Tunable nonlinear optical properties in nanocrystalline Si/SiO_{2} multilayers under femtosecond excitation
 Pei Zhang^{1},
 Xiaowei Zhang^{1},
 Jie Xu^{1},
 Weiwei Mu^{1},
 Jun Xu^{1}Email author,
 Wei Li^{1} and
 Kunji Chen^{1}
DOI: 10.1186/1556276X928
© Zhang et al.; licensee Springer. 2014
Received: 22 October 2013
Accepted: 5 January 2014
Published: 14 January 2014
Abstract
The nonlinear optical properties of nanocrystallineSi/SiO_{2} (ncSi/SiO_{2}) multilayers have been investigated through Zscan technique by using a Tisapphire laser with 50fs 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 selfdefocusing to selffocusing. We propose that the localized states at the ncSi/SiO_{2} 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 ncSi, which can be used as engineering different nonlinear optical devices.
Keywords
Optical nonlinearities Nanocrystalline Si Multilayers Interface state 42.65.k 42.65.Jx 42.65.PcBackground
Linear and nonlinear optical properties in Sibased 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 nearinfrared spectral range has been observed in nanocrystalline Si (ncSi) films and alloptical switch as well as optical amplifier based on ncSi has been realized [6–8]. So far, nonlinear optical properties have been observed in ncSi 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 [9]. Martínez et al. fabricated ncSi films by three different deposition techniques: ebeam evaporation, plasmaenhanced chemical vapor deposition, and lowpressure chemical vapor deposition (LPCVD), and they found that the ncSi films prepared by LPCVD show the saturation absorption property, while the other two samples displayed the reverse saturation absorption characteristics [10]. More recently, Ma et al. observed the tunable nonlinear absorption behaviors by changing either the incident laser intensity or the bandgap of ncSi films [11]. Therefore, it is one of the important issues to further understand the nonlinear optical properties of ncSi 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 ncSi film. Prakash et al. reported the sizedependent nonlinear optical coefficient, and they attributed it to the increase of oscillator strengths because of the quantum confinementinduced localization of electron–hole pairs [6]. Meanwhile, the localized defect states are also proposed to affect the nonlinear optical properties of ncSi films. Ito et al. found that the nonlinear refractive index did not decrease monotonously with the size of ncSi, and they believed that both the quantized electronic states and defect states contributed to the large nonlinear refractive index [12]. In our present work, we systematically studied the nonlinear optical properties of Si/SiO_{2} multilayers during the transition process from amorphous phase to nanocrystalline Si state. We found tunable nonlinear optical behaviors, reverse saturation absorption in the amorphousphase dominant samples, and saturation absorption in the nanocrystallinephase dominant ones, under femtosecond laser excitation. The nonlinear refraction was also simultaneously changed. We proposed that the interface states of ncSi play the important role in the changing of nonlinear optical behaviors.
Methods
The Zscan technique [14] was applied to measure the nonlinear optical coefficients of ncSi/SiO_{2} multilayers. In this experiment, the excitation laser was a Tisapphire laser with 50fs pulse duration at 800 nm, the repetition rate was 1 kHz. The low repetition rate is in favor of reducing the thermal accumulative effect [9]. A lens with 20cm focal length was used to obtain Gaussian beam, the obtained beam waist was about 30 μm.
Results and discussion
where x = z/z_{0}, ${z}_{0}=k{\omega}_{0}^{2}/2$ is the Rayleigh diffraction length, z is the sample position from the focal point, I_{ 0 } is the excitation intensity at the focal point, ${L}_{\mathrm{eff}}=\frac{1{e}^{{\alpha}_{0}L}}{{\alpha}_{0}}$ indicates the effective thickness of the sample, α_{0} 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 [12].
where Δ Φ_{0} = k_{0}n_{2}I_{0}L_{eff} represents the nonlinear phase change. The nonlinear refraction index n_{2} of sample A is 3.34 × 10^{12} cm^{2}/W. Spano et al. also reported the negative nonlinear refraction n_{2} in the order of 10^{13} cm^{2}/W which is one order of magnitude lower than that of our sample [9]. The enhanced nonlinear optical refraction can be attributed to the strong free carrier nonlinearity in our multilayers sample via the twophoton absorption process as we discussed before. The nonlinear refractive index n_{2} in sample B is reduced to about 0.56 × 10^{12} cm^{2}/W, which is consistent with the reduced twophoton absorption process due to the enlargement of optical bandgap and the formation of ncSi. 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 n_{2} of samples C and D are 4.94 × 10^{12} and 3.47 × 10^{12} cm^{2}/W, respectively. It is worth mentioning that we also measured the n_{2} from pure SiO_{2} layer pumped under similar condition in order to exclude the contribution of SiO_{2} layers. The calculated n_{2} is 1.4 × 10^{16} cm^{2}/W, which is much lower than that of Si/SiO_{2} 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 ncSi and SiO_{2} 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 [19]. It was reported that the electrical field building up by the charges trapped at the ncSi/SiO_{2} interface states would enhance the optical nonlinear process [20]. In our proposed model, the interface states between ncSi and SiO_{2} 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.
Conclusions
In summary, we observed the tunable NLA and NLR response in Si/SiO_{2} multilayers during the transition process from the amorphous to nanocrystalline phases under femtosecond excitation at 800 nm. We suggested that the twophoton absorption process dominates in the samples mainly containing amorphous Si phases, while the phononassisted onephoton transition process between the valence band and interface states dominates the nonlinear optical properties in ncSi/SiO_{2} multilayers. The obtained NLA coefficient β is about 10^{7} cm/W and the NLR index n_{2} is about 10^{12} cm^{2}/W for ncSi/SiO_{2} multilayers which is two orders of magnitude larger than bulk Si, which indicate that ncSi/SiO_{2} multilayers can be applied into highsensitive photonic devices such as optical switch and Qswitch laser.
Abbreviations
 NcSi:

Nanocrystalline Si
 NLA:

Nonlinear absorption
 NLR:

Nonlinear refraction
 RSA:

Reverse saturation absorption
 SA:

Saturation absorption
 TPA:

Twophoton absorption
 XTEM:

Crosssectional transmission electron microscopy.
Declarations
Acknowledgements
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 Zscan measurements.
Authors’ Affiliations
References
 Hernández S, Pellegrino P, Martínez A, Lebour Y, Garrido B, Spano R, Cazzanelli M, Daldosso N, Pavesi L, Jordana E, Fedeli JM: Linear and nonlinear optical properties of Si nanocrystals in SiO_{2} deposited by PECVD. J Appl Phys 2008, 103: 064309. 10.1063/1.2896454View Article
 Sun SH, Lu P, Xu J, Xu L, Chen KJ, Wang QM, Zuo YH: Fabrication of antireflecting Si nanostructures with low aspect ratio by nanosphere lithography technique. Nano–Micro Lett 2013, 5: 18–25.
 Almeida VR, Barrios CA, Panepucci RR, Lipson M: Alloptical control of light on a silicon chip. Nature 2004, 431: 1081–1084. 10.1038/nature02921View Article
 Foster MA, Turner AC, Sharping JE, Schmidt BS, Lipson M, Gaeta AL: Broadband optical parametric gain on a silicon photonic chip. Nature 2006, 441: 960–963. 10.1038/nature04932View Article
 Zhang CQ, Li CB, Liu Z, Zheng J, Xue CL, Zuo YH, Cheng BW, Wang QM: Enhanced photoluminescence from porous silicon nanowire arrays. Nanoscale Res Lett 2013, 8: 277. 10.1186/1556276X8277View Article
 Vijaya Prakash G, Cazzanelli M, Gaburro Z, Pavesi L, Iacona F, Franzò G, Priolo F: Nonlinear optical properties of silicon nanocrystals grown by plasmaenhanced chemical vapor deposition. J Appl Phys 2002, 91: 4607–4610. 10.1063/1.1456241View Article
 Martínez A, Blasco J, Sanchis P, Galán JV, GarcíaRupérez J, Jordana E, Gautier P, Lebour Y, Hernández S, Guider R, Daldosso N, Garrido B, Fedeli JM, Pavesi L, Martí J: Ultrafast alloptical switching in a siliconnanocrystalbased silicon slot waveguide at telecom wavelengths. Nano Lett 2010, 10: 1506–1511. 10.1021/nl9041017View Article
 Sirleto L, Ferrara MA, Nikitin T, Novikov S, Khriachtchev L: Giant Raman gain in silicon nanocrystals. Nat Commun 2012, 3: 1220.View Article
 Spano R, Daldosso N, Cazzanelli M, Ferraioli L, Tartara L, Yu J, Degiorgio V, Jordana E, Fedeli JM, Pavesi L: Bound electronic and free carrier nonlinearities in silicon nanocrystals at 1550 nm. Opt Express 2009, 17: 3941–3950. 10.1364/OE.17.003941View Article
 Martínez A, Hernández S, Pellegrino P, Jambois O, Miska P, Grün M, Rinnert H, Vergnat M, IzquierdoRoca V, Fedeli JM, Garrido B: Comparative study of the nonlinear optical properties of Si nanocrystals fabricated by ebeam evaporation, PECVD or LPCVD. Phys Status Solidi C 2011, 8: 969–973. 10.1002/pssc.201000420View Article
 Ma YJ, Oh JI, Zheng DQ, Su WA, Shen WZ: Tunable nonlinear absorption of hydrogenated nanocrystalline silicon. Opt Lett 2011, 36: 3431–3433. 10.1364/OL.36.003431View Article
 Ito M, Imakita K, Fujii M, Hayashi S: Nonlinear optical properties of silicon nanoclusters/nanocrystals doped SiO_{2} films: annealing temperature dependence. J Appl Phys 2010, 108: 063512. 10.1063/1.3480821View Article
 Mu WW, Zhang P, Xu J, Sun SH, Xu J, Li W, Chen KJ: Directcurrent and alternatingcurrent driving Si quantum dotsbased light emitting device. IEEE J Sel Topics Quantum Electron 2014, 20(4):8200106.
 SheikBahae M, Said AA, Wei TH, Hagan DJ, Van Stryland EW: Sensitive measurement of optical nonlinearities using a single beam. IEEE J Quantum Electron 1990, 26: 760–769. 10.1109/3.53394View Article
 Ikeda K, Shen Y, Fainman Y: Enhanced optical nonlinearity in amorphous silicon and its application to waveguide devices. Opt Express 2007, 15: 17761–17771. 10.1364/OE.15.017761View Article
 Wang K, Long H, Fu M, Yang G, Lu P: Sizerelated thirdorder optical nonlinearities of Au nanoparticle arrays. Opt Express 2010, 18: 13874–13879. 10.1364/OE.18.013874View Article
 LópezSuárez A, TorresTorres C, RangelRojo R, ReyesEsqueda JA, Santana G, Alonso JC, Ortiz A, Olive A: Modification of the nonlinear optical absorption and optical Kerr response exhibited by ncSi embedded in a siliconnitride film. Opt Express 2009, 17: 10056–10068. 10.1364/OE.17.010056View Article
 Yin M, Li HP, Tang SH, Ji W: Determination of nonlinear absorption and refraction by single Zscan method. Appl Phys B 2000, 70: 587–591. 10.1007/s003400050866View Article
 Takagahara T, Hanamura E: Giantoscillatorstrength effect on excitonic optical nonlinearities due to localization. Phys Rev Lett 1986, 56: 2533–2536. 10.1103/PhysRevLett.56.2533View Article
 Jiang Y, Wilson PT, Downer MC, White CW, Withrow SP: Secondharmonic generation from silicon nanocrystals embedded in SiO_{2}. Appl Phys Lett 2001, 78: 766–768. 10.1063/1.1345825View Article
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