Hans Zappe

Tunable microfluidic microlenses

A novel type of liquid microlens, bounded by a microfabricated, distensible membrane and activated by a microfluidic liquid-handling system, is presented. By use of an elastomer membrane fabricated by spin coating onto a dry-etched silicon substrate, the liquid-filled cavity acts as a lens whereby applied pressure changes the membrane distension and thus the focal length. Both plano-convex and plano-concave lenses, individual elements as well as arrays, were fabricated and tested. The lens surface roughness was seen to be approximately 9 nm rms, and the focal length could be tuned from 1 to 18 mm. This lens represents a robust, self-contained tunable optical structure suitable for use in, for example, a medical environment.

Effect of lens size on the focusing performance of plasmonic lenses and suggestions for the design

We present a detailed investigation of the effect of lens size on the focusing performance of plasmonic lenses based on metallic nanoslit arrays with variable widths. The performance parameters considered include the focal length, depth of focus (DOF), full-width half-maximum (FWHM) and the maximum intensity of the focal point. 2D FDTD simulation was utilized. The results show that all the lens parameters are greatly affected by the lens size. A larger lens size, with a total phase difference of at least 2π, will produce a better focusing behavior and a closer agreement with the design. The Fresnel number and diffraction theory can be used to explain the effect of lens size. Suggestions are provided for realization of a practical plasmonic lens using the existing nanofabrication techniques.

Design of spherically corrected, achromatic variable-focus liquid lenses

A design method for correcting chromatic as well as spherical aberrations of variable-focus, multi-chamber liquid lenses is described. By combining suitable optical liquids with appropriate radii of the liquids interfaces, liquid lenses with superior, diffraction-limited resolution over a wide focal tuning range are possible. For an infinite object distance, the analytic thin-lens approximation of an achromatic positive/negative varifocal liquid lens is derived and the obtained results are compared with ray-traced optimized designs which consider finite thicknesses and rigid cover glasses. As a design example, the optical performance of a 4mm-diameter positive/negative f /3.6 achromatic liquid lens is given in detail.

Optical MEMS: From Micromirrors to Complex Systems

Microelectromechanical system (MEMS) technology, and surface micromachining in particular, have led to the development of miniaturized optical devices with a substantial impact in a large number of application areas. The reason is the unique MEMS characteristics that are advantageous in fabrication, systems integration, and operation of micro-optical systems. The precision mechanics of MEMS, microfabrication techniques, and optical functionality all make possible a wide variety of movable and tunable mirrors, lenses, filters, and other optical structures. In these systems, electrostatic, magnetic, thermal, and pneumatic actuators provide mechanical precision and control. The large number of electromagnetic modes that can be accommodated by beam-steering micromirrors and diffractive optical MEMS, combined with the precision of these types of elements, is utilized in fiber-optical switches and filters, including dispersion compensators. The potential to integrate optics with electronics and mechanics is a great advantage in biomedical instrumentation, where the integration of miniaturized optical detection systems with microfluidics enables smaller, faster, more-functional, and cheaper systems. The precise dimensions and alignment of MEMS devices, combined with the mechanical stability that comes with miniaturization, make optical MEMS sensors well suited to a variety of challenging measurements. Micro-optical systems also benefit from the addition of nanostructures to the MEMS toolbox. Photonic crystals and microcavities, which represent the ultimate in miniaturized optical components, enable further scaling of optical MEMS.

Micro-machined tunable optical filters with optimized band-pass spectrum

A novel MEMS-based tunable optical filter structure is presented which for the first time combines the advantages of an optimized filter shape function with tunability. Such a filter is essential for monitoring and reconfiguration of optical communication networks. The device is based on a Fabry-Perot interferometer employing multiple solid-state silicon cavities and dielectric Bragg mirrors. It is fabricated as a self-supporting membrane with thin film metal resistors using silicon MEMS technology.

Iris‐Like Tunable Aperture Employing Liquid‐Crystal Elastomers

A liquid-crystal elastomer (LCE) iris inspired by the human eye is demonstrated. With integrated polyimide-based platinum heaters, the LCE material is thermally actuated. The radial contraction direction, similar to a mammalian iris, is imprinted to the LCE by a custom-designed magnetic field. Actuation of the device is reproducible over multiple cycles and controllable at intermediate contraction states.

An all-dielectric tunable optical filter based on the thermo-optic effect

A MEMS based, thermally tunable optical filter, hybridly assembled with a fibre based input/output, is presented. The filter is based on a Fabry?Perot interferometer employing a silicon cavity and silicon based dielectric Bragg reflectors and is fabricated as a free-standing membrane. The filter membrane is fixed to the substrate through micromachined suspension arms, which act as a thermal isolation. Wavelength tuning is achieved through thermal modulation of the cavitys optical thickness using thin film resistors. The filter characteristics measured were a full width at half-maximum value (FWHM) of 1.19?nm at a wavelength of 1530?nm and a finesse exceeding?1000. The tuning efficiency of a single-cavity filter with Bragg mirrors based on silicon nitride and silicon dioxide was measured to be 51.9?pm?K?1. Measurements of static and transient electrothermal behaviour were performed on filter membranes, resulting in maximum temperatures of 700??C and thermal time constants of 5.14?ms. The assembly technique relied on the alignment of optical fibres with the filter array using a novel silicon optical bench approach.

Fabrication and testing of micro-lens arrays by all-liquid techniques

Various all-liquid fabrication strategies to construct polymer micro-lens arrays on glass substrates are presented. Wettability patterns were made on glass substrates and a monomer was deposited selectively on circular domains on the substrate by making use of the hydrophobic effect, resulting in liquid micro-lenses. The liquid micro-lenses were solidified by polymerization. Three different variations of the selective wetting process were developed. The polymer micro-lenses were characterized extensively by optical methods: focal length, surface roughness and optical aberrations of the lenses were determined by scanning white light interferometry. Due to their fabrication from the liquid phase, the root-mean-square surface roughness of the micro-lenses is between 10 and 35 nm, i.e. better than 1/10 wavelength. These measurements allowed comparison of the quality of lenses fabricated by these techniques.

Narrow-linewidth vertical-cavity surface-emitting lasers for oxygen detection.

The use of vertical-cavity surface-emitting lasers (VCSELs) for optical detection of atmospheric oxygen is described. The VCSELs were custom designed for single-mode emission in the 763-nm wavelength range, with low noise and narrow optical linewidth. Using standard wavelength modulation spectroscopy and a second-harmonic detection scheme with a 1-m air path, we determined an oxygen concentration resolution of 0.2%. Because of its small size, low power dissipation, and good tunability characteristics, the VCSEL promises to be an attractive light source for use in compact, low-cost optical sensor microsystems for trace gas detection.

Tunable Pneumatic Microoptics

A new class of tunable and actuated microoptical devices is presented: pneumatic microoptics. Using microelectromechanical system fabrication technology extended by the use of polydimethylsiloxane (PDMS) membranes, tunable microlenses, and lens arrays, actuated micromirrors with large tilt angles and tip-tilt piston mirrors have been designed and fabricated. Actuation is by pressure: Gas- or liquid-filled microfluidic cavities are employed to distend the microfabricated PDMS structures which then act as a lens surface or as an actuator for a micromirror. Thermopneumatic actuation is also employed for completely integrated tunable optical systems in which all actuator and optical components are fabricated on-chip. The technology is particularly promising for microsystem applications in which significant movement is required but high voltages or external fields are impractical. [2007-0301].