Coherent Phonons in Antimony: an Undergraduate Physical Chemistry Solid-State Ultrafast Laser Spectroscopy Experiment

Ultrafast laser pump-probe spectroscopy is an important and growing field of physical chemistry that allows the measurement of chemical dynamics on their natural timescales, but undergraduate laboratory courses lack examples of such spectroscopy and the interpretation of the dynamics that occur. Here we develop and implement an ultrafast pump-probe spectroscopy experiment for the undergraduate physical chemistry laboratory course at the University of California Berkeley. The goal of the experiment is to expose students to concepts in solid-state chemistry and ultrafast spectroscopy via classic coherent phonon dynamics principles developed by researchers over multiple decades. The experiment utilizes a modern high-repetition rate 800 nm femtosecond Ti:Sapphire laser, split pulses with a variable time delay, and sensitive detection of transient reflectivity signals. The experiment involves minimal intervention from students and is therefore easy and safe to implement in the laboratory. Students first perform an intensity autocorrelation measurement on the femtosecond laser pulses to obtain their temporal duration. Then, students measure the pump-probe reflectivity of a single-crystal antimony sample to determine the period of coherent phonon oscillations initiated by an ultrafast pulse excitation, which is analyzed by fitting to a sine wave. Due to the disruption of in-person instruction caused by the COVID-19 pandemic, during those semesters students were provided the data they would have obtained during the experiment to analyze at home. Evaluation of student written reports reveals that the learning goals were met, and that students gained an appreciation for the field of ultrafast laser-induced chemistry

Polarization-Resolved Extreme Ultraviolet Second Harmonic Generation from LiNbO3_3

Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic orders through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hamper the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In non-centrosymmetric LiNbO$_3$, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO$_3$, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO$_{6}$ octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position by $\sim$0.5 Angstrom while the Nb-O bonds are elongated/contracted by displacements of the O atoms by $\sim$0.1 Angstrom. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. This constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems

Coherent Phonons in Antimony: an Undergraduate Physical Chemistry Solid-State Ultrafast Laser Spectroscopy Experiment

Ultrafast laser pump-probe spectroscopy is an important and growing field of physical chemistry that allows the measurement of chemical dynamics on their natural timescales, but undergraduate laboratory courses lack examples of such spectroscopy and the interpretation of the dynamics that occur. Here we develop and implement an ultrafast pump probe spectroscopy experiment for the undergraduate physical chemistry laboratory course at the University of California Berkeley. The goal of the experiment is to expose students to concepts in solid-state chemistry and ultrafast spectroscopy via classic coherent phonon dynamics principles developed by researchers over multiple decades. The experiment utilizes a modern high-repetition-rate 800 nm femtosecond Ti:Sapphire laser, split pulses with a variable time delay, and sensitive detection of transient reflectivity signals using the lock-in technique. The experiment involves minimal intervention from students and is therefore easy and safe to implement in the laboratory. Students first perform an intensity autocorrelation measurement on the femtosecond laser pulses to obtain their temporal duration. Then, students measure the pump-probe reflectivity of a single-crystal antimony sample to determine the period of coherent phonon oscillations initiated by an ultrafast pulse excitation, which is analyzed by fitting to a sine wave. Students who completed the experiment in-person obtained good experimental results, and students who took the course remotely due to the COVID-19 pandemic were provided with the data they would have obtained during the experiment to analyze. Evaluation of student written and oral reports reveals that the learning goals were met, and that students gained an appreciation for the field of ultrafast laser-induced chemistry

Decomposition of contributions from core-levels exhibiting spin-orbit splitting in XUV core-level spectroscopy

Attosecond transient absorption spectroscopy (ATAS) is a versatile technique that allows observing ultrafast charge dynamics in solid-state samples. In ATAS, the transient absorption of a core-level excitation in the extreme UV (XUV) is measured following optical excitation of the sample. The lack of spin-selectivity in the XUV pulse results in overlapping XUV absorption spectra from spin-orbit split core-levels and leads to difficulty in disentangling the spectral signatures. Here, we demonstrate the successful retrieval of the contribution of a single spin-orbit level on the XUV transient absorption signal. Under the approxn. that the spin-orbit split levels yield identical absorption spectra which are energetically shifted and weighted by the degeneracy of the spin-orbit states, the contribution of one single spin-orbit state can be retrieved by the Fourier transform of the measured signal. In this case the energy shift of the spectra from the spin-orbit split states becomes a phase shift. By dividing out this phase factor and taking the inverse Fourier transform, the underlying signal referenced to a single spin-orbit level can be retrieved. We apply this method to an ATAS measurement at the germanium M_(4,5)-edge (30 eV), where the spin-orbit energy splitting (0.57 eV) is comparable to the germanium band gap (0.66 eV). The successful decompn. of the transient absorption signals yields clear, spectrally-resolved signatures of electrons and holes such that carrier dynamics can be simultaneously measured and characterized. The presented method allows decompg. contributions of spin-orbit split core-levels in a transient absorption spectrum. This allows clearer assignment of spectroscopic features and reveals weak signal contributions not visible in the exptl. raw data. In the presented case of germanium M-edge spectroscopy, a clear assignment of features assocd. with electrons, holes and the bandgap becomes possible only after applying this method

Decomposition of contributions from core-levels exhibiting spin-orbit splitting in XUV core-level spectroscopy

Attosecond transient absorption spectroscopy (ATAS) is a versatile technique that allows observing ultrafast charge dynamics in solid-state samples. In ATAS, the transient absorption of a core-level excitation in the extreme UV (XUV) is measured following optical excitation of the sample. The lack of spin-selectivity in the XUV pulse results in overlapping XUV absorption spectra from spin-orbit split core-levels and leads to difficulty in disentangling the spectral signatures. Here, we demonstrate the successful retrieval of the contribution of a single spin-orbit level on the XUV transient absorption signal. Under the approxn. that the spin-orbit split levels yield identical absorption spectra which are energetically shifted and weighted by the degeneracy of the spin-orbit states, the contribution of one single spin-orbit state can be retrieved by the Fourier transform of the measured signal. In this case the energy shift of the spectra from the spin-orbit split states becomes a phase shift. By dividing out this phase factor and taking the inverse Fourier transform, the underlying signal referenced to a single spin-orbit level can be retrieved. We apply this method to an ATAS measurement at the germanium M_(4,5)-edge (30 eV), where the spin-orbit energy splitting (0.57 eV) is comparable to the germanium band gap (0.66 eV). The successful decompn. of the transient absorption signals yields clear, spectrally-resolved signatures of electrons and holes such that carrier dynamics can be simultaneously measured and characterized. The presented method allows decompg. contributions of spin-orbit split core-levels in a transient absorption spectrum. This allows clearer assignment of spectroscopic features and reveals weak signal contributions not visible in the exptl. raw data. In the presented case of germanium M-edge spectroscopy, a clear assignment of features assocd. with electrons, holes and the bandgap becomes possible only after applying this method

Ultrafast extreme-ultraviolet reflection spectroscopy of electro-phonon dynamics in Germanium

Ultrafast transient reflection spectroscopy in the extreme UV (XUV) is developed as a new technique for following bandgap dynamics after photoexcitation. Signatures of carrier and lattice dynamics are obsd. through the measurement of reflectivity changes in the XUV of single-cryst. germanium after exciting the sample with a sub-5 fs visible-to-IR pulse. In the expt., a 5 fs near IR (NIR) pulse excites carriers across the band gap. The subsequent dynamics are probed by measuring the transient reflectivity changes at the Ge M_(4,5) edge around 30 eV, corresponding to transitions from the 3d core levels to the unoccupied conduction and valence band states. The energetically sepd. spectral signatures of photoexcited electrons and holes in the XUV enable to measure the thermalization dynamics of the carriers. Importantly, shifts in photon energy of the electron and hole features allow real time tracking of the energy sepn. of valence and conduction bands change over time. The dynamics obsd. in the expts. suggest dynamics on different time scales. Within 500fs, decays and energetic shifts of the obsd. reflectivity changes occur, which are assigned to electron-electron and electron-optical phonon scattering thermalizing the photoexcited carrier distribution. Within 2 ps, a blue shift of the valence band features is obsd., whereas a red shift of the conduction band features is present. This striking observation is assigned to a renormalization of the band gap due to the lattice expansion of the crystal. This band-gap renormalization sets in once momentum of the excited carriers has been transferred to phonons by electron-phonon and phonon-phonon scattering. Transient reflectivity is shown to overcome the thin-film and x-ray membrane related issues of transmission geometries. It will be shown how transient reflectivity can further be employed to ext. the time-dependent complex dielec. function over the timescales governing carrier-carrier, carrier-phonon, and phonon-phonon interactions

Ultrafast extreme-ultraviolet reflection spectroscopy of electro-phonon dynamics in Germanium

Ultrafast transient reflection spectroscopy in the extreme UV (XUV) is developed as a new technique for following bandgap dynamics after photoexcitation. Signatures of carrier and lattice dynamics are obsd. through the measurement of reflectivity changes in the XUV of single-cryst. germanium after exciting the sample with a sub-5 fs visible-to-IR pulse. In the expt., a 5 fs near IR (NIR) pulse excites carriers across the band gap. The subsequent dynamics are probed by measuring the transient reflectivity changes at the Ge M_(4,5) edge around 30 eV, corresponding to transitions from the 3d core levels to the unoccupied conduction and valence band states. The energetically sepd. spectral signatures of photoexcited electrons and holes in the XUV enable to measure the thermalization dynamics of the carriers. Importantly, shifts in photon energy of the electron and hole features allow real time tracking of the energy sepn. of valence and conduction bands change over time. The dynamics obsd. in the expts. suggest dynamics on different time scales. Within 500fs, decays and energetic shifts of the obsd. reflectivity changes occur, which are assigned to electron-electron and electron-optical phonon scattering thermalizing the photoexcited carrier distribution. Within 2 ps, a blue shift of the valence band features is obsd., whereas a red shift of the conduction band features is present. This striking observation is assigned to a renormalization of the band gap due to the lattice expansion of the crystal. This band-gap renormalization sets in once momentum of the excited carriers has been transferred to phonons by electron-phonon and phonon-phonon scattering. Transient reflectivity is shown to overcome the thin-film and x-ray membrane related issues of transmission geometries. It will be shown how transient reflectivity can further be employed to ext. the time-dependent complex dielec. function over the timescales governing carrier-carrier, carrier-phonon, and phonon-phonon interactions

Attosecond quantum kinetics of photoexcited Germanium

The electronic motion in a semiconductor after light absorption is the central aspect of modern opto-electronics. However, a real-time observation of the initial electronic response following single-photon excitation has so far defied exptl. approaches due to its extreme time and energy scales. Here, attosecond transient reflectivity is developed to measure the attosecond time-resolved dielec. function in the extreme UV following carrier promotion by visible-to-IR sub- 5 fs (fs) pulses from the valence into the conduction band in germanium. The buildup of holes and electrons in the valance and conduction band is monitored on attosecond timescales by the change in the reflection at the M_(4,5) edge (30 eV). The electron and hole features are found to exhibit a 1.4 fs oscillation, which is indicative of a field-induced polarization of the bands. The measurement of the attosecond dielec. function further enables the buildup of screening of the core-hole potential due to the collective electronic motion in the valence and conduction band to be tracked. It is exptl. obsd. through the real part of the dielec. function that a bare, unscreened Coulomb potential is formed instantaneously after photoexcitation. A subsequent broadening of the real part of the dielec. function over a few fs is attributed to the screening of the Coulomb potential due to the collective electronic motion in the valence and conduction bands. Simultaneously, the imaginary part of the dielec. function tracks the buildup of new absorption channels. The expt. shows that two sharp features appear instantaneously due to the change in state-filling in the valence and conduction band, with an addnl. broadening occurring on a few fs time scale. This broadening is attributed to a response time of the collective electronic motion, which screens the Coulomb potential and changes the absorption due to a carrier redistribution. The time scale of the screening clocks in well with the inverse plasma frequency of the electron-hole plasma created by photoexcitation. Similar electronic responses after photoexcitation are anticipated to occur in all materials. The results presented here provide an exptl. measurement of the fundamental timescales of charged particle interactions in solids

Attosecond quantum kinetics of photoexcited Germanium

The electronic motion in a semiconductor after light absorption is the central aspect of modern opto-electronics. However, a real-time observation of the initial electronic response following single-photon excitation has so far defied exptl. approaches due to its extreme time and energy scales. Here, attosecond transient reflectivity is developed to measure the attosecond time-resolved dielec. function in the extreme UV following carrier promotion by visible-to-IR sub- 5 fs (fs) pulses from the valence into the conduction band in germanium. The buildup of holes and electrons in the valance and conduction band is monitored on attosecond timescales by the change in the reflection at the M_(4,5) edge (30 eV). The electron and hole features are found to exhibit a 1.4 fs oscillation, which is indicative of a field-induced polarization of the bands. The measurement of the attosecond dielec. function further enables the buildup of screening of the core-hole potential due to the collective electronic motion in the valence and conduction band to be tracked. It is exptl. obsd. through the real part of the dielec. function that a bare, unscreened Coulomb potential is formed instantaneously after photoexcitation. A subsequent broadening of the real part of the dielec. function over a few fs is attributed to the screening of the Coulomb potential due to the collective electronic motion in the valence and conduction bands. Simultaneously, the imaginary part of the dielec. function tracks the buildup of new absorption channels. The expt. shows that two sharp features appear instantaneously due to the change in state-filling in the valence and conduction band, with an addnl. broadening occurring on a few fs time scale. This broadening is attributed to a response time of the collective electronic motion, which screens the Coulomb potential and changes the absorption due to a carrier redistribution. The time scale of the screening clocks in well with the inverse plasma frequency of the electron-hole plasma created by photoexcitation. Similar electronic responses after photoexcitation are anticipated to occur in all materials. The results presented here provide an exptl. measurement of the fundamental timescales of charged particle interactions in solids

Attosecond kinetics of photoexcited germanium

Attosecond transient reflectivity is developed to observe the photoexcitation dynamics in germanium. Attosecond time-resolved measurements of the dielectric function reveal a few-femtosecond collective electronic response time, which renormalizes the Coulomb interaction between the excited carriers