6754
J . Phys. Chem. 1986, 90, 6754-6756
Phase-Modulated Holography: A New Technique for Investigation of SoitcCState Photochemistry and Hologram Formation Mechanisms J. Pinsl, M. Gehrtz? and Chr. Briiuchle* Znstitut fur Physikalische Chemie der Universitat Miinchen, 0-8000Miinchen 2, FRG (Received: September 22, 1986)
A new holographic technique, called phase-modulated holography (PMH), has been developed. With this method, amplitude and phase gratings generated by photochemical or photophysical changes of molecules dissolved in, e.g. solid matrices, can be monitored separately and simultaneously. Accordingly, the hologram formation mechanism and the underlying photoreaction can be monitored separately and simultaneously allowing conclusions to be made on photoinduced matrix effects and direct measurements of photochemical quantum yields. This feature and an up to eight orders of magnitude improvement of detection sensitivity over conventional phase-insensitive holographic techniques makes PMH the ideal tool for the investigation of new holographic recording materials and of photochemistry and photophysics of molecules in the condensed phase.
+
Ed = -D(iP A ) E , Introduction (1) In the past years grating techniques based on the principle of where E, is the r-beam electric field amplitude before the sample holography have proven to be a powerful tool for the investigation and D is the loss factor due to the average optical density OD of of the photochemistry and photophysics of molecules in the the sample at A. condensed p h a ~ e l -and ~ for the development of new photopolymer holographic recording materials, consisting of a photoreactive solute and a polymer m a t r i ~ . ~Recently ,~ it has been ~ h o w n ~ , ~ D = exp(-1.150D/cos e) that hologram formation in a photopolymer may result not only from the photoreaction of the solute but additionally from a P and A in eq 1 are phase and amplitude diffraction efficiencies photoinduced density change of the matrix. For optimum perwhose temporal growth includes the kinetics of the photoreaction. formance of a photopolymer system for holographic recording it For a simple one-step onephoton reaction in the low reaction yield is important to investigate the photoreaction of the solute as well limit P and A are linear functions of the time t given as the photoinduced matrix effect. This is not easily possible with holographic techniques known to date, because both effects cannot be monitored separately. A ( t ) = ~ ~ ( 2 3 0tE(2.303/4)(d/cos 3 ) ~ ~ e)+zr (4) In this paper we report on a newly developed phase-modulated holographic technique (PMH).’ In contrast to conventional In eq 3 and 4 co is the initial concentration of the educt, Ae = phase-insensitive holographic techniques (PIH) this technique e p - t E is the difference between the extinction coefficients of allows the separate and simultaneous monitoring of the phase product (e,) and educt (tE). Q is the Lorentz-Lorenz correcti~n,’.~ and absorption contributions to the holographic grating and is the total (effective) change of molar r e f r a c t i ~ n ~induced .~,~ measurement of the true sign of either contribution. From the by the photoreaction, Z is the recording intensity, and 4 is the absorption grating, which results from the photoreaction only, the quantum yield. photochemical quantum yield can be determined. From the phase Note the 7r/2 is the phase difference of P and A (the i in eq grating, on the other hand, the total change of molar refraction l ) , Le. the phase difference between dispersive and absorptive arising in the photopolymer can be determined, allowing one to response of a medium, which will be transformed by phase directly make conclusions on the magnitude and the sign of the modulation in the frequency space allowing the separation of P photoinduced matrix effect. Further, PMH offers an eight orders and A . For this purpose the phase of one beam is modulated of magnitude improvement of detection sensitivity for typical (m-beam) with an electrooptic modulator (EOM) (see Figure 1) experimental conditions. Thus even complex reactions associated with a modulation index M and a modulation frequency a,. The with very small refractive index changes, which would be lost by electric field amplitude E , of the m-beam after the sample is PIH, can still be detected with PMH. All features of PMH are demonstrated with several new photopolymer holographic reE,’ = DE, exp[iM sin (w,t)] (5) cording materials, whose properties are fully accounted for elsewhere.6J1 When possible, PMH results are compared directly In the direction of the m-beam, now, a homodyne beat signal P ( t ) with the corresponding PIH results. can be detected with a square law photodetector:
mff
Theory of Phase-Modulated Holography For P M H two laser beams of the same wavelength X are superimposed under the angle 20 on a photoreactive sample (see Figure 1). Their interference pattern leads via a sinusoidal intensity distribution to a sinusoidal distribution of product and educt and thus, in general,’ to a sinusoidal variation of refractive index n (phase grating) and absorption coefficient a (amplitude grating). According to coupled wave theorys for thick holograms each beam is partly diffracted into the direction of the other beam due to the interaction with the amplitude and the phase grating. Considering the reference (r) beam, the diffracted amplitude Ed in m-beam direction is given by8 Present address: IBM Germany, Plant Mainz, Laboratories and New Technologies, POB 2450, D 6500 Mainz, FRG.
0022-3654/86/2090-6754$01.50/0
P ( t ) = lEm’
+ &I2
(6)
Inserting eq 1 and 5 into eq 6 results for the limit of small (1) Brauchle, Chr.; Burland, D.M. Angew. Chem. 1983,95,612. Angew. Chem., In?. Ed. Engl. 1983, 22, 582, and references therein. (2) Bjorklund, G. C.; Burland, D. M.; Alvarez, D. C. J. Chem. Phys. 1980,
73, 4321. (3) Deeg, F. W.; Pinsl, J.; Briuchle, Chr.; Voitlander, J. J . Chem. Phys. 1983, 79, 1229. (4) Tomlinson, W. J.; Chandross, E.A. Adu. Photochem. 1980, 12, 201. ( 5 ) P!nsl, J.; Deeg,F. W.; Briuchle, Chr. Appl. Phys. B 1986, 40, 77. (6) Pinsl, J.; Gehrtz, M.; Reggel, A.; Briuchle, Chr., to be submitted.
(7) Gehrtz, M.; Pinsl, J.; BrHuchle, Chr., submitted for publication in Appl. Phys. B. (8) Kogelnik, H. Bell. Syst. Techn. J . 1969, 48, 2909
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6155
Letters
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Figure 1. Experimental setup for PMH (Ml, M2: mirrors; EOM:
electrooptic modulator). modulation (M
