Multiresolution Spectrally Encoded Terahertz Reflection Imaging Through a Highly Diffusive Cloak

Abstract Turbid media are usually made of wavelength-scale inhomogeneous granular particles, which give rise to many significant imaging challenges. The random variation of the refractive index within such media distorts the spherical beam wavefronts, resulting in smeared and speckly images. The light scattering also limits the depth of focus to only few multiples of the scattering mean free path. Additionally, the scattering-induced spectral artifacts obscure the fingerprints of the chemicals in a sample, preventing accurate imaging and identification of the materials cloaked under a highly diffusive medium. We present the implementation of a multiresolution spectral analysis technique by decomposing the reflection spectrum using a wavelet transform approach for creating spatially- and spectrally-resolved chemical maps through such scattering media. We define a new spectroscopic concept, dubbed the "bimodality coefficient spectrum" of a sample, and show that the broadband wavelet-based reconstruction of the bimodality coefficient spectrum can resolve the signature resonant frequencies through scattering layers. We show that our new approach can achieve spectral images with diffraction-limited resolution. This technique can be used for stand-off detection of materials and spectral imaging in nondestructive testing and biological applications.