2023, Jankowski, Patricia, Höfner, Anja, Hoffmann, Marja Lena, Rohde, Friederike, Rehak, Rainer, Graf, Johanna

The second ‹Bits & Bäume› conference took place in Berlin in 2022. Once again, it provided a space for critical tech and sustainability communities to share ideas and collaborate towards the common goal of shaping digitalisation to foster sustainability. This companion book compiles the insights, work, research and opinions of more than 65 authors with a ‹Bits & Bäume› background, including practitioners, researchers and activists. The articles included in this journal demonstrate the progress made in merging ‹Bits› and ‹Bäume› (Trees) topics since our first publication in 2019 by addressing different sub-areas of the intersections between digitalisation and sustainability. Encompassing a wide range of topics, the articles delve into pressing challenges such as the resource consumption, power implications and democratic governance of digital infrastructures, AI, blockchains, mobile apps, and other software applications, as well as the need to address the unsustainable practices and paradigms of e.g., the platform economy. Offering not only transparency but also solutions, the journal presents practical approaches and concepts related to the necessary transformation, such as the Computer Science for Future programme. It also contains articles commenting on current political developments, such as the EU legislation on sustainability and freedom-related aspects of ICT devices. Further articles highlight the power of and need for an active civil society, aiming to inspire activism. This journal caters for everyone: Are you just getting into the topics around Bits & Bäume? Have you been involved in this field for many years, or are you an expert in one of the areas touched on here? In this journal you will find both introductory topics, such as illustrations on the challenges of today's digitalised society, and also advanced topics, such as conceptual and regulatory discussions. Whatever your background, we think you’ll enjoy the read, learn something new on the way, and get inspired. Ultimately, we are all united by the overarching goal of shaping digitalisation as part of a necessary socio-ecological change; one which contributes to a sustainable and just society.

2018, Röben, Benjamin Malte, Grahn, Holger T.

Terahertz (THz) quantum-cascade lasers (QCLs) are unipolar semiconductor heterostructure lasers that emit in the far-infrared spectral range. They are very attractive radiation sources for spectroscopy, since they are very compact and exhibit typical output powers of severalmWas well as linewidths in the MHz to kHz range. This thesis presents the development of methods to tailor the emission characteristics of THz QCLs and employ them for spectroscopy with highest resolution and sensitivity. In many cases, these spectroscopic applications require that the far-field distribution of the THz QCLs exhibits only a single lobe. However, multiple lobes in the far-field distribution of THz QCLs were experimentally observed, which were unambiguously attributed to the typically employed mounting geometry and to the cryogenic operation environment such as the optical window. Based on these results, a method to obtain a single-lobed far-field distribution is demonstrated. A critical requirement to employ a THz QCL for high-resolution spectroscopy of a single absorption or emission line is the precise control of its emission frequency. This long-standing problem is solved by a newly developed technique relying on the mechanical polishing of the front facet. A QCL fabricated in this manner allows for spectroscopy at a maximal resolution in the MHz to kHz range, but its accessible bandwidth is usually limited to a few GHz. In contrast, a newly developed method to utilize QCLs as sources for THz Fourier transform spectrometers enables highly sensitive spectroscopy over a significantly larger bandwidth of at least 72 GHz with a maximal resolution of typically 100 MHz. The application of QCLs as sources for THz Fourier transform spectroscopy leads to a signal-to-noise ratio and dynamic range that is substantially increased by a factor of 10 to 100 as compared to conventional sources. The results presented in this thesis pave the way to routinely employ THz QCLs for spectroscopic applications in the near future.