Patent Application: US-63113505-A

Abstract:
a tuneable laser device comprises first and second cell walls enclosing a layer of a liquid crystal material having a helical axis substantially normal to the inner surfaces of the cell walls in the absence of an applied field . the liquid crystal contains a fluorescent , phosphorescent , luminescent or rare - earth dye . the device includes electrodes for applying a transverse electric field substantially normal to the helical axis . the invention also provides a method of electrically adjusting the peak wavelength of a photonic band edge laser comprising a chiral nematic liquid crystal material having a helical axis and a fluorescent , phosphorescent , luminescent or rare - earth dye therein and optically pumped by a suitable light source . the method comprises applying an electric field substantially perpendicular to the helical axis so as to deform the helix by means of the flexoelectric effect .

Description:
a sample was prepared using a non - symmetric bimesogen as the nematic liquid crystal host . the non - symmetric bimesogens α -( 2 ′, 4 - difluorobiphenyl - 4 ′- yloxy )- ω -( 4 - cyanobiphenyl - 4 ′- yloxy ) alkanes were synthesized in - house . the cyanobiphenyl mesogen and the 2 , 4 ′- difluorobiphenyl mesogens were connected by a flexible alkyl spacer . we have given the bimesogens the mnemonic ffonocb , where n corresponds to the number of methylene units in the flexible spacer . a preferred value for n is in the range 1 - 20 . in this study we have used ffo8ocb as the nematic host . the ‘ even ’ length spacer means that the bimesogen , ffo8ocb , can lay anti - parallel in the all - trans conformation . the chemical structure is given in fig1 . the nematic host was then mixed with a small concentration (˜ 5 wt %) of high twisting power chiral dopant ( bdh1281 , merck nb - c ) and the highly miscible laser dye , 4 -( dicyanomethylene )- 2 - methyl - 6 -( 4 - dimethylaminostyryl )- 4h - pyran ( dcm , lambda physik ) (˜ 2 wt %), the structure of which is shown in fig2 . the mixture was then heated in a bake oven at 150 ° c . for a period of twenty - four hours . from herein we refer to this mixture as dcm - doped ffo8ocb *. after mixing , the sample was then injected into a 7 . 5 μm - thick ‘ lucid ’ cell by means of capillary action . the substrates of the cell were substantially planar and coated with a rubbed polyimide layer to give an orientation such that the helix axis lies perpendicular to the planes of the glass substrates ( grandjean texture ). the transition temperatures and the chiral nematic phase were identified using optical polarising microscopy and a linkam hot - stage and controller . from optical polarising microscopy the clearing temperature , t c , of the chiral nematic dcm - doped ffo8ocb * was found to be 144 ° c . the experimental set - up for generating and detecting lasing is as follows . sample cells were illuminated with the 532 nm line from a q - switched nd : yag laser ( polaris ii , new wave research ). the beam was focused by an f = 10 cm objective giving a spot size with a diameter of ˜ 160 μm at the sample . in order to adjust the temperature , the sample cell was placed in a custom built heating element / stage , which was controlled by a conventional linkam controller . a fibre - optic bundle then collected the output and the emission was then resolved by a 0 . 04 nm - resolution spectrometer ( hr2000 , ocean optics ). an edge filter was used to separate the pump and the sample laser beams . emission energies of the liquid crystal laser were recorded using a high sensitivity energy meter ( laserstar , ophir ). all energy measurement results are averaged over 50 pulses . having confirmed lasing action in the plain 7 . 5 μm cell for the material involved , an electro - optic cell consisting of gold - deposited electrodes in the plane of the cell was filled and place in the pump beam . in this arrangement , the 10 μm thickness electrodes allow a uniform electric field to be applied perpendicular to both the helical axis ( with the material in the grandjean texture ) and also act as spacer elements onto which the lid of the cell is fixed . these electrodes are separated by a 50 μm wide channel into which the sample is capillary filled . the cell lid is pre - coated with a unidirectionally rubbed layer of ptfe , while the base of the cell is spin - coated with 1 % pva solution in h 2 o . while the electrodes on the base of the cell prevent directional rubbing of an alignment layer , it is found that these two layers in combination provide a grandjean texture in which to induce lasing . a microscope image of the transverse electrode electro - optic cell used is shown in fig3 and the 50 μm wide active region is boxed in the centre . the electro - optic cell was allowed to stabilise to the application of the pump pulse , such that a uniform lasing output from the sample was observed , at which point an electronic pulse , amplified from a signal generator ( tti ), was applied across the active area of the cell to coincide with the pulse from the pump beam . an oscilloscope trace showing the response of a photodiode to the pump pulse in relation to the applied electric field is shown in fig4 . it can be seen from this figure that a period of approximately 200 μs is allowed between the initial application of the electronic pulse and the incidence of the pump laser pulse . this is to allow the material to fully respond in its director deformation to the field before lasing is induced . our previous unpublished work on the flexoelectro - optic effect in the grandjean texture with non - symmetric bimesogens has shown the material response time to be of the order 100 μs 19 . fig5 shows typical reflection band and lasing emission spectra for pbe lasing at the gain maximum of dcm . the reflection band shown was obtained with the application of circularly polarized white light to a sample without dcm , to remove dye absorption effects which typically mask the short wavelength edge . in thin film cells subsidiary interference fringes are observed outside the reflection band , indicative of a well aligned monodomain sample . the figure also shows clearly the precise dependence of the lasing peak relative to the reflection band / photonic band gap . the peak occurs at the first absorption minimum at the long wavelength band edge . fig6 shows the excitation energy dependence of the total emission energy of the dcm - doped ffo8ocb * pbe laser at several different temperatures . the inset of fig6 allows for a closer inspection of the excitation threshold . at low excitation energies , spontaneous emission is observed , and for excitation energies greater than the lasing threshold , represented by the discontinuity in the differential , the total emission energy follows the familiar linear dependence with the input energy up until the saturation limit . at the highest excitation energies (& gt ; 40 μj / pulse ) the cell begins to degrade , although not irreversibly , and the total emission energy starts to drop . the helical pitch was about 350 nm , and is substantially temperature - invariant for these measurements . the thermal dependence of the operating efficiency observed for the pbe laser is also noteworthy . it is shown in fig6 that the operating efficiency decreases at elevated temperatures . since the pbe laser line remains within the spontaneous emission maximum of dcm ( 590 nm to 620 nm ), it is therefore unlikely that this is responsible for the remarkable performance - related temperature dependence . however , the thermal dependence of the operating efficiency of a thermotropic pbe laser can be accounted for by the temperature tuning of the emission efficiency and the quality factor of the chiral nematic 13 , 14 . for this reason field - controlled measurements using the electro - optic cell were carried out at a temperature where the operating efficiency was maximised . fig7 shows the lasing spectra obtained from the sample at a series of applied electric fields . a general trend of increasing red - shift of the lasing peak with applied electric field can be seen . the intensity of the lasing varies slightly due to systematic fluctuation in the response to successive pump pulses recorded by the spectrometer . however , it is thought that the large variation in intensity shown here is principally due to the deforming of the chiral nematic helix which provides the reflection band , by the applied electric field , and degradation of the grandjean texture . it is also thought that the latter is a major contributor to the increased spectral widths of the lasing emission observed in the electro - optic cell compared to that observed in the lucid cell 17 . upon removal of the field , the laser emission line returned immediately to its original zero - field spectral position and intensity . the extent of this red shifting can be clearly seen from the plot of peak laser wavelength against applied field shown in fig8 . the laser is tuned over a range of 8 nm for an electric field of 3 . 4 v / μm , the maximum available from the amplifier used , and the degree of tuning is precisely controlled by the magnitude of the applied field . in conclusion , we have demonstrated photonic band edge lasing in non - symmetric bimesogens which are known to have both high optic - axis tilt angles and fast response times when flexoelectrically coupled to an applied field . we have also demonstrated for the first time electronically controlled tuning of a chiral nematic pbe laser . it is thought that the tuning observed is a result of the flexoelectric deformation of the chiral nematic helix of the material , in a mechanism equivalent to the flexoelectric rotation of the optic axis of a chiral nematic in the uniform lying helix texture . in addition , we have found that a bimesogen with an even or odd number of methylene units in the alkyl spacer is suitable for photonic band edge lasing . the principal limitations to the electronic tuning observed so far are the relatively small field strengths applied , and the lack of complete monodomain uniformity of the sample texture . bimesogens can couple flexoelectrically to fields of up to about 20 v / μm before dielectric coupling becomes dominant , so larger tuning ranges ought to be achieveable with an improved signal amplifier . improvements in the cell alignment layer and annealing of the texture ought also to narrow the spectral width of the lasing peaks considerably , allowing greater resolution of the peak wavelength and its field dependence . it is appreciated that certain features of the invention which are , for clarity , described in the context of separate embodiments , may also be provided in combination in a single embodiment . conversely , various features of the invention which are , for brevity , described in the context of a single embodiment , may also be provided separately , or in any suitable combination . it is to be recognized that various alterations , modifications , and / or additions may be introduced into the constructions and arrangements of parts described above without departing from the scope of the present invention as set forth in the claims . 1 ) v . i . kopp , b . fan , h . k . m . vithana , a . z . genack , opt . lett . 23 , 1707 ( 1998 ) 2 ) a . munoz , p . palffy - 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