“…Figure 7 b exhibits the measured transmission curve of the ReSe 2 /PVA-based SA, from which the saturation intensity and modulation depth were estimated as ~ 11.4 MW/cm 2 and ~ 8%, respectively. Hence, the measured modulation depth is high enough for mode-locking in an anomalously dispersive fiber cavity 49 , 50 . Figure 7 ( a ) Experimental setup for nonlinear transmission measurements and ( b ) the nonlinear transmission curve of the ReSe 2 /PVA-based SA.…”
An experimental investigation into the nonlinear optical properties of rhenium diselenide (ReSe2) was conducted at a wavelength of 1.9 μm using the open-aperture and closed-aperture Z-scan techniques for the nonlinear optical coefficient (β) and nonlinear refractive index (n2) of ReSe2, respectively. β and n2 measured at 1.9 μm were ~ − 11.3 × 103 cm/GW and ~ − 6.2 × 10–2 cm2/GW, respectively, which to the best of our knowledge, are the first reported measurements for ReSe2 in the 1.9-μm spectral region. The electronic band structures of both ReSe2 and its defective structures were also calculated via the Perdew–Becke–Erzenhof functional to better understand their absorption properties. A saturable absorber (SA) was subsequently fabricated to demonstrate the usefulness of ReSe2 for implementing a practical nonlinear optical device at 1.9 μm. The 1.9-μm SA exhibited a modulation depth of ~ 8% and saturation intensity of ~ 11.4 MW/cm2. The successful use of the ReSe2-based SA for mode-locking of a thulium–holmium (Tm–Ho) co-doped fiber ring cavity was achieved with output pulses of ~ 840 fs at 1927 nm. We believe that the mode-locking was achieved through a hybrid mechanism of saturable absorption and nonlinear polarization rotation.
“…Figure 7 b exhibits the measured transmission curve of the ReSe 2 /PVA-based SA, from which the saturation intensity and modulation depth were estimated as ~ 11.4 MW/cm 2 and ~ 8%, respectively. Hence, the measured modulation depth is high enough for mode-locking in an anomalously dispersive fiber cavity 49 , 50 . Figure 7 ( a ) Experimental setup for nonlinear transmission measurements and ( b ) the nonlinear transmission curve of the ReSe 2 /PVA-based SA.…”
An experimental investigation into the nonlinear optical properties of rhenium diselenide (ReSe2) was conducted at a wavelength of 1.9 μm using the open-aperture and closed-aperture Z-scan techniques for the nonlinear optical coefficient (β) and nonlinear refractive index (n2) of ReSe2, respectively. β and n2 measured at 1.9 μm were ~ − 11.3 × 103 cm/GW and ~ − 6.2 × 10–2 cm2/GW, respectively, which to the best of our knowledge, are the first reported measurements for ReSe2 in the 1.9-μm spectral region. The electronic band structures of both ReSe2 and its defective structures were also calculated via the Perdew–Becke–Erzenhof functional to better understand their absorption properties. A saturable absorber (SA) was subsequently fabricated to demonstrate the usefulness of ReSe2 for implementing a practical nonlinear optical device at 1.9 μm. The 1.9-μm SA exhibited a modulation depth of ~ 8% and saturation intensity of ~ 11.4 MW/cm2. The successful use of the ReSe2-based SA for mode-locking of a thulium–holmium (Tm–Ho) co-doped fiber ring cavity was achieved with output pulses of ~ 840 fs at 1927 nm. We believe that the mode-locking was achieved through a hybrid mechanism of saturable absorption and nonlinear polarization rotation.
“…Apart from that, numerical simulations were carried out to investigate the influence of SA recovery time on several parameters in dissipative soliton [ 146 ] and conventional soliton mode‐locking operations. [ 147 ] These parameters include pulse width, spectral bandwidth, peak power and pulse energy. The recovery time should not be too slow, [ 146 ] , otherwise it would result in the destabilization or breaking of the pulse.…”
Saturable absorption is a nonlinear optical phenomenon that is characterized by the reduced optical loss at high light intensity. At low light intensity, the saturable absorber (SA) absorbs light and results in an optical loss. When the light intensity is increased, the available energy states become depleted, leading to a reduction in absorption. For semiconductors, the saturated optical absorption under strong excitation is in conjunction with the Pauli expulsion of occupying carriers at the edge of the filled conduction band thus allowing photons to penetrate through the material without further absorption. [1] When incorporated into a mode-locked laser cavity, the saturable absorption process begins from the inherent noise fluctuations of a laser cavity. A dominant noise spike with the highest intensity will saturate the absorber and decreases the absorbance loss such as the pedestal peaks. [2] Next, this noise spike is amplified in subsequent round trips until a stable pulse train is eventually formed. This favors the generation of ultrafast laser pulses over continuous-wave laser operation. During the formation of ultrafast laser pulses, the SA is operated either as slow or fast SA. The main difference between these two SA configurations is their relaxation time. The relaxation time of a slow SA is longer than its pulse duration measured from the mode-locked laser. The long time constant results in reduced saturation intensity due to slow recovery of absorption. [3] Therefore, the slow SA self-starts the modelocked laser at a lower power threshold. In contrast, the relaxation time of fast SA is shorter than its pulse duration measured from the mode-locked laser. Owing to the fast relaxation time, the electrons in the conduction band will return to the valence band at a period shorter than the pulse duration. This constitutes the higher power threshold to self-start the mode-locked laser.Despite the difficulty to self-start the mode-locking, the fast SA is responsible for the stabilization of the laser. The fast SA has ultrafast carrier relaxation time for the SA to recover from bleaching with minimal time. [4] Besides stabilization, ultrafast relaxation time is also crucial to shortening the pulse duration of the mode-locked laser. [5] Before the advent of ultrashort pulse with relaxation time ranging from a few ps to a few hundred fs, semiconductor saturable absorber mirror (SESAM) was demonstrated with a complex post-fabrication process. For instance, ion implantation is needed to reduce its relaxation time to ps time scale. This encourages the discovery of a material with sub-picosecond recovery time, such as carbon nanotube (CNT) [6] and graphene. [7] Both CNT and graphene have ultrafast and slow recovery from saturation. In a femtosecond laser, the CNT will typically act as a slow SA due to its recovery time of few ps, whereas the graphene will typically act as a fast SA due to its
“…20. The theoretical investigations and experimental evidences having dual and multi-wavelength operation of TISA have been discussed in the recent literature [36,41,86,87,88].…”
Topological Insulators (TIs) exhibit exciting optical properties, which opens up a new pathway to generate ultrashort pulses from fiber lasers. Layered TIs display distinct saturable absorption property due to excited state absorption, as compared to their bulk structures. Moreover, the electronic structures of the films of TIs depend on the thickness of the films due to the quantum confinement of the electrons. By virtue of this, the nanoparticles of TIs play a key role in all-fiber modelocked laser. By tweaking the crystal structures of TIs, it is possible to generate ultrashort pulses across the visible, near-infrared and mid-infrared wavelengths. Starting from the crystal structures and density of states calculations, how different topological insulators can be fabricated and integrated as an efficient passive saturable absorber in all-fiber modelocked lasers with the capability of producing fundamental to high-harmonic pulse generation are described clearly in this review report. Moreover, this report reviews the current state-of-art of TI-based saturable absorbers and their applications in different regimes of modelocked fiber lasers.
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