UC Berkeley

Exploring photoinduced dynamics in solids is crucial to the understanding and development of optoelectronics, photovoltaics, and photocatalytic cells. Core-level transient absorption spectroscopy is a powerful tool to probe the electronic and structural dynamics in atoms, molecules, and condensed matter. After photoexcitation with a sub-5 femtosecond optical pulse, time-delayed broadband subfemtosecond extreme ultraviolet (XUV, 10-200 eV) pulses produced by high harmonic generation excite core-level electrons in the photoexcited sample. The change of absorption spectrum in the XUV provides snapshots of electronic and vibrational states of the system, enabling real-time tracking of electronic and structural dynamics with subfemtosecond resolution. This technique, termed attosecond transient absorption spectroscopy, is utilized througout this work to investigate dynamics of photoexcited semiconductors, metals, and insulators.

In semiconducting 2H-MoTe2, attosecond transient absorption spectroscopy tracks the relaxation and recombination of carriers and coherent lattice displacement by probing the core-level absorption at the Te N4,5 edge (40 eV). Here transient absorption signal below the Te N5 edge can be directly mapped to the energy distribution of photoexcited holes, and a 15±5 fs thermalization time, a 380±90 fs hole relaxation time, and a 1.5±0.1 ps carrier recombination time are experimentally obtained. Coherent phonon excitations causes periodic shifts of the core-level absorption edge and an oscillatory XUV transient absorption feature. Fourier transform of the oscillations in the XUV transient absorption signal reveals the excitations of out-of-plane A1g (5.1 THz) and in-plane E1g (3.7 THz) phonons. By comparison to Bethe-Salpeter equation simulations, the spectral changes are mapped to real-space excited-state displacements of the lattice along the dominant A1g coordinate.

While in semiconducting 2H-MoTe2 the core-level absorption spectrum can be directly mapped to the density of states in the valence shell, core-level absorption in semiconducting WS2 exhibits both direct core-to-band transitions at the W O$_3$ edge (40 eV) and discrete core-exciton transitions at the W N6,7 edge. Here the dynamics of photoexcited carriers and carrier-induced modifications of core-excitons are probed when the XUV pulse arrives after the optical pulse. When the pulse sequence is reversed, core-excitons are excited with the XUV pulse and the optical pulse perturbs the transition dipole of the core-excitons by carrier photoexcitation and field-induced coupling to high-lying core-exciton states or the continuum. Global fitting of the transient absorption signal at the W N6,7 edge yields ~10 fs coherence lifetimes of core-exciton states and reveals that the photoexcited carriers, which alter the electronic screening and band filling, are the dominant contributor to the spectral modifications of core-excitons and that direct field-induced changes play a minor role. A $1.2\pm0.3$ ps hole-phonon relaxation time and a 3.1±0.4 ps carrier recombination time are also extracted from the XUV transient absorption spectra from the core-to-conduction band transitions at the W O3 edge.

Unlike many semiconductors, core-level absorption in metals cannot be generally described by single-particle core-to-band transitions due to electron scattering at the Fermi surface mediated by the core hole potential, which strongly renormalizes the core-level absorption spectrum beyond the single-particle picture. By studying the dynamics of photoexcited electrons in nickel with attosecond transient absorption spectroscopy, it is observed that the core-level absorption lineshape of photoexcited nickel can be described by a Gaussian broadening ($\sigma$) and a red shift ($\omega_{s}$) of the ground state absorption spectrum. Theory predicts, and the experimental results verify, that after initial rapid carrier thermalization, the electron temperature increase ($\Delta T$) is linearly proportional to the Gaussian broadening factor $\sigma$, providing quantitative real-time tracking of the relaxation of the electron temperature. During and after photoexcitation, rapid electron thermalization via carrier-carrier scattering accompanies and follows the nominal 4 fs photoexcitation pulse until the carriers reach a quasi-thermal equilibrium. Entwined with a <6 fs instrumentresponse function, carrier thermalization times ranging from 34 fs to 13 fs are estimated from experimental data acquired at different pump fluences, and it is observed that the electron thermalization time decreases with increasing pump fluence, which is consistent with predictions with Fermi liquid theory. Measurements also reveal an electron cooling time of 640$\pm$80 fs. With hot thermalized carriers, the spectral red shift exhibits a power-law relationship with the change in electron temperature of $\omega_{s}\propto\Delta T^{1.5}$.

For insulators, we outline the experimental results of the decay of core-excitons in NaCl probed by attosecond transient absorption spectroscopy at the Na L$_{2,3}$ edge (32-48 eV). Here, core-excitonic transitions span an energy range over 15 eV. Coherence lifetimes up to 11 fs are observed for core-exciton states below the core-to-conduction band transition onset (36 eV), while the coherence lifetimes of core-exciton states above the core-to-conduction band onset are well within the duration of the optical pulse (<5 fs).

Lastly, we report an experimental technique to simultaneously generate sub-5 fs pulses centered at 400 nm and 800 nm, enabling direct optical excitation of wide band gap semiconductors to probe their dynamics after photoexcitation. By using a dichroic beamsplitter to separate spectral components of a supercontinuum below and above 500 nm and separately compressing each arm with a set of chriped mirrors, laser pulses centered at 400 nm and 800 nm with <4.5 fs pulse duration are simultaneously obtained.

This work summarizes four prototypical experiments of attosecond transient absorption spectroscopy in solids, including a metal, two different semiconductors, and an insulator. The methodologies for the experiments and analyses can be extended and generalized to study electronic and structural dynamics in more complex systems, such as alloys, multilayers, heterostructures, and superlattices.