1. Introduction

Two-dimensional (2D) photonic crystal (PC) slabs constructed from silicon-on-insulator (SOI) wafers have been used to realize ultrahigh-Q nanocavities [1–4] and a very low-loss waveguide [5] due to well-developed nanofabrication techniques. Coupled nanocavity-waveguide systems in silicon (Si) slabs have been used to realize various functional devices with ultra-small sizes, such as photodetectors [6,7], modulators [8,9], and a Raman laser [10]. These devices should have good compatibility with complementary metal oxide semiconductor technology [11,12], and thus Si PC slabs are a promising platform for large-scale-integration photonic chips.

Wavelength-selective filters consisting of nanocavities and waveguides in Si slabs have an ultrasmall footprint for a single port, less than 10 μm × 10 μm, and therefore, they have been intensively studied [13–16]. In 2003, it was demonstrated that a nanocavity with three aligned missing air holes (L3 cavity) has good properties in terms of filtering resolution, radiation pattern, and broad free spectral range (FSR) [17]. Subsequently, a flat-top response using two cascaded nanocavities [18] and a highly efficient in-plane 4-channel filter with 20 nm channel spacing [19,20] were demonstrated. In 2008, a 16 channel drop filter with 5 nm spacing operating in a wide range from 1510 nm to 1590 nm, where a wavelength variation with a standard deviation of ~1 nm was reported [21]. These studies show that PC drop filters have the potential for course wavelength division multiplexing (WDM) applications. However, they have not presented the maximum potential of the PC filters in view of the channel number, wavelength variation, and channel spacing. In particular, a filtering operation with 100 GHz (~0.8 nm) spacing, which is a landmark value in dense WDM, have not been reported yet.

Here, we report 100 GHz-spaced drop filters using 32 arrayed high-Q nanocavities. By increasing the lattice constant of the PC only in the x-direction by 0.375 nm, we successfully controlled the drop wavelengths at ~100 GHz spacing in the c-band. The variation of the operating wavelengths showed a standard deviation less than 0.30 nm. The size of a single channel unit was 15 μm × 6.5 μm, and the total device size was 15 μm × 270 μm, which is 50-times smaller than that of a Si arrayed waveguide grating (AWG) [22] or filters using Si ring resonators [23,24]. A movie showing the device operation demonstrated that Si PC slabs have the potential to make ultra-compact wavelength filters for dense WDM.

2. Configuration of 1 × 32 channel drop filter and experimental setup

Figure 1 shows a laser scanning microscope image of 32 arrayed nanocavities fabricated in series and a scanning electron microscope (SEM) image of a nanocavity. This device consisted of 32 PC units (PC₁, PC₂, ····PC_n, ····PC₃₂), each of which had a line-defect transmission waveguide and a point-defect nanocavity to extract transmitted light with a specific wavelength. A single unit had 16 periods of air holes in the x-direction (~6.5 μm) and 20 periods in the y-direction (~15 μm). The length of the waveguide was 270 μm. In order to achieve 100-GHz spacing of the resonant wavelengths from the neighboring nanocavities, the lattice constants a_n in the x-direction were changed by 0.375 nm from 398.375 nm (PC₃₂) to 410.000 nm (PC₁): a_n (nm) = 410−0.375 × (n−1). On the other hand, the lattice constants in the y-direction were the same, 710 nm, for all units to maintain the structural uniformity. This method enabled fine wavelength control and could reduce the partial reflection of transmitted light at the seams of each unit. The width of the transmission waveguide was 738 nm, and the separation from the nanocavities was 6 rows of air holes in all regions. The thickness of the Si slab was 220 nm.

 

Fig. 1 (Upper) Confocal laser scanning microscope image of a 32-channel drop filter which consists of 32 photonic crystal units, PC₁–PC₃₂. (Lower) SEM view of a 0.2a shifted L3 nanocavity for the unit PC_n. The lattice constant a_n in the x-direction was {410−0.375 × (n−1)} nm. We fabricated three samples with air holes of different radii.

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We fabricated three samples on the same chip with different air hole radii (r) of 110 nm, 113 nm, and 116 nm. We employed a shifted L3 nanocavity with neighboring air holes shifted to the outside of the cavity by 0.2a_n [1]. The design Q factor (Q_des) calculated using the three-dimensional (3D) finite difference time domain (FDTD) method was 1.31 × 10⁵ (1.01 × 10⁵) for the cavity of PC₁ (PC₃₂) with r = 110 nm. We fabricated the samples using the same fabrication procedure described in a previous report [25]. Postprocessing to tune the resonant wavelengths was not used. In order to ensure that any field distortion influencing the accuracy of the electron beam (EB) lithography was steady for all 32 units, the pattern for each unit was drawn at the same position within the EB field by instead displacing the EB stage. The stage position was detected by the laser interferometer with the accuracy of less than 1 nm.

Figure 2 shows the measurement setup used to investigate the spectral properties of the 32 nanocavities. The light from a tunable cw laser was split into two beams. One beam was sent to a high-resolution wavelength meter. The other was modulated by a mechanical chopper at a frequency of ~1 kHz with a 50% duty ratio and was focused by a 0.40-numerical-aperture (N.A.) objective lens on the facet of the excitation waveguide with TE polarization. When the incident wavelength matched the resonant wavelengths of the nanocavities, the transmitted light was extracted in the direction perpendicular to the slab, as shown in Fig. 1. The dropped light was collected by a 0.65-N.A. objective lens ( × 50). The sample was placed on a high-precision 6-axis stage, and the positions of the optical components were adjusted using near-infrared (NIR) cameras such that only the dropped light from a single cavity was injected into the InGaAs photodiode. The intensity of the dropped light was measured by a lock-in amplifier system as a function of the laser wavelength.

 

Fig. 2 Measurement setup used to investigate the drop wavelengths of arrayed nanocavities. Pol: polarizer. OL: objective lens. BS: beam splitter. M: mirror on a flip mount stage. PD: InGaAs photodiode. NIR camera: near-infrared InGaAs camera.

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3. Experimental results and discussion

Figure 3(a) plots the measured drop wavelengths of 32 cavities versus the designed lattice constants for three samples. This graph shows that the resonant wavelengths were precisely proportional to the lattice constant. The slope of the fitted line for the sample with r = 110 nm shows a 0.73 nm increase in wavelength per 0.375 nm increase in lattice constant in the x direction. This value corresponds to a spacing of 92 GHz. Similarly, the slopes for the samples with r = 113 nm and r = 116 nm indicate spacings of 96 GHz and 97 GHz, respectively. The inset shows the resonant wavelengths of the measured nanocavities calculated using 3D FDTD, where wavelength dispersion of Si was considered [26]. Three linear fits indicate 94–96 GHz spacing per 0.375 nm increase in lattice constant, which is in good agreement with the experimental results. The variation in wavelengths from the fitting values had a standard deviation as small as 0.21 nm in the sample with r = 110 nm, which would be negligibly small when applied to course WDM. The values for the samples with r = 113 nm and r = 116 nm were 0.26 nm and 0.29 nm, respectively. Figure 3(b) shows the histogram of wavelength variations for three samples where indicates the standard deviation of 0.25 nm. This value is 4–5-times smaller than the values in a previous report [21]. This development was due to the improvement in EB lithography described above.

 

Fig. 3 (a) Wavelength of dropped light versus the lattice constant in the x direction for 32 arrayed nanocavities. Open circles represent experimental data, and red lines indicate linear fits. The inset is the calculated resonant wavelength of the nanocavities. (b) Histogram of wavelength deviations for the three samples.

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Figure 4 (Media 1) shows near-infrared camera images for the drop filter with r = 110 nm when the transmitted laser wavelength was scanned from 1525 to 1545 nm. The sample was located in air and was illuminated by the lamp. The dropped spots in Figs. 4(a)-4(c) represent emission from the cavities for PC₃₀, PC₂₃, and PC₁₅, respectively. All cavities clearly emitted light with the same pattern. Although the dropped spots have the side lobes in 0.2a_n shifted L3 cavity, single-lobed spot can be obtained in the 0.15a_n shifted L3 cavity as presented in Media 2 (the cavities have the same structural parameters as those in Fig. 1). The movies clearly demonstrate that the transmitted laser light was extracted from the nanocavities in succession, and that two different cavities rarely emit simultaneously. These results clearly show that the device shown in Fig. 1 has the potential to be used in 100 GHz spaced filters.

 

Fig. 4 Near-infrared camera shots of the 32-channel drop filter with r = 110 nm when the wavelength of the transmitted laser was scanned from 1525 to 1545 nm at a speed of 2 nm per second (Media 1). A movie showing the operation of the drop filter consisting of 0.15a shifted L3 cavities is presented in Media 2. Movies were obtained with exposure time of 10 millisecond, 25 frames per second, and camera resolution of 320 × 256 pixels.

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It is important to briefly refer to recent results for other types of drop filters in Si photonics. A 16-channel silica AWG with 200 GHz spacing having dimensions of 4.0 mm × 2.7 mm on a Si chip has been reported [27]. A 32-channel Si AWG with 200 GHz spacing with a smaller size of 400 μm × 500 μm, owing to the high refractive index of Si, has also been demonstrated [22]. This size may be comparable to filters using Si ring resonators when the channel number is 32 [23,24]. Our 32-channel drop filter with 100 GHz spacing had dimensions of 15 μm × 270 μm, which is 50-times smaller than the Si AWG. It is noted that the operating wavelengths can be thermally tuned all together, where the small device size will be advantageous. Furthermore, shifted L3 cavities with a broad FSR of more than 60 nm are attractive for multichannel operation.

Figure 5(a) shows the drop spectra for 32 channels in the sample with r = 110 nm, where each spectrum is normalized independently. Figure 5(b) shows the histogram of the experimental Q values (Q_exp) for 32 cavities, which are estimated from the linewidths and the peak wavelengths for the drop spectra. The values randomly vary centered at 4.7 × 10⁴ ranging between 2.4 × 10⁴ and 7.3 × 10⁴. The Q_exp is determined by three Q factors,

 

Fig. 5 (a) Normalized drop spectra for 32 nanocavities in the sample with r = 110 nm. (b) Histogram of the Q_exp values for 32 channels. (c) Histogram of the drop efficiencies.

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In order to further decrease the wavelength variation, the fabrication accuracy of the air holes should be enhanced, because random fluctuations of the radii and positions of the air holes may be the main cause of these variations [3,29]. The fluctuation in slab thickness may also have some influence. The resonant wavelength of the nanocavity for PC₁ (PC₃₂) with r = 110 nm shifts by 1.09 nm (1.54 nm) per 1 nm change in slab thickness, according to 3D FDTD calculations. The roughness average of the top surface of the fabricated samples is 0.1−0.2 nm within a micrometer-size area, as determined by atomic force microscope (AFM) measurements [25,28]. Because the volume of the nanocavity is very small, the thickness fluctuation in a micrometer area may cause a wavelength variation with a magnitude of ~0.1 nm. In order to compensate for the inevitable wavelength variation, postprocessing to tune the wavelengths may be one approach, for example, local oxidation by laser irradiation or AFM lithography [31,32].

For practical applications, a flat-top spectral shape will be important for increasing the 1 dB bandwidth without crosstalk to neighboring channels, though each spectrum in Fig. 5(a) has a single Lorentzian peak. This will be achieved by using coupled nanocavities with optimum Q factors [18]. Such a demonstration will be a significant step for PC drop filters where the 1 dB bandwidth and the variation of dropped power should be investigated. The in-plane drop operation will be also important. This can be resolved by using structures like those reported in [19,20], where output waveguides and heterostructure interface mirrors for cooperative interference should be integrated. The increase in the device footprint caused by these structural modifications will be slight. By adding waveguided Ge-on-Si photodetectors [22] or Si nanocavity detectors [6,7], it should be possible to construct a demultiplexing (DEMUX) receiver. Also the air-bridge structure used in this study has some demerits, including mechanical instability, low thermal conductivity, and vulnerability to contamination. This can be resolved by utilizing the low-index-glass cladding structure having sufficiently high Q values [33]. By integrating a Ge PD on the glass cladding, it should be possible to make a DEMUX receiver with vertical coupling.

4. Summary

Here, we report 32-channel wavelength filters with 100 GHz spacing using arrayed nanocavities. By increasing the lattice constant only in the x-direction by 0.375 nm, we successfully controlled the drop wavelengths at ~100 GHz spacing. The variation in drop wavelength was less than 0.30 nm in a standard deviation. The total device size is as small as 15 μm × 270 μm, which is 50-times smaller than that of a Si AWG. A movie showing the device operation clearly demonstrates that the transmitted laser light is extracted from the nanocavities in succession and two different cavities rarely emit simultaneously. These results are the first demonstration that PC wavelength filters have the potential for use in dense WDM applications. This study will also contribute to the development of multichannel sensors using arrayed high-Q nanocavities.