Abstract Basics of ultrafast spectroscopy are introduced in this chapter. We first discuss the basic concept of spectroscopy in terms of linear and nonlinear spectroscopy. We then describe definitions and parameters of ultrashort pulses and the ultrafast nonlinear/coherent spectroscopy. The density matrix formalism of light-matter interaction and the Bloch sphere representation of quantum states are also introduced.
Abstract Basics of ultrafast spectroscopy are introduced in this chapter. We first discuss the basic concept of spectroscopy in terms of linear and nonlinear spectroscopy. We then describe definitions and parameters of ultrashort pulses and the ultrafast nonlinear/coherent spectroscopy. The density matrix formalism of light-matter interaction and the Bloch sphere representation of quantum states are also introduced.
Theoretical concepts and framework for describing nonlinear light-matter interactions are the subject of this chapter. A density matrix approach is introduced for describing the nonlinear optical response of quantum systems to an electromagnetic field. Starting with the full description, different approximations are introduced and their limitations discussed from the viewpoint of physics. Basic concepts such as quantum coherences in multi-level systems, nonlinear currents, and nonperturbative light-matter interactions are explained and illustrated by model calculations.
In this thesis we discuss a theoretical description of ultrafast and non-linear spectroscopy. Due to the high intensities and ultrashort pulse durations involved in such experiments, it is necessary to use an explicitly time-dependent formalism. In addition, many of the systems we have considered exhibit strong interactions between the constituent particles, such as electron-electron, electron-nuclear and electron-photon interactions. To properly account for such interactions we have made extensive use of effective models, designed to capture the qualitative aspects of the systems.The thesis is based on six papers, that can be broadly categorized as follows: In Papers III and VI we study the non-linear interaction of light and matter. In Paper III we look at the multi-photon photo-emission from InAs nanowires, and in particular at the polarization dependence of the photo-emission signal on the crystal structure. In Paper VI we discuss second harmonic generation in a two-level system, and compare the fluorescence spectra from a quantum and semi-classical treatment of radiation.The remaining four papers are concerned with ultrafast dynamical processes: In Papers I and II we study atomic desorption from a surface, resulting from strong electron-nuclear interactions. To account for these interactions we consider a finite system that can be solved using exact diagonalization. We show that the desorption yield can be controlled by using a protocol with two ultrashort pulses and varying their delay. To assess finite size effects we compare the exact solution of the finite system to an approximate solution of a semi-infinite system based on non-equilibrium Green's functions and Ehrenfest dynamics. In Paper V we study Auger decay in a real-time description, and propose a protocol to induce the quantum Zeno effect by driving the system during decay. The Zeno effect leads to an increase in the Auger lifetime, that is experimentally measurable and can be controlled by varying the intensity of the driving field. In Paper IV we discuss the role of electron-electron interactions for charge-separation in donor-acceptor systems. We identify a regime where the charge-separation is driven solely by electronic correlations, that are captured by treating interactions to first order beyond a mean-field description.
Abstract An introduction to multidimensional coherent spectroscopy (MDCS) is provided in this chapter. We start out by introducing MDCS in qualitative terms, as an extension of transient absorption spectroscopy. We proceed to discuss the meaning and utility of coherent spectroscopy more generally. We go on to discuss the specifics of MDCS as a specific form of coherent spectroscopy. Pulse sequences involved in generating MDCS plots are introduced. Finally, we discuss density matrix perturbation theory and double-sided Feynman diagrams, and the role of this formalism in simulating and interpreting MDCS.
Spectroscopy, the study of matter via its interactions with light, is a vast subject that includes many of humanity's principle means of probing the fundamental structure of matter. Though usually understood from the perspective of classical light, important effects arise due to the quantum nature of the electromagnetic field. Additionally, the quantum vacuum can serve as an effective couple between molecules in the system, generating many-body interactions and cooperative processes in the material. While many techniques are well-described classically, quantum-field corrections can dominate the desired signals in certain cases, such as cascading in higher-order n-wave mixing signals, and are key for experimental interpretation. Even spontaneous emission and light scattering are best understood from the perspective of quantum fields. In this thesis, we utilize a systematic procedure, based on the density matrix in Liouville space, to give a rigorous description of elementary quantum-field effects in nonlinear spectroscopy, with specific examples made to ultrafast x-ray and optical techniques.
In the previous chapters, we have seen that quantum-optical correlations can produce effects that have no classical explanation. In particular, the matter excitations can depend strongly on the specific form of the quantum fluctuations, i.e., the quantum statistics of the light source. For example, Fock-state sources can produce quantum Rabi flopping with discrete frequencies while coherent-state sources generate a sequence of collapses and revivals in atomic excitations. Hence, not only the intensity or the classical amplitude of the field is relevant but also the quantum statistics of the exciting light influences the matter response. Even if we take sources with identical intensities, the resulting atomic excitations are fully periodic for a Fock-state excitation while a coherentstate excitation produces a chaotic Bloch-vector trajectory with multiple collapses and revivals. In this chapter, we use this fundamental observation as the basis to develop the concept of quantum-optical spectroscopy. We show in Chapter 30 that this method yields a particularly intriguing scheme to characterize and control the quantum dynamics in solids. Since one cannot imagine how to exactly compute the many-body wave function or the density matrix, we also study how principal quantum-optical effects can be described with the help of the cluster-expansion scheme. Quantum-optical spectroscopy Historically, the continued refinement of optical spectroscopy and its use to manipulate the states of matter has followed a very distinct path where one simultaneously tries to control and characterize light with increased accuracy.
An overview of the influence of coherent coupling in light–matter interaction traced by transients in pumpprobe spectroscopy is given, with special emphasis on interest and works provided by the authors. Advantages of this technique used to trace pathways of transfer and decay of quantum coherences in atoms, molecular aggregates, and biological structures, as well as feasibility of the probing of both the lifetime and characteristic lengths (area, volume) of coherent excitations are discussed. Theoretical analysis of the pump-probe measurements is based on the numerical solution of the equation for the reduced density matrix in the representation of atomic, molecular, and excitonic states, considering coherent coupling between light and appropriate quantum states precisely.