We investigated the photochemical stability of an electro-optic (EO) polymer under laser irradiation at 1310nm to reveal photodegradation mechanisms. It was found that one-photon absorption excitation assisted with the thermal energy at the temperature is involved in the photodegradation process, in contrast to our previous studies at a wavelength of 1550nm where two-photon absorption excitation is involved in the photodegradation process. Thus, both the excitation wavelength and the thermal energy strongly affect to the degradation mechanism. In any cases, the photodegradation of EO polymers is mainly related to the generation of exited singlet oxygen.

In this review paper we showcase recent activities on silicon photonics science and technology research in Hong Kong regarding two important topical areas--microresonator devices and optical nonlinearities. Our work on silicon microresonator filters, switches and modulators have shown promise for the nascent development of on-chip optoelectronic signal processing systems, while our studies on optical nonlinearities have contributed to basic understanding of silicon-based optically-pumped light sources and helium-implanted detectors. Here, we review our various passive and electro-optic active microresonator devices including (i) cascaded microring resonator cross-connect filters, (ii) NRZ-to-PRZ data format converters using a microring resonator notch filter, (iii) GHz-speed carrier-injection-based microring resonator modulators and 0.5-GHz-speed carrier-injection-based microdisk resonator modulators, and (iv) electrically reconfigurable microring resonator add-drop filters and electro-optic logic switches using interferometric resonance control. On the nonlinear waveguide front, we review the main nonlinear optical effects in silicon, and show that even at fairly modest average powers two-photon absorption and the accompanied free-carrier linear absorption could lead to optical limiting and a dramatic reduction in the effective lengths of nonlinear devices.

Novel functionality and material were developed for Si-photonics in this study. Ultra-fast silicon all optical switches using two-photon absorption (TPA) were developed in silicon nanowire optical waveguide on silicon-on-insulator substrate. This waveguide can produce high optical intensities that yield optical nonlinearity such as TPA even at input optical powers typically used in fiber optic communication systems. In addition, we fabricated a GaSb based quantum well (QW) on a Si substrate. The emission wavelength of QW was 1.55 µm at room temperature, so that the new function can be developed on Si-photonics using this QW.

An all-optical switch based on a single photonic crystal defect with an air-bridge configuration and two-photon absorption was proposed, fabricated and characterized. In optical measurements, we obtained a sharp defect mode with a quality factor higher than 600 at 1.55 µm. More importantly, we observed its nonlinear response to the excitation of ultrashort pulses by utilizing two-photon absorption. Nonliner refractive index change of about -410-3 was achieved at a pumping power density of 3.6109 W/cm2.

Competition of two-photon and one-photon absorption in Si-APD was studied. Device should be cooled down in order to clearly observe two-photon absorption at low illumination intensity. Passive Geiger mode operation was studied to sensitively detect small number of carriers generated by two-photon absorption. The illumination intensity dependence of the photocurrent pulse count number is well explained by taking into account the two absorption mechanisms and a dead time period that depends on bias voltage.

We report the dispersion of two-photon absorption (TPA) coefficient, (β), in polydiacetylene (12, 8) thin film waveguides in the wavelength range less than the one-photon band-gap with a 100 femtosecond mode-locked Ti: Sapphire laser pulses. The TPA coefficient was found to be 4 cm/GW for TE polarization at 900 nm (1.38 eV) by taking into account a Gaussian intensity distribution as well as a temporal pulse shape. We observed a sharp resonance in β above the first one-photon allowed transition with a full width at half maximum (FWHM) of 90 meV.