1. Introduction

In recent years, photonic devices based on artificial defects formed by the introduction of small regions of disorder in photonic crystals (PCs) [1–14] have been vigorously studied for applications in a wide variety of fields: for example, ultra-small wavelength filters [2–9], switching devices [10–13], and delay devices [14], have been examined for applications in telecommunications, low-threshold nanolasers [15–17], chemical sensors [18], and quantum informatics. In particular, ultra-compact channel drop filters, consisting of point-defects and line-defects in two-dimensional photonic crystal slabs, have attracted much attention; the ultimate sizes of such filters are predicted to be less than 1/10,000 of those of conventional optical devices [2–9]. A surface-emitting channel drop filter has been realized, in which light propagating through a line-defect waveguide is resonantly trapped by a point-defect cavity before being emitted normal to the surface [3]. The concept of in-plane hetero photonic crystals (IP-HPCs), which consist of a series of connected PC regions with different lattice constants, has also been proposed [4]. Such structures have been used to demonstrate multi-wavelength drop operations [4]; and theoretical improvements in drop efficiency using reflection at the heterostructure interface have been calculated [6, 19].

Another configuration to consider is that of an in-plane PC device in which light trapped in a point-defect cavity is extracted into a neighboring waveguide. This configuration may serve as a key platform upon which various functional elements can be integrated, switching, optical memory, etc. and therefore is expected to become increasingly important. One-channel drop operation of an in-plane device having an ultra-high Q nano-cavity [20] between two parallel waveguides has been demonstrated experimentally [5], where the drop efficiency is theoretically limited to a maximum of 25%. Furthermore, one-channel in-plane drop operations with efficiencies of more than 80% have been demonstrated experimentally in devices which utilized destructive interference to eliminate undesired outputs [7, 8]. Another configuration which utilizes resonant-tunneling has been also proposed, and one-channel drop operation with an efficiency of 65±20% has been demonstrated experimentally [9].

In this work we experimentally investigate a method of making an in-plane multi-wavelength add/drop filter device with high efficiency and constant quality factor, based on the concept of heterostructure photonic crystals that we proposed previously.

2. Basic concept

Figure 1 shows a schematic of the proposed in-plane multi-channel add/drop filter, designed to realize high efficiencies and constant quality factors for all channels. Based on the concept of heterostructure photonic crystals [4, 6–8], this device consists of multiple photonic crystal slabs, PC₁, PC₂, PC₃…, with proportional lattice constants, where each photonic crystal slab has a different operating wavelength. When multi-wavelength light is incident on this device and propagates through the input waveguide, the light resonant with the point-defect cavity in each PC region becomes trapped by that cavity; the light trapped in each cavity can then be extracted to a separate output waveguide. Here the heterostructure interfaces between the adjacent photonic crystal regions can act as wavelength-selective mirrors [22]. As shown in Fig. 1, a proportion of the resonant light passes the point-defect cavity without becoming trapped. Also a proportion of the light trapped in the point-defect is coupled out of the cavity into the mode propagating towards the next PC region; through the output waveguide. However, the structure of the filter has been designed so that the light propagating towards the next PC region is reflected back by the heterostructure interface. Adjusting the phase of light reflected at the heterostructure interface can result in destructive interference, reducing the proportion of light propagating from the point-defect cavity back towards port 1, while enhancing the proportion propagating towards the output port. In the ideal case this interference may result in the extraction of resonant light from output ports only. Additionally, multi-channel drop operation can be achieved, as the device can be designed so that wavelengths not resonant with the preceding filter unit are transmitted at the heterostructure interface. This transmitted light can then be dropped by another suitably designed heterostructure filter unit within the device. Furthermore, the operating wavelength can be changed whilst maintaining the efficiency and quality factor, because the operating wavelength of a hetero photonic crystal is proportional to the lattice constant, but the efficiency and quality factors are dimensionless parameters. So, once we have optimized the design of one hetero photonic crystal, we can simply connect multiple proportional structures, and expect the same efficiency for all-channel add/drop operations. Although the heterostructures employed in the following experiments are not exactly proportional because the thicknesses of the filter units are inevitably common, the quality factors and efficiencies are expected to be almost constant for small lattice constant change of several percent [23].

 

Fig. 1. Schematic of the multi-channel add/drop filter described in Section 2

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3. Filter unit structure and optical properties

In this section, we report experimental results of a device based upon the design described in Section 2. Figure 2(a) shows scanning electron micrographs of the fabricated basic filter unit and Figs. 2(b–d) show magnified views of important parts of the device. The device consisted of two photonic crystal slabs, PC₁ and PC₂, with lattice constants; a ₁ = 0.420 μm and a ₂ = 0.415 μm, respectively, as shown in Fig. 2(b). The ratio a ₁/a ₂ = 1.012. Here, this value was chosen for the cut-off wavelength in PC₂ to become shorter than the resonant wavelength of the point-defect cavity in PC₁. The distance between the point-defect cavity and the heterostructure interface was set to d = 5a ₁, as it was found that this distance optimized the phase condition of light reflected at the heterostructure interface. Figure 2(c) shows a magnified view of the point-defect cavity, which consisted of three missing air holes. The positions of the six air holes nearest both edges were fine-tuned to maximize the performance [21]; air hole displacements were 0.173a ₁ at position A, 0.024a ₁ at position B, and 0.173a ₁ at position C (see Fig. 2(c)). The intrinsic quality factor of the point-defect cavity was very high, at almost 100,000. Utilization of a cavity having very high intrinsic quality factor is crucial for in-plane type device in order to suppress an out-of-plane loss [5]. The point-defect cavity was centrally positioned in PC₁, between the input and output waveguides, at a distance of 2.5√3a ₁ from each waveguide. For multi-channel operation, the output waveguide was bent; the sizes of the two air holes at the corner were tuned in order to match the transmission frequency of the bent waveguide to the resonant frequency of the point-defect cavity as shown in Fig. 2(d) [5]. The radii of both air holes were set to r _c = 0.26a ₁, which is smaller than the normal air hole radius of 0.30a ₁. The bending efficiency was estimated to be > 90%. The 2D-PC pattern was formed on the thin Si slab of a SiO₂/Si substrate before the SiO₂ layer under the PC slab was etched away to leave a membrane structure.

 

Fig. 2. Scanning electron microscope images of the fabricated device: (a) top view of the fabricated basic filter unit, showing the hetero photonic crystal structure and the high-Q nanocavity; (b) magnified view of the heterostructure interface; (c) magnified view of the point-defect cavity with very high quality factor of almost 100,000; and (d) magnified view of the bend in the output waveguide, showing the tuned radius of the two air holes at the corner.

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Fig. 3. ‘Drop spectrum’ (red line) and ‘through spectrum’ (blue line) of the fabricated basic filter unit, and ‘through spectrum’ (gray line) of the reference waveguide. Scanning electron micrograph image of the fabricated device showing the reference waveguide and filter unit (centre left); and magnified images of the reference waveguide (top left) and device (below left).

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The ‘drop spectrum’ of the fabricated device was measured by injecting light from a tunable laser into port 1 and observing the emission from port 3. The ‘drop spectrum’ plotted as a red line in Fig. 3, clearly contains an emission peak at 1580 nm. The quality factor of the filter was estimated to be Q_T = 1,400, from the full width at half maximum of the emission peak. The ‘through spectrum’ detected at port 2 of the device (blue line in Fig. 3), may be compared to the ‘through spectrum’ of a reference waveguide, prepared adjacent to the device (gray line in Fig. 3). The reference waveguide had a lattice constant of a ₁ = 420 nm, which was the same as that of the input waveguide in the PC₁ region (as shown in Fig. 3). The transmission intensities of the input and reference waveguides were almost equal across their common transmission wavelengths, however, the cut-off wavelengths of the waveguide modes were slightly different. This difference was caused by the heterostructure of the device: the cut-off wavelength of the device was determined by one of the waveguide modes of PC₂, as wavelengths above 1573 nm (within the region in yellow in Fig. 3) were cut by the heterostructure interface. To estimate the drop efficiency, the peak intensity of the ‘drop spectrum’ observed at port 3 was divided by the intensity of the ‘through spectrum’ of the reference waveguide at the corresponding wavelength. A high drop efficiency η of almost 100% was found.

4. Multi-channel drop operation

Next, we describe the configuration of a fabricated multi-channel add/drop filter. Figure 4 shows scanning electron micrographs of a fabricated four-wavelength filter device. The device consisted of four simply connected filter units, with proportional structures. The first filter unit shown in Fig. 4(b), had an equivalent structure to the basic filter unit described above. Each filter unit was composed of two adjacent PC regions. So, the whole device consisted of a total of five photonic crystal slabs, PC₁ to PC₅, each with a different lattice constant. The individual lattice constants were set to a ₁ = 0.420 μm, a ₂ = 0.415 μm, a ₃ = 0.410 μm, a ₄ = 0.405 μm and a ₅ = 0.400 μm, such that a ₁/a ₂ = a ₂/a ₃ = a ₃/a ₄ = a ₄/a ₅ = 1.012. Within each PC, the distance between the point-defect cavity and the heterostructure interface was set to be five times the lattice constants e.g. d ₁ = 5a ₁ etc,.

 

Fig. 4. Scanning electron microscope images of the fabricated multi-wavelength filter device: (a) top view of the fabricated four-channel filter structure; (b)–(e) magnified views of the 1^st – 4^th filter units with proportional structures.

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The drop spectra of the fabricated device were measured; by injecting light into the input port and observing the emission from the output ports of the filter units. The resulting drop spectra are shown in Fig. 5; red, orange, green and blue lines correspond to the emission observed from the output ports of the 1^st, 2^nd , 3^rd and 4^th filter units, respectively. A single clear emission peak was found in each of the four spectra, at 1516 nm, 1536 nm, 1559 nm, and 1583 nm. The quality factor of the first filter unit was estimated to be Q_T = 900, from the full width at half maximum of the emission peak. Both the resonant wavelength and estimated quality factor are consistent with those of the basic filter unit described above. The change of resonant wavelength between each successive filter unit was caused by the corresponding small changes in lattice constant; the wavelength intervals between adjacent emission peaks were almost equal. The wavelength intervals are around 20 nm, which is determined by the lattice constant ratio of 1.012 utilized here. The intervals can be tuned according to need by changing the ratio of lattice constants and by appropriately designing the position of hetero-interfaces to be used for reflection. (The details will be reported in separate paper.) It is also noteworthy that the quality factors of the filter units were almost equal, Q_T = 900 – 1000.

Also shown in Fig. 5 is the ‘through spectrum’ of a reference waveguide prepared adjacent to the device, with a lattice constant of a ₁ = 420 nm, which is equal to that of the input waveguide of the PC₁ region. The drop efficiency of each filter unit was estimated by dividing the peak intensity in the ‘drop spectrum’ by the intensity of the ‘through spectrum’ of the reference waveguide at the same wavelength. The four estimated drop efficiency were all in the range η = 100 ± 20 %.

 

Fig. 5. ‘Drop spectra’ of the fabricated multi-wavelength filter device (red, orange, green and blue lines correspond to the emission observed from the output ports of the 1^st, 2^nd, 3^rd, 4^th filter units, respectively; and ‘through spectrum’ of the reference waveguide (gray line).

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5. Conclusion

We have experimentally investigated a method to obtain multi-channel drop operation, using in-plane channel drop filters in two dimensional photonic crystal slabs. The basic filter unit, containing an ultra-high Q nanocavity and reflective photonic crystal heterostructure interface, was experimentally determined to have a very high drop efficiency of almost 100%. Furthermore, a four-wavelength filter device, by simply connecting four filter units with structures proportional to that of the basic filter unit, showed multi-channel drop operation with very high efficiencies of almost 100%, and almost equal quality factors, across all channels. Thus, we have experimentally demonstrated a very simple method of making multichannel add/drop filters, using heterostructure photonic crystals, in which the efficiency is improved by reflection at the heterostructure interface, and dimensionless parameters including the efficiency and the quality factor are maintained across all channels.