In non-photochemical spectral hole burning (NPHB) and spectral hole recovery experiments, cytochrome bf protein exhibits behavior that is almost independent of the deuteration of the buffer/glycerol glassy matrix containing the protein, apart from some differences in heat dissipation. On the other hand, strong dependence of the hole burning properties on sample preparation procedures was observed and attributed to a large increase of the electron-phonon coupling and shortening of the excited-state lifetime occurring when n-dodecyl β-d-maltoside (DM) is used as a detergent instead of n-octyl β-d-glucopyranoside (OGP). The data was analyzed assuming that the tunneling parameter distribution or barrier distribution probed by NPHB and encoded into the spectral holes contains contributions from two nonidentical components with accidentally degenerate excited state λ-distributions. Both components likely reflect protein dynamics, although with some small probability one of them (with larger md) may still represent the dynamics involving specifically the -OH groups of the water/glycerol solvent. Single proton tunneling in the water/glycerol solvent environment or in the protein can be safely excluded as the origin of observed NPHB and hole recovery dynamics. The intensity dependence of the hole growth kinetics in deuterated samples likely reflects differences in heat dissipation between protonated and deuterated samples. These differences are most probably due to the higher interface thermal resistivity between (still protonated) protein and deuterated water/glycerol outside environment.

The terahertz spectral range (frequencies of 0.1-10 THz) has recently emerged as the next frontier in non-destructive imaging and sensing. Here, we review amplitude-based and phase-based sensing modalities in the context of the surface wave enhanced sensing in the terahertz frequency band. A variety of surface waves are considered including surface plasmon polaritons on metals, semiconductors, and zero gap materials, surface phonon polaritons on polaritonic materials, Zenneck waves on high-k dielectrics, as well as spoof surface plasmons and spoof Zenneck waves on structured interfaces. Special attention is paid to the trade-off between surface wave localization and sensor sensitivity. Furthermore, a detailed theoretical analysis of the surface wave optical properties as well as the sensitivity of sensors based on such waves is supplemented with many examples related to naturally occurring and artificial materials. We believe our review can be of interest to scientists pursuing research in novel high-performance sensor designs operating at frequencies beyond the visible/IR band. Keywords: terahertz band; surface waves; amplitude sensing modality; phase sensing modality; surface plasmon polaritons; surface phonon polaritons; Zenneck waves; spoof plasmons Recently, the THz spectral range (frequencies of 0.1-10 THz, wavelengths of 3 mm-30 µm) has emerged as the next frontier in non-destructive imaging and sensing. Key advantages when operating in the THz band are the ability to detect the complex electric field of THz waves (phase and amplitude), availability of both pulsed (sub-1 ps) and continuous wave (CW) THz systems, as well as ability of conducting dynamic measurements with sub-1 ms temporal resolution [6]. Additionally, relative transparency of dry dielectrics to THz radiation, strong absorption of THz waves by water and other polar solutions, as well as the non-ionizing nature of THz light enable many and more graphical than analytical, which is the main reason why we chose a more theoretically elegant approach for the characterization of sensor sensitivity with respect to changes in the analyte refractive index complemented by the study of the surface wave probing depth. Amplitude Sensing ModalityAmplitude sensing modality offers the simplest sensor implementation as it only requires monitoring of the sensor transmission amplitude. A simplified schematic of an amplitude-based sensor is presented in Figure 1. The amplitude sensing detection modality is mostly used to detect variations in the imaginary part of the analyte refractive index.Sensors 2020, 20, x FOR PEER REVIEW 3 of 24 theoretically elegant approach for the characterization of sensor sensitivity with respect to changes in the analyte refractive index complemented by the study of the surface wave probing depth. Amplitude Sensing ModalityAmplitude sensing modality offers the simplest sensor implementation as it only requires monitoring of the sensor transmission amplitude. A simplified schematic of an amplitude-based sensor is presented in Figure 1...

Terahertz spectral range (frequencies of 0.1-1 THz) has recently emerged as the next frontier for non-destructive imaging, industrial sensing and ultra-fast wireless communications. Here, we review several classes of materials such as simple metals, semiconductors, high-k dielectrics, polar materials, zero gap materials, as well as structured materials that can support strongly localised electromagnetic modes at material interfaces in the Terahertz spectral range. We present the basic theory of surface waves, detail the requirement of strong modal confinement and low loss for the surface waves propagating at material interfaces and discuss challenges for excitation of such modes at Terahertz frequencies. A large number of examples related to naturally occurring and artificial materials is then presented. A variety of practical applications is envisioned for surface waves at Terahertz frequencies including non-destructive super-resolution imaging and quality control, high sensitivity sensors capable of operation with small volumes of analytes that are opaque in the visible and near-infrared, as well as design of compact optical circuit for the upcoming ultra-high bitrate THz communication devices.

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