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Abstract
Lithium niobate (LN), possessing wide transparent window, strong electro-optic effect, and large optical nonlinearity, is an ideal material platform for integrated photonics application. Microring resonators are particularly suitable as integrated photonic components, given their flexibility of device engineering and their potential for large-scale integration. However, the susceptibility to temperature fluctuation has become a major challenge for their implementation in a practical environment. Here, we demonstrate an athermal LN microring resonator. By cladding an x-cut LN microring resonator with a thin layer of titanium oxide, we are able to completely eliminate the first-order thermo-optic coefficient (TOC) of cavity resonance right at room temperature (20°C), leaving only a small residual quadratic temperature dependence with a second-order TOC of only 0.37 pm/K2. It corresponds to a temperature-induced resonance wavelength shift within 0.33 nm over a large operating temperature range of (−10 – 50)°C that is one order of magnitude smaller than a bare LN microring resonator. Moreover, the TiO2-cladded LN microring resonator is able to preserve high optical quality, with an intrinsic optical Q of 5.8 × 105 that is only about 11% smaller than that of a bare LN resonator. The flexibility of thermo-optic engineering, high optical quality, and device fabrication compatibility show great promise of athermal LN/TiO2 hybrid devices for practical applications, elevating the potential importance of LN photonic integrated circuits for future communication, sensing, nonlinear and quantum photonics.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Lithium niobate (LN) is one of the most broadly used optical materials due to its wide transparency window, excellent material quality, significant nonlinear optical susceptibility, and strong electro-optic response. In recent years, miniaturization and integration of LN photonic devices on chip [1,2] has led to a variety of interesting applications including electro-optic modulation [3–9], sensing [10,11], spectroscopy [12], RF photonics [13,14], nonlinear and quantum photonics [15–27]. The performance of these devices, however, is impacted by their sensitivity to temperature variations, especially for those phase sensitive ones such as sensing, electro-optic modulation, and frequency conversion. Therefore, achieving passive athermal property without affecting the other merits of LN is highly desirable for LN photonic integrated circuits.
In the past decade, a variety of approaches have been proposed and demonstrated to engineer the temperature sensitivity of integrated photonic devices, where the athermal property is attained by either finely designing the photonic structures [28–30], controlling the mechanical stress [31], hybridizing the device with certain cladding/substrate materials [32–45], or optimizing the heat dissipation [46]. So far, the efforts have been focused on silicon [28,29,34–40,42,43,45,46], silicon nitride [30,41,44], and silica [31–33] platforms. For LN devices, however, athermal operation has not been realized, which is indispensable for their practical applications.
Here we utilize titanium oxide (TiO$_2$) as a cladding material to engineer the thermo-optic property of LN microring resonators. Titanium oxide is chosen because of its chemical and mechanical stability, device compatibility, and a strong negative thermal optic effect. It has been applied in silicon and silicon nitride devices for compensating temperature sensitivity [30,39–46]. As its refractive index is comparable with LN while its thermo-optic coefficient is significantly higher, a thin layer of TiO$_2$ cladding would be adequate for eliminating the thermo-optic sensitivity of the LN devices, leading to the optical energy primarily preserved inside the LN core. This elegant feature is crucial for photonic functionalities that rely on LN properties such as electro-optic modulation, nonlinear frequency conversion, etc. Moreover, as we will show below, the TiO$_2$ cladding introduces fairly minor impact on the optical quality of the devices, in contrast to other device platforms [33–45] where the temperature compensating cladding generally introduce significant extra losses.
2. Device design and fabrication
To engineer the thermo-optic property of a LN waveguide, we propose to coat the whole LN waveguide structure with a uniform thin layer of titanium oxide, as shown in Fig. 1(a). An advantage of this approach is that it does not require a lift-off or etching process [44,47,48] for the titanium oxide layer and is thus easy to fabricate in practice to achieve high optical quality.
Fig. 1. Device geometry of the TiO$_2$-cladded LN waveguide and resonator. (a) Schematic of the cross section of a TiO$_2$-cladded x-cut LN waveguide. (b) Optical mode field profile of the fundamental quasi-TE mode, simulated by the finite-element method. The LN waveguide has dimensions of W = 1.8 ${\mu}$m, H = 600 nm, h = 410 nm, and $\theta = 75^{\circ }$. The TiO$_2$ cladding layer has a thickness of T = 115 nm. (c) Schematic of a TiO$_2$-cladded x-cut LN microring resonator. The white arrows indicate the polarization direction of a quasi-TE cavity mode, which evolves along the resonator and alternates between the ordinary and extraordinary polarization of LN crystal.
The optical property of the hybrid waveguide is generally described by the effective refractive index $n_\textrm{eff}$ of a guided mode, which is given as $n_\textrm{eff} = \sum _{i}^{N} {\Gamma _i n_i}$ where $n_i$ is the refractive index of each material consisting of the hybrid waveguide (for example, SiO$_2$, LN, and TiO$_2$ in this work) and $\Gamma _i$ is the ratio of optical energy inside each layer. In general, $\Gamma _i$ remains fairly invariant within a moderate range of temperature variation and the thermo-optic property of the guided mode is dominated by those of the waveguide materials, which is given by the following expression [42]:
where $\frac {dn_i}{dT}$ is the thermo-optic coefficient (TOC) for each material.For an optical resonator made with the hybrid waveguide, the thermo-optic property of a cavity resonance is described by the temperature sensitivity of the resonance wavelength $\lambda _0$, which is given by [32]:
In the following, we focus on a circular-shaped x-cut LN microring resonator and its fundamental quasi-TE mode to show the operation principle, since it is broadly employed for electro-optic and nonlinear photonic functionalities. But the similar method can be applied as well to other resonator shapes, to quasi-TM modes, and to z-cut devices. We carried out detailed simulations by the finite-element method (FEM) to have a better understanding of the overall thermo-optic property of the hybrid microring resonator. First, since the birefringence of LN material is fairly small (refractive index is about 2.14 and 2.21 at a wavelength around 1550 nm for the extraordinary and ordinary polarization, respectively [51]), the energy ratio $\Gamma _\textrm{LN}$ for LN waveguide core remains fairly constant along the resonator (our simulations show that it changes by only 1% at maximum). As a result, $\frac {d\bar {n}_\textrm{eff}}{dT}$ is thus given by
Fig. 2. FEM-simulated optical and thermo-optic property of the fundamental quasi-TE cavity mode in a circular-shaped TiO$_2$-cladded x-cut LN microring resonator. (a) Simulated thermo-optic coefficient, $\frac {d\lambda _0}{dT}$, of cavity resonance around 1550 nm at room temperature, as a function of waveguide width $W$ and TiO$_2$ layer thickness $T$. The dashed curve corresponds to the cases when $\frac {d\lambda _0}{dT} = 0$. The color bar on the right shows the magnitude scale of $\frac {d\lambda _0}{dT}$. (b) Fraction of optical energy, $\Gamma _\textrm{LN}$, inside the LN waveguide core, corresponding to (a). The color bar on the right shows the magnitude scale of $\Gamma _\textrm{LN}$. The microring resonator is assumed to have H = 600 nm, h = 410 nm, and $\theta = 75^{\circ}$. The mark "*" indicates the case used in device fabrication.
Figure 2(a) shows that, for a waveguide width in the range of (0.5 – 2.5) ${\mu} \textrm{m}$, the $\frac {d\lambda _0}{dT}$ of the hybrid ring resonator can be finely tuned in the range from $-60~\textrm{pm/K}$ to $40~\textrm{pm/K}$, by changing the thickness of the TiO$_2$ cladding layer from 50 to 150 nm. The engineered negative TOC could be potentially useful for certain nonlinear photonic applications such as Kerr frequency comb generation [24]. In particular, as shown by the dashed curve in Fig. 2(a), for each waveguide width, $\frac {d\lambda _0}{dT}$ can be engineered to be zero with appropriate thickness of TiO$_2$ cladding. As shown clearly in the figure, only a fairly thin layer of TiO$_2$ cladding of (70 – 120) nm is needed to achieve zero TOC. Consequently, the optical energy of the cavity mode is dominantly preserved inside the LN waveguide core. As shown in Fig. 2(b), above 80% of the optical energy stays inside the LN core layer. As a specific example indicated by the star in Fig. 2, for a waveguide width of 1.8 ${\mu} \textrm{m}$, a TiO$_2$ cladding layer of 115 nm thick is able to decrease the overall wavelength dependence $\frac {d\lambda _0}{dT}$ to zero. The ratio of optical energy inside each material layer is simulated to be $\Gamma _\textrm{LN}$ = 83.0%, $\Gamma _{\textrm{TiO}_2}$ = 10.2%, and $\Gamma _{\textrm{SiO}_2}$ = 3.2% in the LN core, TiO$_2$ cladding, and SiO$_2$ substrate, respectively. Clearly, the optical field stays dominantly in the LN core, which is crucial for applications relying on the LN properties. This specific example is used in the following to fabricate the hybrid LN microresonator.
The devices were fabricated on a 600-nm-thick x-cut single crystalline LN thin film with 2-${\mu}$m-thick buried silicon oxide layer sitting on a silicon substrate. ZEP-520A resist was used as a mask and the devices were patterned by electron-beam lithography and were then etched down by about 410 nm via argon-ion milling process. To compare the thermo-optic performance, we made two sets of identical devices on separate chips. One chip was cladded with amorphous TiO$_2$ using the physical vapor deposition(PVD) process at a rate of 2 Å/s to achieve a finely controllable thickness. The thickness of the deposited TiO$_2$ was measured with a Woollam ellipsometer to be 120$\pm$5 nm and all the other parameters are consistent with simulation. The layout of a fabricated device is shown in Fig. 3, where the microring resonator has a radius of 100 ${\mu} \textrm{m}$. The external coupling of a device was controlled by adjusting the width of the gap between the microring and the coupling bus waveguide.
Fig. 3. Scanning electron microscopic image of a fabricated TiO$_2$-cladded x-cut LN microring resonator with a radius of 100 ${\mu} \textrm{m}$. The waveguide dimensions are indicated by the mark "*" in Fig. 2. A pulley bus waveguide of 1.2 ${\mu} \textrm{m}$ wide is used to couple light into and out of the resonator, which has a gap of around 0.9 ${\mu} \textrm{m}$ and a coupling length of around 60 ${\mu} \textrm{m}$ to allow a broadband coupling condition.
3. Experimental characterization of the devices
3.1 Experiment testing setup
Figure 4 show the schematic of the experimental setup. A tunable laser (Santec TSL-510) passes through an erbium-edoped fiber amplifier and an variable optical attenuator to control its power, has its polarization adjusted with a polarization controller to align with the quasi-TE polarization of the device, and is then launched onto the chip via a lensed optical fiber to probe the optical property of the device. The output is collected by another lensed fiber and recorded by a photo detector. The fiber-to-chip coupling loss was measured to be 6 and 9 dB/facet for the bare LN chip and the TiO$_2$-cladded LN chip, respectively. The temperature of the LN chip is controlled by a thermo-electric cooler. A fiber-based Mach-Zehnder interferometer is used to calibrate the wavelength tuning amount of the laser when it is scanned. At the same time, a wavemeter (Bristol-621) is employed to characterize the absolute resonance wavelength shift when the device temperature is changed.
Fig. 4. Schematic of the experimental testing setup. The temperature of the device is controlled by a thermo-electric cooler (TEC). EDFA: erbium-doped fiber amplifier; VOA: variable optical attenuator; PC: polarization controller; MZI: Mach-Zehnder interferometer; PD: photo detector.
3.2 Linear optical property
Figure 5(a) shows the transmission spectrum of a bare LN microring resonator (without TiO$_2$ cladding) at room temperature (20$^\circ {\textrm{C}}$) when the laser is scanned at the telecom band. The resonator exhibits a free-spectral range of 1.6 nm, corresponding to a group index of 2.3. As shown in Fig. 5(b), detailed characterization of the cavity resonance around 1542 nm shows that it exhibits an intrinsic and loaded optical Q of $6.5\times 10^5$ and $5.3\times 10^5$, respectively, which corresponds to a low propagation loss of 0.62 dB/cm. The oscillatory background on the device transmission comes from the reflection by the inverse tapers fabricated on the two facets of the chip.
Fig. 5. Laser-scanned transmission spectra of LN microring resonators at room temperature. (a) and (b) show the case of a bare LN microring resonator without TiO$_2$ cladding, where (b) shows the details of a cavity resonance around 1542 nm, with intrinsic and loaded optical Q of $6.3\times 10^5$ and $5.3\times 10^5$, respectively. (c) and (d) show the case of a LN microring resonator with TiO$_2$ cladding, where (d) shows the details of a cavity resonance around the same wavelength of 1542 nm, with intrinsic and loaded optical Q of $5.8\times 10^5$ and $4.6\times 10^5$, respectively. In (b) and (d), the blue dots show the experimental data and the red cures show theoretical fittings.
Figure 5(c) shows the laser-scanned transmission spectrum of a TiO$_2$-cladded LN microring resonator at room temperature. The resonator is identical to that of Figs. 5(a) and 5(b) except with the TiO$_2$ cladding. It exhibits a similar free-spectral range of 1.6 nm, correspond to a group index of 2.3 which is very close to the bare LN microreresonator described above. It indicates that the thin TiO$_2$ cladding has negligible impact on the group velocity property of the devices. Detailed characterization of a cavity resonance around the same wavelength of 1542 nm (Fig. 5(d)) shows that the device exhibits an intrinsic and loaded optical Q of $5.8\times 10^5$ and $4.6\times 10^5$, respectively, which corresponds to a propagation loss of 0.69 dB/cm. It shows clearly the high-quality of TiO$_2$ cladding, which introduces only a minor degradation of intrinsic optical quality by about 11%.
3.3 Thermo-optic property
To explore the thermo-optic property of the devices, we changed the temperature of the devices and monitored the wavelength shifts of the cavity resonances around 1542 nm given in Figs. 5(b) and 5(d). Figure 6 shows the measurement results, where each data point was recorded with at least 10 minute time separation until the resonance became completely steady after the temperature was changed. As shown by the black stars in Fig. 6, the cavity resonance of the bare LN microring resonator exhibits a clear linear dependence on temperature variation. Fitting it with a linear curve (Fig. 6, red curve) results in $\frac {d\lambda _0}{dT}=53~\textrm{pm/K}$. It corresponds to $\frac {d\bar {n}_\textrm{eff}}{dT}=1.5\times 10^{-5}$/K after subtracting the contribution of thermal expansion. This value is consistent with the TOC of bulk LN material [49].
Fig. 6. Temperature dependent wavelength shift of cavity resonances shown in Figs. 5(b) and 5(d), for a bare LN resonator (black stars) and a TiO$_2$-cladded LN resonator (blue circles), respectively. The red curve shows a linear fitting and the green curve shows a quadratic fitting.
In contrast, as shown by the green circles in Fig. 6, the temperature dependence is significantly suppressed in the TiO$_2$-cladded LN microring resonator, leaving only a residual quadratic dependence on temperature. In particular, as shown in Fig. 6, the first-order TOC is completely eliminated right at room temperature (20$^\circ$C). Overall, the wavelength shift remains below 25 pm in the temperature range of (15 – 25)$^\circ$C, which is more than one order of magnitude smaller than that of the bare LN microring resonator. Over a larger temperature range of (15 – 55)$^\circ$C, the wavelength shift is within 0.49 nm, which is only about 22% of the bare LN microresonator. These observations show clearly the effect of TiO$_2$ cladding in suppressing the temperature sensitivity of LN microresonators. Fitting the data with a quadratic temperature dependence (Fig. 6, green curve), we obtained a second-order TOC of $0.37~{\textrm{pm/K}}^2$ for the TiO$_2$-cladded LN microring. This residual quadratic temperature dependence primarily contributes from the TiO$_2$ material and similar phenomenon was observed before in other device platforms [42,43]. The small quadratic temperature dependence infers that the TiO$_2$-cladded LN microring resonator is fairly resistant to the perturbations induced by temperature variations, with its resonance wavelengths shifted within only 0.33 nm over a fairly large operating temperature range of (−10 – 50)$^\circ$C.
4. Conclusion and discussion
In summary, we have demonstrated a high-quality athermal lithium nioate microring resonator. The athermal property is achieved by uniformly coating the LN device with a thin layer of amorphous TiO$_2$. TiO$_2$ exhibits a large negative TOC and a refractive index similar to LN, which make it an ideal cladding material for LN photonic integrated devices. Our simulations show that the TOC of the microring resonator can be flexibly engineered over a significant range in both positive and negative regimes, with only a fairly thin layer of TiO$_2$ cladding. This excellent property preserves the majority of optical power inside the LN waveguide core, which is particularly useful for applications relying on the properties of LN material.
In our fabricated microring reosnators, we were able to completely eliminate the first-order TOC right at room temperature, leaving only a residual quadratic temperature dependence with a second-order TOC of only 0.37 ${\textrm{pm/K}}^2$. As a result, the temperature-induced wavelength shift of cavity resonance remains below 25 pm in the temperature range of (15 – 25)$^\circ {\textrm{C}}$ that is more than one order of magnitude smaller than a bare LN microring resonator, and is within 0.49 nm that is only about 22% of that for a bare LN microring resonator. In particular, the TiO$_2$-cladded LN microring resonators is able to preserve high optical quality, with an intrinsic optical Q as high as $5.8\times 10^5$ that is only about 11% smaller than that of a bare LN resonator. The impact to optical Q is expected to remain fairly minor even when the cladding is applied to devices with higher optical Q, since, with a small portion of optical energy inside, the TiO$_2$ cladding layer contributes only a relatively small fraction of propagation loss. The optical quality can be improved further with further optimization of TiO$_2$ deposition.
The TiO$_2$-cladded LN waveguide structure demonstrated here offers a convenient and practical avenue to manage the thermo-optic properties of LN photonic integrated devices. As the fabrication process of TiO$_2$ layer coating is compatible with other common fabrication processes and can be flexibly applied for various waveguide and device geometries, we expect the demonstrated approach would be of great potential for broad applications of LN photonic integrated circuits, such as in sensing, signal processing, and nonlinear/quantum photonics, wherever engineering of temperature sensitivity plays a crucial role.
Funding
Defense Threat Reduction Agency (HDTRA11810047); National Science Foundation (ECCS-1810169, ECCS1842691, EFMA-1641099).
Acknowledgments
This work was performed in part at the Cornell NanoScale Facility (CNF), a member of the National Nanotechnology Coordinated Infrastructure (National Science Foundation, ECCS-1542081), and at the Cornell Center for Materials Research (CCMR, National Science Foundation, DMR-1719875). The authors thank the staff members in CNF and CCMR for the training and guidance for the equipment and fabrication procedures. The authors also thank Usman Javid and Austin Graf for their help on testing.
Disclosures
The authors declare no conflicts of interest.
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