Uwe D. Zeitner

Ultra-short pulse propagation in complex optical systems.

In application of ultra-short laser pulses the pulse parameters have to be controlled accurately. Hence the manipulation of the propagation behavior of ultra-short pulses requires for specially designed optics. We have developed a tool for the simulation of ultra-short laser pulse propagation through complex real optical systems based on a combination of ray-tracing and wave optical propagation methods. For the practical implementation of the approach two commercially available software packages have been linked together, which are ZEMAX and Virtual Optics Lab. The focussing properties of different lenses will be analyzed and the results are demonstrated.

Advanced mask aligner lithography: Fabrication of periodic patterns using pinhole array mask and Talbot effect

The Talbot effect is utilized for micro-fabrication of periodic microstructures via proximity lithography in a mask aligner. A novel illumination system, referred to as MO Exposure Optics, allows to control the effective source shape and accordingly the angular spectrum of the illumination light. Pinhole array photomasks are employed to generate periodic high-resolution diffraction patterns by means of self-imaging. They create a demagnified image of the effective source geometry in their diffraction pattern which is printed to photoresist. The proposed method comprises high flexibility and sub-micron resolution at large proximity gaps. Various periodic structures have been generated and are presented.

Advanced mask aligner lithography: new illumination system

A new illumination system for mask aligner lithography is presented. The illumination system uses two subsequent microlens-based Köhler integrators. The second Köhler integrator is located in the Fourier plane of the first. The new illumination system uncouples the illumination light from the light source and provides excellent uniformity of the light irradiance and the angular spectrum. Spatial filtering allows to freely shape the angular spectrum to minimize diffraction effects in contact and proximity lithography. Telecentric illumination and ability to precisely control the illumination light allows to introduce resolution enhancement technologies (RET) like customized illumination, optical proximity correction (OPC) and source-mask optimization (SMO) in mask aligner lithography.

152 W average power Tm-doped fiber CPA system.

A high-power thulium (Tm)-doped fiber chirped-pulse amplification system emitting a record compressed average output power of 152 W and 4 MW peak power is demonstrated. This result is enabled by utilizing Tm-doped photonic crystal fibers with mode-field diameters of 35 μm, which mitigate detrimental nonlinearities, exhibit slope efficiencies of more than 50%, and allow for reaching a pump-power-limited average output power of 241 W. The high-compression efficiency has been achieved by using multilayer dielectric gratings with diffraction efficiencies higher than 98%.

POLARIZATION MULTIPLEXING OF DIFFRACTIVE ELEMENTS WITH METAL-STRIPE GRATING PIXELS

Diffractive elements with polarization multiplexing for the visible spectral region are demonstrated. The polarization-multiplexing property of the element is based on the polarization-dependent transmission characteristics of metal-stripe subwavelength period gratings. The proper dimensions of these gratings are estimated by rigorous calculations. The principle of polarization multiplexing by use of metal-stripe subwavelength period gratings is described for a diffractive element that has a binary amplitude transmission per polarization channel and is demonstrated by experimental results.

Materials and technologies for fabrication of three-dimensional microstructures with sub-100 nm feature sizes by two-photon polymerization

The fabrication of sub-100 nm feature sizes in large-scale three-dimensional (3D) geometries by two-photon polymerization requires a precise control of the polymeric reactions as well as of the intensity distribution of the ultrashort laser pulses. The authors, therefore, investigate the complex interplay of photoresist, processing parameters, and focusing optics. New types of inorganic– organic hybrid polymers are synthesized and characterized with respect to achievable structure sizes and their degree of crosslinking. For maintaining diffraction-limited focal conditions within the 3D processing region, a special hybrid optics is developed, where spatial and chromatic aberrations are compensated by a diffractive optical element. Feature sizes below 100 nm are demonstrated.

Development, fabrication, and testing of an anamorphic imaging snap-together freeform telescope

The fabrication chain for the development of an afocal all aluminum telescope using four anamorphic aspherical mirrors is described. The optical and mechanical design are intended to achieve an enhanced system integration with reduced alignment effort by arranging two optical surfaces monolithically on common mirror bodies. Freeform machining is carried out by a hybrid fabrication approach combining diamond turning and diamond milling in the same machine setup. A direct figure correction of diamond turned aluminum mirrors by magnetorheological finishing is presented, resulting in high-precision athermal mirror modules with excellent figure properties. The interferometric system test highlights the diffraction limited telescope performance and the feasibility of the chosen approaches for freeform machining and mechanical integration.

Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks

The application of the phase-shift method allows a significant resolution enhancement for proximity lithography in mask aligners. Typically a resolution of 3 µm (half-pitch) at a proximity distance of 30 µm is achieved utilizing binary photomasks. By using an alternating aperture phase shift photomask (AAPSM), a resolution of 1.5 µm (half-pitch) for non-periodic lines and spaces pattern was demonstrated at 30 µm proximity gap. In a second attempt a diffractive photomask design for an elbow pattern having a half-pitch of 2 µm was developed with an iterative design algorithm. The photomask was fabricated by electron-beam lithography and consists of binary amplitude and phase levels.

Hybrid optics for focusing ultrashort laser pulses

We present a specially designed hybrid optics comprising refraction and diffraction effects for tight spatial and temporal focusing of ultrashort laser pulses. Both aims can be put into practice by having a high numerical aperture (NA=0.45) and low internal dispersion at the same time. We are presenting what we believe is the first experimental realization of such a hybrid short plus focusing optics. The focusing properties are compared with a commonly used microscope objective (20 X, NA = 0.45) in theory and experimentally.

Fabrication of micro-optical surface profiles by using grayscale masks

The fabrication of surface profile may become an interesting technology in the field of micro optics and micromachining. Recently, surface profiles are known and widely used in optics, especially in diffractive optics. In the last few years the demand on deep and arbitrarily shape profiles increased drastically. Laser beam writing and e-beam writing are technologies suitable for the fabrication of such profiles, but only for a limited range of profile depth. Photolithography is also able to realize surface profiles, much deeper profiles can be realized by combining of different technologies. In this paper we report about a strategy for arbitrary deep profile generation as well as results we achieved by using single and combined technologies of special gray scale masks (based on HEBS glass), e-beam lithography and photolithography.