Energy Absorption Interferometry

TL;DR

This paper provides a comprehensive theoretical and practical framework for Energy Absorption Interferometry (EAI), detailing how to extract the complex-valued spatial/polarimetric modes through which a structure absorbs energy. It introduces multiple formalisms—space-domain, k-domain, dual-surface, and phase-space—alongside a rigorous treatment of scattering, crosstalk, phase referencing, and sampling. A key contribution is a full noise-propagation analysis that derives how measurement noise perturbs eigenvalues and eigenmodes, including explicit expressions for eigenvalue shifts, uncertainties, and mode covariance via reduced resolvents. The illustrated model demonstrates how EAI can faithfully recover modal content and absorption efficiencies with realistic noise and sampling constraints, providing practical guidance for characterizing multimode, ultra-low-noise devices such as FIR/optical detectors and imaging arrays.

Abstract

Energy Absorption Interferometry (EAI) is a technique for measuring the responsivities and complex-valued spatial polarimetric forms of the individual degrees of freedom through which a many-body system can absorb energy. It was originally formulated using the language of quantum correlation functions, making it applicable to different kinds of excitation (electromagnetic, elastic and acoustic fields). EAI has been applied in a variety of theoretical and experimental ways. It is particularly effective at characterising the multimode behaviour of ultra-low-noise far-infrared and optical devices, imaging arrays, and complete instruments, where it can be used to ensure that a system is maximally responsive to those partially coherent fields that carry signal whilst avoiding those that only carry noise. Despite its utility there is no comprehensive overview of electromagnetic EAI. In this paper we describe the theoretical foundations of the method, and present a range of new techniques in areas relating to sampling, phase referencing, mode reconstruction and noise. We present, for the first time, an analysis of how noise propagates through an experiment resulting in errors and artefacts on spectral and modal plots. A noise model is essential, because it determines the signal to noise ratio needed to ensure a given level of experimental fidelity.

Figures (29)

• Figure 1: Diagramatic representation of space-domain Energy Absorption Interferometry.
• Figure 2: Diagramatic representation of k-domain Energy Absorption Interferometry. The far-field sources produce a fringe across the SUT that samples one Fourier component of the response.
• Figure 3: Diagramatic representation of phase-space Energy Absorption Interferometry, where far-field and near-field sources are used.
• Figure 4: Far-field EAI of thin-film strip absorber.
• Figure 5: Currents in the films as the angle of incidence of the plane wave is changed. The model is that same as that of Fig. \ref{['fig_powabs_film_prog1_plot1']}, but the sheet impedance of the absorbing film was held constant at 377 $\Omega$ in all cases. Current in the absorbing film (red solid); current in the ground plane (black solid); current in the ground plane when the absorbing film is removed (black dashed).