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Negligible nonlinear absorption in hydrogenated amorphous silicon at 1.55μm for ultra-fast nonlinear signal processing Xin Gai,* Duk-Yong Choi, and Barry Luther-Davies Laser Physics Center, the Australian National University, Canberra, ACT 0200, Australia * xin.gai@anu.edu.au

Abstract: Three-photon absorption (3PA) has been observed as the dominant mechanism for nonlinear absorption in wide-bandgap hydrogenated amorphous silicon (a-Si:H-W) at 1.55μm. The nonlinear index n2 and 3PA coefficient were measured to be 22 × 10−17m2/W and 5.0 × 10−26 m3/W2 respectively at 1.55μm by using the z-scan method. This indicates that the figure of merit (FOM) of this material is intensity dependent. A value FOM>60 is predicted at intensities below 0.5 GW/cm2 which is the maximum practical intensity for high-bit-rate (>160GB/s) alloptical signal processing. The nonlinear phase change in a-Si:H-W has been compared with other common nonlinear materials (c-Si, As2S3, Ge11.5As24Se64.5) for a 2cm long waveguide with a-Si:H-W showing the greatest potential for integrated devices for all-optical processing with a high nonlinear index and negligible nonlinear absorption at intensities < 0.5GW/cm2. ©2014 Optical Society of America OCIS codes: (130.3130) Integrated optics materials; (190.4400) Nonlinear optics, materials; (320.7110) Ultrafast nonlinear optics.

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Received 14 Feb 2014; revised 3 Apr 2014; accepted 3 Apr 2014; published 17 Apr 2014 21 April 2014 | Vol. 22, No. 8 | DOI:10.1364/OE.22.009948 | OPTICS EXPRESS 9948

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1. Introduction Hydrogenated amorphous silicon (a-Si:H) has attracted a lot of attention as a platform for nonlinear optics, mainly because it has a larger third order nonlinearity, n2, compared with other common materials [1–6]. The values for n2 that have been reported are 4~6 times those of crystalline Silicon (c-Si) [7]; 7~13 times those of As2S3 glass [8]; and 3~5 times those of Ge11.5As24Se64.5 (Ge11.5) glass [9, 10]. However, a drawback of a-Si:H is that, like crystalline Si, it is reported to suffer from two photon absorption (2PA) as well as 2PA induced free carrier absorption (FCA) and this ultimately will limit the efficiency of nonlinear devices [1]. The nonlinear phase change calculated over the distance limited by the two photon absorption coefficient (β2PA) is commonly used to define a figure of merit (FOM2PA) as FOM2PA = n2/(β2PAλ). A large FOM2PA indicates that a large nonlinear phase change can be obtained, and this is required for efficient four-wave mixing [11–13]; all-optical switching [14, 15]; and supercontinuum generation [10, 16]. However, a-Si:H has been reported to have a relatively small FOM2PA in the telecommunications band [1] and this suggests it would not be as effective for nonlinear optics as, for example, chalcogenide glasses where 2PA is generally negligible. Significant attempts have been made to reduce 2PA in a-Si:H. It is well known that the properties of a-Si:H vary with the hydrogen concentration produced in different deposition conditions. In particular the band gap energy, Eg, can be changed from 1.5eV up to 2.1eV [17]. Because of their dispersive natures, the values of the nonlinear refractive index, n2, and the two photon absorption coefficient, β2PA, depend on the ratio between the photon energy Ep = ħω and band gap energy Eg and, hence, tuning the bandgap can have a large effect on the nonlinear properties [18]. According to Dinu’s model [18], when Ep/Eg < 0.7, β2PA drops at a much faster rate than n2 with decreasing Ep/Eg, and this results in an increasing FOM2PA. This implies that the main way of improving the performance of a-Si:H waveguides is to increase the bandgap of the material by optimizing the processing conditions. As a result, β2PA in aSi:H has progressively dropped from 4.1e−11m/W in 2010 [1] to 0.25e−11m/W in 2012 [2, 3] although n2 has also dropped by a factor of 2, from 42e−18m2/W to 21e−18m2/W. Nevertheless, this led to an increase in FOM2PA from 0.65 to about 5. Although β2PA was thereby reduced by more than an order of magnitude and a FOM of 5 is generally assumed to be big enough for many nonlinear processes, device performance is never quite as good as would be expected because 2PA-induced FCA degrades the efficiency. This is especially the case for long pulse, CW and high repetition rate pulsed lasers that are used in ultrafast signal processing because the free carrier lifetime is quite long, typically ≈400ps [19]. As a result, an even higher FOM2PA is desirable since this will reduce the rate at which the free carriers are generated and, hence, the accumulated free carrier density Nfc. Taking chalcogenide glasses as an example, the FOM2PA of As2S3 is reported to be >120 [8] and Ge11.5As24Se64.5 is > 60 at 1.55μm [9, 10]. Devices made from these materials are not limited by either 2PA or FCA. It is logical to expect, and predicted by the theoretical models [18, 20], that 2PA should vanish when the photon energy is less than half the band gap energy, Eg (although this condition does not strictly apply to amorphous materials since defects and disorder extend tail states into the band gap). However, even if 2PA has been completely eliminated because the photon energy is well below 0.5Eg, three-photon absorption (3PA) can be present and in some conditions can dominate the nonlinear absorption. This has been shown to be the case in c-Si in the mid-infrared [7, 21] as well as in the other materials like GaAs, ZnO and CdI2 [22]. One thing that needs to be emphasized is that a new figure of merit, FOM3PA, must be introduced which defines the nonlinear phase change due to n2 that can be accumulated over a distance limited by three-photon absorption. Whilst the FOM2PA depends only on material parameters, FOM3PA can be written as FOM3PA = n2/(β3PAIλ) and depends inversely on

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intensity. The impact of an intensity dependent FOM3PA on waveguide nonlinear optics has so far only been discussed in the case of AlGaAs waveguides [23]. In this paper, we report for the first time the observation of 3PA at 1.55μm in a-Si:H films tuned to have a wide band gap: to distinguish this material from that with a narrow bandgap we designate the material used in this study as a-Si:H-W. The dispersion of the third order nonlinearity, (n2 and β2PA) as well as the FCA coefficients have been measured by the z-scan method for wavelengths between 1.15 and 1.55μm. The impact of 3PA on the potential of aSi:H-W for nonlinear processing is discussed. Material preparation and characterization We deposited a 6.5μm thick a-Si:H-W film onto a 100μm thick borosilicate cover glass by plasma enhanced chemical vapour deposition (PECVD) with SiH4 and He gas. The deposition conditions are shown in Table 1. Thinner films 0.2µm thick were also deposited onto oxidized silicon wafers and were used to determine the optical loss of the films. The hydrogen concentration was measured by Fourier transform infrared spectroscopy (FTIR) and the band gap energy was determined using a Filmtek 4000 spectroscopic reflectometer. At a deposition temperature of 250°C, the film contained about 10% hydrogen and had a band gap of 1.73eV which is consistent with the values reported by Fukutani [17]. According to our FTIR results, shown in Fig. 1(a), the hydrogen in the film was mainly associated with Si-H bonds (centred at 2000cm−1) rather than Si-H2 (centred at 2100cm−1). This suggests that the film was of good quality since Si-H passivates Si dangling bonds and has the effect of reducing the defect density whilst Si-H2 causes extra scattering losses [24]. Table 1. Hydrogenated amorphous silicon deposition condition Deposition system

Plasmalab 100 (Oxford)

Precursor gas Carrier gas Process pressure

25 sccm Silane (SiH4) 475 sccm Helium 200 Pa (1500 mTorr)

RF power (13.56 MHz) Substrate temperature Deposition rate

10 W 250 ◦C 20 nm/min

The linear loss at 1.55μm of the a-Si:H-W used in these experiments was measured to be 0.2 ± 0.05dB/cm by prism-coupling a 1.55µm beam into the 0.2µm thick film deposited onto an oxidized silicon wafer and recording the decay in intensity of the propagating beam as it passed through the film using an InGaAs camera, as shown in Figs. 1(b) and 1(c). Also, high resolution transmission electron microscopy (HR-TEM) images were recorded to check for the presence of nano-crystals with no evidence of any nano-structure being found, as shown in Figs. 1(d) and 1(e). This indicates that the a-Si:H-W film was of good quality. A band gap of 1.73eV corresponds to a wavelength of 0.717μm which means that at 1.55µm, Ep/Eg = 0.46 – that is the regime where 2PA should be absent. However, 2PA cannot be completely ruled out because of the characteristic exponential Urbach tail associated with the disorder of this amorphous material which could means some 2PA remains at photon energies π rad are achieved in a-Si:H-W at an intensity of only ≈0.25GW/cm2 and well before significant 3PA and FCA turn on. This is sufficient to achieve very efficient four wave mixing. Table 3. The material parameters used in calculation of the nonlinear phase change. Material β3PA [m3/W2] β2PA [m/W] n2 [m2/W] σFCA [m2] τ0 [ps] −26 −18 a-Si:H-W 400 5.0 × 10 22 × 10 2.0 × 10−21 a-Si:H-N [2] 400 [19] 0.25 × 10−11 22 × 10−18 2.0 × 10−21 400a c-Si [7] 6.7 × 10−18 1.45 × 10−21 1.03 × 10−11 −18 As2S3 [8] 2.9 × 10 Ge11.5 [10] 7.5 × 10−18 a the τ0 value of c-Si is larger than 400ps, but we took the same value as for a-Si:H-W for convenience;

In Fig. 6(b) we show the behaviour in regime B using 2ps duration low duty cycle pulses. In this case, saturation is caused directly by 2PA or 3PA and the generation of free carriers has a negligible affect at low intensities (2.5GW/cm2) and contributes only about a 20% reduction in the nonlinear phase shift at the highest intensities (7.5GW/cm2). Again the aSi:H-W performs the best but its advantage compared with the other materials is not as large as in regime A. However, a nonlinear phase shift of up to 15π rad is predicted which is more than adequate for all-optical switching, parametric amplification and supercontinuum generation.

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Fig. 6. Nonlinear phase change. (a) The nonlinear phase change as a function of intensity for a CW laser and a 2cm long waveguide. Regime A is defined by the maximum practical intensity for high-bit-rate processing (>160Gb/s). (b) The nonlinear phase change as a function of intensity for a 2ps pulse from a 2cm long waveguide. Regime B is defined by the practical intensity for low duty cycle but high peak power processing. The black solid is for α-Si:H-W excluding FCA; the black dashed is for α-Si:H-W with FCA; the red solid is for α-Si:H-N excluding FCA; the red dashed is for α-Si:H-N with FCA; the green solid line is for c-Si with FCA; the light blue line is for As2S3; and the dark blue line is for Ge11.5.

4. Conclusion

In conclusion, we have observed 3PA at 1550nm in a-Si:H for the first time by increasing the band gap energy via control of the deposition conditions. The nonlinearities including n2, β2PA, β3PA, σFCA and σFCN have been measured by the z-scan method using a numerical model to analyse the data. With n2≈22 × 10−17m2/W and β3PA≈5.0 × 10−26 m3/W2, a-Si:H-W has an intensity dependent FOM3PA and a value over 60 can be obtained at an intensity levels appropriate for all-optical signal processing, which is more than one order improvement compared with reported values for a-Si:H in which 2PA dominates. With its large value of n2 and large FOM3PA at low intensity, a-Si:H-W shows negligible nonlinear absorption indicating its better potential for ultra-fast processing than other materials such as a-Si:H-N, chalcogenide glasses, c-Si, etc. Its advantage for high intensity nonlinear optics is not as obvious because the FOM3PA drops with increasing intensity. However, by further increasing the bandgap, β3PA can be reduced and this should create a material that has advantages in all applications involving third order nonlinear optics. Acknowledgments

Dr. Xin Gai is supported by an Australian Research Council (ARC) Discovery project DP130100086 and the ARC Centre of Excellence for Ultrahigh Bandwidth Devices for Optical Systems (project number CE110001018). Dr. Duk-Yong Choi is supported by an ARC Future Fellowship FT110100853. Device fabrication was supported by the ANU node of the Australian National Fabrication Facility (ANFF).

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