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Aktsipetrov et al.

Vol. 14, No. 4 / April 1997 / J. Opt. Soc. Am. B

771

Electroinduced and photoinduced effects in optical second-harmonic generation and hyper-Rayleigh scattering from thin films of bacteriorhodopsin
O. A. Aktsipetrov, A. A. Fedyanin, and T. V. Murzina
Department of Physics, Moscow State University, Moscow 119899, Russia

G. P. Borisevich and A. A. Kononenko*
Department of Biology, Moscow State University, Moscow 119899, Russia Received May 31, 1996; revised manuscript received September 13, 1996 The mechanism of optical second-harmonic generation (SHG) in thin solid oriented and nonoriented films of bacteriorhodopsin (bR) is studied. The role of the random spatial inhomogeneity of the quadratic susceptibility in the mechanism of diffuse and depolarized SHG in thin solid bR films is discussed. A hyper-Rayleigh scattering formalism is used for the description of SHG in these films. Electroinduced and photoinduced effects in SHG are detected, and the saturated concentrations of the electroinduced and the photoinduced spectral forms of this type of bR film are estimated. Nondestructive SHG readout of bR-based optical memories is suggested. © 1997 Optical Society of America [S0740-3224(97)01403-3] PACS number(s): 42.65.Ky, 78.35. c, 78.55.Kz.

1. INTRODUCTION
Optical second-harmonic generation (SHG) has been shown to be a sensitive probe for studying surfaces, interfaces,1,2 and thin films.3 SHG permits detection of the orientation of molecules in monolayers,4 and of the type of packing of layers in multilayered Langmuir ­ Blodgett films5 and the study of the effects of external factors (dc-electric and magnetic fields, illumination, etc.6 ­ 8) on the quadratic nonlinear-optical response of thin films. Here we investigate the mechanism of SHG in thin solid films of bacteriorhodopsin (bR) and use the SHG technique to study electroinduced and photoinduced transformations of quadratic responses of oriented and nonoriented solid bR films. Our interest in studying photochromic bR films stems from our interest in nonlinear-optical properties of bRbased materials both from the basic point of view and because of their possible applications in optoelectronics. The bR molecule is the simplest natural light energy transducer. Photon absorption induces a cyclic transformation of the bR molecule that is accompanied by changes in the bR absorption spectrum.9 The main stages of the photochromic cycle are illustrated by the transformation bR570 M412 bR570, where the initial spectral form of the bR molecule is bR570 and the second basic long-lived form is M412 .10 The wavelengths of absorption bands are written as subscripts. Another transformation to stable modification of the bR molecule, bR570 bRE 630, takes place by application of an external dc-electric field.11 Since 1987 the SHG probe has been applied to the study of the nonlinear-optical properties of bR
0740-3224/97/040771-06$10.00

molecules.12 In the research reported in Ref. 12 secondharmonic (SH) generation and thermoinduced and photoinduced variations of the SH intensity were observed in bR films. Later the SH intensity variations were shown to be attributable to photoinduced modifications of the bR molecule structure.13 ­ 16 These experiments were carried out for bR molecules embedded in a poly(vinyl alcohol) matrix. In addition to bR ­ poly(vinyl alcohol) films, dried solid bR films17 can be used for studies of the electroinduced and photoinduced effects on the nonlinear-optical properties of bR molecules. The linear-optical experiments demonstrate considerable modification of linearoptical properties in the presence of external bias and cw radiation.18 At the same time, to our knowledge, systematic studies of nonlinear-optical properties of these films have not yet been performed. Here we describe experimental studies of SHG in thin solid oriented and nonoriented bR films. The mechanism of SHG in inhomogeneous solid bR films is interpreted in terms of hyper-Rayleigh scattering (HRS). The significant electroinduced and photoinduced variations of the SH intensity are observed, and the estimates of the concentrations of electroinduced and photoinduced bR spectral forms are given.

2. EXPERIMENT
A. Experimental Setup The output of a Q -switched YAG:Nd3 laser at a wavelength of 1064 nm with a pulse duration of 15 ns and a repetition rate of 12.5 Hz was used as the fundamental radiation. SHG was studied both in reflection from and
© 1997 Optical Society of America


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in transmission through the bR films of the IR fundamental radiation. The SH radiation at 532 nm was detected by a double monochromator, a photomultiplier, and gated electronics. The cw radiation of an Ar ­ Kr laser in the wavelength range 488 ­ 647.1 nm was used to excite photoinduced processes in bR films. Oriented bR films as thick as 4 m were prepared from a water suspension of purple membranes (PM's) containing bR molecules. A fraction of PM's with a concentration of 7 mg/mL of water, pH 6.8, was isolated by the method described in Ref. 19. After centrifugation in a sucrose gradient, PM's obtained were purified by liquid chromatography on Sephadex G-50 (crude) and equilibrated with distilled water. PM's were then resuspended three times in bidistilled water and then spun at 80,000 g. The procedure for preparation of solid oriented bR films was similar to that described in Ref. 17. A drop of PM suspension was placed upon a Si(111) substrate in an orienting electrostatic field of 102 V/cm applied between the silicon substrate (anode) and the nickel electrode (cathode). Such a preparation procedure was shown to result in a predominant orientation of flat PM segments in the plane of the substrate.18 Because the electrostatic dipole moment of a PM segment is oriented close to the normal to the plane of the segment, the application of the orienting electrostatic field leads to the orientation of almost all the segments collinearly with the orienting field. Nonoriented bR films of the same thickness were prepared in the absence of an orienting dc-electric field. Dc-electric-field-induced SHG from bR films was measured as the bias was applied to the silicon substrate with respect to the cap In ­ Ga electrode placed upon the surface of the bR film (Fig. 1, upper inset). The In ­ Ga electrode, 0.1 cm thick, was formed as a ring with an inner

Fig. 2. Dependence of the SH reflection intensity from a nonoriented bR film for s -in, s -out polarization combination geometry on the polar angle . Zero polar angle coincides with the specular direction; positive values correspond to polar angles towards the film normal. Inset: sketch of the scattering indicatrix measurement.

diameter of 0.1 cm. The maximum bias, U 4 V, was limited in our experiments by the electric breakdown of the film. B. General Properties of SHG in Solid bR Films Figure 1 shows the dependence of SH transmission intensity through the bR film on the intensity of the fundamental radiation. The dependence is found to be close to quadratic, which indicates that the fundamental radiation at 1064 nm does not give rise to photoinduced transformations of the bR molecules and also that the signal at 532 nm is due to a second-order nonlinear-optical process. The spectral width of the radiation in the vicinity of 2 is found to be less than 1 nm (as determined by the spectral resolution of the monochromator). Thus the radiation detected in our experiment can be attributed to SHG. We measured the SHG scattering indicatrix (the dependence of the SH intensity on the polar angle of the SH wave propagation) by rotating the detection system around the Y axis of the XYZ sample frame, where XZ is the plane of incidence of the fundamental radiation and the Z axis is the normal to the film (see the inset in Fig. 2). The SH radiation from bR films appears to be strongly diffused: The scattering indicatrices in reflection and in transmission do not possess a pronounced specular maximum. Figure 2 shows the scattering indicatrix of the s -polarized SH radiation on reflection of the s -polarized fundamental radiation from a nonoriented bR film. A shallow specular peak is riding on a broad diffuse angular background. To study the anisotropy of the nonlinear-optical response we measure the azimuthal angular dependence of the intensity of SH transmission, I 2 ( ) , by rotating bR

Fig. 1. Dependence of the SH intensity on the fundamental transmission intensity. Upper inset: schematic of the dcelectric-field application to the bR film. Lower inset: dependence of the SH intensity on the azimuthal angle.


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773

samples around the Z axis. The dependence I 2 ( ) appears to be practically isotropic for all biases applied and for both parallel and perpendicular polarizations of the fundamental and the SH waves (see the lower inset in Fig. 1). For the sake of simplicity, all further experimental results are presented for parallel polarization. Figure 3 shows the dependence of the SH intensity on the thickness of nonoriented bR films. The thickness dependence appears to be monotonic and close to a linear function, with steplike feature at thickness tending to zero. By rotating the analyzer we can study polarization diagrams of the SH radiation in reflection and in transmission as the polarization of the fundamental wave is fixed and the polarization of the SH wave is measured (Fig. 3, inset). SHG in bR films appears to be strongly depolarized. C. Effects of dc-Electric Field and cw Radiation on Nonlinear Optical Response of bR Films Figure 4 shows the bias dependences of transmission of the SH intensity through the oriented and nonoriented bR films. The application of bias up to 2 V leads to a drastic decrease of the SH intensity for oriented films. The bias dependence flattens out at U 2 V, where the SH signal is approximately four times smaller than that for the unbiased film. The SH intensity increases for negative biases up to 1.6 V. The electroinduced modification of the SH intensity is reversible. For nonoriented films the SH intensity for unbiased samples is approximately two times smaller than that for oriented samples. The field-induced decrease of the SH intensity for nonoriented films is 0.78. The asymmetric shape of

Fig. 4. Dependences of the SH transmission intensity on bias for oriented (filled circles) and nonoriented (open circles) films. Solid and dashed curves are the fits. Upper inset: schematic of the mutual orientation of the static dipole moment of bR molecule and the external dc-electric field E. Lower inset: graph E of the fit from Eq. (2) for the normalized concentration of bR630 form within the elementary solid angle.

Fig. 5. Dependences of SH intensity on cw intensity W i : 488.0 nm (squares), 514.5 nm (open circles), and 647.1 nm (filled circles).

Fig. 3. Dependence of the SH transmission intensity on the thickness of nonoriented bR films. The solid line is the fit by the linear function. Inset, SHG polarization diagram: the dependence of the SH intensity on the analyzer angle for fixed polarization of the fundamental wave (zero angle corresponds to the parallel polarizations of the fundamental and SH waves).

the bias dependence with respect to zero bias is observed for both types of solid bR film. Figure 5 shows a series of dependences of SH intensity on cw radiation intensity W i at three wavelengths. A substantial decrease of the SH signal is observed as the cw intensity increases. The I 2 ( W i ) dependences are similar for both oriented and nonoriented bR films and


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Fig. 6. Dependence of the SH transmission intensity on bias in the absence of the cw illumination (circles) and as the saturating cw radiation of intensity W sat 150 mW/cm2 (647.1 nm) is switched on at U 2 . 8 V (diamonds).

whereas isotropic SHG from a homogeneous medium for the normal angle of incidence is forbidden by symmetry considerations.20 Second, for both oriented and nonoriented bR films the SH radiation is diffused and depolarized. Third, a linear (within the accuracy of the experiment) dependence of the SH intensity on the film thickness is obtained. These characteristics of SH radiation are typical for HRS.22 Traditionally HRS is studied in solutions and suspensions (including the aqueous suspension of PM's23) and is caused by time-dependent fluctuations of the quadratic susceptibility. In our case the source of HRS can be attributed to the fluctuations of the spatial distribution of the quadratic susceptibility in random inhomogeneous films. Two types of inhomogeneity are expected in the solid bR films: fluctuations of the PM density and fluctuations of the bR molecules' orientation with respect to the plane of the film. The main source of the bR molecule's nonlinearity is the retinal chromophore.12,15 The largest component of the molecular hyperpolarizability tensor is z z z in the molecular coordinate frame with the Z axis oriented along the retina. Thus the intensity of SHG scattered by bR films by a HRS mechanism is given by23 I K
m

demonstrate saturation at W i 100 ­ 250 mW/cm2. The saturated SH signal is approximately two times smaller for all the wavelengths of the cw radiation used. Figure 6 shows the combined effect of the cw radiation and the dc-electric field on SHG. This combined effect is studied as a bias up to U 2.8 V is applied to oriented bR films and a cw radiation of 150 mW/cm2 illuminates the sample. The SH intensity decreases by a factor of 8. Subsequent switching of the illumination and application of bias does not influence the SH intensity for the saturation region of the I 2 ( U , W i ) dependence.

2

N

m

2 zzz

mI

2

KN

0

n
m

m

2 zzz

mI

2

,

(1)

3. DISCUSSION
A. Mechanism of SHG in Thin Solid bR Films The experimental results presented prove that the SH signal is coming from the bulk of bR films. First, one can neglect the contribution to SHG from the Si substrate as well as its possible dependence on the external dc-electric field. This can be done because the anisotropic I 2 ( ) dependence for a Si(111) substrate should be sixfold symmetric,20,21 whereas the analogous dependence for bR film is shown to be isotropic. Second, the pronounced monotonic dependence of the SH intensity on the thickness of bR film indicates that the bulk of the film plays the key role in SHG. At the same time, the nonzero SH signal for a film thickness tending to zero can be explained by the contribution to the total SH signal from the Si ­ bR or bR ­ air interfaces or both. The experiments on photoinduced and electroinduced SHG are performed for films 3 m thick. The corresponding SH intensity considerably exceeds the signal for a film thickness tending to zero. This proves that the SH signal arises primarily from the bulk of the film. The following features of SHG observed in these experiments are important for interpretation of the mechanism of generation. First, sufficient isotropic SHG in transmission at the normal angle of incidence is observed,

where K is a coefficient that depends on the geometry of the SHG experiment and contains the local field corrections at and 2 ; z z z ( m ) is z z z of the m th spectral form of the bR molecule, n m N m / N 0 is the normalized concentration of the m th spectral form, where N m and N 0 are the concentration of the m th spectral form and the total concentration of bR molecules, respectively; denotes averaging over the random ensemble of molecules; and I is the intensity of the fundamental radiation. The sum is taken over the all spectral forms in the bR film. B. Electroinduced Effects in SHG from bR Films The two-photon energy of the fundamental IR radiation at a wavelength of 1064 nm is close to the maximum of the absorption band of the main bR spectral form, bR570 . At the same time, the maximum of the absorption band of E the electroinduced spectral form bR 630 is off the twoE 11,18 photon resonance. The contribution of bR 630 to the SH signal should be smaller than the contribution of bR570 . Thus the transformation of a part of the bR molE ecules into the spectral form bR 630 in the biased samples should lead to a decrease of the SH intensity, and this is observed experimentally. The asymmetry of the bias dependence I 2 ( U ) shown in Fig. 4 can be explained by the properties of the metal ­ dielectric ­ semiconductor (In ­ Ga/bR/Si) structures. Because of the difference in electron Fermi energies of the silicon substrate and the cap metal electrode, the initial band bending and the interface electric field appear near the Si ­ bR interface. Thus some of the bR molecules remain in the electroinduced form, even for the unbiased


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sample. The maximum of I 2 ( U ) dependence is reached at the flat-band voltage, as the initial band bending is compensated for by the applied external bias. In our case this bias is 1.6 V. According to Ref. 18 there should be no reorientation of PM segments for the biases used. Thus the only mechanism for electroinduced changes of SHG in bR films is the E transition of bR molecules into the bR 630 form. E To estimate the concentration of bR 630 in our SHG experiments, assume that the bR molecule is transformed E into the form bR 630 only when there is a positive projection of the original static bR dipole moment onto the external dc-electric-field (Ref. 18). The normalized concentration dn 630 of bR molecules in the electroinduced spectral form with dipole moments oriented within the elementary solid angle d can be written in the following form: dn
630

n

630

U

an 0 / 1 a1

exp n0 / 1

U

0

U U
0

exp

U

, (5)

/d

a / (4

1

exp

E

0

E cos

), (2)

2 ) a , and U 0 are assumed where the parameters , ( 1 to be the same as for the case of nonoriented bR films. Therefore the only adjustable parameter in Eq. (5) is n 0 . Figure 4 shows the corresponding fit for n 0 0.89 calcu2 lated above and ( 1 )a 0.62. A noticeable deviation of this fit from the experimental results is observed. Better agreement with the experimental data is achieved 2 for ( 1 )a 0.71 and n 0 0.92 (see Fig. 4, solid curves). One of the possible reasons for the discrepancy 2 in the parameter ( 1 ) a is the different maximal conE centration of bR 630 in the saturation of the bias dependence for oriented and nonoriented films. The significant value of n 0 obtained from our fit indicates the high orientation of PM in oriented bR films, which is consistent with previous linear-optical studies.18

where and E 0 are numerical factors, is the angle between the static bR dipole moment and the dc-electric field E, and a describes the concentration of the electroinE duced form bR 630 that can be achieved for these films (see the insets in Fig. 4). Equation (2) satisfies the following limiting conditions: dn 630( E 0)/d 0 and dn 630 ( E )/d a /4 1/4 . 1. Electroinduced Effects in SHG from Nonoriented bR Films For the case of nonoriented films the angular distribution of static molecular dipole moments is assumed to be isotropic. After Eq. (2) is integrated over the solid angle , the normalized concentration n 630 and the SH intensity as functions of the applied voltage are given by n
630 2

U U

a/ 2 U C K
2 570

U 1

0 2

C n
630

U

0

U
2

,

(3) (4)

I

1

UI

,

2 2 2 ln 1 exp( ) , where C ( ) 630 570 , and the factor includes here the relation between E and U . The experimental bias dependence for nonoriented bR film is fitted by Eq. (4) with adjustable parameters ( 1 2 ) a , , and U 0 and by consideration that the flat-band voltage of silicon substrate is U fb 1.6 V. The solid 2 curves in Fig. 4 show the fit for ( 1 )a 0.62, 1 2.45 V , and U 0 1.34 V and demonstrate good agreement with the experiment. Thus the minimal n 630 in the saturation region of the bias dependence for nonoriented bR films is estimated to be 0.62.

C. Photoinduced Effects in SHG from bR Films The illumination of the photochromic film by visible cw light initiates the bR molecule photocycle and results in the transformation of some bR molecules into the photoinduced spectral form M412 , which is far off two-photon resonance with the fundamental IR radiation.10 Thus the SH signal decreases under cw illumination. The saturation of I 2 ( W i ) curves shown in Fig. 4 attests to the facts that for this cw intensity all bR molecules are involved in the photocycle and that the dynamic equilibrium sets between the bR570 and the M412 forms. The relative concentrations of bR570 and M412 for oriented bR films can be estimated by use of Eq. (1), as their secondorder hyperpolarizabilities at the SH wavelength are known to be 570 2.1 10 27 and 412 2.8 10 28 esu, respectively.15 Thus the normalized concentration n 412 in our experiments is 0.51. Describing the combined effect of the cw radiation and the dc-electric field on bR films shown in Fig. 6, we take into account that bR molecules undergo two types of process initiated by the simultaneous effect of the electrostatic field and the cw illumination. The first is the elecE troinduced transition to the bR 630 form. The second is the decrease of the M412bR570 transition rate, resulting in an increase of M412 concentration.24 Both processes should result in a decrease of the SH signal because of the decrease in the nonlinear susceptibility of the bR film. Inasmuch as the ratio I 2 ( W i 0)/ I 2 ( W i W sat) for U 0 and U 2.8 V is 1.67 and 2, respectively, one can conclude that the accumulation of the M412 form is observed. D. Nondestructive SHG Readout of Photochromic Memory Systematic studies of photoinduced and electroinduced variations of the nonlinear-optical properties of thin bR films show that photomodified and electromodified states of bR molecules can easily be distinguished by the SHG probe. For example, in the case of bR ­ poly(vinyl alcohol) films the ratio of the SH intensity from bR570 to the SH intensity from M412 is 10.12 On the other hand, the IR fundamental radiation does not cause photochromic

2. Electroinduced Effects in SHG from Oriented bR Films In the case of oriented bR films almost all the dipole moments of PM segments are oriented along the normal to the film.18 This predominant orientation requires introduction of one more adjustable parameter, n 0 , describing the normalized concentration of bR molecules with the static dipole moment oriented collinearly with the orienting electric field. Other dipoles are assumed to be oriented in the opposite direction. Thus for the oriented bR films one can obtain the following expression for n 630( U ):


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Aktsipetrov et al. effects in polydiacetylene Langmuir ­ Blodgett multilayers,'' Thin Solid Films 132, 1 ­ 10 (1985). O. A. Aktsipetrov, O. V. Braginskii, and D. A. Esikov, ``Nonlinear optics of gyrotropic media--SHG in rare-earth iron garnets,'' Kvantovaya Elektron. (Moscow) 17, 320 ­ 325 (1990). W. Stoeckenius, R. H. Lozier, and R. A. Bogomolni, ``Bacteriorhodopsin and the purple membrane of Halobacteria,'' Biochim. Biophys. Acta 505, 215 ­ 278 (1979). K. Bryl, G. Varo, and R. Drabent, ``Photocycle of bacteriorhodopsin immobilized in poly(vinyl alcohol) film,'' FEBS Lett. 285, 66 ­ 70 (1991); R. R. Birge, ``Nature of the primary photochemical events in rhodopsin and bacteriorhodopsin,'' Biochim. Biophys. Acta 1016, 293 ­ 327 (1990). G. P. Borisevich, E. P. Lukashev, A. A. Kononenko, and A. B. Rubin, ``Bacteriorhodopsin (bR570) bathochromic band shift in an external electric field,'' Biochim. Biophys. Acta 546, 171 ­ 174 (1979). O. A. Aktsipetrov, N. N. Akhmediev, N. N. Vsevolodov, D. A. Esikov, and D. A. Shutov, ``Photochromism in nonlinear optics: photocontrolled second harmonic generation by bacteriorhodopsin molecules,'' Sov. Phys. Dokl. 32, 219 ­ 220 (1987). O. A. Aktsipetrov, G. P. Borisevich, A. A. Kononenko, T. V. Murzina, A. B. Rubin, and A. A. Fedyanin, ``Influence of electrostatic field on the second harmonic generation in bacteriorhodopsin films,'' Dokl. Akad. Nauk 337, 675 ­ 679 (1994). Th. Rasing, J. Huang, A. Lewis, T. Stehlin, and Y. R. Shen, ``In situ determination of induced dipole moments of pure and membrane-bound retinal chromophores,'' Phys. Rev. A 40, 1684 ­ 1687 (1989); J. Huang, Z. Chen, and A. Lewis, ``Second harmonic generation in purple membrane ­ poly(vinyl alcohol) films: probing the dipolar characteristics of the bacteriorhodopsin chromophore in bR570 and M412 ,'' J. Phys. Chem. 93, 3314 ­ 3320 (1989). O. Bouevitch, A. Lewis, I. Pinevsky, J. P. Wuskell, and L. M. Loew, ``Probing membrane potential with nonlinear optics,'' Biophys. J. 65, 672 ­ 679 (1993). G. Varo, ``Dried oriented purple membrane samples,'' Acta Biol. Acad. Sci. Hung. 32, 301 ­ 310 (1981). A. A. Kononenko, E. V. Lukashev, A. V. Maximychev, S. K. Chamorovsky, A. B. Rubin, S. F. Timashev, and L. N. Chekulaeva, ``Oriented purple-membrane films as a probe for studies of the mechanism of bacteriorhodopsin functioning. I. The vectorial character of the external electricfield effect on the dark state and the photocycle of bacteriorhodopsin,'' Biochim. Biophys. Acta 850, 162 ­ 169 (1986). D. Oesterhelt and W. Stoeckenius, Methods Enzymol. 31, 667 ­ 678 (1974). O. A. Aktsipetrov, I. M. Baranova, and Yu. A. Il'inskii, ``Surface contribution to the generation of reflected second harmonic for centrosymmetric semiconductors,'' Sov. Phys. JETP 64, 167 ­ 172 (1986). J. E. Sipe, D. J. Moss, and H. M. van Driel, ``Phenomenological theory of optical second- and third-harmonic generation from cubic centrosymmetric crystals,'' Phys. Rev. B 35, 1129 ­ 1141 (1987). S. Keilich, ``Second harmonic light scattering by dense isotropic media,'' Acta Phys. Pol. 33, 89 ­ 104 (1968); ``Higherorder elastic scattering of laser light,'' Acta Phys. Pol. 33, 141 ­ 143 (1968). P. K. Schmidt and G. W. Rayfield, ``Hyper-Rayleigh light scattering from an aqueous suspension of purple membrane,'' Appl. Opt. 33, 4286 ­ 4292 (1994); K. Claus and A. Persoons, ``Hyper-Rayleigh scattering in solution,'' Phys. Rev. Lett. 66, 2980 ­ 2983 (1991). E. P. Lukashev, E. Vozary, A. A. Kononenko, and A. B. Rubin, ``Electric field promotion of the bacteriorhodopsin bR570 to bR412 photoconversion in films of Halobacterium halobium,'' Biochim. Biophys. Acta 592, 258 ­ 266 (1980).

transformations in bR molecules, as is proved by the quadratic dependence of the SH intensity on the fundamental intensity shown in Fig. 1. Thus the SHG probe with IR fundamental radiation does not change the information recorded in photochromic memory and yields its nondestructive readout.

8.

9. 10.

4. CONCLUSIONS
In summary, we have found that SHG in solid bR films is strongly depolarized and diffused, and sufficient isotropic SHG exists at the normal angle of incidence. The dependence of the SH intensity on the film thickness is shown to be monotonic and close to linear. Thus SHG in thin solid bR films is attributed to a HRS mechanism caused by fluctuations of spatial distribution of density and orientation of PM segments. The dc-electric-field and photoinduced effects in SHG and HRS are observed in these thin photo(electro)chromic films. These reversible effects are interpreted in terms of electroinduced and photoinduced transformations of bR molecules. It is shown that the IR fundamental radiation does not change the states of bR molecules and can be used for nondestructive SHG readout of optical memory based on this photochromic material.
11.

12.

13.

14.

15.

ACKNOWLEDGMENTS
The authors are pleased to acknowledge useful discussion with A. B. Rubin and A. N. Rubtsov. This study was supported by Physics of Solid Nanostructures Program grant 1-36, International Science Foundation grant M12300, and Russian Foundation of Basic Research grant 96-0450436a.
16. 17. 18.

*Deceased.

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