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A split nanobeam cavity is theoretically designed and experimentally demonstrated. Compared with the traditional photonic crystal nanobeam cavities, it has an air-slot in its center. Through the longitudinal and lateral movement of half part of the cavity, the resonance wavelength and quality factor are tuned. Instead of achieving a cavity with a large tunable wavelength range, the proposed split nanobeam cavity demonstrates a considerable quality factor change but the resonance wavelength is hardly varied. Using a nanoelectromechanical system (NEMS) comb-drive actuator to control the longitudinal and lateral movement of the split nanobeam cavity, the experimentally-measured change of quality factor agrees well with the simulated value. Meanwhile, the variation range of resonance wavelength is smaller than the full width at half maximum of the resonance. The proposed structure may have potential application in Q-switched lasers.
© 2015 Optical Society of America
1. Introduction
Photonic crystal cavities can achieve light modes with high quality (Q) factors and confine light in a small volume, which is useful for a wide range of applications such as optical switches [1–3], filters [4–6], modulators [7, 8], ultra-small lasers [9, 10], quantum electrodynamics [11, 12], nonlinear optics [13, 14], and biological and chemical sensors [15, 16]. Among these, the ability to tune photonic crystal cavities is an attractive feature for it can actively and dynamically control the cavities’ resonances. Various tuning approaches, including employing microfluidics [17], liquid-crystal [18], magneto-optic [19], thermo-optic [20], and electro-optic [21, 22] effects, have been reported. Additionally, nanoprobes, such as atomic force microscopes (AFM) [23, 24] and scanning near-field optical microscopes (SNOM) [25], have been utilized as tuning tools by perturbing the evanescent field of the cavity. On-chip-integrated microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) driven nanoprobe perturbation and coupled cavity tuning approaches have also been introduced [26–34]. The integrated MEMS/NEMS tuning schemes could achieve a tuning range of up to 24 nm [35]. Generally, resonance tuning through cavity coupling leads to less degradation of Q-factor compared with nanoprobe perturbation approach because light inside the cavities involved in tuning is well confined. However, the resonance shift through coupled cavities is accompanied by a resonance split, which is undesirable for many applications. Recently, a tunable slotted slab photonic crystal cavity has been reported to avoid the problem of resonance split and yet maintain wide-range tuning and high-Q properties simultaneously [36, 37].
The quasi-one dimensional (1D) photonic crystal cavity has also attracted much attention lately for its small size, easy fabrication and better integration with optical waveguides [38]. Recently, many quasi-1D photonic crystal cavities with ultrahigh-Q have been reported [39–41]. Among these, the “zipper” cavity (double-clamped ladder structures) can simultaneously localize mechanical and optical energy at the nanoscale [42]. Besides, there are theoretical analyses and simulations available for quasi-1D photonic crystal cavities [43], which can be tuned by NEMS-based actuators.
Here, we present the design and demonstration of a tunable split nanobeam cavity, which has an air-slot in its center. Instead of achieving a cavity with large wavelength tunable region, the proposed split nanobeam cavity has an ultra-small tunable region of resonance wavelength but a considerable Q factor change. The cavity can be potentially applied in the Q-switched lasers. In this paper, the split nanobeam cavity is firstly simulated and investigated through longitudinal and lateral movements. Next, the fabrication process and characterization procedure are described. Finally, the devices are experimentally characterized and the measured data are compared with the simulation results.
2. Device design and fabrication
The design of the split nanobeam cavity is based on Silicon-on-Insulator (SOI) wafers, which are desirable for future large-scale integration of photonic and electronic devices. The SOI wafer used has a device layer of thickness 260 nm and buried oxide layer of 2 μm. Figure 1(a) shows a global scanning electron microscope (SEM) image of the NEMS-driven photonic crystal cavity. As shown in Fig. 1(b), the split nanobeam cavity is formed in the silicon device layer with an air-slot in the middle, splitting the structure into two symmetrical parts. The oxide layer under the split nanobeam cavity is removed at the end of the fabrication process, thereby creating a released air-suspended cavity. Two tuning mechanisms are investigated. Firstly, the resonance wavelength and Q factor of split nanobeam cavity is tuned through a NEMS actuator that changes the width of the air-slot (ws) in the z direction, i.e. the longitudinal direction. The geometry of the middle holes in the cavity and other dimensions are shown in Fig. 1(b). The detailed SEM images and dimensions of the comb-drive actuator, spring suspension, and the additional air-slot in the waveguide are given in Figs. 1(c)-1(e), respectively.
Fig. 1 (a) Global SEM image of electrically tunable split nanobeam cavity with NEMS comb-drive actuators. The light grey area shows the released region, which is about 1.5 µm undercut enabling the comb-drive to move in the z direction. (b) The magnified SEM image for split nanobeam cavity. The width of the nanobeam, air-slot and middle hole diameter are 700 nm, 80 nm and 224 nm, respectively. (c) The magnified SEM image showing the actuator’s critical dimensions. The width of comb-drive fingers is 195 nm; initial finger overlap is 490 nm; the air gap between two adjacent fingers is 205 nm; the finger number is 41 on each side. (d) The magnified SEM image for NEMS springs. The width and length of the flexible beams are 300 nm and 14 μm respectively; (e) The magnified SEM image showing an addition air-slot in the waveguide, which is introduced to enable the cavity to move in the z direction. The slot width is 60 nm. The slot may introduce some very low Q factor Fabry-Perot (FP) modes. However, these modes have negligible effect on the experimental results because the Q factor of the cavity under investigation is much higher by orders of magnitude.
The NEMS comb-drive actuator [44] is located at the left side of the split nanobeam cavity. The electrostatic pulling force is generated by the potential difference between the two sets of comb-drive fingers. Normally, the movable part of the comb-drive is connected with the ground to avoid bending it towards the grounded substrate while a positive voltage is applied to the fixed electrode of the comb-drive. The left half of the cavity is connected with the movable part of the comb-drive while the right half is fixed. Thus, the air-slot width of the cavity can be widened when a voltage is applied to actuate the left comb-drive or narrowed when the right comb-drive is actuated instead. Thus bipolar tuning is feasible experimentally.
In addition to longitudinal tuning, the tuning of resonance wavelength and Q factor of the split nanobeam cavity through a lateral offset induced by the NEMS comb-drive is also designed and investigated, as shown in Fig. 2(a) for the global view of the device and Fig. 2(b) for the zoom-in view showing the details of the cavity. The driving mechanism is straightforward and the NEMS comb-drive and spring suspension designs are nearly identical to those used in the device shown in Fig. 1.
Fig. 2 (a) Global SEM image of the laterally tunable split nanobeam cavity with a NEMS comb-drive actuator. (b) A magnified SEM image showing the center of the cavity as indicated in the red dotted box in (a). The lower part of the cavity is movable in the x-direction.
The device is fabricated on the SOI wafer mentioned before. E-beam resist ZEP 520A-7 is first coated on the wafer forming a 260 to 280 nm thick layer. The patterns of the split nanobeam cavities and NEMS comb-drive actuators are obtained by electron-beam lithography (EBL). The writing current is 200 pA and the exposure dose is 320 µC/cm2. The device pattern is then transferred to the wafer’s device layer through an inductively-coupled plasma reactive ion etching (ICP-RIE) system, which uses plasma of C4F8/SF6 gas. The etch depth of this step is 260 nm, i.e. the silicon layer is etched through. The residual E-beam photoresist is removed by Microposit 1165 remover. Next, another EBL and ICP-RIE etch are used to fabricate the rib waveguides and grating couplers, except that the etching depth for this step is controlled to be at 80 nm. The residual E-beam photoresist is also removed by 1165. In addition, the other structures for device actuation, including the isolation trenches and electrodes, are fabricated through a series of optical lithography, RIE etching, metal E-beam evaporating, and lift-off processes. Finally, the wafer is diced into 6 × 6 mm chips, and hydrofluoric (HF) acid vapor is used to etch the silicon dioxide below the cavities and actuators. The released regions of the devices shown in Figs. 1 and 2 have a slight difference in color (light grey) compared with the other silicon regions on the SOI wafer indicating that the dioxide under the silicon structures is removed by HF vapor. The etch depth and undercutting width are about 1.5 μm. The optical field is mainly concentrated in the silicon nanobeam region with a small modal volume due to the high refractive index of silicon. It should be noted that the irregular silicon dioxide residue underneath the waveguides and cavities is effectively outside the modal field regions, and thus will not interact with the optical modes.
3. Simulation
There are several designs of ultrahigh Q quasi-1D photonic crystal cavities available, such as lattice constant modulation [40], hole-size modulation [41] and beam-width modulation [42]. The hole-size modulation design is adopted here to construct the cavity. We set the total number of holes in the split nanobeam cavity equal to 79. The lattice period of the cavity is denoted by a, which is fixed at 320 nm. The radius of air-holes is labeled rn (n changes from 0 to 39 corresponding to number of holes from the center to either side in the cavity region). The radius of the air-holes in the cavity region is modulated according to the equation below to achieve a high Q factor and strong coupling to the feeding waveguide [33, 43]:
where r39 = 20 nm, and r0 = 110 nm, respectively. In addition, the width of cavity is w = 660 nm and that of the air-slot ws = 40 nm. With the refractive index of silicon set to 3.476, the three-dimensional (3D) finite-difference time-domain (FDTD) simulations for this split nanobeam structure are carried out using RSoft FullWAVE. The two-dimensional modal profiles in the x-z plane and the resonance wavelengths of the fundamental to fifth order modes are shown in Figs. 3(a) to 3(e), respectively.Fig. 3 Simulated modal profile of the (a) first mode, (b) second mode, (c) third mode, (d) forth mode, and (e) fifth mode.
This structure is favorable for transverse-electric-like (TE-like) band gaps. Therefore we study a TE-like field and represent it with the electric field component in the x-z plane. Increasing the radii of air-holes can shift the high-order mode down into the band gap of the mirror region and a resonance can be formed. Since there is an air-slot along the x-axis for the split nanobeam cavity, the electric field component has a wave node in the air-slot of the dielectric region, as can be seen from the Fig. 3(a), which is totally different from the normal fundamental mode of a nanobeam cavity. Furthermore, for the even modes, there are energy concentrations in the region of the air-slot. Typically, such modes are lossy due to their tendency to radiate energy to free space, which introduces large intrinsic losses in those modes. Thus, the corresponding Q-factors for the fundamental to fifth modes are 1.07 × 106, 700, 3.23 × 105, 900, and 1.72 × 104, respectively. It is obvious that the Q-factors of the even modes are very small compared to those of the odd modes. Therefore, only the odd modes are considered in this paper.
The dependence of Q-factor and resonance wavelength on the slot width is theoretically investigated here. Figure 4(a) shows the simulated resonant wavelength of the cavity as a function of the width of the air-slot, while Fig. 4(b) gives the simulated Q with the change of the air-slot width. The red, blue, and black curves denote respectively the fundamental, third-order, and fifth-order resonances. It can be deduced that narrowing and widening of the air-slot ws produce red and blue shift of resonance wavelength, respectively. However, it is found that the shift of resonance wavelength is slight. For instance, the shift of wavelength of first mode is just 0.532 nm as ws increases from 40 nm to 120 nm, but the Q-factor decreases from 1.07 × 106 to 9 × 104. The variations of the resonance wavelength and Q factor for higher order modes show the same trends compared to those of the fundamental mode when the slot width changes. However, since the modal volume increases as the modal order scales up, which makes the energy concentration in the air-slot region decrease. Thus, the slop of variation decreases as the modal order increases.
Fig. 4 (a) Simulated resonant wavelengths for the fundamental (red), third (blue) and fifth (black) order modes change as functions of slot width (ws) from 0.04µm to 0.12µm. (b) Simulated Q-factor of the fundamental (red), third (blue) and fifth (black) order modes versus slot width (ws) from 0.04µm to 0.12µm.
Subsequently, the dependence of Q-factor and resonance wavelength on the movement in the x direction (mx) is investigated at four different widths of air-slot (ws = 0.04 µm, 0.06 µm, 0.08 µm, and 0.1 µm). Figures 5 (a), 5(c) and 5(e) show the dependences of the resonance wavelengths on the lateral movement, while the changes of the Q factors versus the lateral movement are given in Figs. 5(b), 5(d) and 5(f). The red, blue, black, and cyan curves denote respectively the cavities with ws = 0.04 µm, 0.06 µm, 0.08 µm and 0.1 µm. It can also be seen that the shifts of resonance wavelengths are extremely small. The variations of resonance wavelengths due to mx are 12 pm, 10 pm, 8 pm and 7.5 pm for the fundamental mode when ws = 0.04 µm, 0.06 µm, 0.08 µm and 0.1 µm, respectively. But the Q factors for these modes are changed considerably with little effect on the resonance wavelengths. We believe that the proposed cavities can be potentially applied in the area of Q-switched lasers by introducing the active gain materials.
Fig. 5 Simulated resonant wavelengths for the fundamental (a), third (c) and fifth (e) order modes change as functions of lateral movement from 0 µm to 0.05 µm at four different widths of air-slot, and the simulated Q factors of the fundamental (b), third (d) and fifth (f) order modes versus lateral movement from 0 µm to 0.05 µm at four different widths of air-slot.
4. Characterization
Figure 6 shows a schematic of the setup for testing the tunable split nanobeam cavity. Light from a tunable laser source (ANDO AQ4321D) is launched into a single-mode fiber. A fiber polarization controller is utilized to selectively excite the TE-like modes of the split nanobeam cavity. A pair of XYZ-stages controls the coupling fibers tilt at 10 degrees from the normal direction to the device surface. The fibers are aligned manually with the grating couplers on the device under a microscope. The area, period, filling factor (ratio of groove width to grating period), and groove depth of the grating coupler are 12 μm × 12 μm, 600 nm, 50%, and 80 nm respectively. These parameters are optimized using FDTD simulation for maximum coupling efficiency to a fiber with 10 degrees tilt angle with respective to the grating normal. This grating coupler design has an experimentally-measured peak coupling efficiency of around 30% at a wavelength of 1570 nm and a 3 dB-bandwidth of 105 nm. The light launched from the grating coupler is guided by a rib waveguide, which has the same etched depth (80 nm) as the grating coupler. The width of the rib waveguide then gradually tapers down to 1500 nm, where it is connected to the air-suspended silicon waveguide. The light is then further guided by the air-suspended waveguide and directed to the cavity and the cavity’s output is launched into a multi-mode fiber through the output grating coupler. Finally, the light signal is recorded by an optical spectrum analyzer (ANDO AQ6317C). The tunable laser and spectrum analyzer can synchronously sweep through wavelengths from 1520 nm to 1620 nm with a resolution of 1 pm.
Fig. 6 Schematic of setup used to characterize the tunable split nanobeam cavity. TLS, tunable laser source; FPC, fiber polarization controller; OSA, optical spectrum analyzer.
For NEMS comb-drive actuators designed in our work, the relationship between displacement and applied voltage can be approximately calculated by [44]:
where Fe is the electrostatic force produced by the comb-drive actuator, k is the spring constant of the folded-beam suspensions, n is the number of comb-drive fingers, ε is the permittivity in air, t is the device thickness, Va is the applied voltage, s is the air gap between two adjacent fingers, E is the Young’s modulus of silicon, w and L are the width and length of the folded beams, respectively.The normalized transmission spectrum of the device shown in Fig. 1(a) is experimentally characterized and the results are given in Fig. 7. The red and black curves in Fig. 7(a) give the shape of the resonances for the applied voltages of 0 and 18V, respectively. The simulation results are around 1568.44 nm and 2.3 × 105. However, when the applied voltage is off, the resonance wavelength and Q factor obtained by the Lorentz fit are 1576.053 nm and 5458, respectively. We believe that the increased width of the nanobeam cavity caused by the imperfection of the nanofabrication process may lead to the increase of resonance wavelength and decrease of Q factor. Furthermore, the tilt and rough sidewalls also severely increase the cavity’s intrinsic losses, leading to further reduction in the experimental Q-factors. Figures 7(b) and 7(c) provide the measured resonance wavelengths and Q factors as the voltage is applied to the right side of comb-drive, which makes the slot in the center of the cavity narrower. From the simulation results, the resonance wavelength and the Q factor increase simultaneously when the width of the slot decreases, which agrees with the experimental results. Through an 18V applied voltage, the Lorentz-fitted resonance wavelength is 1576.438 nm. Furthermore, when the width of the air-slot decreases, the measured Q factor increases to 10250, which is about twice the original Q factor.
Fig. 7 (a) The red and black curves give the experimentally-measured transmission spectrum of the split nanobeam cavity shown in Fig. 1(b) at the applied voltage of 0 and 18V, respectively. (b) The red curve shows the measured cavity resonance wavelength as a function of applied voltage Va. (c) The red curve shows the measured Q factor versus applied voltage Va.
In our devices, we slightly enlarge the nanobeam width in the mask design intentionally to compensate the over etch in the nanofabrication process. However, this compensation is difficult to be controlled precisely, which leads to the over-compensation and enlargement of the nanobeam width in the current devices. Hence, the measured resonance wavelengths of cavities are shifted to low frequencies compared to the simulated results. In addition, it also leads to the degradation of Q factors. Furthermore, the rough sidewalls of the air-holes and nanobeams due to the imperfect fabrication process also severely increase the cavities’ scattering losses, thus leading to a further reduction in experimental Q-factors.
The laterally-tunable split nanobeam cavity shown in Fig. 2 is also tested and the results are provided in Fig. 8, which show the measured resonance wavelengths and Q factors as the driving voltage is applied to the right side of the comb-drive resulting in a lateral offset between the two cavity parts. From Fig. 8(a), it can be seen the transmission decreases to one third when the applied voltage is 18V. The Lorentz-fitted resonance wavelength is 1578.385 nm. The region of experimental resonance wavelengths change is about 70 pm, which is well below the full width at half maximum (FWHM) of the resonance peaks (210 pm) and is extremely small. The Q factor reduces to 5446, which is 75% of that of the original cavity. The experimental results in Figs. 8(b) and 8(c) show that the resonance wavelength shifts to red and the Q factor decreases respectively as the nanobeam offset increases with the increasing voltage applied, which are similar to those predicted by the simulation results shown in Figs. 5(a) and 5(b).
Fig. 8 (a) The black and red curves give the experimentally-measured transmission spectrum of the split nanobeam cavity shown in Fig. 2(b) at the applied voltage of 0 and 18V, respectively. (b) The red curve shows the measured cavity resonance wavelength versus applied voltage Va. (c) The red curve shows the measured Q factor as a function of applied voltage Va.
Therefore, we can conclude that the proposed structure can be utilized to tune the Q factor of a split nanobeam cavity with only slight change of the resonance wavelength. Note that the experimental results are not the limit of the proposed resonance tuning approach; rather, it is limited by the stroke of our current NEMS actuator design and air-slot fabrication. With the actuator optimized for stiffness and a smaller width of air-slot, it would be possible to achieve larger Q factor tuning range with relatively small applied voltages as predicted by the simulation results.
5. Conclusion
In this paper, the design and demonstration of a split nanobeam cavity are presented. The split nanobeam cavity is numerically investigated through the longitudinal and lateral movements. Instead of achieving a cavity with a large resonance tunable range, the proposed split nanobeam cavity has a small resonance wavelength shift but a considerable Q factor variation. Using a NEMS comb-drive actuator to control the longitudinal movement of half of the split nanobeam cavity, an experimentally-measured Q factor that can be changed by one order of magnitude is shown. Meanwhile, it is found that the change of Q factor caused by lateral movement is relatively small compared to that of longitudinal movement. However, the range of resonance wavelength shift is smaller than the FWHM of the resonance itself. The performance of device can be improved by reducing nanofabrication errors and fabricating a split nanobeam cavity with the width of air-slot being smaller than 40 nm. Furthermore, an optimized NEMS comb-drive actuator can further increase the tuning range of the cavity, which leads to a larger Q factor change.
Acknowledgments
This work is supported by MOE Research Grant R-265-000-416-112. Devices are fabricated in the SERC Nanofabrication and Characterization Facility (SNFC), Institute of Materials Research and Engineering, A*STAR, Singapore.
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