arXiv subjects

Peter A. Rose

Publications and source records attributed to Peter A. Rose.

7 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

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↗

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↗

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↗

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↗

arXiv2012

Dynamics of Non-classicality Measures in the Decohering Harmonic Oscillator

We show that eigenstates |n> of the harmonic oscillator coupled to a linear Markovian bath demonstrate a non-trivial behavior in the dynamics of measures of non-classicality. Specifically, as the system undergoes decoherence, a time-dependent peak in non-classicality as a function of n emerges. We find this effect studying the dynamics of several non-classicality measures previously presented in the literature which compare quantum states to the set of all classical states. In studying these measures we introduce a novel set of classical states for the purpose of calculations which improve upon the results obtainable using these measures. In addition, following in the footsteps of Kenfack and Źyczkowski, we show that the negative volume of the Wigner function agrees well with all the non-classicality measures while being otherwise calculationally significantly more tractable. Finally, we explore the dynamics of non-classicality of several other states.

quant-ph↗