Volumetric optical memory based on bacteriorhodopsin Jeffrey A. Stuart a,* , Duane L. Marcy a , Kevin J. Wise b , Robert R. Birge b a W.M. Keck Center for Molecular Electronics, Syracuse University, Syracuse, NY 13244, USA b Department of Chemistry, University of Connecticut, Storrs, CT 06269, USA Abstract An architecture for a three-dimensional optical computer memory based on the photoactive protein bacteriorhodopsin (BR) is described, utilizing a branch-off of the BR photocycle to access a long-lived photointermediate capable of serving as an active element for memory storage. This intermediate (the Q-state) is accessed as a result of the sequential absorption of two photons, the first to initiate the BR photocycle, and the second to drive the protein into the branched photocycle from the O-state several milliseconds later. The stability of the Q-state arises predominantly from the fact that it is strongly blue-shifted with respect to other intermediates in the photocycle, making it invisible to the laser wavelengths used to write and read information in the memory. Both proof of principle and second-generation prototypes are currently being developed in the W.M. Keck Center for Molecular Electronics, in collaboration with Critical Link, LLC of Syracuse, NY. The article will focus on the BR-branched photocycle memory architecture, the remaining challenges to fabrication of a commercially viable device, and the ongoing efforts in prototype development, optimization and protein characterization at Syracuse University. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Bacteriorhodopsin; Volumetric optical memory; Q-state 1. Introduction Three-dimensional optical memories have the potential to offer tremendous advantages over the conventional silicon disk and platter-based systems. The performance of current software applications is often limited, not by the quality of the computer processor, but by the ability of the computer memory to manipulate stored information quickly and efficiently. When coupled with the fact that silicon-based technologies will probably reach their prac- tical limit within the next few decades, the need for new memory architectures becomes increasingly apparent. The advantages offered by volumetric optical memories are primarily in the form of storage density and optical data throughput, and are made possible by utilizing light to store and manipulate information in an optically transpar- ent volumetric matrix. The materials comprising both the matrix and the active memory element must meet several criteria in order to work efficiently: (1) the active component of the material which is utilized for memory storage must respond to light such that it can be revers- ibly driven between two stable, long-lasting states, (2) the active memory component must not respond to the actinic wavelengths used to write and read information such that previously stored information is disturbed or degraded in any way (i.e., routine write and read memory operations must be non-destructive), (3) the material comprising the matrix that encapsulates the active memory component must be optically transparent with a low potential for scattering light, (4) the matrix material must remain stable over long periods of time, and (5) the active memory element must possess a high cyclicity and must be switch- able between states with a reasonable efficiency. Cyclicity refers to the average number of times the active memory element can be switched between states before it degrades. Two of the most actively researched materials for volu- metric optical memories are the class of organic photochro- mic spiropyrans [1,2] and the photochromic protein bacteriorhodopsin (BR) [3–9]. An excellent review of volu- metric storage technologies can be found in Ref. [10]. This treatment will focus solely on the BR-based volumetric memory in development at the W.M. Keck Center for Molecular Electronics at Syracuse University. BR has long been known for its unique photochemical and photophysical properties, and over the last 30 years it has become one of the most heavily researched of all proteins. Initial interest was stimulated by its common structural motif and chromophore with rhodopsin, the protein respon- sible for all dim and non-color vision in most vertebrates. Whereas the interaction of light with rhodopsin results in Synthetic Metals 127 (2002) 3–15 * Corresponding author. Tel.: þ1-315-443-3098; fax: þ1-315-443-4070. E-mail address: jastuart@syr.edu (J.A. Stuart). 0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0379-6779(01)00586-0