25,736 research outputs found
Exact polarizability and plasmon resonances of partly buried nanowires
The electrostatic polarizability for both vertical and horizontal
polarization of two conjoined half-cylinders partly buried in a substrate is
derived in an analytical closed-form expression. Using the derived analytical
polarizabilities we analyze the localized surface plasmon resonances of three
important metal nanowire configurations: (1) a half-cylinder, (2) a
half-cylinder on a substrate, and (3) a cylinder partly buried in a substrate.
Among other results we show that the substrate plays an important role for
spectral location of the plasmon resonances. Our analytical results enable an
easy, fast, and exact analysis of many complicated plasmonic nanowire
configurations including nanowires on substrates. This is important both for
comparison with experimental data, for applications, and as benchmarks for
numerical methods
Optical third harmonic generation in black phosphorus
We present a calculation of Third Harmonic Generation (THG) for two-band
systems using the length gauge that avoids unphysical divergences otherwise
present in the evaluation of the third order current density response. The
calculation is applied to bulk and monolayer black Phosphorus (bP) using a
non-orthogonal tight-binding model. Results show that the low energy response
is dominated by mixed inter-intraband processes and estimates of the magnitude
of THG susceptibility are comparable to recent experimental reports for bulk bP
samples.Comment: 9 pages, 5 figure
Field-induced dissociation of two-dimensional excitons in transition-metal dichalcogenides
Generation of photocurrents in semiconducting materials requires dissociation
of excitons into free charge carriers. While thermal agitation is sufficient to
induce dissociation in most bulk materials, an additional push is required to
induce efficient dissociation of the strongly bound excitons in monolayer
transition-metal dichalcogenides (TMDs). Recently, static in-plane electric
fields have proven to be a promising candidate. In the present paper, we
introduce a numerical procedure, based on exterior complex scaling, capable of
computing field-induced exciton dissociation rates for a wider range of field
strengths than previously reported in literature. We present both Stark shifts
and dissociation rates for excitons in various TMDs calculated within the
Mott-Wannier model. Here, we find that the field induced dissociation rate is
strongly dependent on the dielectric screening environment. Furthermore,
applying weak-field asymptotic theory (WFAT) to the Keldysh potential, we are
able to derive an analytical expression for exciton dissociation rates in the
weak-field region
Optical properties of graphene antidot lattices
Undoped graphene is semi-metallic and thus not suitable for many electronic
and optoelectronic applications requiring gapped semiconductor materials.
However, a periodic array of holes (antidot lattice) renders graphene
semiconducting with a controllable band gap. Using atomistic modelling, we
demonstrate that this artificial nanomaterial is a dipole-allowed direct gap
semiconductor with a very pronounced optical absorption edge. Hence, optical
infrared spectroscopy should be an ideal probe of the electronic structure. To
address realistic experimental situations, we include effects due to disorder
and the presence of a substrate in the analysis.Comment: 11 pages, 9 figures, accepted for publication in Phys. Rev.
Iterative approach to arbitrary nonlinear optical response functions of graphene
Two-dimensional materials constitute an exciting platform for nonlinear
optics with large nonlinearities that are tunable by gating. Hence,
gate-tunable harmonic generation and intensity-dependent refraction have been
observed in e.g. graphene and transition-metal dichalcogenides, whose
electronic structures are accurately modelled by the (massive) Dirac equation.
We exploit on the simplicity of this model and demonstrate here that arbitrary
nonlinear response functions follow from a simple iterative approach. The power
of this approach is illustrated by analytical expressions for harmonic
generation and intensity-dependent refraction, both computed up to ninth order
in the pump field. Moreover, the results allow for arbitrary band gaps and
gating potentials. As illustrative applications, we consider (i)
gate-dependence of third- and fifth-harmonic generation in gapped and gapless
graphene, (ii) intensity-dependent refractive index of graphene up to ninth
order, and (iii) intensity-dependence of high-harmonic generation.Comment: 6 pages, 5 figures. Supplemental material: 6 pages, 2 figure
Linear and nonlinear optical response of crystals using length and velocity gauges: Effect of basis truncation
We study the effects of a truncated band structure on the linear and
nonlinear optical response of crystals using four methods. These are
constructed by (i) choosing either length or velocity gauge for the
perturbation and (ii) computing the current density either directly or via the
time-derivative of the polarization density. In the infinite band limit, the
results of all four methods are identical, but basis truncation breaks their
equivalence. In particular, certain response functions vanish identically and
unphysical low-frequency divergences are observed for few-band models in the
velocity gauge. Using hexagonal boron nitride (hBN) monolayer as a case study,
we analyze the problems associated with all methods and identify the optimal
one. Our results show that the length gauge calculations provide the fastest
convergence rates as well as the most accurate spectra for any basis size and,
moreover, that low-frequency divergences are eliminated.Comment: 11 pages, 7 figure
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