Disha Mehtani

Optical properties and enhancement factors of the tips for apertureless near-field optics

Resonant excitation of surface plasmons of a metal or metal-coated tip is crucial for achieving high enhancement of an optical signal with apertureless near-field optics. However, it remains a challenge to measure the optical spectrum of a tip with sub-wavelength dimensions. We present a technique based on total internal reflection microscopy to measure the optical properties of tips. A dependence of the optical resonance on the metal deposited is shown for silver-coated and gold-coated tips. These tips were also used to measure the tip-enhanced Raman spectra of silicon and a polymer blend of poly(3,4-ethylenedioxythiophene) and poly(styrenesulfonate) (PEDOT/PSS) at 514.5 and 647 nm incident wavelengths. Qualitative agreement was observed between the tip-enhanced Raman spectra and the optical resonance of the tip measured with the technique developed.

Scanning nano-Raman spectroscopy of semiconducting structures

Tip-enhanced Raman spectroscopy (TERS) using side illumination is a promising spectroscopic tool for nanoscale characterization of chemical composition, structure, stresses and conformational states of non-transparent samples. Recent progress has shown signal enhancements for a variety of samples, including break-through enhancements of semiconductors. In this work, optimization of the polarization geometry increases contrast between near-field and far-field signals on Si and improves imaging quality. Two-dimensional images of semiconductor nanostructures show reasonable agreement between topographical and TERS images. These recent TERS results using both silver- and gold-coated tips demonstrate localization of the Raman enhancement to within approximately 20 nm of the tip. Also, the enhanced Raman signal of a strained Si layer is separated from an underlying Si substrate, which is encouraging for potential strain distribution analysis of silicon nanostructures.

Tip-enhanced Raman spectroscopy with high contrast

Tip-enhanced Raman spectroscopy (TERS) is emerging as a promising spectroscopic tool for nanoscale characterization of chemical composition, structure, stresses and conformational states. However, its widespread application requires optimization of the technique to reproducibly achieve sufficiently high contrast between near-field and far-field signals. We present a TERS spectrometer, based on side illumination geometry, which demonstrates reproducible enhancements of the Raman signal of the order of 103-104 for a variety of molecular, polymeric and semi-conducting samples using both silver- and gold-coated tips. We estimate the localization of the Raman signal enhancement to be ~20 nm. For thick samples, the contrast is limited by a strong far-field signal (from the laser illuminated spot) that overpowers the near-field signal (enhanced in the vicinity of the tip). Optimizing the polarization geometry and the incident angle, we have achieved a contrast between near-field and far-field signal of 12 times on (100) Si - a level that makes this technique attractive for characterization of silicon nanostructures.

Scanning nano-Raman spectroscopy of silicon and other semiconducting materials

Publisher Summary The chapter presents an overview of developments in the side-illumination tip-enhanced Raman spectroscopy (TERS) technique, and applications of TERS for the characterization of semiconducting materials and structures. The nano-Raman system was modified to measure optical properties of the tips using the principle of total internal reflection microscopy. The signal in TERS consists of two components, including the background signal coming from the entire laser-illuminated spot and the locally enhanced signal that comes from the nanoscale region in the vicinity of the tip. An important parameter for apertureless near-field optics is the enhancement of the signal provided by the tip. Enhancement is an increase in the Raman signal intensity in the small volume around the tip caused by the plasmon resonance of the tip. The experimental realization of optimum resonant enhancement conditions in TERS requires the measurement of the wavelength-dependent optical response of the tip apex. TERS can be effectively applied for the analysis of thin films.

Optical properties of the tips for apertureless near-field microscopy

The local electric field enhancement in the vicinity of a metal-coated or metal tip is a significant factor in the performance of apertureless near-field optical microscopy and spectroscopy techniques. Enhancement, which is related to the generation of localized surface plasmons in the metal tip, can be maximized when the plasmons resonate at the probing wavelength. Thus the resonance frequencies of the tip apex are crucial to near-field optics. However, it remains a challenge to measure the optical properties of the apex of a tip with a radius much smaller than the wavelength of light. A dark-field scattering spectroscopy method is presented in combination with a side-illumination nano-Raman spectrometer to experimentally determine the optical properties of the tip. The dependence of the optical resonance on the metal deposited is shown for silver- and gold-coated tungsten tips as well as gold-coated silicon nitride tips. The enhancement for Si using gold-coated silicon nitride tips is somewhat larger for a wavelength of 647 nm than for a wavelength of 514.5 nm. The former is closer to the plasmon resonance observed for this tip at ~680 nm.