arXiv subjects

Jacob J. Krich

Publications and source records attributed to Jacob J. Krich.

At least 19 recordsLinked to original sources

arXiv2026

Nonlinear order separation in two-dimensional electronic spectroscopy quantifies properties of higher-excited states

Two-dimensional (2D) spectroscopy combines high temporal and spectral resolution, allowing the observation of ultrafast energy transfer and the separation of homogeneous and inhomogeneous broadening. Typically, 2D spectroscopy is dominated by the lowest-order nonlinear signal for a given phase-matching configuration while signals of higher order are present but difficult to access separately. Recently, we introduced a technique to separate nonlinear orders in 2D spectroscopy by systematically varying the intensity of the pump pulses and appropriate post-processing. Here, we unravel the full potential of higher-order 2D spectroscopy by separating multiple nonlinear orders at different multi-quantum positions. As an example, we investigate a squaraine dimer. Using a theoretical model, we find excellent qualitative and quantitative agreement throughout all nonlinear orders and multi-quantum positions. Our simulations demonstrate the sensitivity and information content hidden in the higher-order spectra such as transition dipole moments and energy levels even of highly excited states. Our results pave the way for establishing higher-order spectroscopy as a unique extension of multidimensional spectroscopy, providing access to highly excited states and their properties encoded in successive orders of nonlinearity.

physics.chem-ph↗

arXiv2025

Device-scale modeling of valley photovoltaics

We present a Poisson/drift-diffusion model that includes valley scattering effects for simulating valley photovoltaic devices. The valley photovoltaic concept is a novel implementation of a hot-carrier solar cell and leverages the valley scattering effect under large electric field to potentially achieve high voltage and high efficiency. Fabricated devices have shown S-shaped current-voltage curves, low fill factor, and thus low efficiency. We hence develop the first device model for valley photovoltaics. Our model includes electric-field-dependent valley scattering rates extracted from previous ensemble Monte Carlo simulations. We show that the condition of nonequilibrium carrier populations in the satellite valleys is not enough for valley photovoltaics to achieve high efficiency. We also show that increasing the built-in electric field of the valley-scattering region does not improve efficiency, contrary to previous suggestion.

physics.app-ph↗

arXiv2025

Separating orders of response in transient absorption and coherent multi-dimensional spectroscopy by intensity variation

Interpretation of time-resolved spectroscopies such as transient absorption (TA) or two-dimensional (2D) spectroscopy often relies on the perturbative description of light-matter interaction. In many cases the third order of nonlinear response is the leading and desired term. When pulse amplitudes are high, higher orders of light-matter interaction can both distort lineshapes and dynamics and provide valuable information. Here, we present a general procedure to separately measure the nonlinear response orders in both TA and 2D spectroscopies, using linear combinations of intensity-dependent spectra. We analyze the residual contamination and random errors and show how to choose optimal intensities to minimize the total error in the extracted orders. For an experimental demonstration, we separate the nonlinear orders in the 2D electronic spectroscopy of squaraine polymers up to 11$^{th}$ order.

physics.chem-ph↗

arXiv2024

Radiator Tailoring for Enhanced Performance in InAs-Based Near-Field Thermophotovoltaics

Near-field thermophotovoltaics (NFTPV) systems have significant potential for waste heat recovery applications, with both high theoretical efficiency and power density, up to 40% and $11 \ \mathrm{W/cm^{2}}$ at 900 K. Yet experimental demonstrations have only achieved up to 14% efficiency and modest power densities (i.e., $0.75 \ \mathrm{W/cm^{2}}$). While experiments have recently started to focus on photovoltaic (PV) cells custom-made for NFTPV, most work still relies on conventional doped silicon radiators. In this work, we design an optimized NFTPV radiator for an indium arsenide-based system and, in the process, investigate models for the permittivity of InAs in the context of NFTPV. Based on existing measurements of InAs absorption, we find that the traditional Drude model overestimates free carrier absorption in InAs. We replace the Drude portion of the InAs dielectric function with a revised model derived from ionized impurity scattering. Using this revised model, we maximize the spectral efficiency and power density of a NFTPV system by optimizing the spectral coupling between a radiator and an InAs PV cell. We find that when the radiator and the PV cell are both made of InAs, a nearly threefold improvement of spectral efficiency is possible compared to a traditional silicon radiator with the same InAs cell. This enhancement reduces subgap thermal transfer while maintaining power output.

physics.app-ph↗

arXiv2023

Interpretations of High-Order Transient Absorption Spectroscopies

Transient absorption (TA) spectroscopy has long been an invaluable tool for determining the energetics and dynamics of excited states in atomic, molecular, and solid-state systems. When pump pulse intensities are sufficiently high, the resulting TA spectra include both the generally desired third-order response of the studied material as well as responses that are higher order in the electric field amplitudes of the pulses. It has recently been shown that pump-intensity-dependent TA measurements allow separating the various orders of response of the TA signal, but the information content available in those higher orders has not been described. We give a general framework, intuition, and nomenclature for understanding the information contained in high-order TA spectra. Standard TA spectra are generally interpreted in terms of three fundamental processes: ground-state bleach (GSB), stimulated emission (SE), and excited state absorption (ESA), and we extend those concepts to higher order. Each order introduces two new processes: SE and ESA from highly excited states that were not accessible in lower orders. In addition, each order contain negations of lower-order processes, just as GSB is a negation of the linear absorption. We show the new spectral and dynamical information that is introduced at each order and show how the relative signs of the signals in different orders can be used to identify which processes are dominant.

physics.chem-ph↗

arXiv2022

Lossless plasmons in highly mismatched alloys

We explore the potential of highly mismatched alloys (HMAs) for realizing lossless plasmonics. Systems with a plasmon frequency at which there are no interband or intraband processes possible are called lossless, as there is no 2-particle loss channel for the plasmon. We find that the band splitting in HMAs with a conduction band anticrossing guarantees a lossless frequency window. When such a material is doped, producing plasmonic behavior, we study the conditions required for the plasmon frequency to fall in the lossless window, realizing lossless plasmons. Considering a generic class of HMAs with a conduction band anticrossing, we find universal contours in their parameter space within which lossless plasmons are possible for some doping range. Our analysis shows that HMAs with heavier effective masses are most promising for realizing a lossless plasmonic material.

physics.optics↗

arXiv2022

Absorption spectrum of doped highly mismatched alloys

Highly mismatched alloys (HMA's) are a class of semiconductor alloys with large electronegativity differences between the alloying elements. We predict the absorption spectrum due to transitions between the split bands of a doped highly mismatched alloy with a conduction band anticrossing. We analyze the joint densities of states for both direct and indirect transitions between the split bands. The resulting spectrum has features that reveal the unusual state distribution that is characteristic of HMAs, hence providing valuable insight into their electronic structure. In particular, we predict a peak near the absorption edge, which arises due to the suppression of direct transitions at large momenta. We present analytic forms for the near-absorption-edge and large-energy spectra, showing that they are qualitatively different from those in standard parabolic semiconductors. In particular, as a result of suppressed direct transitions, indirect transitions dominate the spectrum away from the edge of absorption.

cond-mat.mtrl-sci↗

arXiv2021

Efficient quantum dot $\mathbf{k}\cdot\mathbf{p}$ in wurtzite systems including spatially varying elastic and dielectric constants and smooth alloy profile

We present Fourier-space based methods to calculate the electronic structure of wurtzite quantum dot systems with continuous alloy profiles. We incorporate spatially varying elastic and dielectric constants in strain and piezoelectric potential calculations. A method to incorporate smooth alloy profiles in all aspects of the calculations is presented. We demonstrate our methodology for the case of a 1D InGaN quantum dot array and show the importance of including these spatially varying parameters in the modeling of devices. We demonstrate that convergence of the lowest bound state energies is to good approximation determined by the largest wave vector used in constructing the states. We also present a novel approach of coupling strain into the $\mathbf{k}\cdot\mathbf{p}$ Hamiltonian, greatly reducing the computational cost of generating the Hamiltonian.

cond-mat.mes-hall↗

arXiv2020

Efficient numerical method for predicting nonlinear optical spectroscopies of open systems

Nonlinear optical spectroscopies are powerful tools for probing quantum dynamics in molecular and nanoscale systems. While intuition about ultrafast spectroscopies is often built by considering impulsive optical pulses, actual experiments have finite-duration pulses, which can be important for interpreting and predicting experimental results. We present a new freely available open source method for spectroscopic modeling, called Ultrafast Ultrafast (UF$^2$) Spectroscopy, which enables computationally efficient and convenient prediction of nonlinear spectra, including treatment of arbitrary finite duration pulse shapes. UF$^2$ is a Fourier-based method that requires diagonalization of the Liouvillian propagator of the system density matrix. We also present a Runge-Kutta Euler (RKE) direct propagation method. We include open-systems dynamics in the secular Redfield, full Redfield, and Lindblad formalisms with Markovian baths. For non-Markovian systems, the degrees of freedom corresponding to memory effects are brought into the system and treated nonperturbatively. We analyze the computational complexity of the algorithms and demonstrate numerically that, including the cost of diagonalizing the propagator, UF$^2$ is 20-200 times faster than the direct propagation method for secular Redfield models with arbitrary Hilbert space dimension; that it is similarly faster for full Redfield models at least up to system dimensions where the propagator requires more than 20 GB to store; and that for Lindblad models it is faster up to dimension near 100, with speedups for small systems by factors of over 500. UF$^2$ and RKE are part of a larger open source Ultrafast Software Suite, which includes tools for automatic generation and calculation of Feynman diagrams.

physics.chem-ph↗

arXiv2020

Automatic Feynman diagram generation for nonlinear optical spectroscopies

Perturbative nonlinear optical spectroscopies are powerful methods to understand the dynamics of excitonic and other condensed phase systems. Feynman diagrams have long provided the essential tool to understand and interpret experimental spectra and to organize the calculation of spectra for model systems. When optical pulses are strictly time ordered, only a small number of diagrams contribute, but in many experiments pulse-overlap effects are important for interpreting results. When pulses overlap, the number of contributing diagrams can increase rapidly, especially with higher order spectroscopies, and human error is especially likely when attempting to write down all of the diagrams. We present an automated Diagram Generator (DG) that generates all of the Feynman diagrams needed to calculate any $n^{\text{th}}$-order spectroscopic signal. We characterize all perturbative nonlinear spectroscopies by their associated phase-discrimination condition as well as the time intervals where pulse amplitudes are nonzero. Although the DG can be used to automate impulsive calculations, its greatest strength lies in automating finite pulse calculations where pulse overlaps are important. We consider third-order transient absorption spectroscopy and fifth-order exciton-exciton interaction 2D (EEI2D) spectroscopy, which are respectively described by 6 or 7 diagrams in the impulsive limit but 16 or 240 diagrams, respectively, when pulses overlap. The DG allows users to automatically include all relevant diagrams at relatively low computational cost, since the extra diagrams are only generated for the inter-pulse delays where they are relevant. For EEI2D spectroscopy, we show the important effects of including the overlap diagrams.

physics.chem-ph↗

arXiv2020

Plasma frequency in doped highly mismatched alloys

Highly mismatched alloys (HMAs) have band structures strongly modified due to the introduction of the alloying element. We consider HMAs where the isolated state of the alloying element is near the host conduction band, which causes the conduction band to split into two bands. We determine the bulk plasma frequency when the lower-energy band is partially occupied, as by doping, using a semi-analytical method based on a disorder-averaged Green's function. We include the nontrivial effects of interband transitions to the higher-energy band, which limit the plasma frequency to be less than an effective band gap. We show that the distribution of states in the split bands causes plasmons in HMAs to behave differently than plasmons in standard metals and semiconductors. The effective mass of the lower split band $m^*$ changes with alloy fraction, and we find that the plasmon frequency with small carrier concentration $n$ scales with $\sqrt{n}/m^*$ rather than the $\sqrt{n/m^*}$ that is expected in standard materials. We suggest experiments to observe these phenomena. Considering the typical range of material parameters in this group of alloys and taking a realistic example, we suggest that HMAs can serve as highly tunable low-frequency plasmonic materials.

cond-mat.mtrl-sci↗

arXiv2020

Two-Photon Photocurrent in InGaN/GaN Nanowire Intermediate Band Solar Cells

Intermediate band solar cells hold the promise of ultrahigh power conversion efficiencies using a single semiconductor junction. Many current implementations use materials with bandgaps too small to achieve maximum efficiency or use cost-prohibitive substrates. Here we demonstrate a material system for intermediate band solar cells using InGaN/GaN quantum-dot-in-nanowire heterostructures grown directly on silicon to provide a lower cost, large-bandgap intermediate band solar cell platform. We demonstrate sequential two-photon current generation with sub-bandgap photons, the hallmark of intermediate band solar cell operation, through vertically stacked quantum dots in the nanowires. Near-infrared light biasing with an 850 nm laser intensity up to 200 W/cm2 increases the photocurrent above and below the bandgap by up to 19% at 78 K, and 44% at room temperature. The nanostructured III-nitride strategy provides a route towards realistic room temperature intermediate band solar cells while leveraging the cost benefits of silicon substrates.

physics.app-ph↗

arXiv2020

Efficiency Increase in Multijunction Monochromatic Photovoltaic Devices Due to Luminescent Coupling

We present a multijunction detailed balance model that includes the effects of luminescent coupling, light trapping and nonradiative recombination, suitable for treatment of multijunction solar cells and photonic power converters -- photovoltaic devices designed to convert narrow-band light. The model includes both specular and Lambertian reflections using a ray-optic formalism and treats nonradiative processes using an internal radiative efficiency. Using this model, we calculate and optimize the efficiency of multijunction photonic power converters for a range of material qualities and light-trapping schemes. Multijunction devices allow increased voltage with lower current, decreasing series resistance losses. We show that efficiency increases significantly with increased number of junctions, even without series resistance, when the device has an absorbing substrate. Such an increase does not occur when the device has a back reflector. We explain this effect using a simplified model, which illustrates the origin of the decreased radiative losses in multijunction devices on substrates.

physics.app-ph↗

arXiv2019

Simudo: a device model for intermediate band materials

We describe Simudo, a free Poisson/drift-diffusion steady state device model for semiconductor and intermediate band materials, including self-consistent optical absorption and generation. Simudo is the first freely available device model that can treat intermediate band materials. Simudo uses the finite element method (FEM) to solve the coupled nonlinear partial differential equations in two dimensions, which is different from the standard choice of the finite volume method in essentially all commercial semiconductor device models. We present the continuous equations that Simudo solves, show the FEM formulations we have developed, and demonstrate how they allow robust convergence with double-precision floating point arithmetic. With a benchmark semiconductor pn-junction device, we show that Simudo has a higher rate of convergence than Synopsys Sentaurus, converging to high accuracy with a considerably smaller mesh. Simudo includes many semiconductor phenomena and parameters and is designed for extensibility by the user to include many physical processes.

physics.app-ph↗

arXiv2019

Efficiency limits of electronically-coupled upconverter and quantum ratchet solar cells using detailed balance

The intermediate band solar cell (IBSC) and quantum ratchet solar cell (QRSC) have the potential to surpass the efficiency of standard single-junction solar cells by allowing sub-gap photon absorption through states deep inside the band gap. High efficiency IBSC and QRSC devices have not yet been achieved, however, since introducing mid-gap states also increases recombination, which can harm the device. We consider the electronically coupled upconverter (ECUC) solar cell and show that it can achieve the same efficiencies as the QRSC. Although they are equivalent in the detailed balance limit, the ECUC is less sensitive to nonradiative processes, which makes it a more practical implementation for IB devices. We perform a case study of crystalline-silicon based ECUC cells, focusing on hydrogenated amorphous silicon as the upconverter material and highlighting potential dopants for the ECUC. These results illustrate a new path for the development of IB-based devices.

physics.app-ph↗

arXiv2019

Numerical Method for Nonlinear Optical Spectroscopies: Ultrafast Ultrafast Spectroscopy

We outline a novel numerical method, called Ultrafast Ultrafast (UF$^2$), for calculating the $n^\text{th}$-order wavepackets required for calculating n-wave mixing signals. The method is simple to implement, and we demonstrate that it is computationally more efficient than other methods in a wide range of use cases. Resulting spectra are identical to those calculated using the standard response function formalism but with increased efficiency. The computational speed-ups of UF$^2$ come from (a) non-perturbative and costless propagation of the system time-evolution (b) numerical propagation only at times when perturbative optical pulses are non-zero and (c) use of the fast Fourier transform convolution algorithm for efficient numerical propagation. The simplicity of this formalism allows us to write a simple software package that is as easy to use and understand as the Feynman diagrams that organize the understanding of $n$-wave mixing processes.

physics.chem-ph↗

arXiv2018

Efficient Wave Optics Modeling of Nanowire Solar Cells Using Rigorous Coupled Wave Analysis

We investigate the accuracy of rigorous coupled wave analysis (RCWA) for near-field computations within cylindrical GaAs nanowire solar cells and discover excellent accuracy with low computational cost at long incident wavelengths, but poor accuracy at short incident wavelengths. These near fields give the carrier generation rate, and their accurate determination is essential for device modeling. We implement two techniques for increasing the accuracy of the near fields generated by RCWA, and give some guidance on parameters required for convergence along with an estimate of their associated computation times. The first improvement removes Gibbs phenomenon artifacts from the RCWA fields, and the second uses the extremely well-converged far field absorption to rescale the local fields. These improvements allow a computational speedup between 30 and 1000 times for spectrally integrated calculations, depending on the density of the near fields desired. Some spectrally resolved quantities, especially at short wavelengths, remain expensive, but RCWA is still an excellent method for performing those calculations. These improvements open up the possibility of using RCWA for low cost optical modeling in a full optoelectronic device model of nanowire solar cells.

physics.app-ph↗

arXiv2016

Optimizations of GaAs Nanowire Solar Cells

The efficiency of GaAs nanowire solar cells can be significantly improved without any new processing steps or material requirements. We report coupled optoelectronic simulations of a GaAs nanowire (NW) solar cell with vertical p-i-n junction and high band gap AlInP passivating shell. Our frequency-dependent model facilitates calculation of quantum efficiency for the first time in NW solar cells. For passivated NWs, we find that short-wavelength photons can be most effectively harnessed by using a thin emitter while long-wavelength photons are best utilized by extending the intrinsic region to the nanowire/substrate interface, and using the substrate as a base. These two easily implemented changes, coupled with the increase of NW height to 3.5 um with realistic surface recombination in the presence of a passivation shell, result in a NW solar cell with greater than 19% efficiency.

cond-mat.mtrl-sci↗