Optical multidimensional coherent spectroscopy is a powerful tool for studying structure and dynamics in complex systems, such as semiconductors. In optical two-dimensional coherent spectroscopy (2DCS), where the spectrum is presented in a two-dimensional (2D) plane with two frequency axes, an important advantage is the ability to isolate quantum pathways by unfolding a one-dimensional spectrum onto a 2D plane. For many systems, however, the quantum pathways are only partially separated in a 2D spectrum. In order to completely isolate the quantum pathways, we extend 2DCS into a third dimension to generate three-dimensional (3D) spectra in which the spectrum is further unfolded. A 3D spectrum provides complete and well-isolated information of the third-order optical response of the system. The information can be used to fully characterize the quantum pathways and to determine the system’s Hamiltonian. Quantitative knowledge of the Hamiltonian enables prediction and control of quantum processes. For instance, such information is essential for deterministic control and improved performance of coherent control schemes.
As an extension to powerful two-dimensional coherent spectroscopy, optical three-dimensional (3D) coherent spectroscopy has been experimentally implemented and found beneficial in studying various systems in physics and chemistry. A critical challenge is how to interpret 3D spectra and extract useful quantitative information, given the richness and complexity of 3D data. Here, we demonstrate how the information of a system's optical response is manifested in 3D spectra by theoretical simulations of a few representative examples including a homogeneous three-level V system, an inhomogeneous three-level V system, and an inhomogeneous three-level ladder system. These examples show that important parameters of the system can be extracted from the spectral pattern, peak positions, amplitudes, and line shapes. The method developed here can be used to analyze 3D spectra of more sophisticated systems which might be a generalization or combination of the three examples, contributing to develop a general approach for the interpretation of 3D spectra.
Abstract Optical multidimensional coherent spectroscopy provides information about the structure and excitation dynamics in a broad range of material systems. This book focuses on systems of interest in physics, including atomic vapors, semiconductor nanostructures and two-dimensional materials. An introduction to ultrafast spectroscopy and the fundamental concepts of multidimensional coherent spectroscopy applies to all material systems. Relevant experimental implementations, including phase-stabilized crossed-beams, frequency-tagged fully-collinear, three-dimensional and frequency-comb based methods. The use of the method to study atomic vapors, semiconductor quantum wells, semiconductor quantum dots and two-dimensional materials are described in separate chapters. The final chapters survey the use of the method in some other areas and speculate on future directions.
Double-quantum two-dimensional (2D) coherent spectroscopy (MDCS) is a powerful optical method that is used to study the optical properties of atomic and complex molecular systems and semiconductor materials. Double-quantum 2D spectra and particularly the peak line shapes on the spectra can also provide information about many-body interactions. We model 2D spectra by solving the optical Bloch equations and show the effects of correlation between coupled resonances, which also explains the discrepancies between the experimental results reported by multiple groups.
Predicting and controlling quantum mechanical phenomena require knowledge of the system Hamiltonian. A detailed understanding of the quantum pathways used to construct the Hamiltonian is essential for deterministic control and improved performance of coherent control schemes. In complex systems, parameters characterizing the pathways, especially those associated with inter-particle interactions and coupling to the environment, can only be identified experimentally. Quantitative insight can be obtained provided the quantum pathways are isolated and independently analysed. Here we demonstrate this possibility in an atomic vapour using optical three-dimensional Fourier-transform spectroscopy. By unfolding the system's nonlinear response onto three frequency dimensions, three-dimensional spectra unambiguously reveal transition energies, relaxation rates and dipole moments of each pathway. The results demonstrate the unique capacity of this technique as a powerful tool for resolving the complex nature of quantum systems. This experiment is a critical step in the pursuit of complete experimental characterization of a system's Hamiltonian.
Electronic structure and dynamics are captured by optical 2D-Fourier-transform (2DFT) spectroscopy, which tracks the phase of the nonlinear signal during two time delays of a multi-pulse excitation sequence. These Fourier-transformed spectra separate and isolate overlapping and competing contributions to the coherent response. We have developed an ultra-stable platform consisting of nested interferometers with active phase control, allowing for exploration of single- and two-quantum coherences. Phase-resolved spectra are retrieved by all-optical determination of experimental phase ambiguities. GaAs quantum wells show suppression of many-body interactions in cross-linear polarized 2DFT spectra and many-body two-quantum coherences. Potassium vapor also shows unexpected two-quantum coherences.
Optical two-dimensional coherent spectroscopy (2DCS) has become a powerful tool for studying energy-level structure, dynamics, and coupling in many systems including atomic ensembles. Various types of two-dimensional (2D) spectra, including the so-called single-quantum, zero-quantum, and double-quantum 2D spectra, of both
D lines (
D1 and
D2 transitions) of potassium (K) atoms have been reported previously. For rubidium (Rb), a major difference is that the
D lines are about 15 nm apart as opposed to only about 3 nm for K. Simultaneously exciting both
D lines of Rb atoms requires a broader laser bandwidth for the experiment. Here, we report a broadband optical 2DCS experiment in a Rb atomic vapor. A complete set of single-quantum, zero-quantum, and double-quantum 2D spectra including both
D lines of Rb atoms were obtained. The experimental spectra were reproduced by simulated 2D spectra based on the perturbation solutions to the optical Bloch equations. This work in Rb atoms complements previous 2DCS studies of K and Rb with a narrower bandwidth that covers two
D lines of K or only a single
D line of Rb. The broadband excitation enables the capability to perform double-quantum and multiquantum 2DCS of both
D lines of Rb to study many-body interactions and correlations in comparison with K atoms.