Demystifying the memory effect: A geometrical approach to understanding speckle correlations

Abstract

Abstract. The memory effect has seen a surge of research into its fundamental properties and applications since its discovery by Feng et al. [Phys. Rev. Lett. 61, 834 (1988)]. While the wave trajectories for which the memory effect holds are hidden implicitly in the diffusion probability function [Phys. Rev. B 40, 737 (1989)], the physical intuition of why these trajectories satisfy the memory effect has often been masked by the derivation of the memory correlation function itself. In this paper, we explicitly derive the specific trajectories through a random medium for which the memory effect holds. Our approach shows that the memory effect follows from a simple conservation argument, which imposes geometrical constraints on the random trajectories that contribute to the memory effect. We illustrate the time-domain effects of these geometrical constraints with numerical simulations of pulse transmission through a random medium. The results of our derivation and numerical simulations are consistent with established theory and experimentation.

Remote laser-speckle sensing of heart sounds for health assessment and biometric identification

“…Since the scattering surface (the subject’s neck) is subject to small deformations due to heart sound pressure waves propagating in the blood vessels, we can rely on the so-called speckle memory effect [ 11 ], i.e. the speckle pattern does not change shape upon tilting or vibration of the skin surface but translates proportionally to the tilting angle [ 29 ]. We employed a speckle tracking algorithm based on the optical flow [ 28 ], implemented in MATLAB (2018b) Computer Vision Toolbox to retrieve the local displacement map at each pixel of the image and then averaged these vectors to calculate the overall displacement amplitude and direction between consecutive frames, see Fig.…”

Controlling optical memory effects in disordered media with coated metamaterials

“…The usual memory effect in light scattering refers to the short-range angular correlation between a rotated incident beam over small angles and the resulting speckle pattern [1][2][3][4]. This memory effect shows up by tilting the electromagnetic field incident upon a diffuser, leading to a corresponding tilting of the scattered field by the same amount, which is limited by a certain angular-correlation range [1,5]. These spatial correlations find applications in areas where knowledge of the intensity transmission matrices of diffusers is crucial, ranging from optical communications and wavefront shaping [3,6] to adaptive optics [7], and specially in biomedical imaging of structures within soft tissues [8][9][10][11][12].…”