57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Turbid media are found throughout oil exploration, extraction and transportation, including porous rock, drilling mud and aggregates of gas hydrates. Non-invasive methods are required to quantify their microscale structure and dynamics. We are exploring a new approach to the optical spectroscopy of turbid media based on random lasing. When multiply scattering media with gain are pumped with a laser pulse, they emit coherent radiation in many spectral lines. The positions of these spectral lines depend sensitively on the cross section and spatial distribution of scatterers in an undetermined matter.

To lay the groundwork for a random laser correlation spectroscopy, we are characterizing the statistics of random lasing spectra from samples that are nearly identical optically, which have different scatterer dynamics. On one hand, we consider the statistics of random lasers where the scatterers are immobilized in a polyacrylamide gel. On the other, we consider systems were the scatterers diffuse freely in water.

Above the lasing threshold, both of these systems emit light in a number of discrete peaks within the smooth emission spectrum of the weakly-pumped system. The emission spectra of these random lasers vary dramatically from shot-to-shot.

In systems with fixed scatterers, a finite number of modes contribute to light emission, albeit with enormous fluctuations in the relative intensities of the modes from shot to shot. In addition, these quenched systems show strong two-point correlations in the emission intensity, even when modes are well separated in frequency space. The intensity distributions of the weaker lasing modes decay exponentially, while the brightest lasing modes decay faster than an exponential.

In systems with freely-diffusing scatterers, there is a smooth distribution of the center-frequencies of emission modes. Significant correlations between the intensities of distinct laser modes are only observed for near-by modes, and the distribution of intensities decays slower than exponential for all wavelengths.

We have shown that the statistics of spectra at a single point in a system with freely difussing scatterers is equivalent to the statistics of spectra taken at an ensemble of points in a system with fixed scatterers. In other words, slower-than exponential decay statistics observed for lasers with freely-diffusing scatterers is a consequence of mixing a wide number of laser modes with exponential statistics of varying decay parameters. 

We are currently developing a computation method to decompose the emission spectra of systems with freely-diffusing scatterers to identify characteristic decay parameters for a sample. These parameters may provide insight into the underlying scatterer structure and dynamics.