Current research / employment

Harmonic oscillations environ us in a variety of forms, as water waves, sound waves, mechanical vibrations of strings in musical instruments and the motion of pendula like swings and metronomes. Confining these oscillations in an appropriate way, one is able to drastically enhance them. In practice, the stability of resonators is an important requirement. In optics, this reliability is strongly increased using robust monolithic cavities, such as etalons. However, discrete reflections usually limit the quality factors and prohibit the facile and continuous change of coupling strength. These limitations can be lifted in the whispering gallery mode (WGM) configuration (see Fig.), where light is guided via continuous total internal reflection (TIR).

Furthermore, frustrated TIR offers a way to continuously change the optical coupling strength, e.g. using tapered fibres or prisms. Extremely flat surfaces are found in liquid droplets and can be produced by melting or polishing solid material. Thus, surface scattering can be virtually eliminated and the only relevant loss mechanism is material absorption. Ideally, the cavity would confine the light indefinitely. Hence, the resulting high quality WGM resonators are extremely sensitive to perturbations in their environment, allowing e.g. for sensing single atoms in their vicinity. Along this line, the optical modes strongly couple to crystal lattice defects, e.g. by doping. Apart from that, they also sense the slightest polarisation responses of the monolith, they are made of.

Indeed, the second order nonlinear response in bulk material already shows a plethora of dynamical effects, examples being bistability, chaotic behaviour and self pulsing, along with non classical effects such as squeezing and entanglement. It is thus intriguing to investigate these effects in a highly sensitive WGM resonator. We study the nonlinear optical processes in WGM resonators to explore their quantum-optical aspects. Processes as e.g. second harmonic generation, parametric down-conversion, and others, become efficient for very low optical powers and ultimately for individual photons. In this regime, the non-classical nature of the generated or converted light fields is most strongly revealed.

Figure: Top view on a whispering gallery mode resonator and the coupling prism.

Publications M. Förtsch, J. U. Fürst, C. Wittmann, D. V. Strekalov, A. Aiello, M. V. Chekhova, C. Silberhorn, G. Leuchs, C. Marquardt, A versatile source of single photons for quantum information processing, , 1204.3056v2, 1-7, (2012)

G. Lin, J. U. Fürst, D. V. Strekalov, I. S. Grudinin, N. Yu, High-Q UV whispering gallery mode resonators made of angle-cut BBO crystals, Optics Express, 20, 21372-21378, (2012)

J. U. Fürst, D. V. Strekalov, D. Elser, A. Aiello, U. L. Andersen, Ch. Marquardt, G. Leuchs, Quantum light from a whispering-gallery-mode disk resonator, Physical Review Letters, 106, 1-4, (2011)

J. U. Fürst, D. V. Strekalov, D. Elser, M. Lassen, U. L. Andersen, C. Marquardt, G. Leuchs, Naturally-phasematched second harmonic generation in a whispering gallery mode, Physical Review Letters, 104, 153901, (2010)

J. U. Fürst, D. V. Strekalov, D. Elser, A. Aiello, U. L. Andersen, C. Marquardt, G. Leuchs, Low-threshold optical parametric oscillations in a whispering gallery mode resonator, Physical Review Letters, 105, 1-4, (2010)

C. Wittmann, J. U. Fürst, C. Wiechers, D. Elser, H. Häseler, N. Lütkenhaus, G. Leuchs, Witnessing effective entanglement over a 2km fiber channel, Optics Express, 18, 4499, (2010)