Spectroscopy for Materials Characterization. Группа авторов

3 Ultrafast Optical Spectroscopies

       Alice Sciortino and Fabrizio Messina

       Department of Physics and Chemistry – Emilio Segrè, University of Palermo, Palermo, Italy

3.1 Femtosecond Spectroscopy: An Overview

      Traditional steady state spectroscopies, addressed in Chapter 1 of this book, are essential tools to characterize a physicochemical system through the extensive mapping of its energy landscape and transitions. However, these classical methods cannot capture the dynamical aspects driving the response of the system after an initial perturbation. These aspects can only be addressed by time‐resolved spectroscopic techniques, which are methods capable of following in time the evolution of a physicochemical system, after it has been initially brought out of equilibrium by a defined stimulus, such as photon absorption. Indeed, a thorough understanding of these dynamics, as they unravel in time, is often crucial to fully elucidate the behavior of a system of physical, chemical, or biological interest.

      Photoexcitation of any physical system triggers a complex sequence of phenomena occurring over many temporal orders of magnitude after initial photon absorption, and responsible for the ultimate outcome of the photocycle. The primary and most fundamental dynamics, however, usually occur on picosecond (1 ps = 10⁻¹² s) or femtosecond (1 fs = 10⁻¹⁵ s) timescales for many well‐known processes of fundamental interest in photophysics and photochemistry [1]. In particular, it can be argued that femtoseconds are the “fundamental” timescale for any physicochemical process involving short‐range atomic rearrangements, such as chemical reactions or molecular relaxations, because the vibrational period of nuclei always fall in the femtosecond time range (∼10 fs for the OH vibration). Thereby, it is evident that very fast, or rather “ultrafast,” spectroscopic techniques are compulsory to investigate these types of phenomena.

      This chapter addresses a variety of experimental methods usually referred to as ultrafast or femtosecond spectroscopies. These techniques are capable of time resolutions reaching a few femtoseconds, which are essential to reconstruct in detail the course of events initiated by photoexcitation, achieving a comprehensive understanding of the photocycle of any physical system. Examples of typical phenomena which are addressed by femtosecond methods are molecular energy relaxations, such as internal conversion or intersystem crossing [2], solvation dynamics, electron and energy transfer events [3–5], fluorescence quenching [5], dynamics of charge carriers and excitons in semiconductors [6], and photochemical reactions [7]. For these reasons, femtosecond techniques are today well‐established and considered a fundamental tool in spectroscopy.

For example, whenever a molecule in solution phase is electronically excited by UV or VIS light, the surrounding solvent responds to the electronic redistribution within the molecule by a series of short‐ and long‐range rearrangements, collectively named solvation dynamics. These dynamics typically occur on femtosecond and picosecond timescales [3, 4] and are a key component of the behavior of any system in solution phase. As another example, when charge carriers in a bulk or nano‐sized semiconductor are promoted to a high energy state by photon absorption, their successive relaxation involves processes, such as electron–electron or electron–phonon scattering, or Auger recombination, which are of fundamental importance and equally take place on such, ultrafast, timescales [8, 9].

      The general idea behind most femtosecond spectroscopies is to use at least two, or more, light pulses with very short time durations to follow in real time the undergoing dynamics. One of the pulses, for example, is resonant to an electronic transition of the investigated system, and its absorption by the system causes an injection of energy and a quasi‐instantaneous redistribution of the electronic charge. Then, the successive dynamics are studied by taking spectroscopic snapshots of the excited systems at variable delays, by the use of a second light pulse. This can be done by using one of several possible spectroscopic observables, capable of retrieving different types of information, such as the UV/VIS absorption, reflectance or luminescence, or the infrared vibrational absorption. For example, transient absorption (TA) or pump/probe methods, discussed in Section 3.3, exploit a pump/probe approach, where the changes of visible or infrared absorption are detected and followed in time after excitation. Other methods rely on different observables, such as the spontaneous emission (ultrafast fluorescence, discussed in Section 3.4) or the Raman scattering (Section 3.5). From the technical point of view, femtosecond spectroscopies are characteristically nonlinear optical experiments, which make large use of a wide toolbox of laboratory techniques in nonlinear and laser optics, to manipulate, generate, detect, and control femtosecond‐pulsed light beams in various spectral regions, as described in Section 3.2.

      The development of the field of femtosecond spectroscopy has been characterized by a progressive and dramatic improvement in what can be practically achieved. With time, a femtosecond time‐resolved version has been developed for almost any traditional spectroscopic technique, including, in recent times, photoemission spectroscopy [10], X‐ray scattering [11], X‐ray absorption [12], or optical microscopy methods [13]. The last frontier in the field is pushing even more the time resolution of these experiments, to the extent that the first attosecond (1 as = 10⁻¹⁸ s) experiments have been emerging in the last years [10, 14].

      When these methods are properly combined, it is often possible to literally track in real time the flow of charge and energy through time and space after photoexcitation. The potential of these methods is testified by their applications on a wide variety of different systems and nanosystems, such as semiconductor NPs [8], molecules and macromolecules in solution [15], carbon nanomaterials [16], and many others, always providing very useful insight on their photoinduced behaviors. Besides reconstructing the photocycle, a further capability of femtosecond experiments is disentangling homogeneous and inhomogeneous broadenings of the spectral lines, via methods like TA hole burning [17] or four‐wave mixing [18], which do not have an equivalent in traditional spectroscopy.

      The chapter is organized as follows. First, a general presentation of the characteristics of fs laser beams will be presented in Section 3.2, as needed to follow the rest of the chapter. Then, three well‐established ultrafast spectroscopic methods will be described: transient absorption (Section 3.3), femtosecond‐resolved fluorescence (Section 3.4), and femtosecond Raman (Section 3.5). In the final section (Section 3.6), four different case studies will be presented, to illustrate the utility of these techniques in selected real‐case scenarios.

3.2 Ultrafast Optical Pulses

      3.2.1 General Properties

      An ultrafast optical pulse is an electromagnetic pulse characterized by a very short duration (from few femtoseconds to few hundreds of femtoseconds) and a broad spectral distribution (10–100 nm FWHM) in the near‐infrared, visible, or UV spectral range. Generating ultrafast pulses relies on the use of a mode‐locked laser, which