Alvarado Tarun

Near-field enhanced Raman spectroscopy using side illumination optics

We demonstrate near-field enhanced Raman spectroscopy with the use of a metallized cantilever tip and highly p-polarized light directed onto the tip with side illumination optics using a long working distance objective lens. The highly p-polarized light field excites surface plasmon polaritons localized at the tip apex, which results in the enhanced near-field Raman scattering. In this article, we achieved an enhancement factor of 4000 for Rhodamine 6G molecules adsorbed on a silver island film. The side illumination is also applicable to an opaque sample and to near-field photolithography.

Tip-enhanced Raman spectroscopy for nanoscale strain characterization

AbstractTip-enhanced Raman spectroscopy (TERS), which utilizes the strong localized optical field generated at the apex of a metallic tip when illuminated, has been shown to successfully probe the vibrational spectrum of today’s and tomorrow’s state-of-the-art silicon and next-generation semiconductor devices, such as quantum dots. Collecting and analyzing the vibrational spectrum not only aids in material identification but also provides insight into strain distributions in semiconductors. Here, the potential of TERS for nanoscale characterization of strain in silicon devices is reviewed. Emphasis will be placed on the key challenges of obtaining spectroscopic images of strain in actual strained silicon devices. FigureFigure Concept of Tip Enhanced Raman Spectroscopy (TERS), which utilizes the strong localized optical field generated at the apex of a metallic tip when illuminated. TERS has been demonstrated to successfully probe the vibrational spectrum of today’s and tomorrow’s state-of-the-art silicon and next generation semiconductor devices

Depolarization effect in reflection-mode tip-enhanced Raman scattering for Raman active crystals

Reflection-mode tip-enhanced Raman scattering (TERS) has the advantage to characterize any sample, particularly opaque, bulk, and multilayered samples. However, the background signal in reflection-mode TERS is huge due to large focus spots associated with an objective lens that has a long working distance. Moreover, for a multilayered and bulk sample, the Raman signal from the bulk layer interferes with the Raman signal on a thin surface layer. This unwanted bulk background signal reduces the sensitivity of the measurement and makes it difficult to get a high-contrast TERS image in the reflection mode. Here, we demonstrate two techniques to suppress the far-field Raman signals coming from the focus area and bulk silicon germanium substrate. First, we reduce the far-field signal by controlling the polarization state of the incident and scattered Raman as well as manipulating the well-defined polarization of a crystalline sample, which strongly depends on the polarization and propagation of the incident lig...

Controlling the plasmon resonance wavelength in metal-coated probe using refractive index modification.

We present a novel technique to tune the plasmon resonance of metal-coated silicon tips in the whole visible region without altering the tips original sharpness. The technique involves modification of the refractive index of silicon probe by thermal oxidization. Lowering the refractive index of silicon tip coated with metal shift the PRW of the metallic layer to shorter wavelength. Numerical simulation using FDTD agrees well with the empirical results. This novel technique is very useful in tip-enhanced Raman spectroscopy studies of various materials because plasmon resonance can tuned to a specific Raman excitation wavelength.

Apertureless optical near-field fabrication using an atomic force microscope on photoresists

We present an apertureless optical near-field fabrication using an atomic force microscope. With this technique, we can directly pattern 100 nm lines that are smaller than the diameter of the incident far-field diffraction spot on positive photoresist spin coated over glass substrate. The nanostructure fabrication was carried out by the field enhancement generated at the tip apex of an apertureless probe illuminated with a 403 nm laser light. Results also show that the amount of energy required to induce photolysis seems to decrease in the presence of the tip.

Highly efficient tip-enhanced Raman spectroscopy and microscopy of strained silicon

We present a versatile tip-enhanced Raman spectroscopy (TERS) system that permits efficient illumination and detection of optical properties in the visible range to obtain high signal-to-noise Raman signals from surfaces and interfaces of materials using an edge filter. The cutoff wavelength of the edge filter is tuned by changing the angle of incident beam to deliver high incident power and effectively collect scattered near-field signals for nanoscopic investigation in depolarized TERS configuration. The dynamic design of the instrument provides a unique combination of features that allows us to perform reflection or transmission mode TERS to cover both opaque and transparent samples. A detailed description of improvements of TERS was carried out on a thin strained silicon surface layer. The utilization of an edge filter for shorter collection time, specialized tip for higher field enhancement and for elimination of Raman signal from the tip, shorter wavelength, sample orientation relative to probing polarization, and depolarized configuration for higher contrast Raman signal is discussed.

UV-Raman imaging of the in-plane strain in single ultrathin strained silicon-on-insulator patterned structure

Confocal UV-Raman with glycerin-immersed high numerical aperture objective lens was used to probe the local strain in individual strained Si structures. The investigated structures were fabricated from 15 nm thick strained silicon-on-insulator substrates with a tensile strain of 0.8%. Two-dimensional maps of the postpatterning strain were obtained for single structures with lateral dimension of 500 nm. We found that the strain measured at the center partially relaxes and drops to 0.67% as a result of patterning-induced free surfaces. This relaxation increases toward the edges following nearly a parabolic behavior. A different strain behavior was observed for larger structures.

Focused Excitation of Surface Plasmon Polaritons Based on Gap-Mode in Tip-Enhanced Spectroscopy

We propose a novel method to improve field enhancement factor in tip-enhanced spectroscopy techniques using gap mode configuration. In gap-mode, a silver coated tip is placed just above the focus at silver thin film deposited on a dielectric substrate. The enormous electric field enhancement generated at the focus is due to coupling of surface plasmon polaritons at the gap between the 48 nm silver film and silver coated tip. The proposed technique was demonstrated in both radially polarized and linearly polarized beam. Radial polarization was found to be better than linear polarization in gap mode because only p-polarized component can contribute to the excitation of surface plasmon of the silver film. The electric field distributions at the gap below tip are calculated using finite difference time domain method and experimentally visualized by scanning probe microscope. Numerical results are in good agreement with the empirical data.

Temporal coherence behavior of a semiconductor laser under strong optical feedback

Abstract We investigate the temporal coherence properties of a semiconductor laser (SL) under strong continuous-wave optical feedback (OF). The temporal coherence length z c of the 830-nm laser is computed from the inverse Fourier transform of the time-averaged emission spectrum at different feedback strengths. The case of a confocal OF configuration is considered in the experiments. For a given feedback intensity R (0≤ R ≤0.76), z c exhibits a hump-like dependence with increasing bias current I . The peak location of the hump shifts to lower I values with increasing R , but the functional dependence of z c with I is generally maintained. For a fixed I , z c decreases monotonically with R , indicating that feedback may be utilized to control the coherence of the laser light source. For R >0.76, no fixed z c values are obtained because the spectra are very unstable. The power output of the laser is also analyzed using the Lang–Kobayashi (LK) model and the predictions are compared with experimental data. The results are utilized to establish the optimal conditions for the use of the laser as an optical signal detector.

Tip-Enhanced Raman Spectroscopy

This chapter describes Raman spectroscopy with a high spatial resolution beyond the diffraction limit of light.