Tao Dong

The ultrafast manipulation of quantum material has led to many novel and significant discoveries. Among them, the light-induced transient superconductivity in cuprates achieved by melting competing stripe orders represents a highly appealing accomplishment. However, recent investigations have shown that the notion of photoinduced superconductivity remains a topic of controversy, and its elucidation solely through c-axis time-resolved terahertz spectroscopy remains an arduous task. Here, we measure the in-plane and out-of-plane transient terahertz responses simultaneously in the stripe-ordered nonsuperconducting La 2− x Ba x CuO 4 after near-infrared excitations. We find that although a pump-induced reflectivity edge appears in the c-axis reflectance spectrum, the reflectivity along the CuO 2 planes decreases simultaneously, indicating an enhancement in the scattering rate of quasiparticles. This in-plane …

The significant enhancement of ultrafast terahertz optical conductivity and the unexpected formation of a polaronic‐like state in semiconductor Mn3Si2Te6 at room temperature are reported. With the absorption of pump photons, the low‐frequency terahertz photoconductivity spectrum exhibits a significant rise, quickly forming a broad peak and subsequently shifting to higher energy. The short‐lived nature of the broad peak, as well as the distribution of optical constants, strongly points toward a transient polaron mechanism. This study not only provides profound insights into the remarkable photoelectric response of Mn3Si2Te6 but also highlights its significant potential for future photoelectric applications.

We report on nonlinear terahertz third-harmonic generation (THG) measurements on YBa2Cu3O6+x thin films. Different from conventional superconductors, the THG signal starts to appear in the normal state, which is consistent with the crossover temperature T* of pseudogap over broad doping levels. Upon lowering the temperature, the THG signal shows an anomaly just below Tc in the optimally doped sample. Notably, we observe a beat pattern directly in the measured real-time waveform of the THG signal. We elaborate that the Higgs mode, which develops below Tc, couples to the mode already developed below T*, resulting in an energy level splitting. However, this coupling effect is not evident in underdoped samples. We explore different potential explanations for the observed phenomena. Our research offers valuable insight into the interplay between superconductivity and pseudogap.

Near-infrared (NIR) pump-terahertz (THz) probe spectroscopy is used to investigate the charge and spin exciations in a strongly correlated electron compound Na0.5CoO2. This compound exhibits a coexistence of various charge and spin orders arising from intricate interactions among charge, spin, and orbital degrees of freedom. NIR pulses create significantly diverse effects on the charge and spin orders; while the charge order is easily melted,coherent magnon excitations are present in all fluences examined. Furthermore, a novel {\pi} phase shift of the coherent magnon oscillations is observed in the pump-induced change of the terahertz electric field between regions of increasing and decreasing field change. These results unequivocally illustrate that ultrashort laser pulses enable the disentanglement of different interactions within complex systems characterized by multiple orders, providing a fresh perspective on the interplay between itinerant and localized electrons within the Co 3d t2g multiplets.

Utilizing ultrafast light-matter interaction to manipulate electronic states of quantum materials is an emerging area of research in condensed matter physics. It has significant implications for the development of ultrafast electronic devices of the future. However, the ability to induce long-lasting metastable electronic states yet in a fully reversible manner is a long standing challenge. Here, by using ultrafast laser excitation with distinct pulse sequences and fluences, we were able to regulate the symmetry and electronic properties in a polar charge-density-wave material EuTe4. We demonstrated the capability of nonvolatile writing and erasing the polar order, a process that is completely reversible and is achieved at room temperature in an all optical manner. Each induced state brings about modifications to the electric resistance and second harmonic generation intensity. The results point to a distinct dynamical symmetry inversion mechanism in which photoexcitation mediates the polar phases of long-range electronic order. Our finding extends the scope of nonvolatile all-optical control of electronic states to ambient conditions, thus providing possibilities for applications in ultrafast optoelectronics.

In condensed matter physics, materials with kagome lattice display a range of exotic quantum states, including charge density wave (CDW), superconductivity, and magnetism. Recently, the intermetallic kagome metal ScV 6 Sn 6 was discovered to undergo a first-order structural phase transition with the formation of a 3× 3× 3 CDW at around 92 K. The bulk electronic band properties are crucial to understanding the origin of the structural phase transition. Here, we conducted an optical spectroscopy study in combination with band structure calculations across the structural transition. Our findings showed abrupt changes in the optical reflectivity/conductivity spectra as a result of the structural transition, without any observable gap formation behavior. The optical measurements and band calculations actually reveal a sudden change of the band structure after transition. It is important to note that this phase transition is of …

Optical nonlinearities, one of the most fascinating properties of two-dimensional (2D) materials, are essential for exploring novel physics in 2D systems and developing next-generation nonlinear optical applications. While tremendous efforts have been made to discover and optimize second-order nonlinear optical responses in various 2D materials, higher odd-order nonlinear processes, which are in general much less efficient than second order ones, have been paid less attention despite their scientific and applicational significance. Here we report giant odd-order nonlinear optical wave mixing in a correlated van der Waals insulator MnPSe3 at room temperature. Illuminated by two near-infrared femtosecond lasers simultaneously, it generates a series of degenerate and non-degenerate four- and six-wave mixing outputs, with conversion efficiencies up to the order of and for the four- and six-wave mixing processes, respectively, far exceeding the efficiencies of several prototypical nonlinear optical materials (GaSe, LiNbO3). This work highlights the intriguing prospect of transition metal phosphorous trichalcogenides for future research of the nonlinear light matter interactions in 2D systems and for potential nonlinear photonic applications.

Low dimensionality in charge density wave (CDW) systems leads to anisotropic optical properties, in both equilibrium and nonequilibrium conditions. Here, we perform polarized two-color pump probe measurements on a quasi-one-dimensional material ZrTe 3, in order to study the anisotropic transient optical response in the CDW state. Profound in-plane anisotropy is observed with respect to the polarization of probe photons. Below T CDW both the quasiparticle relaxation signal and amplitude mode (AM) oscillation signal are much larger, with E pr nearly parallel to the a axis (E pr∥ a) than for E pr parallel to the b axis (E pr∥ b). This reveals that the E pr∥ a signal provides much a larger response to the variation of the CDW gap. Interestingly, the lifetime of the AM oscillations observed with E pr∥ b is longer than E pr∥ a. Moreover, at high pump fluence where the electronic order melts and the AM oscillations …

Ultrafast light-matter interaction has emerged as a powerful tool to control and probe the macroscopic properties of functional materials, especially two-dimensional transition metal dichalcogenides that can form different structural phases with distinct physical properties. However, it is often difficult to accurately determine the transient optical constants. In this work, we developed a near-infrared pump—a terahertz to midinfrared (12–22 THz) probe system in transmission geometry to measure the transient optical conductivity in 2 H− Mo Te 2 layered material. By performing separate measurements on bulk and thin-film samples, we are able to overcome issues related to nonuniform substrate thickness and penetration depth mismatch and to extract the transient optical constants reliably. Our results show that photoexcitation at 690 nm induces a transient insulator-metal transition, while photoexcitation at 2 μ m has a …

Cuprate high-Tc superconductors are known for their intertwined interactions and the coexistence of competing orders. Uncovering experimental signatures of these interactions is often the first step in understanding their complex relations. A typical spectroscopic signature of the interaction between a discrete mode and a continuum of excitations is the Fano resonance/interference, characterized by the asymmetric light-scattering amplitude of the discrete mode as a function of the electromagnetic driving frequency. In this study, we report a new type of Fano resonance manifested by the nonlinear terahertz response of cuprate high-Tc superconductors, where we resolve both the amplitude and phase signatures of the Fano resonance. Our extensive hole-doping and magnetic field dependent investigation suggests that the Fano resonance may arise from an interplay between the superconducting fluctuations and the …

We report the investigation of spin dynamics in the antiferromagnetic metal EuIn 2 As 2 with the Zintl phase by using time-resolved magneto-optical Kerr effect (TR-MOKE) and transient reflectivity spectroscopy. In TR-MOKE measurement, we observe a spin precession mode with a frequency of about 18 GHz below 1.4 T and a new emerging coherent acoustic phonon mode near 35 GHz at a higher field where the latter shows a field-independent behavior. With temperature increasing, the spin precession disappears above Néel temperature T N, whereas the acoustic phonon persists up to 25 K due to existence of magnetic-field-induced polarized paramagnetism. In contrast, the quasiparticle dynamics of EuIn 2 As 2 does not show dramatic change in the vicinity of T N. By comparing the spin and quasiparticle dynamics, we attribute the appearance of an acoustic phonon in TR-MOKE to photoinduced transient …

We study THz-driven condensate dynamics in epitaxial thin films of MgB 2, a prototype two-band superconductor (SC) with weak interband coupling. The temperature and excitation density dependent dynamics follow the behavior predicted by the phenomenological bottleneck model for the single-gap SC, implying adiabatic coupling between the two condensates on the ps timescale. The amplitude of the THz-driven suppression of condensate density reveals an unexpected decrease in pair-breaking efficiency with increasing temperature—unlike in the case of optical excitation. The reduced pair-breaking efficiency of narrow-band THz pulses, displaying minimum near≈ 0.7 T c, is attributed to THz-driven, long-lived, nonthermal quasiparticle distribution, resulting in Eliashberg-type enhancement of superconductivity, competing with pair breaking.

The kagome magnet TbMn 6 Sn 6 is a new type of topological material that is known to support exotic quantum magnetic states. Experimental work has identified that TbMn 6 Sn 6 hosts Dirac electronic states that could lead to topological and Chern quantum phases, but the optical response of the Dirac fermions of TbMn 6 Sn 6 and its properties remain to be explored. Here, we perform an optical spectroscopy measurement combined with first-principles calculations on a single-crystal sample of TbMn 6 Sn 6 to investigate the associated exotic phenomena. TbMn 6 Sn 6 exhibits frequency-independent optical conductivity spectra in a broad range from 1800 to 3000 cm− 1 (220–370 meV) in experiments. The theoretical band structures and optical conductivity spectra are calculated with several shifted Fermi energies to compare with the experiment. The theoretical spectra with a 0.56 eV shift for Fermi energy are well …

The topological Weyl semimetals (WSMs) with peculiar band structures exhibit novel nonlinear optical enhancement phenomena, even for light at optical wavelengths. While many intriguing nonlinear optical effects are constantly uncovered in type‐I WSMs, few experimental works focuse on basic nonlinear optical properties in type‐II WSMs. Here a static and time‐resolved second harmonic generation (SHG) is performed on the 3D type‐II WSM candidate β‐WP2. Although β‐WP2 exhibits extremely high conductivity and a high carrier density (≈1021 cm−3), the SHG is unscreened by conduction electrons, and a rather strong SHG response is observed, comparing with non‐topological polar metals. Additionally, the time‐resolved SHG experiment traces ultrafast symmetry switch and reveals that polar metal β‐WP2 tends to form an inversion symmetric metastable state after photo‐excitation. Intense femtosecond …

Nonlinear responses of superconductors to intense terahertz radiation has been an active research frontier. Using terahertz pump-terahertz probe spectroscopy, we investigate the c-axis nonlinear optical response of a high-temperature superconducting cuprate. After excitation by a single-cycle terahertz pump pulse, the reflectivity of the probe pulse oscillates as the pump-probe delay is varied. Interestingly, the oscillatory central frequency scales linearly with the probe frequency, a fact widely overlooked in pump-probe experiments. By theoretically solving the nonlinear optical reflection problem on the interface, we show that our observation is well explained by the Josephson-type third-order nonlinear electrodynamics, together with the emission coefficient from inside the material into free space. The latter results in a strong enhancement of the emitted signal whose physical frequency is around the Josephson …

Most superconductors can be categorized into two classes–the conventional Bardeen-Cooper-Schrieffer (BCS) superconductors [1] and the unconventional superconductors including the strongly correlated high-Tc superconductors [2]. For typical high-Tc superconductors, their parent compounds are Mott or charge transfer insulators. Upon chemical substitutions, a superconducting dome emerges close to the strongly insulating phase. The recent realization of strongly insulating states and superconducting dome in the twisted multi-layer graphene [3] and van der Waals materials [4] suggests that certain features of unconventional superconductivity may not be limited to 3d transition metal compounds such as Cu-[2], Fe-[5], and Ni-based superconductors [6]. In this work, we present a compelling case that seemingly unconventional superconducting and normal-state properties can result from conventional mechanisms such as disorder and electron–phonon coupling. The nature of superconductivity in transition metal nitrides has not been settled since their discovery [7]. Although their superconducting properties, including superconducting gaps and upper critical fields, can be explained by the BCS physics, a few early works also reported experimental observations that require theory beyond the BCS picture, eg, the density of states at the Fermi surface [8] and electron–phonon coupling strength [9] are too low to account for their relatively high Tc. Meanwhile, because of their mechanical hardness and durability, superconducting transition metal nitrides have attracted a lot of attention lately for their unique advantages in technological applications …

The advent of intense ultrashort optical pulses spanning a frequency range from terahertz to the visible has opened a new era in the experimental investigation and manipulation of quantum materials. The generation of strong optical field in an ultrashort time scale enables the steering of quantum materials nonadiabatically, inducing novel phenomenon or creating new phases which may not have an equilibrium counterpart. Ultrafast time‐resolved optical techniques have provided rich information and played an important role in characterization of the nonequilibrium and nonlinear properties of solid systems. Here, some of the recent progress of ultrafast optical techniques and their applications to the detection and manipulation of physical properties in selected quantum materials are reviewed. Specifically, the new development in the detection of the Higgs mode and photoinduced nonequilibrium response in the …

BaNi2As2 is a non-magnetic analogue of BaFe2As2, the parent compound of a prototype pnictide high-temperature superconductor, displaying superconductivity already at ambient pressure. Recent diffraction studies demonstrated the existence of two types of periodic lattice distortions above and below the triclinic phase transition, suggesting the existence of an unconventional charge-density-wave (CDW) order. The suppression of CDW order upon doping results in a sixfold increase in the superconducting transition temperature and enhanced nematic fluctuations, suggesting CDW is competing with superconductivity. Here, we apply time-resolved optical spectroscopy to investigate collective dynamics in BaNi2As2. We demonstrate the existence of several CDW amplitude modes. Their smooth evolution through the structural phase transition implies the commensurate CDW order in the triclinic phase evolves …

We study the nonlinear optical response in a superconducting NbN thin film with a strong terahertz (THz) wave. In. addition to the expected third-harmonic generation, we observe a transient oscillation which softens in frequency with temperature increasing towards the superconducting transition temperature T c. We identify this feature as transient Higgs oscillation. To verify this proposal, we introduce a time-frequency resolved technique, a spectrogram for visualizing the THz spectrum. The dynamic decaying behavior of the oscillation is observed, which is consistent with the theoretical expectation of intrinsic Higgs oscillation. Moreover, we observe a higher-order nonlinear optics effect, ie, fifth-harmonic generation, which we assign to the higher-order coupling between the Higgs mode and electromagnetic field.

CuTe is a two-dimensional (2D) layered material; yet it forms a quasi-one-dimensional (quasi-1D) charge-density-wave (CDW) along the a axis in the a b plane at high transition temperature T CDW= 335 K. However, the anisotropic properties of CuTe remain to be explored. Here, we performed combined transport, polarized infrared reflectivity, and ultrafast pump-probe spectroscopy studies to investigate the underlying CDW physics of CuTe. Polarized optical measurements clearly revealed that an energy gap gradually forms along the a axis upon cooling, while optical evidence of a gap signature is absent along the b axis, suggesting pronounced electronic anisotropy in this quasi-2D material. Time-resolved optical reflectivity measurements revealed that the amplitude and relaxation time of photoexcited quasiparticles change dramatically across the CDW phase transition. Performing fast Fourier transformation of …