Multiwall carbon nanotubes act as an addressable 3-D electrode structure to control an array of microlenses in a liquid crystal cell, extending the capability of wavefront sensor technology.
Dr. Tim Wilkinson, University of Cambridge, UK
An exciting breakthrough combines liquid crystals with vertically grown carbon nanotubes to create a reconfigurable three-dimensional liquid crystal device structure. This offers completely new ways to control molecules in liquid crystals, allowing the crystals to move in a variety of directions to create optical components such as lenslet arrays. This technology is still in the early phase of development, but recent trials indicate that potential applications exist in adaptive optical systems such as the wavefront sensors used in optometry, digital video cameras, optical diffusers and emerging head-up display devices.
The sketch on the left shows a carbon nanotube (CNT) electrode in a liquid crystal (LC) cell with no external field; the diagram on the right shows how a positive external voltage (V) creates an electric field (E-field) in the shape of a microlens.
The technology comprises an array of electrodes made from individual vertically aligned multiwall carbon nanotubes. The very high conductivity of the carbon nanotube enables it to act as an electrode, while its needlelike shape creates a strong localized electric field that influences the behaviour of the molecules within the surrounding layer of liquid crystals, forming microlenses. An array of 1000 × 1000 microlenses occupies a chip just 10 × 10 mm in size.
This diagram simulates the electrical field profile surrounding the single carbon nanotube (10 μm high) with an applied field of 1 Vm–1.
The upper electrode acts as an Earth plane for the electric field. It consists of a transparent conducting material — indium tin oxide — on glass, with an alignment layer of low pretilt polyamide that causes the liquid crystal molecules to line up parallel to the upper substrate surface in the absence of an electric field. With an applied voltage, each nanotube electrode creates a Gaussian electric field profile, which reorients a planar aligned nematic liquid crystal. The molecules of the liquid crystal are dielectrically anisotropic, so they experience a torque and change their alignment in response to the electric field. This creates a variation in refractive index within the liquid crystal layer, which acts as a graded index optical element. Changes in the electric field applied via the carbon nanotube control this element, creating a variable-focal-length lenslet array. The ability to combine the microlenses and address them as a pure three-dimensional electrode structure makes this a very exciting concept. It enables the creation of complex reconfigurable refractive index profiles such as a perfect hologram or kinoform.
The 3-D liquid crystal technology was developed at the University of Cambridge’s Centre for Applied Photonics and Electronics, with help from Bill Milne. The work began several years ago, and the challenge was to grow fully conducting nanotube electrodes on a silicon substrate. The aspect ratio of the nanotube is critical in providing the correct field profile, and these multiwall carbon nanotubes are about 2 μm long and 50 nm in diameter. Furthermore, the tube has multiple walls: There are several concentric single-wall carbon nanotubes in every multiwall one, rather like an onion in which each skin is a single layer of graphene rolled into a tube. The 50-nm tubes contain about seven layers. This is very important in overcoming the practical problem that one in every three single-wall carbon nanotubes is semiconducting.
Shown is a simulated field profile of an array of nanotube electrodes at 1 Vm–1.
The multiple layers guarantee electrical conductivity. Electron-beam lithography creates a pattern of dots on a 5-nm-thick nickel catalyst layer on a silicon wafer, and plasma-enhanced chemical vapour deposition grows a multiwall nanotube on each dot, while mild heating of the substrate protects the catalyst from cracking.
There also was a challenge associated with the liquid crystal integration to allow electrical contact with the multiwall carbon nanotubes. The solution was to sputter 400 nm of aluminium onto the array, providing a common electrical connection to all the nanotubes and acting as a mirror for the optics. A thin layer of silica on top of the aluminium stops any background bulk inductive-capacitive switching effect.
We modelled the electric field profile in three dimensions — using the finite element technique — and found that the width of the field is the same order of magnitude as the height of the nanotube and that it is circularly symmetrical about it. When we grow small groups of two, three or four nanotubes within 1 μm of each other, we cause the field profiles to overlap, enabling a larger optical aperture for each lenslet. This provides the opportunity for new applications that, until now, would have been regarded as technically challenging.
This electron microscopy image shows the carbon nanotubes growing in a sparse grasslike array, with four tubes at every site.
An example of where this technology might play an important role is in improving wavefront sensors. The telecoms and optometry industries are making demands on optics that are increasingly becoming more difficult to fulfil. The current available technology is almost at its limit. The liquid crystal lenslets provide a more versatile, simpler and cheaper process that can reconstruct the wavefront dynamically into a more perfect image for detection.
Tim Wilkinson uses an Olympus BR11 microscope to display the switching of a lenslet array in his laboratory at the University of Cambridge. Picture credit: Phil Hands.
The next step is commercialization of the technology, which is under the management of Cambridge Enterprise Ltd., the commercialization office of the University of Cambridge. That company is interested in speaking with potential partners to investigate which applications will be the first adopters of the 3-D liquid crystal technology.
Contact: Dr. Robert Fender, Cambridge Enterprise Ltd., University of Cambridge, UK; fax: +44 1223 764 888; e-mail: email@example.com.