Polarization Tailored Raman Spectroscopy

Conclusion and outlook: In this thesis, we successfully demonstrated a novel measurement scheme, namely polarization tailored Raman spectroscopy (PTRS). We used a tightly focused radially polarized beam, and a tightly focused azimuthally polarized beam to excite phonons within a GaN pillar of sub-wavelength dimensions.

Previously, z-sensitive Raman spectra had only been accessible by rotating the substrate when working with linearly polarized excitation beams. The strong longitudinal electric field component, present at the optical axis in the focal field distributions of the tightly focused radially polarized beam, enabled us to measure those spectra in a reflection configuration under normal incidence, relative to the substrate. The pillar under investigation exhibited Raman spectra, predominantly resulting from the longitudinal electric field component when placed on the optical axis of the tightly focused radially polarized beam. The pillar's diameter was sufficiently small (d::::: 166nm), to yield a relatively weak contribution ofthe lateral field components to the Raman spectra. Moreover, we observed a strong dependence ofthe Raman spectra on the lateral position of the GaN pillar relative to the optical axis for both excitation beams used in this thesis. The presented measurements can be considered a successful proof of principle.

Strang features, which are in compliance with the theoretical predictions, could be observed in the changes of the relative Raman intensities for lateral displacements. Raman spectra were recorded at different lateral positions along a line, as well as in the form of a 2D-scan for the tightly focused radially polarized beam. The Raman line scan conducted with a tightly focused azimuthally polarized beam shows strong influence of a magnetic multipole mode in the particle under investigation. Additional Raman peaks have been observed that may be explained by quasi-TO phonons and surface phonons upon further evaluation.

A tentative fitting scheme for the extraction of the Raman tensor elements of the structure under investigation was proposed. The method is based on a least squares minimization of the difference of the calculated Raman intensity ratios resulting from FDTD simulations and the measured Raman intensity ratios. In the present thesis this method yielded diverging results, because too many parameters were unknown. In combination with thorough characterization of both the exciting field distribution and the geometry ofthe particle under investigation, this evaluation technique could reveal basic material properties ofthe investigated structure.

Possible follow up experiments could include measurements with further excitation beams. Tightly focusing a Hermite-Gaussian TEM 10 mode would lead to interesting field distributions for PTRS. In these focal field distributions, there is a node line along which no lateral electric field is defined. On this line,the z-component exhibits a maximum on the optical axis, similar to the field distribution in the focal plane of the tightly focused radially polarized beam [11].

Examining further GaN pillar samples ofvarying sizes could reveal more details about the lower Raman shifts that were obtained within this thesis. The technique could also be applied to pillars lying on the substrate, by either choosing sufficiently small pillars or suitable excitation beams. Rotating the polarization of linearly polarized light could already yield valuable insight in the phonon modes in a lying pillar. One could also aim to selectively excite particular modes in the structures, similar to reference [12] .

The effects on the Raman spectra resulting from selective excitation by tailoring the excitation fields to the resonances supported by the nanospecimen could become another topic for future examinations.

The selective excitation of higher order multi pole modes in a lying pillar could for example reveal the lattice properties ofthe pillar at different positions along its axis.

Moreover, the interaction of crystal phonons with light carrying optical angular momentum could be further investigated. The application of PTRS could be especially interesting in combination with specimens exhibiting Raman optical activity [38] . In this case, the transverse angular momentum carried by the tightly focused radially polarized beam could be used to observe the effects in more convenient measurement configurations, just like the longitudinal electric field enabled us to do in this thesis.

Furthermore, the polarization tailored Raman spectroscopy technique demonstrated in this thesis could be extended by combining it with other enhancement approaches for Raman spectroscopy. Roy et al. used a radially polarized beam for the excitation of a plasmonic tip to enhance Raman spectra [91]. 

Similarly, one could place a plasmonic nanostructure, i.e. a gold split ring resonator with well known properties, close to the structure under investigation and control the near fields around the Raman specimen by selectively exciting different modes in the plasmonic structure. The selective excitation of dark modes by tailored light could also be considered within the approach of plasmonic nanostructure enhanced Raman spectroscopy.

Finally, incorporating a spatial and angular polarization analysis of Raman signals into the measurement scheme may reveal new insight in the characteristic properties of the Raman effect.