For applications requiring high spectral purity, nanosecond Backward Wave Optical Parametric Oscillators (BWOPO) have been proposed as compact and simple alternatives to conventional cavity-based oscillators. Although plane wave theory predicts that BWOPO should deliver nearly Fourier-transform-limited pulses, we show that in practical systems, the output spectrum is asymmetrically broadened and structured at the subgigahertz scale. In this paper, we show that this behavior arises from infinitesimal distributed phase-matching inhomogeneities and is pump intensity dependent. Experimentally, our BWOPO consists in a periodically poled Rb-doped KTiOPO4 crystal with a quasi-phase-matching period of 560 nm. It is pumped by a single-longitudinal mode laser at 1064 nm, delivering 10 ns pulses, and generates forward and backward parametric waves at 2211 and 2051 nm, respectively. Using shot-to-shot optical heterodyning, we have been able to measure the phase evolution and spectrum of the backward wave with a 5 MHz resolution. We observe an Airy-type spectrum with multiple side-lobes and a cubic phase modulation, the sign of which depends on the pump propagation in the BWOPO sample. It is also observed that the central wavelength of the mainlobe can be tuned as a function of pump intensity. These experimental observations are reproduced by numerical modeling assuming a slight gradient in the phase-matching condition along the propagation direction. Having identified the underlying cause of the spectral broadening, we are now able to propose ways to mitigate this dynamic chirping to generate Fourier-transform-limited nanosecond pulses.
Field-free deterministic switching of perpendicular magnetization is essential for spin–orbit torque (SOT) devices. Lateral structural asymmetry via a wedge-shaped structure has been widely explored to enable field-free switching in heavy-metal/ferromagnet heterostructures. However, due to the coexistence of the Dzyaloshinskii–Moriya interaction (DMI), its mechanism remains obscure, leading to contradictory switching behaviors. Here, we fabricate β-W/Ti/CoFeB(wedge)/MgO devices with a thin Ti insertion layer that strongly suppresses DMI, thereby decoupling lateral asymmetry from DMI-induced chiral spin texture asymmetry. This allows us to present the switching physics governed solely by the wedged ferromagnetic layer. We achieve robust field-free SOT-driven magnetization switching. The switching polarity remains identical across all devices and independent of the sign change of the magnetic anisotropy gradient, revealing a unidirectional in-plane built-in electric field originating from the wedged CoFeB layer. Furthermore, a slight tilt of the magnetic easy axis facilitates switching, particularly in the thinner CoFeB region. Our results demonstrate the universal contribution of the in-plane built-in electric field and provide a comprehensive understanding of field-free switching in wedge-shaped lateral asymmetric structures.
High-power laser optical components usually require low surface reflectance, excellent thermal stability, and reliable laser resistance. Here, large-area (2 × 2 cm2), double-sided fused silica nanopillar metasurfaces were fabricated by nanoimprint lithography for broadband and wide-angle antireflection. The gradient-index nanostructure reduces the average reflectance to below 2.4% across the visible-to-near-infrared range, with a minimum near 720 nm, consistent with interference-assisted impedance matching. The patterned fused silica maintains low reflectance over a wide angular range with weak polarization dependence. Enhanced wettability is also observed, with the water contact angle decreasing to approximately 40°. After annealing at 1100 °C for 30 min under N2, the morphology and reflectance of the monolithic nanostructure remained nearly unchanged, demonstrating robust high-temperature stability. Finally, an estimated laser-induced damage threshold of 23.5 J/cm2 is obtained for defect-free regions under 1064 nm nanosecond irradiation, exceeding reported values for comparable SiO2 micro/nanostructured antireflective surfaces. These results demonstrate a scalable route to broadband, wide-angle, and thermally robust fused silica optics for high-energy laser applications.
Capacitor-less two-transistor (2T0C) architecture provides an efficient pathway for further miniaturization of dynamic random-access memory (DRAM). Monolayer molybdenum disulfide (ML MoS2) has a bandgap of 1.8 eV, and MoS2-based field effect transistors exhibit low leakage current, making them suitable for 2T0C DRAM. However, capacitive coupling in 2T0C DRAM remains a critical issue. In this work, we demonstrate 2T0C DRAM devices using transferred MoS2 as the transistor channel material. A simple illumination process is employed to tune the threshold voltage (Vth) of MoS2 transistors, enabling balanced operation between the write and read transistors in the 2T0C DRAM cell. Furthermore, capacitive coupling at the storage node (SN) is systematically investigated and optimized through a read-pulse compensation strategy. We achieve a retention time of 115 s and a low leakage current of 1.2 × 10−16 A/μm (Vhold = 0 V, initial VSN = 1.0 V). This work demonstrates the feasibility of transferred MoS2-based transistors in the 2T0C DRAM cell, which allows for low-temperature 3D stack and high-density DRAM cell integration.
Common approaches to enhance the carrier mobility of SnO2 thin films include high-temperature deposition and post-deposition annealing, both of which are strongly temperature-dependent. In this work, SnO2 thin films deposited by atomic layer deposition (ALD) with tetrakis(dimethylamino)tin as the metal precursor and H2O, O3, sequential H2O/O3, and O3/H2O as reactants were systematically characterized. The electrical performance of the obtained films was modulated via the control of hydroxyl defects. The results reveal that the dual oxygen-source strategy enables more effective modulation of hydroxyl defects compared with the single oxygen-source process. From an electrical perspective, low-density hydroxyl defects serve as charge carriers, whereas excessive hydroxyl groups act as electron scattering centers. By optimizing the dual oxygen-source process to control hydroxyl defects, SnO2 films deposited at a low temperature of 230 °C demonstrated high electron mobility of 22.78 cm2 V−1 s−1, the highest reported results so far within this temperature range. In addition, the fabricated SnO2 films exhibit a stable optical transmittance above 85% across the full visible spectrum under all deposition temperatures, and the corresponding TFTs with SnO2 active layers deliver good electrical characteristics. Benefiting from its low-temperature compatibility, the proposed dual oxygen-source ALD strategy exhibits promising potential and practical value for photovoltaic applications that cannot tolerate high-temperature processing.
