We present a velocity-gauge model for the generation of even-order high harmonics, and reveal that the even-order harmonics originate from the multiple-step transitions among the energy bands in momentum space, while the odd-order harmonics are mainly fro
The resolution of astronomical imaging from large optical telescopes is usually limited by the blurring effects of refractive index fluctuations in the Earth’s atmosphere. In this letter, we develop a lucky imaging system to restore astronomical images th
The basic form of a mirror as a cast metal object, with a highly polished reflective surface and relief decorations at the back, has remained unchanged throughout most of China’s history. The earliest known Chinese mirrors date back to approximately 2000