[show abstract][hide abstract] ABSTRACT: In several optical systems, a specific Point Spread Function (PSF) needs to be generated. This can be achieved by shaping the complex field at the pupil. The Extended Nijboer-Zernike (ENZ) theory relates complex Zernike modes on the pupil directly to functions in the focal region. In this paper, we introduce a method to engineer a PSF using the ENZ theory. In particular, we present an optimization algorithm to design an extended depth of focus with high lateral resolution, while keeping the transmission of light high (over 60%). We also have demonstrated three outcomes of the algorithm using a Spatial Light Modulator (SLM).
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: Microscale strain gauges are widely used in micro electro-mechanical systems (MEMS) to measure strains such as those induced by force, acceleration, pressure or sound. We propose all-optical strain sensors based on micro-ring resonators to be integrated with MEMS. We characterized the strain-induced shift of the resonances of such devices. Depending on the width of the waveguide and the orientation of the silicon crystal, the linear wavelength shift per applied strain varies between 0.5 and 0.75 pm/microstrain for infrared light around 1550 nm wavelength. The influence of the increasing ring circumference is about three times larger than the influence of the change in waveguide effective index, and the two effects oppose each other. The strong dispersion in 220 nm high silicon sub-wavelength waveguides accounts for a decrease in sensitivity of a factor 2.2 to 1.4 for waveguide widths of 310 nm to 860 nm. These figures and insights are necessary for the design of strain sensors based on silicon waveguides.
IEEE Journal of Selected Topics in Quantum Electronics 01/2014; 20(4):1-10. · 4.08 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have performed an in vitro and in vivo study, based on laser speckle contrast analysis, to detect fluid pulsation in the presence of artifacts caused by the relative motion between the sample and the illumination source. We observe that the pulsation signal is clearly detectable for a range of motion amplitudes and oscillation frequencies; however, for higher amplitudes and oscillation frequencies of motion, the signal, due to pulsation, becomes increasingly difficult to detect.
[show abstract][hide abstract] ABSTRACT: We developed a fast phase retrieval algorithm that is suitable for
real-time applications such as adaptive optics. The phase retrieval
model is developed by linearising the pupil function in the
approximation of small aberrations and is valid for low-NA focused
field. The linear model in conjunction with a particular choice for the
position of the single out-of-focus measurement plane and an efficient
control algorithm, significantly reduces the computation time for phase
retrieval. The experimental results demonstrate the validity of the
described approach for fast correction of aberrations.
Journal of the European Optical Society - Rapid Publications vol 8 13070, 5 pages. 10/2013;
[show abstract][hide abstract] ABSTRACT: In this paper we experimentally demonstrate the proof of concept for predictive control of thermally induced wavefront aberrations in optical systems. On the basis of the model of thermally induced wavefront aberrations and using only past wavefront measurements, the proposed adaptive optics controller is able to predict and to compensate the future aberrations. Furthermore, the proposed controller is able to correct wavefront aberrations even when some parameters of the prediction model are unknown. The proposed control strategy can be used in high power optical systems, such as optical lithography machines, where the predictive correction of thermally induced wavefront aberrations is a crucial issue.
[show abstract][hide abstract] ABSTRACT: Using the subspace identification technique, we identify a finite dimensional, dynamical model of a recently developed prototype of a thermally actuated deformable mirror (TADM). The main advantage of the identified model over the models described by partial differential equations is its low complexity and low dimensionality. Consequently, the identified model can be easily used for high-performance feedback or feed-forward control. The experimental results show good agreement between the dynamical response predicted by the model and the measured response of the TADM.
[show abstract][hide abstract] ABSTRACT: Non-interferometric phase retrieval from the intensity measurements in Coherent Fourier Scatterometry (CFS) is presented using a scanningfocused spot. Formulae to determine the state of polarization of the scattered light and to retrieve the phase difference between overlappingscattered orders are given. The scattered far field is rigorously computed and the functionality of the method is proved with experimentalresults.
Journal of the European Optical Society Rapid Publications 07/2013; 8(2013):13048. · 0.93 Impact Factor
[show abstract][hide abstract] ABSTRACT: 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.
Journal of optics 05/2013; 15(7):075707. · 1.99 Impact Factor
[show abstract][hide abstract] ABSTRACT: Through-focus phase retrieval methods aim to retrieve the phase of an optical field from its intensity distribution measured at different planes in the focal region. By using the concept of spatial correlation for propagating fields, for both the complex amplitude and the intensity of a field, we can infer which planes are suitable to retrieve the phase and which are not. Our analysis also reveals why all techniques based on measuring the intensity at two Fourier-conjugated planes usually lead to a good reconstruction of the phase. The findings presented in this work are important for aberration characterization of optical systems, adaptive optics and wavefront metrology.
[show abstract][hide abstract] ABSTRACT: We report an experimental and theoretical study on the alignment error
tolerance of a 2 μm-size solid immersion lens (SIL) illuminated by
different types of focused spots. Tightly confined focal spots are of
great interest for improving the performance of many optical systems, so
that a study on the alignment tolerance is of interest. In particular,
it was found that micro-SILs can be largely misaligned with respect to
the optical axis of an objective lens focusing light onto it and yet
allow for a reasonably good immersed spot. In fact, a displacement of
approximately 400 nm, i.e. one fifth of the lens diameter, is tolerable.
The measurements are compared with a rigorous finite element method
model for a micro-SIL, showing an excellent agreement.
[show abstract][hide abstract] ABSTRACT: 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.
[show abstract][hide abstract] ABSTRACT: We carry out performance characterisation of a commercial push and pull deformable mirror with 48 actuators (Adaptica Srl). We present a detailed description of the system as well as a statistical approach on the identification of the mirror influence function. A new efficient control algorithm to induce the desired wavefront shape is also developed and comparison with other control algorithms present in literature has been made to prove the efficiency of the new approach.
[show abstract][hide abstract] ABSTRACT: A detailed study on the influence of the focusing lens numerical
aperture on the InSb super resolved spot is presented. The super
resolution effect is described using the threshold model, where the
material permittivity locally changes under laser exposure, being
rigorously simulated in a finite element method program. The
superresolved InSb focused spot is investigated for a wide range
focusing lenses systems, from high to low numerical apertures. An
extension of the model to account for disc motion is discussed. Such
dynamic super-resolution near-field structure (Super-RENS) threshold
model is suitable for readout signal analysis when combined with a
``quasi-scalar'' readout program, also described. Interesting properties
of the InSb Super-RENS focused spot, such as sub-diffraction limited
spot size and the general conditions to achieve it, are shown. An
analysis on the Super-RENS readout signal is also given, with useful
applications in optical data storage. Thus, this work greatly helps in
understanding the InSb scatterer type of Super-RENS effect for optical
data storage and other super resolution applications.
Japanese Journal of Applied Physics 11/2012; 51(11):2501-. · 1.07 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have found an alternative way of achieving a doughnutlike focused spot by simply melting a subwavelength scatterer in a polycarbonate/ZnS sample. The near-field microscopy technique is used to directly measure the induced doughnut spot in the near-field regime. A numerical model based on rigorous solution of the Maxwell's equations is proposed to study the phenomena. The simulations help to understand the optical mechanism behind the spot formation.
[show abstract][hide abstract] ABSTRACT: 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.
New Journal of Physics 10/2012; 14(10):103024. · 4.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: Marcatili's famous approximate analytical description
of light propagating through rectangular dielectric waveguides, published
in 1969, gives accurate results for low-index-contrast waveguides. However,
photonic-integrated circuit technology has advanced to high-index-contrast
(HIC) waveguides. In this paper, we improve Marcatili's model by adjusting
the amplitudes of the components of the electromagnetic fields in his description.
We find that Marcatili's eigenvalue equation for the propagation constant
is also valid for HIC waveguides. Our improved method shows much better agreement
with rigorous numerical simulations, in particular for the case of HIC waveguides.
We also derive explicit expressions for the effective group index and the
effects of external forces on the propagation constant. Furthermore, with
our method, the phenomenon of avoided crossing of modes is observed and studied.
Journal of Lightwave Technology 07/2012; 30(14):2388-2401. · 2.56 Impact Factor
[show abstract][hide abstract] ABSTRACT: Incoherent Fourier Scatterometry (IFS) is a successful tool for high accuracy nano-metrology. As this method uses only far field measurements, it is very convenient from the point of view of industrial applications. A recent development is Coherent Fourier Scatterometry (CFS) in which incoherent illumination is replaced by a coherent one. Through sensitivity analyses using rigorous electromagnetic simulations, we show that the use of coherence and multiple scanning makes Coherent Fourier Scatterometry (CFS) more sensitive than Incoherent Fourier Scatterometry (IFS). We also report that in Coherent Fourier Scatterometry it is possible to determine the position of the sample with respect to the optical axis of the system to a precision dependent only on the experimental noise.
Journal of the European Optical Society Rapid Publications 07/2012; Vol 7. · 0.93 Impact Factor
[show abstract][hide abstract] ABSTRACT: We study decoupled and coupled types of surface plasmons in the near UV
wavelength range (λ = 193, 365, 405nm) in circular metal film
diaphragms composed of concentric sub wavelength nanoslit grooves.