We present the mathematical description, design, and development of an instrument that precisely determines the backscattering coefficient (bb) in water using a custom integrating cavity to collect light scattered in the backward hemisphere-a true bb meter. The design allows us to directly measure bb in a medium while not making any assumptions about the shape of β(θ) and/or of its scattering particulates. The concave surface of the quartz aperture to the integrating cavity minimizes reflection losses. The output signal is a direct linear function of bb. This bb meter is the first instrument to make a direct measurement of bb with <1% accuracy and it is compatible with several modes of routine oceanic deployment.

Data for the spectral light absorption of pure water from 250 to 550 nm have been obtained using an integrating cavity made from a newly developed diffuse reflector with a very high UV reflectivity. The data provide the first scattering-independent measurements of absorption coefficients in the spectral gap between well-established literature values for the absorption coefficients in the visible (>400  nm) and UV (<200  nm). A minimum in the absorption coefficient has been observed in the UV at 344 nm; the value is 0.000811±0.000227  m-1.

We report the development of a diffuse reflecting material with measured reflectivity values as high as 0.99919 at 532 nm and 0.99686 at 266 nm. This material is a high-purity fumed silica, or quartz powder, with particle sizes on the order of 40 nm. We demonstrate that this material can be used to produce surfaces with nearly Lambertian behavior, which in turn can be used to form the inner walls of high-reflectivity integrating cavities. Light reflecting off such a surface penetrates into the material. This means there will be an effective "wall time" for each reflection off the walls in an integrating cavity. We measure this wall time and show that it can be on the order of several picoseconds. Finally, we introduce a technique for absorption spectroscopy in an integrating cavity based on cavity ring-down spectroscopy. We call this technique integrating cavity ring-down spectroscopy.

We report the development and testing of a new commercially available diffuse reflecting material with reflectivities in the visible comparable to industry-leading products. This new diffuse reflector consists of solid quartz in which there is a dense distribution of tiny pockets of air. The multiple reflections by the quartz-air interfaces of these air pockets transforms a highly transmissive base material into a highly diffuse reflecting material.

A number of imaging modalities rely on the exact knowledge of both the absorption and scattering properties of cells and organelles. We report a simple method for accurate and precise measurement of the optical absorption coefficient of biological samples, even in the presence of strong scattering. The technique is based on cavity ring-down spectroscopy, but the traditional mirrored cavity is replaced with a high-reflectivity integrating cavity. The Lambertian behavior of the cavity walls creates an isotropic field inside the cavity, thereby eliminating the effects of scattering in the sample. Thus, integrating cavity ring-down spectroscopy (ICRDS) provides a true, direct measurement of the absorption coefficient, as opposed to the net attenuation. We demonstrate the effectiveness of this technique by measuring the absorption coefficient of retinal pigmented epithelium cells. Furthermore, we demonstrate that ICRDS is insensitive to scattering effects using suspensions of copolymer microspheres and an absorbing dye solution. These results are compared with measurements made using a more traditional transmission-style setup. This technique will have an impact on the field of nanoscience, where optical characterization of nanoparticles is still done using a conventional spectrometer that is only capable of providing measurements of the extinction coefficient.

Clean water is paramount to human health. In this article, we present a technique for detection of trace amounts of human or animal waste products in water using fluorescence emission cavity-enhanced spectroscopy. The detection of femtomolar concentrations of urobilin, a metabolic byproduct of heme metabolism that is excreted in both human and animal waste in water, was achieved through the use of an integrating cavity. This technique could allow for real-time assessment of water quality without the need for expensive laboratory equipment.

Random lasers are a developing class of light sources that utilize a highly disordered gain medium as opposed to a conventional optical cavity. Although traditional random lasers often have a relatively broad emission spectrum, a random laser that utilizes vibration transitions via Raman scattering allows for an extremely narrow bandwidth, on the order of 10 cm −1 . Here we demonstrate the first experimental evidence of lasing via a Raman interaction in a bulk three-dimensional random medium, with conversion efficiencies on the order of a few percent. Furthermore, Monte Carlo simulations are used to study the complex spatial and temporal dynamics of nonlinear processes in turbid media. In addition to providing a large signal, characteristic of the Raman medium, the random Raman laser offers us an entirely new tool for studying the dynamics of gain in a turbid medium.

Propagation of light in a highly scattering medium is among the most fascinating optical effect that everyone experiences on an everyday basis and possesses a number of fundamental problems which have yet to be solved. Conventional wisdom suggests that non-linear effects do not play a significant role because the diffusive nature of scattering acts to spread the intensity, dramatically weakening these effects. We demonstrate the first experimental evidence of lasing on a Raman transition in a bulk three-dimensional random media. From a practical standpoint, Raman transitions allow for spectroscopic analysis of the chemical makeup of the sample. A random Raman laser could serve as a bright Raman source allowing for remote, chemically specific, identification of powders and aerosols. Fundamentally, the first demonstration of this new light source opens up an entire new field of study into non-linear light propagation in turbid media, with the most notable application related to non-invasive biomedical imaging.

A new diffuse reflector has been developed whose reflectivity in the 250 nm to 1 micron spectral region is so high that ring-down spectroscopy of scattering aerosols in an integrating cavity is now possible.