Topological nodal-line semimetals attract growing research attention in the photonic and optoelectronic fields due to their unique topological energy-level bands and fascinating nonlinear optical responses. Here, to the best of our knowledge, we first report the saturable absorption property of topological nodal-line semimetal HfGeTe and the related pulse modulation in passively Q-switched visible lasers. Few-layer HfGeTe demonstrates outstanding saturable absorption properties in the visible-light band, yielding the saturation intensities of 7.88, 12.66, and 6.64 µJ/cm² at 515, 640, and 720 nm, respectively. Based on an as-prepared few-layer HfGeTe optical switch and a Pr:LiYF₄ gain medium, Q-switched visible lasers are also successfully achieved at 522, 640, and 720 nm. The minimum pulse widths of the green, red, and deep-red pulsed lasers are 150, 125.5, and 420 ns, respectively. Especially for the green and red pulsed laser, the obtained pulse width is smaller than those of the low-dimensional layered materials. Our work sheds light on the application potential of topological nodal-line semimetals in the generation of visible pulsed lasers.

Electron–phonon coupling can tailor electronic transition processes and result in direct lasing far beyond the fluorescence spectrum. The applicable time scales of these kinds of multiphonon-assisted lasers determine their scientific boundaries and further developments, since the response speed of lattice vibrations is much slower than that of electrons. At present, the temporal dynamic behavior of multiphonon-assisted lasers has not yet been explored. Herein, we investigate the Q-switched laser performance of ytterbium-doped YCa4O(BO3)3 (Yb:YCOB) crystal with phonon-assisted emission in nanosecond scales. Using different Q-switchers, the three-phonon-assisted lasers around 1130 nm were realized, and a stable Q-switching was realized in the time domain from submicroseconds to tens of nanoseconds. To the best of our knowledge, this is the longest laser wavelength in all pulse Yb lasers. The minimum pulse width and maximum pulse energy are 29 ns and 204μJ, respectively. These results identify that the electron–phonon coupling is a fast physical process, at least much faster than the present nanosecond pulse width, which supports the operation of multiphonon-assisted lasers in the nanosecond range. In addition, we also provide a simple setup to create pulse lasers at those wavelengths with weak spontaneous emission.

High-harmonic generation in the ultraviolet region is promising for wireless technology used for communications and sensing. However, small high-order nonlinear coefficients prevent us from obtaining high conversion efficiency and functional photonic devices. Here, we show highly efficient ultraviolet harmonic generation extending to the fifth order directly from an epsilon-near-zero indium tin oxide (ITO) film. The real part of the annealed ITO films was designed to reach zero around 1050 nm, matching with the central wavelength of an Yb-based fiber laser, and the internal driving electric field was extremely enhanced. A high energy conversion efficiency of 10-4 and 10-6 for 257.5 nm (fourth-order) and 206 nm (fifth-order) ultraviolet harmonic generation was obtained, which is at least 2 orders of magnitude higher than early reports. Our results demonstrate a new route for overcoming the inefficiency problem and open up the possibilities of compact solid-state high-harmonic generation sources at nanoscale.