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The complex conductivity of YBa$_2$Cu$_3$O$_{7-\delta}$ samples representing the optimally doped, the underdoped and the overdoped stoichiometry was measured using time-domain terahertz spectroscopy. In the normal state, the frequency dependence is described by the Drude model. Below the critical temperature $T_\mathrm{c}$, the two-fluid model was successfully employed to fit all the spectra, from 5 K up to $T_\mathrm{c}$. The temperature behaviour of fundamental parameters such as the scattering time $\tau$, the superfluid fraction $f_\mathrm{s}$ and the conductivity $\sigma$ was investigated at given frequencies. The real part of the conductivity $\sigma_1(T)$ exhibits a coherence-like peak at low frequencies, which slightly shifts with increasing frequency whereas its height decreases. It can be observed for all three stoichiometries and its exact shape depends on the quality of the sample. A further analysis shows that this peak is a consequence of the competition between the scattering time $\tau(T)$ and the superfluid fraction $f_\mathrm{s}(T)$. The decrease of the superfluid fraction towards $T_\mathrm{c}$ depends on the reduced temperature with a power law close to 2, suggesting a dirty d-wave superconductor case for all levels of doping., Comment: 7 pages, 6 figures

Hexagonal boron nitride (hBN) has emerged as a promising two-dimensional (2D) material for many applications in photonics. Although its linear and nonlinear optical properties have been extensively studied, its interaction with high-intensity laser pulses, which is important for high-harmonic generation, fabricating quantum emitters, and maskless patterning of hBN, has not been investigated. Here we report the first study of dielectric breakdown in hBN monolayers induced by single femtosecond laser pulses. We show that hBN has the highest breakdown threshold among all existing 2D materials. This enables us to observe clearly for the first time a linear dependence of breakdown threshold on the bandgap energy for 2D materials, demonstrating such a linear dependency is a universal scaling law independent of the dimensionality. We also observe counter-intuitively that hBN, which has a larger bandgap and mechanical strength than quartz, has a lower breakdown threshold. This implies carrier generation in hBN is much more efficient. Furthermore, we demonstrate the clean removal of hBN without damage to the surrounding hBN film or the substrate, indicating that hBN is optically very robust. The ablated features are shown to possess very small edge roughness, which is attributed to its ultrahigh fracture toughness. Finally, we demonstrate femtosecond laser patterning of hBN with sub-wavelength resolution, including an isolated stripe width of 200 nm. Our work advances the knowledge of light-hBN interaction in the strong field regime and firmly establishes femtosecond lasers as novel and promising tools for one-step deterministic patterning of hBN monolayers., Comment: 21 pages in total. 16 pages in the main text, the rest are supplementary. 6 figures in the main text, 5 figures in the supplementary data

Mid-infrared spectroscopy probes molecular vibrations to identify chemical species and functional groups. Therefore, mid-infrared hyperspectral imaging is one of the most powerful and promising candidates for chemical imaging using optical methods. Yet high-speed and entire bandwidth mid-infrared hyperspectral imaging has not been realized. Here we report a mid-infrared hyperspectral chemical imaging technique that uses chirped pulse upconversion of sub-cycle pulses at the image plane. This technique offers a lateral resolution of 15 $\mu$m, and the field of view is adjustable between 800 $\mu$m $\times$ 600 $\mu$m to 12 mm $\times$ 9 mm. The hyperspectral imaging produces a 640 $\times$ 480 pixel image in 8 s, which covers a spectral range of 640-3015 cm$^{-1}$, comprising 1069 wavelength points and offering a wavenumber resolution of 2.6-3.7 cm$^{-1}$. For discrete frequency mid-infrared imaging, the measurement speed reaches a frame rate of 5 kHz, the repetition rate of the laser. As a demonstration, we effectively identified and mapped different components in a microfluidic device, plant cell, and mouse embryo section. The great capacity and latent force of this technique in chemical imaging promise to be applied to many fields such as chemical analysis, biology, and medicine., Comment: 22 pages, 10 figures

We theoretically investigate the real-time transient responses of a two-dimensional (2D) electron gas with anisotropic Rashba spin-orbit coupling (SOC) to laser pulses. Through explicitly monitoring the time-dependent photocurrents and spin polarization under different linear polarizations of the laser pulse, we find that the transient breaking of the mirror symmetry in combination with the anisotropy of the Rashba SOC results in significant distinction between the charge-mediated and the spin-mediated contributions to the photocurrents. Such distinction is obtained by analyzing the dependence of the symmetry-breaking induced (transverse) components of the photocurrents on the linear polarization angle of the laser pulse. This suggests a possibility of inferring spin-mediated processes in photocurrents without the use of circularly polarized lights. Moreover, the interplay between transient symmetry breaking and the anisotropy of the Rashba SOC also leads to transiently nonzero spin polarization components that are otherwise zero in the steady-state limit and the linear response regime. Especially, the out-of-plane spin polarization component can be induced or turned off by controlling the relative orientation of the linear polarization with respect to the symmetry axis of the 2D electronic system, without involving material-intrinsic magnetization effects. Our findings demonstrate the efficacy of a particular coordination between the polarization of the ultrafast laser pulses and the spatial symmetry of the electronic materials in directing the real-time charge and the spin responses that are fundamental to the development of ultrafast spintronics in solid states.

Transition metal dichalcogenides are known to possess large optical nonlinearities and driving these materials at high intensities is desirable for many applications. Understanding their optical responses under repetitive intense excitation is essential to improve the performance limit of these strong-field devices and to achieve efficient laser patterning. Here, we report the incubation study of monolayer MoS${}_{2}$ and WS${}_{2}$ induced by 160 fs, 800 nm pulses in air to examine how their ablation threshold scales with the number of admitted laser pulses. Both materials were shown to outperform graphene and most bulk materials; specifically, MoS${}_{2}$ is as resistant to radiation degradation as the best of the bulk thin films with a record fast saturation. Our modeling provides convincing evidence that the small reduction in threshold and fast saturation of MoS${}_{2}$ originates in its excellent bonding integrity against radiation-induced softening. Sub-ablation damages, in the forms of vacancies, lattice disorder, and nano-voids, were revealed by transmission electron microscopy, photoluminescence, Raman, and second harmonic generation studies, which were attributed to the observed incubation. For the first time, a sub-ablation damage threshold is identified for monolayer MoS${}_{2}$ to be 78% of single-shot ablation threshold, below which MoS${}_{2}$ remains intact for many laser pulses. Our results firmly establish MoS${}_{2}$ as a robust material for strong-field devices and for high-throughput laser patterning.

Laser direct writing is an attractive method for patterning 2D materials without contamination. Literature shows that the femtosecond ablation threshold of graphene across substrates varies by an order of magnitude. Some attribute it to the thermal coupling to the substrates, but it remains by and large an open question. For the first time the effect of substrates on femtosecond ablation of 2D materials is studied using MoS$_{2}$ as an example. We show unambiguously that femtosecond ablation of MoS$_{2}$ is an adiabatic process with negligible heat transfer to the substrates. The observed threshold variation is due to the etalon effect which was not identified before for the laser ablation of 2D materials. Subsequently, an intrinsic ablation threshold is proposed as a true threshold parameter for 2D materials. Additionally, we demonstrate for the first time femtosecond laser patterning of monolayer MoS$_{2}$ with sub-micron resolution and mm/s speed. Moreover, engineered substrates are shown to enhance the ablation efficiency, enabling patterning with low-power femtosecond oscillators. Finally, a zero-thickness approximation is introduced to predict the field enhancement with simple analytical expressions. Our work clarifies the role of substrates on ablation and firmly establishes femtosecond laser ablation as a viable route to pattern 2D materials.

Quantum correlations between spatially separated parts of a $d$-dimensional bipartite system ($d\geq 2$) have no classical analog. Such correlations, also called entanglements, are not only conceptually important, but also have a profound impact on information science. In theory the violation of Bell inequalities based on local realistic theories for $d$-dimensional systems provides evidence of quantum nonlocality. Experimental verification is required to confirm whether a quantum system of extremely large dimension can possess this feature, however it has never been performed for large dimension (e.g., $d\geq 1000$). Here, we report that Bell inequalities are experimentally violated for bipartite quantum systems of extremely high dimensionality with the usual ensembles of polarization-entangled photon pairs. Our entanglement source violates Bell inequalities for extremely high dimensionality of $d>4000$. The designed scenario offers a possible new method to investigate the entanglement of multipartite systems of large dimensionality and their application in quantum information processing.

Pulsed terahertz (THz) radiation, generated through optical rectification (OR) by exciting [110] ZnTe crystal with ultrafast optical pulses, typically consists of only a few cycles of electromagnetic field oscillations with a duration about a couple of picoseconds. However, it is possible, under appropriate conditions, to generate a long damped oscillation tail (LDOT) following the main cycles. The LDOT can last tens of picoseconds and its Fourier transform shows a higher and narrower frequency peak than that of the main pulse. We have demonstrated that the generation of the LDOT depends on both the duration of the optical pulse and its central wavelength. Furthermore, we have also performed theoretical calculations based upon the OR effect coupled with the phonon-polariton mode of ZnTe and obtained theoretical THz waveforms in good agreement with our experimental observation., Comment: 9 pages, 5 figures