# Sum-over-states vs quasiparticle pictures of coherent correlation spectroscopy of excitons in semiconductors; femtosecond analogues of multidimensional NMR

## TL;DR

This paper compares the sum-over-states and quasiparticle approaches to understanding 2D correlation spectroscopy of excitons in semiconductors, providing a formal connection between the two methods using Green's functions.

## Contribution

It derives Green's function expressions linking the sum-over-states and quasiparticle pictures for 2DCS signals in semiconductors, clarifying their relationship.

## Key findings

- Established formal connection between the two theoretical pictures
- Linked the approaches to Frenkel and vibrational excitons
- Provided Green's function framework for 2DCS analysis

## Abstract

Two-dimensional correlation spectroscopy (2DCS) based on the nonlinear optical response of excitons to sequences of ultrafast pulses, has the potential to provide some unique insights into carrier dynamics in semiconductors. The most prominent feature of 2DCS, cross peaks, can best be understood using a sum-over-states picture involving the many-body eigenstates. However, the optical response of semiconductors is usually calculated by solving truncated equations of motion for dynamical variables, which result in a quasiparticle picture. In this work we derive Green's function expressions for the four wave mixing signals generated in various phase-matching directions and use them to establish the connection between the two pictures. The formal connection with Frenkel excitons (hard-core bosons) and vibrational excitons (soft-core bosons) is pointed out.