Hao Li, Biqin Dong, Zhen Zhang, Hao F. Zhang, and Cheng Sun

Supplementary Information

A transparent broadband ultrasonic detector based on an optical micro-ring resonator for photoacoustic microscopy

Hao Li, Biqin Dong, Zhen Zhang, Hao F. Zhang, and Cheng Sun

Estimation of pressure sensitivity of the micro-ring resonator ultrasonic detector

The estimation of sensitivity is based on Eq. (1):

, (1)

(dneff/dP) is the pressure-induced effective RI change in the polymer waveguide. Assuming the optical waveguide has a square-shaped cross-section with a side length of 800 nm, its effective RI under various ultrasound pressure was calculated(using Comsol Multiphysics) by consideringboth the deformation of polymer waveguide and elasto-optic effect (Fig. S1). Linear fit shows that (dneff/dP) is -6.8×10-5MPa-1in the waveguide been designed.(dr/dneff) is calculated by Eq. (3). (dr/dneff)=r/neff=780nm/1.5=520 nm. The off-resonance laser output is 0.72 W and the ADP gain is 2.5×105 V/W the corresponding (dT/dr)=(0.72 W)×(2.5×105 V/W)×(11.2 nm-1)=2.1 V/nm where 11.2 nm-1 is the normalized (dT/dr) obtained from Fig. 1f. The sensitivity (dT/dP) is calculated by Eq. (1) which is (-6.8×10-5 MPa-1)×(520 nm)×(2.1 V/nm)=-75 mV/MPa.

Fig. S1. Calculated effective RI of the TM mode in a square-shaped SU-8 polymer (n=1.58) waveguide (800×800 nm) with respect toultrasound pressure. Waveguide is on fused quartz substrate (n=1.46) and immersed in water (n=1.33).

Frequency linearity

Fig. S2compared the MRR frequency response with two piezoelectric transducers with center frequencies of 15 MHz and 75 MHz, respectively. The MRR was placed on top of one of the transducers with water in between to detect the ultrasound waves emitted from the transducer. The echo signals reflected by the MRR cover slip were received by the transducerfor comparison. The ultrasound signals detected by the MRR closely resembles the echo signals, which suggests the linear frequency response of our MRR detector up to 75 MHz.

Fig. S2. Frequency linearity test of the MRR. (a) Time-resolved comparison of MRR and pulse-echo detections at the center frequency of 15 MHz; (b) Time-resolved comparison of MRR and pulse-echo detection at the center frequency of 75 MHz; (c) Frequency response comparison of MRR and pulse-echo detections at the center frequency of 15 MHz; (d) Frequency response comparison of MRR and pulse-echo detections at the center frequency of 75 MHz.

Long-term detection stability

The PA detection stability of the MRRultrasonic detector was examined for 15 min. We maintained the PA laser excitationenergy and focused the light onto a uniform carbon black thin film. After the system reached the thermal equilibrium (10 mins), the laser induced PA waves were recorded for 15 minutes. The measured PA signal amplitudes exhibit modest fluctuation within 1.6 % (RMS) as shown in Fig. S3. No reduction in the sensitivity has been found experimentally, which suggests that the MRR ultrasonic detector is stable for long term PA imaging.

Fig. S3 MRR detected PA signal amplitude variation over 15 minutes.

Carbon black (CB) thin film targets

Fig. S4a illustrates the schematicof the custom targets fabricated on the glass substrate. The CB objects patterned by electron beam lithography were coated with a 1.5-m SU-8 layer. Fig. S4b-d shows the scanning electron microscopy (SEM) images of the target before coated with SU-8 layer. The arrangement of alltargets with various scales is shown in Fig. S4b.Eachtarget consists of three rows of elements alternately arranged by two sets of bars and one solid square. Each set of bars consists of three bars with widths of 8 m and a lengths of 40 m separated by spaces equal to the widths. The target being imaged by PAM is located at the bottom-right corner (highlightedby the dashed-square in Fig. S4b). High magnification images of the target are shown in Fig. S4c and S4d.

Fig. S4. Structure of the fabricated PAM target.(a)Schematic of the cross-section of the patterned CB thin film. (b) SEM image of the target before coated with SU-8 layer;scale bars: 200 µm.(c) Magnified image the area been imaged by PAM;scale bars: 20 µm. (d) Higher magnified image shows the width of each bar is 8 µm;scale bar: 10 µm.