Abstract
A method of reference-free speckle spectroscopy based on the statistical analysis of intensity spatial fluctuations of the spectrally-selected multiple-scattered fluorescence radiation is examined in the case of the finite-band spectral selection of fluorescence light emitted by the laser-pumped random medium, and detection conditions far from the ideal case. Intensity fluctuations are recorded during point-to-point scanning of the surface of a random multiple-scattering medium, which is characterized by the dependences of the second- and third-order statistical moments of intensity on the wavelength of detected spectrally selected light. In turn, the statistical moments of intensity fluctuations are determined by the average propagation path of fluorescent radiation in the medium. This makes it possible to analyze the features of the light-medium interactions at a scale of the order of the transport mean free path of radiation propagation in the medium. Depending on the spectral selection conditions, the method is applicable for characterizing micro- or nano-structured fluorescent layers with thicknesses from tens of micrometers to several millimeters. In the examined case, the finite-band spectral selection results in the values of coherence length of the detected fluorescence radiation compared with the ensemble-averaged absolute value of the path-length difference between the stochastically interfering and spectrally selected partial contributions to the fluorescence field. In addition, non-ideal detection conditions (usage of a multimode optical fiber in the light-collecting unit) cause additional strong damping of the detected speckle intensity fluctuations. These factors lead to a remarkable suppression of spatial fluctuations of the fluorescence intensity in the course of spatially- and spectrally-resolved surface scanning of the laser-pumped probed random medium. Nevertheless, with appropriate procedures of the intrinsic noise reduction and data correction, the obtained spectral dependencies of the normalized third-order statistical moment of the band-limited fluorescence intensity clearly indicate the fluorescence propagation features in the probed multiple-scattering random media (such as a strong influence of the scattering strength and multiple self-absorption–re-emission events on the average propagation path of light in the medium).The possibilities of noise reduction and data correction in the case of applying the band-limited reference-free spectroscopic instrumentation with low spectral and spatial resolution are illustrated by the experimental results obtained using the Rhodamine-6G-doped and continuous wave (CW)-laser-pumped layers of the densely packed titania and silica particles.
Highlights
Nowadays, the diffusing light techniques are one of the most powerful tools for characterization of the structure and dynamics of the complex random media at the microscopic level
The goal of this work is to examine the workability of the band-limited reference-free speckle spectroscopy (BLRFS) applied as the probing technique for the continuous wave (CW)-pumped fluorescent random media in the case of a strong damping of the detected signal due to non-ideal detection conditions and data corruption by an intrinsic noise of the speckle-spectroscopic system
We studied the statistical properties of spatial fluctuations of spectrally selectable fluorescence radiation propagating in the random media
Summary
The diffusing light techniques are one of the most powerful tools for characterization of the structure and dynamics of the complex random media at the microscopic level (for spatial scales comparable with the wavelength of the probe light). The suppression effect associated with the blur of the dynamic speckle patterns in the scattered, partially coherent light fields, can be used to recover the path-length distributions of the probe light in the static multiple-scattering media. This opens the way for evaluation of the structural properties of the examined scattering system using an appropriate scattering model, which establishes the relationships between the structural properties of the system (e.g., the average size of the scattering units and their volume fraction), and its optical transport parameters (e.g., the transport mean free path of light propagation in the system and the absorption length, [20]). Beginning from this work, several investigations were conducted on the improvement and expansion of this approach [22,23,24,25]
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