Resonant Microbubble as a Microfluidic Stage for All-Optical Photoacoustic Sensing

Microbubble Resonators for All-Optical Photoacoustics of Flowing Contrast Agents

“…The coupling of this resonance was optimized using the polarization controller, obtaining the shape shown in Figure 2a. At variance with [19], where the peristaltic pump was off during the experiment, the resonance featured a wobble on a time scale of the order of seconds: we ascribe this wobble to the pulsating liquid flow inside of the MBR and to the tension applied on the capillary stem by the peristaltic pump. The probe laser was then set on the half-maximum wavelength work-point and the wavelength scan stopped, obtaining a baseline trace (black curve in Figure 2c).…”

“…This conversion was based on the WGM shape recorded before enabling the pump laser (Figure 2a) and consisted in approximating the WGM fringe to a parabola around the half-maximum work-point (Figure 2b). This empirical method (green trace, Figure 2b) allows for a more faithful representation of the experimental fringe if compared to an overall data fitting using the theoretical shape for WGM resonances (red trace, Figure 2b), as shown in [19]. To take into account the resonance wobble induced by the flowing colloid, we adjusted the work-point of the parabolic approximation for each PA acquisition.…”

“…A sketch of the experimental setup is shown in Figure 1 [19]. A home-made MBR (diameter 540 µm, volume 82 nl) was connected to a microfluidic circuit where a colloidal suspension of GNR in water was put into motion through a peristaltic pump (Minipuls 3, Gilson, Middleton, WI, USA).…”

“…Finally, the pump laser was enabled and the WGM read-out changed to a shape shown as a blue curve in Figure 2c, featuring a sharp peak followed by fast oscillations. These oscillations are caused by the shift of the WGM resonance frequency induced by the PA wave triggered through the pump laser [19]. This interpretation is corroborated by the clear PA signal shown in Figure S1 of the Supplementary Materials, which was obtained after immersing the MBR in water, filling it with GNR, coupling it with a standard US transducer (Olympus Panametrics, mod V382-SU-F, sensor diameter 0.5 inch, frequency range 3.5 MHz, focal distance 0.83 inch, 40-dB amplifier mod 5676, Tokyo, Japan), and triggering PA emission with the same pump laser [40,42].…”

“…In this context, we recently introduced a Whispering Gallery mode (WGM) microbubble resonator (MBR) as a microfluidic platform for material inspection via their PA response, achieving all-optical PA detection and high sensitivity towards the material spectral absorbance [19]. Microbubble resonators are manufactured in house by heating a pressurized glass capillary [20][21][22] and thus producing a spherical bulge extending from the capillary stem: this bulge is the resonator itself.…”