arXiv subjects

Rodrick Kuate Defo

Publications and source records attributed to Rodrick Kuate Defo.

15 recordsLinked to original sources

arXiv2026

Optimizing Finite Structures to Suppress the Photonic Density of States

We propose a topology-optimization framework for optimizing finite structures of arbitrary shape by combining density-based methods with level-set approaches. We first optimize regular polygonal structures to suppress the photonic density of states and find that the best performing polygon is consistent with a tiling of space with hexagonal unit cells. We next show that introducing cavities into hexagonal structures further suppresses the photonic density of states, particularly when the cavity is also hexagonal. Such a result would find application in the design of fiber-optic cables. We then describe an approach for optimizing arbitrary x-simple or y-simple designs that can recover finite supercells of a hexagonal unit cell. Our approach can therefore discover the symmetry of photonic-crystal primitive unit cells that significantly suppress the photonic density of states for a given set of material parameters within a single optimization.

physics.optics↗

arXiv2025

Structure-Adaptive Topology Optimization Framework for Photonic Band Gaps with TE-Polarized Sources

Leveraging our structure-adaptive topology optimization framework based on the integration of the photonic density of states over a frequency window for the TM polarization of light [see A. Bahulikar et al., arXiv:2411.09165 (2025)], we show that the $\Gamma$-point and full Brillouin zone integration schemes can also recover two-dimensional photonic crystals for TE polarization. For the $\Gamma$-point formalism, we employ the scalar magnetic field formulation of the electromagnetic wave equation with independent sources polarized in the x and y directions. For the full Brillouin zone formalism, we employ the vector electric field formulation of the electromagnetic wave equation, again with independent sources polarized in the x and y directions. This work can simultaneously treat frequency-dependent optical response, allow for targeted optimization for a given frequency and reciprocal lattice vector pair, and inherently encourage binarized designs.

physics.optics↗

arXiv2025

Ultracoherent self-assembled diamond nanomechanics reveals superfluid dynamics

From gravitational-wave detection, protein force microscopy, to exploration of quantum-classical boundaries, many anticipated discoveries in fundamental science require improving measurement sensitivity limits. Through the fluctuation-dissipation theorem, mechanical dissipation sets the acoustic noise for this limit. Yet, even in high-purity crystals, the microscopic mechanisms responsible for the acoustic loss remain poorly understood. Tension-induced dissipation dilution offers a route to ultralow acoustic loss, but is challenging to implement in crystalline materials including single-crystal diamond. Here we realize a strain-engineered diamond nanomechanical platform using a liquid-assisted van der Waals self-assembly process that harnesses intrinsic surface forces to apply tensile stress exceeding 1 GPa. At cryogenic temperatures these resonators achieve quality factors beyond 10 billion (intrinsic material quality factors beyond 100 million). This exceptional coherence turns them into a sensitive probe for residual dissipation, elucidating three distinct two-level-system channels and one topological dissipation channel from a surface superfluid helium film. Our work shows how advancing mechanical coherence opens access to new regimes of physics in hybrid quantum systems, precision metrology, and condensed-matter physics.

cond-mat.mes-hall↗

arXiv2024

Theoretical Investigation of Yield-Enhancing Equilibrium Negatively Ionized Tin-Vacancy Center Preparation Pathways in N-Doped Diamond

The elucidation of the mechanism of Sn$V^-$ formation in diamond is especially important as the Sn$V^-$ color center has the potential to be a superior single-photon emitter when compared to the N$V$ and to other Group IV color centers. The typical formation of the Sn$V$ involves placing Sn in diamond by ion implantation, but the formation of a charged Sn$V$ species requires an additional complication. This complication is related to the energy cost associated with electronic transitions within the host diamond. Effectively, producing the Sn$V^-$ charge state using an electron obtained from a band edge of the host diamond is less energetically favorable than having the Sn$V^-$ receive an electron from a neighboring donor dopant. Among donor dopants, substitutional N (N$_\text{C}$) is always present in even the purest synthetic or natural diamond sample. The mechanism of electron donation by N$_\text{C}$ has been proposed by Collins for charging the N$V$ in diamond and it has been used to interpret many experimental results. Therefore, in this paper we use DFT to explore the pathways for the formation of the Sn$V^-$ charge state due to electron donation arising from the presence of N$_\text{C}$ in the host diamond. Explicitly, defect concentrations are calculated in equilibrium in each of the explored pathways to determine the yield of the Sn$V^-$ throughout each of the pathways. The importance of our work is to suggest experimental ways of enhancing the yield of charged states like the Sn$V^-$ in diamond for transformative applications in optoelectronics and quantum information.

cond-mat.mtrl-sci↗

arXiv2024

Structure-Adaptive Topology Optimization Framework for Photonic Band Gaps with TM-Polarized Sources

We present a structure-adaptive topology optimization framework for engineering photonic band gaps with TM-polarized sources based on computation of the photonic density of states with a uniform source substituting for the standard Dirac delta function sources in formalisms analogous to $\Gamma$-point integration and to integration over a full Brillouin zone. We generalize the limiting uniform and Dirac delta function sources to more general collections of sources, such that the union of the sources in a given collection is hyperuniform. The uniform-source approach necessarily leads to the fastest computations. We also demonstrate how our approach can be generalized to the treatment of the frequency-dependent optical response of materials. Finally, we show that we can recover known two-dimensional photonic crystals for the TM polarization. A key advantage of our work is its ability to optimize for a specific midgap frequency and band gap in a structure-adaptive manner. Our work leverages the insight that the determination of the minimum supercell size and the minimum precision to which the frequencies within the photonic band gap must be sampled will lead to the observation of photonic-crystal structures when the $\Gamma$-point formalism for the uniform-source approach is employed. Additionally, our $\Gamma$-point and full Brillouin zone formalisms for the uniform-source approach inherently encourage binarized designs even in gradient descent.

physics.optics↗

arXiv2024

Computing Using Shallow NV-Center Charges in Diamond

The static electric dipole-dipole coupling between donor-acceptor pairs (DAPs) in wide-bandgap semiconductors has recently emerged as a means of realizing a quantum science platform through optically controllable, long-range interactions between defects in the solid state. In this work, we generalize DAPs to consider arbitrary dopant populations and demonstrate that the charge of the NV center in diamond is well suited for quantum science. Explicitly, we leverage experimental results [see Z. Yuan et al., PRR 2, 033263 (2020)] to show that shallow NV centers can be efficiently initialized to a given relative population of the negative and neutral charge states and that modulating the surface termination would allow for control of the timescale over which the initialization and subsequent computations would occur. Furthermore, we argue that the observation of electroluminescence from the neutral charge state of the NV center [see N. Mizuochi et al., Nat. Photon. 6, 299 (2012)], but not from the negative charge state, implies the ability to interface with the NV center's charge in a manner analogous to the spin interface enabled by the spin-state dependent fluorescence of the NV center.

cond-mat.mtrl-sci↗

arXiv2023

Suppressing electromagnetic local density of states via slow light in lossy quasi-1d gratings

We propose a spectral-averaging procedure that enables computation of bandwidth-integrated local density of states (LDOS) from a single scattering calculation, and exploit it to investigate the minimum extinction achievable from dipolar sources over finite bandwidths in structured media. Structure-agnostic extinction bounds are derived, providing analytical insights into scaling laws and fundamental design tradeoffs with implications to bandwidth and material selection. We find that perfect LDOS suppression over a finite bandwidth $\Delta\omega$ is impossible. Inspired by limits which predict nontrivial $\sqrt{\Delta\omega}$ scaling in systems with material dissipation, we show that pseudogap edge states of quasi-1d bullseye gratings can -- by simultaneously minimizing material absorption and radiation -- yield arbitrarily close to perfect LDOS suppression in the limit of vanishing bandwidth.

physics.optics↗

arXiv2023

Charge-State Stability of Color Centers in Wide-Bandgap Semiconductors

The NV$^-$ color center in diamond has been extensively investigated for quantum sensing, computation, and communication applications. Nonetheless, charge-state decay from the NV$^-$ to its neutral counterpart the NV$^0$ detrimentally affects the robustness of the NV$^-$ center and remains to be fully overcome. In this work, we provide an $ab~initio$ formalism for accurately estimating the rate of charge-state decay of color centers in wide-bandgap semiconductors. Our formalism employs density functional theory calculations in the context of thermal equilibrium. We illustrate the method using the transition of NV$^-$ to NV$^0$ in the presence of substitutional N [see Z. Yuan $et~al$., PRR 2, 033263 (2020)].

cond-mat.mtrl-sci↗

arXiv2023

Theoretical Investigation of Charge Transfer Between Two Defects in a Wide-Bandgap Semiconductor

Charge traps in the semiconductor bulk (bulk charge traps) make it difficult to predict the electric field within wide-bandgap semiconductors. The issue is the daunting number of bulk charge-trap candidates which means the treatment of bulk charge traps is generally qualitative or uses generalized models that do not consider the trap's particular electronic structure. The electric field within a wide-bandgap semiconductor is nonetheless a crucial quantity in determining the operation of semiconductor devices and the performance of solid-state single-photon emitters embedded within the semiconductor devices. In this work we accurately compute the average electric field measured at the location of N$V^-$ charged defects for the substitutional N (N$_\text{C}$) concentration of $n_{\text{N}_\text{C}} \approx 1.41\times10^{18}$ cm$^{-3}$ for the commonly used oxygen-terminated diamond (see [D. A. Broadway $et$ $al$., Nature Electronics 1, 502 (2018)]). We achieve this result by evaluating the leading-order contribution to the electric field far away from the surface, which comes from the N$_\text{C}$ defects that induce the ionization of the N$V^-$. Our results use density-functional theory (DFT) and the principle of band bending. Our work has the potential to aid both in the prediction of the functioning of semiconductor devices and in the prediction and correction of the spectral diffusion that often plagues the optical frequencies of solid-state single-photon emitters upon repeated photoexcitation measurements. Our results for the timescales involved in thermally driven charge transfer also have the potential to aid in investigations of charge dynamics.

cond-mat.mtrl-sci↗

arXiv2022

Maximum Electromagnetic Local Density of States via Material Structuring

The electromagnetic local density of states (LDOS) is crucial to many aspects of photonics engineering, from enhancing emission of photon sources to radiative heat transfer and photovoltaics. We present a framework for evaluating upper bounds on LDOS in structured media that can handle arbitrary bandwidths and accounts for critical wave scattering effects with no heuristic approximations. The bounds are solely determined by the bandwidth, material susceptibility, and device footprint, with no assumptions on geometry. We derive an analytical expression for the maximum LDOS consistent with the conservation of energy across the entire design domain, which upon benchmarking with topology-optimized structures is shown to be nearly tight for large devices. Novel scaling laws for maximum LDOS enhancement are found: the bounds saturate to a finite value with increasing susceptibility and scale as the quartic root of the bandwidth for semi-infinite structures made of lossy materials, with direct implications on material selection and design applications.

physics.optics↗

arXiv2021

Physical limits on electromagnetic response

Photonic devices play an increasingly important role in advancing physics and engineering, and while improvements in nanofabrication and computational methods have driven dramatic progress in expanding the range of achievable optical characteristics, they have also greatly increased design complexity. These developments have led to heightened relevance for the study of fundamental limits on optical response. Here, we review recent progress in our understanding of these limits with special focus on an emerging theoretical framework that combines computational optimization with conservation laws to yield physical limits capturing all relevant wave effects. Results pertaining to canonical electromagnetic problems such as thermal emission, scattering cross sections, Purcell enhancement, and power routing are presented. Finally, we identify areas for additional research, including conceptual extensions and efficient numerical schemes for handling large-scale problems.

physics.optics↗

arXiv2021

Calculating the Hyperfine Tensors for Group-IV Impurity-Vacancy Centers in Diamond: A Hybrid Density-Functional Theory Approach

The hyperfine interaction is an important probe for understanding the structure and symmetry of defects in a semiconductor. Density-functional theory has shown that it can provide useful first-principles predictions for both the hyperfine tensor and the hyperfine constants that arise from it. Recently there has been great interest in using group-IV impurity-vacancy color centers X$V^-$ (where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy) for important applications in quantum computing and quantum information science. In this paper, we have calculated the hyperfine tensors for these X$V^-$ color centers using the HSE06 screened Hartree-Fock hybrid exchange-correlation functional with the inclusion of core electron spin polarization. We have compared our results to calculations which only use the PBE exchange-correlation functional without the inclusion of core electron spin polarization and we have found our results are in very good agreement with available experimental results. Finally, we have theoretically shown that these X$V^-$ color centers exhibit a Jahn-Teller distortion which explains the observed anisotropic distribution of the hyperfine constants among the neighboring $^{13}$C nuclear spins.

cond-mat.mtrl-sci↗

arXiv2020

Enhanced cavity coupling to silicon monovacancies in 4-H Silicon Carbide using below bandgap laser irradiation and low temperature thermal annealing

The negatively charged silicon monovacancy $V_{Si}^-$ in 4H-silicon carbide (SiC) is a spin-active point defect that has the potential to act as a qubit or quantum memory in solid-state quantum computation applications. Photonic crystal cavities (PCCs) can augment the optical emission of the $V_{Si}^-$, yet fine-tuning the defect-cavity interaction remains challenging. We report on two post-fabrication processes that result in enhancement of the $V_1^{'}$ optical emission from our 1-dimensional PCCs, indicating improved coupling between the ensemble of silicon vacancies and the PCC. One process involves below bandgap illumination at 785 nm and 532 nm wavelengths and above bandgap illumination at 325 nm, carried out at times ranging from a few minutes to several hours. The other process is thermal annealing at $100^o C$, carried out over 20 minutes. Every process except above bandgap irradiation improves the defect-cavity coupling, manifested in augmented Purcell factor enhancement of the $V_1^{'}$ zero phonon line at 77K. The below bandgap laser process is attributed to a modification of charge states, changing the relative ratio of $V_{Si}^0$ (dark state) to $V_{Si}^-$ (bright state), while the thermal annealing process may be explained by diffusion of carbon interstitials, $C_i$, that subsequently recombine with other defects to create additional $V_{Si}^-$s. Above bandgap radiation is proposed to initially convert $V_{Si}^{0}$ to $V_{Si}^-$, but also may lead to diffusion of $V_{Si}^-$ away from the probe area, resulting in an irreversible reduction of the optical signal. Observations of the PCC spectra allow insights into defect modifications and interactions within a controlled, designated volume and indicate pathways to improve defect-cavity interactions.

physics.optics↗

arXiv2020

How carbon vacancies can affect the properties of group IV color centers in diamond: A study of thermodynamics and kinetics

Recently there has been much interest in using Group IV elements from the Periodic Table to fabricate and study X$V$ color centers in diamond where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy. These Group IV color centers have a number of interesting spin and optical properties which could potentially make them better candidates than N$V^-$ centers for important applications in quantum computing and quantum information processing. Unfortunately, the very same ion implantation process that is required to create these X$V$ color centers in diamond necessarily also produces many carbon vacancies ($V_{\rm C}$) which can form complexes with these color centers ($V_{\rm C}-$X$V$) that can dramatically affect the properties of the isolated X$V$ color centers. The main focus of this work is to use density-functional theory (DFT) to study the thermodynamics and kinetics of the formation of these $V_{\rm C}-$X$V$ complexes and to suggest experimental ways to impede this process such as varying the Fermi level of the host diamond material through chemical doping or applying an external electrical bias. We also include a discussion of how the simple presence of many $V_{\rm C}$ can negatively impact the spin coherence times ($T_2$) of Group IV color centers through the presence of acoustic phonons.

cond-mat.mtrl-sci↗

arXiv2015

\textit{Ab-initio} Tight-Binding Hamiltonian for Transition Metal Dichalcogenides

We present an accurate \textit{ab-initio} tight-binding hamiltonian for the transition-metal dichalcogenides, MoS$_2$, MoSe$_2$, WS$_2$, WSe$_2$, with a minimal basis (the \textit{d} orbitals for the metal atoms and \textit{p} orbitals for the chalcogen atoms) based on a transformation of the Kohn-Sham density function theory (DFT) hamiltonian to a basis of maximally localized Wannier functions (MLWF). The truncated tight-binding hamiltonian (TBH), with only on-site, first and partial second neighbor interactions, including spin-orbit coupling, provides a simple physical picture and the symmetry of the main band-structure features. Interlayer interactions between adjacent layers are modeled by transferable hopping terms between the chalcogen \textit{p} orbitals. The full-range tight-binding hamiltonian (FTBH) can be reduced to hybrid-orbital k $\cdot$ p effective hamiltonians near the band extrema that captures important low-energy excitations. These \textit{ab-initio} hamiltonians can serve as the starting point for applications to interacting many-body physics including optical transitions and Berry curvature of bands, of which we give some examples.

cond-mat.mes-hall↗