Removal of pitch jumps from a chiral nematic liquid crystal
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Removal of pitch jumps from a chiral nematic liquid crystal Alison Ford, Stephen Morris and Harry Coles CMMPE, University of Cambridge, Department of Engineering, 9 JJ Thomson Avenue, Cambridge, CB3 0FA. Background.

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Removal of pitch jumps from a chiral nematic liquid crystalAlison Ford, Stephen Morris and Harry ColesCMMPE,University of Cambridge,Department of Engineering,9 JJ Thomson Avenue,Cambridge, CB3 0FA


Background
Background

In this poster we present a new method for eliminating pitch jumps from a chiral nematic liquid crystal when subjected to a variation in temperature. The transmission spectra are examined as a function of temperature for three different chiral nematic liquid crystals. E49 (commercially available from Merck) is used as the nematic host for two of these samples, each doped with a different chiral dopant. The temperature dependence of these two samples exhibit opposite trends and distinct discontinuous changes in the reflection wavelength. This discontinuous change is known as a pitch jumps. A combination of the two chiral dopants into the same host can eliminate the pitch jump effect for these samples.

  • When used for applications such as temperature sensors, these pitch jumps are desirable because they indicate a clear discrete change in temperature. A good example of such an effect can be observed in fore-head thermometers, as shown.

  • In an unbound state, the pitch of a chiral nematic can vary continuously as the temperature is varied. However, when subjected to boundary conditions such as those imposed by an alignment layer within a cell, the pitch varies discontinuously, termed pitch jumps.

  • Pitch jumps arise due to a competition of forces between the bulk liquid crystal and the surface anchoring condition. The helical structure reorients from N turns to N  1 turns. This effect has been studied in detail by Belyakov and Zink.

  • However, pitch jumps are not always preferable. For applications such as the chiral nematic band edge laser, where lasing occurs at the band edge (as shown in the figure on the right), a continuous variation in wavelength with temperature is preferred.

N turns

N -1 turns

H.Zink, V.A.Belyakov, J. Exp. Theor. Phys, 85 (2), 285, 1997

V.A.Belyakov and E.I.Kats, J. Exp. Theor. Phys,91 (3), 488, 2000

V.A.Belyakov, P.Oswald and E.I.Kats, J. Exp. Theor. Phys, 96 (5), 915, 2003

  • Pitch jumps can be eliminated by increasing the cell thickness or decreasing the surface anchoring energy of the cell.

  • However, both of these methods decreases the quality of the liquid crystal laser emission and therefore is not a satisfactory solution.

    P.Shibaev, V.Kopp, A.Genack and E.Hanelt, Liq. Cryst. 30, 1391, 2003

    S. M. Morris, A. D. Ford, B. J. Broughton, M. N. Pivnenko and H. J. Coles, Proc. SPIE,5741, 118 (2005)


Experimental techniques
Experimental Techniques

A single host was used, E49 + D1* which exhibited the following phase transition temperatures (measured on cooling):

I – (104 ºC) – N + I – (100 ºC) – N – (< 30 ºC) – Cr

The sample was filled into a planar aligned 7.5 mm thick cell and the transmission spectra were measured as a function of temperature. A typical transmission spectrum is shown in the figure on the right.

The central wavelength (l) is defined according to:

where n is the average refractive index and P is the pitch.

l = nP

A micrograph showing the discontinuous change in colour indicating a pitch jump.

Temperature dependence of the refractive indices

Temperature dependence of the pitch

The temperature dependence of the long and short wavelength band edges is shown in the figure on the right. The regions of continuous wavelength change are a result of the temperature dependence of the refractive indices whilst the discontinuous changes are due to the temperature dependence of the pitch.

On heating a N*LC, the pitch can either increase or decrease due to two competing mechanisms: thermal expansion causing an increase in the pitch and an increase in the average angular separation of molecules along the helix axis causing a decrease in the pitch. The dependence also depends on the other mesophases. For example on cooling a sample with an underlying smectic phase, the pitch will diverge on approaching the smectic phase.


A single nematic host with different chiral dopants
A Single Nematic Host with Different Chiral Dopants

E49 + D2*

E49 + D1*

  • E49 was used as the nematic host for two chiral nematic samples. Each sample was doped with the same percentage of a different chiral dopant, D1* and D2*.

  • The chiral dopant D1* exhibits a higher helical twisting power than D2*. This is apparent from the figure on the right.

  • The transmission spectra were measured as a function of temperature from which the temperature dependence of the long wavelength band edge was measured and is shown in the figure on the right.

  • It is clearly shown that the two samples exhibit different temperature dependencies whereby a decrease in temperature induces a pitch dilation for E49 + D1* whilst E49 + D2* exhibits a pitch contraction.

Different Nematic Hosts with a Single Chiral Dopant

  • The chiral dopant D2* was doped into two different nematic hosts, E49 and M21.

  • The transmission spectra were measured as a function of temperature from which the temperature dependence of the long wavelength band edge was measured and is shown in the figure on the right.

  • These two samples exhibit different temperature dependencies. A decrease in temperature induces a pitch contraction for E49 + D2* whilst M21 + D2* exhibits a pitch dilation.

E49 + D2*

M21 + D2*

  • The temperature dependence of the chiral nematic liquid crystal is not determined by the chiral dopant but rather by the combination of the chiral dopant and the nematic host.


Pitch jump elimination
Pitch Jump Elimination

E49 + D2*

E49 + D1*

A chiral nematic sample was prepared by doping E49 with D1* and D2*. The temperature dependence of the transmission spectrum was measured and the results are shown in the right hand figure below. These results shown that a combination of the two different chiral dopants has eliminated the pitch jumps exhibited by the two individual samples (as shown in the left hand figure below).

E49 + D1* + D2*

Conclusions

In this poster we have shown that the temperature dependence of a chiral nematic liquid crystal is determined by the combination of the nematic host and the chiral dopant. Pitch jumps that arise due to a competition of forces can be eliminated by using a combination of chiral dopants that exhibit opposite temperature dependencies in the same nematic host. This is a simple process for eliminating pitch jumps, whilst maintaining the high quality alignment and strong anchoring energy associated with a thin cell and planar alignment.


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