<title>Guided-mode resonance in a thin azopolymer film via a surface grating</title>

Abstract

ABSTRACT A slab waveguide is made by spin coating a thin film of azopolymer onto a glass slide. A surface grating is then optically generated and acts as optical coupler. The structure exhibits resonance coupling as a function of the incident probe wavelength and angle of incidence. The transmission of the film is observed to decrease by more than 50% at resonance. Thewavelength at resonance is seen to vary linearly with the angle of incidence for small angles.Keywords: Azopolymer, resonance, waveguide, surface grating 1. INTRODUCTION It has recently been found that high efficiency surface gratings can be directly inscribed onto the surface of thinpolymer films using a low power laser beam13. Thin polymer films containing azobenzene side groups are spin coated ontosubstrates. These are then exposed for a few minutes to a two beam interference pattern produced from an argon laser beamat powers of about 50 mW/cm2 at room temperature, which is well below the glass transition temperatures of the polymerfilms. This results in the formation of stable surface gratings on the polymer surface. The polymer films are transparent in thered and near infrared, and the gratings act as phase gratings in this region of the electromagnetic spectrum. The grating depthis controlled by adjusting the exposure time. The grating spacing is also easily controlled and spacings from 3000 nm to 300n.m are readily obtained using the 488nm line of an argon laser. The gratings are stable for temperatures up to the glasstransition temperatures of the polymer.Surface phase gratings can be inscribed into the polymer films that act as slab waveguide structure. Thus light canbe coupled into the slab waveguide by properly matching the diffracted waves from the gratings to the wave propagationmodes of the waveguides. This coupling is achieved by the selection of the physical parameters of both the waveguide andthe grating, and we have shown that the azopolymer films can be used for this purpose4'5. The coupling is a resonancecondition which has been studied for many years69. In the present study we have focussed on the changes in the transmissionproperties of thin film structures which occur at resonance. Theoretical predictions and some experiments have shown thatvery high changes in these optical properties can be expected and the potential use of resonant grating structures as opticalfilters is being considered.In the present work we investigate resonant structures made from azopolymer thin films with surface relief gratings.The ease of fabrication of the films and of the control of the grating parameters make these structures very attractive for bothfundamental studies as well as device fabrication.