Abstract
Undoubtedly, Raman spectroscopy is one of the most elaborate spectroscopy tools in materials science, chemistry, medicine and optics. However, when it comes to the analysis of nanostructured specimens or individual sub-wavelength-sized systems, the access to Raman spectra resulting from different excitation schemes is usually very limited. For instance, the excitation with an electric field component oriented perpendicularly to the substrate plane is a difficult task. Conventionally, this can only be achieved by mechanically tilting the sample or by sophisticated sample preparation. Here, we propose a novel experimental method based on the utilization of polarization tailored light for Raman spectroscopy of individual nanostructures. As a proof of principle, we create three-dimensional electromagnetic field distributions at the nanoscale using tightly focused cylindrical vector beams impinging normally onto the specimen, hence keeping the traditional beam-path of commercial Raman systems. In order to demonstrate the convenience of this excitation scheme, we use a sub-wavelength diameter gallium-nitride nanostructure as a test platform and show experimentally that its Raman spectra depend sensitively on its location relative to the focal vector field. The observed Raman spectra can be attributed to the interaction with transverse and pure longitudinal electric field components. This novel technique may pave the way towards a characterization of Raman active nanosystems, granting direct access to growth-related parameters such as strain or defects in the material by using the full information of all Raman modes.
Highlights
Raman spectroscopy has become a widely spread and well established method
The mode quality is increased with the help of a Fourier filter consisting of two lenses and a pinhole
This linear scan measurement already indicates that a position-dependent interaction of the focal fields with the nanostructure can be realized with the chosen excitation beam
Summary
Raman spectroscopy has become a widely spread and well established method. Key factors that enabled a huge variety of commercial and scientific applications of this technique are the availability of lasers providing narrow spectral line-widths at high powers in combination with highly sensitive spectrometers and interference filters capable of blocking the excitation wavelength while transmitting and measuring the Raman scattered light [1]. Besides the strong longitudinal electric field component on the optical axis of a high numerical aperture objective lens, the focal field features a ring-like distribution of transverse (in-plane) electric fields around the optical axis as it will be discussed in detail later on Owing to their spatial degree of freedom, such light beams tailored at nanoscale dimensions have been proven already to be versatile tools in the linear or nonlinear study of individual nanostructures [28, 29, 30, 31, 32, 33, 34], in microscopy and imaging [35, 36], Angstrom-localization of nanoparticles [37, 38], beam steering and guiding [39, 40], and the highly efficient coupling of individual photons to ions [41], to only name a few. Paraxial spatial modes of light carrying orbital angular momentum were utilized for Raman spectroscopy [45]
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