Optical fiber based devices are a major area of interest within the development of optical fiber amplifiers and lasers. These devices generally cover a wavelength range stretching from the S-band (1460 nm to 1530 nm)1–3 to L-band region (1565 nm to 1625 nm)4–6. The advancement of optical fiber lasers has seen variety of laser output reported in a various laser configuration including pulsed7–12 and multi-wavelength13–16. Tremendous achievement in the field of near-infrared (IR) fiber lasers have kept this field growing to cater the ever-growing demands from laser end-users. This include expanding the wavelength operation to 2.0 µm region. Eye-safe 2.0 µm laser sources are of particular interest due to its potential in medical, space, defence and industrial applications. A key aspect of 2.0 µm light sources is its ability to be strongly absorbed in the human eye’s vitreous part, reducing the possibility to harm the retina 17. This property is much needed in free-space optical communication system 18 or in LIDAR (Light Detection and Ranging) 19–21. Additionally, 2.0 µm laser sources play a critical role in the near-infrared (IR) gas spectroscopy and sensing owing to the efficient light absorption of atmospheric gaseous (e.g., carbon dioxide, CO2) at 2.0 µm wavelength range 22–24. 2.0 µm fiber laser source can be generated from thulium-doped 25 and thulium/holmium-doped fiber lasers 26. The thulium and thulium/holmium doped fiber, exhibit broad bandwidth covering almost 500 nm, from 1.7 µm to 2.2 µm. This feature allows a wide selection range of laser operations in the eye-safe spectral region, including the continuous-wave (CW) mode and the Q-switching mode. Such broad bandwidth is also known to be capable of generating femtosecond pulses in the mode-locking regime 27–32.
Mode-locked fiber lasers as highly versatile light sources have attracted enormous attention due to its ability to access a wide range of scientific and industrial processes including optical communications, sensing, material processing and medical treatment 33–38. So far, two main approaches utilized in the operation of mode-locked fiber lasers are based on active and passive techniques. Compared to active, passive technique has the intrinsic advantages of high stability and reproducibility for robust ultrashort optical pulses. Saturable absorber (SA) plays a vital role in passive mode-locking technique. Presently, SA can be classified into two broad types namely, artificial, and real. Artificial SAs such as nonlinear loop mirrors 39–43 and nonlinear polarization rotation 44,45 are based on nonlinear effects with the properties of high damage threshold and low cost. These SAs provide a good platform for operation of high energy laser. However, the vulnerability of the laser system towards environmental perturbation has limit its practical applications. Real SAs, made up of materials that exhibit intensity-dependent transmission are regarded as a more effective way to generate mode-locked pulses. Specifically, two-dimensional (2D) material-based SAs have been widely employed as effective SAs due to their excellent optical properties, including wide absorption band and ultrafast recovery time. Following the successful exfoliation of graphene and its first ultrafast application, other 2D materials including topological insulators (TIs) 31,46−49, transition metal dichalcogenides (TMDs) 50–55, metal chalcogenides 56, antimonene 57 and MXenes have been explored for their unique saturable absorption property in ultrafast laser generation.
Transition metal carbides and/or nitrides, which are widely known as MXenes, are a member of the 2D material group that possess unique properties that could be altered by simply manipulating the composition and surface termination elements 58,59. In general, MXenes consist of few-atoms-thick layers of transition metal carbides, nitrides, or carbonitrides with composition of Mn+1XnTx, where M stands for an early transition metal (such as: Ti, V, Cr, Nb, etc.), X stands for carbon and/or nitrogen, n = 1, 2, or 3, and Tx is the surface termination groups ((–O), (–F), and (–OH)) 60. Being in the family of MXene, niobium carbide (Nb2C) has received extensive research attention in the last few years due to its unique physical and chemical properties that are valuable in various applications 61. Theoretically, it has been predicted that Nb2C demonstrate a great reduction of lattice thermal conductivity resulted from the abnormal electron–phonon scatterings with intensities close to that of phonon–phonon scatterings 62. In a study conducted by Lin et. al.63 has revealed that the Nb2C possess strong optical response in the near infrared region as it shows high photothermal conversion efficiency that can be used in biomedicine, particularly for cancer phototherapy. In another report, Wang et. al. 64 has investigated the broad-band nonlinear optical response and the ultrafast carrier dynamics of Nb2C over the wavelength ranging from visible to the near-infrared region. Their finding discloses the dependency of the nonlinear optical response of Nb2C on wavelength and excitation intensity. The unique nonlinear absorption response inversion properties of Nb2C, that is the ability to shift from saturable absorption to two-photon absorption in near infrared region has facilitate its vast applications in nonlinear photonics, in particular as an optical switch 65.
In this work, high-quality few layer Nb2C nanosheets were fabricated by the liquid phase exfoliation method and deposited onto a tapered fiber using a drop-casting method, forming an all-fiber SA device. Based on Nb2C-coated microfiber SA, passive mode-locking operation in TDFL and THDFL were successfully generated. Both laser systems show stable mode-locked pulses with the former system operating at 1948 nm and the later operating at 1952 nm. These results suggest that the Nb2C-coated microfiber could perform as a practical and a high-performance SA for an ultrafast fiber laser generation in the 2.0 µm region. This could further promote the development of MXene-based optical devices in the photonics technology.
Characterization Of Nbc Mxene
The surface morphology of the Nb2C MXene in the powder form was investigated by field emission scanning electron microscopy (FESEM) and the elemental composition of Nb2C MXene was determined by energy dispersive X-ray (EDX) analysis. These studies were performed using a Hitachi Model SU8220 FESEM which was equipped with EDX detector and operating at 2.0 kV. Figure 1 shows the FESEM image of Nb2C Mxene captured at the magnification of 20.0k. It can be observed that Nb2C MXene exhibits a sheet-like structure with multiple layers stacked together, whose size was about 2.0 µm.
Figure 2 represents the EDX elemental mapping images of Nb2C MXene where the distribution of each element can be clearly seen. The Niobium (Nb) and Carbon (C) maps verified the presence of Nb and C elements, thus confirming the successfully formation of Nb2C MXene.
An atomic force microscope (Park System NX-10 AFM) was used to measure the thickness of the Nb2C MXene. The measurement was done under non-contact mode. Initially, 10 µL of Nb2C MXene solution was deposited on the Si substrate using the spin coating technique. The sample was dried at room temperature overnight before being used for thickness measurement. The AFM topography image of Nb2C MXene together with its corresponding lateral height measurement were presented in Fig. 3(a) and Fig. 3(b), respectively. The obtained result demonstrates that the Nb2C MXene exhibits few-layer structure of the Nb2C sheet with a thickness of about 28 nm and lateral size of approximately 1.2 µm.
The measurement of nonlinear optical absorption of MXene Nb2C-coated microfiber SA was examined using the twin-detector measurement technique. A 1950 nm Toptica FemtoFerb femtosecond laser with a repetition rate of 30 MHz and a pulse width of 100 fs was employed as the seed laser. The laser source was connected to a variable attenuator and subsequently to a 3 dB optical coupler for beam splitting. One port of the 3 dB coupler was connected to the Nb2C-coated microfiber SA and another port was connected directly to a microfiber without the SA as the reference port. Both transmitted powers were measured using optical power meter. The experimental data was recorded and fitted using the saturation model equation below:
where I, Isat, αns, αs signify the laser input intensity, saturation intensity, non-saturated loss and modulation depth. From the fitting of the measured data shown in Fig. 4, the saturation intensity and modulation depth obtained were 0.4 MW/cm2 and 6.77 %, respectively.