Saturable absorption and visible pulse modulation of few-layer topological nodal-line semimetal HfGeTe

Qiming Zhao; Shouyan Zhang; Shuxian Wang; Gang Wang; Haohai Yu; Huaijin Zhang

doi:10.3788/COL202422.031601

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Abstract

Topological nodal-line semimetals attract growing research attention in the photonic and optoelectronic fields due to their unique topological energy-level bands and fascinating nonlinear optical responses. Here, to the best of our knowledge, we first report the saturable absorption property of topological nodal-line semimetal HfGeTe and the related pulse modulation in passively Q-switched visible lasers. Few-layer HfGeTe demonstrates outstanding saturable absorption properties in the visible-light band, yielding the saturation intensities of 7.88, 12.66, and 6.64 µJ/cm² at 515, 640, and 720 nm, respectively. Based on an as-prepared few-layer HfGeTe optical switch and a Pr:LiYF₄ gain medium, Q-switched visible lasers are also successfully achieved at 522, 640, and 720 nm. The minimum pulse widths of the green, red, and deep-red pulsed lasers are 150, 125.5, and 420 ns, respectively. Especially for the green and red pulsed laser, the obtained pulse width is smaller than those of the low-dimensional layered materials. Our work sheds light on the application potential of topological nodal-line semimetals in the generation of visible pulsed lasers.

Keywords

Q-switching Pr doping saturable absorption topological nodal-line semimetals visible pulsed laser

1. Introduction

Visible lasers are located in the visible-light band (380 nm to 760 nm) and manifest a wide range of important applications from scientific research to daily life, such as display, precision measurement, medical surgery, and optical communication^[1–5]. In the past few decades, the generation of visible lasers has mainly been based on the nonlinear frequency conversion strategy from mature infrared lasers^[6]. The invention of blue light-emitting diodes yielded the Nobel Prize in Physics in 2014 and made direct blue-laser-diode-pumped visible lasers possible^[7]. Afterward, Pr-, Tb-, and Dy-based visible lasers aroused widespread research interest because they match good absorption of blue laser diodes and compact laser resonator design^[8,9]. Along with the exploration of gain media, the continuous-wave blue-laser-diode-pumped visible lasers attain watt-level laser output^[10]. In addition, pulsed visible lasers also attracted growing attention owing to their large energy density and high peak power, especially for the passive pulse modulation on the strength of saturable absorption^[11–13]. Varieties of new-type visible saturable absorbers have been developed to enhance the relevant modulation properties^[14]. Nonetheless, although different material optimization strategies have been implemented in thermal conductivity, saturation recovery time, broadband response, saturation power intensity, and pulsed modulation depth, the modulation performance of those saturable modulation devices remains unsatisfactory in the visible-light band so far^[15].

Topological materials are one novel class of quantum materials with topological quantum states protected by space–time symmetry and have become a research hotspot in the fields of materials science and condensed matter physics^[16]. More strikingly, topological materials exhibit fascinating physicochemical properties when space or time symmetry is broken, such as anomalous Hall effects, topological phase transitions, and topological superconductivity^[17–19]. Topological insulators ( ${Bi}_{2} {Se}_{3}$ , ${Bi}_{2} {Te}_{3}$ ) and topological semimetals are also widely used as pulse laser modulators, but the modulation effect in the visible band is not satisfactory^[20]. For example, graphene, as one representative of topological materials, has attracted widespread attention over the past decades due to its excellent optoelectronic properties^[21]. Graphene owns zero bandgap, linearly dispersing Dirac points, excellent thermal conductivity, high carrier mobility, and a short saturation recovery time^[22–25]. Those enviable properties enable graphene to support broadband pulse modulation, which has been confirmed from the visible to the infrared regions. However, the absorption of monolayer graphene is only 2.3%, resulting in a low modulation depth^[25,26]. The increase in layer number will bring additional unsaturated losses, which is not beneficial for the modulation performance of related pulsed lasers.

With the exploration of topological materials, topological nodal-line semimetals have been proposed in recent years^[27]. The WHM-type (W = Hf, Zr; H = Ge, Si, Sn; M = Se, Te, O, S) topological nodal-line semimetals attracted great research interest due to their unique three-dimensional electronic band structure and novel physical phenomena, such as giant magnetoresistance, frequency-independent optical conductivity, and temperature-independent nodal-line plasmons^[28–30]. Their Fermi velocities are comparable to graphene ( $\sim 10^{6} m / s$ ), and the related carrier recovery time ( $\sim 0.6 ps$ ) is much faster than that of graphene ( $\sim 1.67 ps$ ), indicating their sensitive response capability^[25,31,32]. Moreover, the linear dispersion range near their Dirac points is approximately 2.5 eV, covering from the visible to the mid-/far-infrared bands^[33]. Recent studies have verified the remarkable nonlinear response capability of WHM-type topological materials in nonlinear frequency conversion^[34]. However, the saturable absorption and relevant pulse modulation of WHM-type nodal-line semimetals have not been studied so far. Here, we successfully prepared few-layer topological nodal-line semimetal HfGeTe and studied their saturable absorption properties in the visible-light region. On that basis, we performed relevant pulsed modulation at different visible wavelengths (522, 640, and 720 nm). The obtained results verified the application potential of the as-prepared few-layer nodal-line semimetal HfGeTe in the visible pulsed lasers, especially in the green-light band.

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2. Experiments and Methods

The HfGeTe single crystals (Hf:Ge:Te = 1:0.92:1) were grown by a high-temperature solution method^[35,36]. The obtained HfGeTe crystal was subjected to ultrasound in ethanol (30°C, 24 h) and centrifuged (6000 r/min, 5 min). The supernatant after centrifugation was directly spun onto quartz glass and silicon slice.

The morphology of the as-prepared HfGeTe samples was characterized by atomic force microscopy (AFM) (Dimension Icon, Bruker, Inc.). The Raman spectra of HfGeTe samples were characterized by a Raman spectrometer (iHR-550, Horiba, Inc.). The absorption spectra were investigated by a UV-visible-infrared spectrophotometer (UV-2600i, SHIMADZU, Inc.).

The nonlinear optical properties of HfGeTe/quartz glass samples were studied using the Z-scan technique. A 1030 nm femtosecond laser (200 fs, 100 kHz; BFL-1030-10H, Tianjin BWT, Inc.) equipped with a periodically polarized lithium niobate (PPLN) crystal was used as the pump source. The beam was focused by a convex lens ( $R = 50 mm$ ) and was irradiated onto the HfGeTe/quartz glass sample. The sample was placed on a computer-controlled linear mobile stage (M2DU-200 Stage, Data Ray, Inc.). The power transmitted through the sample was measured using a power meter (7N6153A sensor, Newport, Inc.).

As shown in Fig. 1, in the laser experiments a polished Pr:YLF laser crystal without coating ( $a$ cut, 0.43% doping concentration, 3 mm × 3 mm × 6 mm) was used as the gain medium. The input mirror (M1) and output coupling mirror (M2) were planar–concave mirrors with a radius of curvature of 50 mm. As shown in Fig. 1, the output average power was measured using the optical power meter (1919-R, MKS/Newport, Inc.). The pulse width and repetition rate of the pulsed laser were measured using an oscilloscope (TDS3012, Tektronix, Inc.).

Figure 1.Q-switched laser schematic design. Pr:YLF crystal, 444 nm; LD, LSR444SD, 3.5 W; M1, plane coating, R_444 nm < 0.1%, concave coating, T_444 nm > 99.9%, and R_{522 nm (640 nm or 720 nm)} > 99.9%; M2, concave coating, T_{522 nm (640 nm or 720 nm)} = 4%.

3. Results and Discussion

3.1. Characterization of HfGeTe

As shown in Fig. 2(a), HfGeTe belongs to one kind of PbFCl-type (space group $P 4 / nmm$ ) structure, and the related crystal growth is along the $c$ -axis direction. The cleavage plane is an Hf-Te layer, and the single-layer cell thickness is about 8.5 Å. As shown in Fig. 2(b), the bulk electronic band structures of HfGeTe were calculated based on density functional theory^[33,37,38]. It can be seen that the linear dispersion band region of the nodal-line Dirac points is relatively large (approximately 2.5 eV), which is conducive to the wideband absorption response covering from the visible regions to the mid-/far-infrared regions. As shown in Fig. 2(c), AFM was used to study the quality of the as-prepared few-layer HfGeTe sample. The surface was relatively uniform, and the related thickness was several nanometers. In addition, as shown in Fig. 2(d), two Raman spectrum modes existed in HfGeTe; one is ascribed to the motion in the $a b$ plane [ $E_{g}$ mode ( $99 {cm}^{- 1}$ )], and the other is mainly due to the out-of-plane motion along the $c$ axis [ $A_{1 g}$ modes (119.5 and $137.3 {cm}^{- 1}$ )], which are consistent with the reported Raman vibrations of HfGeTe^[33], indicating that the ultrasonic-treated sample still maintains the crystal structure of HfGeTe.

Figure 2.(a) Crystal structure of HfGeTe; (b) bulk electronic structure of HfGeTe without spin-orbit coupling (SOC)^[32,36,37]. Two kinds of Dirac points: red circle, a diamond-shaped Dirac nodal line; blue square, a quartic degenerated Dirac point. (c) Atomic force microscopy morphology of the HfGeTe/silicon slice; (d) Raman spectrum of the HfGeTe/silicon slice and HfGeTe bulk crystal.

The saturable absorption mechanism at the Dirac points is described in Fig. 3(a). The electrons in the valence band are continuously excited into the conduction band, yielding a saturable absorption property. Finally, the Pauli exclusion principle blocks the interband transition, and the absorption reaches the saturation state. As shown in Fig. 3(b), the absorption spectrum of the as-prepared HfGeTe sample with quartz glass substrate was measured by comparing the absorption spectra of quartz glass. Compared with quartz glass, the as-prepared sample exhibits obvious absorption in the visible-light region. From that figure, it can be observed that the as-prepared HfGeTe/quartz glass sample has a wide-range response, from 300 to 1000 nm. This wide absorption range is attributed to the extremely narrow bandgap of HfGeTe, which can be used as broadband saturable absorbers modulators for the visible-light band. To analyze the relationship between saturable absorption and wavelength, three different wavelengths (515, 640, 720 nm) were obtained by a 1030 nm femtosecond laser and a PPLN crystal. The saturable absorption properties of HfGeTe/quartz glass samples at those three different wavelengths were studied. The saturable absorption process was studied by a two-level analysis model, and the relationship of transmission versus power density can be described as^[39] $T = A \exp (\frac{- δ T}{1 + I / I_{sat}}),$ (1)where $T$ is the normalized transmittance of the experimental sample, $I_{sat}$ is the saturation absorption intensity, $δ T$ is the absolute modulation depth of the experimental sample, $I$ is the power intensity of the incident laser, and $A$ is the constant of the experimental data. As shown in Fig. 3, the saturable absorption properties of the HfGeTe sample were measured at three different visible wavelengths (515, 640, 720 nm), and the experimental data were fitted using Eq. (1). The fitting results indicate that the modulation depth $δ T$ of the HfGeTe sample is 6.0% at 515 nm, 7.9% at 640 nm, and 5.9% at 720 nm. And the calculated saturation power intensity $I_{sat}$ is $7.88 μJ / {cm}^{2}$ at 515 nm, $12.66 μJ / {cm}^{2}$ at 640 nm, and $6.64 μJ / {cm}^{2}$ at 720 nm. The relatively larger absolute modulation depth and saturation intensity at 640 nm compared with the saturable absorption performance at 515 nm and 720 nm suggest that the HfGeTe sample might have better modulation potential at this wavelength.

Figure 3.(a) Schematic diagram of nonlinear saturable absorption at the Dirac point; (b) absorption spectra of the HfGeTe/quartz glass and quartz glass; relationship between transmittance and optical intensity at (c) 515 nm, (d) 640 nm, and (e) 720 nm, respectively.

3.2. $Q$ -switched pulsed laser with saturable absorber HfGeTe

The output average power values of three different wavelengths versus pump power are shown in Fig. 4 (the inset is the lasing spectrum). The maximum $Q$ -switched output power obtained is 28 mW (522 nm), 25 mW (640 nm), and 16 mW (720 nm), respectively. As shown in Fig. 4, for the three lasing wavelengths, their pulse repetition frequency shows a gradually increasing trend, while their pulse width shows a declining trend. The shortest pulse widths at three lasing wavelengths are obtained as the related output power reaches the maximum value, corresponding to the relevant largest repetition frequency. As shown in Fig. 5(a), the minimum pulse width of the 522-nm laser is 150 ns, which is smaller than those of ${Co}^{2 +} : {MgAl}_{2} O_{4}$ (205 ns), $V^{3 +} : YAG$ (440 ns), and ${MoS}_{2}$ (579 ns)^[40-42]. As shown in Fig. 5(b), the smallest pulse width of the 640-nm laser is 125.5 ns, which is smaller than those of low-dimensional materials [ ${MoS}_{2}$ (227 ns), ${Bi}_{2} {Se}_{3}$ (210 ns), monolayer graphene (709 ns), etc.]^[43–45]. As shown in Figs. 4(f) and 5(c), the minimum pulse width of the 720-nm laser is 420 ns, which is larger than that of ${Bi}_{2} {Se}_{3}$ (368 ns) and CdTe/Cds QDs (235 ns)^[44,46]. In addition to the single-pulse waveforms of three different $Q$ -switched lasers presented in Figs. 5(a)–5(c), the relatively uniform 640-nm pulse train in Fig. 5(d) further indicates the excellent modulation performance of the HfGeTe saturable absorber.

Figure 5.(a) Pulse profile at 522 nm under a pump average power of 3.74 W; (b) pulse profile at 640 nm under a pump average power of 2.87 W; (c) pulse profile at 720 nm under a pump average power of 2.43 W; (d) pulse train of 640-nm pulsed laser under a pump average power of 2.87 W.

Figure 4.Average output power versus pump power at (a) 522 nm, (c) 640 nm, and (e) 720 nm. The inserted figure is the related lasing spectra. Change of pulse width and pulse repetition frequency of the (b) 522-nm, (d) 640-nm, and (f) 720-nm Q-switched lasers.

As for $Q$ -switched processes, the relationship of the pulse modulation depth $δ T$ and the pulse width $τ_{p}$ could be represented by an analytical model of semiconductor saturable absorbers^[47], $τ_{p} = \frac{3.25 T_{R}}{δ T},$ (2)where $T_{R}$ is the round-trip time. As measured in the above section, the saturable modulation depth of the HfGeTe sample was 6.0%, 7.9%, and 5.9% at the three different lasing wavelengths (522, 640, and 720 nm), respectively. In addition, the laser cavity of the three passively $Q$ -switched lasers shows almost no change (145 mm). Based on the experimental data in Fig. 3(b) and Eq. (2), it can be concluded that the pulse modulation of the HfGeTe saturable absorber would demonstrate the smallest and largest pulse widths at 640 and 720 nm, respectively. The aforementioned laser results are consistent with the theoretical analysis. In addition, with the pulse repetition rate, pulse width, and output average power, the values of output pulse energy and output peak power can be calculated. As shown in Fig. 4, the maximum repetition frequency is 540 kHz (522 nm), 536 kHz (640 nm), and 187 kHz (720 nm), respectively. The maximum pulse energy is 51.9 nJ (522 nm), 46.6 nJ (640 nm), and 85.6 nJ (720 nm), respectively, which is comparable to the reported pulse energies of most two-dimensional materials. The output peak power corresponds to 0.35 W (522 nm), 0.37 W (640 nm), and 0.2 W (720 nm), respectively.

As Fig. 6(a) shows, the 640-nm saturation energy density of HfGeTe is $12.66 μJ / {cm}^{2}$ , which is comparable to those of semiconductor saturable absorber mirrors (SESAMs) but lower than those of some low-dimensional saturable absorbers. With respect to $\Pr^{3 +}$ -based red pulsed lasers shown in Fig. 6(b), the as-prepared HfGeTe saturable absorber has a better modulation performance than those low-dimensional saturable absorbers in red-light bands even if under the close modulation depth ( $\sim 10 %$ ). Moreover, few-layer nodal-line semimetal HfGeTe also yields a better modulation performance than those ${Co}^{2 +}$ - and $V^{3 +}$ -doped bulk crystals from the aspect of pulse width. Such a result verifies the application potential of this class of topological nodal-line semimetals in visible pulse modulation. We believe that the modulation performance of such kinds of saturable absorbers in the visible-light band can be further improved by optimizing material preparation and device design.

Figure 6.(a) Saturation energy density of several low-dimensional saturable absorbers; (b) pulse widths of Pr-based red pulsed lasers based on different saturable absorbers; the numbers correspond to Refs. [42–46,48–56] (42, MoS₂; 43, WS₂; 44, Bi₂Se₃; 45, graphene; 46, CdTe/CdS quantum dots; 48, graphene oxide colloids; 49, Au nanorods; 50, few-layer MXene Ti₃C₂T_x; 51, 1T-titanium selenide; 52, Bi₂Se₃; 53, black phosphorus; 54, black phosphorus; 55, graphene oxide; 56, graphene).

4. Conclusions

In summary, we prepared few-layer nodal-line semimetal HfGeTe and studied its saturable absorption properties in the visible-light band. Then, green (522 nm), red (640 nm), and deep-red (720 nm) pulsed visible lasers were successfully achieved using a few-layer HfGeTe saturable absorber. The as-prepared HfGeTe saturable absorber had a better modulation performance in green- and red-light bands. Especially for the green pulsed laser, the obtained minimum pulse width was 150 ns, which is superior to ${Co}^{2 +}$ - and $V^{3 +}$ -doped crystals and low-dimensional materials. To our knowledge, this is the first pulse modulation report of the topological nodal-line semimetal HfGeTe optical switch. Our obtained results indicate that the topological nodal-line semimetals have promising applications in the generation of visible pulsed lasers, which will broaden their application scope in the fields of photonics and optoelectronics.

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