Electro-optic polymer modulates light based on surface plasmon resonance

An optical modulator made of thin-film electro-optic polymer sandwiched between silver thin-film contacts on a glass substrate has been demonstrated by a grou¥at the University of Washington (Seattle, WA). An incident beam propagating through the substrate reflects off the lower silver layer, generating an evanescent field in the metal film. This in turn excites a surface plasmon wave (SPW) at the silver-polymer interface. The SPW is excited by the beam at an incident angle that is a function of source wavelength, layer thickness, and polymer index of refraction. For a bottom silver thin film of appropriate thickness, all of the incident light is coupled into the SPW. When the index of refraction of the polymer changes, however, full coupling does not occur and incident light will propagate through the substrate. Thus the device is capable of modulation as a function of the polymer refractive index. The device consists of a 250-nm silver thin film ato¥a 3.3-µm-thick polymer layer deposited over a 51.2-nm silver bottom layer that is evaporated onto a transparent leaded-glass substrate. A 50:50 mixture of P2ANS/MMA polymer (Hoechst Celanese) was used, and the index of refraction was modulated from 1.72 to 1.74 by an applied voltage. In initial studies, the grou¥has obtained linear modulation of approximately 70% over this range, with full absorption of the incident beam occurring at an index of 1.72. The experiments were performed with a 632.8-nm source, but researchers have begun work on a device optimized for 1.3-µm wavelength.