Four-Port Resonant Tunnelling Bottle Microresonator Device

Summary form only given. The phenomenon of resonant tunnelling has attracted significant interest due to its intriguing underlying physics and multidisciplinary applications in the theory of nuclear reactions [1], electron propagation through semiconductor devices, quantum wire and quantum dot structures [2, 3], as well as propagation of light through optical microresonators [4-6]. The applications of resonant tunnelling phenomenon in microresonator photonics is of special interest. The simplest photonic device exhibiting resonant tunnelling behaviour consists of an optical microresonator and two semi-infinite waveguides separated from the microresonator by effective potential barriers, which can be fabricated inside the photonic crystals. In order to observe a similar type of resonant tunnelling in a whispering gallery mode microresonator, the latter should be evanescently coupled to two microfibers, i.e., present a four-port device [5, 6]. The four-port resonant tunnelling bottle microresonator device demonstrated in this report is illustrated in Fig. 1(a). It consists of a bottle microresonator (BMR) coupled to the waists (microfibers) of two biconical tapers oriented transversely to the BMR axis. The bottle microresonator was fabricated by nanoscale effective radius variation (ERV) of a 125 μm diameter optical fibre by local annealing with a CO
₂
laser [7]. The tapers were fabricated in a microheater. Coupling between the microfibers and the BMR was tuned by translating the microfibers along their axis and along the axis of the BMR as well. In contrast to the four-port BMR device demonstrated previously [7], the spectrum of our BMR was not overcrowded and well localized because of its nanoscale ERV. First, we characterized the BMR by scanning it with a single microfiber 1 (MF1) and measured the resonant transmission power (λ, z
₁
) as a function of wavelength A and microfiber coordinate along the BMR z 1 . The spectrogram (λ, z
₁
) is shown in Fig. 1(b). Next, the MF1 was attached to the BMR at the fixed position (dashed red line in Fig 1(c)), while microfiber 2 (MF2) was translated along the BMR and the resonant transmission power P12 (λ, z
₂
) from MF1 to MF2 was measured. The spectrogram P12 (λ, z
₂
) is shown in Fig. 1(c). The experimental results are in a good agreement with our theoretical modelling shown in Fig. 1(d). Small magnitude of the observed transmission power is caused by the fact that microfibers and BMR were strongly undercoupled. In summary, we have proposed and demonstrated a four-port resonant tunnelling device based on BMR with nanoscale ERV. The measured spectrogram of our device is in a good agreement with theoretical modelling. The spectrograms obtained allowed us to predictably tune the coupling between the input-output microfibers and BMR by translating the microfibers along their axis and along the BMR axis. Following this proof-of-concept demonstration, we will optimize our device to arrive at the resonant transmission power Pi2 (λ, z
₂
) close to unity.