All-optical switching and filtering based on liquid crystals and photosensitive composite organic materials

Integrated optic devices using low driving power are crucial to develop new photonic systems both for optical communications and for sensing. The large electro-optic effect and nonlinear optical properties of liquid crystals (LC) and doped LC novel materials allow the realization of low cost optoelectronic devices. We present our recent experimental and theoretical results on optical channel waveguides based on LC made on both glass and silicon substrates. Devices made of waveguides using a LC core can operate both in linear [1] and nonlinear optical regime at 1550 nm wavelength spectral region [2]. As linear waveguides they can behave both as variable optical attenuators and as optical switches by exploiting the electrooptic effect in a nematic LC (NLC). An applied voltage lower than just 5 V was required to drive an electro-optic switch with an on-off contrast of more than 40 dB. Optical Freedericks transition was also experimentally observed when an input optical power of just 25 mW was fiber coupled to the same NLC waveguide. Modelling of the waveguide was obtained by minimizing the LC total free energy, including the dielectric energy at optical frequencies by using finite elements. Preliminary simulations of waveguides using azodye doped LC indicate that a further reduction of driving power is possible. All-optical tunable filters can also be obtained by using photosensitive composite materials. Recent experimental findings will be presented on an integrated optic filter, which combines the simple and low cost ion-exchange waveguide technology with a composite LC methyl red (MR) azo-dye photosensitive compound to obtain full optical tuneability. The structure of the integrated optical filter recalls a previous POLICRYPS based electro-optic tuneable filter [3]. The prototype showed a pass-band with more than 20 dB signal suppression at the Bragg wavelength with a bandwidth of the transmitted notch of about 3 nm. A tuning range of 6.6 nm was observed by applying a pump signal of 45 mW [4]. Full consistency between experiment and simulation results was found. The measured shift is nearly double from previous results achieved in electro-optically tunable POLICRYPS based filters, because of the nematic liquid crystals methyl red properties. These devices can trigger the development of a new generation of low power, compact and low cost all-optical components for next generation fiber optic telecom and sensor systems. References [1] D. Donisi, B. Bellini, R. Beccherelli, R. Asquini, G. Gilardi, M. Trotta, and A. d’Alessandro, “A switchable liquidcrystal optical channel waveguide on silicon”, IEEE J. Quantum Electron. 46, 762–768 (2010). [2] A. d’Alessandro, R. Asquini, M. Trotta, G. Gilardi, R. Beccherelli and I. C. Khoo, “All optical intensity modulation of near infrared light in a liquid crystal channel waveguide”, Appl. Phys. Lett. 97, 093302 (2010). [3] A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating”, Opt. Express 16, 9254–9260 (2008), Patent US 7925124 B2 (2011). [4] G. Gilardi, L. De Sio, R. Beccherelli, R. Asquini, A. d’Alessandro, and C. Umeton, “Observation of tunable optical filtering in photosensitive composite structures containing liquid crystals”, Opt. Lett. 36, 4755–4757 (2011).