Toralf Scharf

An Adaptive Microlens formed by Homeotropic Aligned Liquid Crystal with Positive Dielectric Anisotropy

Abstract 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 crystal director field. This produces an axially symmetric profile of the extraordinary refractive index. This director configuration is expected to have lens properties. Under the influence of an electric field, the liquid crystal cell becomes a concave (diverging) lens. The director structure was investigated by polarising microscope. We have simulated the director profile with a finite element method and compared the calculated director profile with the measurements. The properties are discussed from the viewpoint of the director orientation in the spatial non-uniform electric field.

Multilevel nematic liquid crystal phase gratings

Planar-aligned nematic liquid crystals cells with an array of parallel electrodes can perform phase modulation with profiles going from binary to blazed gratings. A nematic liquid crystal with high birefringence (BL006) allows reducing the thickness of liquid crystal cells and thus reducing switching times. But the discrete electrodes and the small thickness have negative consequences on the shape of the phase profile. Liquid crystal phase gratings with 192 separately controlled electrodes were fabricated. The electrode distances and widths were 3 or 4 micrometers at a cell thickness of 6 micrometers . Perpendicular and parallel alignment of the liquid crystal with respect to the electrode grating were investigated. Far field diffraction measurements and phase measurements with a Mach-Zehnder interferometer were performed to characterize the gratings. The director distribution was modeled in 2 dimension and the resulting phase profiles were calculated with a Jones matrix method. It allows a comparison with the measurements. It was found that in the liquid crystal grating cell the in- plane electric field has a large influence on the optical properties. The in-plane electric field between the high- and low-voltage electrodes forms an unfavorable director deformation that limits the diffraction efficiency to approximately 65% for linear polarized light. Both, the measured and the simulated phase profiles, show a typical structure where valleys appear and worsen the optical performances. The optimization of such a grating is a compromise of large thickness, that smooths the valley structures, and small thickness, that reduces the in-plane switching effect and the switching time.

Emerging light fields from liquid crystal microlenses

Liquid crystal (LC) microlenses have a variable focal length with a change in applied voltage. These lenses have suitable dimensions and focal length variations for a wide range of applications where the absence of moving parts is favored. In this article we present the three-dimensional imaging of the emerging light fields from these lenses for the first time. This novel technique is evaluated as a measure of the optical performance of the microlenses. It is compared with the results obtained on an interferometer custom built for the investigation of the optical properties of micro-optics. These combined results make this the most detailed investigation of the optical properties of liquid crystal microlenses to date. The results are centered on a microlens with a diameter of 60 μm at the lower (0.09) and higher (0.11) numerical apertures achievable with these lenses. In previous studies on LC microlens cells the main area of investigation was the focal length ranges available for particular applications. Images have been taken with these lenses and interference patterns between the extraordinary and ordinary rays propagating through the cells have been presented to show the lensing properties of the cells. However, a detailed examination of their optical properties show that although this type of lenses can produce images at certain voltages they act as diffraction limited lenses only over a short voltage range. It is also shown that the optical properties can be improved by variation of the driving frequency of the applied voltage.

On the coupling and transmission of transverse and longitudinal fields into fully metal-coated optical nano-probes

Fully-metal-coated near-field optical probes, based on a cantilever design, have been studied theoretically and experimentally. Numerical simulations prove that these structures allow non-zero modal emission of the electromagnetic field trough a 60 nm thick metallic layer, that is opaque when deposited on flat substrates. The far-field intensity patterns recorded experimentally correspond to the ones calculated for the fundamental and first excited LP modes. Moreover, this study demonstrates that a high confinement of the electromagnetic energy can be reached in the near-field, when illuminated with radially polarized light. Finally, it was verified that the confinement of the field depends on the volume of the probe apex. The coupling and transmission of transverse and longitudinal fields into the probes has been also investigated. Two kinds of probes with different metal coating roughness are considered. Transverse and longitudinal field distributions are obtained by focusing azimuthally and radially polarized beams produced by means of a liquid crystal plate. The focal plane is scanned using microfabricated probes in a collection mode configuration. It is found that the roughness of the metal coating plays an important role in the coupling strength of transverse fields into the probes: the relative coupling efficiency for transverse fields diminishes with a rough metal coating, while that of longitudinal fields does not.

Blue phases as photonic crystals

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.

Reflection of multidomain structured cholesteric liquid crystals

The simulation of polymer-dispersed cholesteric liquid crystals is carried out using a model, which takes into account the domain structure of the layer. The model is based on the 4x4 matrix calculation method of Berreman. One considers different orientations of the helical axis in multi- and single-domain configurations. The distribution of the helical axis orientations in the multi-domain model allows the description of optical properties of multi-stable director configurations.

Semiconductor-based narrow-line and high-brilliance 193-nm laser system for industrial applications

We present a novel industrial-grade prototype version of a continuous-wave 193 nm laser system entirely based on solid state pump laser technology. Deep-ultraviolet emission is realized by frequency-quadrupling an amplified diode laser and up to 20 mW of optical power were generated using the nonlinear crystal KBBF. We demonstrate the lifetime of the laser system for different output power levels and environmental conditions. The high stability of our setup was proven in > 500 h measurements on a single spot, a crystal shifter multiplies the lifetime to match industrial requirements. This laser improves the relative intensity noise, brilliance, wall-plug efficiency and maintenance cost significantly. We discuss first lithographic experiments making use of this improvement in photon efficiency.

Improved calibration of vertical scanning optical profilometers for spherical profiles measurements

A new method for calibrating optical scanning profilometers is presented. Especially adapted to spherical and aspherical profile measurements, it shows an increase of accuracy bigger than one order of magnitude for radius of curvature measurements. Calibration of vertical scaling is obtained with a reduction of its uncertainty by a factor larger than 2, which also demonstrates the advantage of this method for any surface measurements. Using commercially available reference balls, this method is easily implementable.

High power modular LED-based illumination system for lithography applications

Mask-aligner lithography is a technology used to transfer patterns with critical dimensions in the micrometer range from below 1 micron for contact printing to a dozen of microns in proximity printing. This technology is widely used in the fabrication of MEMS, micro-optical components, and similar fields. Traditionally, the light sources used for mask-aligners are high-pressure mercury arc lamps, which emit in the UV rang of the spectrum with peaks at 365 nm, 405 nm and 435 nm, respectively the g-, h- and i- lines. These lamps suffer from several disadvantages (inefficient, bulky, dangerous), which makes alternatives interesting. In recent years, high power UV LEDs at the same wavelengths appeared on the market, opening the door to new illumination systems for mask-aligners. We have developed a modular 250 W LED-based illumination system, which can advantageously replace a 1 kW mercury arc lamp illumination. LEDs, arranged in a 7×7 grid array, are placed in the entrance apertures of individual reflectors, which collimate the individual irradiation to an output angle of 10°. A subsequent fly’s eye integrator homogenizes the illumination in the mask plane. It is followed by a Fourier lens, superimposing the individual channels in the mask plane, and a field lens to ensure telecentric illumination. This multisource approach allows the shaping of the source by switching individual illumination channels, determining the illumination angles and the spatial coherence in the mask plane. This concept can be used, for example, to do source-mask optimization. Compared to mercury arc lamp illumination, our system is simultaneously more efficient, compact, versatile, economic and sustainable. In our contribution, we present the design of the system as well as lithographic test prints done with different illumination patterns.

Corrugated grating on organic multilayer Bragg reflector

Polymeric multilayer Bragg structures are combined with diffractive gratings to produce artificial visual color effects. A particular effect is expected due to the angular reflection dependence of the multilayer Bragg structure and the dispersion caused by the grating. The combined effects can also be used to design particular filter functions and various resonant structures. The multilayer Bragg structure is fabricated by spin-coating of two different low-cost polymer materials in solution on a cleaned glass substrate. These polymers have a refractive index difference of about 0.15 and permit multilayer coatings without interlayer problems. Master gratings of different periods are realized by laser beam interference and replicated gratings are superimposed on the multilayer structure by soft embossing in a UV curing glue. The fabrication process requires only polymer materials. The obtained devices are stable and robust. Angular dependent reflection spectrums for the visible are measured. These results show that it is possible to obtain unexpected reflection effects. A rich variety of color spectra can be generated, which is not possible with a single grating. This can be explained by the coupling of transmission of grating orders and the Bragg reflection band. A simple model permits to explain some of the spectral vs angular dependence of reflected light.