Tunable nonlinear optical properties in nanocrystalline Si/SiO2 multilayers under femtosecond excitation
© Zhang et al.; licensee Springer. 2014
Received: 22 October 2013
Accepted: 5 January 2014
Published: 14 January 2014
The nonlinear optical properties of nanocrystalline-Si/SiO2 (nc-Si/SiO2) multilayers have been investigated through Z-scan technique by using a Ti-sapphire laser with 50-fs pulse duration at 800 nm as a pump laser. It is interesting to note that with increasing the annealing temperature to make the sample change from amorphous phase to nanocrystalline state, the nonlinear absorption turns the reverse saturation absorption into saturation absorption while the nonlinear optical refraction is also changed simultaneously from self-defocusing to self-focusing. We propose that the localized states at the nc-Si/SiO2 interfaces play the key role in the observed switching behaviors. Our results demonstrate that the tunable optical nonlinearities can be achieved by controlling the microstructures of nc-Si, which can be used as engineering different nonlinear optical devices.
KeywordsOptical nonlinearities Nanocrystalline Si Multilayers Interface state 42.65.-k 42.65.Jx 42.65.Pc
Linear and nonlinear optical properties in Si-based materials have attracted much attention in the recent years since they can be potentially applied in many kinds of optoelectronic devices by using the mature Si technology [1–5]. However, bulk crystalline Si has a weak nonlinear optical effect due to the low Kerr coefficient, which will restrict its actual applications. Recently, the enhanced nonlinear optical effect in the near-infrared spectral range has been observed in nanocrystalline Si (nc-Si) films and all-optical switch as well as optical amplifier based on nc-Si has been realized [6–8]. So far, nonlinear optical properties have been observed in nc-Si films prepared by various techniques such as chemical vapor deposition (CVD) and sputtering methods. It is found that the observed nonlinear optical behaviors are strongly dependent on the film microstructures as well as the measurement conditions [9–11]. For example, Spano et al. reported the change of nonlinear refraction indices from positive to negative with changing the film composition and measurement conditions . Martínez et al. fabricated nc-Si films by three different deposition techniques: e-beam evaporation, plasma-enhanced chemical vapor deposition, and low-pressure chemical vapor deposition (LPCVD), and they found that the nc-Si films prepared by LPCVD show the saturation absorption property, while the other two samples displayed the reverse saturation absorption characteristics . More recently, Ma et al. observed the tunable nonlinear absorption behaviors by changing either the incident laser intensity or the bandgap of nc-Si films . Therefore, it is one of the important issues to further understand the nonlinear optical properties of nc-Si films especially under the ultrafast laser excitation.
Usually, spatially confined exciton due to quantum confinement effect is considered to play a dominant role in enhanced nonlinear optical property of nc-Si film. Prakash et al. reported the size-dependent nonlinear optical coefficient, and they attributed it to the increase of oscillator strengths because of the quantum confinement-induced localization of electron–hole pairs . Meanwhile, the localized defect states are also proposed to affect the nonlinear optical properties of nc-Si films. Ito et al. found that the nonlinear refractive index did not decrease monotonously with the size of nc-Si, and they believed that both the quantized electronic states and defect states contributed to the large nonlinear refractive index . In our present work, we systematically studied the nonlinear optical properties of Si/SiO2 multilayers during the transition process from amorphous phase to nanocrystalline Si state. We found tunable nonlinear optical behaviors, reverse saturation absorption in the amorphous-phase dominant samples, and saturation absorption in the nanocrystalline-phase dominant ones, under femtosecond laser excitation. The nonlinear refraction was also simultaneously changed. We proposed that the interface states of nc-Si play the important role in the changing of nonlinear optical behaviors.
The Z-scan technique  was applied to measure the nonlinear optical coefficients of nc-Si/SiO2 multilayers. In this experiment, the excitation laser was a Ti-sapphire laser with 50-fs pulse duration at 800 nm, the repetition rate was 1 kHz. The low repetition rate is in favor of reducing the thermal accumulative effect . A lens with 20-cm focal length was used to obtain Gaussian beam, the obtained beam waist was about 30 μm.
Results and discussion
where x = z/z0, is the Rayleigh diffraction length, z is the sample position from the focal point, I 0 is the excitation intensity at the focal point, indicates the effective thickness of the sample, α0 is the linear absorption coefficient, and L is the real thickness. The calculated β is 7.0 × 10-8 cm/W, which is comparable to the value reported previously .
where Δ Φ0 = k0n2I0Leff represents the nonlinear phase change. The nonlinear refraction index n2 of sample A is -3.34 × 10-12 cm2/W. Spano et al. also reported the negative nonlinear refraction n2 in the order of 10-13 cm2/W which is one order of magnitude lower than that of our sample . The enhanced nonlinear optical refraction can be attributed to the strong free carrier nonlinearity in our multilayers sample via the two-photon absorption process as we discussed before. The nonlinear refractive index n2 in sample B is reduced to about -0.56 × 10-12 cm2/W, which is consistent with the reduced two-photon absorption process due to the enlargement of optical bandgap and the formation of nc-Si. However, for samples C and D, the positive nonlinear refractive index is obtained suggesting that different nonlinear optical process dominates the nonlinear response, the obtained n2 of samples C and D are 4.94 × 10-12 and 3.47 × 10-12 cm2/W, respectively. It is worth mentioning that we also measured the n2 from pure SiO2 layer pumped under similar condition in order to exclude the contribution of SiO2 layers. The calculated n2 is 1.4 × 10-16 cm2/W, which is much lower than that of Si/SiO2 multilayers. It is suggested that the enhanced optical nonlinearity is mainly resulted from the ultrathin Si layers. As debated before, the SA is obtained in samples C and D, and we attributed it to the existence of interface states between the nc-Si and SiO2 layers. Takagahara et al. theoretically predicted that excitons localized at disorders or impurities could increase its oscillator strength, which led to the large optical nonlinearity . It was reported that the electrical field building up by the charges trapped at the nc-Si/SiO2 interface states would enhance the optical nonlinear process . In our proposed model, the interface states between nc-Si and SiO2 layers can also localize the excitons to suppress the two photon absorption process, which can result in the enhanced nonlinear optical refraction index as obtained in our case.
In summary, we observed the tunable NLA and NLR response in Si/SiO2 multilayers during the transition process from the amorphous to nanocrystalline phases under femtosecond excitation at 800 nm. We suggested that the two-photon absorption process dominates in the samples mainly containing amorphous Si phases, while the phonon-assisted one-photon transition process between the valence band and interface states dominates the nonlinear optical properties in nc-Si/SiO2 multilayers. The obtained NLA coefficient β is about -10-7 cm/W and the NLR index n2 is about 10-12 cm2/W for nc-Si/SiO2 multilayers which is two orders of magnitude larger than bulk Si, which indicate that nc-Si/SiO2 multilayers can be applied into high-sensitive photonic devices such as optical switch and Q-switch laser.
Reverse saturation absorption
Cross-sectional transmission electron microscopy.
This work is supported by 973 project (2013CB632101), NSFC (no. 11274155), and PAPD; we acknowledge Z. L. Wang and X. Chen for the assistance with the Z-scan measurements.
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