arXiv subjects

Ramazan Sahin

Publications and source records attributed to Ramazan Sahin.

22 records · Page 2Linked to original sources

arXiv2019

Ultra-high resolution aSNOM imaging at off-resonant wavelengths

An atomic force microscope~(AFM) tip, with a few nm-thick noble metal coating, gives rise to strong electric-field at the near-field of tip apex, i.e. hot spot, when illuminated with a beam of light linearly polarized in the axial direction. This strong near-field enables resolving molecular landscape or nano-scale defects on crystal surfaces in apertureless scanning near field optical microscopy or tip enhanced Raman spectroscopy applications. However, strong near fields appear only at certain illumination wavelengths at which material and geometry dependent plasmon resonances take place. Once the metal coated tip is manufactured, optimal operation wavelength remains fixed since the material and geometry of the tip apex remains fixed. Here, we show for the first time a method which renders an AFM tip useful at wavelengths off-resonant to its plasmon resonances. The technique relies on decoration of the tip with appropriate auxiliary molecules. For instance, a tip originally bearing a plasmon resonance at $λ_{\rm p}=581$ nm can be effectively operated off-resonantly at $λ_{\rm exc}=532$ nm, when it is decorated by an appropriate auxiliary molecule. Furthermore, the near-field is found to be strongest just below the auxiliary molecule which enables a single-molecule-size ultra-high spatial resolution imaging. We demonstrate the phenomenon with exact solutions of 3D Maxwell equations. We also show why such an enhancement takes place.

cond-mat.mes-hall↗

arXiv2019

Direct micro-structuring of Si(111) surfaces through nanosecond-laser Bessel beams

We present here a nanosecond laser ablation of Si(111) wafer with diffraction-free (Bessel-$J_0$) beams. First, the Axicon a conical shaped optical element for beam shaping is characterized with a visible and infrared light sources. Then, ablation profiles are obtained with Bessel beams generated for $α=1^{\circ}$ and $α=20^{\circ}$ base angles, then they are characterized via a Scanning Electron, an Atomic Force and an Optical Interferometric Microscopes. Experimental results compared with theoretical predictions obtained by using Bessel functions. Results show that Bessel beams give possibility of straightforward micro-structuring of Si(111) samples because the only central spot of Bessel beams could damage the surface provided that the laser pulse energy becomes in energy-range we found in our experiments. Moreover, our results clearly indicate that reduced HAZ area due to thermal expansion in ns pulse regime are natural outcome of Bessel beams as opposed to Gauss beams of same spot size. Lastly, nanosecond pulses indicate two-ablation regimes regarding the pulse energy, the quality of structures can be enhanced by using high quality Gauss beams ($M^{2}\approx1$) and the size of fabricated structures can be much reduced by using larger base angle of Axicon (such as $α=40^{\circ}$) in our scheme.

physics.app-ph↗

arXiv2018

Nanosecond laser ablation of Si(111) under an aqueous medium

Nanosecond laser ablation of p-type Si(111) wafer is presented under an ambient, a water and a glycerin conditions. Effects of pulse energy, number of pulses and type of environment are systematically analyzed with an Optical Profilometer, a Scanning Electron Microscope and an Atomic Force Microscope. Obtained results are compared with Zemax-EE simulations. Self-assembly nano-ripple formation has been observed in an ambient condition. However, these ripples and heat affected zone (HAZ) area disappeared in aqueous medium which enhances the quality of ablation. Moreover, no remarkable oxidation was obtained in ablation zone even in an ambient condition based on the SEM-EDX analysis. Ablation profiles and their depths for different environments are compared with false color technique. Although ablation diameters are comparable each other, their depths are small enough in aqueous medium to control depth profile of damaged area. According to our results, best ablation profiles (precise edges without HAZ) are achieved under an aqueous medium especially in glycerin conditions both in low and high pulse energy regime.

physics.app-ph↗

arXiv2018

Exact solution of an exciton energy for a monolayer medium

We present exact solutions of an energy spectrum of 2-interacting particles in which they seem to be relativistic fermions in 2+1 space-time dimensions. The 2x2 spinor equations of 2-interacting fermions through general central potential were separated covariantly into the relative and center of mass coordinates. First of all, the coupled first order differential equations depending on radial coordinate were derived from 2x2 spinor equations. Then, a second order radial differential equation was obtained and solved for Coulomb interaction potential. We apply our solutions to exciton phenomena for a free-standing monolayer medium. Since we regard exciton as isolated 2-interacting fermions in our model, any other external effect such as substrate was eliminated. Our results show that the obtained binding energies in our model are in agreement with the literature. Moreover, the decay time of an exciton was found out spontaneously in our calculations.

cond-mat.mes-hall↗