Browsing by Author "Apalkov, V."

Now showing 1 - 10 of 10

Circularly-polarized-pulse-driven ultrafast optical currents in monolayer hexagonal Boron Nitride (h-BN)
(Solid State Communications, 2022) Hewageegana, P.; Apalkov, V.
We predict the fundamentally fastest, ultrafast optical currents in monolayer hexagonal Boron Nitride (h-BN) by a circularly-polarized single-oscillation optical pulse. The femtosecond currents in gapped graphene and transition metal dichalcogenides have been discussed. However, the extension of the gapped graphene model for the large bandgap () has not been shown before. The strong optical pulse redistributes electrons between the bands and generates femtosecond currents during the pulse. The pulse generates both direction and direction currents due to charge transfer through the system. Thus, femtosecond ultrashort laser pulses provide an effective tool to manipulate and study the transport properties of electron systems and enhance the conductivity in solids at an ultrafast time scale with high temporal resolution. Ultrafast currents and charge transfer in monolayer h-BN may provide a fundamental basis for petahertz-band information processing.
Circularly-polarized-pulse-driven ultrafast optical currents in monolayer hexagonal Boron Nitride (h-BN)
(Solid State Communications, 2022) Hewageegana, P.; Apalkov, V.
We predict the fundamentally fastest, ultrafast optical currents in monolayer hexagonal Boron Nitride (h-BN) by a circularly-polarized single-oscillation optical pulse. The femtosecond currents in gapped graphene and transition metal dichalcogenides have been discussed. However, the extension of the gapped graphene model for the large bandgap () has not been shown before. The strong optical pulse redistributes electrons between the bands and generates femtosecond currents during the pulse. The pulse generates both direction and direction currents due to charge transfer through the system. Thus, femtosecond ultrashort laser pulses provide an effective tool to manipulate and study the transport properties of electron systems and enhance the conductivity in solids at an ultrafast time scale with high temporal resolution. Ultrafast currents and charge transfer in monolayer h-BN may provide a fundamental basis for petahertz-band information processing.
Electron localization in graphene quantum dots
(Physical Review B, 2008) Hewageegana, P.; Apalkov, V.
We study theoretically a localized state of an electron in a graphene quantum dot with a sharp boundary. Due to Klein?s tunneling, the ?relativistic? electron in graphene cannot be localized by any confinement potential. In this case the electronic states in a graphene quantum dot become resonances with finite trapping time. We consider these resonances as the states with complex energy. To find the energy of these states we solve the time-independent Schr�dinger equation with outgoing boundary conditions at infinity. The imaginary part of the energy determines the width of the resonances and the trapping time of an electron within quantum dot. We show that if the parameters of the confinement potential satisfy a special condition, then the electron can be strongly localized in such quantum dot, i.e., the trapping time is infinitely large. In this case the electron localization is due to interference effects. We show how the deviation from this condition affects the trapping time of an electron. We also analyze the energy spectra of an electron in a graphene quantum ring with a sharp boundary. We show that in this case the condition of constructive interference can be tuned by varying internal radius of the ring, i.e., parameters of confinement potential.
Enhanced mid-infrared transmission through a metallic diffraction grating
(Journal of Physics: Condensed Matter, 2008) Hewageegana, P.; Apalkov, V.
We study theoretically an enhancement of the intensity of mid-infrared light transmitted through a metallic diffraction grating. We show that for s-polarized light the enhancement of the transmitted light is much stronger than for p-polarized light. By tuning the parameters of the diffraction grating, the enhancement of the transmitted light can be increased by a few orders of magnitude. The spatial distribution of the transmitted light is highly nonuniform with very sharp peaks, which have spatial widths of about 10 nm.
Graphene Quantum Dots
(2010) Hewageegana, P.; Apalkov, V.
A quantum dot in topological insulator nanofilm
(2014) Herath, T.M.; Hewageegana, P.; Apalkov, V.
Quantum dot photodetectors with metallic diffraction grating: Surface plasmons and strong absorption enhancement
(Physica E: Low-dimensional Systems and Nanostructures, 2008) Hewageegana, P.; Apalkov, V.
We report our theoretical study of the effect of metallic diffraction grating on the sensitivity of quantum dot photodetectors in terahertz frequency. We have found that the effect of diffraction grating is much stronger for s-polarization than for p-polarization. For s-polarization the sensitivity of photodetectors can be enhanced by metallic diffraction grating by a few orders of magnitude. Due to strongly inhomogeneous distribution of electromagnetic field the quantum dots should be placed at special points, i.e. hot spots, where the field intensity is maximum.
Second harmonic generation in disordered media: Random resonators
(Physical Review B, 2008) Hewageegana, P.; Apalkov, V.
We theoretically study the effect of random resonators on the conversion efficiency of fundamental mode propagating through disordered nonlinear dielectric film. Only resonators with double-resonant properties, i.e., which can trap both the fundamental and second harmonic modes, contribute to local generation of the second harmonic light of high intensity. We calculate the density of such resonators. The parameters of the random media under which all the random resonators with the given quality factors have the double-resonant properties are found.
Theoretical study of terahertz quantum well photodetectors: Effect of metallic diffraction coating
(Infrared Physics & Technology, 2008) Hewageegana, P.; Apalkov, V.
The possibility of enhancement of sensitivity of quantum well photodetectors by adding metallic diffraction coating on top of the dielectric layer of photodetectors is studied. With the grating the spatial distribution of the intensity of electromagnetic wave within the active region of the photodetector is highly non-uniform with the intensity variation over a few orders of magnitude within a period of the grating. This effect is due to the coupling of surface plasmon with incident electromagnetic wave. At terahertz frequencies the average intensity of the transmitted radiation wave through the grating strongly depends on the dielectric constant of metal.
Trapping of an electron in coupled quantum dots in graphene
(Physical Review B, 2009) Hewageegana, P.; Apalkov, V.
Due to Klein?s tunneling the electronic states of a quantum dot in graphene have finite widths and an electron in quantum dot has a finite trapping time. This property introduces a special type of interdot coupling in a system of many quantum dots in graphene. The interdot coupling is realized not as a direct tunneling between quantum dots but as coupling through the continuum states of graphene. As a result the interdot coupling modifies both the positions and the widths of the energy levels of the quantum dot system. We study the system of quantum dots in graphene theoretically by analyzing the complex energy spectra of the quantum dot system. We show that in a double-dot system some energy levels become strongly localized with an infinite trapping time. Such strongly localized states are achieved only at one value of the interdot separation. We also study a periodic array of quantum dots in graphene within a tight-binding mode for a quantum dot system. The values of the hopping integrals in the tight-binding model are found from the expression for the energy spectra of the double quantum dot system. In the array of quantum dots the states with infinitely large trapping time are realized at all values of interdot separation smaller than some critical value. Such states have nonzero wave vectors.