One of the key technologies to evolve in the displays market in recent years is liquid crystal over silicon (LCOS) microdisplays. Traditional LCOS devices and applications such as rear projection televisions, have been based on intensity modulation electro-optical effects, however, recent developments have shown that multi-level phase modulation from these devices is extremely sought after for applications such as holographic projectors, optical correlators and adaptive optics. Here, we propose alternative device geometry based on the flexoelectric-optic effect in a chiral nematic liquid crystal. This device is capable of delivering a multilevel phase shift at response times less than 100 μsec which has been verified by phase shift interferometry using an LCOS test device. The flexoelectric on silicon device, due to its remarkable characteristics, enables the next generation of holographic devices to be realized.
© 2009 Optical Society of America
Traditional liquid crystal on silicon (LCOS) devices , such as the advanced grating chip shown in Fig. 1, can deliver multi-level phase modulation based on planar aligned nematic liquid crystals (LCs) but, due to cell geometry and visco-elastic properties, are only capable of achieving frame rates of around 100 Hz. Ferroelectric LCOS devices, on the other hand, can deliver frame rates in excess of 10 kHz, but are limited to binary phase modulation due to the two stable states that are available through surface stabilization. Consequently, an electro-optic effect that offers both analogue phase modulations with frame rates in excess of 1 kHz is central to advancements in holographic projection and adaptive optics [2–4].
It is known that the flexoelectro-optic effect in chiral nematic LCs, when in the uniform lying helix (ULH) geometry, is a fast switching, in-plane deflection of the optic axis that is linear with an externally applied electric field [5, 6]. The flexoelectro-optic effect is characterized by the tilt angle, ϕ, of the optic axis and the response time, τ. To a first approximation these can be expressed in terms of the macroscopic physical properties as
where p is the pitch of helix, γ is the relative effective viscosity for the distortion of the helix, e is the average flexoelectric coefficient (e = ½(es+eb)) and K is the average of the splay and bend elastic constants for the material defined as K = ½(K 11+K 33) where es and eb are the flexoelectric coefficients and K 11 and K 33, are the elastic constants for splay and bend deformation of the material, respectively.
These expressions show that the tilt angle of the optic axis is linear in the applied electric field and its magnitude is governed by the so-called flexoelastic ratio. The response times, however, can be minimized by selecting a short-pitch chiral nematic liquid crystal. In this paper, we present results of a new LCOS device based upon flexoelectro-optic switching and demonstrate that a good ULH texture can be obtained using a similar approach to that adopted for conventional glass cell structures. The switching characteristics based on intensity measurements are found to agree with the results obtained for a control glass cell confirming flexoelectro-optic switching. Furthermore, we characterize flexoelectro-optic switching in the ULH geometry on silicon by observing the phase change from interference fringes in the far-field. The results demonstrate fast, analogue multi-level phase modulation, a feature not observed in previous LCOS devices. Such a device would be of significance for the development of a range of applications including adaptive optics and next-generation holographic projection.
2. Fabrication of the LCOS device
The LCOS device was formed using a standard silicon very large scale integration (VLSI) process to create a silicon backplane which contained two parallel aluminum pixels and an addressing circuitry for the bottom substrate. This allowed the device to be used in reflection mode whereby the aluminum pixels on the silicon addressing circuit acted as both an electrode, with which to apply the electric field across the liquid crystal layer, and a ‘mirror’ that enabled the interaction optically with the LC material. On the top of the LCOS device was a glass substrate which was coated with a patterned electrode layer of indium tin oxide (ITO) on the inner side of the glass. A low pre-tilt polyimide alignment layer (AM4276) was rubbed along the long edge of the aluminum pixels on both substrates. The cell gap of the empty cell was created by using 5 μm spacer balls doped in the ultraviolet cured glue seal. The cell gap was then measured using a Fabry-Perot interference technique. The size of the aluminum pixels were 2mm × 6mm. The structure of the LCOS device is shown in Fig. 2.
3. Liquid crystal mixtures
The nematic LC mixture used in this study is BL048 (Merck). The chiral nematic LC mixture used in this study consisted of the commercially available nematic LC mixture BL006 (Merck KGaA) and a low concentration (1 wt %) of the high twisting power chiral dopant BDH1305 (Merck KGaA). The pitch of this sample was greater than 600 nm. The resultant mixture was then filled into an empty LCOS device by vacuum-assisted capillary action. After filling the mixture into the LCOS device, a Grandjean texture was obtained at room temperature in the absence of an applied electric field. The ULH texture was obtained in the LCOS device by cooling the mixture from the isotropic phase to room temperature (at 27°C) under the influence of a bipolar square wave (2.5 Vμm-1) at a frequency of 100 Hz. Mechanical shearing across the device was used to improve the alignment.
4. Experimental procedure
4.1 Intensity Measurements
The experimental apparatus used to prepare the ULH texture in the LCOS device, and on which the measurements of the flexoelectro-optic response were carried out, included an Olympus BH-2 reflection polarizing microscope and a Linkam hot-stage, which allowed the temperature to be controlled to within an accuracy of 0.1°C. The flexoelectro-optic response of the LCOS device was measured using a photodiode mounted in the phototube of the microscope, a digitizing oscilloscope (HP54501A, Hewlett-Packard), and an amplified output from a waveform generator (TGA1230, Thurlby-Thandar) in combination with a high voltage amplifier built in-house.
4.2 Phase modulation
The experiment carried out for phase measurements was similar to Young’s double slits experiment but differed in that it used light reflected from the LCOS device, Fig. 3. The light source was a polarized laser source mounted on a rotator, and the input laser polarization was aligned with the optic axis of the ULH texture in the chiral nematic liquid crystal sample. A microscope objective (x5) with a numerical aperture of 0.12 was used to gather the reflected light through the double silts which covered the aluminum pixels of the LCOS device. The double silts for the NLC sample were positioned with a 0.5 mm gap between them and each slit was ~0.4mm × 0.5mm. The double silts for the chiral nematic liquid crystal sample were also separated by a 0.5 mm gap and had the same area. The aluminum pixels were covered with a ‘double slit’ mask to maximize the phase difference between the two pixels, in such an approach, only one of the pixels was driven by an applied electric field and the other pixel acted as a reference (i.e. no field applied). A charge-coupled device (CCD) camera (Logitech, QVGA) was used to examine the far-field interference pattern of the test device. The fringes were then recorded in the far-field whereby a separation between two maxima was 2π in phase. For the response time measurements, the CCD camera was replaced with a photodetector (Thorlabs’ DET210) with an active area of 0.8 mm2 and the microscope objective was changed to an x40 microscope objective with a numerical aperture of 0.65. The photo-detector was connected to a digitizing oscilloscope (Agilent 54624A) which displayed both the output waveform and measured response time of the phase modulation simultaneously.
5. Characterization of the Flexoelectric-LCOS device
To verify flexoelectro-optic switching in the LCOS test device, the tilt angle and response time were determined from an intensity-based modulation with an electric field at a frequency of 100 Hz. An optical micrograph of the ULH texture on the aluminum pixels taken between crossed polarizers at an applied field of 2Vμm-1 is shown in Fig. 4 indicating a relatively good alignment of the optic axis in the plane of the device. This is an encouraging result as it demonstrates that a lying helix can be obtained on silicon substrates. The dark and light states were obtained by rotating the device between crossed polarizers to align the optic axis at 45° to the transmission axes of the polarizers (light state) and then to align the optic axis with the transmission axis of one of the polarizers (dark state).
Figure 5 demonstrates the optical response of the chiral nematic LC in the ULH texture on the LCOS device. The tilt angle and the response time as a function of the applied electric field are plotted separately in Fig. 5. Measurements were taken for comparison of the same chiral nematic LC mixture but in a conventional 5 μm glass cell at room temperature (T = 30°C) at several different frequencies. It is shown that there is a trade-off between tilt angle and response time in that higher frequencies result in incomplete switching of the optic axis before the polarity of the field is reversed hence smaller tilt angles are observed. These combined results confirm that the electro-optic response in the LCOS device was the same as that observed for the conventional glass cell.
For the LCOS test device, the tilt angle is found to be linearly proportional to the applied electric field, which in accordance with Eq. (1), verifies flexoelectro-optic switching. When the applied electric field (100 Hz) was increased to 4 Vμm-1, the mixture exhibited a tilt angle of 17°, and for phase measurements would give the maximum interferometric contrast between the two switched states. The response times were also measured at the same temperature for different frequencies. These response times correspond to the average of the τ10–90 necessary to achieve 10%–90% of the total value of transmitted light intensity. As the applied electric field was increased, we saw a slight decrease in the response times of the material in a glass cell which is typically observed for flexoelectro-optic switching at large tilt angles .
6. Multi-level phase modulation
For the purposes of comparison, and to highlight the benefits of flexoelectro-optic switching on silicon, phase measurements were first carried out using a nematic LC based LCOS device. Fig. 6a shows the CCD camera images of the far-field interference pattern from the LCOS device at different electric field strengths. A straight line is drawn on the image as a reference. As the electric field strength increased, the phase difference between the two pixels changed due to the dielectrically driven reorientation of the LC molecules. Consequently, this reorientation then causes the fringes seen in Fig. 6a to shift accordingly. The phase shifted angle as a function of the applied field is plotted in Fig. 6b where it can be seen that the phase shift was several orders of π, but the responses were not linear in the applied field as expected from nematic-based LCOS devices. Furthermore, the time required for the LC to respond at 500 Hz was 40 ms, see Fig. 6c.
The performance of the chiral nematic LCOS device, on the other hand, is very different, Fig. 7. This figure includes the far-field interference pattern, the electric field dependence of the phase shift, and the response time. These results demonstrate the multi-level phase modulation capability of the device. Unlike the nematic-LCOS device, the phase shift was linear in the applied electric field as expected from the flexoelectric response. At an electric field strength of 4 Vμm-1, a phase shift of 127° (~2π/3) was observed. Since a tilt angle of 15°C leads to a 2π/3 phase shift it is straightforward to realize that for 2π phase shift tilt angles of 45° are required; this is readily achievable using bimesogenic mixtures. The response time of the phase modulation as a function of the applied electric field is plotted in Fig. 7c. The speed of phase modulation in the LCOS device is frequency dependent due to the fact that the time between polarity reversals in the applied electric field is less than the total response time and consequently the optic axis is unable to reach its maximum possible deflection before the polarity reverses and the optic axis is forced to rotate in the opposite direction. The response time for flexoelectro-optic switching is determined by the pitch and the viscoelastic ratio as described by Eq. (2) which is the limit of the response time for maximum deflection of the optic axis. To determine the response time for a given phase change it is necessary to compare Fig. 7c with Fig. 7b. For example, for a square wave of 2 Vμm-1 and frequency of 1 kHz applied to the LCOS device, giving a phase shift of ~ 40° the response time of the phase modulation was measured < 1ms. An increase in the frequency results in shorter response times but smaller tilt angles. This fast response shows the capability of linear multi-level phase modulation in a LCOS device operating at kHz frame rates.
In terms of future work, the next objective is to achieve multi-level phase modulation using bimesogenic mixtures that have been developed in recent years and to demonstrate good uniform alignment using these compounds. The bimesogenic mixtures enable improved response due to the combination of a low dielectric anisotropy and a large flexoelastic ratio which ensures strong flexoelectric coupling of the LC to the applied field whilst at the same time minimizing dielectric coupling. Temperature independent phase shift is also desirable for practical applications; however, this can easily be achieved by ensuring that the pitch is temperature independent and is not thermochromic . In addition, future research into new liquid crystal materials, such as blue phases , chiral doped systems  and multi-color switching materials  will further improve speeds, phase modulation and other optical effects that may lead to even faster frame rates.
In summary, we have demonstrated for the first time an electric field-induced multi-level phase modulation using the flexoelectro-optic effect in LCOS device which is capable of frame rates in excess of several kHz. Such an electro-optical effect is a revolution in the implementation of LCOS devices, allowing for the first time, new applications such as holographic projection and adaptive optics to exploit this frame rate. The limited frame rate of existing phase modulating electro-optical effects has been a major restriction in the use of LCOS phase modulators. By employing flexoelectro-optic based LCOS devices one can now modulate the phase of light at frame rates well above those detected by the eye, allowing the improvement of image quality in holographic projectors as well as the implementation of realtime adaptive ophthalmic imaging for the high resolution diagnosis of retinal disease.
The authors would like to thanks members of the Centre of Molecular Materials for Photonics and Electronics for useful discussions.
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