We have fabricated compact optical modulators consisting of a Si waveguide with a VO2 cladding layer. These devices showed a sharp decrease in transmittance at around 60 °C, which is attributable to the metal–insulator transition of the VO2 cladding layer. By systematically varying the length of the device, we evaluated the transmission losses per unit length of the device to be 1.27 dB/µm, when the VO2 cladding layer was in the insulating (ON) state and 4.55 dB/µm when it was in the metallic (OFF) state. Furthermore, we found that the device showed an additional loss in the OFF state, which is attributable to a structural effect. As a result, an 8-µm-long device showed a large extinction ratio of more than 33 dB.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Vanadium dioxide (VO2) shows a temperature-driven metal–insulator transition (MIT) corresponding to a structural change from a high-temperature tetragonal phase to a low-temperature monoclinic phase at around 68 °C [1,2]. Because this MIT of VO2 is accompanied by a discontinuous change in its electrical conductivity of several orders of magnitude, it has attracted much attention for use in electronic applications such as transistors, memories, gas sensors, or uncooled bolometers [3–7]. The MIT is also accompanied by a marked change in optical transmittance in the infrared region that can be utilized in optical applications. Several optical devices based on VO2 integrated with silicon (Si) photonics have been demonstrated [8,9].
Si photonics is an optical integration technique based on the use of Si as an optical medium. Because Si waveguide optical devices have good compatibility with conventional Si–CMOS processing technology, monolithically integrated optical circuits several centimeters square in size and containing many optical devices can be realized. Conventional Si waveguide optical switches and modulators have resonant structures or interferometer configurations, and their resonant or interference characteristics are controlled by means of thermally induced changes in the refractive index of Si. Recently, the integration of functional materials with Si waveguides has been intensively investigated as a means of adding new functionalities and for realizing future miniaturization. As promising candidates, phase-change materials [10–12], two-dimensional materials [13,14], liquid crystals , and transparent conductors  have been studied. Among these materials, VO2 exhibits excellent properties in terms of applications in optical modulators, particularly its marked change in extinction coefficient (more than one order of magnitude) at the MIT [8,17]. In addition to being temperature driven, the MIT of VO2 can also be induced by electrical or optical stimuli on a sub-picosecond timescale . This suggests that the integration of VO2 with Si waveguides might permit the fabrication of active and ultrafast optical switches and modulators.
To date, several types of VO2-integrated optical switches and modulators have been developed that demonstrate large transmittance modulations [8,9,11,19–23]. For instance, an extinction ratio of ~10 dB at a wavelength of 1550 nm has been reported for an optical switch in which a 500-nm-long section of the Si waveguide was replaced with VO2 . However, this optical switch showed a large insertion loss of more than 5 dB, even in its ON state, due to the relatively large extinction coefficient of VO2 (~0.3) in its insulating state.
Another type of device geometry is that of a Si waveguide with a thin VO2 cladding layer [8,9]. In this geometry, light propagates through the Si waveguide and, consequently, insertion losses in the ON state are suppressed. Devices with this geometry have been reported to have beneficial characteristics, including relatively small insertion losses and micrometer-scale footprints. Nevertheless, the large extinction ratio expected from theoretical calculations has not been demonstrated. Moreover, there have been few systematic investigations on the characteristics of such devices, such as the effects on their properties of the device length and temperature, which are needed to establish design guidelines for hybrid VO2/Si devices.
In this study, we developed compact micrometer-scale optical modulators consisting of Si waveguides with a thin VO2 cladding layer. Through fabrication of VO2 films with a high quality in terms of changes in their refractive index across the MIT, we achieved large extinction ratios of more than 33 dB for 8-µm-long optical modulators. We also investigated the dependence of the transmittance of the optical modulators on the length of the device and the temperature. The transmittance decreased linearly with increasing device length, which permitted us to evaluate the transmission loss (TL) per unit length of the optical modulators. The experimentally determined TLs for the ON and OFF states were in a fairly good agreement with the values obtained from computational simulations. Furthermore, the device-length dependence of the transmittance revealed that the optical modulators had an additional TL for the OFF state due to a structural effect, which enhanced the extinction ratio.
A schematic depiction of an optical modulator consisting of a Si waveguide with a thin VO2 cladding layer is shown in Fig. 1(a). To optimize the thickness of the VO2 cladding layers, we calculated the dependence of the optical transmittance of the device on the thickness of the VO2 layer by the finite-difference method with a grid-mesh spacing of 10 nm. For these calculations, we used a device geometry in which the VO2 layer was present only on the top of the Si waveguide and was absent from the side walls, as will be discussed later. The height and width of the waveguide were set to 220 nm and 400 nm, respectively, and the thickness of the SiO2 layer underneath the waveguide was set to 2 µm. These dimensions coincide with those of the waveguides fabricated in this study. Moreover, we used refractive-index values for VO2 obtained experimentally from a VO2 film on Si; these were 3.21 + 0.26i in the insulating phase and 2.34 + 3.54i in the metallic phase at λ = 1550 nm .
Figure 1(b) shows a typical result for a computational simulation of the electric-field distribution in the TM mode (λ = 1550 nm) in the optical modulator, in which the 30-nm-thick VO2 cladding layer is in the insulating (ON) state. For TM-mode transmission, in general, only the properties of cladding material located above and below the Si waveguide have marked effects on the transmission characteristics because the TM mode has a vertical electric field. Therefore, in this device, changes in the properties of VO2 are capable of effectively modulating the transmission characteristics. Figures 1(c) and 1(d) show the top views of simulation results of light propagation in the optical modulator when the 30-nm-thick VO2 cladding layer is in the insulating (ON) state and the metallic (OFF) state, respectively. The input light propagates in the waveguide for the ON state, whereas it decays markedly in the OFF state. The calculated values of the TL are 1.00 dB/µm for the ON state and 6.69 dB/µm for the OFF state. The simulation also shows that the TL and extinction ratio per unit length depend on the thickness of the VO2 cladding layer, as shown in Fig. 1(e). Here, the extinction ratio is defined as the difference of the TLs in the ON and OFF states. Whereas the TL in the ON state increases monotonically with increasing the VO2 thickness, that in the OFF state shows a maximum at a thickness of 40 nm. This is due to an increased contribution of the transverse electric (TE) element with increasing the VO2 thickness which results in the extinction ratio having a maximum value at a VO2 thickness of 40 nm. Note that the TL of the TE mode is calculated to be 3.89 dB/µm in the OFF state. The smaller TL in the TE mode than in the TM mode is due to the absence of a VO2 layer on the side walls of the Si waveguide. The simulation predicts that if the VO2 cladding layer covers the Si waveguide conformally, the TL of the TM mode will be 1.37 dB/µm for the ON state and 7.52 dB/µm for the OFF state. On the basis of these simulation results, we fabricated devices with a designed VO2 thickness of 30–40 nm.
3. Device fabrication
The Si waveguide was formed from a silicon-on-insulator (SOI) wafer with a 220-nm-thick silicon layer on top of a 2-µm-thick buried oxide layer. To fabricate the 400-nm-wide wire waveguide, we used an i-line stepper in conjunction with inductively-coupled-plasma reactive-ion etching (ICP-RIE) with a gas mixture of SF6 and C4F8 . The first cladding material of SiO2 was fabricated by plasma-enhanced chemical vapor deposition (PECVD) with a tetraethyl orthosilicate gas precursor . The thickness of the SiO2 cladding was set at 1.8 µm [Not shown in Fig. 1(a)]. Subsequently, sections of the SiO2 cladding were wet etched with buffered hydrofluoric acid solution (BHF) to open windows in which the Si waveguide was exposed . The width of the windows was ~4 µm and their length was varied in the range 3−8 µm. Si waveguides without a window were also prepared on the same chip for reference. A VO2 film was then deposited on the waveguide by pulsed laser deposition (PLD) at 450 °C and an oxygen pressure of 20 mTorr . We have previously confirmed that a VO2 layer deposited under these optimized conditions exhibits a marked change in its refractive index across the MIT . The VO2 thickness of the device used in the transmission measurements was evaluated to be 30 nm by a cross-sectional TEM measurement. This thickness is slightly smaller than the optimal one (40 nm) estimated by the computational simulation, but a 30-nm-thick-VO2 device is expected to have a large extinction ratio comparable to a 40-nm-thick-VO2 one [Fig. 1(e)]. The VO2 layer was attached to the Si waveguides in the window sections only, as shown in Fig. 1(a). In other regions, the VO2 layer was spatially separated from the Si waveguides by the 1.8-µm-thick cladding layer of SiO2, so that the impact of the VO2 layer on the optical transmittance in the waveguide was almost negligible. Finally, the VO2 layer was covered by a second SiO2 cladding layer ~1 µm thick, deposited by sputtering [Fig. 1(a)]. Note that the PECVD method could not be used to produce this second cladding layer because of proton incorporation into the VO2 layer during the process; this incorporation would suppress the MIT and stabilize the metallic phase of VO2, even at room temperature [27,28].
The structures of the fabricated devices were investigated by cross-sectional secondary ion microscopy (SIM) and transmission electron microscopy (TEM). Figures 2(a) and 2(b) show cross-sectional SIM images of a device along and across the waveguide, respectively. These images show that a continuous VO2 layer was present on the Si waveguide. BHF etching for window fabrication produced a slope structure in the first SiO2 cladding layer that was 1.6−2 µm wide at the edge of the window. Such slope structures are commonly formed by wet-etching processes. Because the separation distance between the VO2 cladding layer and the Si waveguide is relatively small in the slope region, light absorption by the VO2 cladding layer in that region might generate an additional TL for the OFF state, as will be discussed later. Note that the device geometry used in the simulations shown in Fig. 1 did not include a slope structure at the edge of the widow.
A cross-sectional TEM image of a device is shown in Fig. 2(c). The image shows that the VO2 cladding layer was formed only on the top of the Si waveguide, and not on its side walls. This is due to the highly directional characteristics of the PLD process. In addition, we found that some voids were formed in the second SiO2 cladding layer beside the Si waveguide. This phenomenon might also be due to the highly directional characteristics of the sputtering process. Although these voids might generate a spatial distribution in the effective index of the cladding layer, they are not expected to have a critical effect on the characteristics of the device. This is because the refractive index of voids (air) is much smaller than that of Si, and therefore the SiO2 layer containing the voids acts as a cladding material.
To evaluate the crystallographic phase as well as the MIT in the VO2 cladding layer, we performed Raman scattering measurements using a micro-Raman system with a 532-nm excitation source in a backscattering geometry . The incident light was focused on the VO2 cladding layer through SiO2 by a 100 × objective lens to a laser-spot diameter of ~1 µm. Figure 2(d) shows Raman spectra of VO2 measured at temperatures (T) of 25 °C and 80 °C, which are, respectively, below and above the MIT temperature (TMI). Sharp Raman peaks for the monoclinic structure of VO2 were observed in the spectrum at T = 25 °C [29,30], indicating that the VO2 layer was of high quality. These peaks vanished at T = 80 °C when the crystallographic structure of VO2 changed from monoclinic to tetragonal due to the MIT.
5. Transmission measurements
The optical transmittance of the device was evaluated by using the amplified spontaneous emission of the light source in the wavelength range λ = 1530–1610 nm. The transverse magnetic (TM) mode was used for all measurements, because only the top surface of the Si waveguide was covered by VO2. The light was input and output through a tip-lensed optical fiber at the edge of the chip, and the output signal was transferred to a power meter and an optical-spectrum analyzer. To evaluate the temperature dependence of the modulation characteristics, the devices were placed on a Peltier temperature-controlled stage. Each measurement was performed after the stage temperature stabilized at a set temperature, and the temperature variation was less than 0.2 °C during the measurement.
Figure 3(a) shows the incident-light-power dependence of the transmittance of a device with 3-µm-long windows along the waveguide. Hereafter, this device is referred to as the 3-µm-long device. The transmission measurements were carried out at T = 20 °C and 80 °C. The transmittance was derived by subtracting the contribution of a waveguide without a window at each temperature. Note that the TL of the waveguide without a window was approximately 10 dB, almost independent of the temperature. At T = 20 °C, the transmittance decreased markedly at incident-light powers in excess of 2 mW. A similar behavior has been reported previously . This result is attributable to a photothermal effect in which the device was heated to above the TMI of VO2 by the incident light. In fact, no decrease in the transmittance was observed at T = 80 °C, and the values of the transmittance at higher incident-light powers (> 3 mW) for T = 20 °C were coincident with those for T = 80 °C. Furthermore, a hysteretic behavior was observed in the incident-light-power dependence of the transmittance at T = 20 °C, indicating a first-order phase transition of VO2. From these results, to suppress the photothermal effect as much as possible, we used a small incident power of 0.2 mW for the transmittance measurements at low temperatures below the TMI.
The temperature dependences of the transmittance for devices with different window lengths are shown in Fig. 3(b). The results were recorded during the heating process. The transmittance decreased sharply at around 60 °C and was nearly constant at above 65 °C, irrespective of the device length. The sharp decrease in the transmittance is attributable to the MIT of the VO2 cladding layer, although it occurred slightly below the TMI of bulk VO2, possibly due to the photothermal effect. The magnitude of the decrease in the transmittance, which corresponds to the extinction ratio, increased with increasing device length. The device with the longest device length of 8 µm (8-µm-long device) had an extinction ratio of more than 33 dB. From the results of the temperature dependence, we can also plot the device-length dependence of the transmittance for each temperature, as shown in Fig. 3(c). The transmittance decreased linearly with increasing device length. This linearity permits us to evaluate a TL per unit length from the gradient of a plot of the transmittance against the device length. The absolute value of the TL is plotted as a function of the temperature in Fig. 3(d). In the temperature range 20–50 °C, the TL was nearly constant at approximately 1.3 dB/µm, which was slightly larger than the TL value of 1.00 dB/µm obtained from the computational simulations for the ON state [Fig. 1(c)]. This result might be attributable to a deterioration in the quality of VO2 during the device-fabrication process, causing an increase in the extinction coefficient of VO2 in the insulating state . On the other hand, when temperature was increased to above 50 °C, the TL increased and reached 4.55 dB/µm at T = 80 °C. This value is slightly smaller than the simulated one of 6.67 dB/µm for the OFF state [Fig. 1(d)]. This result might be due to a spatial variation of the VO2-layer thickness and/or the deterioration in the quality of VO2, causing a decrease in the extinction coefficient of VO2 in the metallic state . Although there is a small difference between the experimental and the simulated TL values, these results confirm that the MIT of the VO2 cladding layer drives the optical modulation of the devices.
Here, it should be noted that the ordinate intercept for a linear extrapolation of a plot of the transmittance versus the device-length characteristics had a finite value for T > 20 °C, as shown in Fig. 3(c). The absolute value of this intercept (i.e., the additional TL) is plotted as a function of the temperature in Fig. 3(d). The additional TL increased sharply at around 50 °C, and was in the range 6–9 dB at high temperatures. This large additional TL enhanced the extinction ratio of the device, as described briefly below.
As can be seen in Fig. 3(c), the TL values were 1.27 dB/µm and 4.55 dB/µm at T = 20 °C and 80 °C, respectively. From these values, the extinction ratios were estimated to be 9.8 dB for the 3-µm-long device and 26.2 dB for the 8-µm-long device. On the other hand, the corresponding experimental extinction ratios were 16.7 dB (20.2 dB – 3.5 dB) for the 3-µm-long device and 33.9 dB (43.9 dB – 10.0 dB) for the 8-µm-long device, which exceeded the estimated values by 7–8 dB. This difference corresponds to the additional TL at high temperatures. As mentioned earlier, the VO2 cladding layer at the edge of the window might provide an additional TL at high temperatures. In the edge region, the VO2 cladding layer was formed on a thin SiO2 cladding layer [Fig. 2(a)]. Because of the large extinction coefficient of VO2 in its metallic state, the VO2 cladding layer might absorb light through the thin SiO2 layer at the edge of the window. In fact, simulations based on a device geometry with inclusion of the edge structure predict that the VO2 cladding layer at the edge region should provide an additional TL for the OFF state. However, the calculated value of the additional TL is 3–4 dB, which is approximately half the experimental one. Moreover, a sharp increase in the additional TL was observed at around 50 °C, which is slightly below the TMI of bulk VO2 [Fig. 3(d)]. This discrepancy in temperature might be due to a photothermal effect and/or to a lower TMI of the VO2 cladding layer on the SiO2 cladding layer at the edge of the window. Further experimental and theoretical studies are therefore needed to fully elucidate the origin of the additional TL.
Finally, we discuss the bandwidth characteristic of the devices. Figure 4 shows the wavelength dependence of the transmittance for the devices at various temperatures, along with the simulation results. In the wavelength range 1530–1610 nm, the devices showed a decrease in transmittance with increasing T, although the experimental results at T = 80 °C deviated from the simulation results for the OFF state. This result indicates that the devices have a broadband performance of over 80 nm.
For practical applications, the MIT of the VO2 cladding layer should be induced by an external stimulus. Moreover, conformal deposition of the VO2 layer on the Si waveguide by using chemical vapor deposition or atomic layer deposition techniques is required to realize optical modulation for the TE mode, which would be preferable in Si photonic platforms. Studies on these issues will be conducted in future.
We have fabricated micrometer-scale optical modulators based on a Si waveguide with a VO2 cladding layer. As predicted by simulations, the devices exhibited optical modulation induced by the temperature-driven MIT of the VO2 cladding layer. The transmittance decreased linearly with increasing device length independently of the temperature, which ensures the quality and reproducibility of our devices. From the linear dependence of the transmittance on the device length, the TL per unit length of device was determined to be 1.27 dB/µm for VO2 in its insulating phase (ON state) and 4.55 dB/µm for VO2 in its metallic phase (OFF state), which are in fairly good agreement with the results of the simulation. For the 8-µm-long device, the device with the longest window length in this study, the extinction ratio was more than 33 dB. The dependence of the transmittance on the device length revealed that an additional TL at high temperatures enhanced the extinction ratio in our devices. Although the origin of this additional TL is not yet fully understood, its efficient use might improve the extinction ratio. Further study on the origin of this phenomenon should therefore provide guidance on the design of a compact micrometer-scale optical modulator with a large extinction ratio.
Japan Society for the Promotion of Science (JSPS) (No. 16K04955, 18H03686).
We thank Emiko Omoda for the preparation of the Si waveguides. Part of this work was conducted at the AIST Nano-Processing Facility, which is supported by the ‘Nanotechnology Platform Program.’
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