Graphene/α-In₂Se₃ heterostructure for ultrafast nonlinear optical applications

1. Introduction

In parallel with the research wave of graphene and graphene-like 2D materials, 2D heterostructures have attracted great attentions over the past decade. 2D heterostructure can be artificially assembled by stacking two 2D materials with radically different chemical compositions, band structures or lattice orientations on top of each other [1,2]. The coupling effect and the interfacial charge transfer directly affect their optoelectronic properties, and thus govern the optoelectronic performance of the heterostructure devices [3,4]. Due to their exotic optical functionalities, plenty of photoactive applications using 2D heterostructure have been reported, including photodetectors, photovoltaic solar cells and light-emitting diodes, etc [5–7]. Among them, the ultrafast photonic applications of 2D heterostructure have attracted increasing attention, and the related ultrafast nonlinear optical properties (including enhanced saturable absorption, ultrafast photoresponses, large modulation depth, low saturable loss, etc.) have been widely investigated [8–17]. In 2015, Mu et al. showed that the modulation depth of graphene/Bi₂Te₃ heterostructure is much higher than monolayer graphene, and both the photocarrier dynamics and the nonlinear optical modulation could be tunable by changing the coverage of Bi₂Te₃ on graphene [8]. Afterwards, the MoS₂/graphene nanocomposites were fabricated and investigated by Jiang et al. [9]. Their experiments indicate that both the advantage of ultrafast electron relaxation, broadband response from graphene, and the advantage of strong light-matter interaction from MoS₂, can be integrated together by composition. Subsequently, a large number of studies have focused on the ultrafast nonlinear optical properties of different kinds of heterostructure and their related applications [10–14,16,17]. These works reveal that the heterostructure technique could combine the advantages of different 2D materials, hence realize novel 2D materials with optimized optoelectronic properties. Besides, it can provide a flexible platform for reaching tunable optical properties by stacking different 2D materials, modifying the thickness of material or tuning the coverage of the materials. It should be noticed that in 2019, few-layered $\alpha $-In₂Se₃, a III-VI compound with large effective nonlinear absorption coefficient (β_eff ∼ −3.9 × 10³ cm/GW), has been demonstrated as a promising saturable absorber (SA) in ultrafast all-solid-state bulk lasers by Sun et al. [15]. However, it has a relatively long intra-band carrier relaxation time (270 ps). Inspired by the heterostructure engineering, here, we fabricate high-quality few-layered graphene/α-In₂Se₃ heterostructure (GIHS) and investigate its linear/nonlinear optical response. Large effective nonlinear absorption coefficient (β_eff ∼ −1.2 × 10⁴ cm/GW) comparable to that of α-In₂Se₃ [15], together with ultrafast intra- and inter-band electron relaxation (78 fs and 14 ps, respectively), are experimentally demonstrated. In addition, using GIHS as a SA in an all-anomalous-dispersion fiber laser for mode-locking, stable soliton pulses centered at 1559.7 nm with 158 fs pulse duration are obtained.

2. Material fabrication and characterization

Few-layered α-In₂Se₃ nanosheets were synthesized by liquid phase exfoliation (LPE) method [18]. 0.1 g α-In₂Se₃ powder (Alfa aesar, 99.99%) was added to 10 ml alcohol (30%) and subjected to bath sonication at 300 W for 24 hours. After centrifuging at 8000 rpm for 30 minutes to remove the deposit, few-layered α-In₂Se₃ nanosheets were obtained. Figure 1(a) depicts the typical multi-layered structure and morphology of α-In₂Se₃ nanosheets taken by transmission electron microscopy (TEM). Figure 1(b) displays the precise lattice structure of α-In₂Se₃ with a hexagonal honeycomb structure examined through high-resolution TEM (HRTEM). The lattice spacings are measured to be d₁=d₂=3.32 Å, d₃=3.10 Å, in accordance with the (1120) plane of α-In₂Se₃, corresponding to the P63/mmc space group. Figure 1(c) shows the selected area electron diffraction (SAED) pattern of α-In₂Se₃ nanosheets. Well-resolved six-fold rotational symmetric diffraction pattern reveals its good single-crystalline nature and hexagonal structure.

 

Fig. 1. (a) TEM image, (b) HRTEM Image, (c) SAED image of LPE α-In₂Se₃ nanosheets. (d) AFM Image of GIHS. Above: height variation near the edge of graphene (red line); Below: height variation of α-In₂Se₃ nanosheets on graphene substrate (blue line). (e) Raman spectrum of GIHS. (f) Optical transmission spectrum of graphene, α-In₂Se₃, GIHS and quartz substrate.

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In order to investigate the optical properties of the GIHS, three samples (graphene, α-In₂Se₃ and GIHS) were fabricated on quartz substrate. The graphene we used is synthesized by chemical vapor deposition (XFNano XF024) and transferred to a quartz substrate using a wet transfer method [19]. The α-In₂Se₃ sample is prepared by dropping α-In₂Se₃ dispersion on the substrate. To fabricate GIHS, we drop α-In₂Se₃ dispersion on the CVD graphene and wait several minutes for drying. The atomic force microscopy (Park NX10) results of GIHS is shown in Fig. 1(d). The measured thickness of graphene and α-In₂Se₃ nanosheet are 5.3 and 3 nm, respectively. Figure 1(e) depicts the Raman spectrum collected from the GIHS sample by a Raman spectrometer (Witec alpha300) with 532 nm excitation laser source. The coexistence of typical Raman peaks of graphene (G and 2D, located at 1583.8 and 2721.4 cm⁻¹) and α-In₂Se₃ nanosheets (located at 104, 182 and 201 cm⁻¹) indicates the high quality of graphene and α-In₂Se₃ nanosheets and the successful fabrication of GIHS. The optical transmission spectrum of graphene, α-In₂Se₃, GIHS and pure quartz substrate are measured by a spectrometer (Agilent Cray 5000), as shown in Fig. 1(f). The transmittance of graphene is about 76.9% at 1030 nm, corresponding to 8 layers (∼5.3 nm) [20], which is in accordance with the AFM results. The transmittance of the α-In₂Se₃ sample is about 80.6%. From the data reported before [15], its average thickness is estimated to be ∼3 nm and confirmed by AFM measurements. Similarly, the transmittance of GIHS is 67.8% and the average thickness is ∼8 nm. All these parameters are summarized in Table 1.

Table 1. Nonlinear optical properties of graphene, α-In2Se3 and GIHS samples at 1030 nm

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The nonlinear saturable absorption properties were measured by a typical open-aperture Z-scan system with a femtosecond laser (Light Conversion Ltd Pharos) centered at 1030 nm, with 226 fs pulse duration and 1 kHz repetition rate. The corresponding normalized Z-scan curves are shown in Fig. 2. We can see the obvious saturable absorption characteristics of graphene, α-In₂Se₃ and GIHS, while no signal is observed from glass substrates with pump fluence as high as 189 GW/cm². At the pump power density of 189 GW/cm², the nonlinear absorption coefficient β_eff of GIHS is derived to be −1.2 × 10⁴ cm/GW by fitting the curve with equation $T({z,S = 1} )\approx 1 - \beta {I_0}{L_{eff}}/(2\sqrt {2({1 + {z^2}/z_0^2} )} $ [21], which is larger than graphene (−5.7 × 10³ cm/GW). The modulation depth of graphene and GIHS are fitted to be 16% and 31.5% respectively.

 

Fig. 2. Open-aperture Z-scan measurement of (a) graphene, (b) α-In₂Se₃ and (c) GIHS.

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To investigate the transient optical response of the GIHS, pump–probe technology is employed to uncover the ultrafast carrier dynamics. The fundamental beam (1030 nm, ∼170 fs pulse duration) from Yb: KGW laser (Light Conversion Ltd Pharos) is separated to two paths. One is introduced into a noncollinear optical parametric amplifier to generate pump pulse at 800 nm, while another is focused onto a YAG crystal to produce white light continuum (520 ∼ 950 nm) as probe light. Photo-bleaching signature deriving from Pauli blocking indicates the as prepared GIHS exhibits strong saturable absorption, as shown in Fig. 3. Fitting the experimental data by a biexponentially decaying function, the intra-band relaxation time (τ₁, related to electron–electron scattering and electron–phonon scattering) and inter-band relaxation time (τ₂, related to carrier recombination and thermo-phonon cooling) of GIHS is determined to be 78 fs and 14 ps respectively, which is faster than that of α-In₂Se₃ (e.g., τ₁=7.6 ps, τ₂=97 ps [15]). Note that graphene is a fast SA and α-In₂Se₃ is a slow one. Taking advantage of the fast charge transfer and relaxation channel in graphene, the formation of GIHS could speed up the carrier recombination in α-In₂Se₃.

 

Fig. 3. (a) Transient absorption dynamics of Graphene, α-In2Se3 and GIHS with pump and probe wavelengths of 800 nm and 750 nm, respectively. (b) Long scale of transient absorption dynamics of α-In2Se3 and GIHS.

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3. Mode-locked fiber laser applications

To fabricate the in-line GIHS on fiber facet, a step-by-step transfer method was used. Few-layered graphene was exfoliated from bulk highly oriented pyrolytic graphite (HOPG) and transferred to the end-facet of fiber connector via PDMS. The α-In₂Se₃ dispersion (1mg/ml) was dropped on the surface of graphene and waited several minutes for drying. It should be noticed that we use exfoliated graphene here since it is convenient to fabricate and its surface condition is even better than CVD graphene used in previous experiments. They have almost the same thickness (∼5.3 nm, 8 layers) and possess similar optical properties. The optical image of in-line GIHS is shown in Fig. 4(a). The GIHS coated fiber connector (FC) is connected to a clean FC with a fiber adapter to form in-line GIHS mode-locker. Balanced twin-detector equipment is used to measure its saturable absorption properties. The optical source is a mode-locked fiber laser worked at 1550 nm with 2 ps pulse duration and 10 MHz repetition rates. Fitting with the equation $\alpha = {\alpha _{ns}} + {\alpha _s}/({1 + I/{I_s}} )$ [22], the modulation depth and saturable intensity of in-line GIHS are 8.4% and 315.8 MW/cm² respectively, as shown in Fig. 4(b).

 

Fig. 4. (a) Optical image of GIHS on fiber facet. (b) Nonlinear saturable absorption curve of in-line GIHS SA and graphene. (c) Schematics of the typical erbium-doped fiber laser for passive mode-locking operation with GIHS SA. (d) Output spectrum with 0.02 nm spectral resolution. (e) Oscilloscope trace of output pulse trains; inset: RF spectrum; (f) Interferometric autocorrelation trace of output pulses after amplification and compression.

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The schematic of the all-fiber GIHS ring laser is shown in Fig. 4(c). A 0.43-m-long erbium-doped fiber (EDF, Liekki Er110-4/125) with group velocity dispersion (GVD) of 12 ps²/km is served as gain medium. The pump source is a 750 mW/980 nm laser diode (LD), which is injected into the cavity through a 980/1550 nm wavelength division multiplexer/isolator hybrid (WDM + ISO). The as prepared in-line GIHS is spliced into the laser cavity as a SA. A polarization controllers (PC) is engaged to adjust the polarization state of the propagation light for mode-locking optimization. The total cavity length is 5.3 m consisting of single mode fiber (SMF-28e) with GVD parameter −23 ps²/km, and the net cavity dispersion is about −0.102 ps². Taken by a 10% optical coupler (OC), the optical pulse signal is monitored by a commercial optical spectrum analyzer (Yokogawa AQ6370D), a 3-GHz digital oscilloscope (LeCroy WavePro7300) together with a 1 GHz photodetector (Thorlabs DET01CFC), a radio frequency (RF) spectrum analyzer (Rigol DSA1030) and a commercial autocorrelator (APE Pulsecheck-USB-50). Due to the saturable absorption of the GIHS, self-starting mode-locking operation is obtained when the pump power reaches up to a mode-locking threshold of 35 mW. The optical damage of the GIHS SA appears when the pump power is beyond 280 mW. Figure 4 summarized the mode-locking characteristics under the pump power of 104 mW. The maximum output power is 2.72 mW, corresponding to the pulse energy of 0.07 nJ. Figure 4(d) presents the optical spectrum of the passive mode-locked output, which operates at 1559.7 nm central wavelength with a 3-dB bandwidth of 5.5 nm. The Kelly sidebands are symmetrically distributed at both sides of the spectrum, which indicates the mode-locking operation works in soliton state. The oscilloscope trace of output pulse trains and its Fourier-transform spectrum are shown in Fig. 4(e). The period is 26.94 ns, matching exactly well with total cavity length of 5.3 m. The inset of Fig. 4(e) shows the RF spectrum around the fundamental repetition rate of 38.556 MHz with 100 Hz resolution bandwidth (RBW). The signal-to-noise ratio (SNR) is ∼45 dB, which indicates good mode-locking stability of the GIHS laser. The mode-locking operation can work continuously over 12 h under ambient environment. The output pulses are amplified by a home-made Erbium-doped fiber amplifier (EDFA) and compressed by a section of dispersion compensation fiber. The measured interferometric autocorrelations (IACs) has a full width at half maximum (FWHM) width of 245 fs, as shown in Fig. 4(f), which is the narrowest pulse duration we have measured. Assuming a Sech² pulse profile, the soliton pulse duration is estimated to be about 158 fs. The special advantages of GIHS is that it has combined advantages of ultrafast relaxation (τ₁ ∼ 78 fs, τ₂ ∼ 14 ps) together with large effective nonlinear absorption coefficient (β_eff ∼ −1.2 × 10⁴ cm/GW). Besides, GIHS could be a broadband SA. Recently, many novel 2D materials with unique and attractive optical characteristics have been reported for mode-locking applications [23–26]. We believe these materials could enrich the family of 2D heterostructure for ultrafast nonlinear optical applications.

Conclusion

We report nonlinear optical properties of few-layered GIHS and use it as a SA for ultrafast laser pulses generation. The GIHS owns remarkable characteristics including ultrafast relaxation (τ₁ ∼ 78 fs, τ₂ ∼ 14 ps), large effective nonlinear absorption coefficient (β_eff ∼ −1.2 × 10⁴ cm/GW) and relatively large modulation depth. By integrated it into Erbium-doped fiber laser system, stable mode-locking soliton pulses with 158 fs pulse duration and 2.72 mW average power is successfully generated. The excellent nonlinear optical properties of the GIHS endorses it as a promising candidate for ultrafast nonlinear photonic applications.