Liquid crystal devices tailored for specific imaging applications

In the late 1980s, spatial light modulators (SLMs) began to be used in a variety of imaging systems and optical signal processing applications, such as CCD and CMOS cameras, detector arrays, and spectrometers. As these reduce in size and cost, the whole field of optical imaging and sensing is being revolutionized. Liquid crystal (LC) devices are entering this field as key elements for controlling the wavefront, wavelength, polarization, phase, intensity, and coherence of light. Liquid crystal devices can control these parameters in a compact and fast manner, thus allowing multi-functionality in the same system. My group and I have designed a variety of LC devices for specific imaging applications, including for high-resolution cell nuclei images and cancer diagnosis.

Our initial vision was to enrich optical microscopy with LC devices to build a multi-functional system. Figure 1 shows one concept for an optical microscopy system that can become multimodal by incorporating LC devices into a Linnik microscope (a reflecting microscope that uses interference between a reference and a signal beam for enhanced resolution, contrast, and depth information). Interferometry is the superposition of two optical fields by which an interference pattern is obtained depending on the phase relation between them. Thus, controlling the phase difference allows a higher signal-to-noise ratio to be obtained by what is called phase shift interferometry. The Linnik system functions as a standard microscope when the reference beam path is blocked. Incorporating a tunable filter permits multi-spectral or hyperspectral imaging, and if the filter is narrowband, then spectral domain full-field optical coherence tomography (FF-OCT) becomes possible with the same system.

Figure 1. A multimodal optical microscopy system possible by integrating into it liquid crystal (LC) devices based on the Linnik interference microscope. AS: Aperture stop. FS: Field stop. LC PM: LC phase modulator. P/Z-scan: Pathlength/axial scan, respectively. PSI: Phase shift interferometry.

Using a nematic phase modulator and fast driving scheme gives four phase differences between the reference and sample beams to obtain the envelope of the interferogram accurately. Based on this system, we built an FF-OCT system demonstrating high-resolution images of cell nuclei.¹ Figure 2 presents sample images of cell nuclei obtained with bright field microscopy and our FF-OCT system, showing the great improvements in contrast and resolution.

Figure 2. Sample image of cell nuclei obtained with bright field microscopy (BF, left) and ultra-high-resolution spectral-domain full-field optical coherence tomography (FF-OCT, right) showing improvements in contrast and resolution of FF-OCT.

The second optical modality into which we integrated LC devices is a multispectropolarimetric system used for cancer diagnosis.^{2, 3} Polarimetric imaging deals with imaging at different incident polarization states and analyzing the emerging polarization properties of the optical field. Our system is based on scattering and uses oblique illumination (see Figure 3). At each wavelength selected by the tunable filter, polarimetric images are obtained at multiple polarization states. Tissue scatters and absorbs the different wavelengths differently. Consequently, it is necessary to control the wavelength over a wide range, both to cover the absorption characteristics of the existing chromophores within the tissue and to differentiate between different depths in the tissue. Green light, for example, is absorbed strongly in proteins and blood. It also penetrates less because it scatters the most. Hence it carries information mainly on tissue of the external layers and some chromophores, while the red and IR wavelengths penetrate deeper and are absorbed less, so carry information on deeper layers. Because the polarization of the scattered light depends on the depth within the tissue at which it is scattered and on the orientation of anisotropic scatterers such as collagen fibers, it is also necessary to control the incident polarization state.

Figure 3. Multi-spectropolarimetric scatterometric system with integrated LC devices used for skin cancer diagnosis.

To tune the wavelength over a wide band, we developed two modifications of the well-known Lyot-Ohman (LO) filter. The standard LO filter, made up of a set of alternate birefringent plates and polarizers, can be tuned by modifying the birefringence of the plates: see Figure 4(a). Our first modification uses an additional LC retarder between crossed polarizers to eliminate one high-order peak, thus almost doubling the tuning range: see Figure 4(b–d).⁴ The second uses an odd sequence stack of retarders oriented at 45° between crossed polarizers (see Figure 5).⁵

Figure 4. Two schemes of LC tunable filters exhibiting wide dynamic range. (a) The traditional Lyot-Ohman (LO) filter with (b) an additional LC retarder between crossed polarizers to eliminate higher-order peaks. d is the thickness of the thinnest retarder, d_e is the thickness of the additional eliminator retarder, and J_out is the Jones vector of the output light. (c) Transfer functions of the first two retarders in the Lyot stack (dotted red for the first and dashed green for the second) and their multiplication result (blue). (d) The transfer function of the LO filter (dashed green), the additional retarder (dotted red), and their multiplication (blue) showing how the 3rd order is eliminated.

Figure 5. (a) A stack of odd sequence retarders (d is the thickness of the thinnest retarder and j is the number of the retarder) between crossed polarizers. (b) The experimental and theoretical transfer functions of the odd sequence filter showing the tunability over both the visible and the near-IR.

To reduce the bandwidth and improve the speed, LC tunable filters (LCTFs) based on resonance effects are preferable to nonresonant filters such as the LO filter. Usually a single LC layer is required, such as in the Fabry-Pérot and guided-mode resonance filters.⁶ Sometimes the polarization dependence is not desired for tunable filters, and we have proposed polarization-independent tunable filters using polarization conversion mirrors.⁷ We have demonstrated a polarization-independent LCTF by polarization diversity using a Wollaston prism.⁸ (Polarization diversity means splitting the two orthogonal polarizations in space, modifying one of them so they become identical, passing them through the LC filter, and then recombining them.)

To control the polarization, we have developed several devices based on a combination of two or more LC retarders:^{1, 9} see Figure 6 for the structure and operation of a wavelength-independent polarization rotator. The first retarder has variable retardation, Γ, while the second retarder is a quarter-wave plate (QWP). Vertically polarized incident light will be rotated by Γ/2, and it is possible to simplify the device by having the QWP achromatic.¹⁰

Figure 6. (a) Structure of the LC polarization rotator. (b) Polarization rotation versus voltage at different wavelengths in the visible and near-IR. LCVR: LC variable retarder. QWP: Quarter-wave plate. Γ/2: Rotation of vertically polarized light.

Finally, SLMs with a large number of pixels are available, but their high cost hinders widespread use. We have developed lower-cost SLMs with a small number of pixels in transmission mode that nevertheless enhance the image. Figure 7 shows two such SLMs: the first is an electrically addressed SLM (EASLM) made up of eight annular electrodes, and the second is an optically addressed SLM that uses ultrathin chalcogenide glass film as a photosensor.¹¹ We have used the EASLM for tunable focusing, extended depth of focus, and overextended depth of focus using time multiplexing of several phase masks.^12–14

Figure 7. Schemes of two transmissive spatial light modulators (SLMs) made of a small number of pixels and cost-effective for optical imaging applications. (a) Annular electrically addressed SLM made of eight rings with total radius R. (b) Single-pixel optically addressed SLM that uses ultrathin chalcogenide film photosensor.

In summary, we have harnessed LC devices for non-display applications, in particular for optical imaging, which we have identified as a field ripe to be transformed by tailored, cost-effective LC devices. The next steps include improving the alignment quality and speed of the LC devices as well as reducing the number of voltage channels to be controlled. We also plan to develop other applications for 3D imaging and medical diagnosis using these systems.

The author acknowledges support of the Ministry of Science and the following students and postdocs for their excellent work on the subject matter: Miri Kerzhner Gelbaor, Sivan Issac, Marwan AbuLeil, Asi Solodar, Shahar Mor, Riki Moses, Ofir Aharon, Avner Safrani, Arun Kumar, Ashok Chaudhary, and Iftach Klapp.