Department of Physical Chemistry, Fritz Haber Institute of the MPG 

Lattice Dynamics Group

Alex Paarmann
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Research in the Lattice Dynamics Group

Surface Phonon Polaritons in Polar Dielectric Heterostructures

Surface phonon polaritons (SPhP) are recently investigated as an alternative building block for mid-infrared (MIR) nanophotonic applications, promising to possibly solve the intrinsic loss problem of plasmonics. SPhPs arise in polar dielectrics due to IR-active phonon resonances, leading to negative permittivity between transverse and longitudinal optical phonon frequencies, a region called the reststrahlen band. SPhPs exhibit tremendous field enhancements, driving the lattice atoms into a strongly non-linear regime. Hence, SPhPs might grant a frequency-tunable access to vibrational-driven transient material phases.

In contrast to plasmon polaritons in metals, the limitation of the SPhP dispersion to the reststrahlen region leads to strong modulation of the dielectric response around SPhP frequencies. In consequence, combining different polar dielectrics with overlapping reststrahlen bands leads to a variety of phenomena such as mode-splitting, index-sensing, and wave-guiding, allowing for the construction of new hybrid materials with custom-designed polaritonic response.

Here, we use linear and nonlinear MIR spectroscopy for studying SPhPs in polar dielectric heterostructures, employing Otto-type prism coupling. In this structure, shown in the figure, the variable air gap grants extrinsic control over the critical conditions of the SPhP excit-ation. Our experiments employ the intense, tunable and narrowband MIR pulses from the FHI free-electron laser (FEL), revealing the resonant second harmonic generation (SHG) arising from the optical field enhancement associated with propagating SPhPs at polar dielectric interfaces.

Furthermore, the investigation of a nanoscale AlN film on SiC yields strong coupling and mode splitting of the SiC SPhP and the AlN ultrathin film epsilon near zero (ENZ) polariton. Such a system uniquely combines the low-loss, highly confined SPhP with the remarkable wave propagation characteristics of an infinite-wavelength ENZ mode, opening up a new platform of deep sub-wavelength integrated THz photonics based on strongly coupled ENZ SPhPs.

 

Second-Harmonic Phonon Spectroscopy

Nonlinear optical spectroscopy is a powerful tool to study crystalline solids as it opens up additional experimental degrees of freedom compared to linear techniques. These can be exploited to gain information about the sample’s crystal symmetry while the nonlinear field dependence provides improved sensitivity. In that context, second-harmonic generation (SHG) as the simplest nonlinear optical process assumes a particularly prominent role and constitutes a promising alternative to already established techniques like, e.g., sum-frequency generation (SFG).

In the infrared (IR) to terahertz (THz) spectral region, optic phonons modes, which themselves carry symmetry information, can be directly and excited in non-centrosymmetric media cause a resonant enhancement of the SHG yield at their respective transversal optical (TO) and longitudinal optical (LO) phonon frequencies over several orders of magnitude, providing the means of a highly sensitive phonon spectroscopy.

Making use of the FHI free-electron-laser (FEL) as an intense and widely tunable IR-THz light source, we employ second-harmonic phonon spectroscopy to study polar dielectrics. While tuning the FEL frequency provides the SHG spectra, scanning the sample’s azimuthal angle causes a strong modulation of the SHG signal which is indicative of the sample’s crystal symmetry. Additionally, by independently controlling the polarization states of both, the incoming FEL beams and the SHG signal, we can selectively detect different symmetry components, e.g. originating from different sublattices.

Our current research focuses on the study of structural phase transitions in ferroelectrics and multiferroics, which typically occur at cryogenic temperatures. Thereto, a helium bath cryostat provides full temperature control ranging from room temperature down to 1.4 K.

Vibrational Sum-Frequency and Time-resolved Spectroscopy

The combination of the FEL radiation with a table-top laser allows us to investigate the vibrational structure of material by sum-frequency generation (SFG) spectroscopy. There, the narrowband FEL radiation matches vibrational modes for resonant excitation, whereas the table-top light pulses are used for up-conversion in a nonlinear process. To this end, a near-infrared femtosecond table-top laser synchronized to the FEL pulses (timing jitter < 100 fs) is employed. As an example, SFG spectra for different polarizations of incoming and outgoing light are depicted below. By analyzing also the azimuthal dependence, we can draw conclusions on the structural symmetry. Since centro-symmetric media do not contribute to the SFG signal (within the dipole approximation), SFG spectroscopy is an ideal tool to investigate interfaces and adsorbates, where the inversion symmetry is broken. Furthermore, time-resolved studies by mid-infrared FEL pump-table top probe experiments are a method to probe structural dynamics at surfaces.