17 research outputs found
Advancing time- and angle-resolved photoemission spectroscopy: The role of ultrafast laser development
In the last decade, there has been a proliferation of laser sources for time-
and angle-resolved photoemission spectroscopy (TR-ARPES), building on the
proven capability of this technique to tackle important scientific questions.
In this review, we aim to identify the key motivations and technologies that
spurred the development of various laser sources, from frequency up-conversion
in nonlinear crystals to high-harmonic generation in gases. We begin with a
historical overview of the field in Sec.1, framed by advancements in light
source and electron spectrometer technology. An introduction to the fundamental
aspects of the photoemission process and the observables that can be studied is
given in Sec.2, along with its dependencies on the pump and probe pulse
parameters. The technical aspects of TR-ARPES are discussed in Sec.3. Here,
experimental limitations such as space charge and resultant trade-offs in
source parameters are discussed. Details of various systems and their approach
to these trade-offs are given in Sec.4. Within this discussion, we present a
survey of TR-ARPES laser sources; a meta-analysis of these source parameters
showcases the advancements and trends in modern systems. Lastly, we conclude
with a brief discussion of future directions for TR-ARPES and its capabilities
in elucidating equilibrium and non-equilibrium observables, as well as its
integration with micro-ARPES and spin-resolved ARPES (Sec.5).Comment: 104 pages, 27 figure
Charge transfer state emission dynamics in blue-emitting functionalized silicon nanocrystals
We explore the dynamics of blue emission from dodecylamine and ammonia functionalized silicon nanocrystals (Si NCs) with average diameters of ∼3 and ∼6 nm using time-resolved photoluminescence (TRPL) spectroscopy. The Si NCs exhibit nanosecond PL decay dynamics that is independent of NC size and uniform across the emission spectrum. The TRPL measurements reveal complete quenching of core state emission by a charge transfer state that is responsible for the blue PL with a radiative recombination rate of ∼5 × 10^7 s^(−1). A detailed picture of the charge transfer state emission dynamics in these functionalized Si NCs is proposed
Terahertz Spectroscopy and Brewster Angle Reflection Imaging of Acoustic Tiles
A Brewster angle reflection imaging apparatus is demonstrated which is capable of detecting hidden water-filled voids in a rubber tile sample. This imaging application simulates a real-world hull inspection problem for Royal Canadian Navy Victoria-class submarines. The tile samples represent a challenging imaging application due to their large refractive index and absorption coefficient. With a rubber transmission window at approximately 80 GHz, terahertz (THz) sensing methods have shown promise for probing these structures in the laboratory. Operating at Brewster's angle allows for the typically strong front surface reflection to be minimized while also conveniently making the method insensitive to air-filled voids. Using a broadband THz time-domain waveform imaging system (THz-TDS), we demonstrate satisfactory imaging and detection of water-filled voids without complicated signal processing. Optical properties of the tile samples at low THz frequencies are also reported
Establishing non-thermal regimes in pump-probe electron-relaxation dynamics
Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the
electronic structure of solids under optical excitation, and is a powerful
technique for studying the coupling between electrons and collective modes. One
approach to infer electron-boson coupling is through the relaxation dynamics of
optically-excited electrons, and the characteristic timescales of energy
redistribution. A common description of electron relaxation dynamics is through
the effective electronic temperature. Such a description requires that
thermodynamic quantities are well-defined, an assumption that is generally
violated at early delays. Additionally, precise estimation of the non-thermal
window -- within which effective temperature models may not be applied -- is
challenging. We perform TR-ARPES on graphite and show that Boltzmann rate
equations can be used to calculate the time-dependent electronic occupation
function, and reproduce experimental features given by non-thermal electron
occupation. Using this model, we define a quantitative measure of non-thermal
electron occupation and use it to define distinct phases of electron relaxation
in the fluence-delay phase space. More generally, this approach can be used to
inform the non-thermal-to-thermal crossover in pump-probe experiments.Comment: 18 pages, 10 figure
Direct determination of mode-projected electron-phonon coupling in the time-domain
Ultrafast spectroscopies have become an important tool for elucidating the
microscopic description and dynamical properties of quantum materials. In
particular, by tracking the dynamics of non-thermal electrons, a material's
dominant scattering processes -- and thus the many-body interactions between
electrons and collective excitations -- can be revealed. Here we present a new
method for extracting the electron-phonon coupling strength in the time domain,
by means of time and angle-resolved photoemission spectroscopy (TR-ARPES). This
method is demonstrated in graphite, where we investigate the dynamics of
photo-injected electrons at the K point, detecting quantized energy-loss
processes that correspond to the emission of strongly-coupled optical phonons.
We show that the observed characteristic timescale for spectral-weight-transfer
mediated by phonon-scattering processes allows for the direct quantitative
extraction of electron-phonon matrix elements, for specific modes, and with
unprecedented sensitivity.Comment: 19 pages, 4 figure
Nematicity dynamics in the charge-density-wave phase of a cuprate superconductor
Understanding the interplay between charge, nematic, and structural ordering
tendencies in cuprate superconductors is critical to unraveling their complex
phase diagram. Using pump-probe time-resolved resonant x-ray scattering on the
(0 0 1) Bragg peak at the Cu L3 and oxygen K resonances, we investigate
non-equilibrium dynamics of Qa = Qb = 0 nematic order and its association with
both charge density wave (CDW) order and lattice dynamics in
La1.65Eu0.2Sr0.15CuO4. In contrast to the slow lattice dynamics probed at the
apical oxygen K resonance, fast nematicity dynamics are observed at the Cu L3
and planar oxygen K resonances. The temperature dependence of the nematicity
dynamics is correlated with the onset of CDW order. These findings
unambiguously indicate that the CDW phase, typically evidenced by translational
symmetry breaking, includes a significant electronic nematic component.Comment: 16 pages, 4 figure
Fano interference of the Higgs mode in cuprate high-Tc superconductors
Despite decades of search for the pairing boson in cuprate high-Tc
superconductors, its identity still remains debated to date. For this reason,
spectroscopic signatures of electron-boson interactions in cuprates have always
been a center of attention. For example, the kinks in the quasiparticle
dispersion observed by angle-resolved photoemission spectroscopy (ARPES)
studies have motivated a decade-long investigation of electron-phonon as well
as electron-paramagnon interactions in cuprates. On the other hand, the overlap
between the charge-order correlations and the pseudogap in the cuprate phase
diagram has also generated discussions about the potential link between them.
In the present study, we provide a fresh perspective on these intertwined
interactions using the novel approach of Higgs spectroscopy, i.e. an
investigation of the amplitude oscillations of the superconducting order
parameter driven by a terahertz radiation. Uniquely for cuprates, we observe a
Fano interference of its dynamically driven Higgs mode with another collective
mode, which we reveal to be charge density wave fluctuations from an extensive
doping- and magnetic field-dependent study. This finding is further
corroborated by a mean field model in which we describe the microscopic
mechanism underlying the interaction between the two orders. Our work
demonstrates Higgs spectroscopy as a novel and powerful technique for
investigating intertwined orders and microscopic processes in unconventional
superconductors
Electron-phonon coupling in the time domain : TR-ARPES studies by a cavity-based XUV laser
The electron-phonon interaction is ubiquitous in crystalline materials, leaving fingerprints on both physical and electronic properties. In the study of materials, angle-resolved photoemission spectroscopy (ARPES) can uniquely access the electronic band structure, and the electron interactions encoded within. However, disentangling the contributions from different degrees of freedom -- such as electron-electron and electron-phonon – can be very challenging. Extension of ARPES into the time domain via pump-probe spectroscopy allows one to access the electronic structure on an ultrafast timescale: this is advantageous as the intertwined interactions in equilibrium become separated in the time domain.
In this thesis, we use time-resolved (TR)-ARPES to study the electron-phonon interaction in graphite. Specifically, we observe spectral features arising from the photoexcitation of electrons from the valence band to the conduction band, followed by quantized energy-loss processes corresponding to the emission of strongly-coupled optical phonons. The transfer of spectral weight from an identifiable initial state to a final state is the direct manifestation of a microscopic two-body scattering process from which we can extract the mode-projected electron-phonon matrix element. The spectral features observed in this study arise from the non-thermal (i.e. non-Fermi-Dirac) occupation of electrons. We use Boltzmann simulations to map out various regimes in graphite where non-thermal features arise. These non-thermal signatures are not unique to graphite but are ubiquitous in pump-probe experiments and intrinsically tied to the dominant scattering processes, their timescales, and corresponding bottlenecks.
Our results were made possible by a custom-built state-of-the-art laser source featuring cavity-enhanced high-harmonic generation. The source has three key features: First, high photon energies capable of mapping the whole Brillouin zone of materials; second, a high repetition rate that minimizes the space-charge effect; and last, a balanced time and energy resolution capable of studying subtle spectral features. All three elements were crucial in discerning the spectral features related to electron-phonon scattering in graphite, the first observation of its kind. With these results, we demonstrate that the maturation of high-harmonic sources can now offer a tunable table-top source with unprecedented intensity, repetition rate and resolution.Science, Faculty ofGraduat