## 2 Materials and Methods
### pNDE Imaging System  
The developed pNDE system consisted of three major components: (i) a picosecond pulsed laser for the generation of ultrasound signals in composite samples, (ii) a PZT ultrasonic receiver and signal amplifiers, and (iii) a data acquisition, processing, and imaging subsystem. A picosecond laser(COMPILER 532/266, Passat, Ltd., Vaughan, ON, Canada) was used to provide ultrafast laser pulses with a pulse duration of less than 7 ps. The laser pulse repetition rate (PRR) was adjusted between 1 and 400 Hz. The generated photoacoustic signals were captured by two PZT ultrasound transducers in the experiments. In the multi-scale damage detection test, a transducer (V2062, Olympus NDT, Waltham, MA, USA) with a center frequency of 125 MHz and a bandwidth greater than 87% \(-\)6 dB was used for high spatial resolution. In the macro-scale damage detection test, a transducer (U8517149, Olympus NDT) with a center frequency of 20 MHz and a bandwidth greater than 50% \(-\)6 dB was used. A low-noise preamplifier (ZFL-1000 LN+, Mini-Circuits, Brooklyn, NY, USA) with a bandwidth of 0.1-1000 MHz at \(-\)3 dB and a typical gain of 20 dB was used to prepare weak electrical signals from the transducer and deliver the noise-tolerant output signals to the second-stage amplifier (ZFL-500+, Mini-Circuits) with a bandwidth of 0.05-500 MHz at \(-\)3 dB, and a gain of 25 dB to further improve the signal-to-noise ratio (SNR). Finally, the pre-processed ultrasonic signals were recorded by the data acquisition card (NI PCI-5153 EX, National Instruments, Austin, TX, USA). The schematic diagram of the developed pNDE system is shown in Figure 1a. In addition, Figure 1b illustrates the developed pNDE hardware system. For micro-scale damage detection, the CFRP composite samples were horizontally placed on the bottom glass window of the 3D-printed water container (Figure 1b). The ultrasound transducers were submerged in the water to keep good coupling of the ultrasound propagation between the sample and the transducer. The PPR of the laser was set at 30 Hz so that an adequate signal-to-noise ratio was obtained. A sampling rate of 500 MHz was used to record the ultrasonic signals during data collection at each scanning location. One set of pNDE damage detection data included \(500\times 500\) positions with a step length of 10 \(\upmu\)m per step and detected micro-scale damage in an area of 5 mm \(\times\) 5 mm. All the ultrasonic data was collected when the scanning stage (LMS203 Fast XY Scanning stage, Thorlabs) traveled in the X-Y plane and was controlled using an in-house LABVIEW software. Back projection-based photoacoustic reconstruction was performed in MATLAB [37; 38; 39; 40]. It is noted that  
Figure 1: Developed photoacoustic non-destructive evaluation (pNDE) system. (**a**) A schematic of the pNDE system, showing the following key components: UT, ultrasonic transducer; SS, scanning stage; OL, objective lens; AMP, amplifiers; DAQ, data acquisition card; Laser, green laser (532 nm). (**b**) Experimental setup of the pNDE system, the transducer and the 3D-printed water container are further magnified to demonstrate the sample set up. (**c**) A schematic of A-scan, B-scan, and C-scan for ultrasonic signal presentation. both the pulsed laser beam and ultrasound transducer were kept stationary, while the composite sample was shifted on a 2D translation stage at an average translation speed of 0.05 mm/s. The position-synchronized output of the translator triggered the pulsed laser and turned the laser on and off during the scanning. Similar experimental procedures were adopted for the pNDE detection of macro-scale impact-induced damage in composites. The PRR of the laser was set at 30 Hz, a sampling rate of 500 MHz was used to record the ultrasonic signals, and the translation speed of the stage was 0.05 mm/s. Laser-induced ultrasonic signals were generated and then recorded using an ultrasonic probe.