We demonstrate, for the first time to our knowledge, correlated photon-pair generation in a lithium tantalate (LiTaO3) micro-ring resonator (MRR) via spontaneous four-wave mixing (SFWM), marking a significant step toward integrated quantum photonics on this scalable and mature platform. Our results open the door to monolithically unifying classical and quantum photonic functionalities within a single LiTaO3 circuit.

Nitride materials, valued for their structural stability and exceptional physical properties, have garnered significant interest in both fundamental research and technological applications. The fabrication of high-quality nitride thin films is essential for advancing their use in microelectronics and spintronics. Yet, achieving single-crystal nitride thin films with excellent structural integrity remains a challenge. Here, we introduce a straightforward yet innovative metallic alloy nitridation technique for the synthesis of stable single-crystal nitride thin films. By subjecting metal alloy thin films to a controlled nitridation process, nitrogen atoms integrate into the lattice, driving structural transformations while preserving high epitaxial quality. Combining nanoscale magnetic imaging with scanning nitrogen-vacancy (NV) magnetometry, X-ray magnetic linear dichroism, and comprehensive transport measurements, we confirm that the nitridated films exhibit a robust antiferromagnetic character with a zero net magnetic moment. This work not only provides a refined and reproducible strategy for the fabrication of nitride thin films but also lays a robust foundation for exploring their burgeoning device applications. Nitride materials, prized for their stability and unique properties, face challenges in achieving high-quality single-crystal thin films for microelectronics and spintronics. Here, the authors present an innovative metallic alloy nitridation technique, producing stable single-crystal nitride films with robust antiferromagnetic properties, paving the way for advanced device applications.

Quantum fluctuations are pivotal in driving quantum phase transitions, exemplified by the quantum melting of Wigner crystals into Fermi liquids in electron systems. However, their impact on superconducting systems near zero temperature, particularly in the superconductor-insulator/metal transition, remains an open question. In this study, through electric transport measurements on the two-dimensional (2D) superconductor (SnS)_{1.17}NbS_{2}, we demonstrate that quantum fluctuations induce vortex displacement from their mean position, leading to the quantum melting of vortex solid near zero temperature. Quantitative analysis suggests the magnetic field-induced anomalous metal originates from this quantum melting transition, with energy dissipation governed by quantum fluctuations-driven vortex displacements. Remarkably, further extending this analysis to various 2D superconductors yields the same results, and many properties of anomalous metal can be qualitatively understood within the framework of quantum melting. The connection between the quantum melting of vortex solids and dissipative anomalous metal opens a pathway toward understanding quantum phase transitions through vortex dynamics, providing insights on both fields.

Recently, the infinite-layer nickelates, which have similar crystal structure as cuprates, was successfully synthesized and exhibited superconductivity. Obviously, this new kind of Ni-based superconductors will create new platform to investigate the complex superconducting mechanisms in the cuprates. In the nickelates, there exist strong Coulomb interactions, which may result in different symmetry-breaking orders, like charge density waves (CDWs), spin density waves (SDWs). These orders may contribute to the superconductivity. Therefore, it's of importance to find out the possible ordered state in the nickelates. Here, we study the photoinduced change in reflectivity of the parent compound of the nickelate superconductor-NdNiO2 films. Above 160 K, we observe an additional decay process, which coexist with the decay process at low temperature. At higher temperature, the second decay process dominate the relaxation. This phenomenon may reflect the existence of competing order in the temperature region in the NdNiO2 film. Further efforts should be made to find out the physical origin of the orders.

Utilizing ultrafast optical pump-probe spectroscopy, we present a comprehensive investigation on the photoexcited quasiparticle dynamics in single-crystal CeAgSb2. We elucidate the emergence of collective c-f hybridization between conduction electrons and localized f moments below a critical temperature T & lowast; (approximate to 65 K), as indicated by the onset of an indirect gap of '9 meV via the fluence-dependent quasiparticle relaxation and phonon renormalization processes. Our experimental data also suggest that the c- f hybridization fluctuation should appear at a much higher-temperature value, i.e., T dagger (approximate to 180 K). Below '50 K, we further detect an additional quasiparticle relaxation channel with sub-ps timescale, which can be attributed to the crystalline electric field (CEF) excitation. These observations provide important insights into the Kondo lattice coherence and CEF effect in CeAgSb2, shedding light on its unique properties and behaviors.

CsV3Sb5 is a recently discovered Kagome metal that exhibits a combination of superconductivity with a critical temperature (TC) of 2.5 K and charge density wave (CDW) order at TCDW = 94 K. In this study, we investigate the collective excitations and quasiparticle dynamics in CsV3Sb5 using ultrafast optical pump-probe spectroscopy. According to our results, in addition to the CDW phase related electron and coherent phonon dynamics, we found another electronic symmetry breaking by nematic phase transition at T* ( 20 K), which can mediate the relaxation of carriers. At temperature below T*, a gap about 4 meV related to the nematic order is open, which determines the low-temperature dynamics of carriers in the rst 2 picosecond (ps). Moreover, we also observed the abrupt change of electron dynamics caused by the CDW phase transition at TCDW. Furthermore, we identify the presence of a CDW induced mode at 1.3 THz below TCDW, as well as two collective modes at 3.1 THz and 3.8 THz below 80 K. It is noteworthy that these two collective modes appear to be associated with the breaking of C6 rotational symmetry. These observations suggest the rich phase order below TCDW and emphasize the need for a deeper understanding of CDW and nematicity order in the Kagome metals.

We report the ultrafast optical pump-probe spectroscopy measurements on the single-crystal cerium films. Our experimental results of temperature-dependent quasiparticle dynamics reveal development of the hybridization between localized f moments and conduction electrons, i.e., evolving from fluctuating hybridization to collective hybridization. Exotic phonon renormalization is discovered to appear at the emerging temperature (T†) of fluctuating hybridization apart from its known presence at the coherent temperature (T*), and can hardly be explained by the mean-field theory. The quasiparticle relaxation at high temperatures indicates coexistence of nonthermal electron-electron scattering and inelastic Kondo scattering, while its behavior at low temperatures suggests onset of α phase in the γ-phase dominated film. We also extract the indirect gap below T*, representing the appearance of collective hybridization. Our findings provide novel information about the hybridization and phase evolution in the heavy fermion systems.

We investigate the quasiparticle dynamics in MnBi2Te4 single crystal using the ultrafast optical spectroscopy. Our results show that there exist anomalous dynamical optical responses below the antiferromagnetic (AFM) ordering temperature TN. In specific, we reveal that both the initial carrier decay and recombination processes can be modulated via introducing the AFM order in sub-picosecond and picosecond timescales, respectively. We also discover a long relaxation process emerging below TN with a timescale approaching to the nanosecond regime, and can be attributed to the T-dependent spin-lattice interaction. There also emerges an unusual phonon energy renormalization below TN , which is found to arise from its coupling the spin degree via the exchange interaction and magnetic anisotropy. Our findings provide key information for understanding the dynamical properties of non-equilibrium carrier, spin and lattice in MnBi2Te4.

We measure the time-resolved terahertz spectroscopy of GeSn thin film and studied the ultrafast dynamics of its photo-generated carriers. The experimental results show that there are photo-generated carriers in GeSn under femtosecond laser excitation at 2500 nm, and its pump-induced photoconductivity can be explained by the Drude-Smith model. The carrier recombination process is mainly dominated by defect-assisted Auger processes and defect capture. The first- and second-order recombination rates are obtained by the rate equation fitting, which are (2.6 +/- 1.1) x 10(-2) ps(-1) and (6.6 +/- 1.8) x 10(-19) cm(3)& sdot;ps(-1), respectively. Meanwhile, we also obtain the diffusion length of photo-generated carriers in GeSn, which is about 0.4 mu m, and it changes with the pump delay time. These results are important for the GeSn-based infrared optoelectronic devices, and demonstrate that GeSn materials can be applied to high-speed optoelectronic detectors and other applications.

Nonlinear optical properties are investigated using the static and time-resolved second harmonic generation in the topological material molybdenum phosphide（Mo P） with three-component fermions. Giant second harmonic generation signals are detected and the derived χ （2） value is larger than that of the typical electro–optic material.Upon optical excitation, no photoinduced change of the symmetry is detected in MoP, which is quite different from previous observations in several other topological materials.

Strong second-order optical nonlinearities often require broken material centrosymmetry, thereby limiting the type and quality of materials used for nonlinear optical devices. Here, we report a giant and highly tunable terahertz (THz) emission from thin polycrystalline films of the centrosymmetric Dirac semimetal PtSe 2 . Our PtSe 2 THz emission is turned on at oblique incidence and locked to the photon momentum of the incident pump beam. Notably, we find an emitted THz efficiency that is giant: It is two orders of magnitude larger than the standard THz-generating nonlinear crystal ZnTe and has values approaching that of the noncentrosymmetric topological material TaAs. Further, PtSe 2 THz emission displays THz sign and amplitude that is controlled by the incident pump polarization and helicity state even as optical absorption is only weakly polarization dependent and helicity independent. Our work demonstrates how photon drag can activate pronounced optical nonlinearities that are available even in centrosymmetric Dirac materials.

The study of the interaction between terahertz (THz) radiation and quantum materials has been an active area of research due to its potential for understanding fundamental physics and the development of novel technologies. In this review, we focus on the time-dependent photocurrents, behind which are the up-to-date understanding of the physical processes. We provide the recent advancements in revealing the unique properties of quantum materials via the THz emission spectroscopy. Because the theoretical interpretation of some new experimental results is still evolving, this review is intended to inspire further research in this exciting and rapidly growing field.

We present a comprehensive investigation utilizing ultrafast optical pump-probe spectroscopy to elucidate the intricate quasiparticle dynamics in the CeAgSb2 single crystal. Through our experimental endeavors, a nuanced two-phase evolution in the hybridization phenomena linking localized f moments and conduction electrons has been unveiled. This intricate process is manifest in two distinct manifestations: firstly, the discernible manifestation of hybridization fluctuations spanning the temperature range of approximately 150 K (denoted as T†) down to approximately 65 K (referred to as T*); secondly, the notable emergence of a collective hybridization phenomenon below the critical temperature, T*, as indicated by the onset of an indirect gap in the electronic structure. These discerning observations provide invaluable insights into the interplay of electronic configurations as a function of temperature in CeAgSb2, shedding light on its unique properties and behaviors.

Here, using the ultrafast optical pump-probe spectroscopy, we have studied the quasiparticle dynamics in the type-II Dirac semimetal NiTe2. Anomalous dynamic optical responses are detected around a critical temperature T* (-60 K), which is also evidenced by the transport experiment. In specific, our results reveal a phonon -assisted electron-hole recombination process showing an anomaly at T* <^>' 60 K. We discover an unusual phonon renormalization behavior, manifested by the T-dependent phonon energy and lifetime exhibiting anomalies at T*. We unveil that the observed anomalous T-dependencies can be attributed to the sudden shift of the phonon mode involved in the recombination, which originates from the abrupt change of electronic structure near the Fermi surface. These findings can provide deeper insight into the nonequilibrium carrier and lattice properties in the topological materials.

Spintronic terahertz emitters are broadband and efficient sources of terahertz radiation, which emerged at the intersection of ultrafast spintronics and terahertz photonics. They are based on efficient spin-current generation, spin-to-charge-current conversion, and current-to-field conversion at terahertz rates. In this Editorial, we review the recent developments and applications, the current understanding of the physical processes, and the future challenges and perspectives of broadband spintronic terahertz emitters.

In this work, broadband terahertz (THz) wave emissions have been detected from the trigonal layered PtBi2 on the excitation of the femtosecond laser pulses. Such THz generation is found to arise from the dominated linear photogalvanic effect, which is further discovered to strongly depend on the unique electronic structures of PtBi2. Furthermore, an effective nonlinear susceptibility of PtBi2 is also obtained and is nearly two orders of magnitude larger than that of the traditional nonlinear crystal for THz generation.

We have investigated the quasiparticle dynamics and collective excitations in the quasi-one-dimensional material ZrTe5 using ultrafast optical pump-probe spectroscopy. Our time-domain results reveal two coherent oscillations having extremely low energies of ħω1 ∼0.33 meV (0.08 THz) and ħω2 ∼1.9 meV (0.45 THz), which are softened as the temperature approaches two different critical temperatures (∼54 K and ∼135 K). We attribute these two collective excitations to the amplitude mode of photoinduced dynamic charge density waves in ZrTe5 with tremendously small nesting wave vectors. Furthermore, a peculiar quasiparticle decay process associated with the ħω2 mode with a timescale of ∼1–2 ps is found below the transition temperature T* (∼135 K). Our findings provide pivotal information for studying the fluctuating order parameters and their associated quasiparticle dynamics in various low-dimensional topological systems and other materials.

The microscopic degrees of freedom of electron, lattice, spin and orbit play a vital role in the macroscopic properties of superconducting materials. In superconducting systems, especially unconventional superconducting materials, these degrees of freedom lead to enormous collective excitations and ordered states with different energy scales. Examples for the collective excitations are phonons, magnons, charge density waves, spin density waves, spin fluctuations, nematic fluctuations, and so on; ordered states are superconducting state, pseudogap state, nematic phase, antiferromagnetic/ferromagnetic order, and so on. The collective exciations can make vital contribution to the formation of the ordered states. In particular, different types of collective excitations are entangled in the frequency domain and interact with each other, and at the same time are coupled with electrons (or quasiparticles), resulting in the complex and rich physics in the equilibrium and non-equilibrium states. The uniqueness of ultrafast optical spectroscopy is that it covers both a wide energy range and high time resolution. Using the linear and nonlinear optical responses during the interaction between light (electromagnetic waves) and superconducting materials, the quasi-equilibrium or non-equilibrium dynamical properties can be detected and controlled at the resonant or non-resonant conditions. Because of the flexibility of the table-top optical systems, it is not only used in superconducting materials, but also widely employed in various other inorganic and organic material systems. Because the non-equilibrium theory, especially the theoretical research on correlated electronic materials, is still in the stage of rapid development, this review mainly introduces the commonly used table-top ultrafast optical spectroscopy and the related analytical theories that are currently widely used, focusing on discussion of the universal trends and developments emerging from experimental data of the ultrafast optical spectroscopy on superconducting materials, which include the conventional superconductors, cuprate superconductors, iron-based superconductors, and heavy fermion superconductors.

Researchers have shown great interest in two-dimensional crystals recently, because of their thickness-dependent electronic and optical properties. We have investigated the Raman and photoluminescence spectra of free-standing monolayer and bilayer MoS2, as a function of pressure. As the enforcement of layer interaction, an electronic and a crystal phase transition were revealed at ∼6 GPa and ∼16 GPa, respectively, in bilayer MoS2, while no phase transition in the monolayer is observed. The electronic phase transition at ∼6 GPa is supposed to be a direct interband changing to an indirect Λ-K interband transition, and the new structure shown at ∼16 GPa is not metallized and supposed to be a transformation from stacking faults due to layer sliding like 2Hc to 2Ha. The different pressure-induced features of monolayer MoS2, compared with bilayer MoS2, can help to get a better understanding about the importance of interlayer interaction on modifying the optical properties of MoS2 and other fundamental understanding of 2D materials.

We report the ultrafast optical pump-probe spectroscopy measurements on the recently discovered quantum critical ferromagnet CeRh$_6$Ge$_4$. Our experimental results reveal the two-stage development of the hybridization between localized $f$ moments and conduction electrons with lowering temperature, as evidenced by (1) the presence of hybridization fluctuation for temperatures from $\sim$85 K ($T^*$) to $\sim$140 K ($T^\dagger$), and (2) the emergence of collective hybridization below the coherence temperature, $T^*$, marked by the opening of an indirect gap of 2$\Delta$ $\approx$12 meV. We also observe three coherent phonon modes being softened anomalously below $T^*$, reflecting directly their coupling with the emergent coherent heavy electrons. Our findings establish the universal nature of the hybridization process in different heavy fermion systems.