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PIXELLATION OF OPTO-THERMAL BISTABLE DEVICES FOR SWITCH POWER AND CROSSTALK
REDUCTION
A. Kar, R. Harris, G. Buller, S. Smith, A. Walker
To cite this version:
A. Kar, R. Harris, G. Buller, S. Smith, A. Walker. PIXELLATION OF OPTO-THERMAL
BISTABLE DEVICES FOR SWITCH POWER AND CROSSTALK REDUCTION. Journal de
Physique Colloques, 1988, 49 (C2), pp.C2-443-C2-446. �10.1051/jphyscol:19882105�. �jpa-00227615�
PIXELLATION OF OPTO-THERMAL BISTABLE DEVICES FOR SWITCH POWER AND CROSS- TALK REDUCTION
A.K. KAR, R.M. HARRIS*, G.S. BULLER, S.D. SMITH and A.C. WALKER
Department of Physics. Heriot-Watt University, Riccarton, FB-Edinburgh EH14 4AS. Scotland, Great-Britain
Edinburgh Instruments Ltd., Research Park, Riccarton.
GB-Edinburgh EN14 4AP, Scotland, Great-Britain
Abstract
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We report the reduction of switch power in an optically bistable nonlinear interference filter (NLIF) by pixellation. Pixellation was achieved by machining grooves on NLIF using an excimer laser. We also compare the experimental results with a theoretical model.1
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INTRODUCTIONOptotherrnal nonlinear interference filters (NLIF) can be operated as optically bistable devices /1-3/ and have been exploited to demonstrate proptotype optical digital circuits /4,5/. To realise the full processing potential of such devices, they must be configured in large arrays (e.g. up to lo3 x lo3) such that highly parallel interconnections can be achieved with the use of refractive or holographic free-space optics. We have noted previously /4,6,7/ that this is likely to require pixellation of the active region: i.e.
physical division into discrete areas acting as independent optical switch elements. This process consists of, for example, etching or laser micro-machining film and substrate.
Clearly, the substrate can be compound in which case the pixel so formed is made of a different material for the heat-sink as shown in Figure 1.
SEMI-INSULATING/"-, HIGH
SUBSTRAT HERMAL CONDUCTIVITY
INTERFERENCE FILTF
WORK-PIECE
UV. LENSES \
ARGON FLUORDE EXCWER LASER j193m)
r t L
COMPUTER-CONTRCiLED X-Y STAGE .RESOLUTION turn
Fig. 1
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Design for low cross-talk and Fig. 2-
Experimental arrangement for low switching energy optical logic micro-machining of NLIF.array (dimensions of the pixels are of the order of 1 p ) .
The objective of this pixellation is to inhibit transverse thermal conduction and, thus, reduce the effective size of each switch element. This leads to lower operating powers
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19882105
JOURNAL DE PHYSIQUE
and, by suppressing cross-talk, higher packing densities. This latter advantage is particula ly significant in interference filters where the nonlinear refractive mechanism /2,3/ d h e r m a . 1 , and hence a long range coupling occurs via heat diffusion. Thus
calculations have shown that with optical pixellation only, i.e. illumination of a uniform sample with an array of separate beams, packing densities are limited to 7 x 7 bistable elements operating independently in a uniform area of 4 cm2 /8/. By physically
pixellating the active area itself one could have bistable arrays of 250 x 250 pixels per square centimetre /7,8/. The insulating layer between the NLIF and the heat sink, which is to be pixellated, must be a non-crystalline material, for low thermal conductivity.
consequently direct etching, to form deep narrow grooves, cannot be achieved with chemical etching techniques. Instead, we have used a laser micro machining technique to process glass substrates, both before and after deposition of the multilayer device.
2
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MICRO-MACHINING OF NLIF USING EXCIMER LASERSThe experimental arrangement is shown in Figure 2. The output of the ArF excimer laser, operating at 193 nm and capable of delivering 160 mJ per pulse, was focussed using a series of research grade fused silica lenses. These lenses gave a 90% transmission at 193 nm. Aperturing was used to eliminate any higher order transverse modes and the beam was focussed on to the interference filter after spatial filtering. The interference filter was mounted on a high precision computer controlled X-Y translation stage which was capable of giving a resolution of 1 p.
Arrays containing up to 100, 10 - 30 p n ~ square elements (10 x 10) have been fabricated by this method on a pre-deposited NLIF. However, deterioration of the filter properties during processing appears to offset the gains induced by the pixellation technique. A variety of approaches, in which the substrate is structured before deposition of the NLIF, are being investigated and include various combinations of insulating layer and heat-sink materials. One example is a structure comprising a sapphire substrate with, cemented on one surface, a layer of glass, polished down to a 30 p n ~ thickness. This was laser machined with an ArF excimer laser to produce 25 ~ I U square pixels, by cutting grooves through the thickness of the glass, and finally an NLIF was deposited on the glass pixels.
The NLIF operated at 820 nm and included a top A1 layer so that it worked as a BEAT device /9/.
3
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EXPERIMENTAL RESULTSThe first experiment was performed on a single pixel pre-deposited unoptimized NLIF. The pixel had an area of 80 x 100 pa and a depth of 60 pm. Bistability was first observed in the uniform area of the same NLIF using the output of an Ar+ laser operating at 514 nm and focussed to a spot radius of 35 p. The angle of incidence was a 40". The sample was then translated parallel to itself to observe the bistability in the pixel: the two loops are shown in Figure 3. The switching powers (off and on) were 83 mW and 157 mW for the uniform sample, 57 mW and 103 mW for the pixel giving experimental improvements on pixellation for down-switching and up-switching of 69% and 66% respectively. (NB These switch powers are considerably higher than the few mW critical powers that can be achieved with small spots and optimized NLIF [6,9,10].
The second experiment was performed on pixels in a 10 x 10 array. The pixel dimensions were 25 x 25 p' and a depth of 20 ~IU. The experiments were on a 820 nm BEAT geometry NLIF /9/, using the output from an Ar ion pumped dye laser. The dye laser was operating between 800 to 850 nm. The filter was used at normal incidence with an incident spot diameter of 20 1x11. The detuning was varied by tuning the dye laser output. Nearly all the pixels were tested for optical bistability and highly consistent, repeatable results were obtained for critical switch power and detuning. The comparison of critical switch power on pixels and on uniform area of the NLIF is shown in Figure 4(a). The critical switch power for pixel and uniform area are 2.4 mW and 4.2 mW respectively. the
improvement on switch power after pixellation is a 57%. Fig. 4(b) shows a low power high contrast, large loop bistable characteristic that can be achieved with pixellation.
By using a uniform illumination over the whole pixel, the critical switch power was 3.2 mW, which corresponds to a switch power of 5 pW/pma. This implies that for a 5 x 5 array of 5x5 p m z pixels would require only 3 mW of optical input power
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readily available from a diode laser.Fig. 3 - Experimental bistable characteristics of (a) uniform and (b) pixellated samples
( A = 514 nm), showing shift of bistable region to lower power when pixellated.
INPUT POWER (mW)
INPUT POWER (mW)
Fig. 4(a)
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Experimental critical Fig. 4(b)-
Low power high contrast, switching characteristics of a uniform large loop bistable characteristic on and pixellated samples ( A = 820 m) a pixel.showing shift of critical switching region to lower power when pixellated.
4 - DISCUSSIONS
Abraham et a1 /7,8/ has done extensive theoretical studies of the effect of physical pixellation on NLIFs. Considering a uniform illumination on a cylindrical pixel
Abraham et a1 /7/ obtained an expression for a figure of merit, p, which scales the power levels in terms of the geometry (depths 1 and radius w), the thermal conductivity of a cylindrical pixel (Kp) and thermal conductivity of the substraste (Ks) as
p
1.13 Ksl/kpw+
1 (1)and the switching power on a pixel as
JOURNAL DE PHYSIQUE
These theoretical calculations were later extended by Abraham et a1 /8/ to include a rectangular pixel illuminated by a Gaussian beam. If a and b are the dimensions of a rectangular pixel, t en they show that, for a Gaussian input with spot size
A
s < w' (w' = (ab/rr)
,
the switching powers are given bywhere % r1.13 Ks!?s/ w V 2
KP
+ 11 [ l-
exp (-w'~/s~)]and Psubstrate = 12~1% (Tf-To) KsSl/A(Tf)
I
0 YI iw 150 m
KXDENT POWER P. (mW)
Fig. 5 - Bistability of film temperature as a function of the input power. Full line corresponds to uniform sample, broken line to pixel.
where Tf and T, are the film and ambient temperature respectively. The numerical results are shown in Figure 5 with a = 80 pm, b = 100 p. P = 60 pm, Kp =
Ks =O.Oll W/cnl°C and s = 35 p. From this the deduced values of
p'
are 1.47 for switch up and 1.62 for switch down. The results can be compared with the experimentallydetermined values of p ' for switching down and switching up of 1.45 and 1.52 respectively as shown in figure 3. The experimental and theoretical results are within 10% of each other.
In conclusion, we have demonstrated that the physical pixellation of NLIFs reduces the switching power of the bistable device and with properly optimized filters switching powers could be reduced to < 100 pw for a small pixel /lo/. Packing densities of
>
104 cm-2 should be achievable with a sapphire substrate as shown by Abraham et a1 /7/.Assuming smaller 1 ~ I U dimension pixels, switch rates of 10lb s-1 w-1 cm-2 should be feasible in the near future.
ACKNOWLEDGEMENTS
We are grateful to Drs. E. Abraham, J.G.H. Mathew and F.A.P. Tooley for many useful discussions. This work was partially supported by grants from the office of U.S. Naval Research and Iluropean Commission Stimulation Action Prograrmne.
REFERENCES
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/2/ Smith, S.D., Mathew, J.G.H., Taghizadeh, M.R., Walker, A.C., Wherrett, B.S. and Hendry, h., Opt. C o r n . ,
51,
357 (1984)./3/ Olbright., G.R., Peyghambarian, N., Gibbs, H.M., MacLeod, H.A. and Van Milligen. V., Appl. Phys. Lett.,
45,
1031 (1984)./4/ Smith, S.,D., Janossy, I., MacKenzie, H.A., Mathew, J.G.H., Reid, J.J.E., Taghizadeh, M.R., Tooley, F.A.P. and Walker, A.C., Opt. Eng., 24, 569 (1985).
/5/ Smith, S.D., Walker, A.C., Tooley, F.A.P. and Wherrett, B.S., Nature,
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27 (1987)./6/ Walker, A.C., Taghizadeh, M.R., Mathew, J.G.H., Redmond, I., Campbell, R.J., Smith S.11.. Dempsey, J. and Lebreton, G., Opt. Eng., 27, 38 (1988).
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