Midinfrared plasmonic sensing allows the direct targeting of unique vibrational fingerprints of molecules. While gold has been used almost exclusively so far, recent research has focused on semiconductors with the potential to revolutionize plasmonic devices. We fabricate antennas out of heavily doped Ge films epitaxially grown on Si wafers and demonstrate up to 2 orders of magnitude signal enhancement for the molecules located in the antenna hot spots compared to those located on a bare silicon substrate. Our results set a new path toward integration of plasmonic sensors with the ubiquitous CMOS platform.

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Heavily doped semiconductor thin films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in the 5 to $50\mu \mathrm{m}$ wavelength range at least. In this work, we investigate the electrodynamics of heavily $n$-type-doped germanium epilayers at infrared frequencies beyond the assumptions of the Drude model. The films are grown on silicon and germanium substrates, are in situ doped with phosphorous in the ${10}^{17}$ to ${10}^{19}$ ${\mathrm{cm}}^{-3}$ range, then screened plasma frequencies in the 100 to $1200{\mathrm{cm}}^{-1}$ range were observed. We employ infrared spectroscopy, pump-probe spectroscopy, and dc transport measurements to determine the tunability of the plasma frequency. Although no plasmonic structures have been realized in this work, we derive estimates of the decay time of mid-infrared plasmons and of their figures of merit for field confinement and for surface plasmon propagation. The average electron scattering rate increases almost linearly with excitation frequency, in agreement with quantum calculations based on a model of the ellipsoidal Fermi surface at the conduction band minimum of germanium accounting for electron scattering with optical phonons and charged impurities. Instead, we found weak dependence of plasmon losses on neutral impurity density. In films where a transient plasma was generated by optical pumping, we found significant dependence of the energy relaxation times in the few-picosecond range on the static doping level of the film, confirming the key but indirect role played by charged impurities in energy relaxation. Our results indicate that underdamped mid-infrared plasma oscillations are attained in $n$-type-doped germanium at room temperature.

The infrared conductivity of ${\mathrm{V}}_2{\mathrm{O}}_3$ is measured in the whole phase diagram. Quasiparticles appear above the Néel temperature $T_N$ and eventually disappear further enhancing the temperature, leading to a pseudogap in the optical spectrum above $425\mathrm{K}$. Our calculations demonstrate that this loss of coherence can be explained only if the temperature dependence of lattice parameters is considered. ${\mathrm{V}}_2{\mathrm{O}}_3$ is therefore effectively driven from the “metallic” to the “insulating” side of the Mott transition as the temperature is increased.

We studied the effect of surface roughness on terahertz reflectance spectra of explosives at both quasinormal and oblique incidence by Fourier-transform spectroscopy. The optical constants of 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane and 1,3,5 trinitro-1,3,5 triazacyclohexane were determined from the reflectance spectra of pure optically flat pellets and clear spectral signatures were found in the $0.5–8\mathrm{THz}$ range. In the case of a realistic surface roughness of several tens of microns, the observation of the spectral signatures at quasinormal incidence is hindered by the specular reflectance roll-off at terahertz frequencies. On increasing the angle of incidence, the roll-off occurred at higher frequency and the surface roughness was optically measured. Oblique incidence geometries can benefit the development of terahertz standoff detection devices.