(Fig. 3B) and 2y (Fig. 3C) directions. All three experiments were performed with pristine films. The upper panels show the sum SI of two adjacent intensity maxima as shown in Fig. 2. The angle of incidence of the fundamental beam was chosen such that the Fresnel factors are higher at the lens-prism interface compared with the prism-air interface (6). Thus, the area of increased SHG intensity directly reflects the size of the contact area. The middle panels show maps of DI. For normal loading of the lens (Fig. 3A), DI changes sign across the con- tact area, indicating that the forces are axially symmetric with respect to the initial point of contact, causing a radial alignment of the mol- ecules. Because we only probed alignment along the y axis, uDIu for molecules aligned along the x axis is zero and gradually increases with increasing alignment along y. This is seen in the radially symmetric case in which DI changes sign at y 5 0. In contrast, rolling causes a unidirectional alignment over the whole contact area (Fig. 3, B and C). It is evident from the alignment seen under both normal loading and rolling conditions that shear forces act on the film. However, their origin is not clear at present. Microslip due to the difference in the elastic constants of the prism and the lens seems to be too small to account for the alignment, and, furthermore, an alignment pattern different from those depicted in Fig. 3, B and C, is expected from standard models in contact mechanics (23). Thus, other mechanisms should be consid- ered, such as squeezing of the organic film, which would produce a lateral force originat- ing from the point of highest pressure out- ward. Also, interfacial contaminations such as an adsorbed water film that is squeezed out of the contact area under loading could give rise to an additional shear force. The results presented here demonstrate that the tribological properties of surface coatings and lubricants between two bodies can be monitored in situ to study molecular changes as a function of loading conditions and to monitor their durability or wear. They can be extended to sum frequency generation (SFG) (28) studies, which will then allow the study of confined organic films more relevant to technical applications than the model sys- tem presented here. References and Notes 1. B. Bhushan, J. N. Israelachvili, U. Landman, Nature 374, 607 (1995). 2. R. Maboudian, MRS Bull. ( June 1998), p. 47. 3. J. A. Harrison and S. S. Perry, MRS Bull. ( June 1998), p. 27. 4. S. A. Joyce, R. C. Thomas, J. E. Houston, T. A. Michal- ske, R. M. Crooks, Phys. Rev. Lett. 68, 2790 (1992). 5. Q. Du et al., Phys. Rev. B 51, 7456 (1995). 6. R. Frankel, G. E. Butterworth, C. D. Bain, J. Am. Chem. Soc. 120, 203 (1998). 7. A. Berman, S. Steinberg, S. Campbell, A. Ulman, J. Israelachvili, Tribol. Lett. 4, 43 (1998). 8. J. N. Israelachvili and D. Tabor, Nature 241, 148 (1973). 9. J. N. Israelachvili, J. Vac. Sci. Technol. A 10, 2961 (1992). 10. G. Liu and M. Salmeron, Langmuir 10, 367 (1994). 11. S. Karaborni, Phys. Rev. Lett. 73, 1668 (1994). 12. K. J. Tupper and D. Brenner, Langmuir 10, 2335 (1994). 13. K. J. Tupper, R. J. Colton, D. W. Brenner, Langmuir 10, 2041 (1994). 14. R. Henda, M. Grunze, A. J. Pertsin, Tribol. Lett. 5, 191 (1998). 15. J. I. Siepmann and I. R. McDonald, Langmuir 9, 2351 (1993). 16. S. H. J. Idziak et al., Science 264, 1915 (1994). 17. P. M. Cann and H. A. Spikes, Tribol. Trans. 34, 248 (1991). 18. G. J. Johnston, R. Wayte, H. A. Spikes, Tribol. Trans. 34, 187 (1991). 19. Y. R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984). 20. R. M. Corn and D. A. Higgins, Chem. Rev. 94, 107 (1994). 21. The sample preparation followed (29) with the use of a solution of N-[3-(trimethoxysilyl)propyl 2,4-dini- trophenylamine] in toluene. 22. D. S. Chemla and J. Zyss, Eds., Nonlinear Optical Properties of Organic Molecules and Crystals, vol. 1 (Academic Press, Orlando, FL, 1987). 23. K. L. Johnson, Contact Mechanics (Cambridge Univ. Press, Cambridge, 1985). 24. The following data were used in the calculation. The radius of the curvature of the lens was 250 mm. Elastic moduli of the SF10 glass prism and the BK7 glass lens were 64 GN/m 2 and 82 GN/m 2 , respectively. The Pois- son ratios were 0.23 for SF10 and 0.206 for BK7. 25. B. U. Felderhof, A. Bratz, G. Marovsky, O. Roders, F. Sieverdes, J. Opt. Soc. Am. B 10, 1824 (1993). 26. Because a detailed treatment is beyond the scope of this report, we make the simplifying assumption that the signal originates only from the organic layer. This does not hold exactly (27) but can be neglected for the purpose of this report. 27. A. Legant, thesis, University of Heidelberg, Heidel- berg, Germany (1999). 28. C. D. Bain, in Modern Characterization Methods of Surfactant Systems, B. P. Binks, Ed. (Dekker, New York, 1999), pp. 336 –373. 29. K. Bierbaum et al., Langmuir 11, 512 (1995). 30. This work was funded by the Office of Naval Re- search. Additional support from the Deutsche For- schungsgemeinschaft and the Fond der Chemischen Industrie is gratefully acknowledged. We are grateful to W. Eck for purifying the DNS. 1 September 1999; accepted 22 November 1999 Ultrafast Mid-Infrared Response of YBa 2 Cu 3 O 7-d R. A. Kaindl, 1 M. Woerner, 1 T. Elsaesser, 1 * D. C. Smith, 2 J. F. Ryan, 2 G. A. Farnan, 3 M. P. McCurry, 3 D. G. Walmsley 3 Optical spectra of high-transition-temperature superconductors in the mid- infrared display a gap of in-plane conductivity whose role for superconductivity remains unresolved. Femtosecond measurements of the mid-infrared reflec- tivity of YBa 2 Cu 3 O 7-d after nonequilibrium optical excitation are used to dem- onstrate the ultrafast fill-in of this gap and reveal two gap constituents: a picosecond recovery of the superconducting condensate in underdoped and optimally doped material and, in underdoped YBa 2 Cu 3 O 7-d , an additional sub- picosecond component related to pseudogap correlations. The temperature- dependent amplitudes of both contributions correlate with the antiferromag- netic 41-millielectronvolt peak in neutron scattering, supporting the coupling between charges and spin excitations. A number of energy-sensitive studies on high- transition-temperature (high T C ) cuprate super- conductors including tunneling (1), angular-re- solved photoemission (2), neutron scattering (3), Raman scattering (4 ), and infrared reflectivity (5– 8) suggest that understanding the elementary excitations in the mid-infrared energy range (\v ’ 40 to 200 meV) in the vicinity of the superconducting gap is essential for clarifying the mechanisms behind formation of the super- conducting condensate. In particular, the low- energy electromagnetic response of such cup- rates contains valuable information on electron- ic excitations and their correlated dynamics. When temperature T is lowered below the su- perconducting transition in optimally doped ma- terials, the most pronounced changes of the reflectivity R(v,T ) for light of frequency v po- larized parallel to the superconducting CuO 2 planes [(ab)-plane reflectivity] appear in the spectral range around \v ’ 100 meV. These reflectivity changes (Fig. 1A) are directly con- nected with a strong depression of the in-plane conductivity (Fig. 1C), which has been attribut- ed to the opening of a gap for electronic transi- tions involving inelastic collisions. A straight- forward association with the superconducting gap, however, is hampered by the observation that, in underdoped cuprates, such features al- ready occur at temperatures T * substantially higher than T C (Fig. 1, B and D) and are termed pseudogap (5– 8). While stationary infrared spectroscopy mea- sures the total of all contributions to the mid- infrared reflectivity and cannot clearly discern spectrally similar components in the pseudogap 1 Max-Born-Institut fu ¨r Nichtlineare Optik und Kurz- zeitspektroskopie, D-12489 Berlin, Germany. 2 Depart- ment of Physics, Clarendon Laboratory, Oxford Uni- versity, Oxford OX1 3PU, UK. 3 Department of Pure and Applied Physics, Queen’s University of Belfast, Belfast BT7 1NN, UK. *To whom correspondence should be addressed. R EPORTS 21 JANUARY 2000 VOL 287 SCIENCE www.sciencemag.org 470