S. F. Pereira

85% efficiency for cw frequency doubling from 1.08 to 0.54 μm

Conversion efficiency of 85% has been achieved in cw second-harmonic generation from 1.08 to 0.54 microm with a potassium titanyl phosphate crystal inside an external ring cavity. An absolute comparison between the experimental data and a simple theory is made and shows good agreement.

Realization of the Einstein-Podolsky-Rosen paradox for continuous variables in nondegenerate parametric amplification

The Einstein-Podolsky-Rosen (EPR) paradox is demonstrated experimentally for continuous variables by employing a nondegenerate optical parametric amplifier (NOPA). Such a system is analogous to and under some ideal conditions is in one-to-one correspondence with the original system discussed by EPR. In particular, the quadrature-phase amplitudes for a signal beam are inferred in turn from those of a spatially separated but strongly correlated idler beam, where these optical amplitudes are analogous to canonical position and momentum variables. The variances for the two inferences are measured and their product is observed to be below the limit of unity associated with the Heisenberg uncertainty relation, in apparent contradiction with quantum mechanics according to the argument of EPR. The smallest product of inference variances achieved in the experiment is (0.70±0.01). Various other types of quantum noise for this system are also investigated, and a theory of a narrowband NOPA is presented with losses included. A comparison between experiment and this theory shows relatively good agreement. The question of a local hidden-variables description of the system is discussed.

Calculation of the vectorial field distribution in a stratified focal region of a high numerical aperture imaging system

We present an algorithm for calculating the field distribution in the focal region of stratified media which is fast and easy to implement. Using this algorithm we study the effect on the electric field distribution of an air gap separating a solid immersion lens and a sample, where we analyse the maximum distance for out-of-contact operation. Also, we study how the presence of a metallic substrate affects the field distribution in the focal region; the interference effects of the reflected field could be used as an alternative for 4Pi-microscopy.

Coherent Fourier scatterometry: tool for improved sensitivity in semiconductor metrology

Incoherent Optical Scatterometry (IOS) is a well-established metrology technique in the semiconductor industry to retrieve periodic grating structures with high accuracy from the signature of the diffracted optical far field. With shrinking dimensions in the lithography industry, finding possible improvements in wafer metrology is highly desirable. The grating is defined in terms of a finite number of geometrical shape parameters (height, side-wall angles, midCD etc.). In our method the illumination is a scanning focused spot from a spatially coherent source (laser) within a single period of the grating. We present a framework to study the increment in sensitivity of Coherent Fourier Scatterometry (CFS) with respect to the IOS. Under suitable conditions, there is a more than fourfold enhancement in sensitivity for grating shape parameters using CFS. The dependence of scanning positions on the sensitivity analysis is also highlighted. We further report the experimental implementation of a Coherent Fourier Scatterometer. The simulated and experimental far fields are compared and analyzed for the real noise in the experimental configuration.

Determination of wavefront structure for a Hartmann Wavefront Sensor using a phase-retrieval method

We apply a phase retrieval algorithm to the intensity pattern of a Hartmann wavefront sensor to measure with enhanced accuracy the phase structure of a Hartmann hole array. It is shown that the rms wavefront error achieved by phase reconstruction is one order of magnitude smaller than the one obtained from a typical centroid algorithm. Experimental results are consistent with a phase measurement performed independently using a Shack-Hartmann wavefront sensor.

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.

Time evolution of the diffraction pattern of an ultrashort laser pulse

An analytical expression for the time evolution of the diffraction pattern of an ultrashort laser pulse passing through a circular aperture is obtained in the Fresnel regime. The diffraction is not constant in time as the pulse travels through the aperture. This may have implications in experiments involving fast dynamics. Examples of the evolution of the diffraction pattern are given.

Interferometric coherent Fourier scatterometry: a method for obtaining high sensitivity in the optical inverse-grating problem

In recent times, coherent Fourier scatterometry has been considered as an emerging optical grating scatterometry technique for semiconductor metrology since it shows large sensitivity owing to its scanning ability. However, further utilization of coherence is possible by making additional measurements using the principle of temporal phase-shifting interferometry. In this paper, through numerical simulation, we show how scanning and interferometry can be coupled together to improve the sensitivity of coherent Fourier scatterometry, to extend its range of applicability and to obtain sufficient information to calculate the complex scattering matrix for all angles of incidences inside the numerical aperture of a microscope objective.

Reconstruction of sub-wavelength features and nano-positioning of gratings using coherent Fourier scatterometry

Optical scatterometry is the state of art optical inspection technique for quality control in lithographic process. As such, any boost in its performance carries very relevant potential in semiconductor industry. Recently we have shown that coherent Fourier scatterometry (CFS) can lead to a notably improved sensitivity in the reconstruction of the geometry of printed gratings. In this work, we report on implementation of a CFS instrument, which confirms the predicted performances. The system, although currently operating at a relatively low numerical aperture (NA = 0.4) and long wavelength (633 nm) allows already the reconstruction of the grating parameters with nanometer accuracy, which is comparable to that of AFM and SEM measurements on the same sample, used as reference measurements. Additionally, 1 nm accuracy in lateral positioning has been demonstrated, corresponding to 0.08% of the pitch of the grating used in the actual experiment.

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.