Topological insulator (TI) Bi2SeTe2 nanosheets with very regular hexagonal morphology were synthesized by a hydrothermal route. Open aperture (OA) z-scan method was performed to measure the saturable absorption (SA) characteristics of the as-prepared TI Bi2SeTe2 nanosheets. The measured modulation depth, saturation intensity and nonsaturable loss of the sample were 61.9%, 4.46 GW/cm2 and 4.5% respectively. An ultrafast intraband scattering time of ~50 fs was obtained through simulating the SA curve, which indicates the TI Bi2SeTe2 nanosheets may be a good candidate for mode-locking material.
© 2015 Optical Society of America
Intense ultrashort laser pulses are powerful tools for physics and chemistry researches. They are widely used in, for instance the micromachining [1, 2], pump-probe measurements with high time resolution , and femtosecond coherent anti-Stokes Raman spectroscopy (CARS) . For the pulsed laser generation, it is well known that the introducing of saturable absorbing dyes into the laser resonator forces the laser to operate in a ‘mode-locked’ rather than in a continuous mode. Though picoseconds pulses are in fact being generated with dye saturable absorber who recovers on a nanosecond to picosecond timescale, it cannot do much shortening on pulse duration shorter than its own recovery time. It has been demonstrated that, saturable absorbers with ultrafast saturable absorption (SA) are needed in the ultrafast pulse generation systems. The pulse duration could be shortened from pico- to sub-picosecond by using semiconductor saturable absorber mirrors (SESAM) as such absorbers . Also carbon nanotubes (CNT) with ultrafast recovery times (~1 ps) were demonstrated to be suitable saturable absorbers for generating femtosecond laser pulses . In this decade, the atomic layer graphene, which was demonstrated possessing ultrafast SA , has been employed as a saturable absorber in mode-locked fiber laser for the generation of ultrashort soliton pulses . However, such saturable absorbers have to be fabricated by complex and costly processes, such as metal-organic vapour-phase epitaxy (MOVPE) or metal-organic chemical vapour deposition (MOCVD).
Topological insulator (TI), which is characterized by its bulk insulating state with a small band gap and a surface Dirac-like band structure , has been proved to have excellent SA properties, such as low saturation intensity, wavelength-independent saturable absorbing characteristics. Due to the narrow gap in the bulk of binary TI (for Bi2Te3 is ~0.15 eV), both the surface and the bulk absorb the excitation light, and can be saturated under strong excitation . The ultrafast time-resolved optical spectroscopy measurements have shown the carrier interband scattering time was less than 500 fs in several binary bismuth compounds [11, 12]. Further, femtosecond laser pulses (~660 fs) with centre wavelength located at middle infrared range have been achieved by using TI as saturable absorbers , which promoted the TI nanosheets in the application of ultrashort pulses generation. Recently, ternary bismuth compounds which belong to tetradymite were investigated on the bulk resistivity [14, 15], surface quantum oscillations , and Femi level tuning [17, 18]. Some of the unique physical properties and promising applications in ternary TIs were discovered, such as the surface states became dominated in TI Bi2Te2Se resulted from the reduced contribution of bulk carriers . However, few researches are done on the optical characteristics of ultrafast SA of such ternary TIs, which could also be promised as excellent mode-locking materials.
In this investigation, we synthesized TI Bi2SeTe2 nanosheets via a solvothermal route. The as-fabricated Bi2SeTe2 nanosheets with regular hexagonal morphology were analysed by X-ray diffraction (XRD), revealing highly crystalline, phase-pure of the product, and absence of impurities. Open aperture (OA) z-scan method was performed to measure the SA characteristics of the ternary bismuth compounds, and the measured modulation depth, saturation intensity and nonsaturable loss were to be 61.9%, 4.46 GW/cm2 and 4.5% respectively. The physical mechanism of ultrafast SA of this TI Bi2SeTe2 nanosheets was well described by the intraband carrier-carrier (c-c) scattering with a scattering time of ~50 fs. Q-switching erbium-doped fiber laser experiment was achieved by using TI Bi2SeTe2 nanosheets as saturable absorber, which indicated the prepared TI Bi2SeTe2 nanosheets may be a good candidate for mode-locking material.
The starting materials for the solvothermal synthesis of TI Bi2SeTe2 nanosheets were BiCl3, Na2SeO3, Na2TeO3, (2:1:2 mole ratio) and NaOH. These raw materials with analytical grade were respectively dissolved in ethylene glycol with the assistance of ultrasound. Firstly, vinyl pyrrolidone (PVP) was added into ethylene glycol solution of BiCl3 until dissolved in a beaker, and then the ethylene glycol solution of Na2TeO3, and Na2SeO3 were introduced together into the beaker. Then, the ethylene glycol solution of NaOH was added. After stirring for sever minutes, the mixed solution was poured into a Teflon-lined stainless steel autoclave, which was maintained at 180 °C for 36 h. Gray products were collected and washed with ethanol by centrifugations. For preparing the nonlinear absorption measurements, the as-prepared Bi2SeTe2 nanosheets were dispersed in isopropyl alcohol. After ultrasonicated for 2 hours, the dispersion solution was dropped onto a piece of fused silica with thickness of 0.5 mm. Later, the glass plate was coated a uniform film of TI nanosheets, and then was placed in a drying oven for 1 hour.
SEM images of the as-prepared Bi2SeTe2 nanosheets are shown in Fig. 1(a). It can be clearly observed that large-scale isolated sheet-like nanomaterials with large lateral dimensions are widespread. The nanosheets are almost two-dimensional structures with intact surface texture. An SEM image with larger magnification is shown in the inset of Fig. 1(a). We observed that the Bi2SeTe2 nanosheets prepared by our method exhibited very symmetric hexagonal morphology, indicating relatively higher stability. To characterize the phase purity of the nanosheets, XRD measurement was performed for the samples. Figure 1(b) shows the XRD pattern of the Bi2SeTe2 nanosheets. Compared with JCPDS data card No. 29-0247, these ternary products have been found to exhibit rhombohedral crystal geometry (space group: R-3m (166)) with no detectable impurities of other phases. Figure 1(c) shows the linear absorption spectrum of as-prepared Bi2SeTe2 nanosheets, showing a broad transmission range from visible to middle infrared. Figure 1(d) shows the Raman spectrum of the Bi2SeTe2 nanosheets in the range of 50-240 cm−1 using 785 nm excitation (Renishaw inVia) at room temperature. Three typical Raman peaks of Bi2SeTe2 nanosheets centered at ~73 cm−1, ~116 cm−1, 152 cm−1 were found in the spectrum, corresponding to the out-plane vibrational mode A11g, in plane vibrational mode E2g, and the outplane vibrational mode A21g of Te-Bi-Se-Bi-Te lattice vibration, respectively .
An OA z-scan configuration showed in the inset of Fig. 2(a) was applied to study the nonlinear absorption characteristics of the as-fabricated Bi2SeTe2 nanosheets. A Coherent Legend Elite Ti: sapphire regenerative amplifier system was used as the femtosecond laser source, which emitted laser pulses with a repetition rate of 1 kHz, a pulse width of 130 fs (carefully monitored with interferometric second harmonic auto-correlation), and a central wavelength of 800 nm. The incident laser beam passed through a combination of a half wave plate and a polarizing beam splitter, which could adjust the incident light intensity. A lens with a focus length of 150 mm was used to focus the incident beam, generating a beam waist of 15 μm. The TI Bi2SeTe2 sample was mounted on a linear translation stage. A dual-detector power meter (Thorlabs PM320E) controlled by a computer simultaneously monitored the laser power before and after the glass plate with TI sample while the TI Bi2SeTe2 sample transferred through z = 0 position.
3. Experimental results and discussion
Figure 2(a) shows the normalized OA z-scan curves under different incident power densities varied from 0.16 to 26.4 GW/cm2. Sharp and narrow peak at the beam focus was observed from each OA z-scan curve showing the characteristic of SA in the Bi2SeTe2 nanosheets. No optical damage was observed when the incident laser intensity was increased to 26.4 GW/cm2, and the optical damage threshold of the sample was estimated to be five times larger. A compared OA z-scan curve was measured using the fused silica substrate at 26.4 GW/cm2, which further confirmed the SA originated from the Bi2SeTe2 nanosheets.
The mechanism of SA can be explained by Pauli blocking, as shown in Fig. 2(b). As a new class of quantum matter, binary or ternary TIs possess narrow band gap along the high-symmetry directions on the bulk state . When irradiated by light with photon energy larger than Eg, both the surface and the bulk of Bi2SeTe2 absorbs the incident light, and the electrons in the valence band are excited in to the conduction band at the bulk state. After photo-excitation, intraband carrier-carrier scattering and carrier interband scattering relax the photoexcited carriers. Therefore, the incident light photons can be continuously absorbed through the excitation of the electrons from the valence band to the conduction band. However, under more intensive irradiation of light with photon energy larger than Eg, the states in the valence band become depleted, while the finial states in the conduction band are partially occupied. Further transition of electrons from the valence band is Pauli blocked and no absorption can be observed, leading to a SA in Bi2SeTe2 nanosheets.
Noting that no obvious trend is observed in the SA curves in Fig. 2(a) even the incident power densities varied about two orders of magnitude. We attribute this to the ultrafast SA of Bi2SeTe2 nanosheets even if the intensity of the incident light is very low. In order to extract the SA parameters, we used the following model. The OA z-scan curve was obtained by continuously moving the translation stage where the fused silica plate placed. That means the relative distance between the glass plate and focus point is correspondingly changed, resulting in the variation of the laser beam size as well as the laser transmittance through the glass plate covered with Bi2SeTe2 nanosheets. Therefore, we could only translate the sample position z to the corresponding laser intensity in any measured OA z-scan curve, the dependence between the transmittance and the incident laser intensity can be obtained as well as the SA parameters.
For TI Bi2SeTe2, we only consider the one photon absorption:Eq. (1), we could obtain the laser transmittance through the Bi2SeTe2 sample :Figure 3(a) shows dependence between the normalized transmittance and incident laser intensity, which is translated from the OA z-scan curve under the intensity of 26.4 GW/cm2. The saturable trend of absorption can be obviously seen when the incident laser intensity increased. Simultaneously, this relationship is fitted which denotes the characteristics of SA of Bi2SeTe2 sample :22], however, the modulation depth is much larger. By using the saturable absorber with high modulation depth in the optical resonator, the pulse wave breaking effect can be significantly suppressed, and large energy dissipative solitons may be formed, indicating the as-prepared Bi2SeTe2 have promising applications for high power pulse generation.
In the case of light excitation with intense fentosecond laser pulse (1.55 eV), a nonequilibrium population of electrons in the conduction band and holes in valence band are created with momentum conservation. Subsequently, the nonequilibrium carriers scattering in the conduction band occurs as in graphene and semiconductor saturable absorbers . The excited electrons scatter to lower energy in a femtosecond time period, populating the surface state and conduction band. After several picoseconds, the surface state and conduction band populations decay and energetically relax towards the bottom of their respective bands. Shortly after the recombination of the photoexcited carriers, the TI absorbs light again. Further, it has been demonstrated that the surface states disperse into bulk states and the relaxation time of TIs charge carriers is thus mainly determined by interband scattering from the bulk conduction band whose recombination time was more efficient . Therefore, an ultrafast intraband scattering process after photoexcited may dominate the ultrafast SA of TI.
Over the cross-section at the pulse propagation direction z, the instantaneous power absorbed by Bi2SeTe2 obeys Fermi’s golden rule :10]. The early evolution of photonexcited carriers scattering can be described by an ultrafast SA model in graphene :26]. We assume both the temporal and spatial profiles of incident femtosecond laser pulse follow a Gaussian distribution, and then use Eq. (6) to fit the SA curve in Fig. 2(a). Figure 3(b) shows the SA curve measured at 26.4 GW/cm2 was fitted by Eq. (6) in which was the only fitting parameter. A good fitted value of (~50 fs) was obtained. The nonequilibrium carriers thermalized through intraband c-c and c-optical phonon (c-op) scattering to form Fermi-Dirac distribution. Therefore, comprises contributions from both the intraband c-c scattering and the dissipative c-op coupling, i.e. , where and refer to the lifetimes for c-c scattering and c-op coupling respectively. Recent researches show that the typical relaxation time of c-op interaction in TI is about several picoseconds , which is much larger than . Therefore, , referring the ultrafast intraband c-c scattering time is about 50 fs. This c-c scattering dynamics process is very fast that occurs once the sample absorbs light and reaches to saturation. Since the decay rate at CB represent the band-emptying rate, the ultrafast intraband scattering leads to the ultrafast SA of the as-prepared Bi2SeTe2 nanosheets.
To perform the applications of the as-fabricated TI sample, Bi2SeTe2 nanosheets were exploited as a saturable absorber in a passively Q-switching erbium-doped fiber laser experiment. The experimental setup is similar to Ref . Briefly, the TI Bi2SeTe2 nanosheets were incorporated with polymethyl methacrylate (PMMA) to fabricate a flexible film, and then the film was sandwiched between two fiber connectors and integrated into the laser cavity for Q-switching operation. The 976 nm pump light was coupled into a 2-m-long gain fiber by a wavelength division multiplexer. The stable Q-switching pulses were started at pump power reached to ~27 mW. Figure 4 shows the Q-switching pulse-train pumped at 40 mW with the repetition rate of 8.7 kHz, pulse duration of 16.4 μs, and output power of 65 μW.
In conclusion, two-dimensional TI Bi2SeTe2 nanosheets were fabricated by a hydrothermal route, which is a flexible method for the morphology controlling. SEM imaging and XRD pattern of the as-prepared Bi2SeTe2 nanosheets showed very regular hexagonal morphology and highly crystalline, respectively. The characteristic of strong SA of Bi2SeTe2 nanosheets was measured by OA z-scan method. The ultrafast intraband scattering processes in Bi2SeTe2 nanosheets were theoretical simulated. Q-switching erbium-doped fiber laser experiment was achieved, which promoted the as-prepared Bi2SeTe2 nanosheets to be a good candidate material for future photonic devices.
This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 11404114, 51132004, 61307026), the China Postdoctoral Science Foundation (Grant No. 2014M550435), Fundamental Research Funds for the Central Universities (Grant Nos. 2014ZB0027, 2013ZM0001), Guangdong Natural Science Foundation (Grant No. S2011030001349).
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