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Subject: Optical- ×

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    Experimental observation of Berry phases in optical Möbius-strip microcavities
    (London [u.a.] : Nature Publ. Group, 2022) Wang, Jiawei; Valligatla, Sreeramulu; Yin, Yin; Schwarz, Lukas; Medina-Sánchez, Mariana; Baunack, Stefan; Lee, Ching Hua; Thomale, Ronny; Li, Shilong; Fomin, Vladimir M.; Ma, Libo; Schmidt, Oliver G.
    The Möbius strip, a fascinating loop structure with one-sided topology, provides a rich playground for manipulating the non-trivial topological behaviour of spinning particles, such as electrons, polaritons and photons, in both real and parameter spaces. For photons resonating in a Möbius-strip cavity, the occurrence of an extra phase—known as the Berry phase—with purely topological origin is expected due to its non-trivial evolution in parameter space. However, despite numerous theoretical investigations, characterizing the optical Berry phase in a Möbius-strip cavity has remained elusive. Here we report the experimental observation of the Berry phase generated in optical Möbius-strip microcavities. In contrast to theoretical predictions in optical, electronic and magnetic Möbius-topology systems where only Berry phase π occurs, we demonstrate that a variable Berry phase smaller than π can be acquired by generating elliptical polarization of resonating light. Möbius-strip microcavities as integrable and Berry-phase-programmable optical systems are of great interest in topological physics and emerging classical or quantum photonic applications.
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    Coupled mechanical oscillator enables precise detection of nanowire flexural vibrations
    (London : Springer Nature, 2023) Sharma, Maneesha; Sathyadharma Prasad, Aniruddha; Freitag, Norbert H.; Büchner, Bernd; Mühl, Thomas
    The field of nanowire (NW) technology represents an exciting and steadily growing research area with applications in ultra-sensitive mass and force sensing. Existing detection methods for NW deflection and oscillation include optical and field emission approaches. However, they are challenging for detecting small diameter NWs because of the heating produced by the laser beam and the impact of the high electric field. Alternatively, the deflection of a NW can be detected indirectly by co-resonantly coupling the NW to a cantilever and measuring it using a scanning probe microscope. Here, we prove experimentally that co-resonantly coupled devices are sensitive to small force derivatives similar to standalone NWs. We detect force derivatives as small as 10−9 N/m with a bandwidth of 1 Hz at room temperature. Furthermore, the measured hybrid vibration modes show clear signatures of avoided crossing. The detection technique presented in this work verifies a major step in boosting NW-based force and mass sensing.
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    Laser cooling in Yb:KY3F10: a comparison with Yb:YLF
    (Washington, DC : Optica, 2022) Püschel, Stefan; Mauerhoff, Felix; Kränkel, Christian; Tanaka, Hiroki
    Laser cooling by anti-Stokes fluorescence is a technology to realize all-solid-state optical cryocoolers. We grew Yb3+-doped KY3F10 (Yb:KYF) crystals as novel laser cooling media and compare their cooling performance to Yb3+-doped LiYF4 (Yb:YLF) crystals also grown in our institute. We present temperature-dependent absorption and emission cross sections as well as the fluorescence lifetime of Yb:KYF, and calculate its material figure-of-merit for laser cooling. Yb:KYF exhibits a higher figure-of-merit than Yb:YLF at temperatures below 200 K. This is because, in contrast to Yb:YLF, the excitation transition from the second-highest Stark level of the ground state is best-suited for cryogenic cooling in Yb:KYF. Thus, it has the potential to achieve unprecedentedly low temperatures below the boiling point of liquid nitrogen. In this work, we observe the first laser cooling of Yb:KYF, and obtain a background absorption coefficient of ∼10−4 cm−1, which is among the lowest ever reported for Yb3+-doped fluoride crystals. A simple model calculation predicts that our Yb:KYF and Yb:YLF crystals can potentially be cooled down to ≈100 K in a high-power cooling setup. Our Yb:KYF crystals still leave room for further improvement through the optimization of the growth process and the use of purer raw materials.