EP2016459A1

EP2016459A1 - Liquid crystal devices

Info

Publication number: EP2016459A1
Application number: EP07732527A
Authority: EP
Inventors: William Alden Crossland; Neil Collings; Paul Andrew Robertson
Original assignee: Cambridge Enterprise Ltd
Current assignee: Cambridge Enterprise Ltd
Priority date: 2006-04-24
Filing date: 2007-04-24
Publication date: 2009-01-21
Also published as: TW200801758A; WO2007122409A1; JP2009534974A; KR20090057940A; GB0608055D0; CN101449203A; BRPI0710770A2

Abstract

An electrical component comprising a substrate (30) carrying a LC cell (10) wherein the LC cell (10) comprises liquid crystal material (20) containing anisotropic particles (25), at least one conductive member (36) disposed on the substrate (30) and at least one conductive member (34) disposed over the LC cell (10), and means for affecting the alignment of the anisotropic particles (25) whereby the permittivity between the conductive members (34, 36) is varied.

Description

Liquid Crystal Devices

The present invention relates to an electrical component, a planar waveguide, an antenna, a beam shaper and to an electrically tunable dielectric.

Embodiments are suitable for use in the terahertz frequency or millimetre wavelength region; others for Very High Frequency (VHF), Ultra High Frequency (UHF) and microwaves.

In one aspect, the present invention provides an electrical component comprising a substrate carrying a Liquid Crystal (LC) cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

The component may be used in frequency agile antennas, steerable antennas, tuneable filters, polarisation variable antennas, voltage controlled oscillators, variable delay lines, automatic impedance matching circuits and active temperature compensation for microwave circuits.

hi another aspect the present invention provides a planar waveguide comprising a substrate carrying a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

In a further aspect the present invention provides an antenna comprising a substrate carrying a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

In a still further aspect the invention provides a beam shaper for free space propagation of Terahertz frequency waves comprising a substrate carrying a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

The means for affecting the alignment may comprise the conductive members.

In a yet further aspect the invention provides an electrically tunable dielectric comprising a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, and control electrodes for affecting the alignment of the anisotropic particles.

The LC cell may comprise Polymer Dispersed Liquid Crystal (PDLC) material.

The anisotropic particles may comprise Carbon Nanotubes (CNT).

The present invention will be more clearly understood by reference to the following description, together with the accompanying drawings, in which:

Fig Ia is a diagram of a liquid crystal cell showing anisotropic particles in "no field" conditions;

Fig Ib is a diagram of a liquid crystal cell showing anisotropic particles in "field on" conditions Fig 2 shows a cross-sectional view of a test cell;

Fig 3 shows a perspective diagram of a patch antenna embodying the invention; Fig 4 shows a cross sectional view along lines III-III ' of Fig 2;

Fig 5 shows a cross section through a stripline waveguide embodying the invention;

Fig 6 shows a beamshaper embodying the invention.

Referring to Figs Ia and Ib, a liquid crystal cell 10 has a layer 20 of liquid crystal material that has anisotropic particles 25 dispersed within it. hi this embodiment, the particles are CNT. In other embodiments, dye doping is used. In yet other embodiments, the liquid crystal material is a PDLC material without CNT. The liquid crystal material 20 is bounded by opposing generally flat and mutually parallel glass substrates 30, 32 carrying Indium Tin Oxide (ITO) electrodes 34, 36, and the substrates are spaced by spacers 33, 35. The electrodes are accessed via conductors 37, 38.

The spacing s between the substrates is substantially less than the extent e of the substrates.

In Fig Ia, the anisotropic particles 25 are aligned generally parallel to the substrates by the liquid crystal material 20.

In Fig Ib, a field is applied between the ITO electrodes 34, 36. to cause the molecules of the liquid crystal material 20 to tilt, and to thereby draw the particles 25 into a rotated position- here about 35 degrees on average to the plane of the substrates. The angle of 35 degrees is not an essential feature of this apparatus. A change in the alignment of the anisotropic particles results in the permittivity between the ITO electrodes 34, 36 being varied.

The birefringence of a range of commercial liquid crystals in the millimetre wave region has been shown by K C Lim et al [Liq Crystal 14(1993) p327-337] to be 46-67 per cent of visible region values. Moreover, custom liquid crystals have been synthesized with enhanced dielectric anisotropy (Δε=l) and tan δ losses similar to FR4 in the mm wave region (see Weil et al [Electronic Letters 39(24) 1732-4. (2003)]).

In making experimental devices according to the present invention, a variety of materials systems were used for making the liquid crystal layer, based on a nematic host and carbon nanotube materials. A suitable material is fabricated by mixing carbon nanotubes and a liquid crystal. The carbon nanotubes are dispersed within the liquid crystal by subjecting the mixture to sonication.

Referring to Fig. 2, a test cell 400, thicker than conventional test cells (up to lmm) has a support substrate 401, supporting a copper ground plane 410 that extends across its upper surface. A liquid crystal material layer 420 is disposed on the ground plane, and a cover layer 430 is supported by spacers 435 to define the liquid crystal cell, hi other cells, a polymer is used instead of glass. Top electrodes 440 are of copper- e.g. microstrip copper track. They have a width w and the LC cell has a thickness of h, so that the characteristic impedance Z₀ of the micro-strip transmission line, of which the top electrodes are a part, is defined by

1/2.

Z₀ ~ h / [(w + 2h) 6₀ c₀ €_τ ^ιu] and

Electrical length, βL ~ 2pL/λg with λ_g = c_o / [f e_r l/2η ]

hi order to electrically switch the liquid crystal and tune the device, the following liquid crystals are suggested: electrically controlled birefringence; and two-frequency nematics.

hi one embodiment, to extend the tuning range, carbon nanotube (CNT) doping of the liquid crystal host was employed, since prior work on dispersing anisotropic metallic microtubules in a liquid crystal host has shown an increase of greater than 50% of the birefringence of the liquid crystal at 30 GHz due to a 0.2% dispersion, (see AM Lackner et al. [Liq. Crystal. 14 (1993) 351-359]). Ordered CNTs in organic media are also fundamental to other applications of CNT composites with liquid crystals or polymers or reactive monomers in such applications as conducting polymers or optical non linear materials for holography.

It is necessary to obtain a good and stable alignment of the CNT so that the dielectric anisotropy is maximised. Functionalisation techniques of the CNTs in organic media (e.g. including liquid crystals and reactive monomers) used chemical modification (covalent functionalisation), polymer wrapping (noncovalent functionalisation), and using surfactants for organic media. Other problems include understanding the detailed spectral behaviour of the absorptive and dielectric properties.

It is envisaged that electro-optic devices made from these materials have applications ranging from the manipulation of radiation e.g. in medical imaging, adaptive antennae in microwave, radio and radar applications e.g. satellites and mobile phones.

Embodiments envisage the use of polymer dispersed LCs (with or without CNT doping) to realise a 'solid' substrate on which microwave circuits can be fabricated, akin to existing PTFE and fibreglass substrates, suitable for microstrip, stripline, slotline and other planar waveguiding techniques. This version enables simpler fabrication of the microwave circuits, at the expense of a narrower tuning range, as compared to the LC 'cell' type structure in the proposal. In some cases, a limited tuning range may be preferable, e.g. for fine tuning an oscillator.

An example for radio LANs and/or mobile phones is a steerable antenna, for example to enable a higher density of users in a given area, rn the case of phones, a steerable antenna allows reduction in radiation exposure by the user. Other non-limiting examples of applications of the invention are microstrip & stripline circuits, phase shifters, matching circuits, tuneable and steerable patch antennas, filters and circulators.

Referring to Figs. 3 and 4, a patch antenna 100 has a dielectric substrate 110 with a copper electrode 114 on its upperside. A liquid crystal cell 120 is disposed on the upper side of the substrate 110 over the copper electrode 114. It is bounded on its four sides by spacers 124 composed of a glue seal with ferrite particle loading. Liquid crystal material 128 forms the active material, and comprises CNT dispersed in use Merck BL037, in this embodiment.

On top of the liquid crystal cell is a copper electrode 122 forming the patch. The electrode 122 is of a suitable thickness to minimise conduction losses over the frequency of operation in order to achieve an acceptable Q-factor.

A connecting conductor 140 enables energy to be input to or extracted from the patch 122.

hi use the ferrite particles form a magnetic field boundary thus helping eliminate crosstalk. High frequency, e.g. gigahertz, signals are applied to the patch 122, and a lower frequency (e.g. dc) bias is applied to control the permittivity of the CNT-doped LC 128. The bias may be actual dc or may be a low frequency varying potential, bearing in mind that the LC material response time is in the order of milliseconds.

Turning to Fig 5, a first waveguide or transmission line 220 is formed on a dielectric substrate 210 having an earth plane 214 on its upperside, and a copper conductor 226 forming an earth plane. CNT doped LC material 228 forms a cell bounded at the side by spacers 224 and bounded above by an upper substrate 218 supporting a copper line electrode 222 on its underside.

Operation is generally similar to operation of the first embodiment.

A second transmission line embodiment is shown in Fig 6. In this embodiment the line electrode 222 is bounded on both its upper and lower surfaces by liquid crystal material, and is between two earth planes 220,224. Other waveguide and similar structures are envisaged. Turning to Fig 7, a beamshaper 300 consists of a generally square matrix of patch antenna electrodes 310, spaced apart in a plane and spaced above a backplane 301. In some embodiments only a small number of antenna elements are needed, for example 4 or 5 elements.

In Fig 8, an alternative arrangement to Fig 1 is shown which is relevant when using two frequency materials. The figure illustrates the situation where only low frequency fields and only high frequency (much faster than the response time of the liquid crystal material) fields are applied.

In Fig 8a, the anisotropic particles 25 are aligned substantially perpendicular to the mutually parallel glass substrates 30, 32. This is caused by the low frequency electric field applied between the ITO electrodes 34, 36. The near perpendicular alignment is purely illustrative.

hi Fig 8b, the anisotropic particles 25 are aligned substantially parallel to the mutually parallel glass substrates 30, 32. This is caused by a high frequency electric field applied between the ITO electrodes 34, 36.

hi one embodiment each patch electrode 310 has its own liquid crystal cell beneath it; in another less preferred version a single liquid crystal cell is provided. Conductors 304a-d feed both signal and bias to each patch electrode. The bias causes the value of permittivity for each patch to be set so as to vary the signal distribution and direct a radiated beam in known fashion.

Embodiments of the invention have now been described. The invention is not however to be taken as limited to details of the embodiments.

Claims

1. An electrical component comprising a substrate carrying a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

2. A planar waveguide comprising a substrate carrying a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

3. An antenna comprising a substrate carrying a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

4. A beam shaper for free space propagation of terahertz frequency waves comprising a substrate carrying a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, at least one conductive member disposed on the substrate and at least one conductive member disposed over the LC cell, and means for affecting the alignment of the anisotropic particles whereby the permittivity between the conductive members is varied.

5. An electrically tunable dielectric comprising a LC cell wherein the LC cell comprises liquid crystal material containing anisotropic particles, and control electrodes for affecting the alignment of the anisotropic particles.

6. An electrical component according to claim 1, a planar waveguide according to claim 2, an antenna according to claim 3, or a beam shaper according to claim 4, wherein the means for affecting the alignment of the anisotropic particles comprises the conductive members.

7. An electrical component according to claim 1, a planar waveguide according to claim 2, an antenna according to claim 3, a beam shaper according to claim 4, or an electrically tunable dielectric according to claim 5, wherein the LC cell comprises PDLC material.

8. An electrical component according to claim 1, a planar waveguide according to claim 2, an antenna according to claim 3, a beam shaper according to claim 4, or an electrically tunable dielectric according to claim 5, wherein the anisotropic particles comprise CNT.