ELSEVIER Synthetic Metals 84 (1997) 547-548 Femtosecond electron-transfer holography in C&polymer blends Eric Maniloff’, Dan Vac&, Duncan McBranchl, Hsing-Lin Wang’, Ben jamin Mattesl, and Alan J. Heege? ‘Chemical Sciences and Technology Division, Los Alatnos National Lab, Los Alamos, NM 87545 , USA %stitute for Polymers and Organic Solids, University of California, Santa Barbara, SantaBarbara, CA 93106, uspI Abstract Holographic recording has recently been demonstrated in conducting polymer/C60 blends. Results are presented that demonstrate an improved signal-to-noise ratio is obtained when holographic detection is used to observe the dynamics of photo-induced absorption. Keywords: Non-linear optical methods, poly(phenylene vinylene) and derivatives, Fullerenes and derivatives, Photoinduced absorption spectroscopy, Laser Spectroscopy There has been considerable recent interest in holographic gratings for a variety of optical applications, particularly in the areas of interconnection networks, optical memories, and optical computing[l,2]. The vast majority of research has concentrated on inorganic crystals, however recently there has been considerable interest in holography using organic materials. Advantages of organic holographic materials include ease of fabrication, cost, and the ability to more readily tailor material properties to a particular application. Most of the organic holographic materials studied to date respond on time scales ranging from milliseconds to minutes. Charge transfer holography has recently been demonstrated in Ceo/polymer blends [3]. The recorded holograms rely on charge transfer from the polymers to Cm [4]. These materials are interesting from a technical perspective, since the active step in the holographic recording procedure occurs in less than 300 fs. The nature of the excited states of conducting polymers, and the mechanism of the slow relaxation of the charge transfer state have been a subject of considerable recent interest. Holographic detection is interesting as a sensitive probe of the materials dynamics. The experimental results presented here demonstrate the improvement in signal-to- noise ratio (SNR) obtained when holographic detection is used. Almost any material which undergoes a change in either absorption or refractive index under illumination can be used as to record holograms. The most commonly studied organic holographic recording materials are photochromic materials, which undergo a shift in their absorption peak after photoinduced isomerization. A number of papers have been published demonstrating the capability of these materials to record images, however the response time of these materials typically ranges from milliseconds to minutes. Organic photorefractive materials have also been a topic of recent interest, due to their large diffraction efficiency, however these materials also respond slowly to incident light because of their requirement that charges drift over distances of several microns to record the holograms. Because of the slow response of most holographic materials, if such materials are illuminated with pulsed light they respond only to the time averaged field. The newly discovered phenomenon of charge transfer from conducting polymers to C6u opens up an exciting new class of holographic recording materials. Because the recording takes place in times less than 300 fs, the materials can be used to record holographic gratings of individual femtosecond laser pulses. Holographic detection provides a sensitive probe of absorption 0379-6779/97/%17.00 0 1997 Elsevier Science Sk All rights reserved PII SO379-6779(96)04044-1 and refractive index changes in materials. Since holographic detection is a background-free measurement, it can provide a more sensitive probe than (PIA) measurements. An alternative framework for analyzing transient grating measurements is that of nonlinear optics. Within this framework four-wave mixing phenomena that are due to excited states (rather than to a coherent electronic response) are referred to as incoherent x(3) processes[S]. In conducting polymer/C60 blends, the absorption modulation arises from an excited state of the blend, and it is therefore possible to obtain more control over the nonlinearity than in a single species material. Charge transfer holography experiments have been performed using a number of conducting polymers and oligomers. In the materials which we have studied, the peak absorption of the polymer is centered between 400 and 500 nm, and the induced absorption is strongest in the near infrared (close to 800 nm). The experiments are therefore conducted using a nondegenerate four wave mixing setup, in which the pump wavelengths excite electrons across the ft-X* energy gap, and the absorption coefficient at the probe wavelength is spatially modulated by the induced absorption. Nondegenerate four wave mixing allows a significantly increased diffraction efficiency as opposed to the more common degenerate approach. In the ideal nondegenerate experiment, the pump waves are strongly absorbed, while the probe wave is fully transmitted in the absence of the pump wave. If the probe wave is strongly modulated by the induced absorption, a large diffracted signal results. A simple analytical treatment has shown that a two order of magnitude increase in the maximum diffracted signal is possible for non-degenerate (compared to degenerate) four-wave mixing using incoherent nonhnearities[3]. Experiments demonstrating holographic recording were conducted using an amplified Ti:sapphire laser (Clark-MXR CPA 1000) operating at 800 nm, with a 150 fs pulsewidth at the sample plane. The second harmonic (400 nm) was used for the two pump waves. The fundamental was used as a probe wave, since this wavelength lies outside the absorption band but is near the maximum of the induced absorption for the polymers studied. The sample was kept in vacuum to inhibit photo-bleaching processes, and was illuminated over an area of approximately 1 mm2. The probe wave was passed through a computer controlled delay arm in order to probe the time dynamics of the recorded gratings. All three incident waves were vertically polarized. Diffracted signal intensities were detected by chopping one of the pump waves at a frequency of 140 Hz, and measuring the