Large wavelength shifts have been measured in the reflectivity spectra of Bragg mirrors etched in porous silicon after exposure of the mirrors to vapor from organic solvents. The shift in the Bragg wavelength of the mirror arises from refractive index changes, induced by capillary condensation of the vapor in the mesoporous silicon, in the layers of the mirrors. Modeling of the reflectivity changes shows that the layer liquid volume fraction occurring in the measurements was 0.29 for acetone and 0.33 for chlorobenzene. Time-resolved measurements show that condensation occurs on the time scale of tens of seconds.

The stability of derivatized mesoporous silicon mirrors in simulated human blood plasma has been assessed. The rate at which they are dissolved in‐vivo is predicted to be tunable by surface chemistry over timescales of weeks to years, and high reflectivity can be maintained until the bottom of the multilayer stack starts to corrode. Such biodegradable optical components could be utilized to direct and define optical path lengths for therapeutic treatments and minimally‐invasive diagnostics.

Sets of 5 to 10 random fecal samples were obtained from animals of various management groups of nine dairy herds. Titanium content of feces and soils to which the animals had access was determined by x-ray fluorescence spectroscopy. Titanium of feces was the indicator of soil ingestion, which was calculated for 60% digestibility of the total ration dry matter. Mean soil ingestion +/- standard error as a percent of dry matter intake by groups of lactating cows ranged from .14 +/- .02 to .53 +/- .05 for those confined to concrete, .35 +/- .06 to .64 +/- .18 for those housed in freestall barns with soil bedding, and .60 +/- .07 to .96 +/- .22 for those with access to unpaved lots with no vegetation. Mean soil ingestion as a percent of dry matter intake by groups of yearling heifers and dry cows ranged from .52 +/- .11 to .81 +/- .19 for those confined to concrete, .25 +/- .04 to 2.41 +/- .26 for those with access to unpaved lots with no vegetation, 1.56 +/- .21 to 3.77 +/- 1.50 for those with access to unpaved lots with sparse vegetation, and 1.38 +/- .33 to 2.43 +/- .50 for those on pasture but receiving supplemental feed. Sensitivity of the method depended on the titanium content of the soils which was four to five times greater in clay than in sandy soils. Over all observations, the percent soil of feces was related to the percent ash of feces with the orthogonal linear regression equation, Soil = .69 Ash--5.1 (squared correlations, .62).

The sensitivity of the optical reflectivity of porous silicon structures to the refractive index of liquid within the pores is studied for a single layer, a Bragg mirror and a microcavity. Sucrose solutions of concentration in the range 0.05 to 1.0% by weight are introduced into the pores within a flow cell in order to change the refractive index of the liquid in the pores from 1.3330 to 1.3344. Optimum wavelengths for detection via reflectivity changes are determined based on a signal to noise analysis. The optical thickness of the single layer is also monitored by measuring the fringe spacing via a Fourier transform technique. It is just possible to detect the effect of a change in refractive index of liquid in the pores of 0.00007 for both the reflectivity and optical thickness approaches.

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