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PDF, Low-Dimensional Materials As Saturable Absorbers for Pulsed PERSPECTIVE • OPEN ACCESS Recent citations Low-dimensional materials as saturable absorbers - Saturable Absorption Dynamics of Highly Stacked 2D Materials for Ultrafast Pulsed for pulsed waveguide lasers Laser Production Young In Jhon and Ju Han Lee To cite this article: Ziqi Li et al 2020 J. Phys. Photonics 2 031001 View the article online for updates and enhancements. This content was downloaded from IP address 170.106.40.139 on 26/09/2021 at 10:10 J. Phys. Photonics 2 (2020) 031001 https://doi.org/10.1088/2515-7647/ab8a5a Journal of Physics: Photonics PERSPECTIVE Low-dimensional materials as saturable absorbers for pulsed OPEN ACCESS waveguide lasers RECEIVED 22 December 2019 Ziqi Li, Chi Pang, Rang Li and Feng Chen REVISED School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Shandong, Jinan 250100, People’s Republic of China 28 February 2020 E-mail: [email protected] ACCEPTED FOR PUBLICATION 17 April 2020 Keywords: 2D materials, carbon nanotubes, plasmonic nanoparticles, saturable absorbers, waveguide lasers, mode-locking, Q-switching PUBLISHED 22 May 2020 Original content from Abstract this work may be used Low-dimensional (LD) materials, such as 2D materials, carbon nanotubes, and nanoparticles, have under the terms of the Creative Commons attracted increasing attention for light modulation in photonics and optoelectronics. The high Attribution 4.0 licence. nonlinearity, broad bandwidth, and fast response enabled by LD materials are critical to realize Any further distribution of this work must desired functionalities in highly integrated photonic systems. Driven by the growing demand for maintain attribution to the author(s) and the title compact laser sources, LD materials have recently demonstrated their great capacity as saturable of the work, journal absorbers in pulsed (Q-switched or mode-locked) laser generation in waveguide platforms. We citation and DOI. review the recent advances of pulsed waveguide lasers based on LD materials. A perspective is also presented in this rapidly growing research field. 1. Introduction With the nanoscale confinement of electrons, low-dimensional (LD) materials, including zero-dimensional (0D) nanoparticles, one-dimensional (1D) nanotubes, and two-dimensional (2D) layered materials, have exhibited extraordinary physical properties that are absent from their three-dimensional (3D) counterparts [1–3]. For example, under light excitation, the near-field and optical nonlinearity of noble metallic nanoparticles could be strongly enhanced due to localized surface plasmon resonance (LSPR) [4, 5]. 2D materials have shown strong light-matter interactions due to the reduced dielectric screening [6–8]. One of the remarkable applications of LD materials is that they could be used as efficient and broadband optical modulators [9–11], which is essential for the development of various devices in nonlinear optics and ultrafast photonics. Optical waveguides are basic components in integrated photonics [12]. The integration of LD materials in optical waveguides could enable highly-desired ultrafast laser sources that are compatible with miniaturized photonics systems. Jia et al have recently presented a comprehensive review of pulsed waveguide lasers [13]. While the field still growing rapidly, a number of new research works are emerged based on LD material-based saturable absorbers (SAs) and numerous opportunities are ready to boom in the next few years. In this Perspective, we mainly focus on the recent progress and future outlook on the research of nonlinear optical properties of LD materials and their key role in solid-state waveguide platform. This paper will be organized as follows. First, the nonlinear optical properties of LD materials as well as the solid-state waveguide systems are briefly described. Then, we demonstrate the role of the LD materials as SAs in pulsed waveguide laser operation with different operation regimes. The majority will be focused on the solid-state waveguide lasers with laser crystals as gain media. Some brief descriptions of pulsed lasers based on semiconducting III–V materials and quantum dot layers are presented as well. Finally, we highlight key challenges and possible solutions for future research in LD material-based SAs and pulsed waveguide lasers. 2. Fundamentals of LD saturable absorbers and optical waveguides Owing 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 [14–23]. 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 point. With either open-aperture or close-aperture configuration, we can obtain the nonlinear parameters, such as modulation depth, saturation intensity, and nonlinear refractive index. The carrier lifetime is also an essential factor for SAs. The carrier dynamics and relaxation time could be investigated by ultrafast pump-probe measurements and subsequent exponential fitting of the experimental data. 3+ Since the groundbreaking invention of ruby (Cr :A12O3) laser in 1960, lasers have attracted widespread attention and played an important role along with the modern technological evolutions [24]. Optical waveguides are the basic components in integrated photonics, in which the light field could be confined in a very small volume [12, 25]. Waveguiding structures with different structures can be fabricated by a few well-established techniques, such as femtosecond laser writing, ion beam implantation, ion exchange, pulsed laser deposition, and molecular beam epitaxy, in which the first two techniques are most commonly used for solid-state waveguides. The femtosecond laser direct writing is one of the most effective methods for preparing complex three-dimensional (3D) microscale waveguide structures [12, 26]. Due to the high energy density at the focal point, the energy of femtosecond laser pulses can be deposited inside the crystal through non-linear absorption processes (including excessive photon absorption, tunneling ionization, avalanche ionization, etc), triggering a localized lattice structure (refractive index) change. Ion beam irradiation is also a commonly used method for the preparation of crystal optical waveguides [1, 17, 18]. The deposition of ion energy causes localized lattice distortion, thereby inducing corresponding changes in the refractive index of the crystal. Based on the fabricated optical waveguides, miniature photonic devices could be achieved, such as beam splitters, directional couplers, and frequency converters [27–31]. Compact laser sources based on the waveguide platform have recently shown its great potential for the development of integrated photonics [13]. Lasers could be experimentally achieved based on different waveguiding configurations such as straight, curved, and Y-branch structures [32–38]. With the microscale waveguiding volume, waveguide lasers are expected to have better laser performances, such as higher slope efficiency and lower lasing thresholds [39]. According to the operation regime, lasers based on waveguide platforms can be classified into continuous-wave and pulsed waveguide lasers. As discussed earlier [13], LD nanomaterials could be integrated with the solid-state waveguide in two configurations, i.e. transmission absorption (directly attached in the end face) and evanescent field absorption (near field interaction with propagating waves). The combination of LD materials and the optical waveguides could enable the design and optimization of ultrafast laser sources
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