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Abstract
By using an Open-Aperture Z-scan technique with both femtosecond and nanosecond laser pulses at 1064nm, the nonlinear optical properties of GaAs and Bi-doped GaAs, including the saturable absorption property and reverse saturable absorption property are systematically measured and analyzed directly in detail. Compared to pure GaAs, Bi-doped GaAs has a lower saturation intensity, wider saturable absorption energy region, lower two-photon absorption coefficient, better saturable absorption response and stronger optical limiting response. The results suggest that the incorporation of Bismuth in GaAs is an effective way of improving the nonlinear optical properties of GaAs, which provide crucial experimental evidence for that the characteristics of the passively Q-switched laser with Bi-doped GaAs saturable absorber is better than pure GaAs.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Compared with other saturable absorbers, GaAs has the advantages of stable photochemical property and saturable absorption, good thermal conductivity, and high damage threshold. It has been widely used as the saturable absorber for passively mode locking or Q-switching 1 µm lasers [1–5]. It is well accepted that saturable absorption around 1 µm is due to the EL2 defect located in band gap, which yields a deep level, 0.82 eV below the GaAs band edge [6]. However, the concentration of EL2 deep-level defects is very low, and it is a challenge to control the amount of EL2 defects in GaAs saturable absorber to design its Q-switching parameters [7–9].
Alloying is an effective way of modifying the properties of a material. Incorporation of Bismuth in GaAs introduces many interesting properties, including large band gap reduction (~90 meV per x = 0.01) and large spinorbit splitting energy increase [10–15]. Theoretical calculations on the effects of Bismuth alloying in GaAs saturable absorber have been reported previously [16]. In addition, a passively Q-switched Nd:GGG laser with a minimum pulse duration of 1.2ns and a passively Q-switched, mode-locked Nd:GGG laser have been realized by using Bi-doped GaAs saturable absorber. It is found that the incorporation of Bismuth in GaAs is an effective way of improving the performance of GaAs as saturable absorber [17,18]. However, the nonlinear absorption mechanisms of Bi-doped GaAs, such as the single-photon absorption and two-photon absorption [19], are still not well established as far as we know. The Open-Aperture Z-scan technique, using single excitation wavelength, is widely used to resolve saturable absorption dynamics directly. Probing the nonlinear absorption characteristics of Bi-doped GaAs may provide crucial experimental evidence to answering the question that why performance of Bi-doped GaAs as saturable absorber is better than GaAs. Therefore, it is necessary to study the saturable absorption characteristics of Bi-doped GaAs by using Open-Aperture Z-scan technique.
In this paper, we investigate the nonlinear optical properties of Bi-doped GaAs by performing single-beam Z-scan experiments with nanosecond laser pulses and femtosecond laser pulses at 1.06μm [20]. The saturable absorption response and stronger reverse saturable absorption response are analyzed systematically. The two-photon absorption coefficient and saturable intensity have been obtained. For comparison, the Open-Aperture Z-scan experiments with Bi-doped GaAs or pure GaAs are achieved under the same conditions.
2. Experimental preparation and setup
2.1. Characterisation of Bi:GaAs and GaAs
Bismuth is doped into GaAs using ion bombardment at 500 keV. The sample is subsequently annealed in a rapid thermal processor in nitrogen ambient (The annealing temperature is 700 °C,the duration is fixed at 60 s) [21]. Samples in this work, including Bi:GaAs and GaAs, have the same thickness of 600 μm and the size of them is 26 mm × 26 mm.
Characterisation of Bi:GaAs and GaAs are shown in Fig. 1. Figure 1(a) shows the calculated ion ranges of Bismuth alloying in GaAs at 500 keV with a dose of 1×1014 ions/cm2. The thickness of the implantation layer is about 100 nm. The profile of transmission spectrum is obtained by an infrared spectrometer and shown in Fig. 1(b). The transmittance of Bi:GaAs is 49.2% and the transmittance of GaAs is 48.5% at the wavelength of 1064 nm. It can be seen that the transmittances of two samples are almost equal, which means incorporation of Bismuth in GaAs using ion bombardment has less effect on transmission spectrum of GaAs. Figure 1(c) and 1(d) show the scanning electron microscopy images of GaAs and Bi:GaAs. It is clearly that incorporation of Bismuth in GaAs do not cause damage to the surface.
Fig. 1 Characterisation of Bi:GaAs and GaAs. (a) The calculated ion ranges of Bismuth alloying in GaAs (b) The profiles of transmission spectrum for Bi:GaAs and GaAs, Scanning electron microscopy image of (c) GaAs and (d) Bi:GaAs.
2.2. Experimental setup
The laser Z-scan technique is used as an effective method for investigating the nonlinear properties of materials [22], which includes nonlinear absorption, scattering, and refraction. The experimental setup of the Open-Aperture Z-scan measurement is shown in Fig. 2. A Gaussian beam is produced by the pump source. D1 and D2 are two energy meters (Rjp-765 energy probe linked to an Rj-7620 ENERGY RATIOMETER, Laser probe). A lens with focal length of 25 cm is set in front of the sample to focus the incident beam. At the focal point, the sample experiences maximum pump intensity, which gradually decreases in either direction from the focus. Total transmittance through the sample as a function of incident intensity is measured while the sample is gradually moved through the focus of a lens along the z-axis. In our nanosecond laser Z-scan measurement, the pump source is a nanosecond laser at wavelength of 1064 nm with a pulse width of 4ns and the pulse repetition rate of 10 Hz. For studying the reverse saturable absorption of the two samples, the pump source is changed by a femtosecond laser at wavelength of 1064 nm with a pulse width of 190 fs and the pulse repetition rate of 1 kHz.
3. Nonlinear optics results and discussion
The nonlinear absorption properties of GaAs crystal and Bi-doped GaAs were investigated by the Open-Aperture Z-scan system. Figure 3(a) and 3(b) show typical Open-Aperture Z-scan measurement data for the two samples under the excitation of different intensities at the wavelengths of 1064 nm for ns pulses. The scatters are the experimental data, while the solid lines are the theoretical fits. All Open-Aperture Z-scan traces exhibit symmetric peaks and valleys with respect to the focus, typical of an induced positive nonlinear absorption effect. A transition from saturable absorption to reverse saturable absorption is observed with increasing the input laser fluence. The experimental Z-scan data are fitted with the saturable model that includes both the saturable absorption and reverse saturable absorption effects [23,24].
Where is the sample length, is the linear absorption coefficient, is the saturable intensity and is the TPA coefficient. Otherwise, we can use peak intensity and the diffraction length of the beam to determine the intensity :Thus, the experimental Z-scan data with the saturable model that includes both the saturable absorption and TPA effects was surveyed with modified Eq. (3):Fig. 3 Typical open-aperture Z-scan data of (a) GaAs and (b) Bi:GaAs with normalized transmission plotted as a function of sample position z under under ns laser pulses at 1064 nm.
As shown in Fig. 3(a), under the peak intensity about18 MW/cm2, optical transmittance of GaAs crystal increases as the sample being gradually moved through the focus point firstly, and then the optical transmittance decreases pronounced near the focus point because the intensity increases rapidly. The sample GaAs crystal shows mainly a saturable absorption with a reverse saturable absorption generating. With the increase of the input laser fluence (is 27 MW / cm2), the nonlinear behavior of GaAs completely switches to reverse saturable absorption, which means that the two-photon absorption plays dominant role.
As shown in Fig. 3(b), the response is typical of saturable absorption response at the same peak intensity about 18 MW / cm2, in which the optical absorbance decreases with the increase of the incident laser flux and becomes saturated above a certain threshold. With the increase of the input laser fluence (is 27 MW / cm2), reverse saturable absorption response also can be observed. It is clearly that the optical transmittance of GaAs reduced faster than that of Bi:GaAs at elevated power levels.
The corresponding fitted parameters are listed in Table 1. The saturable intensity for Bi:GaAs is lower than that of pure GaAs and the TPA coefficient of Bi: GaAs is smaller than GaAs. The reverse saturable absorption data do not fit very well with the model, possibly due to the multiple origins of the reverse saturable absorption behavior.
Table 1. Parameters of the theoretical calculation of the two samples at 1064um
Figure 4 displays the nonlinear absorption of pure GaAs and Bi:GaAs as a function of the excitation intensity for ns pluses. The scatters are the experimental data, while the solid lines are the theoretical fits. Saturable absorption and reverse saturable absorption show similar behaviors obviously. The two samples show saturable absorption response at low input laser fluence. The Bi:GaAs exhibits a stronger saturable absorption response in comparison with pure GaAs, with the amplitude enhancement of normalized transmission almost one times larger than GaAs as shown in Fig. 4. With increasing the input intensity, switch over from saturable absorption to reverse saturable absorption behavior is observed clearly. A higher excitation fluence is required to achieve the transition from saturable absorption to reverse saturable absorption for Bi:GaAs than GaAs, which means the reverse saturable absorption threshold of Bi:GaAs is higher than GaAs. It suggests that Bi:GaAs has a wider saturable absorption energy region than pure GaAs. It has been reported that the incorporation of Bi into GaAs leads to a reduction of bandgap, and the gap decreases with the increase of Bi concentration, due to the Bi induced intraband repulsions [16]. It suggests that the Bi alloying induced bandgap narrowing effect make the saturable absorption more efficient.
Fig. 4 Normalized transmission as functions of input laser fluence for pure GaAs and Bi:GaAs for ns pluses.
As a result, the measured data give evidences of the better laser characteristics with Bi-doped GaAs as saturable absorber, which can produce higher output power, shorter pulses, higher single pulse energies and higher peak powers [17,18].When the intracavity power increases, the two-photon absorption is gradually strengthened to limit the transmittance of light, and the Q-pulse is subjected to the highest loss. Compared to GaAs, the saturable absorption of Bi:GaAs is less affected, which means lower loss. On the other hand, the pulse broadening caused by two-photon absorption has less effect due to the smaller two-photon absorption coefficient of Bi: GaAs.
For further studying reverse saturable absorption of Bi:GaAs, we conducted open-aperture Z-scan experiments with femtosecond laser pulses, the sample peak power density was up to 10 GW / cm2, showing the optical limiting characteristics.
Figure 5(a) and 5(b) show the experimental data and fitting curves of pure GaAs and Bi:GaAs under femtosecond laser pulses at the intensity of 10 GW / cm2. The Z-scan results are fitted using the analytical solution of the propagation Eq. (4) [25]:
Where is the effective length, is the sample length, is the nonlinear absorption coefficient.Fig. 5 Typical open-aperture Z-scan data of (a) GaAs and (b) Bi:GaAs with normalized transmission plotted as a function of sample position under femtosecond laser pulses.
From the fitting curves, we can see that the experimental data fits perfectly. At the z-position ‘far’ away from the focal point, this means low input fluence and the samples exhibit linear optical behavior. The samples exhibit nonlinear optical behavior at the focal point. At this intensity, dips are observed in the both transmission curves, implying that reverse saturable absorption in the sample dominates nonlinear properties of the material, which means that the free carrier absorption and two-photon absorption play dominant role [16].
Figure 6 shows the normalized transmission as functions of input laser fluence for pure GaAs and Bi:GaAs for fs pulses. The scatters are the experimental data, while the solid lines are the theoretical fits. It is evident that the normalized transmission reduced gradually with the increasing of input laser fluence at 1064 nm for the two samples. It should be noted that the reverse saturable absorption response of GaAs under ns pulses was better than that of Bi:GaAs (Fig. 3).However, as shown in Fig. 6, at the same energy level, the normalized transmittance of Bi: GaAs is slightly lower than GaAs, demonstrating its advantage in optical limiting. This opposite phenomenon can be ascribed to the different nonlinear optical mechanisms under fs and ns pulse laser irradiation. The reverse saturable absorption response under fs mainly arises from nonlinear scattering, while under ns mainly from two-photon absorption in our experimental conditions. The difference in the normalized transmittance between pure GaAs and Bi:GaAs is not so apparent. The probable reason is that the incorporation of Bismuth in GaAs has little effect on nonlinear scattering and has great effect on single photon absorption and two-photon absorption.
Fig. 6 Normalized transmission as functions of input laser fluence for pure GaAs and Bi:GaAs for fs pulses.
4. Conclusion
In summary, we have investigated the nonlinear optical properties of GaAs and Bi-doped GaAs by performing Open-Aperture Z-scan experiments with nanosecond laser pulses and femtosecond laser pulses at 1.06um. Key optical nonlinear parameters of SA in the GaAs and Bi-doped GaAs have been determined, including TPA coefficient and saturable intensity. For both input pulses, the results demonstrate that the Bi:GaAs exhibits better nonlinear optical performance than GaAs at 1064 nm. The saturable absorption energy region of GaAs is broadened by the incorporation of Bismuth in GaAs. These results give evidences for explaining the performance of Bi doped GaAs as saturable absorber.
Acknowledgments
National Natural Science Foundation of China (NSFC) (61575109, 21473103); Natural Science Foundation of Shandong Province (ZR2014FM035).
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