Fig. 1. Schematic of porous silicon microcavity (a) and SEM images showing the morphology of (b) mesoporous silicon layers and (c) macroporous silicon layers. In the schematic, the yellow layers are low porosity (high refractive index) and the red layers are high porosity (low refractive index). In the SEM images, the darker regions represent the void space and the bright area is the silicon matrix. Therefore, the brighter layers have lower porosity than the darker ones. The pore openings of the macropores are much larger than those of the mesopores.

Fig. 2. Reflectance spectra, before liquid crystal infiltration, of (a) mesoporous silicon microcavity measured by a spectrophotometer and (b) macroporous silicon microcavity measured by an optical spectrum analyzer with a fiber coupled Xenon arc lamp. Tuning of the resonance wavelength after liquid crystal infiltration enables optical modulation.

Fig. 3. (a) Resonance wavelength red shift as a function of applied voltage for mesoporous and macroporous silicon microcavities with positive anisotropy E7 liquid crystals. The liquid crystals rotate more freely in the macroporous silicon, which leads to the larger wavelength shift. Electrical contact is made to the crystalline silicon and ITO-coated glass on top of the microcavity (inset). (b) Resonance wavelength red shift as a function of applied voltage for mesoporous silicon microcavities with negative anisotropy ZLI-4788 liquid crystals.

Fig. 4. Thermal tuning of mesoporous and macroporous silicon microcavities with liquid crystals. The resonance shift takes place at different temperatures depending on the phase transition temperature of the infiltrated liquid crystal, as shown in (a)-(c). The refractive index increases when liquid crystals change from the ordered nematic phase to disordered isotropic phase (inset (a)). (d) For a resonance with a measured Q-factor of 400, the resonance shift resulting from the E7 phase transition corresponds to a 14 dB extinction ratio.

Fig. 5. The extinction ratio is calculated as the change in transmission at the resonance wavelength (inset). As the Q-factor of the resonance increases, the achievable extinction ratio increases, for a given refractive index change. The magnitude of the refractive index change determines the magnitude of the resonance wavelength shift. When the refractive index change becomes too large, the resonance wavelength begins to shift beyond the stopband, which reduces the extinction ratio. For a microcavity with a Q-factor of 600, the maximum attenuation is achieved for Δn=0.1.