An in-plane, three-port filter consisting of input/output waveguides and two point-defect cavities in a 2D PC slab is designed and fabricated, where a new feedback method is introduced, and its transmission properties are measured. The measured minimum output wavelength spacing between two channels is 1.5 nm, which is realized by slightly adjusting the size of the resonant cavities. The measured resonant wavelengths of two cavities agree well with the calculated ones and the quality factors of the cavities are almost the same. It is believed that this kind of filter may be useful in optical integrated circuits with high density.
©2006 Optical Society of America
In recent years, two dimensional (2D) photonic crystal (PC) slabs[1–4] have attracted much attention for their in-plane photonic crystal band gap (PBG) and easier fabrication with the mature micro-fabrication techniques than their 3D counterparts. By introducing artificial defects, various photonic crystal devices can be realized, such as waveguides[5–7], resonators[8, 9], directional couplers[10, 11] and channel drop filters[12–20], which are being examined for the applications in a wide variety of fields, for example photonic integrated circuits, telecommunications and quantum informatics. In particular, compact channel drop filters are key components for the extraction of light trapped in a point-defect cavity to another neighboring waveguide. Over the past few years, different kinds of channel drop filters have been designed and discussed. A surface-emitting channel drop filter has been reported, in which light tunnels through a line-defect waveguide into the resonant cavity and then is emitted vertically. For the difficulty of collecting the light emitted vertically from the surface, this kind of filter is not suitable in the integrated circuits. Another design is the in-plane channel drop filter in which light tunnels through the resonant cavity into neighboring line-defect waveguide and then is collected horizontally. Noda et al.[14, 15] demonstrate a multi-channel-drop-filter by cascading different lattice constants of photonic crystals (heterostructure) and drop efficiency is improved greatly by the reflection feedback of the heterostructure interfaces. Nevertheless, it is a rather tough job to optimize the distance between the resonator and the hetero-interface and, furthermore, corresponding waveguide bends have to be introduced which can cause serious propagation loss in 2D PC slab. Notomi et al.[16, 17] use width-tuned waveguides as a feedback method to implement a three-port filter system and achieve a high transmittance. Their simulations show that multi-wavelength output is principally possible. However, there is no experimental evidence.
In this paper, we report the design, fabrication and measurement of an ultra compact three-port filter in 2D PC slabs by closing the bus waveguide for 100% reflection feedback. In our design there is no waveguide bends, where the propagation loss depends strongly on the sample roughness, so that the propagation loss is greatly reduced. Therefore, this design can result in a quite high drop efficiency. The minimum wavelength interval of 1.5 nm between two output ports is achieved by carefully controlling the sizes of cavities and almost equal quality factors of around 1000 are obtained across the two output channels. The fact implies that this filter can be used in optical integrated circuits with high density.
2. Design, fabrication and optical properties
The samples are fabricated on a SOI wafer by using electron beam lithography (EBL). The SOI wafer consists of a 235-nm thick silicon layer placed on top of a silica layer, which is deposited on a bare silicon substrate. The 2D PC pattern with a triangular lattice of air holes is defined in a thin film of polymethylmethacrylate (PMMA) with molecular weight of 495K by using EBL (Raith150) under an acceleration voltage of 10 KV, and then directly transferred into silicon membrane by inductively coupled plasma etching under the atmosphere of SF6 and C4F8 gases. After the dry-etching procedure, the resist is removed in terms of O2 plasma. In order to reduce the insertion losses of device, both edges are cleaved and polished. Finally, the insulator layer beneath the PC area is removed by HF acid to form an air-bridged structure.
The beam from a tunable laser source, whose wavelength can be changed from 1500 nm to 1640 nm, is coupled into the input waveguide by a tapered optical fiber and the output light is collected by another fiber. The intensity of vertically scattered light is monitored with an infrared camera. Detailed experimental arrangement is described in Ref. 6.
According to the coupled-mode analysis in Ref. 19, in photonic crystal based channel drop-filters a high dropping efficiency can be achieved by closing both waveguides, which can then work as perfect reflection mirrors. Meanwhile, it has been known that in drop filters the dropping efficiency depends strongly on the relative position of waveguide and micro-cavity. In our new design, where the bus waveguides are closed, the efficiency will be affected by the relative position between the closing waveguide boundary and center of resonator. Therefore, we first study the transmission properties of two-port-resonant-tunneling filters. These filters include two closed bus waveguides, as input and output channel, and a three-point-defect cavity between them. To investigate the dependence of dropping performance on the relative position of waveguide boundary and resonator, we design different types of filters, where the lengths of bus waveguides are different. One type is shown in Fig. 1(a). P3 and P2 are input and output waveguides, respectively, and symmetrically arranged in respect to the center of resonator. The light is coupled to P3 and goes out at P2 when the input frequency is resonant with the cavity. In order to enhance the quality factor of resonator the two air holes at the defect-cavity edges are shifted outward by 0.2a apart from the regular positions, here a is the lattice constant of the PC. The distance between the centers of the defect cavities and the neighboring waveguide is 3 rows of holes in the y direction. The distance from the closing waveguide boundary to the center of the resonator in the x direction is -4a. The corresponding coupling regions are on the left hand of the waveguide boundary. Another type, as shown in Fig. 1(b), has the similar structure as previous one, but the lengths of two waveguides become shorter. The distance between the waveguide boundary and the center of the resonator in the x direction is 4a. In this case, the coupling regions are on the waveguide boundary. The lattice constant of the PC and the radius of the air hole are 420 nm and 130 nm, respectively.
In order to compare the dropping performance, the transmission spectra are measured at the outlet P2 for both filters. The results are shown in Figs. 2(a) and (b), respectively. The two filters have quite different filtering performance although the same resonant cavity is used. In Fig. 2(a), the transmission peak locates between 1530.5 nm and 1538 nm. Moreover, multiple oscillating peaks appear at the top, which may be caused by the interference between the light coupled out from the resonator and that reflected back from the waveguide boundary. In Fig. 2(b), a single sharp peak appears at 1537nm and its full width at half maximum is only 2nm. It implies that a suitable length of closed waveguide, i.e. suitable relative position and distance between the closed waveguide and resonator, is important for the mode matching among waveguides and resonator.
According to the above experiments, a filter with two output wavelengths is designed. Figure 3 shows the scanning electron microscope (SEM) images of the filter. Port 3, port 1 and port 2 are the input and two output waveguides, respectively, which are formed by filling one row of air holes along the Γ-K direction of PC. C1 and C2 are two point-defect cavities. The distance between the centers of the defect cavities and the neighboring waveguide is 3 rows of holes in the y direction. As seen in Fig. 3, C1 consists of three missing air holes, where the two air holes at the cavity edges are shifted outward by 10 nm apart from the regular positions. Similarly, those of C2 are shifted by 20nm. The slight shift of air holes is conducive to confine light inside the cavity and leads to a high quality factor. Meanwhile, the different shifts of two cavities make the resonant wavelengths slightly different. Although it is impossible to directly observe spatial differences of the two cavity sizes in the SEM images, the effect really appears in the optical characteristics of our filters. The distance from the closing waveguide boundary in port 3 to the center of the resonator in the x direction is 4a, here a is the lattice constant of the PC. The lattice constant of the PC and the radius of the air hole are 430 nm and 145 nm, respectively.
The transmission spectra at the two output ports 1 and 2 are measured and the results are shown in Fig. 4. Two single sharp drop peaks corresponding to C1 and C2 appear within the wavelength range of 1510–1550 nm, which indicates that in-plane two channel drops have been successfully achieved. For a broad range of wavelengths single mode operation is possible, which is very useful in various applications. The resonant wavelengths of C1 and C2 are 1529.5 and 1531nm, respectively. The wavelength spacing of the two cavities is about 1.5 nm and might be further reduced by continuously changing the size of the cavity. The full widths at half maximum of the peaks are 1.5 and 1.4 nm and the corresponding quality factors are about 1020 and 1090, respectively. The experimental results show that the slight shift of air holes at the cavity edge indeed can help to localize the light in the cavity and increase the Q factor. This is consistent with the design rule of high-Q nanocavities as proposed by Noda et al. [8, 9] that the light should be confined gently in order to confine strongly. Although the difference in cavity size is only about 20nm, the change in Q factors has become clearly visible. To estimate the drop efficiency, a reference straight waveguide of the same parameters is positioned near the three-port filter. By keeping the same intensity of input light, the transmission intensities of the reference waveguide and port 1 are 0.330 and 0.158 µw, respectively, when the input wavelength is set at 1529.5 nm. The drop efficiency of port 1 is roughly estimated to 48% and a similar result has been obtained at port 2.
The three-dimensional finite-difference-time-domain (3D-FDTD) method is used to calculate the resonant frequencies of C1 and C2. The refractive index of silicon is 3.4 and other parameters are taken the same values as in experiments. The TE-like transmission spectra are plotted in Figs. 5(a) and (b) for port 1 and port 2, respectively. It shows that both ports possess almost the same band gap range for TE-like polarization, from 1130 nm to 1590 nm, and the resonant wavelengths of C1 and C2 are 1521.8 nm and 1524.2 nm, respectively. The spacing between the calculated resonant wavelengths is 2.4 nm, while the measured value is 1.5 nm. In addition, the measured resonant wavelengths of C1 and C2 are slightly larger than the simulated ones. These differences may be attributed to the fluctuation of air hole diameters and the lattice constant that are inevitable in the sample. To understand the symmetry of the cavity mode, we investigate the dispersion relation for the structure using the 3D-FDTD method. The result of the guided-mode band diagrams along with the cavity defect modes is displayed in Fig. 6. Here the frequency f and wavevector k are normalized by a. A single, nonleaky guiding mode exists within the frequency range from 0.278 to 0.287 (a/λ). This mode is an even mode where the Ey field component is even with respect to the mirror reflection plane passing through the waveguide central axis. Photons propagating through the waveguide that are resonant with C1 (0.2826 a/λ) and C2 (0.2821 a/λ) are trapped by the cavity and tunnel to the output channel waveguides. The position of these two closely-spaced cavity modes is represented in Fig. 6 by the horizontal line. At the resonant frequency, the guided mode has a pretty large propagation speed. This can facilitate coupling of energy via the cavity instead of dispersion into the air background and lead to a high dropping efficiency of the three-port filter.
To directly see the propagation behavior of the waveguides and cavities, we observe the light transmission from top of the sample by using an infrared camera. Figure 7 shows the image of the filter performance, as light enters the filter from port 3. It is clearly seen in Figs 7(a) and (b) that light in the input waveguide drops upwards and downwards to the cavity C1 and C2 when the input light wavelengths are set at 1529.5 and 1531nm, respectively. Meanwhile, one bright spot will appear at port 1 and port 2, respectively. On the other hand, in the off-resonant case, i.e. the input light wavelength is set neither at 1529.5 nm nor at 1531 nm, the light in the input waveguide cannot couple to C1 and C2 and strong reflection occurs, as shown in Fig. 7(c). Correspondingly, the bright spots at port 1 and port 2 disappear simultaneously. It turns out that this scattering-light analysis technique can become a powerful supplemental tool to probe the optical properties of PC functional devices to the usual quantitative transmission spectrum technique.
In summary, we have investigated theoretically and experimentally the characteristics of a three-port filter which consists of input/output waveguides and two point-defect cavities in a 2D PC slab. A new feedback method is introduced into this filter and a better performance is proved. The minimum wavelength spacing of 1.5 nm is realized and almost equal quality factors for two output channels are obtained. This work reveals that the three-port filter may be a promising element for future optical integrated circuits with very high density in optical communication systems.
This work is supported by the National Key Basic Research Special Foundation of China (Grant No. 2001CB610402), the National Natural Science Foundation of China (Grant Nos. 10404036, 10515419 and 60345008), and National Center for Nanoscience and Technology, China (Grant No. 2003CB7169). The support from the supercomputing Center, CNIC, CAS is also acknowledged.
References and Links
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