A color filter incorporating a subwavelength patterned grating in a metal film perforated with a square array of circular apertures on a quartz substrate was accomplished. Its performance was enhanced by applying a dielectric overlay to the grating layer so as to match the refractive indices of the media on either side of it. The device was designed by utilizing the finite-difference time-domain method and implemented by adopting the electron-beam direct-writing technique. Two different devices were fabricated with the structural parameters: the grating height of 50 nm and the pitch of 340 nm for the red color and 260 nm for the green color. For the red color filter the center wavelength was 680 nm and the peak transmission 57%, while for the green color one the center wavelength was 550 nm and the peak transmission 50%. It was confirmed the introduction of the index matching overlay led to an increase of ~15% in the transmission efficiency and helped combine double bands into a single dominant band as well, thereby improving the color selectivity of the filter.
©2007 Optical Society of America
A color filter has played a vital role as a crucial element for various applications like complementary metal-oxide-semiconductor (CMOS) image sensors, liquid crystal display devices, light emitting diodes, and so forth [1–3]. So far it was mostly realized by using spin-cast dye films. Recently there was a report claiming that a lowpass filter (or an infrared cutoff filter), which permits the transmission of the short-wavelength light spanning the visible band, could be created with a one-dimensional (1D) subwavelength wire-grid in metals . Also a bandpass color filter was reported based on a 1D grating in silicon . These devices may feature prominent advantages in terms of the outstanding compatibility with the prevalent CMOS process, the flexible integration with other devices, and the low cost. The devices based on a 1D grating structure are, however, susceptible inevitably to the polarization dependency. Meanwhile, a filter based on a two-dimensional (2D) subwavelength metal grating is expected to enable a bandpass filter, which is obtained by inducing a localized surface plasmon so as to provide significant amount of transmission at specific wavelengths [4–8]. So far the research for those devices has been concentrated on the operation in the infrared spectral region instead of the visible region.
In this paper, we focused on the demonstration of a color filter working in the visible spectral range, which was fabricated on a quartz substrate by taking advantage of an aluminum (Al) film perforated with a periodic square array of circular apertures with the subwavelength pitch. It features an inherently polarization independent operation. A medium, whose refractive index is identical to that of the substrate, was applied to the patterned grating to boost the transmission efficiency and to suppress redundant spectral peaks, thereby improving the color selectivity of the filter . It was theoretically designed and analyzed by adopting a finite-difference time-domain method and fabricated through the electron-beam direct-writing technique as well.
2. Device design and simulation results
For an array of subwavelength holes in metals formed on a dielectric substrate, its optical transmission is believed to be proportional to (r/λ)4 where r is the radius of the hole and λ the light wavelength, and it is usually extremely small over the spectral range where the subwavelength condition is satisfied since there is no propagating mode . However, at certain resonant wavelengths denoted as λmax, which are dependent upon the pitch of the array and the dielectric constants of the metal and adjacent dielectric media, it could be enhanced substantially through the surface plasmon resonance induced at the dielectric-metal interfaces. That is, the light impinging upon the subwavelength holes in metals is first coupled to the surface plasmons on the incident surface, transmitted through the holes to the other surface, and re-emitted from it. As a result, it is confirmed that such a device as an array of subwavelength holes in metals will be capable of working as a spectral bandpass filter centered at λmax.
In this work we aimed to achieve a red-color and a green-color filters operating in the visible range exploiting a square subwavelength grating in Al. Figure 1 depicts the schematic and cross-sectional configuration of the proposed devices. A patterned grating is formed on top of a quartz substrate by perforating an Al thin film with a square array of circular apertures with the smaller pitch than the optical wavelength. And a dielectric overlay is placed on top of the metal grating. When an incoming white light impinges on the device, a specific colored light may be obtained at the output independent of its state of polarization. Initially there is a discrepancy between the metal-air interface and the metal-dielectric interface on the top and the bottom of the grating layer respectively without the overlay. When an overlay having the same dielectric constant as that of the substrate is applied on to the grating, however, the interfaces on both sides of the grating become identical. Therefore, the difference in energies associated with the surface plasmon on either side of the metal film will be minimized to help boost the light transmission. As known from Fig. 1, the structural parameters related to the device are denoted as: Λ for the pitch of the grating, G for the diameter of each aperture hole, W for the metal stripe in between two adjacent aperture holes, and H for the grating height. Here the Λ is shorter than the optical wavelength (450 ~ 750 nm). From the cross-sectional view of the device, the refractive indices for the substrate, the metal layer and the overlay are denoted as ns, nm and nh respectively.
The proposed device was analyzed by means of a commercialized tool based on the finite-difference time-domain method, OptiFDTD (Optiwave, Canada). The dispersion information associated with the Al was derived from the Lorentz-Drude model . For the complex refractive index of the metal given as nm = nr + jni, the nr and the ni are known to range from 0.5 to 1.8 and from 4.3 to 7.1 respectively in the visible band, indicating that it functions actually as a good conductor instead of a perfect conductor. The quartz substrate was assumed to possess a constant refractive index of 1.5 with negligibly small loss. And the grating height-equal to the thickness of the metal layer-was chosen to be 50 nm through out this work.
The period of the grating was primarily varied to find out the desired center wavelength of the color filters. Additionally, the duty ratio defined as the ratio of the metal stripe width to the pitch was adjusted to tailor the device performance. As addressed earlier, a dielectric overlay with the same refractive index as that of the quartz substrate was introduced to the metal-air interface of the grating film to enhance the spectral performance of the filter. The device design was basically carried out to acquire a spectral response with sufficient transmission efficiency and desirable roll-off characteristics. Determined parameters were: For the Dev I used as a red color filter, Λ = 340 nm, G = 230 nm, and W = 110 nm; for the Dev II used as green color filter, Λ= 260 nm, G = 170 nm, and W = 90 nm. The theoretical transfer characteristics of them are described in Figs. 2(a) and 2(b). They exhibit no dependency upon the state of two polarizations, transverse electric (TE) and transverse magnetic (TM), because there exists obviously a geometrical symmetry for the proposed device in terms of the shape of the holes and the configuration of the array. Here for the TE and TM polarizations, the electric field is assumed to be in the x-direction and y-direction respectively. It is noted the center wavelengths for the Dev I and the Dev II were 630 nm and 500 nm respectively for the case of a metal-air interface on top of the grating layer (nh=1). As a result of the application of the index-matching overlay, the peak transmission efficiency was increased by 15% for the Dev I and for the Dev II it was enhanced by 12%, while the center wavelength was long-wavelength shifted by about 50 nm for both of them. The spectral shape appeared to be simplified as well. That is, for the Dev I shown in Fig. 2(a), before the application of the overlay two distinct peaks were witnessed originating from the difference in the energies associated with the surface plasmon modes on both sides of the grating layer. Upon the application of the overlay, however, the discrepancy in the surface plasmon energies vanished so that they were merged into a single dominant peak.
3. The device fabrication and experimental results
The fabrication procedure for the proposed color filters is briefly described in Fig. 3: An Al film of 50 nm thickness was deposited on a 4” quartz substrate via the RF sputtering. A thin e-beam resist layer was spin-coated on top of the metal film, and the desired 2D grating pattern was generated in it by means of the electron-beam writing machine. And the metal layer was dry etched with the e-beam resist pattern serving as a soft-mask, and it was subsequently eliminated. Finally an overlay was produced by applying an index matching material to the top side of the grating layer as demanded. The area of the grating pattern was as big as 100 μm × 100 μm. The scanning electron micrographs of the fabricated gratings are depicted in Fig. 4. For the Dev I in Fig. 4(a), the measured grating pitch Λ = 342 nm, the metal stripe width W = 124 nm, the aperture diameter G = 218 nm. And for the Dev II in Fig. 4(b), the achieved grating pitch Λ = 260 nm, the metal stripe width W = 97 nm, the aperture diameter G = 163 nm. These experimental results offer a good agreement with those of the design.
Each of the prepared filter was first mounted onto a translation stage, and an unpolarized light beam from a halogen lamp (LS-1, Ocean Optics) was launched normally into it and the corresponding output beam was detected with a spectrum analyzer (USB 4000-VIS-NIR, Ocean Optics). Here a Glan-Thompson polarizer was inserted in between the device and the spectrum analyzer for the purpose of selecting either polarization state of the TE and TM polarizations. We did not observe any significant discrepancy in the observed transfer characteristics of the device between the two polarizations as anticipated. Measured spectral outputs for the two devices are given in Fig. 5. As shown in Figs. 5(a) and 5(b), the Dev I gave the center wavelength of 620 nm, the 3-dB bandwidth of 200 nm and the peak transmission efficiency of about 42%. And for the Dev II, the center wavelength was 500 nm, the bandwidth 200 nm, and the peak transmission efficiency about 30%. The above results were obtained for the case of an air-metal interface on top of the grating layer.
In accordance with the theoretical investigation discussed above, an overlay was then incorporated into the metal grating by utilizing an index matching oil (Cargille Labs), whose refractive index (nh=1.5 at 588 nm) is approximately the same as that of the quartz substrate. Figure 6 shows the observed transfer curves for the two devices with the index-matching overlay included. As known in Fig. 5(a), for the Dev I, the center wavelength was shifted by ~60 nm to become 680 nm, the peak transmission efficiency was elevated by the amount of 15% to reach 57%, while the bandwidth was remaining at 210 nm. And for the Dev II, as plotted in Fig. 5(b), the center wavelength was shifted by ~50 nm to become 550 nm, the peak transmission efficiency was increased by about 20% to become 50%, while the bandwidth nearly stayed at 230 nm. For the Dev I the double peaks were merged into a single dominant peak with the help of the index-matching overlay, as predicted. In addition, Fig. 5 illustrates the output images obtained from the two devices for an incoming white light. The Dev I provided a red-like color image having the center wavelength of 680 nm as desired. The Dev II however exhibited a yellow-like color image with the center wavelength of 550 nm instead of a green-like color image, which is believed to result from the unwanted excessive inclusion of the yellow color bands due to the slow roll-off in the longer-wavelength region. The quality of filtered colors is anticipated be upgraded by diminishing the bandwidth of the device to improve the roll-off characteristics of the filter, which can be possible by thickening the metal layer . For this purpose we investigated theoretically the influence of the grating thickness upon the bandwidth and the transmission efficiency of the filter. As plotted in Fig. 6(a) and 6(b), for the device with Λ = 260 nm, the peak transmission efficiency was degraded by 15% but the bandwidth was remarkably reduced from 240 nm to 120 nm as the grating height was changed from 50 nm to 200 nm. It should be noted however that there is a trade-off between the grating height and the bandwidth from the viewpoint of practical fabrication.
In summary, we have proposed and realized a couple of visible color filters by taking advantaging of a subwavelength patterned grating in Al films with a periodic square array of circular holes embedded. An index matching overlay was introduced to improve the transmission efficiency by better than 15% and to suppress unwanted sub peaks. The performance of the device will be further improved by optimizing its structural parameters like the metal thickness, the duty ratio etc.
This research work was supported by Center for Nanoscale Mechatronics & Manufacturing of the 21st Frontier Project supported by Korean Ministry of Science & Technology, by the nano IP/SoC promotion group of Seoul R&BD Program, and by a research grant from Kwangwoon University in 2007.
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