Not all optically useful crystals are solid. Liquid crystals are at the heart of vital components for polarization and phase control in precision optical and high-power laser systems.
TOM BAUR AND CHRISTOPHER HOY MEADOWLARK OPTICS
Liquid crystals are undoubtedly most widely used in display technologies. But beyond
this context, the ability of
liquid crystals to avoid certain critical size limitations that affect most solid crystals enables their usefulness for applications in precision optics. Their use in flat panel displays such as television screens
and computer monitors, in fact, demonstrates that liquid crystals do not face these size limitations. This allows them to be particularly effective as optical devices, enabling users to take advantage of important properties of laser light.
Liquid crystal spatial light modulators (SLMs) enable precise spatial beam control. Spatial patterning attenuates laser beam hot spots and eliminates the potential for damage growth on downstream optics in high-power systems. Courtesy of National Ignition Facility at Lawrence Livermore National Laboratory.
Applications using liquid crystals to optimize these properties — polarization,
phase, intensity, and direction — are growing. Liquid crystals are increasingly replacing mechanical motions of fixed optical elements. Less than 10 V applied to a thin, 1- to 10-µm layer of liquid crystal between glass plates will control and change laser light properties without any mechanical motions or high voltage. As a result, designers and developers are harnessing these properties to reduce size, weight, and power in laser-enabled optical systems.
Liquid crystal waveplates
Waveplates, or retarders, are historically made using solid birefringent crystal. Most commonly, the crystal materials
used to fabricate these elements are quartz, mica, magnesium fluoride, and/or sapphire. Recently, birefringent polymers have started to replace these materials.
These waveplates are fixed in the
action that they have on a laser beam — or on any light beam. They are polarization modifiers, and the modification is fixed for a given wavelength and crystal orientation.
The use of liquid crystals in this application enables the electrical modification of the crystal structure. This serves as a beneficial in-application advantage; modifying the crystal structure changes how a thin crystal layer modifies the polarization of a light beam.
A liquid crystal variable retarder (LCVR) is a common liquid crystal structure for this function (Figure 1). It provides a waveplate retardance that varies with applied voltage. An electric field is provided to the thin liquid crystal layer through transparent indium tin oxide electrodes on the inner surfaces of the glass cell windows. The electric field induces a dipole in the rod-shaped liquid crystal molecules, which produces a torque that tilts the molecules out of the plane of the windows. This effect changes the birefringence of the liquid crystal layer for rays at normal incidence. A change in the waveplate retardance results in a subsequent polarization change produced on a transmitted beam.
Figure 1. The function of a liquid crystal variable retarder (LCVR) and, specifically, the effect of a small voltage on liquid crystal alignment. This voltage increase from 0 to 2.5 V alters the birefringence of the liquid crystal layer, thereby altering the polarization of the transmitted light. Courtesy of Meadowlark Optics.
Applications of LCVRs
As mentioned, LCVRs are electrically variable polarization modifiers. Adding a linear polarizer following the LCVR makes the device a variable intensity filter when the incoming laser light is linearly polarized. It is preferable that this takes place with a polarization direction at 45° to the alignment direction of the liquid crystal molecules, which are rod-shaped. A transmission variation of two to three orders of magnitude is nonmechanically varied in this way with a voltage varying from 0 to 10 V. Welding helmets, for example, darken in this way when an arc is struck. The LCVR becomes a variable beamsplitter if the linear polarizer on the exit beam is a polarizing beamsplitter cube. This serves to expand the liquid crystal application to that of a nonmechanical beam path director.
Numerous application examples demonstrate the variable transmission function of LCVRs. They can be used for the control of laser intensities in systems for surgery or texturing of metal surfaces, as well as for laser tattoo removal. Additionally, the variable transmission function can be extended to create combinations of LVCRs that together provide a wavelength-tunable bandpass filter or, in some cases, a notch filter.
In addition to protecting imaging sensors and eyes from laser damage (both tunable and notch filters), tunable bandpass filters are useful for applications spanning hyperspectral microscopy imaging, astronomy, and remote sensing of Earth resources. One lesser-known liquid crystal tunable filter application that Meadowlark has advanced is in grading the color of diamonds, which exhibit colors that range from white to pink to yellow (Figure 2).
Figure 2. Clear diamonds surround a rarer yellow diamond. Pink is also a less common — though occasionally found — color for diamonds, and all are graded for color using a liquid crystal tunable filter. Courtesy of Meadowlark Optics.
An additional example of liquid crystals enabling useful beam dynamics
involves beam path switching. For example, this occurs in the lidar system(s) used for guiding and docking a space capsule to the International Space System. In this case, the function is used to facilitate a “switch” from a long-range lidar beam path to a close-in path for the final docking maneuver.
As long as they experience protection from strong UV radiation, liquid crystals survive well on space missions — but this is not the only example of their utility for applications in extreme conditions. Liquid crystals are useful in very high-power laser systems, such as those that are used for laser fusion research. Two prominent inertial confinement fusion research facilities in the U.S. — the Laboratory for Laser Energetics (LLE) at the University of Rochester and the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory — use liquid crystal devices. The LLE uses liquid crystals for polarization control and the NIF uses them for control of beam shape1-2. In the NIF application, high-powered laser beams are patterned prior to amplification to minimize hot spots and avoid further damage to defects on downstream optical elements (opening image). A spatially variable electrical field across the thin liquid crystal layer enables spatial beam intensity patterning. Meanwhile, the LLE has shown liquid crystal tolerance to pulsed fluences as high as 30 J/cm2 at a wavelength of 1053 nm in 1-ns pulses3.
A time-varying liquid crystal retardance followed by a linear polarizer encodes polarization information onto the time variation of polarizer transmission. This combination functions as a polarimeter for the measurement of both linear and circular polarization. Solar astronomers use this polarization information to measure and map magnetic field distribution in sunspot regions with the aim of predicting solar magnetic storms. Past storms show the potential to cause severe damage to the electrical grid by inducing power line currents that can damage transformers.
Liquid crystal alignment can also be twisted. An applied volage will destroy the twist and remove it in the polarization direction (Figure 3).
Figure 3. This twisted liquid crystal configuration is often used in simple digital displays. Applying <10 V destroys the twist, which allows light to pass through the exit polarizer. Removing the voltage reestablishes the twisted configuration in a few milliseconds. Courtesy of Meadowlark Optics.
It should be mentioned that the described liquid crystal devices are fabricated using liquid crystals in the nematic phase. Both the variable retarders and the twisted alignments switch usually in a few milliseconds. By contrast, ferroelectric liquid crystals switch even faster, in ~100 µs or less. These devices have a retardance that is unchanged by voltage. Instead, the fast and slow axes rotate in-plane by a fixed amount with the applications of only a few volts. A 45° rotation is most useful here since it can function as a shutter for linearly polarized light when the liquid crystal device is followed by a linear polarizer.
Spatial light modulators
LCVRs including those that have been described find myriad applications in precision optics. These liquid crystal components are essentially single-pixel light modulators, used to modulate the properties of an optical beam uniformly across the entire aperture.
But what happens when the aperture is subdivided into many small pixels?
In many ways, the answer lies with liquid crystals’ most recognizable application: Displays and image projectors themselves are pixelated spatial light modulators (SLMs). Here, the liquid crystal is used for intensity modulation by rotating polarization in line with a polarizer.
A powerful aspect of SLMs for precision optics arises when the liquid crystal is used to modulate phase. When the liquid crystal is aligned such that each individual pixel acts as an LCVR, the spatially varying retardance can be used to create a pixel-by-pixel shaping of the phase of the incident wavefront. Figure 4 illustrates the anatomy of a reflective SLM. A pixelated silicon backplane is fabricated through large-scale-integration integrated circuit processes. A liquid crystal is sandwiched between the resulting backplane and a uniform transparent electrode.
Figure 4. A liquid crystal on silicon (LCOS) spatial light modulator (SLM) consists of a layer of liquid crystal overlaying a pixelated backplane. By applying varying electric fields at each pixel, the local birefringence can be controlled. This enables pixel-by-pixel shaping of the reflected wavefront. The fastest SLMs can provide full 2π phase modulation at millisecond timescales. Courtesy of Meadowlark Optics.
By arbitrarily shaping the phase of an incident wavefront, a phase-modulating SLM is a programmable optic that can mimic conventional optics. These include lenses or prisms for nonmechanical beam focusing and steering, Zernike basis modes for wavefront correction, and complex phase objects for mode conversion and display of computer-generated holograms. Also, unlike their display counterparts, SLMs for precision optics can provide analog phase modulation at kilohertz rates, modulate wavelengths from UV through MIR, and withstand kilowatt-level average powers.
Since their initial demonstrations in the late 1980s, liquid crystal on silicon (LCOS) SLMs have steadily evolved to meet the needs of demanding scientific applications. Use cases for SLMs are numerous and wide-ranging in this application space. For example, SLMs are used in the field of neuroscience to holographically project ultrashort-pulse lasers into the brains of awake and behaving animals. Modern commercially available
SLMs provide full analog wavefront shaping at up to 800 Hz in the NIR, which enables optical control of 3D neural circuits with the single-cellular precision and millisecond-scale timing of natural brain activity.
Similarly, SLMs can create arrays of optical traps using computer-generated holograms. In conventional optical trapping, a focused laser uses optical force gradients to hold a small particle, which can range from atoms to biological cells. Using an SLM, hundreds of optical traps from a single laser can be generated and independently controlled. This technique is finding increased deployment for quantum computing, where SLMs are used to create arrays of optical traps for neutral atoms and ions, and optically address these qubits to implement logic gates. Improvements to UV and short blue wavelength-tolerant SLMs are aiding this application.
In addition to increasing tolerance to the shorter wavelengths of light that traditionally degrade many liquid crystal molecules, emerging materials are pushing the useful range for liquid crystals further into the IR. Recent improvements in the MIR band (λ = 3 to 5 μm) are opening opportunities for hardware-in-loop testing of IR sensors and modalities for chemical sensing based on IR vibrational spectroscopy.
Finally, manufacturers continue to
improve the power handling of LCOS SLMs. Some components in this class have recently demonstrated robust modulation of >1-kW average laser power (Figure 5). In terms of applications, beam shaping for laser manufacturing and laser accelerators is among the most obvious beneficiaries to these increases in the average and peak power handling levels of LCOS SLMs.
Figure 5. Liquid crystal on silicon (LCOS) spatial light modulators (SLMs) are used with high-power lasers. The thermal image of a Meadowlark Optics 1024- × 1024-pixel SLM under steady-state illumination of 1-kW average power at 1070 nm (left). A plot of phase modulation versus grayscale pixel value shows a full 2π of phase modulation maintained at up to 1.4 kW under the conditions described (right). Courtesy of Meadowlark Optics.
Geometric phase optics
The phase modulators previously described rely on changing the effective refractive index along the optical path. Liquid crystals can also be used to induce another sort of phase delay that occurs when two optical paths undergo different transformations of polarization between the same beginning and ending polarization states. This delay is known as the Pancharatnam-Berry phase, or geometric phase. It is realized in practice by patterning a liquid crystal waveplate so that its
slow axis orientation varies across the surface of the optic. As a result, these geometric phase optics are also sometimes referred to as diffractive waveplates.
Geometric phase optics exhibit two fundamentally distinguishing properties. First, their theoretical diffraction efficiency is 100%, with diffraction efficiencies ≥99.5% routinely achieved in practice. This is because a geometric phase grating provides a smooth phase function — without the discontinuities
of blazed gratings. Second, geometric phase optics are inherently polarization-sensitive and typically designed so that one handedness of circularly polarized light diffracts into the +1 order of the grating while the opposite handedness diffracts into the −1 order.
When the slow axis varies linearly across the optic, the resulting polarization grating can be used to steer light by
controlling the handedness of the incident polarization. By stacking up many LCVRs and polarization gratings, it is possible to create transmissive electro-optics to nonmechanically steer light to many discrete angles with both large
optical apertures and large steering angles. These enabled capabilities, along with the inherent ruggedness and low power consumption associated with these optics, make beam steering using polarization gratings a popular choice for steering long-range lidar sensors and free-space optical communications systems (Figure 6)4-5. Steering large optical apertures also permits the steering of large beams for long-range applications and enables handling of high laser powers when low-loss coatings are used.
Figure 6. Nonmechanical beam-steering assemblies using polarization gratings and liquid crystal variable retarders (LCVRs). Polarization gratings direct light based on the handedness of circular polarization (left). By combining them with LCVRs to control polarization, these gratings can be used to make transmissive nonmechanical beam-steering assemblies. The two nonmechanical beam-steering assemblies, designed for lidar applications with >50-mm clear aperture and 120° × 25° field of regard (right). Courtesy of Meadowlark Optics.
In the same manner, if the slow axis
rotation varies radially instead of linearly, it is possible to generate polarization lenses that focus or defocus light depending on the incident polarization state. These lightweight and flat lenses have applications in wearable displays for
augmented, virtual, and mixed reality
as well as nonmechanical refocusing of imaging systems. More complex patterning of the retarder axis can lead to a
wide range of useful geometric phase optics, such as highly efficient vortex waveplates used in coronagraphs to reveal exoplanets orbiting distant stars. BEAM Engineering for Advanced Measurements Co. (BEAM Engineering) in Orlando, Fla., has pioneered this challenging application of liquid crystals. Meadowlark Optics and BEAM Engineering are among the liquid crystal technology companies that have trialed and achieved additional useful applications of polarization gratings4,6-8.
The view ahead
The durability of liquid crystals heightens their importance as components for
polarization and phase control in precision optical and laser systems, especially those with high powers. They enable beam steering, polarimetry, wavefront shaping and correction, polarization switching for shuttering, and variable flux control.
Importantly, the use of liquid crystals allows all these functions to be achieved nonmechanically without the use of
moving parts. It is therefore apparent in theory and in application that liquid crystal optics are ideal for rugged environments.
And, in addition to uniform, single-pixel devices, a host of applications are enabled by SLMs consisting of a million or more individually electrical addressable pixels. With emerging applications in optics, integrated photonics, and materials science (as well as in displays) the photonics community and those who use photonics products should expect further increases in the use of liquid crystals in the coming years.
Meet the authors
Tom Baur is the founder and board chairman of Meadowlark Optics. He focuses on product development, bringing new materials to the photonics community for polarization control and improving metrology for polarization
components; email: [email protected].
Christopher Hoy is director of business
development at Meadowlark Optics. He
focuses on developing new opportunities
for Meadowlark polarization solutions and oversees contract R&D activities; email: [email protected].
Acknowledgment
The opening image shows custom liquid
crystal modulators designed by scientists at
the National Ignition Facility (NIF) of the
Lawrence Livermore National Laboratory (LLNL) and built by Meadowlark Optics. The components were developed and deployed for the Laser Energy Optimization by Precision
Adjustments to the Radiant Distribution (LEOPARD) project, managed by LLNL.
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