The Mechanisms Governing Energy Transfer at the ATR Crystal–Sample Interface

A recent discussion with Thomas Mayerhöfer examined the role of the evanescent field in attenuated total reflectance (ATR) spectroscopy, alongside his contributions to infrared (IR) spectroscopy. Mayerhöfer, who is a researcher at the Leibniz Institute of Photonic Technology, has advanced the field through wave optics and dispersion theory, publishing over 120 papers.¹ His latest study, featured in Applied Spectroscopy and conducted with his colleague Jurgen Popp, who is the scientific director at the Leibniz Institute of Photonic Technology, challenges conventional ATR theory by demonstrating that the evanescent field disappears when the rarer medium is absorbing.^1,2 The published paper redefined the concept of the critical angle under such conditions. In their paper, Mayerhöfer and Popp suggests that evanescent waves play little to no significant role in ATR measurements.

Recently, we sat down with Mayerhöfer to talk about his paper more in depth. Through these conversations, Mayerhöfer shed more insight into the conventional ATR theories.

In this interview clip, Mayerhöfer discusses the interaction of evanescent waves with materials, noting that waves can travel into a medium with a higher refractive index, even if absorbing. He highlights his ongoing research in spectroscopy, particularly ATR spectroscopy's sensitivity to nanometer-thick layers, and its connection to spectroscopic ellipsometry.

This interview is the final part of a five-part interview with Mayerhofer. The first part of our conversation focused on defining what the evanescent field is and how it relates to ATR spectroscopy. The second part of our conversation with Mayerhofer focused on what led him to re-examine several spectroscopic methods, including ATR spectroscopy. The third part of our conversation examined the transition from transmission spectroscopy to ATR spectroscopy, highlighting the importance of sample thickness for signal detection. The fourth clip explored how an optical model accounts for thin-layer thickness on the order of the penetration depth, so spectra reflect partial-reflection effects.

References

1. Mayerhofer, T.; Popp, J. Complex-Valued Chemometrics for Composition Analysis. Spectroscopy 2025, 40 (6), 16–21. DOI: 10.56530/spectroscopy.wn4265d4
2. Mayerhofer, T.; Popp, J. Understanding the Role of the Evanescent Field in Attenuated Total Reflection (ATR) Spectroscopy. Appl. Spectrosc. 2026, 80 (2), 125–132. DOI: 10.1177/00037028251358400