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Tunable optofluidic liquid metal core microbubble resonator

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Abstract

This study introduces design and coupling techniques, which bridge an opaque liquid metal, optical WGM mode, and mechanical mode into an opto-mechano-fluidic microbubble resonator (MBR) consisting of a dielectric silica shell and liquid metal core. Benefiting from the conductivity of the liquid metal, Ohmic heating was carried out for the MBR by applying current to the liquid metal to change the temperature of the MBR by more than 300 °C. The optical mode was thermally tuned (>3 nm) over a full free spectral range because the Ohmic heating changed the refractive index of the silica and dimeter of the MBR. The mechanical mode was thermally tuned with a relative tuning range of 9% because the Ohmic heating changed the velocity and density of the liquid metal.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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Supplementary Material (1)

NameDescription
Visualization 1       Light emission phenomenon when high current is applied on mercury.

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Figures (7)

Fig. 1.
Fig. 1. (a) Schematic of experimental setup. The bubble stems are connected with Au microwire electrodes to control the current of the liquid metal core. VOA: variable optical attenuator; FPC: fiber polarization controller; PD: photoelectric detector; OSC: oscilloscope; RTSA: real-time spectrum analyzer; DAQ: data acquisition. The current of the liquid metal core is controlled by a current source. The liquid is pumped into the microchannel of the MBR using a syringe pump. (b) Schematic of liquid metal MBR. (c) Optical photograph of liquid metal MBR coupled with tapered fiber.
Fig. 2.
Fig. 2. (a) Typical transmission spectrum of empty MBR. (b) Transmission spectrum of liquid metal MBR.
Fig. 3.
Fig. 3. Broadband tuning resonant wavelength by Ohmic heating liquid metal core. (a) Resonant mode spectrum changes as current of mercury increased by step of 50 mA. Inset shows sample emitting blue-green light when the current was 420 mA. (b) Resonant wavelength shift (inset shows Q change) as function of current.
Fig. 4.
Fig. 4. Simulated result for (a) 3-D and (b) 2-D shell displacement field distribution of fundamental mechanical mode of optofluidic liquid metal core MBR; (c) 2-D fluid pressure field distribution of fundamental mechanical mode of optofluidic liquid metal core MBR. Black solid lines indicate boundary of MBR shell. Mechanical frequency versus (d) sound speed (mercury density was fixed at 13539.2 kg/m3) and (e) density of mercury (sound speed was fixed at 1458.9 m/s) were calculated.
Fig. 5.
Fig. 5. Mechanical power spectrum when optofluidic core is filled with (a) air and (b) mercury. (c) Mechanical power spectrum changes caused by increasing the current of mercury with a step of 40 mA. (d) Experimentally measured data (※) versus different currents; red line indicates quadratic fitting of measured data, and blue dotted line indicates calculated result of relative mechanical frequency shift as function of current applied to liquid metal core.
Fig. 6.
Fig. 6. Long time data acquisition of mechanical oscillation spectrogram (a) when current was not applied to liquid-meal core and (b) when current I = 120 mA was applied to liquid metal core.
Fig. 7.
Fig. 7. (a) Mechanical power spectrum with different input pump laser power. (b) 3-dB bandwidth δfm as function of input pump laser power.

Equations (3)

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δ λ  =  λ 0 1 n 0 d n d T δ T ,
c  =  1440 0.7 ( T 50 ) ;
ρ  =  13595.05 2.43 T .
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