The PI proposes using palladium and metal oxide nanoclusters as catalytic coatings for fast, high sensitivity/specificity, chemical to optical transduction. Coated, optically pumped, nanoscale VCSELs amplify and encode gas induced changes into an output power and/or wavelength shift for remote readout.
Intellectual Merit: This effort merges new research in nanoscale materials and device architectures to transform current scientific understanding of nanophotonic sensors. Gas induced lattice expansion of palladium nanoclusters is a hot research topic for electrical hydrogen sensors, but due to potential sparking, optical sensors are advantageous. The PI proposes expanding current research by characterizing the complex refractive index change caused by lattice expansion and testing sensors based on this effect.
Broader Impacts: Hydrogen is an attractive alternative fuel, but there is widespread public concern about its safe production and usage due to hydrogen's low flammability point of 4% in air. Poor response speed, sensitivity, and reliability of current hydrogen sensors could derail the future hydrogen fuel economy. Thus, this project can produce broad societal impact by demonstrating high performance sensors. Additionally, the research offers rich opportunities for teaching and training of graduate and undergraduate researchers in several disciplines. Participation of underrepresented groups will be broadened through on campus recruitment, REU internships, and K-12 outreach activities.
We investigated several optical hydrogen gas sensor architectures for applications such as fuel cell leak detection and monitoring industrial emissions. We quantified each sensor’s performance in terms of sensitivity, response time, minimum detection limit, and specificity against systematic drift. We demonstrated several successful sensor designs and also characterized the optical and mechanical properties of palladium (Pd), the sensing material. Through this project, we were able to train and educate numerous graduate, undergraduate, high school, and K-8 students in topics ranging in their depth and specialization from photonic sensors, to integrated optics and computational electromagnetism, to the broader field of electrical engineering, and extending to experimental research in general. Intellectual Merit: We first investigated a Pd coated vertical cavity surface emitting laser (VCSEL) with a 2-dimensional photonic crystal (PhC). The laser mode’s effective index depends on the Pd film’s refractive index. Thus, hydrogen changes the laser’s output power and wavelength at fixed bias current. In collaboration with Prof. Choquette and his research group, we deposited a thin Pd layer on the top distributed Bragg reflector of PhC VCSELs. We simulated, fabricated, and tested PhC VCSEL sensors and observed a 60% output power increase and a 52 pm redshift when exposed to 4% H2. These results demonstrate that the functionalized PhC VCSEL is a suitable H2 sensor. Additionally, simulation tools developed for this sensor can be used to design vastly different PhC VCSEL devices for other applications, e.g. optical communications or chip-to-chip interconnects. We determined that the single-mode nature of the PhC VCSEL was the primary source of the performance difference between the PhC VCSEL and standard VCSEL sensors. Therefore, we pursued further enhancing the PhC VCSEL’s single-mode nature by adding metal in the etched air holes to improve the side mode suppression ratio (SMSR). We simulated and measured implant-confined PhC VCSELs and demonstrated a 4dB improvement in SMSR at a fixed current ratio above threshold. We also simulated, fabricated, and measured edge-emitting laser sensors that have waveguide ridges etched to the waveguide core and a thin Pd blanket layer. The evanescent field overlaps and interacts with the Pd film. Our simulations and measurements quantify the impacts of ridge width and operating temperature on sensitivity. We observed that the sensitivity and response time both improve for narrower ridge widths. Additionally, the device responds faster at higher temperatures. We also designed, fabricated, and tested fiber optic based hydrogen sensors for use in harsh environments. The sensor consists of a nano-aperture etched into a Pd layer that coats an optical fiber facet. We quantified the changes in transmission and reflection and used polarization to reduce the effects of systematic drift. We demonstrated a reflective fiber sensor capable of accurately quantifying the hydrogen concentration over a wide dynamic range that spans from 1% down to 90 ppm. This was our most sensitive sensor geometry. Through three REU supplements, we were able to investigate a Fabry-Pérot etalon sensor with Pd. We fabricated samples and measured changes in the normal incidence reflection and transmission optical spectra for different hydrogen concentrations. Our simulations and experimental results showed good agreement; the H2 concentration can be determined by the resonant wavelength shift or the amplitudes of the minima in the interference pattern. We also demonstrated a 1-2 minute response time for hydrogen concentrations below 1.5%. Finally, to better understand the material properties of the Pd film, shared by all our sensors, we used diffraction phase microscopy to monitor the hydrogen induced lattice expansion of the Pd thin-film in-situ. We computed the film expansion due to hydrogen, which is a key material parameter in the design and simulation of Pd-based hydrogen sensors. We observed as small as 1nm thickness expansion for 0.1% hydrogen exposure. Broader Impacts: The project results have been disseminated in 4 journal and 14 conference publications. In addition, there are also currently 2 journal papers and 1 book chapter under review. This project has enabled the training and development of 4 graduate students, resulting in 1 PhD thesis and 2 MS theses and has provided research opportunities for 7 undergraduates resulting in 4 BS theses. The PI developed a new interdisciplinary course, "Principles of Experimental Research," and offered it three times. The course seeks to train first year graduate students and advanced undergraduates to be more independent researchers earlier in their careers. The PI along with his graduate and undergraduate assistants created the Girls Learning Electrical Engineering (GLEE) camp for high school girls. A total of 71 campers have attended the residential weeklong camp in its four years of operation. In summer 2011, a new outreach program was launched to engage K-8 students in STEM activities including nanotechnology and the science of sports. We partnered with 4-H club to offer a 3-day Electrical and Computer Engineering program for high school students in summer 2012 and 2013.
