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AN ELECTRON-BEAM CONTROLLED SLM BASED ON A BISTABLE NONLINEAR INTERFERENCE
FILTER
A. Walker, S. Smith, R. Campbell, J. Mathew
To cite this version:
A. Walker, S. Smith, R. Campbell, J. Mathew. AN ELECTRON-BEAM CONTROLLED SLM
BASED ON A BISTABLE NONLINEAR INTERFERENCE FILTER. Journal de Physique Collo-
ques, 1988, 49 (C2), pp.C2-47-C2-50. �10.1051/jphyscol:1988211�. �jpa-00227620�
AN ELECTRON-BEAM CONTROLLED SLM BASED ON A BISTABLE NONLINEAR INTERFERENCE FILTER
A.C. WALKER, S.D. SMITH, R.J. CAMPBELL and J.G.H. MATHEW Department of Physics, Heriot-Watt University, Riccarton, GB-Edinburgh EX14 4AS, Scotland, Great-Britain
Abstract
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A nonlinear interference filter has been incorporated into a CRT and e-beam controlled switching of individual elements in a 2 x 2 array of optically bistable pixels demonstrated.1
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INTRODUCTIONOptothermal optically nonlinear interference filters (NLIFs) have been exploited as bistable switches for some years /1-3/. Recently we have developed an electron-beam tuned interference filter spatial light modulator (ETIF-SLM) based on such nonlinear elements /4/. A number of ETIF-SLM devices have been made for operation at 514 nm and 633 nm wave- lengths. These prototype devices were based on standard, Thorn-EM1 type 8541, vidicon tubes which had, in place of the photo-cathode, the NLIF deposited on the inside surface of the faceplate prior to sealing onto the vacuum tube (see Figure 1). The NLIFs
incorporated ZnSe spacers and were matched by a h/4 TiO, layer into an optically opaque Si layer (to make a BEAT type of construction /5/) and the final top layer was % 100 nm of A1 added to provide a return current path. Figure 2 shows the normal-incidence reflection spectrum of both types of filter, around their working wavelengths, after sealing into the tube. For these initial studies the e-beam power was increased by opening the gun
aperture to give 0.1 - 1 mA of beam current and the target voltage set to 500 V
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1 kV,providing 50 mW to 1 Watt of e-beam power.
VACUUM TUBE ELECTRON BEAM Al COATING NLIF
CATHODE ELECTRON FOCUSSING AND GLASS FACE-PLATE
DEFLECTION (AR-COATED)
512 516 520 524 526 6 2 6 6 3 0 634 6 3 6 WAVELENGTH nrn
Fig. 1. Schematic of the ETIF-SLM. Fig. 2. Normal incidence reflection spectra for the two tves of NLIF used.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988211
C2-48 JOURNAL DE PHYSIQUE
Two modes of operation of the ETIF-SLM have been demonstrated: optically-linear and optically-nonlinear. In the optically-linear case the read beam is of insufficient irradiance to induce optical tuning of the interference filter and instead provides a purely passive interrogation of the SLM reflectivity. In the optically-nonlinear mode higher optical irradiances are used (e.g. by breaking up the read beam into an array of focussing beamlets) in order to bias the NLIF into the strongly nonlinear operating regime. If each pixel is held close to a bistable switch then the e-beam can be used to set their state (high or low reflectivity) as required. The SLM then has the interesting property of storing a binary image for as long as the read beam is held on.
Alternatively, by operating in the (nonlinear) transphasor regime / 6 / , in which instead of bistable switching there exists a region of differential gain, then a (non-latching) grey- scale image could be created with a minimum of e-beam input power.
Figure 3 shows examples of the optically-linear mode of operating using 633 nm radiation from a He-Ne laser. In the first experiment the SLM was set up on-resonance, by adjusting the angle of incidence to give minimum reflectivity. The effect of the scanned e-be&,. \iss then to tune the filter off-resonance and hence induce a locally higher reflectivity. The trace reproduced in Fig. 3a was obtained by projecting the light reflected from the SLM surface to an image plane containing a pinhole and detector. As the e-beam was slowly scanned across the corresponding point in the object plane the variation in reflectivity was recorded. A contrast of 11:l was obtained - in good agreement with the spectrum of the filter (Fig. 2b). Fig. 3b shows the equivalent result for a second test in which the initial detuning of the filter was set to a wavelength slightly less than 633 nm. In this case, as expected assuming a simple shift of the peak to longer wavelengths, the
reflectivity initially decreased before finally increasing. These results were obtained using beam curLents of 0.18
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0.36 mA and a target voltage of 1 kV.TIME (mS) TIME (mS)
Fig. 3. Variation of the reflectivity of the ETIF-SLM (633 nm) as the e-beam is scanned through the monitored region for (a) zero and (bf approximately - 1 nm initial detuning.
The optically nonlinear mode of operation was demonstrated using the SLM designed for 514 nm. A 55 mW beam from an argon-ion laser was focussed to a % 30 pm diameter spot which biased the filter into its bistable region. Thus on scanning the e-beam through the illuminated regfion a latching switch action was obtained, as shown in fig. 4. The
switching time of 2. 1 ms was limited by the relatively large e-beam spot-size and slow scan-rate: 1.7 w ps-l. The reflection transfer-characteristi2 of the NLIF is shown in figure 5 along with the modified response obtained on applying rle electron beam. The shift in the bistable region is the result of the change induced by the e-beam in the detuning of the intial peak resonance from the optical wavelength. It can be seen that not only can a switch to the on-resonant (low reflection) state be induced by the e-beam but also, by biasing both optically and electronically, a switch in either direction can be initiated by an increase or decrease',of the e-beam current. Thus individual pixels in a large array could be switched on or off, as necessary, without disturbing the remainder.
This is demonstrated in Figure 6 where a pixel was repeatedly switched from one state to the other by varying the beam current.
INCIDENT POWER (mW1
Fig. 4. Latching response induced by the Fig. 5. Reflection characteristics with scanning electron beam. the (stationary) e-beam off (solid line)
and on (broken line).
To demonstrate the basic concept of a bistable ETIF-SLM, a simple 2 x 2 pattern of light spots (514 run) was focussed onto the NLIF using a holographic lenslet array /7/. The focal spots were each % 50 p n in diameter and arranged at the corners of a 2 mm square.
It was shown that the e-beam could switch specific elements in this simple array while leaving the others in their original state.
TlME (s)
Fig. 6. Repeated switching of a pixel by varying the incident e-beam power. The highly reflective (off resonance) state corresponds to a decrease in current while the low reflectivity (on resonance) state is produced by an increase in the current.
It is anticipated that much larger pixel arrays will require reticulation of the NLIF, as discussed by Walker et a1 /7/. Such physical pixellation will minimise both cross-talk, between close-packed elements /8/, and reduce operating powers possibly down to 10 pJ w-2
/6,7/. Thus 10 x 10 pm pixels could operate with 100 pW optical bias power and 10 ps time constant
(TI.
This implies an optical input power requirement for, say, 105 pixels of 10 watts and a switch energy of % 1 nJ per pixel. For a pixel, optically biased into the bistable region, to be switched, the electron beam need only deposit a significant fraction of this energy while scanning over it. This can be accomplished in times much shorter than r , as demonstrated previously when using 30 ps laser pulsed to switch a NLIF device /9/. Thus an e-beam current of 1 mA with a 10 kV target voltage (10 watts average power) would be able to address 105 such pixels in 10 ps. An overall frame-time of 30 ps(37) would be sufficient to ensure the establishment of equilibrium before clocking the next processor stage and preparing to input new data to the ETIF-SLM. This would be compatible with an overall data processing rate of 1. 3 x 10P bits s-* cme2
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a factor of 1.100 times higher than existing liquidcrystal light-valve SLMs.
C2-50 JOURNAL DE PHYSIQUE
The authors acknowledge the valuable assistance of J. Wardley and colleagues at Thorn-EM1 Electron Tubes Ltd., and the financial support of the European Commission (CODEST) through their Stimulatj.on Action programme.
REFERENCES
/1/ Karpushko F.V. and Sinitsyn, G.V., J. Appl. Spectroscopy (USSR) 29, 1323 (1978).
/2/ Smith, S.D., Mathew. J.G.H., Taghizadeh, M.R., Walker, A.C., Wherrett, B.S., and Hendry, A., Optics Commun.
2 ,
357 (1984)./3/ Olbright, C.R., Peyghambarian, N., Gibbs, H.M., Macleod, H.A., and Van Milligen, F., Appl. Phys. Lett. 45, 1031 (1984).
/4/ Walker, A.C:. and Smith, S.D., Patent application no. 86 777, Luxembourgh (1987).
/5/ Walker, A.C:., Optics Commun. 59, 145 (1986).
/6/ Tooley, F.A.P., Smith S.D. and Seaton C.T., Appl. Phys. Lett. 43, 807 (1983).
/7/ Walker, A.C., Taghizadeh, M.R., Mathew, J.G.H., Redmond, I., Campbell, R.J., Smith, S.D., Dempsey, J. and Lebreton, G., Opt. Eng. 27, 38 (1988).
/8/ Abraham, E., Opt. Lett.
11,
689 (1986)./9/ Bigot, J.Y., Daunois, A., Leonelli, R., Sence, M., Mathew J.G., Smith, S.D. and Walker, A.C:., Appl. Phys. Lett.
