1. Introduction

Microelectromechanical systems (MEMS) methods [1] in conjunction with photonics provide interesting solutions in display technology. Well-known examples of MEMS-based systems for display include the Texas Instruments digital micromirror device (DMD) and the Silicon Light Machines grating light valve (GLV) technologies. The DMD pixel is a flat mirror that is tilted on an axis so that the illuminating light is directed towards the screen or away from it under electronic MEMS control [1, 2]. Rapid tilting of the mirrors allows display of a large number of gray levels as well as a multitude of colors if the input white light is directed through a color wheel. This produces high contrast color images on display screens via suitable projector systems. The GLV system employs a MEMS reflection grating whose diffraction efficiency is controlled by applied voltage [3, 4]. A suspended ribbon structure in which alternate ribbons are deflected to control the level of diffracted intensity constitutes the GLV element. The GLV applies the first diffraction orders of the phase grating, possibly collecting the light in both the +1 and -1 orders for enhanced efficiency with additional optical components. Three different grating periods of the GLV are used to produce red, green, and blue colors for superposition on a display screen; thus 3 distinct pixels provide these 3 primary colors. The diffraction efficiency of each pixel is controlled rapidly by the MEMS feature thus achieving effective color mixing and balancing on the screen.

The interferometric modulator (IMOD) technology is a recent addition to the class of MEMS-based displays [5]. An IMOD pixel is basically a bistable optical element comprising a thin-film stack over a glass substrate and a movable reflective membrane. MEMS actuation varies the cavity dimension between the stack and the membrane to generate constructive interference of a desired color (red, blue, or green). Collapsing the cavity produces a dark state due to absorption of light. These reflective displays may be attractive in portable devices where low power consumption is required [5].

Tuning and switching of the resonance wavelength in guided-mode resonant (GMR) gratings can be realized by perturbing the physical parameters including refractive index of grating layers or surrounding media, layer thickness, period, or fill factor as described in [6–8]. Shu et al. reported analysis of a tunable structure consisting of two adjacent photonic crystal films, each composing a two-dimensional waveguide grating that could be displaced laterally or longitudinally by a mechanical force [9]. Each periodic waveguide admitted guided-mode resonances whose coupling could be mechanically altered for spectral tuning. Carr et al. theoretically studied laterally deformable nanomechanical gratings under resonance conditions and, using a fixed input wavelength, found intensity modulation and polarization effects on application of grating lateral shifts [10]. Later, they fabricated a prototype device in amorphous diamond with 600 nm period and nanoscale features and suggested uses for inertial sensing and modulation [11]. More recently, Kanamori et al. fabricated two-dimensional, polarization independent GMR filters that could be moved, via MEMS actuators, within an air gap to add or drop selected wavelengths in a bit stream. The silicon-on-insulator device had ~65% efficiency at 1545 nm [12].

In this paper, new display pixels based on tunable resonant leaky-mode nanoelectromechanical (NEMS) elements are presented. These elements are spectrally selective subwavelength devices with no higher propagating diffraction orders thus operating in the zero order regime. N/MEMS actuation methods may potentially be applied to tune their spectral response through mechanical micro/nano alteration of the structural parameters. Previously, we presented the characteristics of MEMS-tunable, guided-mode resonance (GMR) structures and explained their operational principles [8]. It was shown that such systems are highly tunable with only nanoscale displacements needed for wide-range tuning. Working with a silicon-on-insulator (SOI) device and fixed parameters, we quantified the level of tunability per unit movement for an example resonant structure. It was found that effective MEMS-based tuning can be accomplished by variation of grating profile symmetry, by changing the waveguide thickness, or both. Numerical examples of the particular MEMS-tunable, leaky-mode structure treated showed that the resonance wavelength could be shifted spectrally across ~300 nm with a horizontal movement of ~120 nm and the reflectance tuned in excess of 30 dB with a vertical movement of ~200 nm in the 1.3–1.7 µm wavelength band. The analysis showed that these structurally dynamic elements can function effectively as tunable filters, variable reflectors, and modulators in the optical telecommunication band [8].

It is clear that analogous tunable devices can be designed in numerous other material systems for operation in arbitrary spectral regions. Of course, as the operational wavelength diminishes the associated finer-feature patterning demands stricter tolerances in fabrication. In the present contribution, we investigate theoretically the operation of N/MEMS-tunable GMR membranes in the visible spectral region using a low-loss example medium. A chief motivation for the study is potential implementation of these compact tunable pixels in future display systems. In this application, a tunable pixelized display can be realized by employing a broadband illumination source and suitably N/MEMS-activated, reflection/transmission GMR filters. Advances in nanometric fabrication processes may allow utilization of such methods to spectrally tune the resonance position in the visible region.

In summary, widely tunable GMR pixels for display systems are introduced. Example results using a silicon nitride (Si3N4) periodic membrane are provided. We show that by altering the refractive index profile of the membrane, conceptually with a N/MEMS actuation approach, a high degree of wavelength tunability within the visible spectrum is achieved. We examine the effect of structural symmetry, device thickness, and input light polarization on the spectral response. These pixels provide tunable spectra with constant reflection efficiency.

2. Influence of structural symmetry in GMR devices

When an incident light beam becomes phase matched to a waveguide’s leaky mode through a grating, a guided-mode resonance (GMR) takes place. Under normal illumination, two counter-propagating leaky modes are created. These modes are excited by evanescent diffraction orders and the structure works in the second stop band [13]. The radiated fields generated by these leaky modes in a symmetric structure can be in phase or out of phase at the edges of the band [14]. At one edge, there is a zero phase difference and hence the radiation is enhanced while at the other edge, there is a π phase difference inhibiting the radiation. In this case, if β=β_R+jβ_I is the complex propagation constant of the leaky mode, we have β_I=0 at one edge, implying that no leakage is possible at that edge. Resonating the incident light in a layer with an asymmetric grating profile yields a nonzero β_I at each edge [15–17].

The proposed tunable GMR pixel, in its general state, is an asymmetric grating structure exhibiting a resonance at each edge of the band. However, as the element approaches symmetry points under dynamic mechanical tuning, one band-edge resonance disappears. Figure 1(a) shows a schematic view of a subwavelength GMR device under normal incidence indicating leaky-mode excitation and their reflective reradiation. Figure 1(b) depicts schematically the dispersion diagram of the device at the second stop band as well as the associated resonances. GMR structures with two different media per period may be categorized as symmetric whereas elements with multi-part periods can be generally taken as having asymmetric profiles.

3. Device structure

Figure 2 shows the structure of the tunable GMR pixels under study. It is a membrane consisting of air (n_L=1.0) and silicon nitride (Si₃N₄, n_H=2.1) parts. One of the high-index (n_H) parts of the structure is fixed and the other part is mobile using MEMS actuation as indicated. For each pixel, the period (Λ), thickness (d), and fill factors (width of each grating subdivision divided by the period) F₁ and F₃ are assumed to be fixed. F₂ and F₄ are taken to be the variable fill factors, although the operation of the device can be characterized by F₂ alone. Here, F₂ is considered to be the variable fill factor and thus the tuning parameter. By changing F2, the refractive index distribution and symmetry of the structure change. While the average refractive index (in an effective-medium sense) remains unchanged, other refractive index Fourier components change accordingly. The second Fourier component of the refractive index profile has the main influence on the width of the second (leaky) stop band [8].

 

Fig. 1. (a) A general schematic view of a subwavelength GMR device under normal incidence. When phase matching occurs between evanescent diffraction orders and a waveguide mode, a reflection resonance takes place. I, R, and T denote the incident wave, reflectance, and transmittance, respectively. (b) Schematic dispersion diagram of a GMR device at the second stop band (blue). For a grating with an asymmetric profile, a resonance occurs at each edge of the stop band as shown. For a symmetric element, a resonance appears only at one edge. Here, K=2π/Λ, k₀=2π/λ, and β is the propagation constant.

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Fig. 2. Proposed idealized silicon nitride membrane structure for the tunable GMR pixels addressed in the paper with F₁=0.15 and F₃=0.1. For the blue-green pixel, Λ=0.385 µm and d=0.2 µm. For the red pixel, Λ=0.5 µm and d=0.25 µm.

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Two sets of parameters are selected for operation of the device in the blue-green and red portions of the visible spectrum. In both sets, F₁=0.15, F₃=0.1, with operation at normal incidence illumination assumed. For the blue-green pixel we set Λ=0.385 µm and d=0.2 µm. For the red pixel, Λ=0.5 µm and d=0.25 µm. The computed results provided are based on idealized conditions (infinite number of periods and plane wave illumination) as in the rigorous coupled-wave analysis [18] code applied. In practice, of course, the incident light beam may be ~Gaussian with finite diameter and the pixels will have finite extent.

4. Results

Figure 3 illustrates the reflection spectra of the two pixels for TE-polarized illumination. For the wavelength ranges shown, the two gratings work in the subwavelength (Λ<λ) regime and hence only zero-order reflection appears without higher diffraction orders being present. The reflectance map R(λ,F₂) has similar appearance for both color regions as the pixel parameters are not very different. In each case, the longer-wavelength region shows a clear, well-shaped reflectance spectrum that responds favorably to tuning by variation of F₂. This branch corresponds to the lower stop band edge [8, 13] with the resonant leaky mode being the fundamental TE₀ mode. The blue-green pixel of Fig. 3(a) has a tuning capability of ~100 nm over the 0.45–0.55 µm range whereas the red pixel in Fig. 3(b) shows a tuning range of ~100 nm across 0.60–0.70 µm, corresponding to a rate of ~0.10 µm wavelength tuning per ~0.125 µm lateral shift. As shown in Fig. 3, the R(λ,F₂) maps are symmetric around the F₂=0.375 profile symmetry point.

 

Fig. 3. Reflection spectra of the tunable pixels versus the air-gap fill factor F₂ for TE polarization (electric field vector perpendicular to the incidence plane). (a) 2D map of reflectance R(λ,F₂) for the blue-green pixel with Λ=0.385 µm and d=0.2 µm. (b) R(λ,F₂) for the red pixel where Λ=0.5 µm and d=0.25 µm.

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At shorter wavelengths, as shown in Fig. 3, the patterns exhibit secondary branches due to appearance of higher resonant modes. At F₂~0 there is a wide-band reflectance region arising from a mixture of leaky modes. Centered on the profile symmetry point F₂=0.375 there appears a second band of high reflectance due to a TE1 type resonant leaky mode. This elliptical region, just as the fundamental band, has a lower-frequency continuous edge and a gap at the upper edge around the symmetry point. Calculations show that the leaky modes (~TE₀ and TE₁) constituting these bands are excited mainly by the first (±1) diffraction orders. The leftmost branch, which vanishes at the symmetry point, supports mixed modes.

Figure 4 displays samples of the reflectance spectra for the blue-green and red pixels for selected values of the fill factor from the two-dimensional maps of Fig. 3. It is apparent that the resonances are well shaped with useful linewidths of several nanometers maintaining their shape well on tuning. The minimum useful pixel size is determined by the parameters used in each case. The key idea is to resonate the input light effectively within the lateral size of the pixel. As the leaky mode has a finite decay length, the lateral size has to exceed this characteristic length. For example, in case of the blue-green pixel, we can estimate the decay length (L_d) by the formula L_d~Λλ/4πΔλ [19] where Δλ is the spectral linewidth in Fig. 4(a). This yields L_d ~3 µm with a transverse pixel size of 10 µm being feasible in this case.

For illumination with practical, imperfectly collimated sources, there needs to be an accommodating angular aperture. Figure 5 shows angular spectra for two example values of F₂ for the red pixel. It is seen that this pixel will work properly with ~±4° tolerance in angle of incidence which is reasonable for display applications.

Although the example pixels presented are designed primarily for TE polarization, their behavior for TM polarization may be of interest. Figure 6 quantifies the TM polarization response for the red pixel, which differs significantly from the TE response. The tuning curve (band diagram) has two well-defined branches both related to the TM₀ mode. As in the TE-polarization case, there is a discontinuity around the profile symmetry point F₂=0.375 occurring at λ=0.615 µm. Importantly, TM polarization provides a different tuning range than TE polarization. Therefore, using incident polarized light and switching between the TE/TM states, one pixel may cover an extended wavelength band. If the light reflected off this pixel, and other similarly designed pixels, is used without cut-off filters, interesting color-mixing schemes may be implemented.

 

Fig. 4. Samples of the reflection spectra for (a) blue-green and (b) red pixels for different values of F₂ from the 2D maps in Fig. 3.

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Fig. 5. Angular spectra of the resonant peaks for two different values of F₂ and wavelength for the red pixel.

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Fig. 6. Reflection spectra map R(λ,F₂) of the red pixel for TM polarization.

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Figure 7 displays the mode field patterns and profiles inside the red pixel structure at the middle of the tuning diagram (symmetry point F₂=0.375) for TE and TM polarizations. In Fig. 7(a), the mode on the main branch is a TE0 type excited mainly by the first diffraction order thus denoted TE_0,1. Similarly, the mode in Fig. 7(b) can be considered to be a TE_1,1 mode; this mode resides on the sub-branch in Fig. 3(b). In the TM case at the symmetry point, as shown in Fig. 7(c), the mode is TM_0,1 with λ=0.5051 µm (upper band edge) and with no resonance appearing on the right branch. As seen in Figs. 6 and 7(c), this pixel exhibits high-Q behavior (Q=λ/Δλ) with the attendant narrow linewidth and high resonance field value in the leaky mode whose evanescent tail extends several wavelengths out of the structure in this case.

 

Fig. 7. Mode patterns (left) and cross-sectional mode profiles (right) for the red pixel. (a) λ=0.7125 µm (main branch TE), (b) λ=0.5365 µm (sub-branch TE) and (c) λ=0.5051 µm (TM). F₂ is equal to 0.375. Electric field profiles due to the zero, first, and second diffraction orders are shown as blue, red, and purple, respectively.

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Grating thickness (d) is a key parameter that determines the number and level of confinement of admitted modes in a GMR structure. To show the influence of grating thickness on the modal behavior of the red pixel, we reduce the thickness to 0.15 µm with resulting reflectance map shown in Fig. 8(a). This decrease in thickness reduces the number of modes supported with a resulting simpler reflectance behavior. Also, in this case, the tuning range falls to ~50 nm. In contrast, by increasing the thickness to 0.35 µm as illustrated in Fig. 8(b), the leaky waveguide can support additional modes as well as confine others more tightly, such that the regions with mixed modes appear narrower with improved shapes as these computed results demonstrate. In all of the R(λ,F₂) pixel diagrams, the symmetry point is present independent of the grating thickness.

 

Fig. 8. Reflectance spectra map R(λ,F₂) of the red pixel for (a) d=0.15 µm and (b) d=0.35 µm. Note that λ=0.5 µm defines the border between the zero order (subwavelength) and multiorder diffraction regimes for this structure.

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5. Conclusions

In this paper, tunable pixels based on the GMR effect are described. The pixels are designed as periodic membranes with subwavelength, nanoscale features. To tune the spectral response of these pixels, MEMS-type actuation can be applied to impart relative motion between the two grating constituents. This modifies the refractive index distribution and symmetry of the structure resulting in rapid change of the resonance wavelength. The computed tuning maps show that this structure is capable of tuning over ~100 nm in each of the blue-green and red parts of the visible spectrum with minimal motion. The reflectance spectra show well-shaped resonances with appropriate linewidths. Computed results show that the pixel thickness has a direct effect on the modal behavior and the tuning range. The state of polarization of the input light strongly affects the tuning range as well as other spectral characteristics of the pixel. These pixels exhibit angular acceptance tolerance of ~±4°, which is useful for display applications. The mechanical tuning action directly varies the resonance wavelength while maintaining constant reflection efficiency. It is of interest to extend these basic ideas to other materials systems in the visible region and to explore tunable devices in other spectral regions. A goal of the project is to design a single pixel to cover the entire visible spectral region. Additional challenges remain in prototype fabrication and N/MEMS system design. Widely tunable RGB pixels of this kind are potential candidates for N/MEMS-based display systems.