Browsing by Author "Hewageegana, P."

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Absolute phase effect in ultrafast optical responses of metal nanostructures
(APPLIED PHYSICS A: MATERIALS SCIENCE & PROCESSING, 2007) Stockman, M.I.; Hewageegana, P.
We predict that nonlinear ultrafast electron photoemission by strong optical fields and, potentially, other nonlinear optical responses of metal nanostructures significantly depend on the absolute (carrier?envelope) phase of excitation pulses. Strong enhancement of the local optical fields produces these responses at excitation intensities lower by order(s) of magnitude than for known systems. Prospective applications include control of ultrafast electron emission and electron injection into nanosystems. A wider class of prospective applications is the determination of the absolute phase of pulses emitted by lasers and atoms, molecules, and condensed matter at relatively low intensities.
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.
Expanding universe with a time-dependent cosmological constant
(4th International Research Symposium on Pure and Applied Sciences, Faculty of Science, University of Kelaniya, Sri Lanka, 2019) Jayawardane, S.; Katugampola, K. D. W. J.; Hewageegana, P.
It is possible to examine the expansion of universe with an accelerating epoch. The prophecy of an expanding universe from Einstein's Field Equations were affirmed by the perceptions of Edwin Hubble and Adam G. Reises in mid twentieth century. Late perceptions of Saul Perlmutter on expansion of the universe with an increasing speed in a specific epoch has awakened the search for new cosmological models that reflect universe expansion with an acceleration
Graphene Quantum Dots
(2010) Hewageegana, P.; Apalkov, V.
Investigation of temperature and density profile of brine in evaporating salt pans at Palavi in north-western Sri Lanka
(2001) Hewageegana, P.; Amarasekara, C.D.; Jayakody, J.R.P.; Punyasena, M.A.
Measurements of temperature and density variations with depth in salt pans having a stable salinity gradient are presented. The density variation of brine has been correlated to the temperature variation with depth. The convective and nonconvective zones were identified. Gradients in salinity and temperature in the nonconvective zone were observed. Temperature and salinity were observed to remain the same throughout the convective bottom layer of brine in the ponds. The investigation has been extended to different ponds filled to different depths and storage periods of brine. A maximum temperature of around 55 0C was observed in the bottom layers while the surface remained as low as 29 0C. This is a consequence of positive salt-density gradient, which suppresses convection and allows a temperature gradient to develop downwards. The temperature measurements of different ponds filled to different heights indicate that the convective bottom layer temperature of a pond will increase if the pond is filled with brine to a higher level. These results indicate that the large area deep salt pans with long term storage periods can be used as solar ponds to collect and store solar energy in addition to the usual salt production. Based on our results we speculate that the bottom temperatures of these ponds could be increased to about 80 0C by filling the ponds to a height of about 1.5 meters. Thermal energy stored in the convective bottom layer of these ponds could be extracted for many applications. The moderate temperature desalination of seawater especially in the areas where fresh water is in short supply is identified as one of the promising applications of thermal energy stored in solar pond-salt pans.
Localized modes of an electron by topological insulator defect
(Faculty of Science, University of Kelaniya, Sri Lanka, 2016) Kumara, R.D.M.M.; Hewageegana, P.
Topological insulators, a new class of materials discovered recently, have very unique topological properties. While the surface states are gapless and are topologically protected by the time reversal symmetry, the bulk of topological insulator shows a full gap. We study the properties of reflected and transmitted electron waves on the surface of topological insulator nanofilm with a defect. We show that an electron incident on a defect produces modes, which are localized at the defect. Such modes result in an enhancement of electron density at the defect. If an electron with a given energy E is incident on a region of the TI nanofilm which does not support electron propagating waves at energy E, then there is a strong enhancement of local electron density near the boundary of such region. We calculate the reflectance and transmittance of an electron from such a defect for different parameters of the topological insulator nanofilm and different parameters of the defect. The amplitude of the electron wave strongly depends on the position of electron energy. If electron energy is in the energy gap of defect, then the reflectance is 1. This result corresponds to total internal reflection. If the electron energy is not in the energy gap of defect, then the reflectance is small and the electron is almost totally transmitted. The critical points dividing these two boundaries are defined by the condition that the electron energy is at the band edge of energy dispersion. The dependence of the electron wave on the parameters of the system, such as thickness of the nanofilm or electron energy, is dominated near the critical points.
A modified equation for Roche limit for celestial bodies with a fluid-like structure
(Faculty of Science, University of Kelaniya Sri Lanka, 2023) Thammitage, S. U. R.; Perera, P. A. A.; Hewageegana, P.
The Roche limit is a concept in astronomy that describes the minimum distance that a celestial body (say satellite) can approach another celestial body (say planet) without being torn into small pieces by tidal forces. This happens when the tidal forces generated on the satellite by the gravitational fields of the planet exceed the self-attractional forces of the satellite. If the planet and satellite are of similar chemical composition, the theoretical Roche limit is about 2.5 times the radius of the planet. Generally, two models have been used for the derivation of Roche limit. If the satellite is assumed to be a solid object, the Roche limit is given by 𝑑 = 1.22𝑅𝑝 (𝜌𝑝 𝜌𝑠) 1 3 and if the physical properties of the satellite are akin to those of a fluid, the expression turns out to be 𝑑 = 2.44𝑅𝑝 (𝜌𝑝 𝜌𝑠)13. Here 𝜌𝑝 and 𝜌𝑠 are the densities of the planet and satellite, respectively and 𝑅𝑝 is the radius of the planet. These equations have been derived in real physical space for the case where the motion of the satellite and the planet are in the same orbital plane, that is both objects must be in the same equatorial plane. But satellites or asteroids orbit different planes or can enter from another plane like Pluto’s orbit. Therefore, a new equation for Roche limit for fluid like satellites, when the equatorial plane of the satellite is tilted to the orbital plane was developed using the tidal generating potential equation and self-gravitational potential equation. This equation takes the form 𝑑 = 2.423𝑅𝑝 (𝜌𝑝 𝜌𝑠) 13 (cos2 𝛼 – 1 3)13, where 𝛼 is the angle between the orbital plane of the satellite and its equatorial plane.
Nanolocalized Nonlinear Electron Photoemission under Coherent Control
(Nano Letters, 2005) Stockman, M.I.; Hewageegana, P.
We theoretically show that two-photon coherent control yields electron photoemission from metal nanostructures that is localized in nanosize hot spots whose positions are controllable on a nanometer scale, in agreement with recent experiments. We propose to use silver V-shapes as tailored nanoantennas for which the position of the coherently controllable photoelectron emission hot spot can be deterministically predicted. We predict that the low-frequency, high-intensity (quasi-stationary) excitation of the photoemission leads to an exponentially high contrast of the coherent control.
Plasmonic enhancing nanoantennas for photodetection
(Infrared Physics & Technology, 2007) Hewageegana, P.; Stockman, M.I.
This paper discusses the use of plasmonic nanostructured systems as nanoantennas for photodetection. Even though semiconductors and their heterostructures have many useful properties and widely used in photodetection, their electron density is very small compared to that of metals and, therefore, they have low absorption cross sections. The idea of using metal nanostructured antennas is to combine the high optical responses of metals with the functional electric properties of semiconductors.
Polarizability per Unit Length of a Nanowire including Nonlocal Effects
(University of Kelaniya, 2012) Hewageegana, P.
It is well known that the electronic and optical properties of very small structures such as, nanospheres, nanowires, nanocavities and nanocylinders are very different from those of the corresponding bulk materials because of the surface effects. Nanosystems possess unique properties different from those of macroscopic materials when characteristic lengths govern their properties. Therefore, the spatial dispersion becomes much more important where the characteristic size of the particle or distances between them becomes comparable to the characteristic scale of the system. We have developed a method for calculating transverse static polarizability (per unit length) of a bulk nanowire by taking into account the temporal and spatial dispersion. To describe these phenomena, we developed an analytical theory based on local random-phase approximation and plasmon pole approximation. This theory is very general in the sense that it can be applied to any material which can be characterized by a bulk dielectric function of the form ) , (  k . The theory is applied to calculate the transverse static polarizability of dielectric and metallic nanowires.
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.
Research in Buddhism and Physics
(Research in Buddhism and Physics) Galmangoda, Sumanapala; Hewageegana, P.
Theory of Perception According to Buddhism everything in the world is impermanent. Everything is momentary. But there is a difference of life-span of mind and matter. The life-span of consciousness is termed as mind-moment (cittakkha%a). The mind-moment is so infinitesimal that millions of mind-moments can occur during the period of blinking eyes. Further each mind-moment consists of three sub-moments: Moment of arising Moment of existence Moment of destruction The dissolution of the mind-moment provides the condition for arising of another consciousness and it also passes the three moments. Material phenomena also pass through the same stages of arising, existence and destruction. But the time that requires for passing the three stages of material phenomena is equal to seventeen mind-moments. The stages of arising and destruction are equal in duration regarding both material and mental processes. Therefore the moment of existence of material phenomena is equal to forty-nine sub-moments of mental phenomena. The equation can be presented as follows: Approximately the duration of mind-moment and sub-moment is as follows: 1 sub-moment = 0.000250 seconds (250?10-6 seconds) or 0.250 milliseconds or 250 microseconds 1 mind-moment = 0.000750 seconds (750?10-6 seconds) or 0.750 milliseconds or 750 microseconds Therefore 17 mind-moments or 51 sub-moments is 0.013 seconds (13?10-3 seconds) or 13 milliseconds. Recent research at Massachusetts Institute of Technology in USA has proved that the duration of life-span of material phenomena (arising, existing and destruction) is 0.013 second or 13 milliseconds. (Mary C. Potter, Brad Wyble, Carl Erick Hagmann and Emily S. McCourt, Massachusetts Institute of Technology, Journal: Attention, Perception and Psychophysics, 2014) According to Theravada theory of perception explained in the Abhidhamma a full process of sense-perception consists of 17 mind-moments or 51 sub mind-moments. During this period of time material phenomena arises, exists and disappears. The 17 mind-moments or 51 sub mind-moments are equal to 0.013 seconds (13?10-3 seconds) or 13 milliseconds. Note that average time taken for blink of an eye is 0.250 seconds (250x10-3 seconds) or 250 milliseconds. Therefore, within a blink of an eye 20 material phenomena may arise, exist and disappear. It is amazing that the 21st century discovery in science approximately equal to Abhidhammic analyses of Sri Lankan Theravada tradition belonging to the 5th century A. D. According to the Theravada Abhidhammic theory of sense-perception two kinds of processes have been identified. i. Perception related to five senses ii. Perception between mind and thoughts The processes of perception are not complete all the time. A complete process of perception between five physical senses and five sense-objects (material) takes 17 mind-moments and each of them consists of three sub-moments. The 17 mind-moments are as follows: i. Stream of sub-consciousness ii. Vibration of sub-consciousness iii. Breaking of sub-consciousness iv. Stimulation of five sense-consciousnesses v. Sense-consciousness vi. Receiving consciousness vii. Investigating consciousness viii. Determining consciousness ix. x. xi. xii. xiii. xiv. xv. xvi. xvii. An article of the Buddhist theory or perception will be published very soon. Reference ? *1 millisecond = 1/1000 second **1 microsecond = 1/1000000 second Ed. Bhikkhu Bodhi, A Comprehensive Manual of Abhidhamma, Buddhist Publication Society, Kandy, Sri Lanka, 1999, pp. 148-162 G. D. Sumanapala, An Introduction to Theravada Abhidhamma, Singapore, 1998 G. D. Sumanapala, Abhidhammic Interpretations of Early Buddhist Teachings, Singapore, 2005
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.
Strong photoelectron emission from silver nanostructures
(Journal of Science of the University of Kelaniya Sri Lanka, 2010) Hewageegana, P.
Localized optically-nonlinear photoelectron emission from metal (silver) nanostructures under two-pulse emission has been developed for relatively low energy. For two-photon emission from random metal nanostructures, it has been shown that the coherent control allows one to move nanosize hot spots whose positions are controllable on a nanometer scale. It is proposed to use silver random planer composite and introduce this photoemission process as an ultrafast process, on femtosecond or subfemtosecond scale.
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.