Journal ArticlePhotoinduced dichroism caused by bleaching of the interband transition in tran s-(CH)x was observed in the picosecond time domain,, The bleaching is induced instantaneously and decays with time as £~1 / / 2 , changing to a lower power after ~50 psec. Although the  photoinduced bleaching persists to long times, polarization memory is lost after ~10"9 sec. The results provide a measurement of the diffusion constant of the charged excitations; D^ 2x 10"2 cm2 sec"1 at 300 and 80 K

Strait, J.

Vardeny, Zeev Valentine

English

The University of Utah: J. Willard Marriott Digital Library

VOLUME 49, NUMBER 22 PHYSICAL REVIEW LETTERS 29 NOVEMBER 1982 Picosecond Photoinduced Dichroism in Trans- (CH) x: Direct Measurement of Soliton Diffusion Z. Vardenylal and J. Strait Division oj Engineering and Department oj Physics, Brown University, Providence, Rhode Island 02912 and D. Moses, T.-C. Chung, and A.J. Heeger Institute jor Polymers and Organic Solids, University oj California at Santa Barbara, Santa Barbara, Calijornia 93106 (Received 2 August 1982) Photoinduced dichroism caused by bleaching of the interband transition in trans-(CH)x was observed in the picosecond time domain, The bleaching is induced instantaneously and decays with time as t- 1/ Z, changing to a lower power after ~50 psec. Although the photoinduced bleaching persists to long times, polarization memory is lost after ~10- 9 sec. The results provide a measurement of the diffusion constant of the charged excita-tions; DC" 2x 10- z cmz sec- 1 at 300 and 80 K. PACS numbers: 78.20.Di, 78.40.Ge Soliton-antisoliton (S-5) pairs can be generated in trans-(CH)x either by charge-transfer doping or by photoexcitation. Dynamical calculations which follow the time evolution of a (CH)x chain after injection of an electron-hole (e-h) pair have been carried out by Su and Schrieffer.1 They found that the photoinjected e-h pair evolves to S-5 in a time of the order of ~ 10- 13 sec. Because of e-h symmetry, the formation of a soliton caus-es the formation of a localized electronic state at midgap.2 The resulting soliton can be neutral with spin ~ or charged + or - with spin O. Initial confirmation of these ideas came from photo excitation experiments.3,4 The photoproduc-tion of S -5 pairs implies the photogeneration of states in the gap, as observed experimentally.5,6 The reversed spin-charge relation is consistent with the results of ESR experiments? which dem-onstrated that the quantum efficiency (QE) for photoproduction of spins is below the QE for photo-production of charge carriers. Therefore, the photogenerated charge carriers are spinless and are identified as charged S -5 pairs. In this study, we investigate the time evolution of the photoexcited carriers in the picosecond time domain by using the pump and probe tech-nique (both polarized) with subpicosecond resolu-tion. We find photoinduced bleaching (A a < 0) in the interband transition region at 2 eV. The bleaching is generated instantaneously and decays with time as t- 1/2 followed by a slower decay. We observe photoinduced dichroism for the first time in a macroscopically isotropic solid, A a II "* A a 1-, where II and -1 refer to the polarization of the probe being parallel or perpendicular, respec-tively, to that of the pump. This photoinduced anisotropy is lost in the nanosecond time range. The polarization memory results from the photo-generation of localized states in regions of the polymer where the (CH)x chains are parallel to the pumping polarization. The eventual loss of photoinduced anisotropy at later times results from diffusion of the solitons over sufficiently large distances (~103 A) that the excited (CH)x chains are random with respect to the pump po-larization. This effect directly measures the diffusion coefficient; D"" 2 x 10- 2 cm2 sec- 1 at 300 and 80 K. The picosecond pump and probe measurements were done in the time domain 0.2 psec <t < 1.5 nsec. For t > 1 psec, we used a cavity-dumped passively mode-locked dye laserS delivering 0.7-psec-duration pulses, linearly polarized, at liw = 2 e V, 2 nJ per pulse, and repetition rate of 5 x 105 sec -1. For t < 1 psec, we used a different configuration (a ring laser) that delivers 0.2-psec-duration pulses (0.3 nJ) with 80 mHz repeti-tion rate. 9 The probe beam was delayed mechan-ically and then passed through a polarization ro-tator which set its polarization at 45° with respect to that of the pump. A polarizer was used on the transmitted probe beam to analyze Aa both for parallel and perpendicular polarization. lO The pump and probe beams (beam diameter ~ 40 !lm) were carefully adjusted to maintain complete spatial overlap on the sample. ll The samples were semitransparent films (~0.1 IJ.m thick) on glass or sapphire, initially polymerized as cis-(CH)x and subsequently isomerized to trans-(CH)x' Taking into account losses in the light path and © 1982 The American Physical Society 1657 VOLUME 49, NUMBER 22 PHYSICAL REVIEW LETTERS 29 NOVEMBER 1982 reflection losses at the sample, the photogener-ated carrier density per 2-nJ pulse (assuming a QE of unity) is 3X 1018 cm- 3• The photoinduced A a(T) produced with the ring laser for parallel and perpendicular polarizations at 300 K is shown up to 4 psec in Fig. 1 (Aa -AT / T, where T is the transmission and AT is the photoinduced change). At 300 K and at 80 K, AT / T "" 4x 10- 4 • The bleaching is induced instantane-ously; i.e., <2x10- l3 sec. Moreover, the initial decay in A a is extremely fast; by 2x 10- 13 sec, decay is evident. Since the films are macroscop-ically isotropic, the most unusual feature is that Aall*Aa.l' The ratio, p=Aa.l/Aall, attc><tp is p "" 0.5, with little change in the picosecond time range. Electron microscopy studies12 of thin (CH)x films indicate that the fibrils lie predominantly in the plane. For such a polycrystalline uniaxial material, the initial polarization ratio, p(O), should be t .10. 11 With trans -(CH)x we observed p(O) "'" 0.5 even for times shorter than 1 psec. The higher value arises from a combination of incomplete crystallinity (amorphous regions) and deviations from completely uniaxial behavior even in the crystalline regions. With neglect of the latter, an amorphous fraction! = 0.33 would raise p(O) to 0.5. Although analysis of x-ray scattering data from stretch-oriented free-standing films yields 13 ! "" 0.2, the short polymerization time (-1 sec) for the thin semitransparent films may lead to a somewhat higher value for! . In a traditional semiconductor, the photoex-cited carriers are delocalized and consequently lose polarization memory in times14 as short as -4 -3 -2 FIG. 1. Time dependence of the photoinduced bleach-ing in trans-(CH)x for parallel and perpendicular polar-ization at 300 K. 1658 10- 14 sec. Even in amorphous silicon/o where the band-edge states are localized, photoinduced anisotropy is not observed. Our observations in trans-(CH)x therefore imply that the electronic structure is quasi-one-dimensional and that lo-calization of the photoexcited carriers takes place extremely rapidly. This is consistent with S-S pair production. 1 Moreover, since these lo-calized photoexcitations are spinless,7 we identify them as S-S pairs. In Fig. 2weplotAall(T), A Cl'.l(T) , and Aa=Aall +A a.l from 1 psec to 1.5 nsec. The data indicate the existence of a slow depolarization process; A all gradually approaches A a.l (at 1.5 nsec, p "" 1). The curve A a(t) shows the true decay, not influenced by polarization-memory relaxation. At 300 K, A Cl'(t) decays as t- 1/2 up to about 50 psec and then slows to rO. 37• At 80 K, the r l12 behavior extends only to 12 psec and then slows to t-O. 25 • The relative response, A a(1500 psec)/ ACl'(l psec), is 5.7x10- 2 (80 K) and 3.8xlO- 2 (300 K). The initial, more rapid decay of Aa may be due to geminate recombination of S-S (or e-h) pairs. However, attempts to influence the decay with electric fields up to 5 xl 04 V cm - 1 were unsuc-cessful (at 300 and at 80 K). Moreover, the slow-er decay at 80 K (which results in a higher steady-state QE at lower temperatures) is in disagree-ment with geminate recombination. 15 Mele16 has extended the dynamical calculations to include e-h pair injection well away from the band edges. He finds that simultaneous relaxa-tion of the injected carriers toward the band edge plus the onset of distortions which lead to soliton " c " .!J ..s ~If- 10 5 " <1 10 t (ps) • .6Q II 4 /':,OJ. o /':,0" + /':,0J. 100 1000 FIG. 2. Mil (t) (circles), M~(t) (triangles), and /':,0' (t) = /':,0'11 + /':,O!~ (squares) at 300 K. VOLUME 49, NUMBER 22 PHYSICAL REVIEW LETTERS 29 NOVEMBER 1982 formation expands the time scale beyond the 10- 13 sec obtained for band-edge injection. 1 Thus, the initial r 112 decay may be providing direct infor-mation on the short-time evolution of the photo-excited carriers into S-S pairs. Defining the degree of polarization as P(t) =(Aall -t..a.l.)/(t.all +t.a.l.), we eliminate the nat-ural decay of both t. a II and t. a.l.' A semilog plot of P(t) is shown in the inset to Fig. 3. The in-duced polarization decays with a time constant of about 1200 psec. Note, however, that as demon-strated in Fig. 3, the photoinduced bleaching sig-nal persists out to longer times,5 implying a relatively long lifetime for the photoexcitations. The recovery of induced dichroism after pico-second laser pulses has been studied in liquids and gases. 17 In a fluid, the anisotropic orienta-tional distribution created by an intense linearly polarized picosecond pulse transforms to an iso-tropic one via the rotational motion of the mole-cules. At high concentrations the anisotropy can decay also by intermolecular energy transfer be-tween molecules of differing orientations. In trans-(CH)x the optical absorption is aniso-tropic as a result of the quasi-one-dimensional electronic structure. 18 After photoexcitation of a soliton on a (CH)x chain, the interband absorp-tion remains uniaxial; thus we expect that only the component of the optical electric field parallel to the local chain direction will be effective in pumping. Consequently, initially after the pump It. alii> It. a .l.1. Since the (CH)x chains are parallel to one another within a given fibril20 and oriented parallel to the fibril axis /0 inter chain excitation ::i ci f-"-f-<l -200 p 200 600 tips) tips) 1000 1400 FIG. 3. t:!.T/T at 300 K for 0< t < 1400 psec, showing the decay of polarization memory. The inset is a semi-log plot of P (t) at 300 and 80 K. transfer within a fibril will not lead to polariza-tion decay. The loss of polarization memory oc-curs, therefore, by diffusion of the photogenerated solitons along the fibrils over distances sufficient-ly great that the fibril direction has rotated by 1T / 4. Since electron microscopy has characterized such distances as typically -103 A, the corre-sponding diffusion constants must be relatively large. We assume that, at time t, the soliton arrives through diffusion to a position in the fibril form-ing an angle c.p with respect to its initial position. Geometrically, c.p(t)=L(t)/R, where L(t) is the diffusion length, L(t) = (2Dt )112, and R is the radius of curvature of the fibril. Averaging Over all fibril orientations with respect to the pump po-larization in a planar geometry, one obtains P(c.p) =~ cos(2c.p). Averaging over the random diffusive motion yields (P(c.p»=~(cos2c.p)=~ exp(- 2(c.p2». Thus, P(t) =P(O) exp(- 4Dt/R2). (1) Best fits to the data (see Fig. 3) yield D /R2 = 2.2 X 108 sec- 1 at 300 K and D/R 2 =1.7x108 sec- 1 at 80 K. Taking R ~ 103 A, D(300 K)= 2.2x10- 2 cm2 sec- 1 and D(80 K) = 1. 7 x 10- 2 cm2 sec- 1 • Kivelson21 described the diffusion of charged solitons in trans-(CHt in terms of intersoliton electron hopping among the localized midgap states. The inter chain diffusion of photogener-ated solitons over large distances may be lim-ited by a similar mechanism. In this case, how-ever, since the charged solitons are not bound (to ions), phonon-assisted hopping would be less important implying a weaker temperature de-pendence, similar to that of neutral solitons. The values quoted above for D are, in fact, compar-able to those obtained for neutral solitons from magnetic measurements. 22 In conclusion, we found that the band-to-band transition in trans -(CH)x is bleached instantane-ously within our resolution of 0.2 psec, in agree-ment with S-S pair production. Analysis of the decay of the photoinduced anisotropy indicates soliton diffusion over distances"" 103 A with dif-fusion coefficient of about 0.02 cm2 sec- 1 • This work was supported by the National Science Foundation-Materials Research Laboratory pro-gram and by the National Science Foundation under Grant No. DMR-79-09819. This work was also supported by the Defense Advanced Re-search Projects Agency- U. S. Office of Naval Research through a grant monitored by the U. S. Office of Naval Research. We are grateful to 1659 VOLUME 49, NUMBER 22 PHYSICAL REVIEW LETTERS 29 NOVEMBER 1982 Dr. Robin Ball for helpful comments. (a)Permanent address: Physics Department, Tech-nion, Haifa, Israel. lW. P. Su and J. R. Schrieffer, Proc. Nat. Acad. Sci. U.S.A. 77, 5626 (1980). 2W. P-:-Su, J. R. Schrieffer, and A. J. Heeger, Phys. Rev. Lett. 42, 1698 (1979), and Phys. Rev. B 22, 2099 (1980). - -3S. Etemad, T. Mitani, and M. Ozaki, T.-C. Chung, A. J. Heeger, and A. G. 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Picosecond photoinduced dichroism in trans-(CH)x: direct measurement of soliton diffusion

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Picosecond photoinduced dichroism in trans-(CH)x: direct measurement of soliton diffusion

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