Perovskite-structured barium strontium titanate (Ba_1-xSr_xTiO₃, x = 0.1–0.9) films have been epitaxially fabricated by using a pulsed-laser deposition technique. The third-order nonlinear optical properties were studied through a z-scan method, allowing the resolution of the nonlinear refractive and absorptive contributions to the responses. Although all the samples show almost the same value of nonlinear absorption coefficient, the extracted nonlinear refractive index of the sample of x = 0.3 is apparently larger than that of other samples. Dependency of the nonlinear optical properties on the Ba/Sr ratio is discussed in terms of the crystal phase transformation and metal-oxygen bond length of the selected materials.

We report VO_x/NaVO3 nanocomposite as a novel saturable absorber for the first time, to the best of our knowledge. The efficient nonlinear absorption coefficient and the modulation depth are determined by the Z-scan technology. As a saturable absorber, a passively Q-switched Nd-doped bulk laser at 1.34 µm is demonstrated, producing the shortest pulse duration of 129 ns at a repetition rate of 274 kHz. In the passively Q-switched Tm:YLF laser with the prepared saturable absorber, the shortest pulse duration was 292 ns with a repetition rate of 155 kHz. Our work confirmed the saturable absorption in VO_x/NaVO3 for possible optical modulation in the near-infrared region.

As one of the greatest inventions in the 20th century, ultrafast lasers have offered new opportunities in the areas of basic scientific research and industrial manufacturing. Optical modulators are of great importance in ultrafast lasers, which directly affect the output laser performances. Over the past decades, significant efforts have been made in the development of compact, controllable, repeatable, as well as integratable optical modulators (i.e., saturable absorbers). In this paper, we review the fundamentals of the most widely studied saturable absorbers, including semiconductor saturable absorber mirrors and low-dimensional nanomaterials. Then, different fabrication technologies for saturable absorbers and their ultrafast laser applications in a wide wavelength range are illustrated. Furthermore, challenges and perspectives for the future development of saturable absorbers are discussed and presented. The development of ultrafast lasers together with the continuous exploration of reliable saturable absorbers will open up new directions for the mass production of the next-generation optoelectronic devices.