Publikationen

We evaluate prospects, performance and temperature limits of a new approach to macroscopic scale laser refrigeration. The considered<br>refrigeration device is based on exciplex-mediated frequency up-conversion inside hollow-core fibers pressurized with a dopant - buffer<br>gas mixture. Exciplexes are excited molecular states formed by two atoms (dopant and buffer) which do not form a molecule in the<br>ground state but exhibit bound states for electronically excited states. The cooling cycle consists of absorption of laser photons during<br>atomic collisions inducing light assisted exciplex formation followed by blue-shifted spontaneous emission on the atomic line of the bare<br>dopant atoms after molecular separation. This process, closely related to reversing the gain mechanism in excimer lasers, allows for a large<br>fraction of collision energy to be extracted in each cycle. The hollow-core fiber plays a crucial role as it allows for strong light-matter<br>interactions over a long distance, which maximizes the cooling rate per unit volume and the cooling efficiency per injected photon while<br>limiting re-absorption of spontaneously emitted photons channeled into unguided radiation modes. Using quantum optical rate equations<br>and refined dynamical simulations we derive general conditions for efficient cooling of both the gas and subsequently of the surrounding<br>solid state environment. Our analytical approach is applicable to any specific exciplex system considered and reveals the shape of the<br>exciplex potential landscapes as well as the density of the dopant as crucial tuning knobs. The derived scaling laws allow for the identification<br>of optimal exciplex characteristics that help to choose suitable gas mixtures that maximize the refrigeration efficiency for specific<br>applications.<br>

We compare the properties of the broadband supercontinuum (SC) generated in twisted and untwisted solid-core photonic crystal fibers when pumped by circularly polarized<br>40 picosecond laser pulses at 1064 nm. In the helically twisted fiber, fabricated by spinning the preform during the draw, the SC is robustly circularly polarized across its entire<br>spectrum whereas, in the straight fiber, axial fluctuations in linear birefringence and polarization-dependent nonlinear effects cause the polarization state to vary randomly with the wavelength. Theoretical modelling confirms the experimental results. Helically twisted photonic crystal fibers permit the generation of pure circularly polarized SC light with excellent polarization stability against fluctuations in input power and environmental perturbations.

Spontaneous parametric down-conversion (SPDC) has been one of the foremost tools in quantum optics for over five decades. Over that time, it has been used to demonstrate some of the curious features that arise from quantum mechanics. Despite the success of SPDC, its higher-order analogs have never been observed, even though it has been suggested that they generate far more unique and exotic states than SPDC. An example of this is the emergence of non-Gaussian states without the need for postselection. Here we calculate the expected rate of emission for nth-order SPDC with and without external stimulation (seeding). Focusing primarily on third-order parametric down-conversion, we estimate the photon detection rates in a rutile crystal for both the unseeded and seeded regimes.