Liquid Crystal Variable Retarders
Meadowlark Optics’ standard liquid crystal products provide tunable retardation by changing the effective birefringence of the material with applied voltage, thus altering the input polarized light to any chosen elliptical, linear or circular polarization.
Our precision Liquid Crystal Variable Retarders are constructed using optically flat fused silica windows coated with transparent conductive Indium Tin Oxide (ITO). Our ITO coating is specially designed for maximum transmission from 450 - 1800 nm. A thin dielectric layer is applied over the ITO and gently rubbed, to provide for liquid crystal molecular alignment. Two windows are then precisely aligned and spaced a few microns apart. The cavity is filled with birefringentnematic liquid crystal material, electrical contacts are attached and the device is environmentally sealed. We mount the Liquid Crystal Variable Retarder in an anodized aluminum housing such that the fast and slow axes are both at 45° relative to a convenient mounting hole.
Anisotropic nematic liquid crystal molecules form uniaxial birefringent layers in the liquid crystal cell. An essential feature of nematic material is that, on average, molecules are aligned with their long axes parallel, but with their centers randomly distributed as depicted in figure 4-6(a). With no voltage applied, the liquid crystal molecules lie parallel to the glass substrates and maximum retardation is achieved. When voltage is applied, liquid crystal molecules begin to tip perpendicular to the fused silica windows as depicted in figure 4-6(b). As voltage increases, molecules tip further causing a reduction in the effective birefringence and hence, retardance. Molecules at the surface, however, are unable to rotate freely because they are pinned at the alignment layer. This surface pinning causes a residual retardance of tens of nanometers, even at high voltage (20 volts). Each Liquid Crystal Variable Retarder is supplied with retardance versus voltage performance data for your specified wavelength.
We achieve zero (or any customresidual) retardance with a subtractive fixed polymer retarder, refereed to as a compensator, attached to the liquid crystal cell. Negative retardance values are sometimes preferred, for example, when converting between right- and left-circularly polarized states, although this can also be achieved with a cell designed to operate between ¼- and ¾-wave of retardance. Figure 4-8 illustrates retardance as a function of voltage for a typical Liquid Crystal Variable Retarder with and without an attached compensator. Placing a compensated Liquid Crystal Variable Retarder between two high extinction polarizers creates an excellent optical attenuator, with convenient electronic control.
As with any anisotropic material, retardance is dependent upon thickness and birefringence. Liquid crystal birefringence is a function of wavelength, drive voltage and temperature. The overall retardance of a liquid crystal cell decreases with increasing temperature (approximately -0.4% per ºC) and/or increasing voltage.
Response Time
Liquid Crystal Variable Retarder response time is a function of several parameters, including layer thickness, viscosity, temperature,
variations in drive voltage and surface treatment. Response time is proportional to the square of the layer thickness and therefore,
the square of the total retardance.
Response time also depends upon direction of the retardance change. If the retardance increases (decreasing voltage), response time is determined solely by mechanical relaxation of the molecules, which is a function of the viscosity. If retardance decreases in value (increasing voltage), response time is much faster due to the increased electric field across the liquid crystal layer. Typical response time for our standard visible Liquid Crystal Variable Retarder is shown in figure 4-7b. It takes about 5 ms to switch from one-half to zero waves (low to high voltage) and about 20 ms to switch from zero to one-half wave (high to low voltage).
Response time can be improved by using custom materials with high birefringence and a thinner liquid crystal layer. At higher temperature, material viscosity decreases, also contributing to a faster response. However, retardance decreases by approximately 0.2% to 0.3% per °C increase in temperature. For speed critical applications, see our Swift LC devices.
The Transient Nematic Effect (TNE) can also be used to improve response times. With this drive method, a highvoltage spike is applied to accelerate the molecular alignment parallel to the applied field. Voltage is then reduced to achieve the desired retardance. When switching from low to high retardance all voltage is temporarily removed to allow the liquid crystal molecules to undergo natural relaxation and then increased to provide the desired retardance. Response time achieved with the transient nematic effect is also shown in figure 4-7c. Our Four Channel Digital Interface conveniently provides the necessary TNE voltage profiles.
