By Nelson TabirYan and Sarik Nersisyan
It's a challenging task to block direct sunlight in an optical system while retaining the ability to observe scenery surrounding the sun. A compelling need for such an artificial solar eclipse arises in many surveillance and reconnaissance applications, as well as in everyday situations such as driving. Liquid-crystal (LC) spatial light modulators (SLMs) are capable of solving this problem when placed at an internal focus of an optical assembly. An LC-SLM is a complex multilayer system in which an electric field provided by an external power supply modulates the optical-axis direction of a thin layer of LC according to the intensity pattern of the light propagating through it. The ability to sense light is achieved by adding a photosensitive semiconductor layer onto one of the substrates sandwiching the LC film. (see Fig. 1). The substrates also contain transparent electrodes for application of the electric voltage to the liquid crystal.
LC-SLMs are expensive
The large potential of LC-SLMs for optical applications was demonstrated years ago by using them for realization of wavefront conjugation of laser beams, transverse pattern formation, optical correlation, and many other processes.1 The high cost of LC-SLMs hinders their development, however, and limits the commercialization of optical systems that incorporate those elaborate devices. In addition to the expense, hypersensitivity to optical radiation makes LC-SLMs impractical for sunlight-mitigation applications.
The common perception that liquid crystals are not sensitive enough to respond to the electric field of a lightwave was countered in the 1980s with the demonstration of several nonlinear optical phenomena due to reorientation of the optical axis of liquid crystals induced by laser radiation itself.2 Thus, optically controlled spatial light modulation could be realized using a bare LC cell at a fraction of the cost of a conventional LC-SLM; no special photosensitive layers and electronic circuits were required. For instance, laser radiation of submilliwatt and milliwatt power is capable of modulating its own phase by over 100 rad because of interaction with a LC film of the proper material (see Fig. 2).
Studies have led to the development of LC-material systems in a wide range of photosensitivities, some comparable to that of semiconductors.3 Those LC materials mark the advent of spatial-light-integrated modulators (SLIMs), in which the two main functions of an SLMlight modulation and photosensitivityare integrated into a single layer of LC material.
All-optical processes add remarkable versatility to the operation of SLIMs, harnessing the usefulness inherent in light-matter interaction phenomena. In particular, the capability arises of varying the refractive index of the LC due to reorientation of the optical axis, as well as varying the principal values of the refractive indices and the magnitude of optical anisotropy of the LC.
Moreover, not only the effective refractive index, but also the state of transparency of the LC can be modulated all-optically. Photochromic and photo-opalescent LC materials allow, respectively, their light-absorbing and scattering properties to be controlled by light.4 For example, a laser beam propagating through a photo-opalescent LC material shows self-extinction (see Fig. 3). Nanoparticulate networks with reconfigurable internal structure embedded in "supra-nonlinear" LC-materials add more capabilities to the operation of SLIMs, such as bistability and configurability of optical properties.
Sunlight-addressed LC
We have developed a device for selectively blocking out the sun while observing the scenery around it. Called the Eclipsor, the device makes use of these novel LC materials (see Fig. 4). Sunlight mitigation is performed by a thin LC film with appropriate photosensitivity, replacing the complex structure of an LC-SLM. The only power source required for operation of the Eclipsor is the radiation of the sun itself shining into the aperture of the device.
In operation, the LC switches between its birefringent and isotropic states under exposure to focused sunlight. The LC film is sandwiched between a crossed polarizer and analyzer. At low light levels, the LC film is birefringent. Hence, it makes the system transmissive; one can look through it in the same way as through conventional sunglasses. At high light levels, when the sun is shining into the imaging device, the film becomes locally isotropic and the direct sunlight cannot propagate through the crossed polarizers. At the same time, all the rest of the film remains birefringent, maintaining high transmission. Thus, the Eclipsor effectively protects the eye (or other sensor) from being blinded.
Many candidate LC materials were tested. The chosen compositions perform both under the hot Florida summer sun as well as from behind the windows of an air-conditioned office. These materials are photostable and their orientation is not damaged by birefringent-to-isotropic transformations. The device reacts quickly, making it possible to look directly at the sun, with only transient stress on the eye during the emergence time of the eclipsed area that blocks the sun.
REFERENCES
NELSON TABIRYAN is president and SARIK NERSISYAN is senior research scientist at BEAM Company, 686 Formosa Ave., Winter Park, FL 32789; email: [email protected].