Tin oxide (SnO<sub>2</sub>) thin film gas sensors that function at room temperature have been fabricated on nanostructured substrates. After femtosecond laser irradiation of the surface of the SnO<sub>2</sub>, the sensitivity to gases, for example, carbon monoxide, increased noticeably. The dependence of the sensitivity on the number of laser pulses has been investigated. It is believed that the femtosecond laser pulses generate defects in a thin layer on the SnO<sub>2</sub> sensor surface. These defects may result in a potential energy well creating surface bound states for electrons to move on the surface, which increases the sensitivity to gases.
SnO<sub>2</sub> thin film room-temperature gas sensors have been fabricated on silicon nanospike surfaces prepared by
femtosecond pulsed laser irradiation. The surface morphologies of the as-fabricated silicon nanospikes and SnO<sub>2</sub> thin
film gas sensors indicate that the surface roughness increased significantly after the SnO<sub>2</sub> layer was deposited. The
surface morphology and electric field distribution of the silicon nanospikes were studied with atomic force microscopy
(AFM) and the electric force microscopy (EFM), respectively. The comparison between AFM morphology and EFM
images shows that the aspect ratio of the nanostructures in the EFM image was larger than that in the AFM image, which
indicates that the nanospikes on the silicon surface can induce an enhanced electric field around their sharp features. The
electric field around the tips is further enhanced when there is electric current flowing through the SnO2 layer. The
enhanced electric field and increased surface area on the nanospike structures are the main contributors to the high
sensitivity of these room temperature gas sensors.
SnO<sub>2</sub> gas sensors were fabricated on polyurethane (PU) polymer surfaces with nanospike structures. These nanospikes
are replicated with a low-cost soft nanolithography method from silicon nanospike surfaces formed by femtosecond
pulsed laser irradiation. The hydrophobicity of the sensing surface was enhanced by a monolayer coating of silane
(1H,1H,2H,2H-perfluorooctyltrichlorosilane, PFOTS). The resulting self-cleaning behavior enabled sensing in
environments with high moisture and heavy particulate content, while performing cleaning-in-place operations to
prolong the lifetime of the sensors. Failure studies were performed to quantify the effects on the sensitivity of water
washing. Contact angle measurements showed that the hydrophobicity was weakened after many cycles of droplet
washing due to wear of the PFOTS film and/or damage of the nanoscale spike structure. It was also found that the
baseline signal increased with droplet washing, while the sensitivity changed randomly within about 7.5%, so that the
sensitivity of the gas sensor remained at a constant level after several thousand cycles of water washing.
The metal oxide semiconductor thin film gas sensors have been successfully fabricated on a nanospiked silicon
surface formed with femtosecond laser irradiations. The sensors show significant response to CO gas at room
temperature. It is well-known that the C-O is polarized with positive charges on oxygen atom and negative charges on
carbon atom. When the currents pass through the semiconductor sensitive layer, some electrons may accumulate on the
tips of the nanospikes to maintain the same electric potential on the surface, which results in strong local electrical fields
near the tips of the nanospikes. Then more CO molecules will be pulled onto the tips of the nanospikes and this will
enhance the sensitivity of the sensor. A gate bias enhancement has been studied on silicon/oxide layer/semiconductor
architecture with the underlying silicon substrate as the back gate. The bias voltage applied on the gate can further
enhance the sensitivities of the gas sensors by alternating the electron (or hole) concentration on the surfaces of the metal
oxide semiconductor thin film.
Raman spectroscopy is a technology that can detect and distinguish materials based on the materials' Raman
scattering. However, the signal produced using this technology is usually too small to be useful. The Raman
spectrum signal can be enhanced by creating rough patches on the surface of the material. In this paper, a novel
method to produce nanometer-sized features on optical materials such as glass, fused silica, and quartz substrate is
presented. Using a femtosecond laser, the transparent materials are sputtered and deposited. When the materials
cool down, they produce structures with nano-features. These nano-features on optical materials can make
designing optical sensing systems much easier. Scanning electron microscope photos of nano-structures on quartz
substrate and optical fiber show that features less than 100 nm in size have been successfully fabricated. The 3D
micro- and nano-structures of the sensor were studied using a confocal Raman spectrum microscope and focused
ion-beam milling. Raman spectrum signals show that the strength of the signal generated by Raman scattering was
greatly enhanced compared to substrates without nano-features.
We have successfully fabricated SnO<sub>2</sub> thin film CO gas sensors on nanospiked polyurethane (PU) polymer surfaces
that are replicated with a low-cost soft nanolithography method from nanospiked silicon surfaces formed with
femtosecond laser irradiations. The sensors show sensitive responses to the CO gas at room temperature because of the
sharp structures of the nanospikes. This is much different from the sensors of SnO<sub>2</sub> thin film coated on smooth surfaces
that show no response to the CO gas at room temperature. To make the nanostructure sensor surface behave self-cleaning
like lotus leaves, we deposited a silane monolayer on the surface of the sensors with the
1H,1H,2H,2H-perfluorooctyltrichlorosilane (PFOTS) which has low surface energy. The contact angle measurement
conducted on the PFOTS monolayer-coated SnO<sub>2</sub> gas sensors indicates that a super-hydrophobic surface formed on the
nanospike sensor. The CO gas response sensitivity of the PFOTS-coated SnO<sub>2</sub> sensors is almost the same to that of the
as-fabricated SnO<sub>2</sub> sensors without the PFOTS coating. Such a super-hydrophobic surface can protect the sensors
exposed to moisture and heavy particulates, and can perform cleaning-in-place operations to prolong the lifetime of the
sensors. These results show a great potential to fabricate thousands of identical gas sensors at low cost.
Starting from the nonlinear Shroedinger equation describing the evolution of non-paraxial perturbations co-propagating with a strong background inside a nonlinear Kerr medium, we have deduced the small signal gain coefficient of the non-paraxial perturbation superimposed on the strong background wave. The results indicate that both the cut-off frequency and the asymptotic value of the gain coefficient of the non-paraxial perturbation are smaller than that of the paraxial counterpart. In addition, it is also shown that the gain coefficient degenerates to the nonlinear gain coefficient of paraxial perturbations under the paraxial approach. Furthermore, under the condition that the perturbation travels far enough inside the nonlinear medium, the gain coefficient degenerates further to the asymptotic gain coefficient predicted by the Bespalov and Talanov theory. The gain coefficient obtained in this work provides a more general solution to the study of perturbations.
Microfluidic channels on borosilicate glass are machined using femtosecond lasers. The morphology of the ablated surface is studied using scanning microscopy. The results show micron scale features inside the channels. The formation mechanism of these features is investigated by additional experiments accompanied by a theoretical analysis of the thermal and fluid processes involved in the ultrafast laser ablation process. These studies indicate the existence of a very thin melting zone on glass and suggest that the surface morphology is formed by the plasma pressure-driven fluid motion of the melting zone during the ablation process.
Based on the exact solutions of Maxwell’s equations, we have studied the basic theoretical properties of submicron and nano-diameter air-cladding silica-wire waveguides. The single-mode condition and the modal field of the fundamental modes have been obtained. Silica wires with diameters of 100-1000nm and lengths ranging from hundreds of micrometer to over 1 millimeter have been fabricated. SEM examination shows that these wires have uniform diameters and smooth surfaces, which are favorable for optical wave guiding. Light has been sent into these wires by optical coupling, and guiding light through a bent wire has also been demonstrated. These wires are promising for assembling photonic devices on a micron or submicron scale.