Controlling high-harmonic generation from strain engineered monolayer phosphorene

Phosphorene, a well-studied 2D allotrope of phosphorus, features unique properties such as widely tunable bandgap, high carrier mobility, and remarkable intrinsic in-plane anisotropy. Utilizing these structural and electronic properties, we investigate ultrafast electron dynamics and high harmonic generation (HHG) from phosphorene subject to band structure engineering through external strain, based on ab initio time-dependent density-functional theory approach. We show that strong field processes in such systems can be optimized and controlled by biaxial tensile and compressive strain engineering, that results in electronic structure modification. While -10% strain resulted in closing of band gap, 2% strain increased the gap by 22% with respect to 0.9 eV in pristine phosphorene, consequently affecting the high harmonic yield. With reduction of gap, by applying strain from 2% to -10%, the valence band near $\Gamma-$point becomes more flat and discreet, resulting in large electronic density of states and enhanced electronic excitation, which reflects in their ultrafast sub-cycle dynamics under laser excitation. Moreover, due to its intrinsic in-plane anisotropy, harmonic yield with laser polarization along the armchair (AC) direction is found to be higher than that of the zigzag (ZZ) direction for all the strain cases. Nearly, an order of magnitude enhancement of harmonic intensity is achieved for -10% strain along AC direction. The current study expands the research possibilities of phosphorene into a previously unexplored domain, indicating its potential for future utilization in extreme-ultraviolet and attosecond nanophotonics, and also for efficient table-top HHG sources.Comment: 16 pages, 10 figures, 1 tabl

Effect of curvature on structures and vibrations of zigzag carbon nanotubes: a first-principles study

First-principles pseudopotential-based density functional theory calculations of atomic and electronic structures, full phonon dispersions and thermal properties of zigzag single wall carbon nanotubes (SWCNTs) are presented. By determining the correlation between vibrational modes of a graphene sheet and of the nanotube, we understand how rolling of the sheet results in mixing between modes and changes in vibrational spectrum of graphene. We find that the radial breathing mode softens with decreasing curvature. We estimate thermal expansion coefficient of nanotubes within a quasiharmonic approximation and identify the modes that dominate thermal expansion of some of these SWCNTs both at low and high temperatures

Tunable ultrafast thermionic emission from femtosecond-laser hot spot on a metal surface: role of laser polarization and angle of incidence

Ultrafast laser induced thermionic emission from metal surfaces has several applications. Here, we investigate the role of laser polarization and angle of incidence on the ultrafast thermionic emission process from laser driven gold coated glass surface. The spatio-temporal evolution of electron and lattice temperatures are obtained using an improved three-dimensional (3D) two-temperature model (TTM) which takes into account the 3D laser pulse profile focused obliquely onto the surface. The associated thermionic emission features are described through modified Richardson-Dushman equation, including dynamic space charge effects and are included self-consistently in our numerical approach. We show that temperature dependent reflectivity influences laser energy absorption. The resulting peak electron temperature on the metal surface monotonically increases with angle of incidence for P polarization, while for S polarization it shows opposite trend. We observe that thermionic emission duration shows strong dependence on angle of incidence and contrasting polarization dependent behaviour. The duration of thermionic current shows strong correlation to the intrinsic electron-lattice thermalization time, in a fluence regime well below the damage threshold of gold. The observations and insights have important consequences in designing ultrafast thermionic emitters based on a metal based architecture.Comment: 17 pages, 7 figures, 1 tabl

Superior Photo-thermionic electron Emission from Illuminated Phosphorene Surface

This work demonstrates that black phosphorene, a two dimensional allotrope of phosphorus, has the potential to be an efficient photo-thermionic emitter. To investigate and understand the novel aspects we use a combined approach in which ab initio quantum simulation tools are utilized along with semiclassical description for the emission process. First by using density functional theory based formalism, we study the band structure of phosphorene. From the locations of electronic bands, and band edges, we estimate the Fermi level and work function. This leads us to define a valid material specific parameter space and establish a formalism for estimating thermionic electron emission current from phosphorene. Finally we demonstrate how the emission current can be enhanced substantially under the effect of photon irradiation. We observe that photoemission flux to strongly dominate over its coexisting counterpart thermionic emission flux. Anisotropy in phosphorene structure plays important role in enhancing the flux. The approach which is valid over a much wider range of parameters is successfully tested against recently performed experiments in a different context. The results open up a new possibility for application of phosphorene based thermionic and photo-thermionic energy converters

Achieving high molecular alignment and orientation for CH3_3F through manipulation of rotational states with varying optical and THz laser pulse parameters

Increasing interest in the fields of high-harmonics generation, laser-induced chemical reactions, and molecular imaging of gaseous targets demands high molecular “alignment” and “orientation” (A&O). In this work, we examine the critical role of different pulse parameters on the field-free A&O dynamics of the CH[Formula: see text] F molecule, and identify experimentally feasible optical and THz range laser parameters that ensure maximal A&O for such molecules. Herein, apart from rotational temperature, we investigate effects of varying pulse parameters such as, pulse duration, intensity, frequency, and carrier envelop phase (CEP). By analyzing the interplay between laser pulse parameters and the resulting rotational population distribution, the origin of specific A&O dynamics was addressed. We could identify two qualitatively different A&O behaviors and revealed their connection with the pulse parameters and the population of excited rotational states. We report here the highest alignment of [Formula: see text] and orientation of [Formula: see text] for CH[Formula: see text] F molecule at 2 K using a single pulse. Our study should be useful to understand different aspects of laser-induced unidirectional rotation in heteronuclear molecules, and in understanding routes to tune/enhance A&O in laboratory conditions for advanced applications

Dimethylammonium iodide stabilized bismuth halide perovskite photocatalyst for hydrogen evolution

Metal halide perovskites have emerged as novel and promising photocatalysts for hydrogen generation. Currently, their stability in water is a vital and urgent research question. In this paper a novel approach to stabilize a bismuth halide perovskite [(CH3)(2)NH2](3)[BiI6] (DA(3)BiI(6)) in water using dimethylammonium iodide (DAI) without the assistance of acids or coatings is reported. The DA(3)BiI(6) powder exhibits good stability in DAI solutions for at least two weeks. The concentration of DAI is found as a critical parameter, where the I- ions play the key role in the stabilization. The stability of DA(3)BiI(6) in water is realized via a surface dissolution-recrystallization process. Stabilized DA(3)BiI(6) demonstrates constant photocatalytic properties for visible light-induced photo-oxidation of I- ions and with PtCl4 as a co-catalyst (Pt-DA(3)BiI(6)), photocatalytic H-2 evolution with a rate of 5.7 mu molh(-1) from HI in DAI solution, obtaining an apparent quantum efficiency of 0.83% at 535 nm. This study provides new insights on the stabilization of metal halide perovskites for photocatalysis in aqueous solution

A quantum‐chemical perspective on the laser‐induced alignment and orientation dynamics of the CH 3 X (X = F, Cl, Br, I) molecules

Motivated by recent experiments, the laser‐induced alignment‐and‐orientation (A&O) dynamics of the prolate symmetric top CH(3)X (X = F, Cl, Br, I) molecules is investigated, with particular emphasis on the effect of halogen substitution on the rotational constants, dipole moments, and polarizabilities of these species, as these quantities determine the A&O dynamics. Insight into possible control schemes for preferred A&O dynamics of halogenated molecules and best practices for A&O simulations are provided, as well. It is shown that for accurate A&O ‐dynamics simulations it is necessary to employ large basis sets and high levels of electron correlation when computing the rotational constants, dipole moments, and polarizabilities. The benchmark‐quality values of these molecular parameters, corresponding to the equilibrium, as well as the vibrationally averaged structures are obtained with the help of the focal‐point analysis (FPA) technique and explicit electronic‐structure computations utilizing the gold‐standard CCSD(T) approach, basis sets up to quintuple‐zeta quality, core‐correlation contributions and, in particular, relativistic effects for CH(3)Br and CH(3)I. It is shown that the different A&O behavior of the CH(3)X molecules in the optical regime is mostly caused by the differences in their polarizability anisotropy, in other terms, the size of the halogen atom. In contrast, the A&O dynamics of the CH(3)X series induced by an intense few‐cycle THz pulse is mostly governed by changes in the rotational constants, due to the similar dipole moments of the CH(3)X molecules. The A&O dynamics is most sensitive to the B rotational constant: even the difference between its equilibrium and vibrationally‐averaged values results in noticeably different A&O dynamics. The contribution of rotational states having different symmetry, weighted by nuclear‐spin statistics, to the A&O dynamics is also studied

Oxygen vacancies induced photoluminescence in SrZnO2 nanophosphors probed by theoretical and experimental analysis

We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of SrZnO2 (SZO) nanophosphors by combined first principle calculations based on density functional theory and full multiple scattering theory, correlated with experimental results obtained from X-ray absorption and photoluminescence spectroscopies. The band structure analysis from density functional theory revealed the formation of new energy states in the forbidden gap due to introduction of oxygen vacancies in the system, thereby causing disruption in intrinsic symmetry and altering bond lengths in SZO system. These defect states are anticipated as origin of observed photoluminescence in SZO nanophosphors. The experimental X-ray absorption near edge structure (XANES) at Zn and Sr K-edges were successfully imitated by simulated XANES obtained after removing oxygen atoms around Zn and Sr cores, which affirmed the presence and signature of oxygen vacancies on near edge structure