An Inside Look at the Evanescent Field

A recent study published in the journal Applied Spectroscopy reexamined the role of the evanescent field in attenuated total reflection (ATR) spectroscopy, challenging its traditionally assumed importance.¹ Although ATR is typically understood to rely on an evanescent field penetrating the sample, wave optics indicates that this field disappears when the rarer medium is absorbing. In such cases, attenuation of total reflection arises instead from transmission into the absorbing medium.

To learn more about the evanescent field and how it applies to ATR spectroscopy, we sat down with Mayerhofer to discuss his study more in-depth.

What did this study in Applied Spectroscopy explored?

In their study, Thomas Mayerhofer, a researcher at the Leibniz Institute of Photonic Technology and Friedrich Schiller University Jena, and Jurgen Popp, a scientific director of the Leibniz Institute of Photonic Technology, demonstrate that system behavior changes continuously with both the angle of incidence and the imaginary component of the dielectric function, rather than exhibiting abrupt transitions.^1,2 By comparing electric field distributions and spectra for semi-infinite media and finite-thickness layers, they demonstrate that ATR spectra can still be observed below the critical angle, even in the absence of an evanescent field.¹ Additional phenomena, such as tunneling and frustrated total reflection, emerge when a vacuum layer is introduced above a weakly absorbing medium.¹ Based on these findings, the critical angle under absorbing conditions is redefined as the point where the real and imaginary components of the perpendicular wavevector are equal.¹ Overall, deviations from total reflection are attributed primarily to transmission, not evanescent waves.¹

He earned his diploma in chemistry from the University of Regensburg in 1996 and his PhD in physical chemistry from Friedrich Schiller University Jena in 1999.² Since 2007, he has worked to unify and advance infrared spectroscopy by integrating wave optics and dispersion theory.² Mayerhofer’s recent research has focused on refining the theoretical foundations of ATR and pioneering the introduction of complex-valued chemometrics in spectroscopy.²

This interview is the first part of a five-part interview with Mayerhofer.

References

1. 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
2. Mayerhofer, T.; Popp, J. Complex-Valued Chemometrics for Composition Analysis. Spectroscopy 2025, 40 (6), 16–21. DOI: 10.56530/spectroscopy.wn4265d4