- document
-
Haber, A. (author), Polo, A. (author), Ravensbergen, S. (author), Urbach, H.P. (author), Verhaegen, M.H.G. (author)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...journal article 2013
- document
-
Polo, A. (author), Pereira, S.F. (author), Urbach, P.H. (author)We study the phase retrieval (PR) technique using through-focus intensity measurements and explain the dependence of PR on the defocus distance. An optimal measurement plane in the out-of-focus region is identified where the intensity distribution on the optical axis drops to the first minimum after focus. Experimental results confirm the...journal article 2013
- document
-
El Gawhary, O. (author), Wiegmann, A. (author), Kumar, N. (author), Pereira, S.F. (author), Urbach, H.P. (author)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...journal article 2013
- document
- Roy, S. (author), Kumar, N. (author), Pereira, S.F. (author), Urbach, Paul (author) journal article 2013
- document
-
Kim, M.S. (author), Scharf, T. (author), Da Costa Assafrao, A. (author), Rockstuhl, C. (author), Pereira, S.F. (author), Urbach, H.P. (author), Herzig, H.P. (author)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...journal article 2012
- document
-
Polo, A. (author), Haber, A. (author), Pereira, S.F. (author), Verhaegen, M.H.G. (author), Urbach, H.P. (author)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...journal article 2012
- document
-
Costa Assafrao, A. (author), Wachters, A.J.H. (author), Pereira, S.F. (author), Urbach, H.P. (author)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...journal article 2012
- document
-
Kim, M.S. (author), Assafrao, A.C. (author), Scharf, T. (author), Wachters, A.J.H. (author), Pereira, S.F. (author), Urbach, H.P. (author), Brun, M. (author), Olivier, S. (author), Nicoletti, S. (author), Herzig, H.P. (author)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 (?-SIL) made of SiO2. Such structured focal spots are characterized by a doughnut-shaped intensity distribution, whose...journal article 2012
- document
-
Westerveld, W.J. (author), Leinders, S.M. (author), Van Dongen, K.W.A. (author), Urbach, H.P. (author), Yousefi, M. (author)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...journal article 2012
- document
-
El Gawhary, O. (author), Schilder, N.J. (author), Da Costa Assafrao, A. (author), Pereira, S.F. (author), Urbach, H.P. (author)It was predicted a few years ago that a medium with negative index of refraction would allow for perfect imaging. Although no material has been found so far that behaves as a perfect lens, some experiments confirmed the theoretical predictions in the near-field, or quasi-static, regime where the behaviour of a negative index medium can be...journal article 2012
- document
-
Assafrao, A.C. (author), Wachters, A.J.H. (author), Verheijen, M. (author), Nugrowati, A.M. (author), Pereira, S.F. (author), Urbach, H.P. (author), Armand, M.F. (author), Olivier, S. (author)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...journal article 2012
- document
-
Polo, A. (author), Kutchoukov, V. (author), Bociort, F. (author), Pereira, S.F. (author), Urbach, H.P. (author)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....journal article 2012
- document
-
Westerveld, W.J. (author), Pozo, J. (author), Harmsma, P.J. (author), Schmits, R. (author), Tabak, E. (author), Van den Dool, T.C. (author), Leinders, S.M. (author), Van Dongen, K.W.A. (author), Urbach, H.P. (author), Yousefi, M. (author)Recently there has been growing interest in sensing by means of optical microring resonators in photonic integrated circuits that are fabricated in silicon-on-insulator (SOI) technology. Taillaert et al. [Proc. SPIE6619, 661914 (2007)PSISDG0277-786X10.1117/12.738412] proposed the use of a silicon-waveguide-based ring resonator as a strain gauge....journal article 2012
- document
-
El Gawhary, O. (author), Kumar, N. (author), Pereira, S.F. (author), Coene, W.M.J. (author), Urbach, H.P. (author)Scatterometry is a well established technique currently utilized in research, as well as in industrial applications, to retrieve the properties of a given scatterer (the target) by looking at how the light coming from a certain source is diffracted in the far field. Currently the light source is often a discharge lamp that, after wavelength...journal article 2011
- document
-
De Bruin, R. (author), Urbach, H.P. (author), Pereira, S.F. (author)We study focused fields which, for a given total power and a given numerical aperture, have maximum electric field amplitude in some direction in the focal point and are linearly polarized along this direction. It is shown that the optimum field is identical to the image of an electric dipole with unit magnification. In particular, the field...journal article 2011
- document
-
Westerveld, W.J. (author), Urbach, H.P. (author), Yousefi, M. (author)We present an accurate and fast 3D simulation scheme for out-of-plane grating couplers, based on two dimensional rigorous (finite difference time domain) grating simulations, the effective index method (EIM), and the Rayleigh-Sommerfeld diffraction formula. In comparison with full 3D FDTD simulations, the rms difference in electric field is...journal article 2011
- document
-
Cui, M. (author), Zeitouny, M.G. (author), Bhattacharya, N. (author), Van den Berg, S.A. (author), Urbach, H.P. (author)We experimentally demonstrate long distance measurements with a femtosecond frequency comb laser using dispersive interferometry. The distance is derived from the unwrapped spectral phase of the dispersed interferometer output and the repetition frequency of the laser. For an interferometer length of 50 m this approach has been compared to an...journal article 2011
- document
-
El Gawhary, O. (author), Pereira, S.F. (author), Urbach, H.P. (author)Providing a femtosecond optical pulse with a proper transverse spatial profile represents a fast and relatively simple method to force a quantum system to follow a prescribed temporal evolution. In the present work, we show that the quantum system presents a surprisingly high sensitivity with respect to the spatial shape of the pulse. We discuss...journal article 2011
- document
-
Zeitouny, M.G. (author), Cui, M. (author), Janssen, A.J. (author), Bhattacharya, N. (author), Van den Berg, S.A. (author), Urbach, H.P. (author)We investigate general properties of the interferograms from a frequency comb laser in a non-linear dispersive medium. The focus is on interferograms at large delay distances and in particular on their broadening, the fringe formation and shape. It is observed that at large delay distances the interferograms spread linearly and that its shape is...journal article 2011
- document
-
Janssen, O.T.A. (author), Wachters, A.J.H. (author), Urbach, H.P. (author)The incoherent emission of periodically structured Light Emitting Diodes (LEDs) can be computed at relatively low computational cost by applying the reciprocity method. We show that by another application of the reciprocity principle, the structure of the LED can be optimized to obtain a high emission. We demonstrate the method by optimizing one...journal article 2010