Phonon polaritons, hybrid light-matter quasiparticles resulting from strong coupling of the electromagnetic field with the lattice vibrations of polar crystals are a promising platform for mid-infrared photonics, to develop applications such as coherent directional sources of IR light, control of thermal radiation, chemical and biological IR sensors. Furthermore, the hybridisation of phonon polaritons with longitudinal phonons represent an important step toward the development of phonon polariton-based electrically pumped mid-infrared emitters.
This Project is targeted at both theoretical and experimental investigation of polar metamaterials for tuning and hybridization of phonon polaritons for infrared (IR) radiation manipulation. The research team is composed by 22 researchers and professors from Sapienza University with synergic background in physics, chemistry, mathematics and medicine.
We aim to explore emerging 2D polar materials and metamaterials to focus on new phenomena of strong coupling between longitudinal and transverse surface phonon polariton (SPhPs) from systems such as anisotropically porous media and self-assembling materials. To this aim, we plan to acquire an innovative, high resolution, FT-IR spectrometer system with a variable external sample chamber for customized experiments allowing measurements of emissivity spectra with variable angle and polarization state, detection of surface waves propagation at an interface with a dielectric prism (using both Otto scheme and ATR configuration), mapping the reflectance in the kx-ky space, as well as investigating the effect of temperature on polariton resonances. Finally, the innovative technology of the required equipment system will provide the highest flexibility for sophisticated experimental set-ups in polaritonics, and will support the ongoing research on 2D polar materials, metamaterials and their response in the IR, in order to exploit their peculiar spectral features for sensing applications.
We aim to explore emerging 2D polar materials and metamaterials to focus on new phenomena of strong coupling between longitudinal and transverse surface phonon polariton (SPhPs) from one- and two dimensional systems, such as anisotropically porous media and self-assembling materials. The interest for maximizing this coupling is related to the possibility to create electrically pumped IR coherent sources. Indeed, longitudinal phonons can be easily excited by electron-phonon interaction (ohmic losses) although they do not produce radiative electromagnetic fields. On the other hand, transverse phonons intrinsically couple to photons thus forming polaritons. A hybrid transverse-longitudinal phonon could combine both features. In order to investigate the properties of these hybrid modes a complete study of reflection in Otto configuration with a full scan in angle of incidence and polarization of both incident and reflected fields must be performed.
The required equipment is composed by a new-generation, extremely versatile first FT-IR modulus that will be employed to investigate the wavelength range, where the existence condition of SPhPs is met. With respect to conventional FT-IR system, we aim to acquire an extremely innovating FT-IR spectrometer with a variable external sample chamber for customized experiments. Typical resolution of most FT-IR system is 4 cm-1, which is somewhat limiting for detection of sharp polariton resonances. The FT-IR spectrometer composing our experimental system should have spectral resolution of better than 0.3 cm-1 being a fundamental requirement to resolve polaritons spectral resonances. Furthermore several accessorizes will be included in the experimental system, so to build up different experimental configurations for Reststrahlen bands investigation.
First of all, the measurements of NIR to F-IR reflection spectra using different angle and polarization state (IR polarizer and mounter, point 5 and 4 of attached offer) will allow to retrieve the resulting permittivities (ordinary and extraordinary) of investigated films or effective medium and to highlight the spectral range where excitation of SPhPs is allowed.
Concerning with the experimental investigation of polar matrix based metamaterials, the required equipment also allows the study of in-plane directionality of surface waves in the metamaterial structures (inclusions/matrix systems) in the near IR to far IR wavelength interval (1,5-28,5 micron) upon mapping the reflectance in the kx=ky space, using the reflectance accessory with computer-controlled angle adjustment in the 13°-83° degrees range (A513/QA, point 7 of attached offer).
More interestingly, the existence of condition for polariton hybridization will be highlighted using specific configurations such as prism coupling experiment [8]. To this aim, the experimental setup we aim to develop is based on the Otto configuration: unlike in the Kretchmann configuration where a prism is employed, a hemispherical IR transparent (ZnSe) prism is placed on the sample (ZnSe prism, point 6 of attached offer). With respect to attenuated total reflectance (ATR) measurements, in Otto configuration it is possible to test both TE- and TM-polarized incident light, since a polarizer (wire grid polarizer) can be employed for controlling the incident polarization. For beam focusing onto sample surface, a parabolic mirror included in the required equipment can be adjusted. After the incident light reaches the sample through the hemispherical ZnSe prism, reflection spectra can be acquired under different angles of incidence and azimuthal angles with respect to the structure orientation. Such measurements allow to highlight the low reflectance zones outside the air light cone kx2+ky2=k02, where k0=w/c, are potential candidates for SPhPs bands because the SPhPs occur beyond the light line.
Furthermore, for those investigations where experimental configurations are too complex or voluminous for the sample compartment, external accessories and modules are also available for custom experimental setups. For example, within the frame of the previously mentioned collaboration, there is an ongoing research on thermal emission tuning from VO2 films [18]. Using the heating module (A225/Q-DLST point 7 of attached offer), we aim to study how the polariton of this thermochromic polar material can be tuned by taking advantage of its phase transition by performing a scan at different temperatures.
Finally, the innovative technology of the required equipment system will provide the highest flexibility for sophisticated experimental set-ups in polaritonics, and will support the ongoing research on 2D polar materials, metamaterials and their response in the IR, in order to exploit their peculiar spectral features for sensing applications.
[18] M.C.Larciprete et al "Adaptive tuning of infrared emission using VO2 thin films", submitted to Scientific Reports.