Low-dimensional materials as saturable absorbers for pulsed waveguide lasers

Abstract

Low-dimensional (LD) materials, such as 2D materials, carbon nanotubes, and nanoparticles, have attracted increasing attention for light modulation in photonics and optoelectronics. The high nonlinearity, broad bandwidth, and fast response enabled by LD materials are critical to realize desired functionalities in highly integrated photonic systems. Driven by the growing demand for compact laser sources, LD materials have recently demonstrated their great capacity as saturable absorbers in pulsed (Q-switched or mode-locked) laser generation in waveguide platforms. We review the recent advances of pulsed waveguide lasers based on LD materials. A perspective is also presented in this rapidly growing research field. Fundamentals of LD saturable absorbers and optical waveguidesOwing to the diverse electronic structures, LD materials offer versatile options to realize wide applications based on the nonlinear optical response. Currently, semiconductor saturable absorber mirrors (SESAMs) are the most frequently used commercial SAs, processing high damage threshold and prominent stability. However, a variety of LD materials retain a number of advantages to be nonlinear SAs, of which SESAMs © 2020 The Author(s). Published by IOP Publishing Ltd J. Phys. Photonics 2 (2020) 031001 Z Li et al cannot fulfill, such as broadband operation, ultrafast recovery time, low cost, and better compatibility with compact devices such as fibers and waveguides. Recently, an increasing number of research efforts have been made to unfold the nonlinear optical properties as well as its corresponding applications of the emerging LD materials [14][15][16][17][18][19][20][21][22][23]. For semiconducting or semimetallic LD materials, the main mechanism for nonlinear saturable absorption is Pauli blocking. After photoexcitation, non-equilibrium carrier state could be created, in which valence-band electrons can be excited to the conduction band, and a hole can be formed on the valence band. As the increase of the incident light intensity, the photon absorption processes of LD materials experience a transition from linear absorption to saturable absorption. When the light intensity continues increasing and reaches the saturation intensity, the extra photons will no longer be absorbed by the electrons in the valence band, and the transmittance of the SA will not increase but gradually tend to a stable value. For metallic nanoparticles, the dominant mechanism is the LSPR effect arising from the interaction between the electric field of incident light and the surface electrons in the conduction band. The optical nonlinearity could be significantly enhanced while maintaining the ultrafast recovery time. These nonlinear effects are widely used in passively Q-switched or mode-locked lasers. In order to characterize the nonlinear optical properties, femtosecond Z-scan spectroscopy is typically employed by moving the sample along the Z direction through a focused Gaussian beam. The laser intensity can be varied continuously with the maximum at the focal poin...

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