High-Q, submicron-confined chalcogenide microring resonators

We demonstrate high quality (Q) factor microring resonators in high index-contrast GeSbSe chalcogenide glass waveguides using electron-beam lithography followed by plasma dry etching. A microring resonator with a radius of 90 μm shows an intrinsic Q factor of 4.1 × 105 in the telecom band. Thanks to the submicron waveguide dimension, the effective nonlinear coefficient was determined to be up to ∼110 W−1m−1 at 1550 nm, yielding a larger figure-of-merit compared with previously reported submicron chalcogenide waveguides. Such a high Q factor, combined with the large nonlinear coefficient and high confinement, shows the great potential of the GeSbSe microring resonator as a competitive platform in integrated nonlinear photonics. © 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement


Introduction
An abundance of nonlinear optical effects have been realized in integrated photonic platforms [1], revealing their potentials in many applications, including coherent telecommunication [2], quantum optics [3], spectroscopy [4], and optical frequency combs [5,6]. In recent years, tremendous efforts have been made to develop integrated nonlinear platforms using different materials, including Group IV semiconductors like silicon [7], Group III-V semiconductors like AlGaAs [8,9], and dielectrics like silicon nitride [10,11]. Many efforts have been made to develop nonlinear photonic devices on the silicon platform due to their compatibility with well-developed CMOS technology and the high-index contrast [12]. However, the silicon platform suffers from strong two-photon absorption (TPA) and TPA-induced free carrier absorption (FCA). This especially becomes serious when the material is exposed to light with a wavelength below 2.2 µm with photon energy approaching half of the optical bandgap of silicon (∼1.1 eV), where the performance of the materials would be seriously deteriorated due to TPA [13]. This in turn precludes silicon from many nonlinear applications in the telecom windows. Instead, a large bandgap nonlinear material with negligible or low TPA effect, multi-efficient nonlinear processes, and low propagation loss is highly desirable.
Chalcogenide glass (ChG) with inherent wide bandgap is considered promising for nonlinear integrated optics due to their exceptionally high nonlinearities (Kerr, Raman, and Brillouin coefficients), which are two or three orders of magnitude greater than silica and comparable or superior to silicon, low two-photon absorptions (TPA) and no free-carrier effects [14]. More importantly, ChGs have broad transparency from the visible to the mid-infrared up to 20 µm depending on the chemical compositions of the materials [15]. The bandgap value can be adjusted between 1.5 eV and 2.5 eV by changing the composition ratio of chalcogenide glass [16][17][18]. The inherent high refractive index (n = 2∼3) enables a small mode volume without excessive radiative loss. Besides, ChGs are highly scalable and can be easily deposited on any substrates without any "lattice mismatch" issue using low-cost, large-area deposition techniques such as thermal evaporation [19]. ChGs have already been extensively used for many applications, including chemical and biological sensors [20,21], broadband supercontinuum generation [22,23], stimulated Brillouin scattering (SBS) [24][25][26], and Raman amplification [27]. However, so far low propagation losses of 0.05 ∼ 0.5 dB/cm have been achieved only in large dimension ChG waveguides (usually widths of several micrometers) [28,29], limiting the achievable nonlinear coefficient. High index-contrast ChG waveguide with submicron mode confinement and compelling loss performance could provide large nonlinear coefficient and thus could be a very promising platform for nonlinear integrated photonics.
In this paper, utilizing a high index and nontoxic Ge 28 Sb 12 Se 60 platform, we demonstrate high index-contrast microring resonators on an insulator platform using electron-beam lithography followed by plasma dry etching. The resonators exhibit quality factors beyond 4.1 × 10 5 , corresponding to a ∼1.3 dB/cm propagation loss. The waveguides with submicron confinement enable a high nonlinear coefficient of 110 W −1 m −1 at 1550 nm and efficient dispersion engineering for GeSbSe waveguides. The thermo-optic effect of the fabricated microring resonators is also characterized to estimate the thermal drift of the devices.

Design and fabrication
Figure 1(a) shows the 3D cross-sectional view of the proposed waveguide. The waveguide core is GeSbSe, cladded with air from top and side and with silicon dioxide from the bottom. The refractive index of the GeSbSe film was determined to be ∼2.8 at a wavelength of 1550 nm using ellipsometry (J.A. Woollam RC2 XI+). High index-contrast and strip waveguide structure was chosen to provide both strong light confinement and efficient dispersion engineering. In this platform, the waveguide width w and height h are two parameters that can be tailored according to specific applications. Figure 1(b) presents the effective index of TE modes in the GeSbSe waveguides as a function of the width at several different waveguides heigh at a wavelength of 1550 nm. We see that an effective index above ∼1.7 can be achieved over a wide range of parameter space. The dashed line represents the critical structural parameters below which the waveguide can only support the fundamental mode in transverse electric (TE) polarization. We choose the cross-sectional dimension in the submicron scale with a thickness of ChG layer h = 300 nm, enabling strong optical confinement of the waveguide mode in the vertical direction. Another essential factor that needs to be considered for the design is the bending loss. To extract the bending loss, we calculate the fundamental TE modes of the resonator using the finite difference method (MODE Solutions, Lumerical Inc.) as shown in Fig. 1(c). The inset of Fig. 1(c) shows the fundamental TE mode profile of a bending waveguide with a radius of 5 µm, where the waveguide width and height are 720 nm and 300 nm, respectively. Thanks to the high index-contrast of the material, the calculated radiation loss derived from the leakage of the sharp bend is negligible for a ring resonator with a radius larger than 50 µm. Such small radius will benefit more compact devices. Figure 2(a) shows the map of group velocity dispersion (GVD), defined by D=−(λ/c)·(d 2 n eff /d 2 λ), as a function of the waveguide width and wavelength for a 300 nm high waveguide and TE 0 mode. The black solid curve shows the trace of the zero-dispersion wavelengths, with regions to the right side of this curve belonging to the anomalous dispersion regime. We find apparent dispersion tailoring engineering of the proposed waveguides thanks to the high index-contrast characteristic. The zero-dispersion wavelength can be efficiently shifted over more than 200 nm (1500 to 1700 nm) when the width is increased by only 80 nm. Figure 2(b) shows the dispersion parameter at the waveguide width of 720 nm, indicating near-zero anomalous dispersion at 1550 nm that facilitates four-wave mixing and soliton-based nonlinear applications. The inset in Fig. 2(b) shows the simulated optical field of the fundamental TE mode of the 720 nm wide GeSbSe waveguide. Due to the strong mode confinement, the effective mode area (A eff ) is as small as 0.17 µm 2 .
We calculate the waveguide nonlinear coefficient of the waveguide with the definition γ = 2πn 2 /λA eff [1], where n 2 is the nonlinear refractive index [30]. Compared to the wellrecognized low index-contrast waveguides, e.g., silicon nitride waveguide with typical A eff of ∼1.0 µm 2 and n 2 of 2.5×10 −19 m 2 W −1 [11,31] , which is one orders of magnitude smaller than GeSbSe. The nonlinear coefficient γ of the proposed GeSbSe waveguide is calculated to be up to 110 W −1 m −1 at 1550 nm, which is more than 100 folds larger than that of silicon nitride waveguides. This nonlinear coefficient value is also comparable to that achieved in silicon nanowires [32].
We fabricated the photonic device in ChG using the following process [33]. First, bulk GeSbSe glass was prepared by the traditional melt-quenching method [34]. A 300 nm thick GeSbSe film was deposited by thermal evaporation onto a 4-inch silicon wafer with a 2 µm thick thermal oxide layer, followed by a 30 nm thick SiO 2 film deposited by magnetron sputtering onto the surface of the GeSbSe film to protect chalcogenide films from damage during development. Then, the devices were patterned with a 200 nm thick ARP6200 photoresist using electron beam lithography (RAITH e-LINE Plus). The exposed structures underwent a plasma treatment step with O 2 /Ar/CHF 3 gases to remove residues after the development process. Subsequently, the underlying GeSbSe layer was fully etched utilizing a mixture gas of CHF 3 and CF 4 at low pressure via inductively coupled plasma (ICP) etching (Oxford 100). After etching, the resist residue was removed by oxygen plasma cleaning and then soaking the sample in N-methyl-2pyrrolidone (NMP) for 5 min. Figure 3(a) shows a scanning electron microscope (SEM) image of the fabricated GeSbSe waveguide cross section. A vertical sidewall is achieved with a good anisotropy, showing the high quality of the fabrication procedures. Figure 3(b) shows a waveguide coupled Ge 28 Sb 12 Se 60 microring, and the coupling region marked by the red box is enlarged in Fig. 3(c), where a clear interface feature indicates a good pattern definition in our process.

Device characterization
Fully etched fiber-waveguide grating couplers were fabricated to measure the microring resonators.
In the experiments, continuous wave with sub-MHz linewidth from the tunable laser (Santac TSL-550) was coupled into and out of the waveguides by first-order Bragg scatting via grating couplers collected on the other side of the waveguides into a low-noise power meter (Santec MPM210) via single-mode fibers. The power of the coupled light was selected to be low enough (∼ 0.1 mW) to minimize any nonlinear effects. We measured the spectral response of the regular fully etched grating couplers (not shown here). The spectra suffer from severe oscillation ripples with an extinction ratio of ∼5 dB due to the high index contrast of GeSbSe gratings. A sub-wavelength grating coupler was introduced to dramatically reduce the Fresnel reflection coefficient to alleviate this issue [35]. Figure 4(a) shows a scanning electron microscopy (SEM) image of the proposed sub-wavelength grating coupler. The optimized sub-wavelength grating coupler for the TE mode has 920 nm grating period, 538 nm grating width, and 83 nm sub-wavelength grating width. The incident angle for the sub-wavelength TE grating couplers is 10°. Figure 4(b) shows the measured spectral response of a fully etched sub-wavelength grating coupler with slight ripples. The peak coupling efficiency for a single grating is measured to be ∼21% (-6.8 dB) from the experimental data. We analyzed a series of ring resonators with a radius of 90 µm (see Fig. 2(b)) to characterize the optical quality factor at telecom wavelengths. To study the coupling behavior of the resonators, we fabricated identical devices with coupling gaps varying from 100 nm to 300 nm, thus allowing us to examine ring resonators in both the over-coupled and under-coupled regimes [36]. The gap between the ring resonator and waveguide is 180 nm when reaching the critical coupling condition. Figure 5(a) shows a representative transmission spectrum of the resonators for TE 0 mode. Only one mode family with a free spectral range (FSR) of ∼1.35 nm was observed in the spectrum, implying that the resonator with anomalous dispersion can be a single-mode operation. When the coupling gap was optimized for critical coupling, we found an extinction ratio up to 23 dB. Figure 5(b) shows the loaded Q-factor (Q l ) is estimated to be 2.1×10 5 by fitting the individual resonance with a full width at half maximum (FWHM) of 7.4 pm using the Lorentzian model. The intrinsic Q-factor (Q i ) of the resonator is calculated to be 4.1×10 5 by using the equation for under-coupled devices [37], where Q load is the measured loaded Q, and T 0 is the fraction of transmitted power at resonance wavelength. The propagation loss α (in cm −1 ) was calculated by [38], where λ r is the resonant wavelength, and Q in represents the intrinsic quality factor. The group refractive index n g is correlated with the free spectral range (FSR) via, where L is the perimeter of the ring resonator. A small propagation loss of α = 1.3 dB/cm is obtained using Eq. (1). The intrinsic Q i of microring resonator is caused by several factors: intrinsic radiative losses (Q rad −1 ), material absorption losses (Q m −1 ), scattering loss due to surface inhomogeneities (Q s −1 ), i.e., the intrinsic Qi can be expressed as: Here Q i is measured to be 4.1×10 5 , Q rad is calculated to be ∼5.2×10 17 using COMSOL Multiphysics when the radius is 90 um, Q mat is estimated to be 2.9×10 6 through the formula Q mat = 2πn g /αλ [39], where n g is the group index of material, α is the absorption loss of material [40]. Our measured Q i factors are much smaller than both Q rad and Q mat , suggesting that the Q factor is limited by the scattering loss from the surface and edge of the waveguide. We can further enhance the Q factor for the GeSbSe microresonators by optimizing the fabrication process. Here the waveguide loss comes mainly from the sidewall roughness since it can be further reduced by increasing the waveguide width in our experiment. Recently, M. Grayson et al. reported that the GeSbSe waveguide loss could be decreased to 1 dB/cm by the thermal annealing process [41]. However, the cross-sectional geometry of the reflowed waveguide is severely deformed, which prevents them from efficient dispersion engineering on demand of emerging nonlinear applications. B. Zhang et al. have proposed a unique method to optimize the waveguide fabrication process using polymer-assisted thermal treatment, which shows a great potential to reduce the waveguide loss with an unchanged geometry [42]. Table 1 summarizes several submicron waveguides reported with different ChG materials. Here, we use the attenuation-related figure-of-merit (FOM) defined by γ/α [42]. As shown in Table 1, the FOM of the GeSbSe waveguide is the highest one among these platforms, which makes it the most suitable platform for nonlinear parametric processes. In addition, the linear refractive index of GeSbSe is larger than those of the other platforms, enabling compact circuits and efficient dispersion engineering. For the platforms based on GeSbSe, our device shows superior FOM, which is 4-folds higher than that in [43].

Thermal characteristics on the ChG microring
We further characterized the thermo-optical properties of the GeSbSe microring resonator. In the experiment, a thermoelectric controller (TEC) was placed beneath the chip to accurately control the temperature of the substrate. Figure 6(a) shows the measured transmission spectrum of the ring resonators with temperatures range from 20°C to 60°C at a step of 5°C, which shows that the resonant wavelength increases with the temperature. Figure 6(b) shows that the wavelength drift induced by temperature is linear, indicating the temperature-dependent resonant wavelength shift dλ res /dT ∼63.4 pm/K. The temperature-dependent resonance wavelength shift for a ring resonator is mainly attributed to both thermal-optical and thermal expansion effects, which is given by [47]. dλ res dT = λ res (︃ 1 n eff where λ res is the resonant wavelength, n eff is the effective index of resonant mode, and R is the principal radius of the microring. Since the optical mode is highly confined in the ChG waveguide core, the temperature-dependent effective index of optical mode is primarily determined by the thermooptic coefficient of the material, indicating (1/n 0 )C TOC ≡ (1/n 0 )(dn 0 /dT) = (1/n eff )(dn eff /dT) with n 0 being the refractive index of bulk ChG. Using the linear thermal expansion coefficient C CTE ≡ 1/R×dR/dT of 1.4×10 −5 /K for GeSbSe glass at room temperature [33,40], the thermo-optic coefficient C TOC can be approximated from Eq. (3) as 5.66×10 −5 /K, which is in agreement with those reported in [40].

Conclusion
We have successfully fabricated dispersion-engineered chalcogenide microring resonators in high index-contrast GeSbSe ChG waveguide using e-beam lithography and plasma etching. A microring resonator with a radius of 90 µm shows an intrinsic Q around 4.1 × 10 5 at 1550 nm, which is comparable with the previously reported Qs of ChG microrings with different compositions [38,44,45]. Benefitting from the submicron mode confinement, the nonlinear coefficient is up to 110W −1 m −1 , which is one of the highest achievable values among ChG waveguide platforms. In addition, a single grating coupler with high peak efficiency of 21% was achieved. The thermal responses for the resonant wavelength of the microring were measured to be ∼63.4 pm/K, and the thermo-optic coefficient of GeSbSe was extracted to be 5.66 × 10 −4 /K. Hence, ChG microring resonators with advantages of dispersion engineering, high-Q, and high-nonlinearity can find rich applications in power-efficient nonlinear photonics.