynamics and suggest a robust framework for manipulating topological currents via structured light, with implications for lightwave electronics and ultrafast quantum information processing.

physics.optics↗

arXiv2026

Coherent control of chirality in Weyl semimetals

Weyl fermions in inversion-symmetric Weyl semimetals occur in pairs of opposite chirality, leading to symmetric optical responses under circularly-polarised light and a vanishing net photocurrent. Here, we show that tailored two-colour light fields break this symmetry and enable selective excitation of individual Weyl nodes. The interference between a circularly-polarised $\omega$ field and a phase-locked linearly-polarised $2\omega$ field generates a chirality-dependent redistribution of carriers in momentum space, resulting in a nonzero controllable photocurrent. We demonstrate that both the magnitude and sign of the photocurrent can be tuned via the relative phase and field strength of the two colours, and identify an optimal regime in which chiral selectivity is maximised. Our results establish a general route to optically-controlled chiral charge dynamics in Weyl semimetals using polarisation-structured light.

cond-mat.mes-hall↗

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↗

arXiv2024

Towards a universal law for blood flow

Despite decades of research on blood flow, an analogue of Navier-Stokes equations that accurately describe blood flow properties has not been established yet. The reason behind this is that the properties of blood flow seem \`a priori non universal as they depend on various factors such as global concentration of red blood cells (RBCs) and channel width. Here, we have discovered a universal law when the stress and strain rate are measured at a given local RBCs concentration. However, the local concentration must be determined in order to close the problem. We propose a non-local diffusion equation of RBCs concentration that agrees with the full simulation. The universal law is exemplified for both shear and pressure driven flows. While the theory is restricted to a simplistic geometry (straight channel) it provides a fundamental basis for future research on blood flow dynamics and could lead to the development of a new theory that accurately describes blood flow properties under various conditions, such as in complex vascular networks.

physics.flu-dyn↗

arXiv2024

Optical Control of Ultrafast Photocurrent in Graphene

The ability to manipulate electrons with the intense laser pulse enables an unprecedented control over the electronic motion on its intrinsic timescale. Present work explores the desired control of photocurrent generation in monolayer graphene on ultrafast timescale. The origin of photocurrent is attributed to the asymmetric residual electronic population in the conduction band after the end of the laser pulse, which also facilitates valley polarization. Present study offers a comprehensive analysis of the differences between these two observables, namely photocurrent and valley polarization. It is found that the corotating circularly polarized $\omega-2\omega$ laser pulses allow the generation of photocurrent but no valley polarization, whereas counterrotating circularly polarized $\omega-2\omega$ laser pulses yield significant valley polarization without any photocurrent in graphene. Different laser parameters, such as subcycle phase, wavelength, and intensity provide different knobs to control the generation of the photocurrent. In addition, threefold increase in the photocurrent's amplitude can be achieved by altering electronic properties of graphene via strain engineering. Our findings reveal intriguing underlying mechanisms into the interplay between the symmetries of the graphene's electronic structure and the driving laser pulses, shedding light on the potential for harnessing graphene's properties for novel applications in ultrafast photonics, optoelectronic devices, and quantum technologies.

cond-mat.mes-hall↗

arXiv2024

Photocurrent generation in solids via linearly polarized laser

To add to the rapidly progressing field of ultrafast photocurrent, we propose a universal method to generate photocurrent in normal and topological materials using a pair of multicycle linearly polarized laser pulses. The interplay of the fundamental and its second harmonic pulses is studied for the generation of photocurrent in Weyl semimetals by varying the angle between the polarization direction, relative intensity, and relative phase delay. It has been found that the presence of a comparatively weaker second harmonic pulse is sufficient to generate substantial photocurrent. Moreover, significant photocurrent is generated even when polarization directions are orthogonal for certain ratios of the lasers' intensities. In addition, the photocurrent is found to be susceptible to the delay between the two pulses. We have illustrated that all our findings are extendable to non-topological and two-dimensional materials, such as graphene and molybdenum disulfide.

cond-mat.mes-hall↗

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↗

arXiv2024

Valley filtering and valley valves in irradiated pristine graphene

We theoretically study valley-filtering in pristine graphene irradiated by bicircular counter-rotating laser drive. The dynamical symmetry of the graphene and laser drive disrupts graphene's inversion symmetry, which results distinct quasi-energy states and Floquet band occupations in the two valleys. Controlling the relative phase between the bicircular laser drive ultimately allows to blocks the contribution from one valley while allowing the opposite valley currents in the system. For practical realization of valley-based device, we propose configurational setup for valley filters and valley valve consisting of two graphene nanoribbons irradiated by two bicircular counter-rotating laser drives with a relative phase shift. It is observed that the relative phase between the two bicircular laser drives offer a control knob to generate valley-selective currents and transport responses with very high efficiency by an all-optical way. In addition, our findings about valley filter and valley valve are robust against moderate disorder and modest changes in driving laser parameters. Present work opens an avenue to realise light-based valleytronics devices in reality.

cond-mat.mes-hall↗

arXiv2023

Tailoring Photocurrent in Weyl Semimetals via Intense Laser Irradiation

Generating and tailoring photocurrent in topological materials has immense importance in fundamental studies and the technological front. Present work introduces a universal method to generate ultrafast photocurrent in {\it both} inversion-symmetric and inversion-broken Weyl semimetals with degenerate Weyl nodes at the Fermi level. Our approach harnesses the asymmetric electronic population in the conduction band induced by an intense {\it single-color} circularly polarized laser pulse. It has been found that the induced photocurrent can be tailored by manipulating helicity and ellipticity of the employed laser. Moreover, our approach generates photocurrent in realistic situations when the Weyl nodes are positioned at different energies and have finite tilt along a certain direction. Present work adds a new dimension on practical applications of Weyl semimetals for optoelectronics and photonics-based quantum technologies.

physics.optics↗

arXiv2023

How massless are Weyl fermions in Weyl semimetals?

Circularly polarized light fails to generate currents in inversion-symmetric Weyl semimetals with degenerate Weyl nodes. While each node generates current with the direction depending on its chirality, the two currents in the two degenerate nodes of opposite chirality cancel each other. By extension, it is also generally expected that the currents generated at the same Weyl node by the fields of opposite helicity should also observe mirror symmetry and cancel. Surprisingly, here we find that this is not the case. The origin of this effect lies in the nonlinear energy dispersion, which manifests strongly already very close to the Weyl nodes, where linear dispersion is expected to hold and the Weyl fermions are thus expected to be massless. A scheme based on using a trefoil field composed of a counterrotating fundamental and its second harmonic is proposed to control the induced asymmetry at a chiral node from positive to negative, including zero.

cond-mat.str-el↗

arXiv2023

Role of topological charges in the nonlinear-optical response from Weyl semimetals

The successful realization of the topological Weyl semimetals has revolutionized contemporary physics. In recent years, multi-Weyl semimetals, a class of topological Weyl semimetals, has attracted broad interest in condensed-matter physics. Multi-Weyl semimetals are emerging topological semimetals with nonlinear anisotropic energy dispersion, which is characterized by higher topological charges. In this study, we investigate how the topological charge affects the nonlinear optical response from multi-Weyl semimetals. It has been observed that the laser-driven electronic current is characteristic of the topological charge, and the laser polarization's direction influences the current's direction and amplitude. In addition, the anomalous current, perpendicular to the laser's polarization, carries a distinct signature of the topological charges and encodes the information about the parity and amplitude of the nontrivial Berry curvature. We show that the anomalous current associated with the anomalous Hall effect remains no longer proportional to the topological charge at higher laser intensity -- a significant deviation from the linear response theory. High-harmonic spectroscopy is employed to capture the distinct and interesting features of the currents in multi-Weyl semimetals where the topological charge drastically impacts the harmonics' yield and energy cutoff.

cond-mat.str-el↗

arXiv2023

All-Optical Ultrafast Valley Switching in Two-Dimensional Materials

Electrons in two-dimensional materials possess an additional quantum attribute, the valley pseudospin, labelled as $\mathbf{K}$ and $\mathbf{K}^{\prime}$ -- analogous to the spin up and spin down. The majority of research to achieve valley-selective excitations in valleytronics depends on resonant circularly-polarised light with a given helicity. Not only acquiring valley-selective electron excitation but also switching the excitation from one valley to another is quintessential for bringing valleytronics-based technologies in reality. Present work introduces a coherent control protocol to initiate valley-selective excitation, de-excitation, and switch the excitation from one valley to another on the fly within tens of femtoseconds -- a timescale faster than any valley decoherence time. Our protocol is equally applicable to {\it both} gapped and gapless two-dimensional materials. Monolayer graphene and molybdenum disulfide are used to test the universality. Moreover, the protocol is robust as it is insensitive to significant parameters of the protocol, such as dephasing times, wavelengths, and time delays of the laser pulses. Present work goes beyond the existing paradigm of valleytronics, and opens a new realm of valley switch at PetaHertz rate.

physics.optics↗

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 $\omega$ and $2\omega$, 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 $\omega$ and $2\omega$ 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↗

arXiv2023

Tailoring polarisation of attosecond pulses via co-rotating bicircular laser fields

The present work introduces a robust way to generate attosecond pulses with tunable ellipticity via high-order harmonic generation by co-rotating $\omega - 2\omega$ bicircular laser fields. The total electric field of the laser fields exhibits an absence of rotational symmetry, which leads to the generation of high harmonics of the same helicity across a broad range of spectral bandwidth. High-harmonics with the same helicity offer the opportunity to synthesize attosecond pulses with tunable ellipticity. The polarisation properties of the generated harmonics are robust against the variations in driving fields' parameters, such as wavelength, intensity ratio, and the sub-cycle phase between $\omega-2\omega$ fields. Our work opens an avenue to study chiral-sensitive light-matter ultrafast processes on their intrinsic timescale.

physics.optics↗

arXiv2023

Charge Migration in Heterocyclic Five-Membered Rings

This contribution presents numerical simulations of N-electron dynamics in heterocyclic five-membered ring molecules to shed light on the effect of molecular symmetry on charge migration. Laser-driven dynamics is studied using the hybrid time-dependent density functional theory/configuration methodology, and the ensuing field-free charge migration is investigated by means of transient electronic flux density maps. Our results demonstrate that the charge migration in aromatic rings is sensitive to the presence of heteroatoms such as oxygen and nitrogen. Their presence within the ring induces significant modifications of the character in the ground and low-lying electronic states, which is imprinted in the charge migration mechanism.

physics.atom-ph↗

arXiv2022

Probing phonon-driven symmetry alterations in graphene via high-harmonic spectroscopy

High-harmonic spectroscopy has become an essential ingredient in probing various ultrafast electronic processes in solids with sub-cycle temporal resolution. Despite its immense importance, sensitivity of high-harmonic spectroscopy to phonon dynamics in solids is not well known. This work addresses this critical question and demonstrates the potential of high-harmonic spectroscopy in probing intertwined phonon-electron dynamics in solids. A pump pulse excites in-plane optical phonon modes in monolayer graphene and a circularly polarised pulse is employed to probe the excited phonon dynamics that generates higher-order harmonics. We show that the coherent phonon dynamics alters the dynamical symmetry of graphene with the probe pulse and leads the generations of the symmetry-forbidden harmonics. Moreover, sidebands associated with the prominent harmonic peaks are generated as a result of the coherent dynamics. It is found that the symmetries and the characteristic timescale of the excited phonon mode determine the polarisation and positions of these sidebands. Present work opens an avenue in time-resolved probing of phonon-driven processes and dynamical symmetries in solids with sub-cycle temporal resolution.

physics.optics↗

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

Probing the Effect of Molecular Structure Saddling on Ultrafast Charge Migration via Time-Resolved X-ray Diffraction

Metal-corroles are macrocycle organic molecules with numerous practical applications. In particular, copper corroles exhibit an interesting saddled geometry, which has attracted significant attention from theoreticians and experimentalists over the years. The present work is dedicated to understand the effect of structural saddling in a copper corrole on potential probe signals via imaging ultrafast coherent electron dynamics. A linearly polarized pulse is used to trigger the electron dynamics and time-resolved x-ray diffraction is employed to image the triggered dynamics. It is found that the symmetry reduction in the time-resolved diffraction signals and electronic flux densities is a signature of the saddling in a copper corrole during ultrafast charge migration. Moreover, analysis of the electronic flux density reveals that the diagonal nitrogen atoms mediate coherent charge migration between them via a central copper atom. Correlation of the flux densities and the diffraction signals indicates that the signature of the charge migration is encoded in time-resolved diffraction signals. A comparison of the static diffraction signals of nonsaddled planar copper porphyrin and saddled nonplanar copper corrole in their ground states is made.

physics.chem-ph↗