**1. Introduction**

Atomically thin layered materials, usually referred to as 2D materials or low-dimensional materials, are often characterized by unique and unexpected electronic and optical proper‐ ties. The most popular example of a 2D material is graphene, which is composed of a single layer of carbon atoms, forming a 2D honeycomb lattice. Graphene is a fundamental building block of three-dimensional (3D) graphite. The growing interest in novel two-dimensional materials started in 2004 after the discovery of unique electrical properties of graphene [1]. After that

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success, the scientists around the world extensively investigate other thin layered materials, such as topological insulators (TIs),transition metal dichalcogenides (TMDCs), black phospho‐ rus (BP), and many others.

The popularity of 2D materials among researchers is mainly driven by their unique electrical properties and their potential applications in new-generation electronic devices. However, those materials are also characterized by multiple unique optical properties, such as broadband and almost wavelength-independent absorption or optical bistability (saturable absorption with ultrashort recovery time and high modulation depth). Those properties make 2D materials useful in laser technology, e.g., as saturable absorbers for lasers emitting ultrashort optical pulses. The so-called mode-locked lasers, emitting ultrashort pulses in the infrared range, are currently on demand of many industrial, military, and medical applications. They might be used in many various fields, e.g., in medicine and surgery [2], materials processing [3], laser spectroscopy [4, 5], and fundamental science (generation of terahertz waves [6], multiphoton systems for optical imaging [7], or supercontinuum generation [8]). Ultrafast lasers are also main building blocks of optical frequency combs, which are currently used in, e.g., spectrograph calibration enabling detection of extrasolar planets [9] or optical-atomic clocks [10]. The research on novel 2D materials strongly contributes to the development of novel laser sources, enabling generation of shorter pulses and broader bandwidths at new wavelength regions, previously uncovered by any other coherent light source. This chapter explains the fundamentals of ultrashort pulse generation and reviews and summarizes the most important recent achievements in the field of ultrafast lasers incorporating 2D materials.
