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A transparent, fully integrated electrically modulated projection technique is presented based on light guiding through a thin liquid crystal layer covering sub-wavelength gratings. The reported device operates at 10 V with response times of 4.5 ms. Analysis of the liquid crystal alignment shows that director-reorientation occurs over timescales on the order of 90 µs close to the grating surface. The technology is suitable for next generation heads-up-displays and reconfigurable multilayer photonic integrated circuits.
©2013 Optical Society of America
1. Introduction
Today’s information-based society has a high demand for display technologies capable of conveying dynamic and interactive data with added functionality. In order to enable a user’s seamless acquisition of desired information, it is preferable to integrate electronics directly and unobtrusively into transparent surfaces. Displays are a key component of the human–machine interface and transparent displays, combined with transparent electronics [1] and periphery such as batteries [2], loudspeakers [3] and tactile sensors [4] facilitate fully integrated transparent systems for applications that include heads-up-displays (HUDs), automotive driver displays and hand-held electronics. Transparent displays based on organic light emitting diodes [5] and polymer dispersed liquid crystals [6] have been studied in this regard, however, a principal challenge in the miniaturization and integration of displays lies in the fact that the required display area becomes the limiting factor for further reduction of device dimensions. One way to solve this limitation is via the use of projection displays that occupy a small area on the device and project information onto a larger screen. The aforementioned types of transparent displays do not permit projection without additional optics that would increase device dimensions and inhibit transparency. Another type, based on an external display and redirection of the image through a series of gratings in a glass plate, has also been presented [7,8], but this concept does not provide the advantages of coherent laser projection, e.g. the ability to project onto curved surfaces without focusing.
In this article, a transparent projection display based on sub-wavelength gratings and liquid crystals, without the requirement for polarizers or lenses, is presented. The sub-wavelength gratings are used for light redirection, i.e. the projection of light guided along the display plane outwards from the device onto a screen. Modulation of the outcoupling process is achieved by means of an applied electric field causing molecular reorientation within a thin liquid crystal (LC) layer that is in contact with the optical grating. Basic specifications of the device are similar to commercially available products, operating at 10 V with rise and fall-off response times of 0.1 ms and 4.5 ms, respectively. Furthermore, the underlying physics of the liquid crystal director reorientation in close proximity to the grating surface is analyzed, showing that fall-off due to re-alignment occurs on 90 µs timescales under certain conditions, pointing to the potential for significantly faster device response times. Accordingly, the results denote a key step towards the next generation of projection display technology, where the general concept of tunable and selective out-coupling of light from transparent media could furthermore be applied to multilayer photonic integrated circuits, addressable chip-to-chip optical interconnects in electronic processors and cell manipulation in lab-on-a-chip systems.
2. Principle of operation
The principle of operation of the device is shown in Fig. 1(a). Light is incident from an external laser light source at the side and propagates inside and along the transparent chip. At specific locations on the chip, micron-scale rectangular areas, filled with sub-wavelength gratings, are defined. These gratings act as picture elements (pixels), selectively out-coupling light at an angle perpendicular to the glass surface, and an entire projected image may then consist of many individual pixels. Here the concept is presented by considering three gratings (pixels) of 250 x 250 µm size, arranged in a row. Each grating is covered by a transparent electrode which is connected to a larger pad at the side to enable local addressing of an applied voltage.
Fig. 1 (a) Principle of operation for the projection display. Light enters from the side and is controllably out-coupled perpendicular to the device plane at specific locations. (b) Detailed side view drawing of the light propagating inside the device and out-coupled by a grating. The liquid crystal layer enables electrical modulation of individual pixels.
Figure 1(b) illustrates the operating principle and the optical paths in greater detail. The device consists of a substrate (Borofloat glass, n = 1.48, 500 µm thick) and an SU-8 thin film (n = 1.58, 610 nm thick) with imprinted gratings (corrugation depth 120 nm). These are covered by a 45 nm indium tin oxide (ITO) transparent electrode on top of each grating. A liquid crystal layer of 600 nm thickness covers the gratings and finally the device is sealed by a Borofloat glass lid coated on its underside with 45 nm of ITO. Since both the liquid crystal layer and the SU-8 film each have higher refractive indices than the glass substrate, they form a primary optical waveguide for the guided light. However, the spot size of light focused by a 10x microscope objective at the sample edge is much larger than the thickness of the thin-films. Accordingly, the entire chip, including substrate and lid, is “flooded” with light that is guided by total internal reflection at the glass/air interfaces. This secondary waveguide continuously “refills” the lossy (due to scattering and out-coupling) primary waveguide. The exact optical response of the waveguide is not trivial due to the numerous material layers and multimode propagation of guided light. Particularly, distortion of the liquid crystal director causes an anisotropic and inhomogeneous refractive index of the LC layer. However, an approximation of the direction of incident and guided light, without consideration of diffraction efficiency or polarization, is given by Eq. (1), which is derived from consideration of the phase differences between grooves and ridges and the requirement of k-vector conservation [9]:
Here nLC and nSU8 are the refractive indices of the layers, m the diffraction order, λ the free-space wavelength of light and Λ the grating period. θin and θout are the angles of the in-coming and out-coupled light, respectively (see also Fig. 1(b)), measured from the surface normal. By using the fixed values m = −1, nLC = 1.5, θin = 90° (propagation parallel to the device surface) and θout = 0° (perpendicular to device surface), the required grating period is determined. The device is designed for operation at 410 nm, 532 nm and 661 nm, accordingly the required periods are calculated as 274 nm, 355 nm and 441 nm, respectively. The corrugation depth does not influence the diffraction angle but only the efficiency. A depth of 120 nm is used, which has not been optimized, but is comparable to previous work in a similar geometry [10] where a diffraction efficiency of about 0.1% is achieved. This low value is sufficient, since only moderate diffraction efficiency is required for each pixel.By application of a voltage between the lid and the pixels, each pixel may be individually tuned in terms of the intensity of the out-coupled light. The gratings provide dual functionality by facilitating 1) the redirection of light and 2) the uniform alignment of liquid crystal molecules parallel to the grating lines at zero applied voltage. Alignment of liquid crystals with surface topography is dependent on the combination of grating dimensions, choice of material and fabrication method [11], with the precise contribution of each still under investigation. Previous works comparing lines imprinted in SU-8 and polyimide [12,13], with dimensions on the same order of magnitude as presented here, do however point towards a predominance of topographical effects as presented by Berreman [14], i.e. the long-chained nematic LC molecules minimize their mechanical deformation energy by aligning parallel to surface indentations. Since the nematic LCs also prefer parallel alignment with respect to each other, the surface grating dictates the orientation of the molecules throughout the cell. This ordering may diminish away from the grating (towards the lid of the cell) where no anchoring method is applied, but this is not an issue because the optical effect is achieved predominantly in proximity to the grating corrugation. By application of a voltage the LC molecules tilt from being parallel to the grating lines towards perpendicular orientation. Since the nematic LC molecules behave optically like a positive uniaxial crystal, this orientation change affects the refractive index perceived by the polarized light. Thus the index contrast at the grating changes, resulting in changes of the diffraction efficiency [15]. If index matching between the LC and the grating is achieved, then no light is out-coupled aside from omnidirectional scattering. The intensity contrast of the projected pattern is calculated as the Michelson contrast , where Ion and Ioff are the intensities of the out-coupled light by a pixel in the on- and off-state, respectively. The optimal value of 1 is achieved if the out-coupling can be completely suppressed and is therefore limited by how well the refractive index of the LC layer can be matched to the index of the grating.
3. Fabrication
The devices are fabricated on 10 cm glass wafers by nanoimprint lithography [16], making cost-effective fabrication of nanostructures in large volumes feasible. To begin, a 610 nm thick layer of SU-8 is spin-coated onto a Borofloat substrate, after which the gratings are formed by nanoimprint using a stamp fabricated by electron beam lithography. Figure 2(a) shows microscope images of gratings with different periods, depicting how narrow wavelength ranges are selectively out-coupled upwards to the objective. These images are taken from a stamp at an intermediate fabrication stage where the gratings are covered by a 15 nm aluminum layer which enhances the color effect.
Fig. 2 (a) Optical microscope image of gratings with periods optimized for red, green and blue light redirection, illuminated with white light from the side. The grating redirects a narrow part of the spectrum upwards through the microscope such that the gratings appear close to the designed color. (b) Photograph of an assembled device, filled with liquid crystals. The device is essentially transparent, except for the aluminum spacers, which provide electrical connection between lid and substrate. (c) Scanning electron microscope (SEM) image of the imprinted gratings, covered with an ITO electrode.
After nanoimprint, the pattern of the electrodes on top of the gratings and contact pads at the edges are defined by photolithography in a layer of AZ5214E photoresist. A 45 nm layer of ITO is sputter-deposited over the surface and a lift-off step in acetone is subsequently performed to remove the resist layer and the ITO outside the grating regions. The lid is prepared on a separate wafer by defining 600 nm thick spacers of aluminum and a subsequent deposition of 45 nm ITO. The aluminum spacers define the thickness of the liquid crystal layer and provide electrical contact to the lid. After dicing of individual 20 x 15 mm sized chips from the wafer, a lid is pressed onto each sample and fixed with epoxy resin. The epoxy contracts during cross-linking such that the contacts of lid and substrate are pressed together to form an electrical connection. In order to fill the cell with LC, the sample is placed on a hot plate at 60 °C while a drop of nematic liquid crystal (4′-Pentyl-4-biphenylcarbonitrile, 5CB, from Sigma Aldrich) is placed at the edge and fills the cell by capillary forces. When cooling to room temperature, the LC is in nematic phase. Figure 2(b) shows a photograph of the final sample filled with LC and Fig. 2(c) presents an SEM image of an imprinted grating, covered by ITO.
4. Measurements
The basic optical operation of the device prior to the addition of LC is shown in Fig. 3(a). Light is focused onto the glass edge from the right, propagating through the glass and then, at each pixel, projected towards a screen. Since each grating is designed for perpendicular out-coupling at a specific wavelength, one grating cannot be used to project multiple colors. This is addressed by using a stack of three replications with different grating periods, as illustrated in Fig. 3(b). In such a scheme, where the grating with the largest period is at the bottom, gratings on the planes above each level appear transparent due to the sub-wavelength period and light may pass through the rest of the stack unaffected. In Fig. 3(a) the microscope objective is translated to focus onto the corresponding plane for each color. Figure 3(c) shows a CCD camera image of the diffracted light. The vertical divergence, causing an elongated profile, is a result of focusing by the microscope objective and may be reduced by adequate beam collimation. The horizontal divergence is determined by the grating size; here, a 250 µm wide grating results in a line broadening of 740 µm (taken at 1/e2), measured at 20 cm distance, corresponding to a beam half-angle divergence of 1.23 mrad.
Fig. 3 (a) Three-color operation of the transparent projection display, light is focused onto the sample by the microscope objective from the right side and projected onto a screen at 10 cm distance. The device shown here consists of three replications stacked on top of each other and glued onto a microscope slide. For each color the objective is translated to focus light onto the corresponding plane. For this basic demonstration, LC and lid are omitted and a logo is placed behind the sample just below the out-coupled beam in order to display the transparency. The input laser power is 20 mW and the power emitted by each pixel is approximately 2.8 µW, clearly visible in ambient light. Note that only a small fraction of the laser light is incident on the pixels, thus the diffraction efficiency of a single grating is not measured. (b) Drawing of the stacked device, consisting of three samples with different grating periods. (c) CCD camera image of the diffracted light (532 nm).
The operation of samples filled with electrically modulatable LC is investigated in two ways. In the first experiment, the sample is observed in an optical microscope using a white light transmission mode in order to study the physical processes (i.e. not how the display device is intended to operate). The sample is placed between crossed polarizers, serving as polarizer and analyzer, as shown in Fig. 4(a). Outside the grating area the molecules are ordered over short ranges only and disclinations in the orientational order are seen as bright lines. The regions with gratings exhibit less disclinations and the area in general appears darker, indicating that the grating dictates some alignment of the molecules. However, this effect is only occurring close to the grating and decreases further away (close to the lid) such that crystal defects occur and change the polarization of transmitted light, observed as a gray, irregular patterned area. This non-uniform alignment may be improved in future work by application of an alignment layer also on the top surface of the cell. Under an applied voltage between grating and lid, above the threshold for the Frederiks transition [17], the liquid crystals re-orient towards homeotropic alignment (perpendicular to the surface). With a voltage of 36 V applied (arbitrary high value above the threshold), as shown in the upper right image of Fig. 4(a), the molecules align parallel to the electric field such that the light path is along the optic axis and no birefringence is present. The polarization of transmitted light remains unaltered during passage through the cell and light is absorbed by the analyzer, causing completely black appearance of the grating.
Fig. 4 (a) Characterization of the sample in a transmission optical microscope with crossed polarizers. For the case of zero applied voltage, the grating appears darker than the surrounding but not completely opaque due to topographical LC orientation with the grating. With an applied voltage, the LC molecules align parallel to the E-field throughout the cell such that the crossed polarizers block all of the light and the grating area appears completely black. (b) Measurement of the redirected light. The right-most pixel of three is switched between the off-state (left) and the on-state (right) when the applied voltage induces a change in the refractive index of the LC, covering the grating. The temporal response is shown below both (a) and (b).
In the second experiment, shown in Fig. 4(b), the samples are characterized in a configuration as a projection display, with light being coupled in from the side and a fraction of the light is out-coupled by the gratings. When the voltage is applied, reorientation of the anisotropic LC molecules results in a change in the refractive index perceived by the light, and thereby modifies the diffraction efficiency of the grating. In order to suppress electrolytic effects, the electrical signal is a bipolar square waveform at 2 kHz, as shown in the lower part of Fig. 4, together with the response of the transmitted and redirected light.
A notable difference between the two configurations is observed in the temporal response shown in the lower graphs of Fig. 4. For the case of transmitted light, the intensity is constant after the voltage is switched on, but the redirected light intensity has response times fast enough to partly follow the switchover of the driving voltage with rise and fall times in the microsecond range. This discrepancy is explained by the different physical processes: during the on-switch state the LC molecule orientation is induced by the electric field, while for the off-switch state the reorientation is due to relaxation caused by the anchoring force at the grating. This restoring force is larger close to the grating surface and the process occurs on quicker timescales than in the center of the LC layer [18]. In transmission mode the light intensity is affected by the alignment of LC molecules throughout the entire cell from grating to lid. For the redirection scheme instead, only the molecules in close proximity to the grating have an influence on the diffraction efficiency; hence fast reorientation into the unperturbed state after switching-off the voltage is possible in this case.
In Fig. 5(a) the optical behavior is analyzed in more detail at a number of applied voltages and for incident light using both TE and TM polarization. In the case of TE polarization, zero applied voltage corresponds to the minimum of out-coupled light. The molecules are aligned parallel to the grating and as the light wave oscillates along the long (slow) axis of the LC molecules, the perceived refractive index in the LC is 1.7. When the voltage is increased and the molecules tilt towards homeotropic orientation, the perceived refractive index is decreased, reaching 1.5 in the final state. This decrease in perceived LC refractive index corresponds to an increase in refractive index contrast between the LC and the grating (refractive index of ITO, 2.3), causing an increase in the diffraction efficiency, i.e. increase of the diffracted light intensity. The opposite case is seen when TM polarization is used: at zero applied voltage the light polarization is perpendicular to the long axis of the LCs, sensing a low refractive index, i.e. a large index contrast to the grating and high intensity of the out-coupled light. Increasing of the voltage consequently reduces the out-coupled light intensity.
Fig. 5 (a) Time-resolved measurement of out-coupled light intensity for TE and TM polarized light. With increasing voltage, the molecules tilt from parallel to perpendicular alignment and depending on the polarization, different refractive indices are perceived by the light. This results in the opposite voltage-intensity relation for TE/TM polarization. (b) Measurement of the response times for TM polarized light as a function of the applied voltage. (c) Measurement of the voltage threshold for TE and TM polarization.
The response times are measured with rise-times well below 1 ms and fall-times of 4.5 ms, as shown in Fig. 5(b). Switching times are measured between 5% and 95% light intensity upon switching the 2 kHz signal on/off. An investigation of the threshold voltage is presented in Fig. 5(c) and the value is measured to be approximately 6 V for both TE and TM polarization.
The measurements with TE/TM polarization verify that the intensity of redirected light is indeed controlled by adjustments of the refractive index which the light perceives when impinging on the grating. Ideally it should be possible to completely suppress the out-coupled light (apart from scattering) when the refractive index of the grating and the liquid crystal are exactly matched. This could not be achieved with the devices presented here due to the high refractive index ITO layer which is deposited on top of the grating. Since higher refractive index liquid crystal materials are not commonly available, an alternative may be to bury the ITO electrodes below the polymer grating. In Fig. 5(a) and 5(c) it is noted that for operation as a projection display, TM polarized light is more suitable because a higher contrast between the bright and dark state is achieved with a value of 0.67. This effect is assumed to originate from the complex interplay between the optical mode profile in the waveguide layers and the grating efficiency, both as function of LC orientation and light polarization. Numerical simulations of the voltage dependent LC orientation profile, light guiding mechanisms, electric field confinement and grating diffraction efficiency would be required for further investigation. In general, the contrast may be limited by the inhomogeneous liquid crystal alignment, due to the competing forces of surface alignment and electrically induced orientation. Even at large voltages, the LC molecules do not fully orient towards normal [19] and thus the achieved performance at applied voltage in terms of redirected intensity and extinction does not reach the values obtained without applied voltage at TE/TM polarization.
5. Discussion
The time-resolved intensity measurements demonstrate switching times of 5 ms, comparable to existing display devices. Additionally, a sub-100 microsecond decay is observed in the presented device geometry, not only for switching-on, but also for switching-off, which is usually the limiting factor in LC devices. This fast relaxation mode is possibly a surface effect caused by the polarity in the asymmetric cell. Since the optical effect is localized very close to the surface, inside the 120 nm thin grating lines, the LC in this vicinity exhibits a major contribution to the redirected signal. This fast µs-scale effect is of modest amplitude, where additional experiments, especially with different LC layer thicknesses, would reveal whether this modulation may be used practically.
Due to the strong anchoring on the surface, the threshold voltage of 6 V is larger than typical LC displays which have a threshold around 2.5 V [20], but still accessible to voltages used in consumer electronics and significantly below the high voltages reported in some other LC-actuated optical devices [21–24]. It should also be noted that the threshold is rather “soft”, with linear tuning of the light intensity via voltage switching possible from 6 V to 10 V.
For integration of high resolution displays into small devices, the grating size may need to be reduced below the dimensions presented in this work. However, this will correspondingly increase the divergence and limit the maximum projection distance, pointing to a design compromise between resolution and depth of focus to be made. Additional optical elements such as microlenses could be applied to the outside of the device to enhance the projection distance, although this will come at the cost of distorting the image viewed through the otherwise planar transparent materials. Cross-talk between neighboring pixels may also need addressing in future work, since pixels located behind others will experience reduced intensities. This may be compensated in two ways: First, the presence of a primary waveguide that continuously refills the LC waveguide, although more light would be required and much of it not used. Second, an intelligent controller could adjust the intensity of each pixel based on the intensity of all other pixels, similar to current LCD TVs where the backlight is dynamically and locally adjusted to increase image contrast.
Until now the article has focused on projection displays, but the aforementioned properties make the device also suitable to a number of other applications. The integration of photonic systems [25] is rapidly advancing and the technique presented here may be used as a coupler in multilayer photonic circuits [26] where light signals may be selectively redirected from one device layer into another. Optical communication links between electronic processors and external components such as memory or graphics processors may benefit from the technology if only a limited number of high-speed links are available and the LC switch is used to reconfigure the connection of these links between selectable components [27]. Transparent lab-on-a-chip (LOC) systems [28] employ optical techniques for purposes such as spectrographic analysis and cell manipulation, where the technology presented here may be readily integrated into such LOCs to allow for reconfigurable light patterns generated directly inside the chip for precise control of localized spectroscopy [29] or optical forces [30,31] at multiple points. In applications where a tunable light source is available also the wavelength-dependence of the diffraction could be exploited, as the direction of out-coupled light will change if the wavelength is tuned. In this way the emerging beams could be scanned across the target area, e.g. for precise and continuous positioning of optical trapping or sensing points.
6. Conclusion
To conclude, a transparent and fully integrated electronically modulated projection display is reported based on optical gratings with a thin liquid crystal layer. The feasibility of full color projection by mixing of primary colors is demonstrated and the device operates at 10 V with a fall-off response time of less than 5 ms. The underlying physics of the liquid crystal director reorientation with respect to the grating shows 90 µs response times due to the localization of the optical effect in the proximity of the grating surface. The results denote a key step towards a new generation of compact and transparent projection display technology, with possible applications in head-up displays or other technologies where controlled redirection of guided light is required at specific locations from a transparent substrate.
Acknowledgments
This work is supported as part of the EC funded project NaPaNIL (Contract No. 214249). C.L.C. Smith acknowledges financial support from the EU FP7 Marie Curie Fellowship (project number PIIF-GA-2009-254573) and the Danish Research Council for Technology and Production Sciences (grant number 12-126601).
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