Efficient tunable photonic integrated devices are important for the realization of reconfigurable photonic systems. Thermal tuning is a convenient and effective approach, and silicon’s large heat conductivity, thermo-optical coefficient, and CMOS fabrication compatibility make it a good candidate material for tunable optical microcavities, which are versatile elements in low-cost, large-scale photonic integrated circuits. Metal heaters are traditionally used for tuning, and a thick upper-cladding layer is usually needed to prevent light absorption by the metal since that could reduce response speed and heating efficiency. In this paper, we propose and experimentally demonstrate thermally tunable silicon photonic microdisk resonators by introducing transparent graphene nanoheaters, which contact the silicon core directly without any isolator layer. The theoretical and experimental results show that the transparent graphene nanoheaters improve the heating efficiency, the temporal response, and the achievable temperature in comparison with a traditional metal heater. Furthermore, the graphene nanoheater is convenient for use in ultrasmall nanophotonic integrated devices due to its single-atom thickness and excellent flexibility. Both experiments and simulations show that the transparent graphene nanoheater is a promising option for other thermally tunable photonic integrated devices such as optical filters and switches.
© 2016 Optical Society of America
Silicon photonics is one of the most promising technologies to realize low-cost, large-scale photonic integrated circuits (PICs) because of the ability to achieve ultrasharp bends, as well as the compatibility with CMOS fabrication processes [1,2]. Among various silicon photonic integrated devices, an optical microcavity is well known as a versatile element to realize many functionality components, including optical filters/switches [3,4], optical modulators , and optical sensors . Since silicon has a large heat conductivity () and a large thermo-optical (TO) coefficient ( at a wavelength of 1.55 μm) , it is possible and beneficial to achieve efficient thermal tuning in silicon photonic microcavities. Traditionally, metal heaters are usually used for thermally tunable silicon photonic microcavities, and a thick upper-cladding layer is usually required between the metal heater and the silicon core to isolate the metal absorption. However, such a thick upper-cladding layer will introduce some disadvantages due to its poor heat conductivity, e.g., low response speed and low heating efficiency. Furthermore, note that the temperature in the silicon core is much lower than that of the metal heater and consequently the temperature dynamics of the silicon core is limited to some degree. Therefore, the thermal tuning range is limited. In addition, silicon photonic microcavities of ultrasmall size are desired for large-scale integration, in which case the fabrication of nanoheaters becomes complicated with metal. Some approaches have been presented to improve the heating performance. One approach is to introduce free-standing silicon resonators with undercut structures so that the heating efficiency is enhanced significantly . However, it is still difficult to achieve large thermal tuning range, as well as to realize nanoscale microcavities. Furthermore, the free-standing structure is usually not so robust mechanically. Its fabrication is also complicated and might be incompatible with the other elements on the same chip. An integrated silicon heater is another new type of heater formed by silicon regions with different doping levels, and has achieved low-power and high-speed thermal tuning [9,10]. The problem is that such a heater needs complicated fabrication processes to form heater regions with different doping levels, and it can be used only in some specific devices (e.g., adiabatic resonant microrings) whose heating sections need very careful design. Therefore, a simple and efficient heating approach is still desired for thermally tunable silicon photonic integrated devices (including microcavities).
Graphene has been extensively investigated worldwide because this two-dimensional (2D) sheet has many extraordinary properties [11 –14], such as a broadband absorption of per layer for vertically incident light [11,12], a carrier mobility as high as at room temperature (RT) [15,16], a “minimum” conductivity of even when the carrier concentration tends to zero [11,13], a high optical damage threshold value, and excellent mechanical stability . Recent studies also suggest that graphene is expected to have high intrinsic thermal conductivity due to the long-wavelength phonon transportation in its 2D crystal lattices [17,18], and an experimental value of up to at RT has confirmed this conclusion . The excellent thermal property of graphene coupled with its remarkable optoelectronic properties enables many potential applications in thermal management [20,21], such as efficient heat spreaders in electronic and photonic devices [22,23], and transparent and flexible heaters [24 –26].
In this paper, we experimentally demonstrate a thermally tunable silicon photonic microdisk resonator with transparent graphene nanoheaters. The graphene nanoheater is designed to contact the silicon core directly without the thick upper-cladding layer required for traditional metal heaters, so that it is possible to achieve efficient thermal tuning and improved temporal response. In our previous work , we have demonstrated a graphene heat conductor to deliver heat from a traditional metal heater to nanophotonic integrated devices for realizing thermal tuning. In contrast, the graphene nanoheater demonstrated in this paper is used to generate heat by itself (instead of a metal heater), so that it works as a real “transparent graphene nanoheater.” Since there is no heat dissipation due to the delivery process, which exists in the graphene heat conductor, the heating efficiency for the present graphene nanoheater is greatly improved. Moreover, due to the single-atom thickness and excellent flexibility of graphene, it is convenient for the transparent nanoheater to be patterned in a nanoscale shape with CMOS-compatible fabrication processes. This is very useful for ultrasmall nanophotonic integrated devices that need thermal management, e.g., the thermally tunable microdisk resonator in our case. In this paper, thermally tunable silicon photonic microdisk resonators with transparent graphene nanoheaters are designed, fabricated, and characterized.
2. STRUCTURE, DESIGN, AND FABRICATION
Figure 1(a) shows the three-dimensional (3D) schematic illustration of the present thermally tunable silicon photonic microdisk with a transparent graphene nanoheater. The fabrication starts with a commercial SOI wafer with a 250 nm thick top silicon layer and a buried oxide (BOX) layer of 3 μm. We use electron beam lithography (EBL) and inductively coupled plasma (ICP) etching processes to form the submicrometer-patterns. Then a second EBL process and a lift-off process are carried out to create some 100 nm thick titanium metal contacts on the top of the BOX layer. A monolayer graphene sheet, grown by the chemical vapor deposition (CVD) method, is wet-transferred to cover the whole SOI chip (here no accurate alignment is required) [28,29]. A third EBL process followed by an oxygen plasma etching process is then utilized to pattern the graphene sheet , forming a circular nanoheater along the edge of the microdisk and two arms connecting the circular nanoheater with the metal contacts at the sides of the microdisk. As shown in Fig. 1(b), the radii of the microdisk and the graphene nanoheater are chosen as and , respectively, in this example. The widths for the circular nanoheater and the arms are and . These parameters are chosen to make the circular graphene nanoheater overlap less with the whispering gallery mode (WGM) of the microdisk resonator [see the bottom of Fig. 1(b)] , so that no significant loss is introduced from the graphene absorption.
Figure 1(c) shows the microscope images of the fabricated devices, which have grating couplers at the input/output ends to achieve efficient coupling of the probe light . To protect the patterned graphene from any damage or contamination, the photoresist for graphene patterning was left on the chip; it will not introduce any notable influence on the thermal behaviors of the devices since the guided-modal field is well confined to the silicon core. Moreover, the low thermal conductivity of photoresist could help decrease the heat convection of the graphene nanoheater, as shown in , and the heating efficiency can be even improved slightly. Figure 1(d) shows scanning electron microscope (SEM) images of the fabricated microdisk resonator with the graphene nanoheater. It can be seen that the graphene sheet flexibly crosses the step from the silicon core to the top surface of the –BOX layer. This makes the present flexible graphene nanoheater available for complex surfaces with nonplanar nanostructures.
3. RESULTS AND DISCUSSION
As is well known, the resonant wavelength of a microdisk resonator with a radius is described as 32]. For thermal tuning behaviors, when the current for heating is injected through the metal contacts, heat is generated mainly in the part of the graphene nanoheater where the resistance is higher than it is in other parts in the electric circuit. The heat is then transferred directly from the graphene nanoheater to the silicon photonic microdisk below, and consequently the temperature in the silicon region becomes increased. Due to the positive TO coefficient of silicon (), the effective index for the WGM of the microdisk resonator increases. Accordingly, the resonant wavelength of the microdisk has a redshift, as indicated by Eq. (1).
Figure 2(a) shows the experimental result for the dynamic spectral responses of our fabricated microdisk resonator. When the heating power is 0, the free spectral range (FSR) and the -factor of the microdisk resonator are and , respectively. With the heating power varies from 0 to 10.5 mW, the resonant wavelength of the microdisk resonator has a red-shift of . Meanwhile, both the -factor and the extinction ratio of the resonant peak also change. This is partially because the conductivity of graphene varies when the temperature increases [33,34], so that the light absorption of the graphene nanoheater and the propagation loss of the microdisk resonator change correspondingly . The resonant-wavelength shifts with varying heating powers are shown in Fig. 2(b); it increases linearly with heating power . The corresponding heating efficiency , which is defined as the ratio of the resonant-wavelength shift to the heating power , is estimated to be about . This value gives an evaluation for the conversion efficiency from the electrical heating power to the temperature change of the silicon photonic microdisk, which includes the processes of heat generation in the graphene nanoheater, heat transfer from the graphene nanoheater to the silicon core, and heat conduction in the silicon photonic microdisk.
The temporal response of the thermally tunable microdisk resonator is also measured by fixing the wavelength of the probe light as and applying a modulated heating power with an amplitude of . As shown in Fig. 2(c), when , the output power of the microdisk resonator has a minimal value (see the solid line), while the output becomes the maximum when (see the dashed line) because of the thermo-optic redshift. In this way, the probe light is modulated by varying the heating power, and the modulated light is then received by a photodetector (Conquer KG-HSP-04SMFP) in our experimental setup. Figure 2(d) shows the measured output signal from the photodetector when the modulation frequency is about 1 kHz. The 90% rising time and the decaying time of the thermal tuning are about 12.8 and 8.8 μs, respectively.
As heater resistance is the key for heat generation and power consumption, the resistance of the graphene nanoheater is analyzed and discussed below. For an electrical circuit with a total resistance between the two metal contacts, one has the following equation for the heating power when an electrical current is injected :
For the present case, the total resistance includes three parts [36 –38], i.e., the resistance of the graphene circular nanoheater , the resistance of the graphene arms , and the total contact resistance between the graphene arms and the metal pads . Therefore, one has3(a), respectively, as the graphene-ribbon width varies. By fitting the experimental data by using Eq. (3) with (i.e., no circular nanoheater for the test structures with graphene ribbons), , and , one obtains sheet resistance and contact resistance for the graphene sheet in our case. With these values of and , the calculated resistances for the present graphene nanoheater are , , and , respectively. Correspondingly, the theoretical total resistance is about 1.68 kΩ, which is well consistent with the experimental result () from the I–V measurement. Since is negligible, the power distribution to the circular nanoheater and the two arms can be obtained as and , respectively, when a heating power is applied.
Figure 3(b) shows the simulated temperature distribution in the microdisk resonator with the graphene nanoheater by using a 3D simulation tool based on Poisson’s equation. In this simulation, the applied electrical power for heating is set as and it is assumed that the electrical energy is completely converted to thermal energy. The graphene sheet is considered to be a 0.34 nm thick layer with thermal conductivity of , which is for the graphene sheet supported on the substrate in our case (lower than that of a suspended graphene sheet [17,40]). The thermal conductivities of silicon, silica, and photoresist polymer are , , and , respectively. The heat convection coefficient of air is set to . From Fig. 3(b), it can be seen that both the silicon microdisk and the BOX layer below are heated. More details can also be seen clearly from the temperature distributions of the cross sections along the cut lines (Cut 1 and Cut 2) shown in Fig. 3(c). Figure 3(d) shows the temperature distribution of the heated silicon microdisk, and the black dashed curves show the outlines of the graphene nanoheater on the silicon microdisk. It can be seen that the regions near the circular nanoheater along the edge of the microdisk have higher temperature than the center of the microdisk. Fortunately, the guided mode (WGM) of the microdisk is localized at the edge [see the bottom of Fig. 1(b)], which helps to have efficient thermal tuning. It can be also seen that the temperature distribution along the edge of the microdisk is not uniform due to the introduction of the graphene arms. In this case, the effective index of the WGM becomes nonuniform along the cavity. Therefore, when using Eq. (1) to estimate the resonant wavelength of the microdisk, one should replace the effective index by the average effective index , which is given by
In order to make a comparison, we also give a simulation for a thermally tunable silicon photonic microdisk resonator with a traditional metal heater. For this case, the metal heater is assumed to have the same shape as the present graphene nanoheater, and a 1 μm thick upper-cladding layer is inserted between the metal heater and the silicon core to isolate the metal absorption . The simulation result shows that the theoretical heating efficiency is about , and the 90% rising/decaying time is for the metal heater. In contrast, the theoretical heating efficiency for the design with the present graphene nanoheater is about , and the 90% rising/decaying time is . The reason for the difference between them is that the design with the graphene nanoheater does not have the thick upper-cladding layer, which has poor heat conductivity and increases the heating volume significantly. The thick upper-cladding layer also means that the silicon core cannot be as hot as the metal heater. For example, the temperature in the silicon core is , which is lower than that of the metal heater (i.e., ) when the heating power is . For the design with the present graphene nanoheater, the temperature in the silicon core is almost the same as that of the heater. Therefore, the present graphene nanoheater potentially enables much higher temperature increase than the traditional metal heater assuming that they have the same damage threshold of temperature. This is very helpful to achieve a large thermal tuning range.
Noting that the dimension of the circular graphene nanoheater influences the temperature distribution of the microdisk and the heating efficiency of the device significantly, in the following we give a theoretical analysis for the dependence of the heating efficiency on the outer-edge position () and width () of the graphene nanoheater, as shown in Fig. 4(a). It can be seen that the heating efficiency has an enhancement as the width of the circular graphene nanoheater decreases. This can be explained as follows. According to the electrical circuit for heating, the heating power loaded in the circular graphene nanoheater is given by . Therefore, more heating power will be loaded in the circular graphene nanoheater when using a narrower graphene nanoheater that has a larger resistance.
Figure 4(a) also shows that the heating efficiency is improved as the outer-edge position of the graphene nanoheater increases. This is because of the improved overlap between the thermal heating distribution and the WGM field [which locates at the edge of the microdisk; see Fig. 1(b)]. On the other hand, one should notice that the dimension of the circular graphene nanoheater also affects the propagation loss of the WGM field due to the graphene absorption in the microdisk. As shown in Fig. 4(b), when the outer-edge position of the circular graphene nanoheater increases, the propagation loss of the WGM is obviously increased due to the absorption of graphene. Therefore, the circular graphene nanoheater should not be too close to the edge of the microdisk. As shown in Fig. 4(b), the graphene propagation loss could also be reduced slightly by reducing the nanoheater width. However, a very narrow graphene strip is not so robust and fabrication should be done carefully. In our experiment, we choose and as a trade-off. In this case, the propagation loss is negligibly low (i.e., ) and the heating efficiency is about , as indicated by the stars in Figs. 4(a) and 4(b). By using the optimized fabrication processes wet-transferring and patterning for the graphene, it is possible to make a narrow graphene nanoheater so that the heating efficiency can be greatly improved. However, a narrow graphene strip would decrease its breakdown current threshold  and limit the maximal operating current of the graphene nanoheater. Therefore, there is a trade-off when designing the graphene nanoheater.
As a comparison, we also consider the design of using a 100 nm thick gold thin film to replace the graphene sheet. In this case, the gold heater also contacts the silicon core directly. Figure 4(c) shows the calculated metal propagation loss as the dimension varies [44 –46]. It can be seen that the propagation loss increases sharply when the outer-edge position of the metal heater increases to be . Particularly, when the metal heater further moves to be more close to the edge of the microdisk, surface plasmonic mode appears and the metal propagation loss becomes huge. As an example, for the design with and , the propagation loss is up to for the metal heater, which is times higher than the propagation loss () for the graphene nanoheater. Figure 4(d) shows the propagation loss as well as the heating efficiency for the metal heater (top) and the graphene nanoheater (bottom) as the width varies from 0.2 to 2.6 μm. Here . It can be seen that the metal heater always introduces a much higher propagation loss than the graphene nanoheater for the same heating efficiency. Therefore, the metal heater should be far away from the edge of the microdisk to avoid any significant loss. On the other hand, when considering the same requirement for low loss, a graphene nanoheater can be placed closer to edge of the microdisk than a metal heater and a higher heating efficiency is possibly achieved.
As discussed above, the heating efficiency can be improved by reducing the width of the circular nanoheater. Note that the fabrication for a narrow graphene nanoheater is relatively easy with a lithography process and an -plasma dry-etching process, in comparison with the lift-off process for a thick metal heater. According to the equation , one sees that another approach to improve the heating efficiency is reducing the resistance of the graphene arms by increasing width or decreasing the length of the graphene arms . Figure 4(e) shows the calculated heating efficiency of the thermally tunable microdisk resonator as the width of the graphene arms increases. It can be seen that the heating efficiency is up to when the graphene arm width and length are chosen as and , respectively. We also notice that the propagation loss introduced by the graphene arms will increase as the width increases, which will influence the -factor and the extinction ratio of the resonator. Therefore, one should make a trade-off when choosing the width in practice. To decrease the graphene absorption, one approach is to tune its Fermi level by electrical gating or chemical doping . This approach may cause some variation of graphene resistance  and the voltage (or current) applied to the graphene nanoheater should be tuned correspondingly. On the other hand, the heating efficiency of the graphene nanoheater will not decrease because the ratios of and almost do not change, as discussed above.
It is well known that the heating efficiency can be improved further if the heating microdisk is shrunk to minimize the heating volume, which is also desired for realizing ultradense PICs. In this case, an ultracompact heater is usually needed. We realize that the present graphene nanoheater has the advantage of being patterned easily to be nanoscale with CMOS-compatible fabrication processes because of its single-atom thickness and excellent flexibility. This is very helpful for realizing ultrasmall thermally tunable silicon photonic microdisk resonators. As an example, we fabricated ultrasmall microdisks ( and ) with transparent graphene nanoheaters with varied parameters. Figure 5(a) shows the measured dynamic spectral responses of the fabricated 3 μm radius microdisk resonator as the heating power increases from 0 to 2.7 mW, where the transparent graphene nanoheater has parameters of , , and . From Fig. 5(b), which shows the resonance wavelength as the heating power varies, one sees the heating efficiency is about , which is much higher than the experimental result for the fabricated microdisk resonator with . Figure 6 shows the comparison of the experimental heating efficiency for the microdisk resonators (, 3 μm, and 5 μm) with transparent graphene nanoheaters with varied parameters. It can be seen that the heating efficiency is as high as when the radius of the microdisk resonator is reduced to 2 μm. From the experimental and simulation results shown above, it can be seen that the present transparent graphene nanoheater provides an efficient method for thermal tuning for silicon photonic microdisk resonators. In this paper, we choose microdisks instead of microrings to demonstrate thermally tunable optical cavities with graphene nanoheaters because it is easier to transfer a graphene sheet to cover a microdisk than a microring. One should note that the heating efficiency can be further improved when using a microring resonator because of the reduced heating volume, which will be developed with an improved wet-transfer process of graphene in the future.
In summary, we have demonstrated thermally tunable silicon photonic microdisk resonators with transparent graphene nanoheaters. Here the transparent graphene nanoheater is designed to contact directly with the silicon core region, and the excess loss due to graphene absorption is negligible () when the position and dimensions of the graphene nanoheater sitting on the silicon core are optimized. This makes the present transparent graphene nanoheater potentially better than conventional metal heaters. Basically speaking, fabrication processes (like lithography and the dry-etching) for the transparent graphene nanoheaters are also easy in comparison with metal nanoheaters, particularly when used for nanophotonic integrated devices with very limited physical space. Even though the experimental result demonstrated here is not as good as theoretically predicted, it has been shown that the present transparent graphene nanoheater potentially provides high heating efficiency, fast temporal response, and large dynamic range of thermal tuning in comparison with traditional metal heaters. The heating efficiency for the present case can potentially be further improved by, e.g., making the graphene arms suspended [6,16] or inserting a layer of low heat conductivity material (e.g., polymer) between the graphene arms and the –BOX layer to prevent heat from dissipating. As a conclusion, the present heating approach with a graphene transparent nanoheater has the potential to be a very promising option for many applications of energy-efficient thermally tunable/switchable nanophotonic integrated circuits in the future.
National Natural Science Foundation of China (NSFC) (61422510, 11374263, 61431166001); The Program of Zhejiang Leading Team of Science and Technology Innovation (2010R50007); The Doctoral Fund of Ministry of Education of China (20120101110094); The Fundamental Research Funds for the Central Universities.
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