arXiv subjects

G. F. Mkrtchian

Publications and source records attributed to G. F. Mkrtchian.

At least 19 recordsLinked to original sources

arXiv2025

High-harmonic spectroscopy of mobility edges in one-dimensional quasicrystals

Quasicrystals occupy a unique position between periodic and disordered systems, where localization phenomena such as Anderson transitions and mobility edges can emerge even in the absence of disorder. This distinctive behavior motivates the development of robust, all-optical diagnostic tools capable of probing the structural, topological, and dynamical properties of such systems. In this work, focusing on generalized Aubry-André-Harper models and on an incommensurate potential in the continuum limit, we demonstrate that high-harmonic generation phenomenon serves as a powerful probe of localization transitions and mobility edges in quasicrystals. We introduce a new parameter--dipole mobility--which captures the impact of intraband dipole transitions and enables classification of nonlinear optical regimes, where excitation and high-harmonic generation yield can differ by orders of magnitude. We show that the cutoff frequency of harmonics is strongly influenced by the position of the mobility edge, providing a robust and experimentally accessible signature of localization transitions in quasicrystals.

physics.optics↗

arXiv2025

Zeptosecond Electron Pulse Train and Ultrafast Coherent Control of Quantum States via Multiphoton Inelastic Cherenkov Diffraction

We investigate the quantum dynamics of fermionic particles interacting with a laser field in a gaseous medium, in the regime of inelastic diffraction scattering on the phase lattice of a slowed travelling wave, below the critical field of induced Cherenkov process. Using a relativistic quantum kinetic framework and numerical solutions of Dirac equation in the rest frame of the slowed wave, we analyze the evolution of actual electron wave packets and beams at the inelastic scattering on the actual laser pulses of finite duration. Our results reveal coherent multiphoton exchange involving up to $10^{4}$ photons and the emergence of attosecond-zeptosecond electron sub-bunches after the free-space propagation. The pulse compression by such mechanism is robust to laser pulse duration but sensitive to the initial momentum spread of the particles/beams. We propose a mechanism to achieve electron pulses in zeptosecond time scales with potentiality for ultrafast coherent control of quantum states that opens new avenues in high-resolution temporal structuring of electron beams for time-resolved quantum technologies and attosecond-zeptosecond science, as well as, for application in high-resolution electron microscopy.

physics.optics↗

arXiv2025

Berry curvature and shift vector effects at high-order wave mixing in biased bilayer graphene

In this work, we present a microscopic quantum theory that elucidates the nonlinear and nonperturbative optical response of biased bilayer graphene subjected to bichromatic strong laser fields. This response is analyzed using a four-band Hamiltonian derived from \textit{ab initio} calculations. For the laser-stimulated dynamics, we employ structure gauge-invariant evolutionary equations to accurately describe the evolution of the single-particle density matrix across the entire Brillouin zone. The resonant generation of electron-hole pairs by the high-frequency component of the field, combined with the induction of high-order harmonic generation and high-order wave mixing by the strong low-frequency field component, leads to significant alterations in the resulting spectra. These changes are driven by the effects of Berry curvature and the shift vector, which modify the relative contributions of interband and intraband channels, thereby fundamentally reshaping the radiation spectra at high-order frequency multiplication. The numerical results are further supported by approximate analytical calculations, demonstrating that high-order wave mixing can be modeled using the classical trajectory analysis of electron-hole pairs, with Berry curvature and the shift vector significantly influencing the saddle-point equations.

cond-mat.mes-hall↗

arXiv2023

Intense anomalous high harmonics in graphene quantum dots caused by disorder or vacancies

This article aims to study the linear and nonlinear optical response of inversion symmetric graphene quantum dots (GQDs) in the presence of on-site disorder or vacancies. The presence of disorder or vacancy breaks the special inversion symmetry leading to the emergence of intense Hall-type anomalous harmonics. This phenomenon is attributed to the intrinsic time-reversal symmetry-breaking in quantum dots with a pseudo-relativistic Hamiltonian, even in the absence of an external magnetic field. We demonstrate that the effects induced by disorder or vacancy have a distinct impact on the optical response of GQDs. In the linear response, we observe significant Hall conductivity. In the presence of an intense laser field, we observe the radiation of strong anomalous odd and even-order harmonics already for relatively small levels of disorder or mono-vacancy. The both disorder and vacancy lift the degeneracy of states, thereby creating new channels for interband transitions and enhancing the emission of near-cutoff high-harmonic signals.

cond-mat.mes-hall↗

arXiv2023

Long-range correlation-induced effects at high-order harmonic generation on graphene quantum dots

This paper focuses on investigating high-order harmonic generation (HHG) in graphene quantum dots (GQDs) under intense near-infrared laser fields. To model the GQD and its interaction with the laser field, we utilize a mean-field approach. Our analysis of the HHG power spectrum reveals fine structures and a noticeable enhancement in cutoff harmonics due to the long-range correlations. We also demonstrate the essential role of Coulomb interaction in determining of harmonics intensities and cutoff position. Unlike atomic HHG, where the cutoff energy is proportional to the pump wave intensity, in GQDs the cutoff energy scales with the square root of the field strength amplitude. A detailed time-frequency analysis of the entire range of HHG spectrum is presented using a wavelet transform. The analysis reveals intricate details of the spectral and temporal fine structures of HHG, offering insights into the various HHG mechanisms in GQDs.

cond-mat.mes-hall↗

arXiv2023

Intense high-order harmonic generation in giant fullerene molecule C$_{240}$

In this work the extreme nonlinear optical response of a giant fullerene molecule C$_{240}$ in strong laser field is studied. The investigation of high-order harmonic generation in such quantum nanostructure is presented modeling the C$_{240}$ molecule and its interaction with the laser field in the scope of the tight-binding mean-field approach. Electron-electron interaction is modeled by the parametrized Ohno potentail, which takes into account long-range Coulomb interaction. The essential role of many body Coulomb interaction in determining of harmonics intensities is demonstrated. We also consider vacancy-deffected molecule C$_{240}$. The presence of a single vacancy breaks the icosahedral symmetry leading to the emergence of intense even-order harmonics. We examine the dependence of moderate harmonics on laser frequency that shows the multiphoton resonant nature of high harmonics generation. The dependence of cutoff harmonics on both laser intensity and frequency are examined too.

physics.optics↗

arXiv2023

Disorder-induced effects in high-harmonic generation process in fullerene molecules

The objective of this article is to investigate the profound nonlinear optical response exhibited by inversion symmetric fullerene molecules under the influence of different types of disorders described by the Anderson model. Our aim is to elucidate the localization effects on the spectra of high harmonic generation in such molecules. We show that the disorder-induced effects are imprinted onto molecules' high-harmonic spectrum. Specifically, we observe a presence of strong even-order harmonic signals already for relatively small disorders. The odd-order harmonics intrinsic for disorder-free systems are generally robust to minor disorders. Both diagonal and off-diagonal disorders lift the degeneracy of states, opening up new channels for interband transitions, leading to the enhancement of the high-harmonic emission. The second harmonic signal has a special behavior depending on the disorder strength. Specifically in the case of diagonal disorder, the second harmonic intensity exhibits a quadratic scaling with the disorder strength, which enables the usage of the harmonic spectrum as a tool in measuring the type and the strength of a disorder.

physics.optics↗

arXiv2023

Graphene valley polarization as a function of carrier-envelope phase in few-cycle laser pulses and its footprints in harmonic signals

We consider coherent dynamics of graphene charged carriers exposed to an intense few-cycle linearly polarized laser pulse. The results, obtained by solving the generalized semiconductor Bloch equations numerically in the Hartree-Fock approximation, taking into account many-body Coulomb interaction, demonstrate strong dependence of the valley polarization on the carrier-envelope phase (CEP), which is interpolated by the simple sinusoidal law. Then we consider harmonic generation in multi-cycle laser field by graphene preliminary exposed to an intense few-cycle laser pulse. We show that the second harmonic's intensity is a robust observable quantity that provides a gauge of CEP for pulse durations up to two optical cycles, corresponding to 40 $\mathrm{fs}$ at the wavelength of 6.2 $\mathrm{μm}$.

physics.optics↗

arXiv2022

High-order harmonic generation in three-dimensional Weyl semimetals

In this paper, the nonlinear interaction of Weyl semimetal (WSM) with a strong driving electromagnetic wave-field is investigated. In the scope of the structure-gauge invariant low-energy nonlinear electrodynamic theory, the polarization-resolved high-order harmonic generation spectra in WSM are analyzed. The obtained results show that the spectra in WSM are completely different compared to 2D graphene case. In particular, at the non-collinear arrangement of the electric and Weyl nodes' momentum separation vectors, an anomalous harmonics are generated which are polarised perpendicular to the pump wave electric field. The intensities of anomalous harmonics are quadratically dependent on the momentum space separation of the Weyl nodes. If the right and the left Weyl fermions are merged, we have a 4-component trivial massless Dirac fermion and, as a consequence, the anomalous harmonics vanish. In contrast to the anomalous harmonics, the intensities of normal harmonics do not depend on the Weyl nodes' momentum separation vector, and the harmonics spectra resemble the picture for a massless 3D Dirac fermion.

cond-mat.mes-hall↗

arXiv2022

The saddle-point exciton signature on high harmonic generation in 2D hexagonal nanostructures

The disclosure of basic nonlinear optical properties of graphene-like nanostructures with correlated electron-hole nonlinear dynamics over a wide range of frequencies and pump field intensities is of great importance for both graphene fundamental physics and for expected novel applications of 2D hexagonal nanostructures in extreme nonlinear optics. In the current paper, the nonlinear interaction of 2D hexagonal nanostructures with the bichromatic infrared driving field taking into account many-body Coulomb interaction is investigated. Numerical investigation in the scope of the Bloch equations within the Houston basis that take into account $e-e$ and $e-h$ interactions in the Hartree-Fock approximation reveals significant excitonic effects in the high harmonic generation process in 2D hexagonal nanostructures such as graphene and silicene. It is shown that due to the correlated electron-hole nonlinear dynamics around the van Hove singularity, spectral caustics in the high harmonic generation spectrum are induced near the saddle point excitonic resonances.

cond-mat.mes-hall↗

arXiv2021

High harmonic generation in fullerene molecules

Using dynamical Hartree-Fock mean-field theory, we study the high-harmonic generation (HHG) in the fullerene molecules C$_{60}$ and C$_{70}$ under strong pump wave driving. We consider a strong-field regime and show that the output harmonic radiation exhibits multiple plateaus, whose borders are defined by the molecular excitonic lines and cutoff energies within each plateau scale linearly with the field strength amplitude. In contrast to atomic cases for the fullerene molecule, with the increase of the pump wave photon energy the cutoff harmonic energy is increased. We also show that with the increase of the electron-electron interaction energy overall the HHG rate is suppressed. We demonstrate that the C$_{70}$ molecule shows richer HHG spectra and a stronger high-harmonic intensity than the C$_{60}$.

physics.atom-ph↗

arXiv2020

Free electron nanolaser based on the graphene plasmons

In this paper, a possible way to achieve lasing from THz to extreme UV domain due to stimulated scattering of graphene plasmons on the free electrons is considered. The analytical-quantitative description of the proposed FEL scheme is based on the self-consistent set of the Maxwell--Vlasov equations. We study the downconversion as well as the upconversion. It is shown that the coherent downconversion of infrared radiation to THz one can be achieved using a source of very non-relativistic electrons at the resonant coupling with the graphene plasmons. Due to the strongly confined graphene plasmons, the upconversion of mid-infrared to extreme UV radiation can be achieved with the mildly relativistic electron beams. The latter is a promising mechanism for the tabletop short-wavelength free electron nanolaser.

cond-mat.mes-hall↗

arXiv2020

Microscopic quantum description of second-order nonlinearities in 2D hexagonal nanostructures beyond the Dirac cone approximation

Single layers of hexagonal two-dimensional nanostructures such as graphene, silicene, and germanene exhibit large carrier Fermi velocities and, consequently, large light-matter coupling strength making these materials promising elements for nano-opto-electronics. Although these materials are centrosymmetric, the spatial dispersion turns out to be quite large allowing the second-order nonlinear response of such materials to be comparable to the non-centrosymmetric 2D ones. The second-order response of massless Dirac fermions has been extensively studied, however a general approach correct over the full Brillouin zone is lacking so far. To complete this gap, in the current paper we develop a general quantum-mechanical theory of the in-plane second-order nonlinear response beyond the Dirac cone approximation and applicable to the full Brillouin zone of the hexagonal tight-binding nanostructures. We present explicit calculation of the nonlinear susceptibility tensor of 2D hexagonal nanostructures applicable to arbitrary three-wave mixing processes.

cond-mat.mes-hall↗

arXiv2020

High laser harmonics induced by the Berry curvature in time-reversal invariant materials

A new nonlinear scheme of high harmonics generation in a wide class of time-reversal invariant materials with broken spatial inversion symmetry (where recently the nonlinear Hall effect has been established) due to the nontrivial topology of bands is proposed. A microscopic quasiclassical theory describing the nonperturbative optical response of pseudo-relativistic electrons with nonzero Berry curvature of bands to a strong laser field is developed. We analyze the harmonic content of the induced current and show that one can decouple induced laser harmonics solely by the Berry curvature of bands. We also study the dependence of the nonlinear response on the driving wave and system parameters.

cond-mat.mes-hall↗

arXiv2020

On the extreme nonlinear optics of graphene nanoribbons in the strong coherent radiation fields

The generation of high-order harmonics in quasi-one-dimensional graphene nanoribbons (GNRs) initiated by intense coherent radiation is investigated. A microscopic theory describing the extreme nonlinear optical response of GNRs is developed. The closed set of differential equations for the single-particle density matrix at the GNR-strong laser field multiphoton interaction is solved numerically. The obtained solutions indicate the significance of the bandgap width and Fermi energy level on the high-order harmonic generation process in GNRs.

cond-mat.mes-hall↗

arXiv2019

High-order harmonic generation in gapped bilayer graphene

Microscopic nonlinear quantum theory of interaction of coherent electromagnetic radiation with gapped bilayer graphene is developed. The Liouville-von Neumann equation for the density matrix is solved numerically at the multiphoton excitation regime. The developed theory of interaction of charged carriers with strong driving wave field is valid near the Dirac points of the Brillouin zone. We consider the harmonic generation process in the nonadiabatic regime of interaction when the Keldysh parameter is of the order of unity. On the basis of numerical solutions, we examine the rates of odd and even high-harmonics at the particle-hole annihilation in the field of a strong pump wave of arbitrary polarization. Obtained results show that the gapped bilayer graphene can serve as an effective medium for generation of even and odd high harmonics in the THz and far infrared domains of frequencies.

cond-mat.mes-hall↗

arXiv2018

Impact of electron-electron Coulomb interaction on the high harmonic generation process in graphene

Generation of high harmonics in a monolayer graphene initiated by strong coherent radiation field, taking into account electron-electron Coulomb interaction is investigated. A microscopic theory describing the nonlinear optical response of graphene is developed. The Coulomb interaction of electrons is treated in the scope of dynamic Hartree-Fock approximation. The closed set of integrodifferential equations for the single-particle density matrix of a graphene quantum structure is solved numerically. The obtained solutions show the significance of many-body Coulomb interaction on the high harmonic generation process in graphene.

cond-mat.mes-hall↗

arXiv2017

Multiphoton excitation and high-harmonics generation in topological insulator

Multiphoton interaction of coherent electromagnetic radiation with 2D metallic carriers confined on the surface of the 3D topological insulator is considered. A microscopic theory describing the nonlinear interaction of a strong wave and metallic carriers with many-body Coulomb interaction is developed. The set of integrodifferential equations for the interband polarization and carrier occupation distribution is solved numerically. Multiphoton excitation of Fermi-Dirac sea of 2D massless carriers is considered for a THz pump wave. It is shown that in the moderately strong pump wave field along with multiphoton interband/intraband transitions the intense radiation of high harmonics takes place.

cond-mat.mes-hall↗