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DOI: 10.1039/C8TC04635B (Paper) J. Mater. Chem. C, 2018, 6, 12638-12642

Ultrasmall 2D NbSe2 based quantum dots used for low threshold ultrafast lasers

Yihuan Shi† a, Hui Long† b, Shunxiang Liu a, Yuen Hong Tsang *b and Qiao Wen *a
aKey Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China. E-mail: wenqiao@szu.edu.cn
bDepartment of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Kowloon, Hong Kong, China. E-mail: yuen.tsang@polyu.edu.hk

Received 13th September 2018 , Accepted 29th October 2018

First published on 29th October 2018

Abstract

Ultrasmall NbSe2 quantum dots (QDs) with an average diameter of 2.68 nm were successfully fabricated and investigated. By employing the NbSe2 quantum dots (QDs) as a saturable absorber, highly stable ultrafast laser pulse trains with a duration of 765 fs or 380 ps were successfully generated from mode-locking erbium or ytterbium-doped fiber laser systems by using the fabricated NbSe2 QDs. The obtained laser pump power threshold for the mode-locking operation of the Er-doped fiber laser was 15 mW, which is the lowest value recorded among 2D materials based saturable absorbers. Our results give a promising contribution to the research field by demonstrating the strong nonlinear optical properties of these novel ultrasmall NbSe2 QDs. It also opens up new opportunities for the use of these novel materials for nonlinear optics and ultrafast laser photonic technologies.

Introduction

Since the discovery of graphene in 2004, research interests have recently been further extended to other two dimensional (2D) nanostructures, such as transition metal dichalcogenides (TMDs) that offer a rich variety of remarkable electronic and optoelectronic properties.1–9 These newly discovered layered materials allow us to develop next generation electronics and optoelectronics. Among these transition metal dichalcogenide (TMD) materials, MoS2,1 MoSe2,4 WS23 and WSe2,6 which exhibit semiconducting properties, have been extensively investigated owing to their natural bandgap, making them more suitable for transistor applications. Relatively, there have not been many studies that focus on metallic layer 2D nanostructures such as NbS2 and NbSe2. Recently, it was found that the few layered NbSe2 exhibits lower conductivity and semiconducting properties compared to its bulk counterparts.10 NbSe2 nanosheets also have a remarkable photocurrent response.10 The chemical vapor deposition (CVD) grown mono or few layers NbSe2 was confirmed to be a superconductor in between 1–4.56 K depending on its number of layers.11 So far, to the best of our knowledge, no research work has been performed on the nonlinear optical properties of this novel layered NbSe2 or its quantum dot (QD) form.

However, 2D materials based on quantum dots (QDs) offer good design flexibility due to the tunable bandgap via particle size control.12,13 Additionally, the properties of 2D material based QDs, for example the optical nonlinear effect, optoelectronics properties, saturable absorption intensity and photochemical stability can be even better than in the 2D layered form owing to the increased surface area volume ratio,14–16 stronger quantum confinement and edge effects.13–32 Therefore, it is even more interesting to investigate the nonlinear optical applications of 2D NbSe2 based QDs. One of the most important applications of these nonlinear optical materials is their use as a saturable absorber (SA) within a mode-locking fiber laser to generate ultrafast laser pulses. These ultrafast pulsed lasers have very wide applications in various fields ranging from scientific, military, medical and industrial applications. However, the application of NbSe2 QDs for ultrafast laser pulses has not yet been studied. In this work, ultrasmall NbSe2 QDs with an average diameter of 2.68 nm were successfully fabricated for the first time. Then, these fabricated NbSe2 QDs were deposited onto the D-shaped fiber to serve as a SA in two ultrafast fiber laser systems operating at either ∼1 micron or 1.5 micron, via an evanescent field interaction. The achieved results suggest that these novel NbSe2 QDs could be used as nonlinear optical materials, for example as a high-quality broadband saturable absorber.

In our work, NbSe2 QDs with the most frequent diameter of 2.35 nm and a standard deviation of 0.03 nm, were successfully fabricated and investigated. As for the applications of ultrafast photonics, the NbSe2 QDs were deposited onto a D-shaped fiber and used as a SA for an ultrafast fiber laser in two wavelengths of around 1 micron and 1.5 micron, via an evanescent field interaction. The achieved results suggest that these novel NbSe2 QDs could be used as an alternative high-quality broadband nonlinear optical material for ultrafast lasers.

QDs NbSe2 fabrication and characterization

NbSe2 QDs fabrication

NbSe2 powder was purchased from Sigma-Aldrich. In detail, 0.1 g NbSe2 was dissolved in 50 ml NMP solution under vigorous stirring for 12 h. The temperature was kept at 29 °C to prevent degradation of the powder. The solution was centrifuged at a rate of ω = 10[thin space (1/6-em)]000 rpm for 30 min. Then, the NbSe2 nanosheets were obtained at a concentration of 0.085 mg ml−1 in NMP.

X-ray diffractometry (XRD) was performed using a Bruker D8 Advance. Both the transmission electron microscopy (TEM) images and the energy dispersive X-ray spectra (EDS) were measured by using a JEOL Model JEM-2011 transmission electron microscope with an accelerating voltage of 200 kV. The atomic force microscopy (AFM) images were captured using a Dimension (Veeco Nanoscope V) in the tap mode. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific, ESCALAB 250Xi) was measured using a monochromatic Al Kα source to investigate the chemical states of the samples. The UV-Vis absorption spectra were recorded in the wavelength range of 200–900 nm using a scanning spectrophotometer (Shimadzu) at room temperature. The Raman measurements were carried out on a HR800 Raman microscopy system (Horiba Jobin Yvon), using a 488 nm laser with a charged coupled device as the detector.

Fig. 1a shows the atomic structure of the NbSe2. From the top and side views of the structure,11 the layered structure consists of Se–Nb–Se atoms within the plane with strong covalent bonding. There is weak van der Waals bonding between the layers,33 hence a smaller NbSe2 nanosheet or quantum dots can be effectively obtained through mechanical34,35 and liquid36 exfoliation. The purchased NbSe2 powders were characterized using XRD and scanning electron microscopy (SEM) to examine the structure and size. Fig. 1b shows the XRD pattern for the NbSe2 powder, which is a black powder as shown in the inset. Sharp and strong peaks were observed in the XRD pattern. The diffraction peaks located at 2θ = 14°, 28°, 43°and 58° correspond to the planes of (002), (004), (006) and (008), respectively, these results correspond well with the hexagonal phase (JCPDS file no. 65-3484).37Fig. 1c shows the SEM image for the crystal, the flake size is approximately a few micrometers before liquid exfoliation.


image file: c8tc04635b-f1.tif
Fig. 1 (a) Atomic structure of NbSe2, top and side views of the atomic structure. (b) XRD pattern of the commercial NbSe2 powder bought from the Aladdin Reagent Company. The inset of (b) is the digital photo of the NbSe2 powder. (c) SEM image of the NbSe2 powder before liquid exfoliation.

Fig. 2a and b shows the TEM images of the NbSe2 QDs. The images clearly show that the obtained NbSe2 QDs are quite uniform. As presented in Fig. 2b, a single NbSe2 QD with an in-plane lattice spacing of 0.27 nm agrees well with the (102) planes, in which the lattice fringes of the NbSe2 QDs are also clearly shown. The diameter distribution of the QDs mainly ranged from 1.8 to 3.4 nm, with an average diameter of 2.68 nm and a full width at half maximum (FWHM) of 0.65 nm as shown in Fig. 2c, suggesting the prepared NbSe2 quantum dots are ultrasmall and possess an excellent uniformity. These results confirm that the suggested simple ultrasound fabrication method can effectively produce very small and uniform NbSe2 quantum dots from a powder form in the NMP solution. Fig. 2d shows a typical AFM image for the NbSe2 QDs on a Si substrate. The topography image of three randomly selected NbSe2 QDs, labeled as 1, 2 and 3, are shown in Fig. 2d. The height profiles of the selected QDs are plotted in Fig. 2e. The statistical information for the thickness of the QDs are given in Fig. 2f. The data was obtained by analysis of the AFM images for the QDs. The average thickness of these NbSe2 QDs is about 5.12 nm, corresponding to a few (∼4) atomic layers of NbSe2 QDs. Fig. 3 shows two characteristic absorption peaks at 269 and 607 nm. The inset in Fig. 3 shows a photo of the NbSe2 QDs suspension in NMP.


image file: c8tc04635b-f2.tif
Fig. 2 (a) A typical TEM image and (b) high resolution TEM image for NbSe2 QDs obtained after liquid exfoliation. (c) The diameter distribution of the NbSe2 QDs. (d) AFM image of the NbSe2 QDs. (e) The height profile of the section marked in (d). (f) The thickness distribution of the NbSe2 QDs. (X0 = average value.)

image file: c8tc04635b-f3.tif
Fig. 3 Absorption spectrum for a suspension of NbSe2 QDs in NMP.

The nonlinear optical characteristics of the fabricated NbSe2 SA, were investigated using a balanced twin detector measurement system that is shown in the inset of Fig. 4. The laser that was used for the saturable absorption measurement is a homemade femtosecond pulse source with center wavelength of 1575 nm, a repetition rate of 14.7 MHz and a pulse duration of ∼200 fs. As presented in Fig. 4, the saturable absorption curve reveals a modulation depth of 3.72%, a saturation intensity of 3.155 GW cm−2 and that the nonsaturable loss is 30.7%.


image file: c8tc04635b-f4.tif
Fig. 4 Dependence of the transmittance of the NbSe2-SA on the incident pump peak power density.

Ultrafast photonics applications

By changing the solution concentration of the NbSe2 QDs the initial laser transmission of the fabricated SA on top of the D-shaped fiber can be optimized. Successful mode locking operations were achieved in two Yb or Er doped fiber laser systems using the fabricated saturable absorbers (SAs).

Cavity schematic of the fiber laser

We constructed two all-fiber structures with Yb or Er-doped fibers as laser gain medium, adopting the ring cavity design as shown in Fig. 5. The fiber laser includes a wavelength division multiplexer (WDM), laser diode (LD), polarization independent isolator (PI-ISO), polarization controller (PC), optical coupler (OC), doped optical fiber and D-shaped fiber. The D-shaped fiber has an interaction length of 5 mm and the distance from the fiber core boundary to the lowest point of the D-shaped area is 1 μm. The NbSe2 QDs solution was dropped onto the side-projection face of the D-shaped fiber to form the SAs. The NbSe2-SAs were inserted in between the PC and OC. Two different fiber laser systems with different operational wavelengths were constructed by using different Yb or Er-doped optical fibers. In order to generate a laser around 1 micron, a Yb-doped fiber (250 dB m−1@980 nm) of 1 m long was used within a ring fiber laser cavity with a total length of 16.9 m. However, the 1.5 micron could also be generated by using an Er-doped fiber (4.45 dB m−1@980 nm) of 4 m inserted within a ring fiber laser cavity with a total length of 27.1 m.
image file: c8tc04635b-f5.tif
Fig. 5 The cavity schematic for the fiber laser.

Characterization of THE ultrafast Er-doped fiber laser operating at 1.5 micron

By careful adjusting the PC in the cavity, the mode-locking operation of THE Er-doped fiber laser WAs obtained when the pump power reached more than 15 mW. The obtained mode-locking threshold was approximately 330% lower than the previous record,16,38 and is the lowest record amongst 2D materials based SAs. This provides strong evidence to show that NbSe2 QDs have the obvious advantage of much stronger nonlinear effect and a lower SA intensity than other 2D nanomaterial SAs. The character of the mode-locked Er-doped fiber laser is shown in Fig. 6. The central wavelength of the mode-locking pulse is 1556 nm and the 3 dB bandwidth was measured to be 2.45 nm, as shown in Fig. 6a. The autocorrelation trace for the mode-locking operation is given in Fig. 6b. The FWHM of the autocorrelator pulse trace is 1.177 ps, which means the pulse width is 765 fs if a sech2 fit is used, with a time-bandwidth product (TBP) of 0.409. The TBP is close to the theoretical limit value of 0.315, indicating that the mode-locked pulse is slightly chirped. Fig. 6c demonstrates the mode-locked pulse train with a time interval of 130 ns, which is in agreement with the repetition rate of 7.7 MHz. The inset of Fig. 6a shows uniform intensity pulses of a mode-locked laser, which shows the highly stable operation of the laser. Fig. 6d gives the radio frequency (RF) spectrum for the mode-locked pulse with a fundamental repetition rate of 7.7 MHz, corresponding to the cavity length of 27.1 m. The signal-to-noise ratio (SNR) is up to 50 dB, indicating a highly stable mode locking operation. Fig. 6e indicates that the evolution of the optical spectra of the solution pulses is over 7 h, showing the high stability of the Er-doped mode-locked fiber laser. The relationship between the average output power and the pump power is given in Fig. 6f. It shows a good linearity and a slope efficiency of 4.16%.
image file: c8tc04635b-f6.tif
Fig. 6 Characteristics of the Er-doped mode-locked laser: (a) output optical spectrum. (b) Autocorrelation trace. (c) Output pulse trains (d) RF spectrum. (e) Optical spectra measurement at 1 h intervals over 7 h. (f) Variation of output power with respect to the pump power.

Characterization of the ultrafast Yb-doped fiber laser operating at 1 micron

In order to show that NbSe2 QDs are capable of operating over a broad wavelength range, the fabricated NbSe2 QDs SA was used in an Yb-doped mode-locked fiber laser cavity to generate ultrafast laser pulses operating at around 1 micron. Under a pump power up to 175 mW, a stable mode locking laser pulse is produced after carefully adjusting the PC in the laser cavity. Compared with the other mode-locked Yb-doped fiber lasers reported in previous studies,39 the ultrafast fiber laser wavelength of around 1 micron described in this work exhibited a relatively low mode-locking pump power threshold value of 175 mW. As shown in Fig. 7a, the obtained central wavelength was 1033 nm with a 3 dB spectral bandwidth of 0.1550 nm. Fig. 7b illustrates the trace of a mode-locked laser pulse measured using a high speed oscilloscope (LeCroy WavePro, 760 Zi-A). The FWHM of the laser pulse duration was 380 ps. Fig. 6c presents the mode-locked pulse train. The time interval between the two pulses is 81 ns, which is in good agreement with the laser cavity length of 16.9 m. As depicted in Fig. 7d, we observed a strong signal peak with a repetition rate of 12.3 MHz. The SNR of the generated laser pulses is 43 dB, which indicates the obtained laser pulses with a high stability. The stability of the Yb-doped fiber laser was examined by continuously monitoring its output spectra at 1 h intervals over 7 hours and the results are given in Fig. 7e. It shows no change in the output spectrum over 7 h. The relationship between the average output power and the pump power is given in Fig. 7f. Obviously, a good linear correlation between the output power and the input power was obtained and the slope efficiency was 3.7%.
image file: c8tc04635b-f7.tif
Fig. 7 Characteristics of the Yb-doped mode-locked fiber laser: (a) output optical spectrum; (b) measurement of the laser pulse width; (c) output pulse trains; (d) RF spectrum; (e) optical spectra measurement at 1 h intervals over 7 h; and (f) the variation of the output power with respect to the pump power.

Conclusions

In conclusion, NbSe2 QDs with an average diameter of 2.68 nm were successfully fabricated and investigated. A type of novel optical SA based on broadband nonlinear NbSe2 QDs was designed that can generate an ultrashort pulse at the wavelengths of 1556 nm and 1033 nm in erbium and ytterbium-doped fiber lasers with low mode-locking pump laser thresholds, respectively. Our results confirm that the fabricated NbSe2 QDs exhibits strong broadband nonlinear SA properties. This work opens up new opportunities for the use of these novel NbSe2 QDs for nonlinear optics and ultrafast laser photonics devices.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is financially supported by the Shenzhen-Hong Kong Innovation Cooperation Project (Grant No. SGLH20150205162842428), the Hong Kong Innovation and technology fund under Grant GHP/007/14SZ, the Research Grants Council of Hong Kong, China (Project Number: GRF 152109/16E PolyU B-Q52T), the Science and Technology Innovation Commission of Shenzhen (JCYJ20170412111625378, JCYJ20170302153540973 and GRCK2017042110412823), the National Special Foundation of China for Major Science Instrument (61227802) and the Hong Kong Polytechnic University (Project number: G-YBVG, G-YBFR).

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Footnote

† These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2018
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