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Abstract
We investigate an optical phase shifter based on Ge2Sb2Te5 (GST) integrated with a Si waveguide at mid-infrared (MIR) wavelengths. Since the optical absorption of both amorphous and crystalline GST can be reduced at a longer wavelength, we demonstrate that the optical loss of the phase shifter can be reduced at MIR wavelengths. The measured optical loss per π phase shift of a phase-change material (PCM) phase shifter at 2.32 µm wavelength is 2.6 dB/π, which is more than 80 times smaller than that at 1.55 µm wavelength (21.7 dB/π) and more than 5 times smaller than that at 1.92 µm wavelength (9.7 dB/π). Moreover, resonance wavelength tuning of an add-drop micro-ring resonator using a PCM phase shifter at 2.32 µm wavelength is demonstrated owing to the low-loss optical phase shift. These findings reinforce the applicability of the approach toward a low-loss optical phase shifter based on PCMs operating at MIR wavelengths on a Si photonic platform for quantum computing, sensing, and optical communication.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Mid-infrared (MIR) silicon (Si) photonics is expected to be a new platform for on-chip sensing [1–3], communication [4], light detection and ranging (LiDAR) systems [5], and so forth. One of the most promising applications of MIR Si photonics is quantum photonics because of (1) the low propagation loss of waveguides due to the reduction of two-photon absorption and Rayleigh scattering, and (2) the large nonlinear refractive index of Si at MIR wavelengths [6]. However, for the large-scale on-chip implementation of quantum algorithms [7–9], a low-loss optical phase shifter used to replace a thermo-optic phase shifter [10] is essential as a key building block of programmable photonic integrated circuits (PICs) [11]. In particular, a non-volatile low-loss optical phase shifter is strongly desired to maximize the performance of MIR Si programmable PICs in terms of energy efficiency and scalability.
Phase-change materials (PCMs) are expected to be used for non-volatile optical devices on a Si photonics platform using abnormally large contrasts of optical properties between amorphous and crystalline phases [12]. In particular, Ge2Sb2Te5 (GST), one of the most widely used chalcogenide PCMs [13,14], has been used in tunable metamaterials at visible to infrared wavelengths [15], and integrated Si photonics at near-infrared (NIR) wavelengths [16,17]; for example, optically and electrically controlled photonic switches [18–23], all-photonic memory [24,25], in-memory computing [26], and photonic tensor cores [27,28] have been reported. However, the large optical absorption of amorphous and crystalline GST in NIR wavelengths has hindered the realization of optical devices without optical attenuation including a low-loss optical phase shifter. Recently, several new wide-gap chalcogenide PCMs such as Ge2Sb2Se4Te1 (GSST) [29,30], Sb2Se3 [31,32], and Sb2S3 [33,34] have been proposed. Although these new PCMs exhibit reduced optical absorption at NIR wavelengths and are promising for realizing a low-loss optical phase shifter, the basic material properties such as stability, cycling endurance, and switching speed have not been investigated sufficiently. On the other hand, these basic material properties of GST have already been investigated to a sufficient degree since it has been commercially used in rewritable optical disks and phase-change random access memories (PCRAMs) [35], offering the advantages of GST over other newly developed PCMs. Therefore, it is valuable to explore the possibility of a low-loss optical phase shifter based on matured GST.
Recently, we have proposed the use of GST as a non-volatile optical phase shifter operating at MIR wavelengths [36]. Since the optical absorption of amorphous and crystalline GST at 2 µm wavelength is smaller than that at 1.55 µm wavelength, the optical loss per π phase shift at 2 µm wavelength can be significantly reduced compared with that at 1.55 µm wavelength. On the basis of this concept, in this study, we investigate the optical loss of GST at a longer wavelength. The measured optical loss per π phase shift at 2.32 µm wavelength is more than 80 times smaller than that at 1.55 µm wavelength and more than 5 times smaller than that at 1.92 µm wavelength, realizing a low-loss optical phase shifter. As a result, we demonstrate the resonance wavelength tuning of an add-drop micro-ring resonator (MRR) using a PCM phase shifter at 2.32 µm wavelength.
2. Measurement of the optical property of GST
First, we measured the optical property of GST from visible to MIR wavelengths. A 20-nm-thick GST film in an amorphous state was deposited on a bulk Si substrate by radiofrequency (RF) sputtering, and GST in the cubic crystalline state was prepared by heating an amorphous GST sample at 210 °C using a hotplate. Note that a 20-nm-thick SiO2 layer was deposited on the GST layer to prevent the oxidation of the GST film. The refractive indices and extinction coefficients of the amorphous and crystalline GST at the wavelength range from 200 nm to 2500 nm were measured by spectroscopic ellipsometry. Figures 1(a) and (b) show the measured refractive indices and extinction coefficients, respectively. Although the refractive index contrast slightly decreases from the NIR range to the MIR range, the extinction coefficients considerably decrease towards the MIR range. The extinction coefficient of amorphous GST vanishes at a wavelength of 1820 nm. Figure 1 (c) shows the calculated material figure-of-merit (FOM) defined as
where $\mathrm{\Delta} n$ and ${\mathrm{\kappa }_\textrm{c}}$ are the change in the refractive index induced by the phase transition and extinction coefficient of crystalline GST, respectively. The FOM continues to increase from the NIR to MIR ranges, indicating that a low-loss phase shifter is achievable at a MIR wavelength using GST.
Fig. 1. Measured spectra of (a) refractive index and (b) extinction coefficient of crystalline and amorphous GST. (c) FOM ($\mathrm{\Delta} n/{\mathrm{\kappa }_\textrm{c}}$) of GST as a function of wavelength.
3. Numerical analysis of the PCM phase shifter
On the basis of the measured optical property of GST, we numerically analyzed a PCM phase shifter using the Lumerical MODE eigenmode solver. The minimum mesh step of 5 nm and Metal boundary condition was used. Figure 2 shows the schematic view of a PCM phase shifter on a standard 220 nm Si-on-insulator (SOI) wafer. The thickness of the buried oxide (BOX) layer and the slab layer of the rib waveguide is 2 µm and 70 nm, respectively. To pattern waveguides and grating couplers (GCs) with a single etching, a rib waveguide with a slab thickness of 70 nm is used. The widths of the rib waveguide are 600 nm and 800 nm for the operating wavelengths of 1.92 µm and 2.32 µm, respectively, so that the single-mode condition is satisfied for each operating wavelength [37]. The electric field intensity profiles of the fundamental transverse electric (TE) mode of a PCM phase shifter with amorphous GST at 1.92 µm wavelength, crystalline GST at 1.92 µm wavelength, amorphous GST at 2.32 µm wavelength, and crystalline GST at 2.32 µm wavelength are shown in Figs. 3(a)– (d), respectively. Since the refractive index of crystalline GST is larger than that of amorphous GST, the overlap between the electric field and the GST layer is larger when GST is in the crystalline state than when it is in the amorphous state. As a result, the mode-mismatch losses of a PCM phase shifter with amorphous GST are 0.07 dB and 0.05 dB for the operating wavelengths of 1.92 µm and 2.32 µm, respectively, and with crystalline GST, it is 0.78 dB and 0.39 dB, respectively. The calculated lengths of a PCM phase shifter for a π phase shift are 2.3 µm and 3.9 µm, and the losses for a π phase shift are 10.3 dB and 6.2 dB for the operating wavelength of 1.92 µm and 2.32 µm, respectively. Note that the loss for a π phase shift consists of the optical absorption and mode-mismatch loss in crystalline GST.
Fig. 3. Electric field intensity profile of the fundamental TE mode of a PCM phase shifter with (a) amorphous GST at 1.92 µm wavelength, (b) crystalline GST at 1.92 µm wavelength, (c) amorphous GST at 2.32 µm wavelength, and (d) crystalline GST at 2.32 µm wavelength. Color bars show the normalized electric field intensity.
4. Fabrication of the PCM phase shifter
The process flow of the PCM phase shifter is shown in Fig. 4 (a). First, Si rib waveguides with GCs were formed on a standard 220 nm SOI wafer by electron-beam (EB) lithography with an OEBR-CAN EB resist (Tokyo Ohka Kogyo) and inductively coupled plasma reactive ion etching (ICP-RIE) with CHF3 and SF6, followed by the formation of a 720-nm-thick SiO2 cladding by plasma-enhanced chemical vapor deposition (PECVD). 2-µm-wide windows were opened in the SiO2 cladding by EB lithography with an OEBR-CAP EB resist (Tokyo Ohka Kogyo), ICP-RIE with Ar and C4F8, and wet etching with BHF. This two-step etching was adopted to prevent over-etching into Si waveguides. Then, 20-nm-thick GST and 20-nm-thick SiO2 capping layers were deposited by RF sputtering. Lastly, unwanted SiO2 capping and GST layers on the grating couplers and waveguides except for the phase shifter regions were removed by ICP-RIE with Ar and C4F8. Since no lift-off process is used, this process flow will be suitable for mass production. A scanning electron microscopy (SEM) image of a fabricated PCM phase shifter is shown in Fig. 4 (b). Since the image was captured after the initiation of the phase transition of GST, nucleation can be observed. The length of the phase shifter was varied from 2 µm to 20 µm.
5. Evaluation of the PCM phase shifter
The phase transition of GST was confirmed by Raman spectroscopy. Figure 5 shows the measured Raman spectra of a 20-nm-thick GST film with a 20-nm-thick SiO2 capping layer on the Si substrate prepared at different annealing temperatures up to 270 °C. The unpatterned sample was prepared in the same way as in the GST and SiO2 deposition in the process flow of the PCM phase shifters. Note that each measurement was performed after leaving the sample at room temperature for a sufficient time. These spectra show that the phase transition from an amorphous state to the cubic crystalline state occurred at 110–130 °C, whereas the transition from the cubic crystalline state to the hexagonal crystalline state occurred at 190–210 °C. The transition to the crystalline state can be confirmed by the disappearance of peaks at 75 cm-1 and 150 cm-1, whereas that to the hexagonal crystalline state can be confirmed by the emergence of a peak around the wavenumber of 160 cm-1.
Fig. 5. Measured Raman spectra of 20-nm-thick GST film prepared at different annealing temperatures up to 270 °C.
To evaluate the loss and phase shift of the PCM phase shifter, we fabricated straight waveguides with a phase shifter and asymmetric Mach–Zehnder interferometers (AMZIs) with a phase shifter on one of the arms. Figure 6 shows a schematic of the measurement setup. As a light source, an amplified spontaneous emission light source (AdValue Photonics, AP-ASE-2000) and a superluminescent diode (SLD, FrankFurt Laser Company, FNPL-2000-2400-XXS) were used for 1.92 µm and 2.32 µm wavelengths, respectively. Light from the light source was input to the device under test through a single-mode fiber (SMF, Thorlabs, SM2000) via a uniform GC after passing through a polarization controller (PC, Thorlabs CPC900). The etching depth of GC was 150 nm, and the pair of pitch Λ and filling factor FF, the definition of which is shown in [37], of GC {Λ, FF} was {830 nm, 0.7} and {1240 nm, 0.55} for the 1.92 µm and 2.32 µm wavelengths, respectively. The output light from the grating coupler was coupled again into an SMF, and the power spectrum of the output light was measured using an optical spectrum analyzer (OSA, Thorlabs, OSA 203B).
Fig. 6. Schematic of the measurement setup and optical microscopy images of a straight waveguide and an AMZI with PCM phase shifters (ASE, amplified spontaneous emission; SLD, superluminescent diode; PC, polarization controller; DUT, device under test; OSA, optical spectrum analyzer).
Figures 7 (a) and (b) show the transmission spectra of the AMZIs with a 16-µm-long PCM phase shifter measured after annealing the sample at different temperatures for around 1.92 µm and 2.32 µm wavelength ranges, respectively. At both wavelengths, clear peak shifts due to the phase shift induced by the phase transition in GST were observed. Note that all the spectra were measured at room temperature after cooling the chip, ensuring the non-volatile operation of the PCM phase shifters. To extract the phase shift per unit length of the phase shifter, the relationship between the measured phase shift and the length of the phase shifter at each annealing temperature is plotted as shown in Figs. 7 (c) and (d), and the slope is calculated as summarized in Fig. 7 (e). Although the phase shift per unit length increases up to the annealing temperature of 150 °C, it starts to decrease at the annealing temperature of 170 °C. This can be attributed to the partial transition to the hexagonal crystalline state around this temperature. The measured phase shifts per unit length at 1.92 µm and 2.32 µm wavelengths reach their maximum values of 0.028 π/µm and 0.041 π/µm when the annealing temperature was 150 °C and decreased to 0.019 π/µm and 0.032 π/µm when the annealing temperature was 190 °C, respectively.
Fig. 7. Results of phase shift measurement. Transmission spectra of the AMZIs with a 16-µm-long PCM phase shifter measured after annealing the sample at different temperatures for (a) 1.92 µm and (b) 2.32 µm wavelengths. Relationship between the measured phase shift and the length of the phase shifter for (c) 1.92 µm and (d) 2.32 µm wavelengths. (e) Phase shift per unit length as a function of annealing temperature.
Figures 8 (a) and (b) show the transmission spectra of the straight waveguides with different lengths of the PCM phase shifter after annealing the sample at 190 °C for around 1.92 µm and 2.32 µm wavelength ranges, respectively. The relationship between the measured optical loss and the length of the phase shifter for 1.92 µm and 2.32 µm wavelengths is shown in Figs. 8 (c) and (d), respectively, and the slope is calculated as summarized in Fig. 8 (e). Similar to the phase shift, the optical loss at both wavelengths increases up to the annealing temperature of 150 °C, whereas it starts to decrease at the annealing temperature of 170 °C. The measured losses per unit length at 1.92 µm and 2.32 µm wavelengths with an annealing temperature of 190 °C are 0.21 dB/µm and 0.16 dB/µm, respectively.
Fig. 8. Results of loss measurement. Transmission spectra of the straight waveguides with different lengths of a PCM phase shifter after heating the sample at 190 °C for (a) 1.92 µm and (b) 2.32 µm wavelengths. Relationship between the measured loss and the length of the phase shifter for (c) 1.92 µm and (d) 2.32 µm wavelengths. (e) Loss per unit length as a function of annealing temperature.
The relationship between the loss and the phase shift is shown in Fig. 9. The loss and phase shift were measured after annealing at 190 °C, at which the crystallization to the cubic crystalline state is considered to be finished according to the Raman measurement. By fitting the measured data, we can obtain the loss per π phase shift: 10.3 dB/π at 1.92 µm wavelength and 4.8 dB/π at 2.32 µm wavelength. These experimental values are slightly smaller than the simulated values: 9.6 dB/π at 1.92 µm wavelength and 5.8 dB/π at 2.32 µm wavelength. The discrepancy between the measured and simulated values is due to inaccuracies in spectroscopic ellipsometry measurement. Note that the loss per $\pi $ phase shift is independent of the operating wavelength as shown in the following relationship,
Figure 10 summarizes the loss per π phase shift at different annealing temperatures. The minimum value of 9.7 dB/π is obtained when the annealing temperature is 170 °C at 1.92 µm wavelength, whereas that of 2.6 dB/π is obtained when the annealing temperature is 150 °C at 2.32 µm wavelength. Since the simulated loss per π phase shift at 1.55 µm wavelength is 21.7 dB/π, the measured loss at 2.32 µm wavelength is more than 80 times smaller than that at 1.55 µm wavelength and more than 5 times smaller than that at 1.92 µm wavelength. The increase in loss at a higher temperature can be attributed to the increase of light scattering at grain boundaries associated with the process of crystallization. We note that although we haven’t demonstrated the tunability yet, a phase shift to the reverse direction by amorphization of the GST thin film is known to be achievable through irradiation of visible light as reported by many papers including [20,38].
6. MRR with PCM phase shifter
Since the optical loss can be reduced at 2.32 µm wavelength, we examined the non-volatile shift of resonance wavelengths of an MRR using a PCM phase shifter. A 12-µm-long and 2-µm-wide GST layer was loaded on a ring waveguide of an add-drop MRR with a radius of 30 µm operating at around 2.32 µm wavelength as shown in Fig. 11 (a). The transmission spectra to the through and drop ports are shown in Figs. 11 (b) and (c), respectively. Owing to the phase change in GST, the redshift in the wavelength peak was observed. A 1.4-nm wavelength shift was obtained by annealing the sample at the temperature range from 110 °C to 150 °C. Note that the noise of the transmission spectra to the drop port in the off-resonant range is attributed to the noise floor of the OSA due to the low output power of SLD.
Fig. 11. (a) Optical microscopy image of an add-drop MRR operating at 2.32 µm wavelength. (b) (c) Transmission spectra to the (b) through and (c) drop ports with different annealing temperatures.
Figure 12 shows the relationship between the phase shift and the finesse of add-drop MRRs operating at 1.55 µm, 1.92 µm, and 2.32 µm wavelengths numerically analyzed using the refractive index and absorption measured by spectroscopic ellipsometry. The measured relationship is also plotted in Fig. 12. Since the amorphous and crystalline GST has optical absorption at 1.55 µm wavelength, the finesse of an add-drop MRR operating at 1.55 µm wavelength is so small that it cannot be used practically. On the other hand, add-drop MRRs at 1.92 µm and 2.32 µm wavelengths have large finesse before the phase shift because the extinction coefficient of amorphous GST is zero at both wavelengths. Moreover, since the extinction coefficient of the crystalline GST at 2.32 µm wavelength is smaller than that at 1.92 µm wavelength, the finesse of an add-drop MRR at 2.32 µm wavelength is larger than that at 1.92 µm wavelength, making it more useful for resonance tuning. Although the measured finesse is slightly larger than the simulated one owing to the inaccuracy in spectroscopic ellipsometry measurement, the trend of change in finesse with increasing phase shift agrees with the simulation.
Fig. 12. Relationship between the phase shift and the finesse of add-drop MRRs operating at 1.55 µm, 1.92 µm, and 2.32 µm wavelengths.
7. Conclusions
In this study, we have investigated a GST-based optical phase shifter at MIR wavelengths where the optical absorption of GST is considerably reduced compared with that at NIR wavelengths. We have experimentally demonstrated that the optical loss per π shift of a PCM phase shifter at 2.32 µm wavelength is more than 80 times smaller than that at 1.55 µm wavelength and more than 5 times smaller than that at 1.92 µm wavelength. Owing to the low optical loss, we have successfully demonstrated the resonance wavelength tuning of an add-drop MRR using a PCM phase shifter at 2.32 µm wavelength. Therefore, the optical phase shifter based on PCMs in the MIR range has the potential to be widely used in MIR Si PICs.
Funding
Japan Society for the Promotion of Science (JP20H02198, JP21J20272); Japan Science and Technology Agency (JPMJCR2004); Ministry of Education, Culture, Sports, Science and Technology (JPMXP09A21UT0093, JPMXP09F21UT0021).
Acknowledgments
Part of this work was conducted at Takeda Sentanchi Super cleanroom, The University of Tokyo, supported by the “Nanotechnology Platform Program” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, Grant Number JPMXP09F21UT0021. This study was partially supported by the University of Tokyo Advanced Characterization Nanotechnology Platform in the Nanotechnology Platform Project sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Grant Number JPMXP09A21UT0093.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data supporting the results presented in this paper are not publicly available but may be obtained from the authors upon reasonable request.
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