arXiv subjects

Vadym Apalkov

Publications and source records attributed to Vadym Apalkov.

27 records · Page 2Linked to original sources

arXiv2018

Ultrafast topological phenomena in gapped graphene

In the model of gapped graphene, we have shown how the recently predicted topological resonances are solely related to the presence of an energy band gap at the $K$ and $K^\prime$ points of the Brillouin zone. In the field of a strong single-oscillation chiral (circularly-polarized) optical pulse, the topological resonance causes the valley-selective population of the conduction band. This population distribution represents a chiral texture in the reciprocal space that is structured with respect to the pulse separatrix as has earlier been predicted for transition metal dichalcogenides. As the band gap is switched off, this chirality gradually disappears replaced by an achiral distribution characteristic of graphene.

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arXiv2018

Fundamentally fastest optical processes at the surface of a topological insulator

We predict that a single oscillation of a strong optical pulse can significantly populate the surface conduction band of a three-dimensional topological insulator, Bi2Se3. Both linearly- and circularly-polarized pulses generate chiral textures of interference fringes of population in the surface Brillouin zone. These fringes constitute a self-referenced electron hologram carrying information on the topology of the surface Bloch bands, in particular, on the effect of the warping term of the low-energy Hamiltonian. These electron-interference phenomena are in a sharp contrast to graphene where there are no chiral textures for a linearly-polarized pulse and no interference fringes for circularly-polarized pulse. These predicted reciprocal space electron-population textures can be measured experimentally by time resolved angle resolved photoelectron spectroscopy (TR-ARPES) to gain direct access to non-Abelian Berry curvature at topological insulator surfaces.

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arXiv2018

Femtosecond valley polarization and topological resonances in transition metal dichalcogenides

We theoretically introduce the fundamentally fastest induction of a significant population and valley polarization in a monolayer of a transition metal dichalcogenide (i.e., $\mathrm{MoS_2}$ and $\mathrm{WS_2}$). This may be extended to other two-dimensional materials with the same symmetry. This valley polarization can be written and read-out by a pulse consisting of just a single optical oscillation with a duration of a few femtoseconds and an amplitude of $\sim0.2~\mathrm V/\mathrmÅ$. Under these conditions, we predict a new effect of {\em topological resonance}, which is due to Bloch motion of electrons in the reciprocal space where electron population textures are formed defined by non-Abelian Berry curvature. The predicted phenomena can be applied for information storage and processing in PHz-band optoelectronics.

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arXiv2015

Ultrafast field control of symmetry, reciprocity, and reversibility in buckled graphene-like materials

We theoretically show that buckled two-dimensional graphene-like materials (silicene and germanene) subjected to a femtosecond strong optical pulse can be controlled by the optical field component normal to their plane. In such strong fields, these materials are predicted to exhibit non-reciprocal reflection, optical rectification and generation of electric currents both parallel and normal to the in-plane field direction. Reversibility of the conduction band population is also field- and carrier-envelope phase controllable. There is a net charge transfer along the material plane that is also dependent on the normal field component. Thus a graphene-like buckled material behaves analogously to a field-effect transistor controlled and driven by the electric field of light with subcycle (femtosecond) speed.

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arXiv2014

Wannier-Stark states of graphene monolayer in strong electric field

We find theoretically energy spectrum of a graphene monolayer in a strong constant electric field using a tight-binding model. Within a single band, we find quantized equidistant energy levels (Wannier-Stark ladder), separated by the Bloch frequency. Singular interband coupling results in mixing of the states of different bands and anticrossing of corresponding levels, which is described analytically near Dirac points and is related to the Pancharatnam-Berry phase. The rate of interband tunneling, which is proportional to the anticrossing gaps in the spectrum, is only inversely proportional to the tunneling distance, in a sharp contrast to conventional solids where this dependence is exponential. This singularity will have major consequences for graphene behavior in strong ultrafast optical fields, in particular, leading to non-adiabaticity of electron excitation dynamics.

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arXiv2014

Gap Structure of the Hofstadter System of Interacting Dirac Fermions in Graphene

The effects of mutual Coulomb interactions between Dirac fermions in monolayer graphene on the Hofstadter energy spectrum have been studied. Our studies indicate that the effects of the interaction depend strongly on the amplitude of the periodic potential. For large amplitudes the interaction effects are small and the properties of the system are primarily determined by the periodic potential but for small amplitudes the interaction greatly influences the band gap. The signature of the interaction effects in the Hofstadter system can be probed through magnetization where the Coulomb effects are dominant for small amplitudes of the periodic potential.

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arXiv2014

Interaction of graphene monolayer with ultrashort laser pulse

We study the interaction of graphene with ultrashort few femtosecond long optical pulse. For such a short pulse, the electron dynamics is coherent and is described within the tight-binding model of graphene. The interaction of optical pulse with graphene is determined by strong wave vector dependence of the interband dipole matrix elements, which are singular at the Dirac points of graphene. The electron dynamics in optical pulse is highly irreversible with large residual population of the conduction band. The residual conduction band population as a function of the wave vector is nonuniform with a few localized spots of high conduction band population. The spots are located near the Dirac points and the number of spots depends on the pulse intensity. The optical pulse propagating through graphene layer generates finite transferred charge, which, as a function of pulse intensity, changes its sign. At small pulse intensity, the charge is transferred in the direction of the pulse maximum, while at large pulse intensity, the direction of the charge transfer is opposite to the direction of pulse maximum. This property opens unique possibility of controlling the direction of the charge transfer by variation of the pulse intensity.

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arXiv2013

Proposed Graphene Nanospaser

We show that graphene monolayer coupled to multi-quantum well system can form a graphene spaser, which is a coherent quantum generator of surface plasmons in graphene. The active element of graphene spaser is a multi-quantum well system with a design similar to the design of an active element of quantum cascade laser. Under realistic parameters of the multi-quantum well system, the spasing in graphene monolayer can be achieved at a finite doping of graphene and at a plasmon frequency, 0.15 eV, close to the typical frequency of intersubband transitions in multi-quantum well systems.

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arXiv2012

Metal nanofilm in strong ultrafast optical fields

We predict that a metal nanofilm subjected to an ultrashort (single oscillation) optical pulse of a high field amplitude $\sim 3 \mathrm{V/Å}$ at normal incidence undergoes an ultrafast (at subcycle times $\lesssim 1 \mathrm{fs}$) transition to a state resembling semimetal. Its reflectivity is greatly reduced, while the transmissivity and the optical field inside the metal are greatly increased. The temporal profiles of the optical fields are predicted to exhibit pronounced subcycle oscillations, which are attributed to the Bloch oscillations and formation of the Wannier-Stark ladder of electronic states. The reflected, transmitted, and inside-the-metal pulses have non-zero areas approaching half-cycle pulses. The effects predicted are promising for applications to nanoplasmonic modulators and field-effect transistors with petahertz bandwidth.

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