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Coherent Optical Sources using Micromolecular Ordered Structures: COSMOS

Objective:

Liquid Crystal Phases are unique phases of matter that are fluid yet have a coil-like structure over a wide temperature range as the material changes from an optically isotropic liquid, where the properties are the same in all directions, into a structured phase where the molecules spontaneously align so that their average direction of alignment twists around the coil or helix axis. The ends of the molecules trace out a path that would look like a coiled spring and these materials are called cholesterics. Light travelling along the direction of the helix axis experiences a layer like optical structure with a repeat distance equal to a half turn of the helix. If this is of the order of the wavelength of light then this leads to bright colour reflections, rather like those seen with soap bubbles. This effect is used in liquid crystal forehead thermometers where the materials are chosen to reflect colour changes as the helix expands or contracts. In these materials one can also apply voltages to expand or contract the helix and hence the reflected colours. Since light has wavelike properties and each colour corresponds to a specific length of the wave (or more correctly wavelength), if we add lots of waves together we can generate giant waves and this process is known as amplification. This is essentially the process used in laser devices where mirrors are used to trap waves and amplify the intensity or brightness of the light. If one mirror is slightly imperfect then this bright light of one specific wavelength leaks out along the axis between the mirrors. We use a very intense flash lamp of many wavelengths to get the light into the system in the first place. Also between the mirrors we place what is known as a gain medium that absorbs light of short wavelength and converts it to a longer wavelength. This is essentially the process used in day-glow inks where ultra violet or blue light (ie short wavelengths) is absorbed and through a process known as fluorescence converted to green or red (ie longer wavelengths) emitted light. So in a laser we have light absorption and emission with the mirrors selecting the longer wavelength output light. This is why most lasers used as pointers during lectures are red. What we have done recently is use the cholesteric mirrors to trap and emit light and by including a flourescent dye we have been able to make molecular liquid crystal lasers. Since we can change the reflections by applying electric fields we can change the emitted colour in a very controlled way. You may have noted that liquid crystal displays are very thin, the layer of liquid crystal within the device is less than the diameter of a human hair (i.e. less than 10 microns). This opens the way, using cholesteric liquid crystals, including fluorescent dyes, to make microscopic lasers. We can also make polymer rubbery cholesteric films where we can use mechanical force to change the reflected or emitted laser light. In this proposal we will make new cholesteric and polymer liquid crystals incorporating fluorescent dyes to make highly efficient lasers or light emitting systems that can then be incorporated into microchip devices. We hope to be able to make materials stable over a temperature range of -20 to +80 degrees C where we can switch the output colour with low voltages (less than 10 V in times shorter than a few 1000s of a second). We will study how the efficiency of these devices depends on chemical structure, optical, electrical and mechanical properties, as well as viscosity and elasticity. We will then make microchip and fibre-optic devices as well as larger area 2-D flat laser films. We have discovered highly stable cholesteric materials that have, peculiarly, helices at 3 directions at right angles. We will try to incorporate these into 3-D lasers or light emitting cubes. Once we have mastered control of the properties of these lasers we will explore their uses in medical, sensor, and telecommunications devices.

For further information visit http://www-g.eng.cam.ac.uk/cosmos/

 
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