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WTe 2 is a unique material in the family of transition metal dichalcogenides and it has been proposed as a candidate for type-II Weyl semimetals. However, thus far, studies on the optical properties of this emerging material have been significantly hindered by the lack of large-area, high-quality WTe 2 materials. Here, we grow a centimeter-scale, highly crystalline WTe 2 ultrathin film (∼35 nm) by a pulsed laser deposition technique. Broadband pump-probe spectroscopy (1.2-2.5 μm) reveals a peculiar ultrafast optical response where an initial photo-bleaching signal (lasting ∼3 ps) is followed by a long-lived photoinduced absorption signature. Nonlinear absorption characterization using femtosecond pulses confirms the saturable absorption response of the WTe 2 ultrathin films, and we further demonstrated a mode-locked Thulium fiber laser using a WTe 2 absorber. Our work provides important insights into linear and nonlinear optical responses of WTe 2 thin films.

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The polynomial and sinusoidal motion errors always exist in the unmanned aero vehicle (UAV) SAR due to the small size and low velocity of the platform, causing serious spectrum compressing/stretching and significant spectral replicas of the azimuth signal. The motion errors induce serious blurring of the SAR image and "ghost targets", and can hardly be precisely estimated by the conventional motion compensation (MOCO) method. In this paper, an improved MOCO method is proposed to estimate and eliminate the motion errors in the high resolution UAV SAR without high precision inertial navigation system (INS) data. The time domain range walk correction (RWC) operation in the coarse phase error estimation process of the proposed MOCO method is the key operation that ensures the estimation accuracy of the whole MOCO method. Finally, the validity of the improved MOCO method is verified by computer simulations and real UAV SAR data processing.

The magnitude error and phase error (MEPE) in the transfer function of a stepped-frequency synthetic aperture radar (SAR) system results in a periodic MEPE in the synthesized wideband waveform, which induces the grating lobes in the high-resolution range profile. In this letter, a novel grating lobe suppression method based on the SAR image is proposed. In the paired-echo theory, a single sinusoidal term of the periodic MEPE in the frequency domain induces a pair of grating lobes in the time domain. Based on the magnitudes and phases of a strong scatterer and its grating lobes in the SAR image, the sinusoidal terms in the periodic MEPE can be estimated using the proposed method. By compensating for the estimated sinusoidal terms in the spectrum reconstruction, the corresponding grating lobes can be suppressed to the background level of the SAR image. The validity of the proposed method has been demonstrated using computer simulations and experiments based on real data. Index Terms-Grating lobe suppression (GLS), magnitude error and phase error (MEPE), stepped-frequency synthetic aperture radar (SAR), synthesized wideband waveform (SWW).

A modular robot system is introduced in this paper. The requirements for configuration synthesis are analyzed and classified as hard ones, soft ones and hard-soft ones. The evaluation function is constructed based on the weighted sum of the evaluation results of the specified design requirements. A one-lever Genetic Algorithm (GA) basing on an improved hybrid coding method is presented to carry out the configuration synthesis. In addition an example is given to demonstrate the effectiveness of this method.