Single-shot Ultrafast Optical Imaging And Its Applications To Femtosecond Laser Ablation Observation

Single-shot ultrafast optical imaging(SUOI)can capture transient scenes in real time at greater than 108 frames per second(fps)in the optical spectral range,which is essential for recording irreversible and difficult-to-reproduce transient events and for understanding the underlying mechanisms in physics,chemistry and biology.According to the different ways of acquiring images,SUOI can be divided into two categories.One is direct imaging techniques and the other is computational imaging techniques.Direct-imaging-based techniques separate the information of ultrafast scenes at different moments into different areas of image sensor and capture them in a single exposure.This type of technology usually has excellent spatial resolution but the sequence depth,i.e.,the number of frames obtained in a single exposure is poor,resulting in difficult to obtain complete temporal evolution information of ultrafast scenes.Computational-imaging-based techniques achieve single-shot recording of transient scenes through optical encoding and computational decoding.Among them,the technology based on compressed sensing(CS)has attracted much attention in recent years,and these techniques have ultra-high sequence depth,but the high data compression ratio leads to poor spatial resolution,which is not conducive to the spatially-detailed analysis of ultrafast scenes.The work of this thesis focuses on the improvement of SUOI in two key technical indicators i.e.,spatial resolution and sequence depth.First,the image quality of the compressed ultrafast photography(CUP)technique based on compressed sensing is improved from the two directions of hardware system and software algorithm.A multichannel-coupled imaging scheme is proposed in hardware to improve the sampling rate,and a novel image reconstruction algorithm is developed in software based on the generalized alternating projection,so as to jointly improve the image reconstruction quality of CUP,and these improvements are applied to the high-fidelity reconstruction of femtosecond laser-induced shock wave dynamics.Second,a singleshot chirped spectral mapping ultrafast photography technique with high spatial and temporal resolutions and high sequence depth is developed and applied to the real-time detection of femtosecond laser ablation dynamics.The main research results are as follows:1.The multichannel-coupled compressed ultrafast photography(MC-CUP)technology is designed and developed.MC-CUP uses a lens array to replicate the transient scene into multiple identical sub-scenes,and performs independent spatial encoding for each sub-scene.Then,the sub-scenes encoded independently are transmitted to the streak camera simultaneously for data acquisition.MC-CUP retains the single-shot advantage of CUP while increasing the sampling rate.By using MCCUP to measure the spatiotemporal evolution of a spatially modulated picosecond laser pulse and the ultrafast dynamics of backscattering photons from a 3D ladder structure,it is confirmed that MC-CUP can signifcantly improve the spatial and temporal resolutions.2.The total variation(TV)combined with block-matching 3D filtering(BM3D)image reconstruction algorithm,referred to as TV-BM3 D algorithm,is developed for CUP.The TV-BM3 D algorithm indirectly uses multiple priors of gradient domain sparsity and nonlocal similarity for image reconstruction by integrating TV denoising algorithm and BM3 D denoising algorithm into the framework of generalized alternating projection.Numerical simulations and experimental results show that TVBM3 D algorithm can not only improve the quality of reconstructed images,but also enhance the anti-noise ability of CUP.In addition,TV-BM3 D algorithm is combined with multichannel-coupled imaging scheme to further realize the high-fidelity reconstruction of femtosecond laser-induced shock wave dynamics.3.The chirped spectral mapping ultrafast photography(CSMUP)technology with high spatio-temporal resolution and high sequence depth is developed.CSMUP uses a broadband laser pulse with chirped frequency to actively illuminate the dynamic scenes and a snapshot hyperspectral camera to capture the spectral images.Based on the spectral-temporal mapping relationship of the chirped probe pulse,the spatio-temporal information of the transient scenes can be inverted from the spectral data.It is verified that CSMUP has the imaging speed of 2.5 × 1011 fps,spatial resolution better than 1200lp/mm,and sequence depth of 25 frames,and successfully records the femtosecond laser ablation dynamics of silicon in real time.4.The ultrafast dynamics of femtosecond laser ablation of gold were observed using CSMUP,and the structural phase transition and material ejection processes of gold were simulated using molecular dynamics method based on the two-temperature model.The comparison analysis between experimental results and simulation results shows that the ablation of the gold is dominated by the phase explosion mechanism under low-fluence laser irradiation,while the ablation of the gold is dominated by the fragmentation mechanism under high-fluence laser irradiation.