Frank Burmeister

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

The fabrication of sub-100u2009nm 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 100u2009nm are demonstrated.

Formation of periodic disruptions induced by heat accumulation of femtosecond laser pulses

The absorption and heat accumulation of successive ultrashort laser pulses in fused silica leads to melting of the material. We analyze the structure and formation of disruptions that occur within the trace of the molten material. We employed focused ion beam (FIB) milling to reveal the inner structure of these disruptions. The disruptions consist of several small voids which form a large cavity with a diameter of several tens of micrometer. Based on the observations, we suggest a model explaining the formation of these disruptions as a results of a fast quenching process of the molten material after the laser irradiation has stopped. In addition, we analyzed the periodic and non-periodic formation of disruptions. The processing parameters strongly influence the formation of disruptions.

High numerical aperture hybrid optics for two-photon polymerization

We report on an immersion hybrid optics specially designed for focusing ultrashort laser pulses into a polymer for direct laser writing via two-photon polymerization. The hybrid optics allows for well-corrected focusing over a large working distance range of 577 μm with a numerical aperture (NA) of 1.33 and low internal dispersion. We combine the concepts of an aplanatic solid immersion lens (ASIL) for achieving a high NA with a diffractive optical element (DOE) for correction of aberrations. To demonstrate the improvements for volume structuring of the polymer, we compare the achievable structure sizes of our optics with a commercially available oil-immersion objective (100x, NA=1.4).

Two-photon polymerization of hybrid polymers for applications in micro-optics

Miniaturization and higher integration of opto-electronic components require highly sophisticated optical designs. This creates the demand for freeform technologies like Two-Photon Polymerization (2PP) and new specially adapted materials like hybrid polymers (ORMOCERRs). Recent progress in the fabrication of microoptical structures using 2PP and specially designed hybrid polymers is presented. Among the structures are freeform and aberration-optimized microlenses and multilevel diffractive optical elements. These components are discussed with respect to fabrication process and their resulting optical performance. Furthermore, 2PP-initiated refractive index modification, offering high potential for energy-efficient fabrication of 3D optical interconnects, is discussed.

Breakthroughs in Photonics 2012: Femtosecond-Laser Nanomachining

The latest breakthroughs in the field of nanoscale structuring using ultrashort laser pulses are presented, all exploiting highly nonlinear effects. Significant advances have been made in improving the resolution in two-photon-polymerization, as well as for applying this technique to create functional nanostructures. In glassy materials, the inscription of self-organized periodic nanostructures resulting in artificial birefringence, as well as periodic subwavelength structures for fiber and volume Bragg gratings, has emerged. In addition, latest results in the fascinating field of biophotonics are presented, where tremendous progress has been made allowing the manipulation of living cells.

Formation of disruptions in molten fused silica induced by heat accumulation of ultrashort laser pulses at high repetition rates

The absorption of ultrashort laser pulses at high repetition rates leads to heat accumulation within the irradiated material. Thereby, the focal region as well as a well-defined vicinity of the surrounding material is molten. This method can be used for local bonding of transparent materials. Figure 1 a) shows a top view of laser molten material within fused silica. Within the traces of the molten material, large disruptions can be found as darks spots. The shape of the molten material narrows after the formation of a disruption, which indicates an interruption of the laser heating. Thus, these disruptions induce tensions and reduce the strength of the bonded material. The appearance and periodicity of these micro-bubbles depend on the processing parameters. In addition, disruptions are always generated at the end of a molten line when the laser irradiation stops. So far, the origin and inner structure of these defects were unknown. We investigated the structure and periodicity of these disruptions in fused silica. In addition, we propose a model, which explains their formation.

Hybrid optics for three-dimensional microstructuring of polymers via direct laser writing

We present an immersion hybrid optics specially designed for focusing ultrashort laser pulses into a polymer for direct laser writing via two-photon polymerization. The hybrid optics enables well corrected focusing over a working distance range of 577 μm with a numerical aperture (NA) of 1.33 thereby causing low internal dispersion. We combine the concepts of an aplanatic solid immersion lens (ASIL) for achieving a high NA with the correction of aberrations with a diffractive optical element (DOE). To demonstrate the improvements for volume structuring of the polymer, we compare achievable feature sizes of structures written with our optics and a commercial available oil immersion objective (100x, NA=1.4).