7 research outputs found
First-principles informed phenomenological models of optical and lattice response in materials
In this dissertation, we present work on the first-principles informed phenomenological modeling of the optical properties of materials. We use density functional theory and time-dependent density functional theory calculations to inform parameterized models of the response to light in materials. We include the effect of ultrafast nonequilibrium effects, as well as the importance of quantum mechanical lattice vibrations. Using these models, we validate the approaches, and predict the effect of both ultrafast phenomena as well as quantum mechanical vibrations on the optical properties of bulk and 2D materials. Such modeling opens up avenues for efficient phenomenological approaches to describing optical phenomena in materials while keeping the accuracy of first-principles simulations
Compact Broadband Low-Loss Taper for Coupling to a Silicon Nitride Photonic Wire
We demonstrate an ultra-compact waveguide taper in Silicon Nitride platform.
The proposed taper provides a coupling-efficiency of 95% at a length of 19.5 um
in comparison to the standard linear taper of length 50 um that connects a 10
um wide waveguide to a 1 um wide photonic wire. The taper has a spectral
response > 75% spanning over 800 nm and resilience to fabrication variations;
>200 nm change in taper and end waveguide width varies transmission by <5%. We
experimentally demonstrate taper insertion loss of <0.1 dB/transition for a
taper as short as 19.5 um, and reduces the footprint of the photonic device by
50.8% compared to the standard adiabatic taper. To the best of our knowledge,
the proposed taper is the shortest waveguide taper ever reported in Silicon
Nitride
Ultra-compact low-loss broadband waveguide taper in silicon-on-insulator
A novel design of large bandwidth, fabrication tolerant, CMOS-compatible
compact tapers (15 um) have been proposed and experimentally demonstrated in
silicon-on-insulator. The proposed taper along with linear grating couplers for
spot-size conversion exhibits no degradation in the coupling efficiency
compared to a standard focusing grating in 1550 nm band. A single taper design
has a broadband operation over 600 nm that can be used in O, C and L-band. The
proposed compact taper is highly tolerant to fabrication variations; 80 nm
change in the taper width and 200 nm in end waveguide width varies the taper
transmission by <0.4 dB. The footprint of the device i.e. taper along with the
linear gratings is ~ 250 {\mu}m2; this is 20X smaller than the adiabatic taper
and 2X smaller than the focusing grating coupler
Illumination protocols for non-linear phononics in bismuth and antimony
We study the optical generation and control of coherent phonons in elemental
bismuth (Bi) and antimony (Sb) using a classical equation of motion informed by
first-principles calculations of the potential energy surface and the
frequency-dependent macroscopic dielectric function along the zone-centered
optical phonons coordinates. Using this approach, we demonstrate that phonons
with the largest optomechanical couplings, also have the strongest degree of
anharmonicity, a result of the broken symmetry structural ground state of Bi
and Sb. We show how this anharmonicity, explaining the light-induced phonon
softening observed in experiments, prevents the application of standard
phonon-amplification and annihilation protocols. We introduce a simple
linearization protocol that extends the use of such protocols to the case of
anharmonic phonons in broken symmetry materials, and demonstrate its efficiency
at high displacement amplitudes. Our formalism and results provide a path for
improving optical control in non-linear phononics
GPAW: open Python package for electronic-structure calculations
We review the GPAW open-source Python package for electronic structure
calculations. GPAW is based on the projector-augmented wave method and can
solve the self-consistent density functional theory (DFT) equations using three
different wave-function representations, namely real-space grids, plane waves,
and numerical atomic orbitals. The three representations are complementary and
mutually independent and can be connected by transformations via the real-space
grid. This multi-basis feature renders GPAW highly versatile and unique among
similar codes. By virtue of its modular structure, the GPAW code constitutes an
ideal platform for implementation of new features and methodologies. Moreover,
it is well integrated with the Atomic Simulation Environment (ASE) providing a
flexible and dynamic user interface. In addition to ground-state DFT
calculations, GPAW supports many-body GW band structures, optical excitations
from the Bethe-Salpeter Equation (BSE), variational calculations of excited
states in molecules and solids via direct optimization, and real-time
propagation of the Kohn-Sham equations within time-dependent DFT. A range of
more advanced methods to describe magnetic excitations and non-collinear
magnetism in solids are also now available. In addition, GPAW can calculate
non-linear optical tensors of solids, charged crystal point defects, and much
more. Recently, support of GPU acceleration has been achieved with minor
modifications of the GPAW code thanks to the CuPy library. We end the review
with an outlook describing some future plans for GPAW
Spectroscopic Study of the Reversible Chemical Reduction and Reoxidation of Substitutional Cr Ions in Sr<sub>2</sub>TiO<sub>4</sub>
The solid-state synthesis
and controllable speciation of Cr dopants in the layered perovskite
Sr<sub>2</sub>TiO<sub>4</sub> is reported. We employed a chemical
reduction procedure with NaBH<sub>4</sub> at relatively mild temperatures
(<450 °C) to impart sensitive control over the relative concentration
of Cr<sup>3+</sup> dopants, the charge-state of oxygen-vacancy defects,
and presence of Ti<sup>3+</sup> defects in highly reduced Cr-doped
Sr<sub>2</sub>TiO<sub>4</sub>. The electron paramagnetic resonance
(EPR) spectra of the reduced powder samples reveal a 12-fold increase
in the Cr<sup>3+</sup> concentration within the axially compressed
Ti<sup>4+</sup>-site of the Sr<sub>2</sub>TiO<sub>4</sub> host. The
increase in Cr<sup>3+</sup> content is achieved through the reduction
of higher-valence Cr ions that are either EPR silent or diamagnetic.
The spin-Hamiltonian parameters for Cr<sup>3+</sup> substituted at
the B-site of Sr<sub>2</sub>TiO<sub>4</sub> were refined to <i>D</i> = −201 × 10<sup>–4</sup> cm<sup>–1</sup>, <i>g</i><sub>⊥</sub> = 1.980, and <i>g</i><sub>∥</sub> = 1.978. In addition, the Cr<sup>3+</sup> ion
exhibits a temperature-dependent axial component to the zero-field
splitting of the <sup>4</sup>A<sub>2</sub> ground term that is accounted
for by ligand field theory and an isotropic contraction of the Sr<sub>2</sub>TiO<sub>4</sub> lattice with decreasing temperature. The observed
changes to the electronic structure upon reduction are quantitatively
reversible upon reoxidation of the sample under aerobic annealing
at the same temperature and duration as the reduction conditions.
This temperature dependence of the Cr<sup>3+</sup> content in the
Cr-doped Sr<sub>2</sub>TiO<sub>4</sub> powders is discussed and contrasted
to our recent study on Cr-doped SrTiO<sub>3</sub>