Introduction

Waveguides based on chalcogenide glasses have seen great strides for both linear and nonlinear applications. Particularly significant because their band gap energies lie within the visible and near – infrared, many chalcogenide glasses have the advantage of a negligible two photon coefficient at the 1.55 μm wavelength. When combined with a relatively large nonlinear refractive index, chalcogenide glasses provide a promising means towards high efficiency nonlinear optical signal processing applications. As a consequence of its large optical nonlinearity, Kerr–based all–optical processes which have an intrinsic response time of less than 100 fs can be implemented very efficiently in chalcogenide glasses1,2. The high photosensitivity resulting from the inherent structural flexibility in chalcogenide glasses enables its utilization in writing Bragg gratings in fiber as well as waveguides3. As a result of the aforementioned qualities, chalcogenide glasses are highly promising for all–optical signal processing such as switching, wavelength conversion and regeneration4,5,6,7.

Chalcogenide glasses consist of the chalcogen elements (group 16 in the periodic table) such as sulfur (S), selenium (Se), and tellurium (Te), combined with network–forming elements such as silicon (Si), arsenic (As), germanium (Ge), phosphorus (P) and antimony (Sb). Therefore, the properties of chalcogenides such as their refractive indices, band–edges, and nonlinearity can be tuned according to its composition. Chalcogenide glasses are reported to get higher nonlinear optical properties than oxide glasses due to low vibrational energies of bonds induced by heavy chalcogen atoms. It enables their optical transparencies to extend up to mid – IR wavelength ranges. The glass density is also larger than that in oxide glasses that result in larger linear refractive index of 2–3. The optical merits such as wide transparency and large refractive index facilitate strong mode confinement and complete bandgap engineering for photonic devices. In particular, Ge–based glasses bonded with heavy metal oxide (e.g. Sb) create smaller multiphoton spectra for laser and fiber optic applications, therefore, ternary Ge–Sb–S system (GeSbS) has been widely investigated by its compositional dependence8. Ge23Sb7S70 has been proven as one of promising waveguide materials because of low toxicity compared to As–based chalcogenide glasses9,10,11. In addition, thermally evaporated Ge23Sb7S70 films do not change much after alpha irradiation due to relatively high degree of radiation hardness12. In this way, Ge23Sb7S70 film has been deposited on a silica substrate to fabricate ridge waveguides using photolithography and lift-off, facilitating cost–effective and fine–line pattering13.

Ge23Sb7S70 film – based waveguides have previously been demonstrated for use in mid – IR chemical sensing, and as a host for PbTe – based photodetection12,14. Their nonlinear optical properties however, have not been studied to date at fiber – based experiments. GeSbS is a chalcogenide which possesses optical transparency far into the mid–IR region, negligible two–photon losses at 1.55 μm and relatively large optical nonlinearities15. It was previously put forth by Harbold et al. that n2 is dependent on resonance enhancement of the optical bandgap as well as on the lone pair electron concentration between chalcogen and network–forming elements showing the relation as 1/Eg4 (Eg = electric bandgap energy)16,17,18,19. A similar phenomenon has also been observed in CMOS photonic materials, where the nonlinearity of the films scales inversely with Eg20,21,22,23. In the study on the composition of Ge23Sb7S70-xSex using X–ray photoelectron spectroscopy24, an increase in Ge-Se bond from 2 × 10–21 to 8 × 10−21/cm3 results in an increase in the n2 from 2 × 10−18 to 10 × 10−18 m2/W. Petit et al. summarized the composition of the investigated GeSbS chalcogenide glasses at 1.064 μm showing n2 of 1.7–2.6 × 10−18 m2/W, nonlinear absorption <0.1 cm/GW and bandgap energy of 1.8–2.3 eV, which in aggregate makes the material a good candidate for applications at telecommunication wavelengths or beyond19,25.

Thanks to wide transparencies and low nonlinear absorptions of chalcogenide materials, a number of studies on supercontinuum generations have been implemented26,27,28, in particular, the recent researches focus on getting wideband spectra within the mid – IR range29,30,31. Recently, Yu et al. succeeded to fabricate 18–mm–long GeAsSe rib waveguide then to obtain supercontinuum spanning from 2 to 10 μm of wavelength by pumping 330 fs laser with pump wavelength of ~4 μm32. The potential of on–chip chalcogenide platforms for the generation of wideband spectral broadening using cheaper, more easily available near – IR pumps to realize supercontinuum spectra covering near to mid – IR wavelengths are also of great merit. Within the regime of supercontinuum pumped at the telecommunications wavelength, the dispersion – engineered planar As2S3 waveguide has been reported to generate supercontinuum spanning of 750 nm at −30 dB level at TM mode using a 610 fs fiber laser as a pump of 1.55 μm wavelength33.

In this manuscript, we investigate the nonlinear optical properties of GeSbS including the nonlinear refractive index and nonlinear absorption properties at the shortwave–IR range. Waveguides are also used in nonlinear spectral broadening experiments, and supercontinuum generation extending beyond the 2 μm wavelength is achieved using 500 fs pulses centered at 1.55 μm. The results suggest that GeSbS waveguides could also be used successfully for nonlinear optics applications extending to the mid – infrared range, which suggests the potential of the platform to achieve broadening covering both near– and mid– IR range.

Results

The refractive index of Ge23Sb7S70 film (nfilm) is measured using Fourier transform infrared spectroscopy to be 2.17 at 1.55 μm of wavelength, which depicts a refractive index difference (Δn) of 0.73 between the film and SiO2 substrate. Figure 1(a) shows the measured n, k values of the Ge23Sb7S70 film. The k – value of the film is observed to be negligible from 600 nm past 2.5 μm. Using Tauc’s method, the band gap of the films are extrapolated to be ~2.5 eV. The GeSbS ridge waveguide has a length of 15 mm, a width (Wc) and height (H) of 2 μm and 1.3 μm, respectively, as shown in Fig. 1(b). Input and output ports in the waveguide possess tapered couplers with a width (W) of 15 μm. W changes linearly into Wc over a distance of 5 mm from both input and output ports. The length of the straight section of the waveguide with width, Wc is 5 mm. The input tapers facilitate fiber – waveguide coupling. The effective refractive indices of the waveguide (neff) at quasi – TE mode are calculated using a fully vectorial beam propagation method from RSOFT as shown in Fig. 2(a). neff ranges from 2.12 to 2.05 at wavelengths between 1.3–1.8 μm. Waveguide dispersion (D), derived from neff values obtained in the simulation program, is −60 ps/nm/km at 1.55 μm that lies in normal dispersion as shown in Fig. 2(b). Compared with material dispersion (~−240 ps/nm/km at 1.55 μm), the waveguide dispersion while still normal, experiences a reduction in the magnitude of dispersion, implying the potential to achieve phase matching for parametric processes by engineering the dispersion. In addition, it implies that the waveguide geometry has somewhat impacted the magnitude of the dispersion though not to the extent available in high index contrast platforms such as silicon on insulator or silicon rich nitride on insulator.

Figure 1
figure 1

(a) Measured n and k values for λ = 130 nm–25 μm and (b) Schematic of GeSbS chalcogenide waveguide (Sky color: SiO2 substrate, light green color: Ge23Sb7S70 film).

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Figure 2
figure 2

(a) Refractive index of Ge23Sb7S70 film and effective index of GeSbS waveguide, (b) material and waveguide dispersion (D) calculated from the n value.

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To characterize the nonlinear refractive index of the Ge23Sb7S70 films, self – phase modulation (SPM) experiments are firstly performed with the fabricated GeSbS waveguides. 1.8 ps pulses adjusted for the transverse – electric polarization with a 20 MHz repetition rate centered at 1.55 μm are coupled into the waveguide. The spectra of the output TE signals were observed using an optical spectrum analyzer with a measurement range of 1000 nm–2400 nm. Experimentally measured SPM –broadened spectra as the input peak power (Ppeak) is varied are shown in Fig. 3(a). Small period oscillations observed in the pulse spectra (Fig. 3(a)) are likely due to Fabry Perot oscillations arising from the effective mirrors created at the input and output tapers. Ppeak noted in Fig. 3(a) depicts the coupled input peak power compensated for waveguide loss and output fiber-waveguide coupling loss (4.3 dB per facet). The spectral bandwidth at the –30 dB level is 18, 31 and 35 nm at Ppeak = 26, 67 and 83 W, respectively. The linear increase in the bandwidth as Ppeak is increased implies that nonlinear loss such as two – photon absorption is negligible as proven by previous reports on GeSbS materials at this wavelength. The nonlinear phase shift φNL is approximately estimated by the relation as (M−0.5)π (M = the number of peaks). φNL = 1.5π is achieved approximately at Ppeak ~ 83 W.

Figure 3
figure 3

(a) Experimental and (b) theoretical SPM–induced broadened spectra as a function of input peak power, and (c) the evolution of SPM–induced spectra (wavelength–dependent intensity distribution) in terms of input peak power (d) a linear relation between input and output power is observed, implying negligible nonlinear losses.

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Using the nonlinear Schrӧdinger equation, the pulse evolution over the waveguide length as the input peak power is varied is calculated. The modeled pulse evolution as Ppeak is continuously varied is also presented as Fig. 3(c). It is observed that φNL ~ 1.5π appears when Ppeak ~ 70–100 W. Three peaks are observed when Ppeak ~ 100 W, implying that 1.5π  < φNL < 2.5π. The nonlinear parameter is extracted using the best fit to the measured experimental data. γ is estimated to be 7 W−1/m by comparing the experimental and theoretical SPM spectra shown in Fig. 3(a,b). The waveguide’s effective area, Aeff34 is calculated to be 2.15 μm2, and is an order of magnitude larger than that available in silicon on insulator waveguides21, due to the smaller index difference with the cladding. Further geometric optimization, however, could yield waveguide designs with smaller effective areas and concomitantly, larger values of γ. The nonlinear refractive index (n2) of the GeSbS film is extracted using the expression, γ = 2πn2/(λAeff) to be 3.71 × 10−18 m2/W at the 1.55 μm wavelength, comparable to that previously measured in microstructured GeSbS fibers (=2.8 × 10−18 m2/W)15. The GeSbS waveguide has fabrication errors of Wc ± 50 nm and H ± 10 nm, which impacts both D and Aeff. Since the dispersion length (LD = T02/β2) is much longer than the waveguide length, dispersive effects are negligible compared to the effects of self-phase modulation. Therefore, the uncertainty in D arising from fabrication errors (±3 ps/nm/km) also has a negligible effect on the uncertainty in γ. The effect of the fabrication uncertainty in Aeff (±0.008 μm2) however translates into an uncertainty in n2 of 0.02 × 10−18 m2/W (~0.5% of the value of n2).

Nonlinear loss characterization is also performed with the 1.8 ps fiber laser amplified using an erbium–doped fiber amplifier (EDFA). A linear relation between input and output power is measured up to 7 GW/cm2, implying the absence of nonlinear losses up to this optical intensity as shown in Fig. 3(d). The onset of two photon absorption (TPA) – induced loss has been reported to occur at intensities exceeding 1 GW/cm2 in As – based chalcogenide waveguides such as in Ag–As2Se3 chalcogenide photonic crystal waveguides35. Negligible nonlinear losses characterized at this wavelength agree with our characterization of the material’s bandgap (Fig. 1); the band gap of 2.5 eV for the Ge23Sb7S70 films implies a TPA edge at ~1 μm. Therefore, our GeSbS waveguides can be efficiently utilized for nonlinear optics applications at the telecommunications wavelength.

The ultrafast spectral broadening in GeSbS is also studied by using a 500 fs fiber laser at a repetition rate of 20 MHz and the evolution of supercontinuum generation as a function of input peak power is shown in Fig. 4. The input laser enters into a GeSbS waveguide with a taper to promote fiber-waveguide coupling. W changes linearly into Wc over a distance of 3.5 mm from the output port. The length of the straight waveguide section with width Wc is 7.5 mm. The spectrum broadens to 130, 320, 510 and 840 nm at –30 dB level for Ppeak = 120, 220, 280 and 340 W, respectively. At the maximum coupled power of 340 W, the spectrum measured at the –30 dB level broadens by a factor of 14. At Ppeak = 120 W, the output spectrum broadens 2 times larger than that in 500 fs fundamental source appearing two sidebands at around 1530 and 1590 nm of wavelength in the vicinity of pump wavelength. Additional two more sidebands at around 1480 and 1660 nm appears at Ppeak = 220 W. Sidebands at 1450 and 1750 nm are added at Ppeak = 280 W, and small peaks is shown between two sidebands of 1590 and 1660 nm. At Ppeak of 340 W, several narrow sidebands appeared at Ppeak < 340 W are merged, which further evolve into three broad sidebands at around 1450, 1670 and 1930 nm.

Figure 4
figure 4

Output supercontinuum spectrum in GeSbS waveguide as a function of input peak power.

The output spectrum broadens as input peak power increases up to 340 W.

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In analyzing the supercontinuum generated, SPM alone is insufficient to account for the broadening past 2 μm. We note further that the waveguide operates in the normal dispersion regime, and therefore nonlinear effects such as soliton fission, cascaded four wave mixing which require anomalous dispersion do not come into play. In previous work, microstructured GeSbS fiber was reported to have a Raman detuning of 9.7 THz and a large Raman gain coefficient of 1.8 × 10−11 m/W15, 180 times larger than a fused silica fiber36 and 3 times larger than a As2S3 fiber37. We predict that stimulated Raman scattering (SRS) is one mechanism contributing to the wideband supercontinuum generation, particularly on the red shifted side of the spectrum. To understand how much the Raman effect influences on the wide spectral output, we simulate supercontinuum generation that includes the terms representing group velocity dispersion (GVD), third order dispersion (TOD), SPM, self–steepening and Raman effects in nonlinear Schrödinger equation34.

The simulated supercontinuum spectra as a function of Ppeak are shown in Fig. 5. When we set slope of Raman gain (TR) = 1 ps, the theoretically obtained spectral bandwidth at −30 dB level is 140, 240, 860 and 940 nm at Ppeak = 120, 220, 280 and 340 W, respectively and matches the experimental data closely. At Ppeak = 220 W, a slightly spectral asymmetry emerges and a small Raman – induced side lobe appears. At Ppeak = 280 W, two small peaks around 1.87 and 2.07 μm emerge at power levels 3 orders of magnitude lower than that in the vicinity of 1.55 μm. These cannot be experimentally observed as a result of limitations in the sensitivity of the optical spectrum analyzer used to capture the spectrum. Consequently, the experimentally observed spectrum at Ppeak = 280 W is slightly narrower in bandwidth than the theoretical prediction. Supercontinuum spanning close to an octave and facilitated by cascaded SRS using an input power of 340 W is achieved. The spectrum broadens more extensively along the red – shifted side, and may be accounted for by the cascaded generation of the Stokes lines at wavelengths larger than the input wavelength, as verified from the theoretical curves38. Within our analysis, we expect that the Raman response is a key contributor to the generated supercontinuum. TR on the chalcogenide waveguide is estimated to be 1 ps, whereas the Raman response time of fused silica is less than 100 fs39. Consequently, the adoption of 500 fs input pulses, shorter than the duration of TR is likely to contribute to the supercontinuum.

Figure 5
figure 5

Simulated supercontinuum spectra by input peak power.

The nonlinear Schrödinger equation includes GVD, TOD, SPM, self-steepening, and Raman response effects.

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Discussions

Significant progress has been made in nonlinear all – optical signal processing using chalcogenide glasses on integrated waveguide platforms at the telecommunications wavelength. Of these, As2S3 and As2Se3 have seen the greatest strides40,41,42,43. Specialized methods of synthesizing the glasses and device fabrication are generally the challenges faced when trying to conduct studies on new chalcogenide materials. Studies on the nonlinear optical properties of GeSbS have so far been limited mainly to bulk or fiber studies15, and relatively unexplored in integrated platforms. In this study, the Ge23Sb7S70 film deposition was performed using thermal evaporation from a bulk glass. The high quality of the source glass and deposition process enables the fabricated waveguides to have linear losses which are relatively low, thus facilitating the observation of nonlinear optics phenomena. The use of multi – component materials, such as the Ge – based sulfide glass in this work, allows fabrications flexibility through solution derived glass film processing routes44, avoids the use of arsenic, and has shown the ability to add elements to improve rare – earth solubility needed for active chalcogenide glass development45. Additionally, thermally deposited GeSbS films have been shown to have outstanding adhesion to SiO2 substrates, aiding in the fabrication of high quality photonic structures with smooth sidewalls46. One key advantage of GeSbS is its very wide transparency window, spanning beyond 12 μm. Consequently, utilization of GeSbS – based waveguides are promising candidates for both linear and nonlinear optical applications from the mid – to long – infrared wavelengths.

Conclusions

In summary, GeSbS waveguides with a length of around 10 mm have been used to characterize their nonlinear optical properties and obtain large spectral broadening at the telecommunications wavelength. SPM broadened spectra using peak powers of up to 83 W are achieved and a nonlinear parameter of 7 W−1/m is extracted. The nonlinear refractive index of the GeSbS films is characterized to be 3.71 × 10−18 m2/W at the 1.55 μm wavelength. The supercontinuum generation exceeding 900 nm is achieved using a peak power of 340 W. This large nonlinear broadening in our GeSbS waveguides may be further enhanced by reducing the waveguide effective area or using even shorter input pulses. Even wider supercontinuum can also be generated using pulses which are ≤100 fs in temporal width, which is commonly used in experiments to generate supercontinuum. The relatively large nonlinear parameter and negligible nonlinear losses shown in this paper show that GeSbS waveguides could be a promising platform for nonlinear optics applications at the telecommunications wavelength and beyond.

Methods

Nonlinear Schrödinger equation including Raman effect

where A and ω0 are the field amplitude of hyperbolic secant pulse and carrier frequency, respectively. α is loss coefficient of 0.83 cm−1. The partial differential equation is numerically solved by discretizing in all but one dimension and then integrating the semi–discrete problem as a system of ordinary differential equation and differential–algebraic equation. TR is related to the slope of Raman gain providing the expression as (=R(t), Raman response function). The shock distance zs representing self–steepening effect is 5 cm at Ppeak = 340 W, which is about 2 orders of magnitude larger than LNL at the same peak power. Therefore, self–steepening is unlikely to have a significant effect on the supercontinuum generation. From the calculated dispersion, β2 = 0.073 ps2/m and β3 = 6.42 × 10−4 ps3/m at 1.55 μm.

Device fabrication

GeSbS waveguides are fabricated on Si substrates with a 3 μm thick thermal oxide layer. NR9 photoresist is first spincoated and the waveguides are patterned into the photoresist using a mask aligner. Ge23Sb7S70 film is then deposited on the patterned substrate using a thermal evaporator. Finally, the photoresist is lifted off in acetone.

Additional Information

How to cite this article: Choi, J. W. et al. Nonlinear characterization of GeSbS chalcogenide glass waveguides. Sci. Rep. 6, 39234; doi: 10.1038/srep39234 (2016).

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