Lab‐on‐Fiber Based on Optimized Gallium Selenide for Femtosecond Mode‐Locked Lasers and Fiber‐Compatible Photodetectors

Although the physicochemical properties of gallium selenide (GaSe) have been widely investigated, the property and application exploration of GaSe‐coupled fiber devices are still in its infancy. There are obvious challenges, namely, selecting from multiple GaSe phases and effectively coupling to the unique fiber structure. Herein, lab‐on‐fiber (LOF) based on optimized GaSe is proposed to be used for robust femtosecond pulse generation and fiber‐compatible photodetection. First, based on density functional theory (DFT) calculations, ε‐GaSe is selected as a preferable model material for its suitable band structures, low work function, and high damage threshold. Benefiting from the fiber‐compatible ε‐GaSe combined with micro–nano processing, stable femtosecond soliton pulse (≈303 fs, 16.67 MHz, 51.64 dB) output and multiwavelength (520, 808, and 1550 nm) detection are realized in LOF. These results pave the way for optics research of polyphase semiconductors and design of integrated all‐fiber devices.

Although the physicochemical properties of gallium selenide (GaSe) have been widely investigated, the property and application exploration of GaSe-coupled fiber devices are still in its infancy. There are obvious challenges, namely, selecting from multiple GaSe phases and effectively coupling to the unique fiber structure. Herein, lab-on-fiber (LOF) based on optimized GaSe is proposed to be used for robust femtosecond pulse generation and fiber-compatible photodetection. First, based on density functional theory (DFT) calculations, ε-GaSe is selected as a preferable model material for its suitable band structures, low work function, and high damage threshold. Benefiting from the fiber-compatible ε-GaSe combined with micro-nano processing, stable femtosecond soliton pulse (%303 fs, 16.67 MHz, 51.64 dB) output and multiwavelength (520, 808, and 1550 nm) detection are realized in LOF. These results pave the way for optics research of polyphase semiconductors and design of integrated all-fiber devices.
judge whether the material is damaged by laser at any time. [26][27][28] However, the coupling characteristic researches of vdW materials and fibers are still in its infancy, and it is still a challenge to transplant the planar device technology to the optical fiber tip. To face the demand for miniature sizes, and diverse integrated functions, the LOF forwards higher requirements for fabrication techniques and coupled materials. Besides, more photophysical process analysis is needed to fill the gap between the experimental results and the expectations. [29][30][31][32] As a representative of the vdW family, group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te) have been widely investigated. [33,34] The Mexican hat-type band structure normally occurs in the few-layer MX materials. When the singular density of states at the band edge and the two Fermi wavevectors reach the height of the hat, unique screening will occur. [35] Furthermore, the large wavevector scattering can be suppressed to reduce the activation energy, while the mobility shows an increasing trend. As a typical MX material, 2D GaSe has a field-effect mobility of about 215 cm 2 V À1 s À1 , higher than most chalcogenides. [36] Especially, GaSe exhibited a thicknessdependent optoelectronic property on the contrary to TMDs as well as near-infrared response based on size effect. Based on theoretical studies, it was predicted that the bandgap of GaSe may be widely tuned by inducing mechanical strain or changing the numbers of the layer, [37] that is flexible for photoelectric and ultrafast photonics device applications. The GaSe crystal has high damage threshold for different lasers, [38] which brings an opportunity for the preparation of LOFs. In consideration of its large nonlinear coefficient, [39] it is essential to figure out the nonlinear dynamics mechanism of GaSe and explore potential applications in fiber lasers. To sum up, GaSe possesses high carrier mobility, large screening properties, an adjustable optical bandgap, as well as nonlinear optical response characteristics. These excellent properties render GaSe a reliable vdW material candidate for fiber-integrated photodetector and saturable absorption devices. However, it's worth noting that the GaSe has a variety of structures, that is, β-, ε-, γ-, δ-type, and the phase selection is of great significance in the following study of ultrafast photonics and optoelectronics applications.
In this work, following a comprehensive study of different phases by density functional theory (DFT) calculations, the ε-phase GaSe is selected for its excellent optoelectronic and photonic properties. Then, the high-quality ε-GaSe single crystals obtained by chemical-vapor-phase mass transport is further exploited for LOF construction. The stable femtosecond (303 fs, 16.67 MHz, 51.64 dB) pulse is successfully generated in the ε-GaSe-based erbium-doped fiber laser. Moreover, the fiber-integrated photodetector based on ε-GaSe realizes multiband (520, 808, and 1550 nm) detection. These results indicate that GaSe was deemed to be a suitable alternative for optoelectronic materials and the GaSe-based LOF also shows great potential in optical fiber communication.

Phase Selection of GaSe
The photonic material is the fundamental component in the construction of LOF, such as saturable absorber (SA) for pulsed laser and channel material for photodetectors. Figure 1a presents a schematic presentation of ultrafast pulse generation using LOF. Herein, the large-scale nanomaterial is prepared as a mode locker via the deterministic transfer method. [40] The continuous wave (CW) laser is modulated by SA, constrained by intracavity, and finally converted into high-energy pulses with ultrashort duration. Generally speaking, materials with different energy band structures show different light absorption characteristics, as shown in Figure 1b; the left panel presents material with wide bandgap and is usually transparent to the laser. The right panel shows the absorption rate, which will reach saturation with increasing energy in narrow-gap material. Figure 1c shows a fiber-compatible photodetector that can detect light from the outside and inside the fiber. Therefore, it is critical to analyze the electronic states and potential optical properties of material to establish the structure-activity relationships.
GaSe is reported to have four phases, namely, ε, β, γ, and δ, which are separated by distinct stacking processes between layers ( Figure S1, Supporting Information). In order to validate the feasibility of GaSe for the construction of LOF, the DFT simulations are used to investigate the optical absorption process of different phases and thicknesses. Based on the simulation, the εand β-phases are calculated with bandgap of roughly 0.69 eV(for more details, see Figure S2, Supporting Information), inclined to be photosensitive at the communication band (1.55 μm), while the electrical band structures of the γand δ-phases have bigger values (1.41 and 0.95 eV). Meanwhile, the work functions of these phases in the monolayer and bulk structures are calculated, ε and β have lower work functions, so the ground state electrons are more likely to be excited to the upper level ( Figure S3, Supporting Information). Moreover, the interlayer binding energies of these phases are compared, the ε-phase has the highest value, and it indicate that ε-phase is more stable (more details in Table S2, Supporting Information), which agree well with the findings of other reports. [41] From room temperature to high temperature, the potential energy of ε-phase GaSe fluctuates little, indicating its stability ( Figure S4, Supporting Information). For its adequate band structures, low work function, and high damage threshold, ε-GaSe is an appropriate model material among numerous phases.
Figure 1d-f shows the electronic structure of ε-GaSe, it has a highly isotropic dispersion at the energy gap, that is, the dispersion at the conduction band minimum and the valence band maximum is the same along the G-M and G-K directions. However, the bottom of the conduction band has a much larger dispersion as compared to the rather flatbands of the top of the valence band, resulting in significantly anisotropic effective mass of carriers, and this ultimately leads to significantly higher mobility of electrons than holes (see Table S2, Supporting Information). The electron-hole mobility determines the rate of interband complexation during the relaxing process. The higher the mobility, the faster the electrons can complex with holes during interband relaxation and will lead to higher modulation depths. [42] With the increasing layers, the bandgap of ε-GaSe is converted from indirect to direct, which contributes to efficient absorption. In addition, the number of carriers increases significantly, which promotes the scattering of carriers and phonons. This will further shorten the carrier thermalization process and establish important conditions for self-start mode lock. Moreover, the Mexican-hat-shaped valence band in ε-GaSe makes the valence band electrons concentrated at a deeper energy level. With the increase of layer thickness, there are fewer electronic states in the A energy range and more electronic states in the B energy range (Figure 1f ), which is helpful to realize fast saturation. Even if the conduction band still exists in the unoccupied state, the deep-energy-level electrons under the B energy range cannot jump across the potential well to the conduction band under the excitation of 1550 nm laser (0.8 eV) (see more in Figure S5, Supporting Information). In view of this, we synthesized high-quality ε-GaSe for the subsequent optical experiments.

Synthesis and Characterization
The ε-phase GaSe with optimized thickness was used in the following experiments. The crystal was synthesized by a catalystfree chemical vapor transfer (CVT) approach (seen in schematic Figure 2a). The as-grown GaSe crystal is about 0.5 mm in length and the flakes are light reddish brown (shown in Figure 2b). The X-ray diffraction (XRD) pattern ( Figure 2c) of GaSe crystal presents a preferred direction along the (00 L) direction, which means that the preferred orientation of the crystal is along the c-axis. The diffraction peaks fit well with the hexagonal www.advancedsciencenews.com www.adpr-journal.com space group P-6 m 2 (187) (PDF # 37-0931). The interplanar spacing of (002) is calculated as %0.789 nm. Therefore, it can be judged that the material may be βor ε-phase (Table S1, Figure S1, Supporting Information). Raman spectra ( Figure 2d) were carried out on the ultrathin GaSe flakes and four Raman-active modes are found at %134, 213, 245, and 308 cm À1 , respectively, which match well with the previous literature. [43] A schematic diagram of the vibration modes is shown in Figure S6, Supporting Information. A 1g and A 2g represent planar vibration. E 1g and E 2g relate to the vibration of selenides in the direction out of plane. GaSe nanosheets with different thicknesses can be obtained by the mechanical thinning technique. Figure 2e shows the optical and atomic force microscopy (AFM) images of an exfoliated GaSe nanosheet after it is transferred onto the SiO 2 /Si substrate. The transferred nanosheet shows a triangular shape with a lateral size of %60 μm. The corresponding height profile indicates that the thickness is %8 nm. In Figure S7, Supporting Information, nanosheets with height %12, 20, 71, 82, and 130 nm were also prepared, each showing unique color contrast. These results show that GaSe has a weak interlayer force and is easy to be mechanically thinned, thereby making nanosheet fabrication extremely cost-effective. Figure 2f shows the scanning electron microscope (SEM) characterization diagram. The typical GaSe flake has a 60°corner. After energy-dispersive X-ray (EDX) measurement, it is found that the Ge and Se elemental ratio is 1:1 (Figure 2g). The identified stoichiometric ratio rules out other possible phases, such as GeSe 2 and Ge 2 Se 3 . As shown in Figure 3a,b the measurement of high-angleannular dark-field scanning transmission electron microscopy (HAADF-STEM) is performed to further study the atomic-scale structure of the GaSe nanosheets. Two orthogonal dissociation surfaces are prepared here, one is the fresh c-plane and the other is the cross section cut by the focused ion beam (FIB). The www.advancedsciencenews.com www.adpr-journal.com corresponding enlarged atomic resolution image (Figure 3c) further confirms the structure, which exhibits alternating bright and dull atomic sites due to the difference in the atomic numbers Z of Ga and Se. As shown in Figure S8, Supporting Information, the Se atoms are slightly brighter than Ga atoms due to the larger mass of Se. From the cross-section view of the GaSe sample, there is a periodic length of about 1.58 nm, corresponding to the lattice constant in the vertical direction (Figure 3d). Each unit layer with 0.8 nm thickness (called a tetralayer) is composed of four atomic layers, and a stacking sequence of Se-Ga-Ga-Se is defined as one layer. The atomic arrangement in both directions corresponds well with the simulated structure. This type corresponds to AB stacking sequence. The analysis demonstrates that the basic crystal structure of the as-grown GaSe is indexed to ε-phase ( Figure S1, Supporting Information). As shown in Figure 3f,g, the EDX mappings corresponding to the nanosheet (Figure 3e) confirmed the existence of elemental Ga and Se, respectively, which show that the GaSe nanosheet maintained compositional homogeneity after exfoliation.

Saturable Absorption and Pulse Generation
To further study its optical properties, the absorption spectrum of the materials is measured. As shown in Figure S9, Supporting Information, the GaSe nanosheet presents wideband absorption www.advancedsciencenews.com www.adpr-journal.com from 200 to 1700 nm. The nonlinear absorption is characterized by the balanced twin-detector technique with a homemade picosecond laser system (The central wavelength (λ) is 1550.02 nm, the pulse duration is 1 ps, and the repetition rate is 3.54 MHz). The output light from the laser is equally divided into a reference arm and signal arm by a 50:50 output coupler (OC). On the reference arm, the optical light from one port of the OC is measured directly with a power meter, whereas on the signal arm the light passes through the GaSe nanosheet, and the power of the transmission light is measured. As shown in Figure S10, Supporting Information, the transmittance of the GaSe will reach a saturated state with the increase of power density. The fit curve shows that the modulation depth (α s ) is %12.34% and saturation intensity (I s ) is %1.47 MW cm À2 . Figure S11, Supporting Information, shows the diagram of the GaSe-based passively mode-locked laser. It consists of 2.2 m erbium-doped fiber (EDF) as the gain medium, which is pumped by a 976 nm laser diode (LD) with a maximum power of 830 mW through a 980/1550 nm wavelength division multiplexer (WDM). A polarization-independent isolator (PI-ISO) was integrated by the GaSe-SA and EDF, ensuring unidirectional propagation. A polarization controller (PC) was used to control the polarization state. 20% of the intracavity light energy was extracted out for measurements by a 20:80 OC and the rest circulated in the laser cavity. The pigtails of all the components are SMF-28. The dispersion coefficient of the SMF-28 and EDF was À22 and 28 ps 2 km À1 , respectively. The total cavity length is 15 m, yielding a total cavity dispersion of À0.22 ps 2 . Meanwhile, the passively mode-locked pulse could be obtained when the pump power increased to 300 mW. As shown in Figure 4a-g, the performances of the passively mode-locked pulse are illustrated. According to the mechanism of mode-locking, a pulse output with multistage Kelly sidebands representing a soliton pulse was obtained (Figure 4a), and the central wavelength is 1559 nm.
As shown in Figure 4b, the pulse train with an interval of 60 ns is displayed. The interval does not change with pump power. As shown in Figure 4c, it can be seen that the output power increased linearly with the pump power, and the maximum output power was 6.47 mW as the maximum pulse energy was 0.389 nJ. Besides, the slope efficiency was 1%. When the www.advancedsciencenews.com www.adpr-journal.com pump power is 800 mW, the third harmonic pulse can be produced (Figure 4d). The changes of 3 dB bandwidth and central wavelength versus incident pump power are illustrated in Figure 4e. It could be found that the 3 dB bandwidth has no obvious changes and the central wavelength (around 1559 nm) also hardly changes with the pump power's increase. The radio frequency (RF) spectrum (Figure 4f ) with a resolution bandwidth (RBW) of 100 Hz at a range of 500 MHz verifies the stability of the mode-locked operation at a range of 23 MHz, and the SNR is measured to be 51.64 dB at the fundamental frequency of 16.63 MHz. The peak value of the frequency spectral line is almost kept at this horizontal line, which shows that the laser has high stability and there is no obvious Q-switch instability. The pulse width was evaluated utilizing an autocorrelation instrument (Pulse Check USB 50). As shown in Figure 4g, the pulse duration (t) is with the FWHM of 303 fs. Several representative 2D materials and relevant laser output characteristics in the near-infrared are listed in Table 1 to illustrate the output performance of GaSe-based EDFL. Other family materials, such as graphene, black phosphorus, topological insulator, MXene, etc, are taken into comparison. Also with similar category of sulfur compounds, GaSe-based pulse laser presents relatively outstanding advantages: pulse duration, repetitive frequency or stability. It indicates GaSe is a strong competitor as SA.

Fiber-Compatible Photodetector
The fiber-compatible GaSe photodetector was further fabricated by the mask evaporation process. Cr/Au (10/70 nm) electrodes were deposited by an electron beam evaporator with a customized copper mesh (bar width 35 μm, hole width 48 μm) as a mask. As shown in Figure 5a, the fiber end cap is used as a platform for preparing devices. Figure 5b shows that the fiber core was completely covered with a GaSe nanosheet. The thickness is estimated to be %70 nm according to the contrast. Then, electrodes are fabricated by mask process to form an optical fiber-integrated photodetector (Figure 5c). By the Agilent Technologies B1500A, the electrical measurements are operated at the probe station. As shown in Figure 5d, the I ph -V output curves fit well with linearity at various powers under the illumination of 520 nm laser, which illustrated that the channel material and electrodes were in good ohmic contact. In Figure 5e, under the illumination of 520 nm laser, the time-resolved photocurrent of the GaSe photodetector with the powers ranging from 0.64 to 22.9 nW is obtained. It is clearly observed that the ''ON" and ''OFF" currents are in good stability. As shown in Figure 5f, when the wavelength of illumination laser is 520 nm, the ideal factor (θ) can be inferred to be 0.83, which is close to 1. It could be found that the generated photocurrent mainly contributes to the absorbed photon, due to the photoconductive effect of GaSe photodetector. Besides, the ideal θ represents that the as-synthesized GaSe nanosheet has high crystalline quality without surface trap states. In order to precisely estimate the performance of the optoelectronic device, the results of responsivity (R) and detectivity (D*) are calculated by the following equations. [44] where P in represents the incident power of the laser, P d represents the effective power of the laser, and q represents the electronic charge. The values of responsivity and detectivity under irradiation with various wavelengths are given in Figure 5g, and the constructed GaSe photodetector with the highest responsivity of %17 mA W À1 decreases with the pump power. The highest detectivity is %11 Â 10 8 Jones and declines in a similar trend. It is worth noting that the photoelectric response appeared in the unprecedented detection band at 808 nm and 1550 nm (Figure 5h,i). The response time is in the order of tens of milliseconds, as recorded in Figure S12, Supporting Information. In other works, GaSe on silicon substrate shows layer-dependent bandgap; the bandgap can reach about 0.8 eV as the material is thick. [45] Hence, this GaSe-based LOF device has the potential for multiband and even infrared detection.

Conclusion
In summary, high-quality vdW GaSe crystals were successfully prepared with a CVT method. The GaSe nanosheets were synthesized by a direct exfoliation method conveniently. The stable femtosecond soliton pulse (%303 fs, 16.67 MHz, 51.64 dB) output is recorded in the GaSe-based erbium-doped fiber laser. Moreover, photodetection with near-infrared bands (808 & 1550 nm) is implemented with GaSe-based sensor, which further proves GaSe with excellent performance in fiber-compatible LOF. In all, this work not only provides more choices of materials for photoelectric and SAs, but also extends the Group III metal chalcogenides family for more applications including infrared optics and photonics.

Experimental Section
Simulation Method: The DFT calculations were performed using the Vienna Ab initio Simulation Package (VASP). [46,47] With VASP as a force www.advancedsciencenews.com www.adpr-journal.com constant calculator, the projector augmented wave (PAW) [48] and the generalized gradient approximation (GGA) [49] function were utilized to describe the electron-ion and electron-electron exchange correlation interactions, respectively. The Brillouin zone of multilayer (monolayer, bilayer, and so on) and bulk with different-phase GaSe primitive were grid into 16 Â 16 Â 1 and 16 Â 16 Â 6 k-points for structure optimization and electronic structure calculations. In order to optimize the crystal structures of the different phases, the planewave cutoff energy and strict convergence criterion were set to be 400 eV and 0.01 eV Å À1 , respectively. Besides, 2 nm vacuum layer was set to avoid lattice periodicity of atoms. The interlayer weak van der Waals forces had a strong influence on the interlayer distances of multilayer and bulk structures, while the van der Waals interactions cannot be described by Perdew-Burke-Ernzerhof function due to the dynamical correlation between fluctuations of charge distributions. To better describe the unbound interlayer interactions for multilayer and bulk structures of GaSe with different phases, a semiempirical dispersion potential (D) was added to the conventional Kohn-Sham DFT energy via Grimme's DFT-D2 approach for the dyadic force field. Chemicals and Synthesis: Among some mature preparation methods, [50,51] the CVT approach was adopted here for high yield and high optical quality. First, stoichiometric Se powder (Alfa aesar, 99.99%) and Ga droplets (Alfa aesar, 99.99%) with a total weight of 0.5 g and iodine (transport agent, %3 mg ml À1 ) were put into a quartz tube (OD: 20 mm, ID: 16 mm, L: 150 mm). Then, the sealed quartz tube suffered oxygen/hydrogen welding torch in high vacuum (%1 Â 10 À3 Pa). Then, the tube was heated at 850/800°C in a two-zone horizontal furnace for at least 1 week. Finally, the hexagonal brownish red flakes were obtained after cooling down to room temperature. The layered structure of fabricated GaSe crystal was easy to be cleaved to the nanosheet due to the weak interlayer coupling of vdW force.
Characterization: Raman measurements were carried out by the confocal Raman spectrometer (Renishaw inVia) excited by the 532 nm laser. The test of X-ray diffraction (XRD) was performed in the diffractometer system (Bruker AXS D8 Advance, λ = 1.5406 Å) under Cu Kα irradiation. SEM and EDX characterizations were implemented by an SEM instrument (Quanta FEG 250) with a spot line of 5.0 under the operating voltage of 30 kV. Thermo Scientific Themis ZS/TEM system was utilized to obtain HAADF-STEM images.
Fabrication of On-Fiber Device: First, the GaSe flake was thinned by mechanical exfoliation with blue tape. After that, the thinned GaSe nanosheet was adhered with homemade polydimethylsiloxane (PDMS) stamp. It was worth noting that the thickness and size of GaSe nanosheets could be roughly estimated due to the transparence of PDMS, which could be convenient to fix the target nanosheet to the fiber core with the three-axis cantilever. Finally, the PDMS stamp pressed against the fiber end face should be slowly peeled off.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.