Toralf Scharf | Ecole Polytechnique Federale de Lausanne (original) (raw)

Papers by Toralf Scharf

Optics express, Dec 17, 2012

Bessel-Gauss beams are known as non-diffracting beams. They can be obtained by focusing an annula... more 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, ie there is a larger phase advance of the beam when compared to a referential ...

The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated expe... more The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated experimentally and theoretically. We stabilized Blue Phases and measured their diffraction pattern for different wavelengths of monochromatic light with the help of a conoscopic setup of a polarization microscope. Moreover, the diffraction patterns were calculated with the help of a 4x4 matrix method which allows amplitude and phase investigations.

Journal of The European Optical Society-rapid Publications, 2010

We report on the fabrication and optical characterization of a switchable photonic crystal nanoca... more We report on the fabrication and optical characterization of a switchable photonic crystal nanocavity actuated by liquid crystals. This device acts as a filter presenting a transmission peak around the telecom wavelength λ = 1550 nm. Passing from the isotropic to the anisotropic (oriented crystals) state of the liquid crystals, a shift of ∆λ = 13 nm has been measured, which confirms the theoretical predictions obtained by finite difference time domain simulations. We have developed a photonic crystal nanocavity that can be tuned thanks to the properties of liquid crystals infiltrated in the holes of the photonic device.

Molecular Crystals and Liquid Crystals, 2001

... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 ... more ... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 M. Kitamura, presented at SID 95 Digest, (1995). [ 171 BEA Saleh and MC Teich, Fimkrrnentals ofPhotonics. New York: John Wiley and Sons, Inc., (1991). ...

Shaping light with microtechnology components has been possible for many years. The Texas Instrum... more Shaping light with microtechnology components has been possible for many years. The Texas Instruments digital micromirror device (DMD) and all types of adaptive optics systems are very sophisticated tools, well established and widely used. Here we present, however, two very dedicated systems, where one is an extremely simple MEMS-based tunable diffuser, while the second device is complex micromirror array with new capabilities for femtosecond laser pulse shaping. Showing the two systems right next to each other demonstrates the vast options and versatility of MOEMS for shaping light in the space and time domain.

The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated expe... more The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated experimentally and theoretically. We stabilized Blue Phases and measured their diffraction pattern for different wavelengths of monochromatic light with the help of a conoscopic setup of a polarization microscope. Moreover, the diffraction patterns were calculated with the help of a 4x4 matrix method which allows amplitude and phase investigations.

Journal of The European Optical Society-rapid Publications, 2010

We report on the fabrication and optical characterization of a switchable photonic crystal nanoca... more We report on the fabrication and optical characterization of a switchable photonic crystal nanocavity actuated by liquid crystals. This device acts as a filter presenting a transmission peak around the telecom wavelength λ = 1550 nm. Passing from the isotropic to the anisotropic (oriented crystals) state of the liquid crystals, a shift of ∆λ = 13 nm has been measured, which confirms the theoretical predictions obtained by finite difference time domain simulations. We have developed a photonic crystal nanocavity that can be tuned thanks to the properties of liquid crystals infiltrated in the holes of the photonic device.

Molecular Crystals and Liquid Crystals, 2001

... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 ... more ... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 M. Kitamura, presented at SID 95 Digest, (1995). [ 171 BEA Saleh and MC Teich, Fimkrrnentals ofPhotonics. New York: John Wiley and Sons, Inc., (1991). ...

Shaping light with microtechnology components has been possible for many years. The Texas Instrum... more Shaping light with microtechnology components has been possible for many years. The Texas Instruments digital micromirror device (DMD) and all types of adaptive optics systems are very sophisticated tools, well established and widely used. Here we present, however, two very dedicated systems, where one is an extremely simple MEMS-based tunable diffuser, while the second device is complex micromirror array with new capabilities for femtosecond laser pulse shaping. Showing the two systems right next to each other demonstrates the vast options and versatility of MOEMS for shaping light in the space and time domain.

Molecular Crystals and Liquid Crystals, 1999

A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens sy... more A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens system was built with a homeotropic aligned liquid crystal of positive dielectric anisotropy and with a circular electrode structure on one side of a sandwiched liquid crystal cell. The fringing effect at the electrode etches produces spatial distribution of the electric field. Applying a voltage deforms the liquid

Optical Engineering, 2000

The behavior of nematic liquid crystals in spatially varying electrical fields is investigated. D... more The behavior of nematic liquid crystals in spatially varying electrical fields is investigated. Director profiles of nematic-liquid-crystal phase gratings are simulated for different orientations of the liquid-crystal director field with respect to the electrode grating. With increasing spatial frequency of the grating the fringing electric field becomes important and the diffraction pattern changes. The optical properties are analyzed and measurements are performed.

A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens sy... more A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens system was built with a homeotropic aligned liquid crystal of positive dielectric anisotropy and with a circular electrode structure on one side of a sandwiched liquid crystal cell. The fringing effect at the electrode etches produces spatial distribution of the electric field. Applying a voltage deforms the liquid

Japanese Journal of Applied Physics, 2000

Liquid crystal microlenses with diffraction limited performance are presented. The micro-lenses c... more Liquid crystal microlenses with diffraction limited performance are presented. The micro-lenses consist of a liquid crystal film sandwiched between a pair of conductive substrates where one has hole patterned electrode. The lens diameters are 120 mum and 110 mum and diameter to thickness ratio of the lenses is approximately 2.4. The optical quality was investigated with a Mach-Zehnder interferometer. The

Molecular Crystals and Liquid Crystals, 2005

Interaction of liquid crystals with nanostructured surfaces is studied experimentally. Whereas ph... more Interaction of liquid crystals with nanostructured surfaces is studied experimentally. Whereas photolithographic or AFM methods were used in the past for surface structuring, we use -besides photolithographyessentially replication methods for the creation of well-controlled nanotopologies. We established a laboratory process to prepare surface structures that can be as large as 4x4 inch and allow preparing test cells in a production like environment. We combined different methods and tested for soft embossing and direct writing in photoresist. Soft embossing was made with UV curable materials. The direct writing in photoresist was done by means of laser beam interferometry. Structure parameters were changed and the surfaces were characterized by surface anchoring energy measurements and by electro-optical switching behaviour. The obtained alignment is quite uniform over large surface areas despite of a certain variation of the surface parameters. In the case of interferometrically produced grating structures with periods ranging from 350-800 nm strong anchoring was observed and no pretilt could be found.

Optics Express, 2005

A new optical concept for compact digital image acquisition devices with large field of view is d... more A new optical concept for compact digital image acquisition devices with large field of view is developed and proofed experimentally. Archetypes for the imaging system are compound eyes of small insects and the Gabor-Superlens. A paraxial 3x3 matrix formalism is used to describe the telescope arrangement of three microlens arrays with different pitch to find first order parameters of the imaging system. A 2mm thin imaging system with 21x3 channels, 70 masculinex10 masculine field of view and 4.5mm x 0.5mm image size is optimized and analyzed using sequential and non-sequential raytracing and fabricated by microoptics technology. Anamorphic lenses, where the parameters are a function of the considered optical channel, are used to achieve a homogeneous optical performance over the whole field of view. Captured images are presented and compared to simulation results.

Physical Review Letters, 2007

We propose an artificial three-dimensional material that exhibits a strong resonance in the effec... more We propose an artificial three-dimensional material that exhibits a strong resonance in the effective permeability in the visible spectral domain. This material may be implemented in a two-step procedure. First, a metamaterial made of densely packed metallic nanoparticles is fabricated that shows a Lorentz-type resonance in the permittivity at the collective plasmon frequency. Second, spheres are formed out of this material and arranged in a cubic lattice. This meta-metamaterial exhibits a strong resonance in the permeability which is caused by a Mie resonance associated with the magnetic mode of a single metamaterial sphere. Realization of this material based on self-organization in liquid crystals and the limitations of the approach are discussed.

Microelectronic Engineering, 2008

Herein, a study of combining refractive microlenses arrays and nanostructured antireflective surf... more Herein, a study of combining refractive microlenses arrays and nanostructured antireflective surfaces is presented. Fabrication method is surveyed for obtaining the hybrid surface with functionalities of refractive and antireflective (high-transmissive) element. A standard re-flow method is used for fabricating the microlenses and the subwavelength structure is generated on the curved surfaces by interference lithography. By using an optical fiber detector the increased transmittance over the whole visible spectrum is shown. This kind a hybrid element could have applications in illumination systems, in which the main purpose is to gather light to certain areas.

Applied Optics, 2002

We present designs of static Fourier transform spectrometers that are based on a Wollaston prism ... more We present designs of static Fourier transform spectrometers that are based on a Wollaston prism with a large field of view. Besides the usual advantages of static Fourier spectrometers (large resolving power, large wave-number range, high throughput), these designs also present the advantage of using relatively cheap liquid-crystal technology. The use of twisted liquid-crystal structures gives a large field of view, which in turn gives the ability to collect more light from a divergent light source. Measurements are compared with simulations. Different simulation principles are used. We found new configurations by using twisted structures that show a large field of view.

Current Nanoscience, 2006

High Resolution Interference Microscopy (HRIM) is a technique that allows the characterization of... more High Resolution Interference Microscopy (HRIM) is a technique that allows the characterization of amplitude and phase of electromagnetic wave-fields in the far-field with a spatial accuracy that corresponds to a few nanometers in the object plane. Emphasis is put on the precise determination of topological features in the wave-field, called phase singularities or vortices, which are spatial points within the electromagnetic wave at which the amplitude is zero and the phase is hence not determined. An experimental tool working in transmission with a resolution of 20 nm in the object plane is presented and its application to the optical characterization of various single and periodic nanostructures such as trenches, gratings, microlenses and computer generated holograms is discussed. The conditions for the appearance of phase singularities are theoretically and experimentally outlined and it is shown how dislocation pairs can be used to determine unknown parameters from an object. Their corresponding applications to metrology or in optical data storage systems are analyzed. In addition, rigorous diffraction theory is used in all cases to simulate the interaction of light with the nano-optical structures to provide theoretical confirmation of the experimental results.

Optics Express, 2008

A two step process has been developed for the fabrication of diffraction limited concave microlen... more A two step process has been developed for the fabrication of diffraction limited concave microlens arrays. The process is based on the photoresist filling of melted holes obtained by a preliminary photolithography step. The quality of these microlenses has been tested in a Mach-Zehnder interferometer. The method allows the fabrication of concave microlens arrays with diffraction limited optical performance. Concave microlenses with diameters ranging between 30 µm to 230 µm and numerical apertures up to 0.25 have been demonstrated. As an example, we present the realization of diffusers obtained with random sizes and locations of concave shapes.

Optics express, Dec 17, 2012

Bessel-Gauss beams are known as non-diffracting beams. They can be obtained by focusing an annula... more 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, ie there is a larger phase advance of the beam when compared to a referential ...

The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated expe... more The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated experimentally and theoretically. We stabilized Blue Phases and measured their diffraction pattern for different wavelengths of monochromatic light with the help of a conoscopic setup of a polarization microscope. Moreover, the diffraction patterns were calculated with the help of a 4x4 matrix method which allows amplitude and phase investigations.

Journal of The European Optical Society-rapid Publications, 2010

We report on the fabrication and optical characterization of a switchable photonic crystal nanoca... more We report on the fabrication and optical characterization of a switchable photonic crystal nanocavity actuated by liquid crystals. This device acts as a filter presenting a transmission peak around the telecom wavelength λ = 1550 nm. Passing from the isotropic to the anisotropic (oriented crystals) state of the liquid crystals, a shift of ∆λ = 13 nm has been measured, which confirms the theoretical predictions obtained by finite difference time domain simulations. We have developed a photonic crystal nanocavity that can be tuned thanks to the properties of liquid crystals infiltrated in the holes of the photonic device.

Molecular Crystals and Liquid Crystals, 2001

... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 ... more ... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 M. Kitamura, presented at SID 95 Digest, (1995). [ 171 BEA Saleh and MC Teich, Fimkrrnentals ofPhotonics. New York: John Wiley and Sons, Inc., (1991). ...

Shaping light with microtechnology components has been possible for many years. The Texas Instrum... more Shaping light with microtechnology components has been possible for many years. The Texas Instruments digital micromirror device (DMD) and all types of adaptive optics systems are very sophisticated tools, well established and widely used. Here we present, however, two very dedicated systems, where one is an extremely simple MEMS-based tunable diffuser, while the second device is complex micromirror array with new capabilities for femtosecond laser pulse shaping. Showing the two systems right next to each other demonstrates the vast options and versatility of MOEMS for shaping light in the space and time domain.

The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated expe... more The Liquid Crystalline Blue Phases (LC BPs) and their diffraction patterns were investigated experimentally and theoretically. We stabilized Blue Phases and measured their diffraction pattern for different wavelengths of monochromatic light with the help of a conoscopic setup of a polarization microscope. Moreover, the diffraction patterns were calculated with the help of a 4x4 matrix method which allows amplitude and phase investigations.

Journal of The European Optical Society-rapid Publications, 2010

We report on the fabrication and optical characterization of a switchable photonic crystal nanoca... more We report on the fabrication and optical characterization of a switchable photonic crystal nanocavity actuated by liquid crystals. This device acts as a filter presenting a transmission peak around the telecom wavelength λ = 1550 nm. Passing from the isotropic to the anisotropic (oriented crystals) state of the liquid crystals, a shift of ∆λ = 13 nm has been measured, which confirms the theoretical predictions obtained by finite difference time domain simulations. We have developed a photonic crystal nanocavity that can be tuned thanks to the properties of liquid crystals infiltrated in the holes of the photonic device.

Molecular Crystals and Liquid Crystals, 2001

... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 ... more ... 8 I-88, (1999). [lo] JS Patel and K. Rastani, Opt. Left., 16, pp. 532-534, (1991). ... [ 161 M. Kitamura, presented at SID 95 Digest, (1995). [ 171 BEA Saleh and MC Teich, Fimkrrnentals ofPhotonics. New York: John Wiley and Sons, Inc., (1991). ...

Shaping light with microtechnology components has been possible for many years. The Texas Instrum... more Shaping light with microtechnology components has been possible for many years. The Texas Instruments digital micromirror device (DMD) and all types of adaptive optics systems are very sophisticated tools, well established and widely used. Here we present, however, two very dedicated systems, where one is an extremely simple MEMS-based tunable diffuser, while the second device is complex micromirror array with new capabilities for femtosecond laser pulse shaping. Showing the two systems right next to each other demonstrates the vast options and versatility of MOEMS for shaping light in the space and time domain.

Molecular Crystals and Liquid Crystals, 1999

A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens sy... more A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens system was built with a homeotropic aligned liquid crystal of positive dielectric anisotropy and with a circular electrode structure on one side of a sandwiched liquid crystal cell. The fringing effect at the electrode etches produces spatial distribution of the electric field. Applying a voltage deforms the liquid

Optical Engineering, 2000

The behavior of nematic liquid crystals in spatially varying electrical fields is investigated. D... more The behavior of nematic liquid crystals in spatially varying electrical fields is investigated. Director profiles of nematic-liquid-crystal phase gratings are simulated for different orientations of the liquid-crystal director field with respect to the electrode grating. With increasing spatial frequency of the grating the fringing electric field becomes important and the diffraction pattern changes. The optical properties are analyzed and measurements are performed.

A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens sy... more A novel switchable microlens configuration is proposed and demonstrated. An adaptive microlens system was built with a homeotropic aligned liquid crystal of positive dielectric anisotropy and with a circular electrode structure on one side of a sandwiched liquid crystal cell. The fringing effect at the electrode etches produces spatial distribution of the electric field. Applying a voltage deforms the liquid

Japanese Journal of Applied Physics, 2000

Liquid crystal microlenses with diffraction limited performance are presented. The micro-lenses c... more Liquid crystal microlenses with diffraction limited performance are presented. The micro-lenses consist of a liquid crystal film sandwiched between a pair of conductive substrates where one has hole patterned electrode. The lens diameters are 120 mum and 110 mum and diameter to thickness ratio of the lenses is approximately 2.4. The optical quality was investigated with a Mach-Zehnder interferometer. The

Molecular Crystals and Liquid Crystals, 2005

Interaction of liquid crystals with nanostructured surfaces is studied experimentally. Whereas ph... more Interaction of liquid crystals with nanostructured surfaces is studied experimentally. Whereas photolithographic or AFM methods were used in the past for surface structuring, we use -besides photolithographyessentially replication methods for the creation of well-controlled nanotopologies. We established a laboratory process to prepare surface structures that can be as large as 4x4 inch and allow preparing test cells in a production like environment. We combined different methods and tested for soft embossing and direct writing in photoresist. Soft embossing was made with UV curable materials. The direct writing in photoresist was done by means of laser beam interferometry. Structure parameters were changed and the surfaces were characterized by surface anchoring energy measurements and by electro-optical switching behaviour. The obtained alignment is quite uniform over large surface areas despite of a certain variation of the surface parameters. In the case of interferometrically produced grating structures with periods ranging from 350-800 nm strong anchoring was observed and no pretilt could be found.

Optics Express, 2005

A new optical concept for compact digital image acquisition devices with large field of view is d... more A new optical concept for compact digital image acquisition devices with large field of view is developed and proofed experimentally. Archetypes for the imaging system are compound eyes of small insects and the Gabor-Superlens. A paraxial 3x3 matrix formalism is used to describe the telescope arrangement of three microlens arrays with different pitch to find first order parameters of the imaging system. A 2mm thin imaging system with 21x3 channels, 70 masculinex10 masculine field of view and 4.5mm x 0.5mm image size is optimized and analyzed using sequential and non-sequential raytracing and fabricated by microoptics technology. Anamorphic lenses, where the parameters are a function of the considered optical channel, are used to achieve a homogeneous optical performance over the whole field of view. Captured images are presented and compared to simulation results.

Physical Review Letters, 2007

We propose an artificial three-dimensional material that exhibits a strong resonance in the effec... more We propose an artificial three-dimensional material that exhibits a strong resonance in the effective permeability in the visible spectral domain. This material may be implemented in a two-step procedure. First, a metamaterial made of densely packed metallic nanoparticles is fabricated that shows a Lorentz-type resonance in the permittivity at the collective plasmon frequency. Second, spheres are formed out of this material and arranged in a cubic lattice. This meta-metamaterial exhibits a strong resonance in the permeability which is caused by a Mie resonance associated with the magnetic mode of a single metamaterial sphere. Realization of this material based on self-organization in liquid crystals and the limitations of the approach are discussed.

Microelectronic Engineering, 2008

Herein, a study of combining refractive microlenses arrays and nanostructured antireflective surf... more Herein, a study of combining refractive microlenses arrays and nanostructured antireflective surfaces is presented. Fabrication method is surveyed for obtaining the hybrid surface with functionalities of refractive and antireflective (high-transmissive) element. A standard re-flow method is used for fabricating the microlenses and the subwavelength structure is generated on the curved surfaces by interference lithography. By using an optical fiber detector the increased transmittance over the whole visible spectrum is shown. This kind a hybrid element could have applications in illumination systems, in which the main purpose is to gather light to certain areas.

Applied Optics, 2002

We present designs of static Fourier transform spectrometers that are based on a Wollaston prism ... more We present designs of static Fourier transform spectrometers that are based on a Wollaston prism with a large field of view. Besides the usual advantages of static Fourier spectrometers (large resolving power, large wave-number range, high throughput), these designs also present the advantage of using relatively cheap liquid-crystal technology. The use of twisted liquid-crystal structures gives a large field of view, which in turn gives the ability to collect more light from a divergent light source. Measurements are compared with simulations. Different simulation principles are used. We found new configurations by using twisted structures that show a large field of view.

Current Nanoscience, 2006

High Resolution Interference Microscopy (HRIM) is a technique that allows the characterization of... more High Resolution Interference Microscopy (HRIM) is a technique that allows the characterization of amplitude and phase of electromagnetic wave-fields in the far-field with a spatial accuracy that corresponds to a few nanometers in the object plane. Emphasis is put on the precise determination of topological features in the wave-field, called phase singularities or vortices, which are spatial points within the electromagnetic wave at which the amplitude is zero and the phase is hence not determined. An experimental tool working in transmission with a resolution of 20 nm in the object plane is presented and its application to the optical characterization of various single and periodic nanostructures such as trenches, gratings, microlenses and computer generated holograms is discussed. The conditions for the appearance of phase singularities are theoretically and experimentally outlined and it is shown how dislocation pairs can be used to determine unknown parameters from an object. Their corresponding applications to metrology or in optical data storage systems are analyzed. In addition, rigorous diffraction theory is used in all cases to simulate the interaction of light with the nano-optical structures to provide theoretical confirmation of the experimental results.

Optics Express, 2008

A two step process has been developed for the fabrication of diffraction limited concave microlen... more A two step process has been developed for the fabrication of diffraction limited concave microlens arrays. The process is based on the photoresist filling of melted holes obtained by a preliminary photolithography step. The quality of these microlenses has been tested in a Mach-Zehnder interferometer. The method allows the fabrication of concave microlens arrays with diffraction limited optical performance. Concave microlenses with diameters ranging between 30 µm to 230 µm and numerical apertures up to 0.25 have been demonstrated. As an example, we present the realization of diffusers obtained with random sizes and locations of concave shapes.