Chirality characterizes an object that is not identical to its mirror image. In condensed matter physics, Fermions have been demonstrated to obtain chirality through structural and time-reversal symmetry breaking. These systems display unconventional electronic transport phenomena such as the quantum Hall effect and Weyl semimetals. However, for bosonic collective excitations in atomic lattices, chirality was only theoretically predicted and has never been observed. We experimentally show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide, whose lattice breaks the inversion symmetry and enables inequivalent electronic K and -K valley states. The time-reversal symmetry is also broken when we selectively excite the valley polarized holes by circularly polarized light. Brillouin-zone-boundary phonons are then optically created by the indirect infrared absorption through the hole-phonon interactions. The unidirectional intervalley transfer of holes ensures that only the phonon modes in one valley are excited. We found that such photons are chiral through the transient infrared circular dichroism, which proves the valley phonons responsible to the indirect absorption has non-zero pseudo-angular momentum. From the spectrum we further deduce the energy transferred to the phonons that agrees with both the first principle calculation and the double-resonance Raman spectroscopy. The chiral phonons have significant implications for electron-phonon coupling in solids, lattice-driven topological states, and energy efficient information processing.
The application of transient terahertz (THz) pulses to excite and probe low-energy quantum and collective excitations in materials represents a powerful tool to study both intrinsic interactions and non-equilibrium phases. In the following, we discuss ultrafast multi-THz studies that resolve the dynamics of electronic itineracy and vibrational symmetries in a strongly-correlated nickelate. Many transition-metal oxides exhibit the emergence of “stripes,” corresponding to quasione- dimensional charge, spin and lattice modulations as a manifestation of strong correlations. In our experiments, optical excitation of a stripe-phase nickel oxide triggers the rapid melting of its atomic-scale charge order and results in dynamics that yields insight into the couplings underlying the stripes. The transient optical conductivity is sensitive to both charges and in-plane vibrations and reveals a succession of ultrafast processes, ranging from rapid delocalization and localization of charges, via a time-delayed reaction of vibrational distortions to the electronic quench, up to the multi-picosecond re-establishment of the symmetry-broken phase.
The intriguing electronic properties of two-dimensional materials motivates experiments to resolve their rapid, microscopic interactions and dynamics across momentum space. Essential insight into the electronic momentum-space dynamics can be obtained directly via time- and angle-resolved photoemission spectroscopy (trARPES). We discuss the development of a high-repetition rate trARPES setup that employs a bright source of narrowband, extreme-UV harmonics around 22.3 eV, and its application to sensitive studies of materials dynamics. In the bulk transition-metal dichalcogenide MoSe<sub>2</sub> momentum-space quasiparticle scattering is observed after resonant excitation at the <i>K</i>-point exciton line, resulting in the time-delayed buildup of electrons at the Σ-point conduction band minimum. We will discuss this and other aspects of the non-equilibrium electronic response accessible with the extreme-UV trARPES probe.
The transient dynamics of transition-metal dichalcogenides is of significant interest for clarifying fundamental manyparticle interactions at the nanoscale as well as for novel applications. We report an ultrafast terahertz study up to 7 THz of the lamellar semiconductor MoS<sub>2</sub> to access the non-equilibrium conductivity of photo-excited indirect <i>e-h</i> pairs in this multi-layered parent compound. While the equilibrium transport is Drude-like, near-IR optical excitation results in a complex photo-induced conductivity that consists of two components. Mobile charge carriers dominate the low frequency response below 2 THz, while at low temperatures an additional excess conductivity is observed that is enhanced around 4 THz. Two time scales appear in the dynamics: a slow ns relaxation due to non-radiative recombination and a faster sub-100 ps decay connected to the high-frequency THz feature. We discuss the broad THz peak within a model of intra-excitonic transitions in MoS<sub>2</sub>. It agrees well with the expected binding energy and oscillator strength, yet results in an anomalous temperature dependence of the exciton fraction requiring an electronically inhomogeneous phase.
We discuss equilibrium and ultrafast optical pump-THz probe spectroscopy of the model stripe-ordered system La<sub>1.75</sub>Sr<sub>0.25</sub>NiO<sub>4</sub>. We present a multi-oscillator analysis of the phonon bending mode splitting observed at low temperatures in equilibrium, along with a variational model for the transient THz reflectivity variations. The low temperature splitting is directly related to the formation of the long-range stripe-order, while the background conductivity is reminiscent of the opening of the mid-IR pseudogap. Ultrafast experiments in the multi-THz spectral range show strong THz reflectivity variations around the phonon bending mode frequency (≈11 THz).
We discuss the mid-infrared optical response of a charge and spin-ordered nickelate in the ultrafast time domain. A strong photo-induced modulation of the optical reflectivity is observed on the sub-picosecond timescale, indicating the transient filling and recovery of the pseudogap in the mid-infrared charge transport. A variational Kramers-Kronig analysis of equilibrium reflectivity data is extended to time-resolved experiments, allowing us to extract the optical conductivity despite a comparatively limited frequency range of tunable femtosecond parametric sources. The fast dynamics of the spectral weight transfer supports an electronic origin of the mid-infrared pseudogap in nickelates.
The generation, manipulation and relaxation of optical intersubband excitations in n-type GaAs/AlGaAs and p-type SiGe/Si quantum wells are studied by different techniques of ultrafast spectroscopy in the mid-infrared. For electrons in GaAs/AlGaAs quantum wells, femtosecond time-resolved four-wave-mixing studies demonstrate de-phasing times of coherent intersubband polarizations of several hundreds of femtoseconds which are determined by electron-electron scattering. The measured dephasing times fully account for the width of the stationary intersubband absorption line, giving evidence of a predominant homogeneous broadening. Using phase-locked mid-infrared pulses and phase-resolving detection schemes, coherent optical control of such intersubband polarizations is demonstrated. In a second experiment, we study the intersubband relaxation of heavy holes (HH) in p-type SiGe/Si quantum wells. Intersubband scattering from the HH2 back to the HH1 subband occurs with a time constant of 250 fs determined by scattering with optical phonons through the deformation potential interaction.
Intersubband excitations play an important role for ultrafast carrier dynamics in quasi-two-dimensional semiconductors and for device applications. We present a study of the ultrafast coherent and incoherent dynamics of intersubband excitations in a pure electron plasma by means of femtosecond spectroscopy in the mid-infrared. The different relaxation processes following intersubband excitation of electrons in GaInAs/AllnAs quantum wells are observed in real-time and the relevant microscopic scattering mechanisms are identified. We find a decay of coherent intersubband polarizations on a time scale of several hundreds of femtoseconds which is governed by electron-electron scattering. Electrons excited to the n equals 2 conduction subband undergo intersubband scattering to the n equals 1 subband by emission of longitudinal optical phonons with characteristic time constants of 1 ps. This is followed by thermalization of the backscattered electrons on a similar time scale, involving both electron-electron and electron-phonon scattering. Eventually, the hot electron distribution cools down to lattice temperature within about 50 ps.