We have demonstrated a highly efficient electrically tunable color filter, which provides precise control of color output, taking advantage of a nano-photonic polarization-tailored dichroic resonator combined with a liquid-crystal based polarization rotator. The visible dichroic resonator based on the guided mode resonance, which incorporates a planar dielectric waveguide in Si3N4 integrated with an asymmetric two-dimensional subwavelength Al grating with unequal pitches along its principal axes, exhibited polarization specific transmission featuring high efficiency up to 75%. The proposed tunable color filters were constructed by combining three types of dichroic resonators, each of which deals with a mixture of two primary colors (i.e. blue/green, blue/red, and green/red) with a polarization rotator exploiting a twisted nematic liquid crystal cell. The output colors could be dynamically and seamlessly customized across the blend of the two corresponding primary colors, by altering the polarization via the voltage applied to the polarization rotator. For the blue/red filter, the center wavelength was particularly adjusted from 460 to 610 nm with an applied voltage variation of 2 V, leading to a tuning range of up to 150 nm. And the spectral tuning was readily confirmed via color mapping. The proposed devices may permit the tuning span to be readily extended by tailoring the grating pitches.
© 2013 Optical Society of America
A color filter is perceived to be an indispensable element for CMOS image sensors, display devices, three-dimensional projection systems, biosensors, photovoltaic cells, and so forth [1–6]. Previously, most color filters were made up of spin-cast dyes, which are susceptible to disadvantages, such as low transmission efficiency, heating caused by light absorption, imperfect color purity, and performance degradation due to continued exposure to light. In an attempt to overcome these issues, various appealing approaches that rely on periodic nano-photonic devices have been extensively researched. Notable examples include a plasmonic nano-structure, a periodic nano-hole array in a metallic film, a metal-dielectric-metal resonator, an array of metallic optical nano-antennas, and a guided mode resonance (GMR) structure that utilizes dielectric or metallic gratings [7–14]. GMR based color filters are particularly attractive owing to their high efficiency with a proper bandwidth, flexible optical transfer characteristics, good color purity, compact size, and simplified process [14–17]. However, most of the studies on optical filters have been limited to static cases, where the structure of the ðlters and their optical responses are ðxed. A color filter featuring a dynamic or adaptively controlled spectral response is highly desirable from the view point of miniaturization of the modules involving it. To date, most GMR based filters have been reported to achieve tunable characteristics by virtue of the change in the angle of incidence or grating period, the mechanical displacement of filter elements, and the thermo- and electro-optic effect [17–22]. These schemes inevitably suffered from a limited tuning range and a high drive voltage, yet they required precise alignment of filter elements. Recently, it has been suggested that light polarization may be utilized to produce a color tuning effect with plasmonic nano-antennas, though this poses some issues such as lack of well-defined center wavelength and bandwidth .
In this work, an electrically tunable color filter has been embodied, capitalizing on a polarization dependent dichroic resonator, combined with a twisted nematic liquid crystal (TN-LC) based polarization rotator. The GMR between a planar dielectric waveguide and an asymmetric two-dimensional (2D) subwavelength grating in metal, having two different pitches in the orthogonal directions, underlies the operation principle of the dichroic resonator, which is presumed to produce double resonant peaks depending on the polarization. Hence, two primary colors can be generated for two polarizations by appropriately adjusting the grating pitches. For an arbitrary polarization, the spectral response and output color of the proposed dichroic resonators are determined by the relative contribution of polarization components of incident light. A color filter exhibiting an electrically controlled spectral response has been successfully created by exploiting the proposed dichroic resonator, which is linked with a TN-LC based polarization rotator and a sheet polarizer. The color emerging from the tunable filter could be flexibly mapped upon the color space by modifying the output spectra. To the best of our knowledge, this is the first report on an electrically tunable color filter exploiting a polarization dependent nano-photonic dichroic resonator, rendering a broad operation range throughout the entire visible band. The proposed devices may potentially be applied to active color pixels, polarization detectors, and security tags.
2. Proposed dichroic resonators and their incorporation to tunable color filters
The proposed visible dichroic resonator is composed of a three-layered slab waveguide loaded with a 2D asymmetric rectangular subwavelength grating in Al, as illustrated in Fig. 1.The waveguide engages a lower cladding in quartz, a core in Si3N4, and an upper cladding in SiO2. The 2D asymmetric subwavelength grating is built by arranging minute 40-nm thick Al sheets in a rectangular array, with periods of Λx and Λy along the x and y directions, respectively. The fill factor for the grating, defined as the ratio of the width of the metallic pattern to the grating pitch, is set at 0.7 in light of the transmission in the visible spectral band. The polarization direction of incident light is indicated by an angle θ of the electric field with respect to the x direction, as depicted in Fig. 1. In particular, the Lx and Ly polarizations refer to the cases of θ = 0 and 90°, respectively.
The GMR between the planar waveguide and the diffracted waves induced by the grating is the underlying operation principle of the proposed resonator. Band-pass filtering is known to be attained at wavelengths where the phase matching between the transverse magnetic (TM) and transverse electric (TE) guided mode of the waveguide and the diffracted waves is satisfied [12,13]. The orientations of the electric E-field and magnetic H-field related to the TM and TE guided modes excited in the waveguide are manifested in Fig. 1. For a metal-dielectric resonant structure such as that proposed, the resonant coupling efficiency is thought to be considerably more substantial for the TM guided mode than for the TE guided mode . Hence the primary peak is equivalent to the resonant wavelength λTM for the TM mode, whereas the secondary peak corresponds to the resonant wavelength λTE for the TE mode. The proposed resonator is meant to assume two separate resonant wavelengths, dictated by the grating pitches of Λx and Λy, respectively. The incoming light propagating along the z direction undergoes diffraction in both the x and y directions simultaneously owing to the asymmetric 2D grating. The main transmission peak for the Lx and Ly polarizations stems from the TM guided mode propagating in the x and y directions, respectively [24,25]. Therefore, Λx and Λy will be individually tailored to secure the primary output colors for the two orthogonal Lx and Ly polarizations, respectively.
In order to verify the possibility of tailoring two dominant resonant peaks for two orthogonal polarizations via the grating pitch, we calculated the relation between the resonant wavelength and grating pitch as shown in the upper graph in Fig. 2(a), while the thicknesses of the Si3N4 core (n = 2.023) and SiO2 cladding layers (n = 1.457) were set to be 100 nm each. The resonant wavelengths were numerically estimated under the phase matching condition between the diffracted light by the grating and the guided modes (TE and TM) of the slab waveguide, derived from the dispersion relation of the waveguide. In addition, the transfer characteristics for the Lx polarization, when Λx was varied from 275 to 400 nm and Λy was fixed at 400 nm, are shown in the lower graph of Fig. 2(a). The indicator lines shown in the upper graph show the respective locations of the resonant peaks due to the TE and TM guided modes for the corresponding grating pitches which are considered in the lower graph. Similarly, the results for the Ly polarization are presented in Fig. 2(b), for Λy set to 275 nm and Λx ranging from 275 to 400 nm. The transfer characteristics were analyzed using a finite difference time difference (FDTD) method based simulation tool (FDTD Solutions from Lumerical, Canada). The properties of the Al used for the simulation were derived from the multi-coefficient model provided by the simulation tool. For the Lx polarization, the main peak pertaining to the TM mode was observed to change with the grating pitch Λx, while a low efficiency secondary peak related to the TE mode remained fixed due to the constant period of Λy = 400 nm. In the case of the Ly polarization, the secondary peak changes depending on the period Λx, while the main peak remains constant at λ = 450 nm, which is determined by the period Λy = 275 nm. The major peaks for the Lx and Ly polarizations are proven to be influenced by the grating periods in the direction of the electric field of incident light. It has been thus proven the two dominant peaks or center wavelengths might be controlled through the corresponding grating period.
For a typical dichroic resonator with Λx = 300 nm and Λy = 400 nm, the transmission characteristics are presented in Fig. 3.Primary resonant peaks with efficiency exceeding 85% were obtained for the TM guided mode at λTM = 487 and 631 nm, which pertain to Λx and Λy for the Lx and Ly polarizations, respectively. Meanwhile, secondary transmission peaks associated with the TE guided mode, which are relevant to Λy and Λx, were also observed. As predicted, the transmission is much lower for the TE mode than for the TM mode. For the proposed resonator, the transmission for a polarization angle θ is given by Tθ = T0(λ)cos2θ + T90(λ)sin2θ, where T0 and T90 are the transmissions for the Lx and Ly polarizations, respectively. For example, the transmission for input polarization with θ = 45° is expressed as T45 = (T0 + T90)/2. As a result, the proposed visible dichroic resonator is anticipated to lead to a spectral shape resulting from the polarization-tailored combination of the primary spectral bands.
The proposed tunable color filter consists of a visible dichroic resonator, a polarization rotator relying on a TN-LC cell, and a sheet polarizer, as presented in Fig. 4.Its operation is briefly elucidated as follows: Launched light is linearly polarized along the y direction, rotated by the polarization rotator, and impinges upon the resonator. With no voltage applied to the TN-LC cell, the polarization is rotated 90° in order to produce the Lx polarization before impinging on the resonator. With a sufficiently large voltage over a certain voltage Vo applied, the polarization is preserved as Ly polarization undergoing no rotation. For the applied voltages less than Vo, the Lx and Ly polarizations coexist. In regard to an electrically tunable color filter utilizing the dichroic resonator mentioned in Fig. 3, the optical response is dominated by the transmission in the blue band, with no voltage applied. With the applied voltage, Va, elevated to Vo, the transmission shifts from blue to red because the polarization of incident light changes from Lx to Ly and the transmission is governed by a peak for red. This implies that the color coordinate of the transmitted light can be efficiently adjusted by altering the polarization angle. The CIE 1964 color coordinates corresponding to the spectral responses given in Fig. 3 were calculated as depicted in Fig. 4. Two primary colors are mapped on the blue and red coordinates in accordance with the resonant peaks corresponding to the pitches of Λx and Λy, respectively. For an arbitrary polarization with θ, the output of the filter may be mapped on a line, connecting the two initial coordinates. Consequently, the proposed color filter is capable of accomplishing a continuously varying color, by combining the two particular responses for the Lx and Ly polarizations.
3. Characterization of implemented dichroic resonators and electrically tunable color filters using them
As aforementioned, the resonant wavelength of the proposed visible dichroic resonator can be controlled by changing the two pitches of the metallic subwavelength grating. Thus, the grating pitches in connection with the dichroic resonator were appropriately chosen to be ΛB = 285 nm, ΛG = 355 nm, and ΛR = 395 nm, which are responsible for the blue, green, and red, respectively. The fabrication procedure for the proposed devices is succinctly described here: Si3N4 and SiO2 thin films, each 100 nm thick, were successively formed via physical vapor deposition on a quartz substrate. A 40-nm thick Al thin film was subsequently deposited via e-beam evaporation. The metallic film was then patterned using e-beam lithography accompanied by dry etching, in order to complete a filter device with the effective dimensions of 40x40 μm2. The device size was determined by considering that at least 100 grating periods were estimated to be required to achieve an efficiency of ~80%. Figure 5(a) shows scanning electron microscope (SEM) images of the prepared three dichroic resonators, DCR-BG, BR, and GR, with grating pitches of ΛB and ΛG, ΛB and ΛR, and ΛG and ΛR, respectively, along the x and y directions.
To evaluate the transfer characteristics of the fabricated devices, light from a halogen lamp (AvaLight-HA, Avantes) was properly polarized and collected by an objective lens to impinge on the filter device. The transmitted light was captured by a multimode fiber, serving effectively as a small pin-hole, which is connected to a spectrometer (Avaspec-3648, Avantes). For the test setup, the numerical aperture was observed to be below 0.017, ensuring that normally incident light might dominantly account for the optical response. As shown in Figs. 5(b)–5(d), the optical response was examined in terms of two orthogonal polarizations for the devices. The simulated results are shown by dotted lines, while the measured results are shown by solid lines. In the case of the Lx polarization, the main transmission for the DCR-BG, BR, and GR was centered at λTM, as determined from the grating pitches in the x direction, ΛB, ΛB, and ΛG, respectively. The efficiency was found to exceed 70%. Meanwhile, the secondary resonant peaks, with a low throughput, were also observed at λTE in accordance with the pitches in the y direction. For the Ly polarization, primary resonant peaks at λTM shifted to green, red, and red bands according to the y-direction pitches. Therefore the resonant wavelengths in compliance with the phase matching for ΛB, ΛG, and ΛR were measured to be λTM = 460, 560, and 610 nm and λTE = 488, 588, and 639 nm for the primary and secondary peaks, respectively. Furthermore, the two resonant wavelengths were verified to be formed independently for two orthogonal polarizations, signifying that the demonstrated filter could provide polarization tuned resonant wavelengths. It is noted that the GMR based dichroic resonator is apparently susceptible to the angle of incident light. For the case of an angle of incidence of 5 degrees, the efficiency and center wavelength for the main resonant peak were estimated to decline by half and shift to shorter wavelength by ~20 nm, respectively.
As shown in Fig. 6(a), each of the dichroic resonators has been incorporated into an electrically tunable color filter, by sandwiching a TN-LC cell between the resonator and a sheet polarizer aligned in the y direction. A typical assembled filter displaying its top and bottom sides is shown in Fig. 6(b).
In order to validate the performance of the LC based polarization rotator, we first scrutinized the Lx and Ly polarization components of light radiating out through the rotator over the visible spectral band ranging from 400 to 700 nm, with the voltage applied to the LC cell varying from 0 to 6 V, as plotted in Fig. 7(a).In the case of initially y-polarized light through the sheet polarizer, the light undergoes a maximum rotation of 90° through the TN-LC cell and becomes transmitted as Lx polarization when the drive voltage is less than 2 V. For the applied voltage exceeding 2 V, however, the induced polarization rotation is inversely proportional to the voltage, which induces a decrease of the transmission for the Lx polarization but an increase of the transmission for the Ly polarization, respectively. Figure 7(b) shows the output for both Lx and Ly polarizations measured at λ = 600 nm, indicating that the polarization rotator could function appropriately with drive voltages ranging from 2 to 4 V. For the measurement, the loss caused by the sheet polarizer was ignored, assuming the source light was already polarized along the direction of the sheet polarizer. An optical loss amounting to about 10% was accounted for by the Fresnel reflection at the boundary between the cell and air.
As presented in Fig. 8(a), the manufactured tunable color filters, including Dev-BG, BR, and GR, taking advantage of the dichroic resonators of DCR-BG, BR and GR, respectively, were measured to produce the main transmission peaks at λ = 460, 460, and 560 nm in accordance with the corresponding grating pitches of ΛB, ΛB, and ΛG, respectively, along the x direction, with 2 V applied to the polarization rotator. When the applied voltage Va was elevated to 4 V, the primary transmission peaks were obtained for the Ly polarization according to the corresponding pitches of ΛG, ΛR, and ΛR along the y direction. The total peak transmission for the complete tunable devices was observed to be around 60%, translating into an insertion loss of 40%, in case the source light was initially aligned along the sheet polarizer as aforementioned. For Va = 2.75 V, the optical responses lie between the cases for Va = 2 and 4 V. For the device Dev-BR, accommodating the blue and red spectral bands, the separation between the two peaks extended up to 150 nm. The tuning range of the proposed color filter was readily extended by carefully taking into account the fact that the dichroic resonator offers two individual resonant peaks, which are governed by the corresponding grating pitches. The novelty of our filter is apparently attributed to its property that permits the tunable range to be enlarged by just tailoring the grating pitches.
The output colors for transmitted light could be altered by modifying the spectral responses of the dichroic resonator, which has been carried out with the assistance of the drive voltage applied to the TN-LC cell. As plotted in Fig. 8(b), the CIE 1964 color coordinates were estimated from the transmission spectra of the prepared tunable color filter as a function of the applied voltage. For example, for the case of the Dev-BR the color coordinates shifted from blue to red when the voltage increased from 2 to 4 V. Similarly, the color coordinates for the Dev-BG and GR devices were evolving from blue to green and green to red, respectively. In contrast to the anticipation that the color coordinates for the output color would ideally follow a straight line connecting the two coordinates of the primary colors corresponding to the Lx and Ly polarizations, the locus of the output color transition of the realized devices actually occurred along a slightly bent curve, resulting in a slight deviation from the expected straight line. This is because the prepared LC based polarization controller failed to deliver a satisfactorily uniform transfer characteristic throughout the visible spectral band. Particularly around λ = 400 nm, the polarization control was not as effective as desired. This issue may be mitigated by improving the performance of the LC cell in the visible band.
Highly efficient electrically tunable color filters were embodied tapping into polarization-tailored dichroic resonators in conjunction with a LC based polarization rotator. Three types of dichroic resonators, each featuring double resonant peaks at two primary colors (blue/green, blue/red, and green/red) for two orthogonal polarizations, were fabricated exhibiting high efficiencies of ~75%. The tunable color filter, which utilizes a blue/red resonator, was useful for sweeping the output color coordinates between the blue and red colors, by adjusting the incoming light polarization through the voltages applied to the polarization rotator. The other two devices, based on blue/green and green/red resonators, were demonstrated to function successfully alike. For the prepared dichroic resonator, the overall spectral shape was determined by the weighted sum of the two pass bands associated with their fixed resonant peaks. Hence, the proposed color filter was not continuously tunable across the spectral band, but the color coordinates for the device could be continuously scanned following an almost linear trajectory on the CIE diagram. In the near future, such electrically tunable filtering devices as discussed here may serve as a central element for various applications, including white organic light emitting diodes, sensors, etc.
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2013-008672, 2013-067321) and a research grant from Kwangwoon University in 2013. The authors are grateful to Prof. T. H. Yoon at Pusan National University for his valuable help.
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