Issues in Organics for Nonlinear Optics
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GEORGE I. STEGEMAN AND WILLIAM TORRUELLAS CREOL, University of Central Florida 12424 Research Parkway, Orlando, FL 32826 INTRODUCTION The potential of organic materials for nonlinear optics was recognized early with the identification of charge transfer concepts for second order materials, and s-electron delocalization in conjugated polymers for third order effects.[1,2] Nevertheless, they have not been seriously considered for applications until the past five years over which applications to electro-optics have been vigorously pursued. At this point many of the relevant technological problems for electro-optics have been faced and solved. This progress has been driven by imminent applications in information processing and communications. Frequency conversion, specifically doubling of near infrared lasers into the blue for data storage etc., also relies on the second order nonlinearities. But progress towards efficient doublers has been dominated by other well-established materials such as KTP and LiNbO 3 because of the short time window perceived for applications.[3,4] However, because of the inherently large nonlinearities associated with organic materials, one would expect that ultimately the most efficient doubling devices could be made from organic materials. The situation for third order devices based on organics is simultaneously less advanced but very promising. The current applications seem to be to optical limiters and ultrafast switching for information processing and routing.[5,6] Organic materials, primarily because of their rich excited state absorption spectrum, provide the most useful optical limiting materials.[5] In the ultrafast processing area, only optical fibers or semiconductors used with photon energies below half their bandgap have been proven to satisfy all of the required figures of merit (FOM).[6] Already a few organic materials have been shown to satisfy the FOM at isolated wavelengths and the key problem is the lack of a useful data base. In this paper we will examine the issues faced when organic materials are used for nonlinear optics, primarily from an gptics point of view. That is, we will examine material requirements from the perspective of devices and comment on the questions which in our opinion need to be addressed, and, in some cases, solved. ELECTRO-OPTICS Electro-optic devices require materials in which a refractive index change is produced which is linear in an applied field (DC to microwave frequencies). The geometry of a simple Mach-Zehnder electro-optic switch is shown schematically in Fig. 1. The refractive index in parts of the two paths is different so that there is a net phase difference between the two arms:
A0 1 -2 oc n 3 re
V
(L)
where V is the voltage applied between the electrodes and n 3reff is the electro-optic figure of merit with reff being the effective electro-optic coefficient. For a high throughput device, L 20 db.[15] This is excellent performance! A prototype modulator for cable TV was demonstrated by the IBM group.[16] A number of the issues disc
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