Alberto da Costa Assafrao

Direct measurement of the near-field super resolved focused spot in InSb

Under appropriate laser exposure, a thin film of InSb exhibits a sub-wavelength thermally modified area that can be used to focus light beyond the diffraction limit. This technique, called Super-Resolution Near-Field Structure, is a potential candidate for ultrahigh density optical data storage and many other high-resolution applications. We combined near field microscopy, confocal microscopy and time resolved pump-probe technique to directly measure the induced sub-diffraction limited spot in the near-field regime. The measured spot size was found to be dependent on the laser power and a decrease of 25% (100 nm) was observed. Experimental evidences that support a threshold-like simulation model to describe the effect are also provided. The experimental data are in excellent agreement with rigorous simulations obtained with a three dimensional Finite Element Method code.

Phase anomalies in Bessel-Gauss beams

Bessel-Gauss beams are known as non-diffracting beams. They can be obtained by focusing an annularly shaped collimated laser beam. Here, we report for the first time on the direct measurement of the phase evolution of such beams by relying on longitudinal-differential interferometry. We found that the characteristics of Bessel-Gauss beams cause a continuously increasing phase anomaly in the spatial domain where such beams do not diverge, i.e. there is a larger phase advance of the beam when compared to a referential plane wave. Simulations are in excellent agreement with measurements. We also provide an analytical treatment of the problem that matches both experimental and numerical results and provides an intuitive explanation.

On the Focused Field Embedded in a Super Rens Medium

A rigorous electromagnetic computational model is presented to simulate the near field characteristics of the focused spot in an embedded super rens medium. The super resolution effect is described using a threshold model, where the material refractive index is changed instantaneously under laser exposure above a certain threshold. The simulations reveal interesting features of the Super Rens focused spot and demonstrate how super resolution is achieved. This model is useful for a better understanding of the super resolution effect in optical data storage and other super resolution applications.

Image formation properties and inverse imaging problem in aperture based scanning near field optical microscopy

Aperture based scanning near field optical microscopes are important instruments to study light at the nanoscale and to understand the optical functionality of photonic nanostructures. In general, a detected image is affected by both the transverse electric and magnetic field components of light. The discrimination of the individual field components is challenging as these four field components are contained within two signals in the case of a polarization resolved measurement. Here, we develop a methodology to solve the inverse imaging problem and to retrieve the vectorial field components from polarization and phase resolved measurements. Our methodology relies on the discussion of the image formation process in aperture based scanning near field optical microscopes. On this basis, we are also able to explain how the relative contributions of the electric and magnetic field components within detected images depend on the chosen probe. We can therefore also describe the influence of geometrical and material parameters of individual probes within the image formation process. This allows probes to be designed that are primarily sensitive either to the electric or magnetic field components of light.

Experimental and numerical analysis of the super resolution near-field effect on an InSb sample

The super resolution near-field effect has attracted the attention of many researchers since it offers a relatively simple way to overcome the diffraction limit of optical systems. Although the technique has been widely applied in optical data storage, it can be promptly used in other fields, once some problems are solved. There is an open question of what happens to the focused spot after passing through an activated super resolution layer. In addition, there is a need for a model that can accurately describe the super resolved spot. Hence, in this work, we analyze and discuss these issues, both numerically and experimentally. Coherent far-field scatterometry and near-field scanning microscopy techniques are employed to both monitor the phase transition of the sample and to measure the transmitted super-resolved spots; Rigorous electromagnetic simulations based on the finite element method are used to model the effect. A direct comparison between experiment and simulation is provided.

Application of micro solid immersion lens as probe for near-field scanning microscopy

We present an experimental and theoretical study of the immersing properties of a micron-sized solid immersion lens (μ-SIL) and evaluate its capabilities of functioning as a near-field probe. It was found that the μ-SIL reduces an impinging focused spot by a factor of approximately its refractive index, similarly to a macroscopic solid immersion lens. This reduced immersed spot is used to investigate the visibility of a periodic grating structure. Results show an improvement in the visibility by approximately 30% when compared to confocal microscopy, demonstrating the potential application of these tiny micro-lenses as a near-field probe in scanning microscopy and other high-resolution optical systems.

Coherent Fourier scatterometry for detection of nanometer-sized particles on a planar substrate surface

Inspection tools for nano-particle contamination on a planar substrate surface is a critical problem in micro-electronics. The present solutions are either expensive and slow or inexpensive and fast but have low sensitivity because of limitations due to diffraction. Most of them are also substrate specific. In this article we report how Coherent Fourier Scatterometry is used for detection of particles smaller than λ/4. Merits of the technique, especially, the procedures to improve SNR, its flexibility and its robustness on rough surfaces are discussed with simulated and experimental results.

Longitudinal-differential phase distribution near the focus of a high numerical aperture lens: Study of wavefront spacing and Gouy phase

The longitudinal–differential (LD) phase distribution was investigated near the focus of a high numerical aperture (NA = 0.9) aplanatic lens illuminated with a linearly polarized monochromatic plane wave. The Richards and Wolf method was used to compute field distributions. The LD phase map was obtained by analyzing the deviation of the phase of the simulated wave to the phase of a referential plane wave. The irregular wavefront spacing that is linked to the Guoy phase is discussed and subtle details of the phase features with respect to the spatial domain relative to the focal point are disclosed. The LD phase is used to revisit different definitions of the focal region. The definition is eventually identified that is in agreement with the Gouy phase in the focal region. Our work paves the way towards a coherent notion to quantify the optical action of high NA optical elements that are increasingly important for many applications.

Submicron hollow spot generation by solid immersion lens and structured illumination

We report on the experimental and numerical demonstration of immersed submicron-size hollow focused spots, generated by structuring the polarization state of an incident light beam impinging on a micro-size solid immersion lens (μ-SIL) made of SiO2. Such structured focal spots are characterized by a doughnut-shaped intensity distribution, whose central dark region is of great interest for optical trapping of nano-size particles, super-resolution microscopy and lithography. In this work, we have used a high-resolution interference microscopy technique to measure the structured immersed focal spots, whose dimensions were found to be significantly reduced due to the immersion effect of the μ-SIL. In particular, a reduction of 37% of the dark central region was verified. The measurements were compared with a rigorous finite element method model for the μ-SIL, revealing excellent agreement between them.

The near field characteristics of the focused field embedded in the super-RENS layer applied to lithography

We present a rigorous numerical model to study the near-field characteristics of the focused spot embedded in a Super Resolution Near Field stack layer. The results indicate that a focused spot beyond the diffraction limit can be achieved and its characteristics can be modeled by proper choice of optical parameters.