Here, we demonstrate a new type of microphotoreactor formed by a liquid-core optofluidic waveguide fabricated inside aerogel monoliths. It consists of microchannels in a monolithic aerogel block with embedded anatase titania photocatalysts. In this reactor system, aerogel confines core liquid within internal channels and, simultaneously, behave as waveguide cladding due to its extremely low refractive index of ~1. Light is confined in the channels and is guided by total internal reflection (TIR) from the channel walls. We first fabricated L-shaped channels within silica aerogel monoliths (ρ= 0.22 g/cm3, n=1.06) without photocatalyst for photolysis reactions. Using the light delivered by waveguiding, photolysis reactions of methylene blue (MB) were carried out in these channels. We demonstrated that MB can be efficiently degraded in our optofluidic photoreactor, with the rate of dye photoconversion increasing linearly with increasing power of incident light. For photocatalytic transformation in this reactor system, titania particles were successfully embedded into the mesoporous network of silica aerogels with varying amount of the titania in the structure from 1.7 wt % to 50 % wt. The presence of titania and its desired crystalline structure in aerogel matrix was confirmed by XRF, XRD patterns and SEM images. Band gap of silica-titania composites was estimated from Tauc plot calculated by Kubelka-Munk function from diffuse reflectance spectra of samples as near expected value of ≈ 3.2 eV. Photocatalytic activity and kinetic properties for photocatalytic degradation of phenol in the channels were investigated by a constant flow rate, and longer-term stability of titania was evaluated.
We present a new method to form liquid-core optofluidic waveguides inside hydrophobic silica aerogels. Due to their
unique material properties, aerogels are very attractive for a wide variety of applications; however, it is very challenging
to process them with traditional methods such as milling, drilling, or cutting because of their fragile structure. Therefore,
there is a need to develop alternative processes for formation of complex structures within the aerogels without
damaging the material. In our study, we used focused femtosecond laser pulses for high-precision ablation of
hydrophobic silica aerogels. During the ablation, we directed the laser beam with a galvo-mirror system and,
subsequently, focused the beam through a scanning lens on the surface of bulk aerogel which was placed on a three-axis
translation stage. We succeeded in obtaining high-quality linear microchannels inside aerogel monoliths by
synchronizing the motion of the galvo-mirror scanner and the translation stage. Upon ablation, we created multimode
liquid-core optical waveguides by filling the empty channels inside low-refractive index aerogel blocks with highrefractive
index ethylene glycol. In order to demonstrate light guiding and measure optical attenuation of these
waveguides, we coupled light into the waveguides with an optical fiber and measured the intensity of transmitted light as
a function of the propagation distance inside the channel. The measured propagation losses of 9.9 dB/cm demonstrate the
potential of aerogel-based waveguides for efficient routing of light in optofluidic lightwave circuits.