Metal-dielectric photosystems

Roger Arsenault and Germain Boivin

Thin film photosystems based on copper doped lead iodide are presented. A photodecomposable Fabry-Perot type interference filter is described and also a cerment system in which copper and lead iodide areevaporated simultaneously. These systems do not require any development, and additional informationcan be added subsequent to the first exposure. The proposed systems are generalizable to several metalsand dielectrics.

1. Introduction

Research in new photographic materials has ledto the development of thin film metal-dielectric pho-tosystems. 1 Compared with conventional silver halideemulsions, these systems have certain advantages,2notably, the recording is direct, and there is no devel-opment stage. This direct recording implies that in-formation can be added at any time after the first ex-posure provided the plate has not been fixed. Thesephotosystems also offer interesting possibilities for uvimaging where there would be no loss of resolution dueto Rayleigh scattering, as in conventional emulsions.Compared with the prior art,3 the photosystems pre-sented here have the additional advantage of a goodtransmission, contrast, and compare favorably in termsof exposure requirements.

Certain dielectrics, when illuminated at wavelengthsbelow their absorption edge, will photodecompose, thatis, a solid state reaction occurs which modifies theiroptical properties. The absorbed light produces anexciton or an electron-hole pair, which is free to movewithin the cristal lattice and release the energy requiredfor reaction at preferred sites. Doping with certainmetals increases the number of such sites and modifiesthe reaction to accelerate the process.4

II. Experimental Considerations

We use lead iodide doped with copper as the photo-sensitive material. These are evaporated onto glasssubstrates (held at room temperature) at a rate of 1-2nm/sec in a vacuum of about 10-5 Torr. The absorp-

The authors are with Laval University, Physics Department,Quebec, P.Q. GlK 7P4.

Received 12 July 19770003-6935/78/0301-0736$0.50/0.© 1978 Optical Society of America.

tion edge of lead iodide is at about 520 nm. We can thusrecord in the blue region of the spectrum and read in thered without the need for fixing the plate.

An optical system allows us to measure the trans-mission and reflection of the films during the deposit,and a quartz oscillator gives the relative thicknesses ofthe layers. The resolution of the Fabry-Perot typephotosystem is determined with the holographic ap-paratus shown in Fig. 1. Since the plate is not sensitiveto red light, we simultaneously record with the He-Cdlaser (X = 441.6 nm) and read with the He-Ne laser (X= 632.8 nm).

I11. Proposed PhotosystemsBefore presenting our photosystems, let us first look

at the transmission characteristics of a single film of leadiodide and of a single film of copper of thicknesses ap-proximately equal to those found in the photosystems.We see in Fig. 2 that the transmission of the lead iodidefilm is small below 500 nm and negligible below 420 nm.The transmission of the copper film is roughly constantdown to 300 nm. These results will be useful in dis-cussing the characteristics of our photosystems. Thephotosystems developed are shown in Fig. 3 and arediscussed presently.

A. Null Reflection Photosystem

The contrast by reflection will be maximum if thenonexposed plate has zero reflection. By optimizingthe thickness of the copper and lead iodide layers it ispossible to obtain a null reflection (neglecting the re-flection at the second interface of the substrate). Thewave reflected at the metal-dielectric interface mustexit in phase and with equal amplitude to the wave re-flected at the dielectric-air interface. The thicknessof metal required is determined by first evaporating ametal layer presenting a thickness gradient along oneaxis of a large substrate. We then deposit a dielectriclayer also presenting a thickness gradient, but along the

736 APPLIED OPTICS / Vol. 17, No. 5 / 1 March 1978

non-exposed Plateplate with grating

I1

I 0IiI I,

12 d- Is2 sint e

Fig. 1. Holographic system for recording interference fringes.

Table 1. Image Characteristics for Different Photosystems a

Reflection TransmissionPhotosystem image image

Positive Negative

y = 0.80 y = 0.56

Fabry-Perot Positive Negative-y = 0.70 -y = 0.80

Cermet Very weak Positive-y = 0.66

a The contrast is defined (Ia - Ib)/(Ia + Ib), where Ia is the in-tensity reflected or transmitted after exposure and Ib the corre-sponding intensity before exposure for X = 633 nm. The systems wereexposed down to their saturation points.

Z0

cnUE

2

r

05

0.4

0.2

0.1

Cu-Pbl2-Cu-30

0.300 e

\ 14 days

X(nm)

Fig. 2. Transmission spectrum of copper and lead iodide.

(D 0 ~~ ~~~~~~~R RE

X _500nX< 540s

12 CA~~~~~~~~~~~~~~s~~k dis

Substrate Subst Ce

T T

D 0a) b) c)

Fig. 3. (a) Null reflection photosystem; (b) Fabry-Perot photosys-tem; (c) cermet photosystem.

axis perpendicular to the first to obtain a large numberof possible thickness combinations. The region ofminimum reflection gives a good indication of the metallayer thickness required, and this can be more accu-rately determined afterward by a few evaporations of,uniform thicknesses. We thus deposit a metal layerwhose value is in the region given by the gradientmethod and subsequently optimize the reflection witha uniform layer of dielectric. The reflection is moni-tored during the deposit, and the dielectric is evapo-rated until a minimum reflection is reached. 5 6 A fewevaporations will allow a precise determination of theoptical properties required of the metal layer. Inprinciple, either one of these two methods will sufficein itself. We use the first method to obtain an ap-proximate value; this reduces the number of experi-ments necessary using the second method.

H

0U)

U)z

H

0.200 F-

X - 633

(AX 87 = m10

3- nn exposed

i_.258 j/cm'

)- i581 j /cm'@ 1.291 j/c'

@- 2.260jr/cm'

Exp-osue ct 435.8 nI= 1.0 76 mw /cm'01O00 -

400 500 600(nm)

700

Fig. 4. Transmission spectrum of a Fabry-Perot photosystem. Thethickness of the copper layer is 24 nm. Number 30 means that thetransmittance air-copper film-substrate during the formation of the

filter was 30%.

The photoreaction at the metal-dielectric interfacedestroys the phase-amplitude relationship between thetwo reflected waves. This results in an increase in thereflection of the system (positive image). The contrastsobtained by reflection and transmission are given inTable I. Below 420 nm, very little light reaches themetal-dielectric boundary (see Fig. 2) so the systemshows a reasonable spectral sensitivity only in the regionof 500 nm.

B. Fabry-Perot Photosystem

A layer of lead iodide sandwiched between two layersof copper results in a photodecomposable Fabry-Perottype interference filter. The photodecomposition ofthe second copper layer (Cu II) will modify the trans-mission to produce a contrast between the illuminatedand nonilluminated regions. To optimize the contrast,the transmission maximum is placed at 633 nm (i.e.,optimum for the He-Ne laser). We have developed asimple and direct method of producing these filterswhich does not require a precise knowledge of the re-fractive indices of the materials.7

The transmission spectrum of such a filter (withcopper layers approximately 24 nm thick) before andafter exposure is given in Fig. 4. The speed of ourphotosystem increases with age, probably because of the

1 March 1978 / Vol. 17, No. 5 / APPLIED OPTICS 737

Pbl,- 3X/4

l I I

I

o 0 9I

(0 D.8

in 3.7

a- .62

c- ).5

a) .) 4~0U)Z 0.3I

H ,-,

2 3

Exposure(joules/cm') 435.8 nm

Fig. 5. Normalized amplitude transmission of a Fabry-Perot pho-tosystem at X = 435.8 nm. Number 30 means that the transmittanceair-copper film-substrate during the formation of the filter was

30%..

H

(n

Cu PbI,:Cu

250X(nm)

Fig. 6. Spectral sensitivity of a Fabry-Perot photosystem.

MTF Cu Pb2 Cu A

(11/;) '

1000 2000 /m m

2%

1,15

632. nm

EXPOSURE ( I/cm ) PER BEAM AT X = 4416 nm

Fig. 7. Upper diagram: modulated transfer function of a Fabry-Perot photosystem; lower diagram: diffraction efficiency of the same

system.

diffusion of the copper into the lead iodide layer (Fig.5). This diffusion modifies only slightly the trans-mission curve of the nonexposed plate. From themeasured transmission spectra of the individual layers(Fig. 2), we can deduce that very little reaction will takeplace at the first interface. Also, the transmissionspectrum of the copper layer being fairly constant, wecan expect a reasonable sensitivity in the uv. Themeasured spectral sensitivity is given in Fig. 6. Thiscorresponds to the results of others on pure lead iodideand permits an extrapolation into the uv.1 ,

3 Themodulation transfer function taken with the holo-graphic apparatus of Fig. 1 is given in Fig. 7 and is a goodindication of the resolution of the plates. The smallthickness of the photosystem (0.3 gim) promises asgood a resolution in microphotography. This is not thecase for conventional plates, however, where the depthof focus of a high resolution objective is much smallerthan the emulsion thickness of the plate.

An antireflection layer of MgF2 increases the contrastand speed and also serves as a protection layer. How-ever, this precludes a subsequent fixing.

C. Cermet PhotosystemA simultaneous evaporation of the metal and the di-

electric results in a homogeneous doping. In this case,the refractive index of the whole layer is modified byphotoreaction. A preliminary study has given inter-esting results.

The electron microscope shows crystals of approxi-mately 20 nm for the nonexposed plates. No granula-rity is visible in the optical microscope (X800) afterphotodecomposition. The resolution should be quitehigh.

The contrasts obtained depend in a complex way onthe thickness of the layer and on the concentration ofcopper. To characterize the system in a general man-ner, one would have to determine the refractive indicesand thicknesses for different concentrations of copperbefore and after photodecomposition. Figure 8 showsthe spectral characteristics of a 200-nm layer of thick-ness with a copper concentration of 10%. Similar

0.4-

0.8 Absorption

0.7 AbAa te, *

0.6 -decomposition A

>- 0 .5 - - TE

wit 0a cope cocntrationf1%

RI

0.1 Retection ~

430 450 500 560 600 650 680

Fig. 8. Spectral characteristics of a layer of lead iodide 200 nm thickwith a copper concentration of 10%.

738 APPLIED OPTICS / Vol. 17, No. 5 / 1 March 1978

-

v I

.

characteristics were obtained with different thicknessesand concentrations. The decrease in absorption indi-cates a product of the type CuI-PbI. (The absorptionedge of CuI is at 406 nm.)

It is improbable that we will be able to fix this systemwithout modifying its optical properties considerably.The image characteristics for the different photosys-tems are presented in Table I.

The cermet system requires about half the exposureof the Fabry-Perot system to reach the point of satu-ration. In the cermet system, the copper is more easilyaccessible for reaction with the lead iodide.

The images on all three photosystems were stable forseveral months under normal laboratory conditions.For longer storage, it would be wise to store the platesin a cool, dry area since lead iodide is slightly hygro-scopic.

IV. Discussion

The energy requirements are quite high, about 106

times greater than conventional photography. It mustbe repeated that there is no development stage and,hence, no amplification of the image. The advantagehere is that no distortion is introduced by a wet chemicaldevelopment and a drying process. Our exposure re-quirements compare favorably with the thin film sys-tems developed by Tubbs et al. 8 and by Kostyshin etal.9

The results obtained are reproducible to a high de-gree, a characteristic which is not given by conventionalemulsions where the response from one plate to anotherand even from one region of the same plate to anothervaries considerably.'0

V. Conclusions

These photosystems and their production are gen-eralizable to several materials, thus presenting a largevariety of properties possible. Their weak sensitivity

allows handling at normal laboratory lighting for shortperiods, and information can be added on at any timeprior to fixing.

The reproductivity and image quality offer inter-esting possibilities for holography, microphotography,and possibly uv imaging. Their applications to thestudy of diffusion and solid-state reactions is mani-fested.

The financial support from the Quebec Departmentof Education and from the National Council of Canadais gratefully acknowledged. We also thank J. Thibaultand G. Pigeon for their technical assistance.

References1. R. Dawood, A. J. Forty, and M. R. Tubbs, Proc. R. Soc. A 284,272

(1965).2. J. Malinowsky, Thin Solid Films 13, 313 (1972).3. J. H. Jacobs and R. A. Corrigan, J. Photogr. Sci. 21, 193 (1973).4. W. C. de Gruijter and J. Schoonman, Photogr. Sci. Eng. 17, 382

(1973).5. The complexity of the problem arises from the fact that the phase

change upon reflection at the metal-dielectric interface is neitherzero nor r, but a value between.

6. Theoretical determination of these thicknesses was questionablesince we could not use the indices of refraction given in the lit-erature because of the poor vacuum (-10-5 Torr) and low evap-oration rates.

7. R. Arsenault and G. Boivin, Appl. Opt. 16, 1890 (1977).8. M. R. Tubbs, M. J. Bessley, and H. Foster, Br. J. Appl. Phys. 2,

197 (1969).9. M. T. Kostyshin, E. V. Mikhailovskaya, and P. F. Romanenko,

Sov. Phys. Solid State 8,451 (1966).10. J. Couture, these de malitrise, Universit6 Laval (1976).

Corning Beryllium Handling Lab

1 March 1978 / Vol. 17, No. 5 / APPLIED OPTICS 739