Jonathan B. Mueller

Three-dimensional multi-photon direct laser writing with variable repetition rate

We perform multi-photon direct laser writing as a function of laser repetition rate over many orders of magnitude and otherwise unchanged experimental conditions. These new data serve as basis for investigating the influence of different proposed mechanisms involved in the photopolymerization: two-photon absorption, photoionization, avalanche ionization and heat accumulation. We find different non-linearities for high and low repetition rates consistent with different initiation processes being involved. The scaling of the resulting linewidths, however, is neither expected nor found to depend on repetition rate or non-linearity.

Polymerization Kinetics in Three‐Dimensional Direct Laser Writing

By in-situ measuring the scattered light during microstructure formation, the polymerization kinetics of three-dimensional direct laser writing are investigated in detail. Oxygen quenching, oxygen diffusion, and inhibitor depletion are shown to have substantial impact on the kinetic behavior. For typical photoresists based on multifunctional acrylates, the polymerization occurs in less than a millisecond.

In-situ local temperature measurement during three-dimensional direct laser writing

We present an approach to measure in situ the local temperature increase in the exposed volume during three-dimensional direct laser writing. The method is based on the detection of luminescence from NaYF4:Yb3+, Er3+ co-doped nanocrystals in a confocal scheme. We found the temperature increase to be below a few K within the normal writing regime. If the photoresist is overexposed, significant temperature changes of several hundred K can be observed.

Photorealistic images of carpet cloaks

Using home-built dedicated ray-tracing software, we simulate photorealistic images of sceneries in three dimensions including dielectric carpet cloaks--i.e., continuously varying refractive-index distributions that allow for invisibility cloaking of a bump in a metallic carpet. Results for the ideal and for a simplified cloak are shown. The presented material gives a visual and intuitive impression of the performance of different arrangements and might be ideally suited for communicating the concepts of transformation optics to the general public.

Three-dimensional micro-printing of temperature sensors based on up-conversion luminescence

The pronounced temperature dependence of up-conversion luminescence from nanoparticles doped with rare-earth elements enables local temperature measurements. By mixing these nanoparticles into a commercially available photoresist containing the low-fluorescence photo-initiator Irgacure 369, and by using three-dimensional direct laser writing, we show that micrometer sized local temperature sensors can be positioned lithographically as desired. Positioning is possible in pre-structured environments, e.g., within buried microfluidic channels or on optical or electronic chips. We use the latter as an example and demonstrate the measurement for both free space and waveguide-coupled excitation and detection. For the free space setting, we achieve a temperature standard deviation of 0.5 K at a time resolution of 1 s.

Molecular Switch for Sub-Diffraction Laser Lithography by Photoenol Intermediate-State Cis–Trans Isomerization

Recent developments in stimulated-emission depletion (STED) microscopy have led to a step change in the achievable resolution and allowed breaking the diffraction limit by large factors. The core principle is based on a reversible molecular switch, allowing for light-triggered activation and deactivation in combination with a laser focus that incorporates a point or line of zero intensity. In the past years, the concept has been transferred from microscopy to maskless laser lithography, namely direct laser writing (DLW), in order to overcome the diffraction limit for optical lithography. Herein, we propose and experimentally introduce a system that realizes such a molecular switch for lithography. Specifically, the population of intermediate-state photoenol isomers of α-methyl benzaldehydes generated by two-photon absorption at 700 nm fundamental wavelength can be reversibly depleted by simultaneous irradiation at 440 nm, suppressing the subsequent Diels-Alder cycloaddition reaction which constitutes the chemical core of the writing process. We demonstrate the potential of the proposed mechanism for STED-inspired DLW by covalently functionalizing the surface of glass substrates via the photoenol-driven STED-inspired process exploiting reversible photoenol activation with a polymerization initiator. Subsequently, macromolecules are grown from the functionalized areas and the spatially coded glass slides are characterized by atomic-force microscopy. Our approach allows lines with a full-width-at-half-maximum of down to 60 nm and line gratings with a lateral resolution of 100 nm to be written, both surpassing the diffraction limit.

3D Micro-printing of Optical Temperature Probes

We present printable optical temperature probes to monitor the temperature with a spatial precision on the micrometer scale. Our approach is based on the temperature-dependent upconversion fluorescence from NaYF4:Yb3+, Er3+ co-doped nanocrystals. These nanoparticles are dispersed in a standard photoresist for direct laser writing, allowing for spatially resolved micro-printing of single or multiple probe spots. For demonstration, we decapsulate a fully operational integrated circuit and print temperature probes directly on the semiconductor chip to monitor its local heating. The printability of the probes facilitates an easy integration into diverse systems, especially when aiming at integrated optics or lab-on-a-chip systems.

Chapter 3 – Reaction Mechanisms and In Situ Process Diagnostics

Abstract In this chapter, we present the state-of-the-art knowledge on the microscopic mechanisms of direct laser writing by multiphoton polymerization as well as the underlying experiments. Due to the small reaction volume and the high exposure intensity, the reaction conditions are very specific in this case compared with other applications in the broader fields of laser materials processing and photopolymerization. We will therefore focus on experiments that show how the common photophysical and photochemical reaction models from these other fields must be adapted. These include the experimental discrimination of the excitation mechanism through variation of the laser repetition rate, the measurement of the local heating during the writing process, the detection of the monomer conversion through Raman spectroscopy techniques, the investigation of the influence of oxygen quenching and oxygen diffusion, and the experimental determination of the duration of the polymerization reaction.

From Curved Space to Optical Cloaking

Transformation optics is a powerful approach to manipulate the propagation of electromagnetic waves [1]. Here, the curvature of space is mimicked by an anisotropic metamaterial, which is described by effective medium theory [2, 3]. An interesting application of such metamaterials is optical cloaking. The metamaterial will prevent that light interact with the cloacked object and in the same time will leave the electromagnetic wave unperturbed. We present the basics of transformation optics and two examples of cloaking devices. This talk will be illustrated with full-wave finite element simulations of feasible cloak designs in homogeneous medium approximation as well as in full geometry.