Structure of the sensor array. The main parts of the sensor array are the micro-ring sensors and the bus waveguide. The scanning electron micrographs of a micro-ring sensor and a bus waveguide are shown in Figs. 2(a) and 2(b), respectively. Each micro-ring sensor has a diameter of 40 µm, a cross-sectional height of 850 nm, and a width of 2.4 µm. They are also separated with a center-to-center distance of 400 µm and each of them occupies an effective area of about 0.85 × 40 × 40 µm3. Note that the resonant frequency of each micro-ring sensor was tuned to be slightly different through photosensitive effect, which is detailed in Supplementary Note 3. The width and the height of the bus waveguide are 650 nm and 850 nm, respectively. Both the micro-ring sensor and the bus waveguide were fabricated using chalcogenide material with a composition of Ge25Sb10S65, exhibiting a refractive index of 2.33 around 1,550 nm. This material has a good photoelastic property with Young’s modulus of 31.9 GPa, leading to a good sensitivity to ultrasound. These nano-fabricated structures sit on top of a silica (SiO2) substrate and are covered with a 3-µm-thick polydimethylsiloxane (PDMS) cladding. This cladding is important in protecting micro-rings in an aqueous environment. No other micro/nanostructures are required to enhance the performance of the sensor array. By using the finite element method (COMSOL Multiphysics 5.6), Figs. 2(c) and (d) show the numerically simulated intensity distribution of the propagating modes inside the micro-ring sensor and the waveguide, respectively, exhibiting good mode confinement. For practical considerations, we fabricated many micro-ring sensor arrays with different parameters on one chip, as shown in the microscope image in Fig. 2(e). After examining the performance, the part that only contains one sensor array was cleaved and encapsulated, occupying a small footprint of about 6 mm × 2 mm. As a comparison, a nickel coin with a 2-cm diameter was placed underneath, as shown in Fig. 2(f). In particular, the end of the bus waveguide, which is enclosed in a red box in Fig. 2(f), was attached to a single-mode optical fiber, with an enlarged view shown in Fig. 2(g).
The fabrication process of the sensor array. We firstly prepared an 850-nm-thick Ge25Sb10S65 film and deposited it on a silicon wafer with a 3-µm-thick oxidized layer using the thermal evaporation technique. To prevent oxidation of the chalcogenide film during subsequent processes, 2-nm-thick alumina oxide was deposited through atomic layer deposition (Kemicro, ALD-100A). After spin-coating a layer of positive photoresist (Allresist, ARP6200) onto the chalcogenide film, electron beam lithography (Raith, EBPG5000+) was employed to write the pattern, i.e., the micro-rings and coupling waveguides. The pattern was then transferred onto the chalcogenide film by using reactive ion etching (Oxford Instrument, PlasmaPro 100RIE). The residual photoresist was removed through appropriate treatment with oxygen plasma, forming the basic structure of the on-chip micro-ring sensor array. In order to improve the durability of the chip, we spin-coated 4-µm-thick PDMS (Dow-corning, GZJ-184) to the entire chip for encapsulation. After completing the thermal curing of the PDMS film (curing agent ratio: 23%), the sensor array can now be exposed to the normal environment.
Experimental setup for PAT using the sensor array. The experimental setup of employing the micro-ring sensor array for PAT is shown in Fig. 3(a). A continuous-wave narrow-linewidth tunable laser (Keysight, 8164B, 10-kHz linewidth, 1,550 nm) was chosen as the optical source for the sensor array. Its frequency was tuned to be close to the resonant frequencies of these micro-rings. The output light of the laser was divided into a signal beam and a local oscillator through a polarization-maintaining fiber coupler (Thorlabs, PNH1550R5A1). The signal beam interacted with the sensor array to probe the information carried by the ultrasonic wave. In this work, we employed a DOFC to realize parallel interrogation to the sensor array, which has the advantages of stability, flexibility, and tunability. It is worth noting that the DOFC has been demonstrated previously for ultrafine spectral measurement (0.01 pm resolution) [40] and low-frequency ultrasound detection (165 kHz) [41]. For a fiber-based resonator, a similar comb structure was also created optically with pulsed light and two fiber Bragg gratings for ultrasonic detection [12]. In this work, the generation of the DOFC started with a digital signal with a multi-carrier bandwidth of 40 GHz and a sequence length of 1,536, which was synthesized through the orthogonal frequency division multiplexing method. The digital signal was then fed into an arbitrary waveform generator (Keysight, M8195A) with a sampling frequency of 60 GHz. In this condition, the corresponding spacing of the subcarrier, i.e., the comb tooth spacing, is 39.0625 MHz (60 GHz/1536). The signal was then amplified and periodically sent to an intensity modulator (Xblue, MXER-LM-20). By setting the intensity modulator with a bias control at the node point, a carrier suppressed double-sideband modulated DOFC signal was generated with a bandwidth of about 40 GHz (320 pm @1550 nm). Detailed mathematical descriptions of the generation of the DOFC can be found in Supplementary Note 1. The DOFC then passed through the sensor array to probe PA signals. Having acquired the information from the sensor array, the DOFC was then post-amplified by an erbium-doped fiber amplifier (Amonics, AEDFA-PA-35-B-FA) and filtered by an optical tunable filter (Santec, OTF-350). Finally, the DOFC was combined with the local oscillator, which was subsequently measured by a coherent receiver (Finisar, CPRV122xA) and digitized by an oscilloscope (Lecroy, 10-36Zi-A). A digital signal processing unit performed Fourier transformation to the measured signal and reconstructed the transmission spectrum of the sensor array (also detailed in Supplementary Note 1). It is worth emphasizing that the employment of the DOFC allows the determination of the transmission spectrum in a one-time measurement, without the need for time-consuming frequency sweeping. In the measured transmission spectrum, the amount of resonant frequency shift reflects the amplitude of the received PA signal. Taking 15 micro-rings as an example, Fig. 3(b) plots the measured transmission spectrum as a function of time in the null case. Notably, the resonant frequency of each micro-ring remains as a flat line, indicating a stable detecting environment due to the encapsulation of the sensor array. When the laser-induced ultrasound signal is present to modulate these micro-rings, Fig. 3(c) plots the measured transmission spectrum as a function of time. In this condition, the amount of resonant frequency shift for each micro-ring sensor faithfully represents the amplitude of the PA signal measured by each element. The time delay reflects the relative distance between the ultrasonic source and the micro-ring sensor. As a result, the reconstructed PA signal as a function of time for each micro-ring sensor is illustrated in Fig. 3(d).
Characterization of the micro-ring sensor. We first characterize the performance of the micro-ring sensor in an aqueous environment. As shown in Fig. 4(a), a point ultrasound source was generated by focusing pulsed light (Elforlight, SPOT-10-200-532, 2.6-ns pulse width) onto a 200-nm-thick golden thin film [27, 30]. The micro-ring sensor array was positioned 5 mm aside to measure the PA signal. A motorized linear translational stage was used to control the position of the sensor array. Both the thin film and the micro-ring sensor were immersed inside the water that serves as the coupling medium for the ultrasonic wave. For a single micro-ring sensor, the received PA signal as a function of time is plotted in Fig. 4(b), which can be treated as the impulse response of delta excitation. To remove unwanted signals due to multiple reflections between substrates, we enforced a time window to keep only the first arriving signals (highlighted in the red dashed box in Fig. 4(b)). Thus, the frequency response of the micro-ring sensor can be estimated by taking the Fourier transformation to the time-gated signal, which is plotted in Fig. 4(c). The central frequency locates around 60 MHz, and the − 6 dB bandwidth is estimated to be 175 MHz (the − 3 dB bandwidth is 120 MHz). We also quantified the acceptance angle of the micro-ring sensor by continuously scanning the position of the sensor, while keeping the position of the point ultrasound source. Figure 4(d) shows the amplitude map of the measured PA signal as a function of time and translational distance. By transforming time into frequency and position into acceptance angle, the frequency response of the micro-ring sensor as a function of the acceptance angle is shown in Fig. 4(e). Two white − 3 dB lines are also provided for visualization purposes. As we can see from the figure, a wide detection angle of about ± 30 degrees can be realized even up to 70-MHz bandwidth. Then, we followed the procedures described in Ref. [27] to characterize the sensitivity of the micro-ring sensor, with the assistance of a calibrated hydrophone (Precision Acoustics, NH0200, 20-MHz bandwidth). The noise amplitude spectral density was calculated when no ultrasound was present, which is provided in Fig. 4(f). By dividing the noise amplitude spectral density with respect to the sensitivity, Fig. 4(g) shows that the NEP spectral density is below 2.2 mPaHz− 1/2, leading to the measured NEP of 7.1 Pa within 20-MHz bandwidth. These values are comparable to those of the state-of-the-art optical ultrasound sensors [19, 27]. The detailed calculation process for obtaining these values can be found in Supplementary Note 2. In PAT, the imaging resolution is determined by the detecting bandwidth of the micro-ring sensor. This parameter was quantified by replacing the golden thin film in Fig. 4(a) with two horizontally placed carbon fibers (7–8 µm in diameter) buried inside the agar. The cross-section of the fiber was imaged to provide information on the point spread function of the imaging system. The micro-ring sensor was linearly translated with a step size of 20 µm with a 2-mm range. Universal back-projection (UBP) [42] was employed to synthesize detected PA signals at each position and produce the image in Fig. 4(h), which contains two carbon fibers. One-dimensional profiles for quantifying both the lateral and axial resolutions are illustrated in Figs. 4(i) and (j), exhibiting full width at half maximum of about 80 and 24 µm, respectively. Note that the discrepancy along the two directions is because lateral resolution mainly depends on the center frequency while the axial resolution is primarily determined by the bandwidth [34].
Frequency tuning of the micro-ring sensor array. The micro-ring sensor array used in this work contains 15 micro-rings, serving as a linear array for PAT. Since slight deviations in terms of diameter of these micro-rings (10 nm) are inevitable due to the fabrication precision of electron beam lithography, the resonant frequencies of these micro-rings were randomly displayed at the beginning. To facilitate the demodulation process of the DOFC and enable parallel interrogation, ordering and equally spacing these resonant frequencies are critically important. This requirement can be fulfilled by exploiting the strong photosensitive effect of chalcogenide glasses [43, 44]. Experimentally, we used an optical fiber to illuminate each micro-ring sensor with 532-nm pulsed light. By controlling the illuminating intensity and time duration, the resonant frequency of the micro-ring sensor is reconfigurable with the desired amount. Illuminating light can be removed later after reconfiguring these resonant frequencies. Detailed operational procedures for rearranging the resonant frequencies of these micro-rings can be found in Supplementary Note 3. After the tuning process, it is shown in Fig. 5(a) that the resonant frequencies of these micro-ring sensors are now ordered with respect to their labeling and equally spaced. The average separation between adjacent resonance frequencies is about 1.66 GHz (0.02 nm @ 1550 nm), and these resonant frequencies occupy an overall spectrum range of 23.7 GHz (0.32 nm @ 1550 nm). As shown in Fig. 5(b), after frequency tuning, the quality factors of these micro-ring sensors were also quantified with good consistency, which fell within the range of 5 to 7 × 105. As a result, a typical transmission spectrum of the micro-ring sensor array is shown in Fig. 5(c), exhibiting 15 distinct and equally separated resonant dips within a frequency range of -11 ~ 12.7 GHz (with respect to the center of the DOFC).