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Abstract
Germanene is an analog of graphene, and its independent novel low-bending honeycomb structure gives outstanding advantages such as environmental stability and significant low-frequency optical absorbance. In this paper, the few-layer germanene was successfully prepared by the liquid phase exfoliation method. The saturable absorption characteristics of germanene in the infrared waveband were detected by the open-aperture Z-scan method. With germanene as a saturable absorber, a high-performance passively Q-switched bulk laser was realized at 1.9 µm. The shortest pulse width of 60.5 ns was obtained from continuous-wave pumping, corresponding to a single pulse energy of 6.7 µJ and peak power of 110 W. By utilizing the pulse pumping style with a repletion rate of 10 Hz, the single pulse energy and peak power increased to 45.8 µJ and 328 W, respectively, which exceeded all two-dimensional SA materials reported before. This research manifests that germanene is an excellent SA material for mid-infrared solid-state lasers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, the pulsed laser has been widely used in industry, medicine, military, and scientific research because of its special output mode and high output power. Passive Q-switched technology is an effective means to obtain pulsed laser, with simple structure, easy adjustment, easy amplification, low cost, and other advantages. It takes advantage of the nonlinear absorption effect of material on light to automatically realize the abrupt change of laser Q value and obtain laser pulses with short duration and high peak power, so exploring high-quality saturable absorbing materials is a vital link to realize passively Q-switched (PQS) operation. Dye saturated absorbers were initially used, but they had natural disadvantages, such as poor stability. They were quickly replaced by traditional saturable absorbers such as SESAMs, carbon nanotubes, and Cr:YAG crystals. In 2004, the successful exfoliation of graphene brought a new life to two-dimensional (2D) saturable absorbers. They have the advantages of a broad working band, a simple preparation process, and low cost. Since then, topological insulators [1] and transition metal basalt-based compounds [2] have become a global research hotspot in electronic and optoelectronic devices with remarkable physical and chemical properties. Tremendous research progress has been made on the group III, IV, and V elements represented by black phosphorus (BP) [3,4]. Its electronic band gap can be adjusted according to its thickness. Still, BP samples are volatile in the atmosphere and a single or few layers of BP may degrade within a few hours, which significantly limits its application [5]. In 2012, silicene, an analog of graphene, was successfully synthesized [6]. In 2019, few-layer silicon nanosheets were employed in an all-solid-state passive Q-switched laser, and pulsed lasers were obtained at 0.9, 1.06, and 1.34 um, respectively [7]. As early as 2009 [8], Cahangirov et al. predicted that germanium (Ge) is another cousin of graphene besides silicon, and they belong to the same 2D materials that accommodate Dirac fermions. Unlike some 2D nanomaterials with single-atom thickness planar structures prepared and studied earlier [4,9], germanium in group IV has outstanding advantages such as stability, low synthesis cost, and significant low-frequency optical absorbance due to its independent novel low-bending honeycomb structure [6]. The earliest 2D Ge was reported in 2014 [6]; the ordered 2D multiphase Ge films were grown in situ by germanium molecular beam epitaxy using the surface of gold (111) as a substrate. The growth pattern was similar to silicene on the silver (111) template. Germanene has a large spin-orbit gap, predicted to be up to 43 MeV, which is much larger than silicene (3.9 MeV) [10]. Secondly, germanene is a semiconductor with a tiny energy band gap of 0.76 eV, which can be controlled by a vertical electric field [10]. In addition, germanene exhibits a larger optical damage threshold of 0.2 J/cm2 than that of 0.014 J/cm2 for graphene, allowing for higher-power laser operation [11,12]. It is well known that graphene is unsuitable for use in electronic devices in the semimetal form with gapless [13]; germanene solves the energy band problem encountered in graphene by opening the band gap in the electronic band structure with its strong intrinsic spin orbit interaction [2]. Matthes and colleagues noted that the spin-orbital effect of germanene also plays a crucial role in its optical absorbance in the low-frequency region [14]. In recent years, the nonlinear properties of germanene have been explored in the near-infrared and mid-infrared bands by open-aperture Z-scan technology, which further demonstrates the excellent saturable absorption properties of germanene and its potential for applications in ultrafast nonlinear optics [15,16]. In 2021, Haoran Mu et al. found that germanene has fast carrier relaxation times and large nonlinear absorption coefficients in the near-infrared wavelength range, an order of magnitude higher than graphene, and achieved stable soliton mode-locking at 1550 nm in the fiber laser cavity [17]. Germanene has a smaller band gap than conventional SiGe SA, allowing mode-locked fiber lasers to operate at longer wavelengths. It can also be used as a SA for Er-doped fiber lasers [18]. In 2022, Ruin Zhao et al. achieved mode-locked pulses with a repetition rate of 6.66 MHz in a conventional dispersive EDF laser, demonstrating that germanene has excellent optical properties [18] even in the normal dispersion state. However, by time now, no experimental studies have been reported using germanene as a saturable absorber (SA) for passive Q-switched solid-state lasers.
In this paper, germanene with a large area, high quality, and few layers was prepared from germanium bulk using the liquid phase exfoliation (LPE) method. The saturable absorption properties of germanene in the 1000-2500 nm range were verified using the open-aperture Z-scan technology. High performance 1.9 µm PQS bulk laser was realized with germanene as a SA, which exhibited the narrowest pulse width and the highest peak power among all two-dimensional materials at this waveband.
2. Preparation and structural characterization of germanene
The germanium blocks were prepared into the few-layer germanene by the LPE method. Weighing 50 mg of block germanium with a purity of 99.99% and a small amount of anhydrous ethanol into a mortar, grind the mixture for 6 hours, encapsulate it in a centrifuge tube, and put it into an ultrasonic machine for 8 hours of ultrasonic pulverization. Stand the dispersion solution for two days to remove the precipitated particles. A small amount of the supernatant was added dropwise to the sapphire substrate and dried to obtain the germanene SA. The remaining supernatant was encapsulated in a centrifuge tube and used for sample characterization.
The crystal structure of germanium nanosheets was characterized by X-ray diffraction (XRD), as shown in Fig. 1(a). Seven characteristic peaks match the literature [13], which are (111), (220), (311), (400), (331), (422), and (511), and the XRD patterns exhibit sharp diffraction peaks. This means that the germanene produced has a highly crystalline structure.
Fig. 1. (a) XRD pattern of the germanium. SEM image of (b) layered germanene crystal powder and (c) exfoliated germanene nanosheets. (d) (e) AFM image of the germanene nanosheets and the height profile. (f) Raman spectra of germanene crystal powder, few-layer germanene, and few-layer germanene after 3 months.
The morphology of the prepared germanium powder and germanium nanosheets was observed using field emission scanning electron microscopy (SEM, Hitachi S-4800), as shown in Figs. 1(b) and (c). The germanium powder exhibits a distinct lamellar structure (red arrow) at the edges of the cross-section, with dimensions of only a few microns, as shown in Fig. 1(b). Figure 1(c) shows that the germanene has a sheet-like morphology, which indicates that the van der Waals forces [19] between the layers within the germanium are weak, allowing the germanium with fewer layers to be successfully exfoliated with a transparent morphology.
The surface morphology and thickness information of the germanene were further analyzed using atomic force microscopy (AFM, Nanoscope Multi Mode V, Digital Instruments/Bruker systems), as shown in Figs. 1(d) and 1(e). The average thickness of the germanene is about 20 nm, and the number of layers of the prepared germanene is about 54 due to the thickness of a single layer of germanium of 0.37 nm [17].
To further confirm that the experimental samples are few-layer germanene and to study the structure of germanene, we measured the Raman spectra (excitation wavelength: 632 nm) of germanium powder on glass substrates and germanene, as shown in Fig. 1(f). For germanium powder, the characteristic Raman peak is observed at 293.2 cm−1, corresponding to the double degenerate E2g in-plane vibrational mode [20], and the red curve represents the test results. Compared with germanium powder, the peak of germanene is significantly blue-shifted to 299.2 cm−1 as shown in the blue curve in Fig. 1(f), indicating a smaller thickness of germanene [16,21]. The green curve in Fig. 1(f) shows the Raman spectrum of the few-layer germanene kept in the air for three months, and it can be seen that the peak is unchanged compared with that of the fresh germanene (blue curve). This phenomenon proves that the few-layer germanene has excellent environmental stability. In addition, no noticeable defect peaks were found during the measurement.
3. Linear optical properties of germanene
The linear transmission spectra of the prepared germanene in the 200 - 2000nm range were recorded using a UV/VIS/NIR spectrophotometer (U-3500, Hitachi, Japan), as shown in Fig. 2. For comparison, we recorded the transmittance of the blank sapphire substrate (green curve) under the same conditions. It can be seen from the figure that the UV absorption edge of the sample is less than 200 nm. In the range of 250 - 2000nm, the intrinsic absorption of the sapphire substrate changes little, and the transmittance of germanene increases smoothly with increasing wavelength (65%−75%). The transmittance at 1.9 µm was 73.9% and 88.9% for the sapphire sheet substrate and germanene, respectively. Thus, the net transmittance of germanene is about 83.1% and the scattering loss is 16.9%. The smooth absorption properties of germanene in the UV to MIR range suggest that germanene can be used as a broadband nonlinear optical material.
Fig. 3. Open-aperture Z-scan results of germanene SA at (a) 1000 nm, (c) 1300 nm, (e) 1900nm, and (g) 2500 nm. The variations in the transmittance with incident intensity at (b) 1000 nm, (d) 1300 nm, (f) 1900nm, and (h) 2500 nm.
4. Nonlinear optical properties of germanene
Open-aperture Z-scan experiments were performed at 1000 nm, 1300 nm, 1900nm, and 2500 nm, respectively, to investigate the nonlinear optical properties of the prepared germanene in the infrared band. The laser source was an OPO laser (Horizon mid-band Continuum Inc, America) with a pulse duration of 7 ns and a repetition rate of 10 Hz. Since the pulse width of the pulsed laser obtained in the PQS laser experiment is on the order of nanoseconds, the use of nanosecond laser to characterize the nonlinear optical properties of germanene is of more reference value for its application in PQS laser. Under the same experimental conditions, the blank sapphire substrate was tested without nonlinear absorption, so the nonlinear optical effects observed in the experiment were only from the germanium coating. Figure 3 shows the results of Z-scan experiments on germanene at different wavelengths. The normalized Z-scan transmittance increased with the incident light intensity density increase and showed prominent positive peaks at Z = 0. These phenomena indicate that the prepared germanene has obvious saturable absorption properties in the infrared band, that is, single-photon absorption.
The energy band structure of germanium is shown in Fig. 4. As an indirect band gap semiconductor material [15], its conduction band bottom and valence band top are located at different positions in the K-space, meaning that forming a half-full energy band requires absorption of energy but also a change of momentum. In the process of electron transition, the evolution of the K value leads to a great probability that the electron will release energy to the lattice and convert it into phonons; that is, the electron absorbs photons accompanied by absorbing or emitting phonons, the photons provide the energy required for the electron transition, and the phonons give the momentum. In the experimental process, the germanene sample is affected by the incident laser; electrons in the valence band absorb photons and phonons and transition to the conduction band [16,22]. When the sample is close to the laser focus, the intensity of the incident laser increases. In the strongly excited state, the conduction band is gradually filled with electrons, and the energy transition process of valence band electrons is blocked. According to the Pauli blocking principle [23–25], the absorption reaches saturation at this point and the remaining photons pass freely without being absorbed, increasing the transmittance of the sample. The excited state reverts to the ground state through thermal stabilization and compounding processes so that photons can be absorbed again.
In the open-aperture Z-scan experiments, the normalized transmittance of the sample is defined as T = (1 - αl)/(1 - α0 l) [26].
where α is the total absorption coefficient, α0 is the linear absorption coefficient, and l is the thickness of the sample. Equation (1) is the calculation method of the total absorption coefficient α [27,28], where Is is the saturation intensity and β is the nonlinear absorption coefficient. I(z) is calculated from Eq. (2), where I0 is the power density at the focal point, Z0 is the Rayleigh diffraction length of the beam, and Z is the position coordinate of the sample. The above equation is transformed to obtain Eq. (3). The normalized transmittance of the sample obtained in the open-aperture Z-scan experiment is fitted using Eq. (3), as shown in Fig. 3 (fitted curve). The fitted nonlinear optical parameters such as saturation intensity (Is) and nonlinear absorption coefficient (β) are summarized in Table 1, and several typical 2D nonlinear SAs are listed for comparison. The imaginary part of the third-order nonlinear optical magnetization Imχ(3) can be expressed as [29]:
Table 1. NLO parameters of different 2D SA materials at nanosecond exciting conditions.
It is worth noting that germanene has lower saturation strength IS and larger nonlinear absorption coefficient β than other typical two-dimensional (2D) SA materials such as graphene, BP, MoS2, Te, and hexagonal boron nitride (h-BN), and the advantages are more obvious at 1900nm and 2500 nm. In addition, the third-order nonlinear optical magnetization Imχ(3) of germanene at 1000 nm, 1300 nm, 1900nm, and 2500 nm are − (6.1 ± 2.67) × 10−8 esu, − (1.44 ± 0.1) × 10−7 esu, − (1.12 ± 0.28) × 10−6 esu, and − (1.93 ± 0.42) × 10−6 esu, respectively, which are four orders of magnitude higher than BP. In summary, germanene has the significant advantages of a large saturable absorption coefficient in the infrared wavelength band, low saturation intensity, fast response speed, and stable physical and chemical properties, which is especially suitable for PQS laser operation with high repetition rate, narrow pulse width, and high peak power in the mid-infrared laser, and can work stably for a long term.
5. PQS operation of germanene in 1.9 µm all-solid-state laser
A concave-flat laser cavity at 1.9 µm was built to investigate the PQS properties of germanene. The experimental setup is shown in Fig. 5(a). The laser crystal is a b-cut Tm:YAP crystal with a doping concentration of 3 at.% and dimensions of 3 × 3 × 5 mm3. For better heat dissipation and to keep the laser output stable, the laser crystal is wrapped with indium foil and then loaded into a copper block, cooled by circulating water at a fixed temperature of 15°C. The laser resonant cavity comprises a concave mirror M1 and a flat mirror M2. M1 has a radius of curvature of R = 100 mm and is coated with a dielectric film with anti-reflective to 793 nm and highly reflective to 1.9 µm wavelength, and M2 is coated with a dielectric film with a transmittance of 5% for 1.9 µm wavelength. The laser resonant cavity length is 22 ± 2 mm for all laser experiments. The average output power is measured by a photodiode detector (Model 1621, New Focus, response time 1 ns, and decay time 1.5 ns). The laser pulse characteristics are recorded by a digital oscilloscope (DPO4000, Tektronix Inc.).
The pump source is a fiber-coupled QCW laser diode with a central wavelength of 793 nm, a core diameter of 105 µm, and a numerical aperture of 0.22. The experiments were carried out in three pump modes respectively, i.e. continuous-wave (CW), quasi-continuous-wave (QCW, repetition rate fp = 1000 Hz), and pulse-wave (repetition rate fp = 10 Hz). For all pump modes, stable laser operations could be obtained whether germanene SA was inserted into the resonator or not, and the polarization direction of the laser is horizontal. As shown in Fig. 5(b), the central wavelength of the PQS laser is 1931.4 nm. The detailed experimental results are described as follows.
5.1 CW pumping
The performance of the 1.9 µm PQS laser output based on CW pumping is shown in Fig. 6. The continuous laser output of 4.33 W is obtained at the absorbed pump power of 10.07 W, with an optical-optical conversion efficiency of 43%. As seen in Figs. 6(a)–6(c), the absorbed pump power threshold of the PQS pulsed laser output is 1.65 W. With the absorption pump power increase, the output power increases almost linearly, the pulse duration is shortened from 530.2 ns to 60.5 ns, and the pulse repetition rate increases from 57.95 kHz to 188.2 kHz. When the absorbed pump power is 10.07 W, the average output power of pulsed laser Pout reaches a maximum value of 1.26 W, corresponding to an optical conversion efficiency of 12.5%. The single pulse energy is 6.7 µJ, and the peak power is 110 W. As the increase of pump energy further, the pulsed laser output tends to saturate, and the laser performance decreases with the adjoint pulse generation. When the pump power is reduced to the interval of regular operation (2-10 W), the laser output returns to stability, indicating that the germanene SA is not damaged. Figures 6(d)–6(g) show the PQS pulses trains at different absorbed pump powers and the single-pulse profile at maximum absorbed pump power. The laser has a neat pulse sequence and stable performance in the absorbed pump range of regular operation.
Fig. 6. Performance of 1.9 µm PQS laser based on CW pumping. (a) Output power. (b) Pulse width and pulse repetition rate. (c) Single-pulse energy and peak power. (d), (e), (f) Pulse sequences at different absorbed pump powers. (g) single-pulse waveform at the maximum absorbed pump power.
5.2 QCW pumping (fp = 1000 Hz)
The output performance of the 1.9 µm PQS laser based on the QCW pump with a pump pulse width of 300 µs and a repetition rate fp of 1000 Hz is shown in Fig. 7. The number of PQS pulses (NPP) and pulse width (PW) in each pump pulse were measured during the experiment. The total number of pulses per unit of time (TNP) is the product of the repetition rate of the pump source and the NPP. The single pulse energy (Eout) is calculated by Pout/TNP, and the peak power Ppeak is calculated by Eout/PW. The thresholds of absorbed pump power for QCW operation and the PQS operation are 0.87 W and 1.12 W, respectively. When the absorbed pump power is 5.6 W, the QCW laser output is 1.54 W, and the PQS laser output is 0.65 W. The corresponding optical conversion efficiencies are 27.5% and 11.6%, respectively. As shown in Figs. 7(b) and 7(c), the pulse width decreases with the increase of the absorbed pump power; the shortest pulse width is 122 ns, and the number of PQS pulses under a single pump pulse increases from 9 to 42, and the calculated maximum single pulse energy and peak power are 15.6 µJ and 128 W, respectively. Figures 7(d)–7(f) show the PQS sequences under a single pump pulse with different absorbed pump power, and Fig. 7(g) shows the single pulse profiles at the maximum absorbed pump power. The pulsed output is stable over the entire absorption pump power range. The experimental results of such stable and smooth PQS pulses show that the germanene SA has good modulation capability at 1.9 µm.
Fig. 7. Performance of 1.9 µm PQS laser based on QCW pumping (fp = 1000 Hz). (a) Output power. (b) Pulse width and the number of pulses under each pump pulse. (c) Single-pulse energy and peak power. (d), (e), (f) Pulse sequence under a single pump pulse at different absorbed pump powers. (g) The single-pulse waveform at the maximum absorbed pump power.
5.3 Pulse pumping (fp = 10 Hz)
Under a pulse pumping mode with a pump pulse width of 300 µs and a repetition rate of 10 Hz, the pulse sequence under a single pump pulse and the single-pulse waveform of the 1.9 µm PQS laser are shown in Fig. 8. When the absorbed pump power is 71 mW, the PQS output power is 7.78 mW, corresponding to an optical conversion efficiency of 11.0%. The pulse width is 139.3 ns, the number of PQS pulses under a single pump pulse is 17. The calculated single pulse energy and peak power are 45.8 µJ and 328 W, respectively. During the experiment, the pulse output is neat and stable.
Fig. 8. Performance of 1.9 µm PQS laser based on pulse pumping (fp = 10 Hz). (a) Pulse sequence under a single pump pulse. (b) Single-pulse waveform.
Table 2 summarizes the characteristics of 1.9∼2 µm PQS lasers based on several typical 2D materials. Obviously, PQS lasers based on germanene SA have characteristics of narrow pulse width, large pulse repetition rate, and high peak power. In CW pumping mode, the PQS laser has the smallest pulse width of 60.5 ns, the largest pulse repetition rate of 188.2 kHz, and a peak power of 110 W that second only to the best result from BP. For pulse pumping mode with a repetition of 10 Hz, the pulse width, single pulse energy, and peak power reach 139 ns, 45.8 µJ, and 328 W, respectively, which are superior to all experimental results reported before. Dislike BP, the property of germanene SA is very stable. It can be reused after 3 months at room temperature without performance degradation.
Table 2. Characteristics of 1.9∼2 µm PQS lasers based on typical 2D SA materials
5. Conclusions
In conclusion, few-layer germanene was prepared by LPE method. The nonlinear properties of the germanene were characterized using the open-aperture Z-scan technology, and the nonlinear absorption coefficients were determined to be – 0.16 ± 0.07 cm MW−1 at 1000 nm, – 0.29 ± 0.02 cm MW−1 at 1300 nm, – 1.55 ± 0.39 cm MW−1 at 1900nm, and – 2.02 ± 0.44 cm MW−1 at 2500 nm. Based on germanene SA, the PQS laser output was achieved at 1.9 µm using three different pumping modes. For CW pumping, we obtained the shortest pulse width of 60.5 ns and the largest pulse repetition rate of 188.2 kHz. For pulse pumping with a repetition of 10 Hz, we achieved the largest single pulse energy of 45.8 µJ and the highest peak power of 328 W. By comparison, it can be known that they are the best results among 1.9 µm PQS solid-state lasers based on 2D material SA. This work shows that germanene is an excellent broadband SA material for infrared PQS laser, which is good at short pulse width, large repetition rate, and high peak power operations.
Funding
National Natural Science Foundation of China (61975096).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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