Mapping the dispersion of polaritons, hybrid quasiparticles arising from light-matter coupling, can provide key insights into the material dielectric response, coupling strength, and energy transfer pathways with other excitations. In this work, we present THz pump-Raman probe (TP-RP) as a versatile method for mapping the polariton dispersion in polar non-centrosymmetric materials, demonstrated here for the case of phonon-polaritons in LiNbO_3. By resonantly driving polaritonic modes with a broadband THz pump and probing them with a tunable NIR Raman pulse, TP-RP allows for the extraction of the momentum-dependence of both their frequency and damping rate with high accuracy. The spectral features observed in the pump-probe signal, including the polaritonic response as well as pulse artifacts, are reproduced within a many-body theoretical approach. Applying the technique to study the E(TO_1) phonon of LiNbO_3 enables the combined analysis of theory and experiments to uncover a nontrivial frequency dependence of the phonon intrinsic damping rate, revealing possible anharmonic couplings to other modes.

Recent works have suggested that transient suppression of a charge density wave (CDW) by an ultra-short excitation can lead to an inversion of the CDW phase. We experimentally investigate the dynamics of the CDW in K_0.3MoO_3 by time resolved x-ray diffraction after excitation with optical pulses. Our results indicate a transient inversion of the CDW phase close to the surface that evolves into a highly disordered state in less than one picosecond. Numerical simulations solving the Ginzburg-Landau equation including disorder from strong pinning defects reproduce our main observations. Our findings highlight the critical role of disorder in schemes for coherent control in condensed matter systems.

We present the combination of a broadband terahertz time-domain spectroscopy system (0.1 - 8 THz), a diamond anvil cell capable of generating high pressure conditions of up to 10 GPa and a cryostat reaching temperatures as low as 10 K. This combination allows us to perform equilibrium and time-resolved THz spectroscopy measurements of a sample while continuously tuning its temperature and pressure conditions. In this work, the procedures and characterizations necessary to carry out such experiments in a tabletop setup are presented. Due to the large modifications of the terahertz beam as it goes through the diamond anvil cell (DAC), standard terahertz time-domain spectroscopy analysis procedures are no longer applicable. New methods to extract the pressure dependent material parameters are presented, both for samples homogeneously filling the DAC sample chamber as well as for bulk samples embedded in pressure media. Different pressure media are tested and evaluated using these new methods, and the obtained material parameters are compared to literature values. Time resolved measurements under pressure are demonstrated using an optical pump - THz probe scheme.

Chirality is a fundamental symmetry concept describing discrete states, i.e., left-handed, right-handed, or achiral, and existing at disparate scales and in many categories of scientific fields. Even though symmetry breaking is indispensable for describing qualitatively distinct phenomena, symmetry cannot quantitatively predict measurable quantities. One can continuously distort an object, introducing the concept of chirality amplitude, similar to representing magnetization as the amplitude of time-reversal symmetry breaking. Considering the role of magnetization in emergent phenomena with time-reversal symmetry breaking, chirality amplitude is intuitively a key quantity for controlling chirality-related emergent phenomena. Here, we propose two types of chiral lattice distortions and demonstrate the tunability of their amplitude in ultrafast timescales. Resonant X-ray diffraction with circular polarization is an established technique to measure crystal chirality directly. We quantify the ultrafast change in chirality amplitude in real time after an optical excitation. Using instead a THz excitation, we observe oscillations in the resonant diffraction intensities corresponding to specific phonon frequencies. This indicates the creation of additional asymmetry, which could also be described as an enhancement in chirality amplitude. Our proposed concept of chirality amplitude and its ultrafast control may lead to a unique approach to control chirality-induced emergent phenomena in ultrafast timescales.

Chirality is a fundamental symmetry concept relevant to many scientific fields. It describes discrete states, i.e., left-handed, right-handed, or achiral. In contrast to this discrete classification, objects can be continuously distorted. To describe the deviation from an achiral state, the chirality amplitude has to be addressed, similarly to magnetization being the “amplitude” for time-reversal symmetry breaking. Even though symmetry breaking is the base for many phenomena in science in general, symmetry cannot quantitatively predict measurable quantities. Chirality amplitude is the key quantity for quantifying chirality-related emergent phenomena. Here, we propose two types of chiral lattice distortions and report their ultrafast dynamics. We determine the ultrafast reduction of the chirality amplitude in CsCuCl_{3} after an optical excitation by using resonant x-ray diffraction with circular polarization. An additional symmetry breaking representing a nonhanded chiral structure is observed by resonant x-ray diffraction when exciting the material with an intense, low-cycle terahertz pulse. The latter demonstrates a hidden enantiomorphic pair in a nonhanded chiral structure. The concept of chirality amplitude, as well as the decomposition into two different types of chiral lattice distortions, suggests a unique approach toward controlling chirality-induced emergent phenomena.

Thin films' properties can be greatly influenced by their supporting growth substrates. Even in the so-called strain-free heterostructure films, it is still unclear whether there will be no interfacial electronic reconstructions induced by the underlying substrates. Here, we report the studies of SrTiO3 (STO) films in the freestanding form (FS) with a thickness ranging from 20 to 80 nm. These STO films, by default, are in a strain-free state; they exhibit distinct properties not seen in both bulk and strain-free heterostructure forms. Our films show an enhanced antiferrodistortive (AFD) phase transition temperature with a preferential in-plane rotation axis for the TiO6 octahedra. The anisotropic Ti orbital occupancy around the surface signals the departure of its properties from the bulk. Moreover, we have found that the in-plane ferroelectricity can be strengthened by the reduced dimensionality, establishing that the dimensionality control is an important factor for enhancing STO's ferroelectric response.

We present high-transmittance, narrow bandwidth band-pass terahertz filters fabricated on 1-μm-thick freestanding, yet robust cyclic olefin copolymer (TOPAS) films integrated on 3D printed frames of the same material. This approach allows for very flexible customization of the shape and dimensions of the filters as well as cost-effective scaling of their fabrication volume.

Materials with linear electronic dispersion often feature high carrier mobilities and unusually strong nonlinear optical interactions. In this work, we investigate the THz nonlinear dynamics of one such material, HgCdTe, with an electronic band dispersion heavily dependent on both temperature and stoichiometry. We show how the band gap, carrier concentration and band shape together determine the nonlinear response of the system. At low temperatures, carrier generation from Zener tunneling dominates the nonlinear response with a reduction in the overall transmission. At room temperature, quasi-ballistic electronic dynamics drive the largest observed nonlinear optical interactions, leading to a transmission increase. Our results demonstrate the sensitivity of these nonlinear optical properties of narrow-gap materials to small changes in the electronic dispersion and carrier concentration.

From extreme strain to bending, the possibilities in the manipulation of freestanding films of oxide perovskites bring a novel landscape to their properties and brings them one step closer to their application. It is therefore of great importance to fully understand the inherent properties of such films, in which dimensionality and surface effects can play a major role in defining the properties of the materials ground state. This paper reports the properties of freestanding (FS) films of the canonical oxide, SrTiO3 (STO) with thicknesses 20, 30, 40 and 80 nm. We show that the relaxed ultrathin STO FS films become polar at temperatures as high as 85 K, in contrast to the quantum paraelectric behavior of bulk. Our findings are based on the softening of the ferroelectric mode towards the ferroelectric transition temperature Tc and its consecutive hardening below Tc with further decreasing temperature, probed with THz time domain spectroscopy in transmission mode. We find almost no thickness dependence in Tc. Moreover, we characterize the antiferrodistortive (AFD) phase transition in STO FS by X-ray diffraction (XRD) probing superlattice reflections characteristic for the rotation of the TiO6 octahedra. Our results point to a higher phase transition temperature in comparison to bulk STO, as well as an unbalanced domain population favoring the rotation axis to be in plane. X-ray linear dichroism results further show a preferential Ti xz/yz orbital occupancy at the surface, but with a complete degeneracy in the t2g states in the inner part of the film indicating that the AFD distortion does not strongly affect the t2g splitting. These findings demonstrate that STO FS films have clearly different properties than bulk.

Topological Dirac and Weyl semimetals (D&W SM) have attracted a lot of attention due to their unusual electronic properties, which are strongly dependent on the symmetry of the system. By altering the symmetry of the lattice, using for example a pulse of light, one can expect to modify their topological nature and induce/suppress the related properties “on demand”. Highfield Terahertz (THz) radiation that interacts with vibrational states under near-resonant conditions is a promising pump candidate to induce structural changes in D&W SM compounds, possibly leading to topological transient or permanent changes. There are, however, some challenges to achieving such control. D&W SM have a specific band structure showing a 3D-linear dispersion around nodes, which makes them sensitive to THz-induced carrier multiplication. These carrier multiplication processes can compete with and ultimately screen the interaction of the THz light with vibrational modes. Here, we present recent results of 2D-THz spectroscopy of a bulk Weyl semimetal TaAs in reflection. We highlight THz-induced carrier multiplication effects and study the field and temperature dependence of the number of carriers induced by the THz field. We examine possible carrier multiplication mechanisms which could be involved.

Quantifying the dynamics of normal modes and how they interact with other excitations is of central importance in condensed matter. Spin-lattice coupling is relevant to several sub-fields of condensed matter physics; examples include spintronics, high-Tc superconductivity, and topological materials. However, experimental approaches that can directly measure it are rare and incomplete. Here we use time-resolved X-ray diffraction to directly access the ultrafast motion of atoms and spins following the coherent excitation of an electromagnon in a multiferroic hexaferrite. One striking outcome is the different phase shifts relative to the driving field of the two different components. This phase shift provides insight into the excitation process of such a coupled mode. This direct observation of combined lattice and magnetization dynamics paves the way to access the mode-selective spin-lattice coupling strength, which remains a missing fundamental parameter for ultrafast control of magnetism and is relevant to a wide variety of materials.

The layered transition-metal dichalcogenide WTe 2 is characterized by distinctive transport and topological properties. These properties are largely determined by electronic states close to the Fermi level, specifically to electron and hole pockets in the Fermi sea. In principle, these states can be manipulated by changes to the crystal structure. The precise impact of particular structural changes on the electronic properties is a strong function of the specific nature of the atomic displacements. Here, we report on time-resolved X-ray diffraction and infrared reflectivity measurements of the coherent structural dynamics in WTe 2 induced by femtosecond laser pulses excitation (central wavelength 800 nm), with emphasis on a quantitative description of both in-plane and out-of-plane vibrational modes. We estimate the magnitude of these motions, and calculate via density functional theory their effect on the electronic structure. Based on these results, we predict that phonons periodically modulate the effective mass of carriers in the electron and hole pockets up to 20%. This work opens up new opportunities for modulating the peculiar transport properties of WTe 2 on short time scales.

We use time-resolved hard x-ray diffraction to investigate the structural dynamics of the multiferroic insulator TbMnO$_3$ in the low temperature antiferromagnetic and ferroelectrically ordered phase. The lattice response following photoexcitation at 1.55 eV is detected by measuring the (0 2 4) and (1 3 -5) Bragg reflections. A 0.02% tensile strain, normal to the surface, is seen to arise within 20 - 30 ps. The magnitude of this transient strain is over an order of magnitude lower than that predicted from laser-induced heating, which we attribute to a bottleneck in the energy transfer between the electronic and lattice subsystems. The timescale for the transient expansion is consistent with that of previously reported demagnetization dynamics. We discuss a possible relationship between structural and demagnetization dynamics in TbMnO$_3$, in which photoinduced atomic motion modulates the exchange interaction, leading to a destruction of the magnetic order in the system.

We report the coherent generation and detection of terahertz (THz) pulses featuring a spectral bandwidth in the range of 0.1-9 THz achieved via the use of a high repetition rate (250 kHz), low pulse energy (6.2 µJ) laser system. More specifically, we test and evaluate a solid-state biased coherent detection device in combination with a spintronic emitter. We demonstrate the use of this combination of techniques to measure the ultra-broadband THz frequency optical properties of bulk crystalline materials with time-domain spectroscopy.

Using two-dimensional THz spectroscopy in combination with numerical models, we investigate the dynamics linked to carrier multiplication caused by high-field THz excitation of the low-gap semiconductor InSb. In addition to previously observed dynamics connected with quasi-ballistic carrier dynamics, we observe other spectral and temporal features that we attribute to impact ionization for peak fields above 60 kV/cm, which continue up to the maximum investigated peak field of 430 kV/cm. At the highest fields we estimate a carrier multiplication factor greater than 10 due to impact ionization, which is well-reproduced by a numerical simulation of the impact ionization process which we have developed.

Optical control of material properties is usually limited to the region that absorbs the light. Coupling to lattice vibrations that travel close to the speed of light allows ultrafast modulation of polarization deep inside a ferroelectric material.

In ionic Raman scattering, infrared-active phonons mediate a scattering process that results in the creation or destruction of a Raman-active phonon. This mechanism relies on nonlinear interactions between phonons and has in recent years been associated with a variety of emergent lattice-driven phenomena in complex transition-metal oxides, but the underlying mechanism is often obscured by the presence of multiple coupled order parameters in play. Here, we use time-resolved spectroscopy to compare coherent phonons generated by ionic Raman scattering with those created by more conventional electronic Raman scattering on the nonmagnetic and non-strongly-correlated wide band-gap insulator LaAlO$_3$. We find that the oscillatory amplitude of the low-frequency Raman-active $E_g$ mode exhibits a sharp peak when we tune our pump frequency into resonance with the high-frequency infrared-active $E_u$ mode, consistent with first-principles calculations. Our results suggest that ionic Raman scattering can strongly dominate electronic Raman scattering in wide band-gap insulating materials. We also see evidence of competing scattering channels at fluences above 28~mJ/cm$^2$ that alter the measured amplitude of the coherent phonon response.

The interaction of many-body systems with intense light pulses may lead to novel emergent phenomena far from equilibrium. Recent discoveries, such as the optical enhancement of the critical temperature in certain superconductors and the photo-stabilization of hidden phases, have turned this field into an important research frontier. Here, we demonstrate nonthermal charge-density-wave (CDW) order at electronic temperatures far greater than the thermodynamic transition temperature. Using time- and angle-resolved photoemission spectroscopy and time-resolved X-ray diffraction, we investigate the electronic and structural order parameters of an ultrafast photoinduced CDW-to-metal transition. Tracking the dynamical CDW recovery as a function of electronic temperature reveals a behaviour markedly different from equilibrium, which we attribute to the suppression of lattice fluctuations in the transient nonthermal phonon distribution. A complete description of the system’s coherent and incoherent order-parameter dynamics is given by a time-dependent Ginzburg-Landau framework, providing access to the transient potential energy surfaces.

Phase transitions driven by ultrashort laser pulses have attracted interest both for understanding the fundamental physics of phase transitions and for potential new data storage or device applications. In many cases these transitions involve transient states that are different from those seen in equilibrium. To understand the microscopic properties of these states, it is useful to develop elementally selective probing techniques that operate in the time domain. Here we show fs-time-resolved measurements of V Ledge Resonant Inelastic X-Ray Scattering (RIXS) from the insulating phase of the Mott- Hubbard material V2O3 after ultrafast laser excitation. The probed orbital excitations within the d-shell of the V ion show a sub-ps time response, which evolve at later times to a state that appears electronically indistinguishable from the high-temperature metallic state. Our results demonstrate the potential for RIXS spectroscopy to study the ultrafast orbital dynamics in strongly correlated materials.