The strong light–matter interaction in two dimensional (2D) materials portends an untapped potential for fiber lasers in the long sought-after terahertz to mid-infrared spectral range. Here, the broadband nonlinear optical properties of zero-gap 2D Ti ₂ CT _x MXene—a class of 2D transition metal carbides that exhibit high laser damage threshold—is discussed. Using the open aperture Z -scan method, for the first time the broadband saturable absorption properties of Ti ₂ CT _x in the 800–1560 nm spectral range are demonstrated. Specifically, it is shown that the high absorption and low saturation intensity of Ti ₂ CT _x make it an ideal saturable absorber for mode-locked fiber lasers operating at 1565 and 1051 nm, which exhibited pulse widths of 5.3 ps and 164 ps, respectively. Additionally, a Ti ₂ CT _x saturable absorber was used as a Q-switcher to realize a pulsed fiber laser operating at 2.8 µ m, which showed excellent nonlinear absorption performance in the mid-infrared regime. This study significantly broadens the scope of materials available for non-linear optical applications in the near and mid-infrared regimes.

Van der Waals heterostructures are composed of stacked atomically thin two‐dimensional (2D) crystals to provide unprecedented functionalities and novel physics. Franckeite, a naturally occurring van der Waals heterostructure consisting of superimposed SnS ₂ ‐like and PbS‐like layers alternately, shows intriguing potential in versatile optoelectronic applications. Here, we have prepared the few‐layer franckeite via liquid‐phase exfoliation method and characterized its third‐order nonlinearity and ultrafast dynamics experimentally. We have found that the layered franckeite shows low saturable intensity, large modulation depth and picosecond ultrafast response. We have designed the passive photonic diodes based on the layered franckeite/C ₆₀ cascaded film and suspension configuration and found that the passive photonic diodes exhibit stable nonreciprocal transmission of light. The experimental results show the excellent nonlinear optical performance and ultrafast response of the layered franckeite, which may make inroad for the cost effective and reliable high‐performance optoelectronic devices.

Terahertz video synthetic aperture radar (THz-ViSAR) is emerging as a key technology for dynamic situational awareness and precise imaging of moving targets. However, existing systems prioritize high frame-rates at the expense of the dwell time, which degrades the azimuth resolution. To address this issue, we propose an improved ViSAR framework that can extend the synthetic aperture time and improve the image quality. First, the Keystone transform (KT) is utlized to correct range cell migration, enabling accurate phase error estimation for high-resolution imaging. Moreover, unlike traditional methods without full-aperture imaging, our technique synthesizes a full-aperture phase error model by estimating sub-aperture phase errors, which can contribute to compensate for overall motion errors and generate high-resolution full-aperture SAR images. Airborne field experiments demonstrate the effectiveness of the proposed method, achieving the full-aperture high-resolution imaging while maintaining practical frame-rates in dynamic surveillance scenarios.

Airborne distributed coherent aperture radar (ADCAR) coheres several flexible unit radars to form an equivalent large aperture radar for enhancing the signal gain, spatial resolution and system reliability. However, the ultra-long interelement spacing brings severe grating lobes, which will considerably deteriorate the performance of clutter suppression and target detection. To conquer this issue, we propose a stepped frequency signal based space-time adaptive processing (STAP) approach for clutter and grating lobes suppression. The increased transmitting signal frequency can expand the space frequency and Doppler frequency of both target echo and clutter echo. Therefore, the target and clutter can be distinguished through different expanded frequency ranges, and in turn the grating lobes can be suppressed. To further reduce the peak sidelobe ratio of target response, accumulated pattern synthesis approach is applied in both transmit and receive end. Furthermore, to guide practical implementation, the influence of the non-ideal factors including gain errors, phase errors, platform control errors and grid mismatch are analyzed to provide a feasible boundary for the algorithm. Simulations corroborate the superiorities of the proposed STAP approach for ADCAR.

Terahertz (THz) inverse synthetic aperture radar (ISAR) imaging of space target has the advantage of high resolution and all-day capability for spatial situational awareness (SSA). However, compared with microwave ISAR imaging, micro vibrations of radar platform and target can cause images defocusing and quality degradation in THz band. Commonly, the existing algorithms considered vibration as simple harmonic motion in rigid body, and estimated the vibration parameters for phase error compensation. Nevertheless, these methods are dependent on the prominent points and cannot focus on the complex non-rigid vibration of the space target. Aiming at these problems, an ISAR imaging framework that can be leveraged for complex non-rigid vibration compensation is proposed. First, a compressed sensing (CS) model is established to compensate for the vibration phase error in ISAR imaging. Then, the alternating direction method of multipliers (ADMM) is employed to optimize the CS model. Subsequently, considering that strong scattering points of space targets will cover up the weak scattering information in ISAR images, a sidelobe suppression constraint is appended. Thus, the weak scattering information is retained while removing the sidelobe and noise. It is worth mentioning that the ISAR imaging algorithm is derived to a two dimension (2-D) matrix form to reduce memory usage and to improve computational efficiency. Finally, simulated and measured data are utilized to verify the superiority of the proposed algorithm, and proves the potential of ISAR imaging for practical space targets.

In recent years, terahertz (THz) radar has been widely researched for its high-resolution imaging. However, the traditional inverse synthetic aperture radar (ISAR) imaging algorithms in the microwave band perform unsatisfactorily in the THz band. Firstly, due to THz radar’s large bandwidth and short wavelength, the rotation of the target will result in serious space-varying(SV) range migration and space-varying phase error. Furthermore, it is challenging to accurately estimate the rotational velocity and compensate for phase errors in the presence of severe range migration effects. Therefore, in this paper, a high-precision THz-ISAR imaging algorithm is proposed. The algorithm includes the following step: First, the SV first-order range migration(FRM) is corrected using keystone transform (KT); then, the minimum entropy based on modified newton (ME-MN) is used to estimate the rotational velocity roughly, and the remaining SV second-order range migration(SRM) is corrected to obtain the range profile with the envelope alignment. Finally, the echo after the envelope alignment is processed for the second time based on ME-MN. The target rotation velocity is accurately estimated, and the phase error is compensated to obtain a well-focused imaging result. The validity of the proposed method is verified by numerical simulation and electromagnetic calculation data.