Absorption Detection Using Optical Waveguide Cavities

Cavity ring-down spectroscopy is a spectroscopic method that uses a high quality optical cavity to amplify the optical loss due to the light absorption by a sample. In this presentation we highlight two applications of phase-shift cavity ring-down spectroscopy that are suited for absorption measurements in the condensed phase and make use of waveguide cavities. In the first application, a fiber loop is used as an optical cavity and the sample is introduced in a gap in the loop to allow absorption measurements of nanoliters of solution at the micromolar level. A second application involves silica microspheres as high finesse cavities. Information on the refractive index and absorption of a thin film of ethylene diamine on the surface of the microresonator is obtained simultaneously by the measurements of the wavelength shift of the cavity mode spectrum and the change in optical decay time, respectively.

Optical Characterization of Indium Gallium Nitride for Application in High-Efficiency Solar Photovoltaic Cells

The semiconductor alloy indium gallium nitride (InxGa1-xN) offers substantial potential in the development of high-efficiency multi-junction photovoltaic devices due to its wide range of direct band gaps, strong absorption and other optoelectronic properties. This work uses a variety of characterization techniques to examine the properties of InxGa1-xN thin films deposited in a range of compositions by a novel plasma-enhanced evaporation deposition system. Due to the high vapour pressure and low dissociation temperature of indium, the indium incorporation and, ultimately, control of the InxGa1-xN composition was found to be influenced to a greater degree by deposition temperature than variations in the In:Ga source rates in the investigated region of deposition condition space. Under specific deposition conditions, crystalline films were grown in an advantageous nano-columnar microstructure with deposition temperature influencing column size and density. The InxGa1-xN films were determined to have very strong absorption coefficients with band gaps indirectly related to indium content. However, the films also suffer from compositional inhomogeneity and In-related defect complexes with strong phonon coupling that dominates the emission mechanism. This, in addition to the presence of metal impurities, harms the alloy’s electronic properties as no significant photoresponse was observed. This research has demonstrated the material properties that make the InxGa1-xN alloy attractive for multi-junction solar cells and the benefits/drawbacks of the plasma-enhanced evaporation deposition system. Future work is needed to overcome significant challenges relating to crystalline quality, compositional homogeneity and the optoelectronic properties of In-rich InxGa1-xN films in order to develop high-performance photovoltaic devices.

Absolute Absorption Cross-Section Measurement of a Submonolayer film on a silica microresonator

Conventional absorption spectroscopy is not nearly sensitive enough for quantitative overtone measurements on submonolayer coatings. While cavity-enhanced absorption detection methods using microresonators have the potential to provide quantitative absorption cross sections of even weakly absorbing submonolayer films, this potential has not yet been fully realized. To determine the absorption cross section of a submonolayer film of ethylene diamine (EDA) on a silica microsphere resonator, we use phase-shift cavity ringdown spectroscopy simultaneously on near-IR radiation that is Rayleigh backscattered from the microsphere and transmitted through the coupling fiber taper. We then independently determine both the coupling coefficient and the optical loss within the resonator. Together with a coincident measurement of the wavelength frequency shift, an absolute overtone absorption cross section of adsorbed EDA, at submonolayer coverage, was obtained and was compared to the bulk value. The smallest quantifiable absorption cross section is σmin 2.7 × 10−12 cm2. This absorption cross section is comparable to the extinction coefficients of, e.g., single gold nanoparticles or aerosol particles. We therefore propose that the present method is also a viable route to absolute extinction measurements of single particles.