Ultrafast Photography Based on Spectral-temporal Coupling and Compressed-sensing

基於時頻轉換與壓縮感知的超快成像技術研究

Student thesis: Doctoral Thesis

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Detail(s)

Awarding Institution
Supervisors/Advisors
  • Lidai WANG (Supervisor)
  • Feng Feng Chen (External person) (External Supervisor)
  • Feng Chen (External person) (External Supervisor)
Award date8 Jun 2021

Abstract

Many physical and chemical processes happen in the time scale from picoseconds to attoseconds. Therefore, developing ultrafast photographic techniques holds great significance for researches on these ultrafast processes. However, there still lacks techniques directly observing picoseconds-scale ultrafast processes with high frame rates, large frame numbers, and fine spectral resolutions at the same time. In this thesis, a new ultrafast photographic technique called compressed ultrafast spectral-temporal (CUST) photography is proposed and experimentally realized. CUST can record ultrafast processes with several-THz frame rate and acquire multiple frames with a single shot. Also, CUST can record ultrafast processes with sub-nm spectral resolution. The thesis is organized as follows:

In chapter I, the background and recent developments of ultrafast photography are introduced. In the early days, ultrafast imaging techniques relied on precise mechanical systems and then on the developments of opt-electronic semiconductors. Traditional mechanical or digital cameras can capture ultrafast images with GHz frame rate for maximum, observing microseconds-scale processes. Pump-probe photography or spectroscopy can record ultrafast processes with femtosecond or even attosecond resolution but is only applicable for repeatable processes. The streak camera can realize ultrafast detecting with single-shot with abundant temporal information but the camera is an equipment with very high cost. Aiming at realizing ultrafast frame rate and high frame number in single detection with low cost, the idea of compressed spectral-temporal photography (CUST) is proposed.

In chapter II, two main principles of CUST, spectral-temporal coupling, and compressed sensing, are discussed respectively. The spectral-temporal coupling provides the method for ultrafast detecting and the compressed-sensing theory provides the method for ultrafast information reconstruction. Based on the spectral-temporal coupling, ultrafast processes can be detected and switched into spectral information. Then, the x-y-λ information, namely, x-y-t information, can be reconstructed from the compressed x-y information via compressed sensing. An effective reconstruction algorithm, two-step iterative shrinkage-thresholding (TwIST) ensures the successful reconstruction of the compressed information.

In chapter III, the spectral modulator and pulse stretcher are built to prepare the detecting light source with the spectral-temporal coupling property. A spectral modulator modulates and selects the wavelength components from a femtosecond laser, and a pulse stretcher is made by a pair of gratings, stretching the femtosecond pulse by inducing the group delay dispersion. The results show that through adjusting the grating pair, the high order dispersion can be effectively controlled to a low level, maintaining the linearity of the spectral-temporal coupling.

In chapter IV, the compressed camera is built to record and reconstruct the spectral, namely, the temporal images. The spectral images recording the ultrafast information is encoded, spatially stretched, compressed, and finally reconstructed. Via optimizing the system parameters such as the groove density of the spatially stretching grating (SSG), the spectral images with shape and intensity changes are successfully reconstructed using the TwIST algorithm. The smallest spectral interval of the compressed camera is proved to be in a sub-nanometer scale.

In chapter V, the detecting light source prepared by the spectral modulator and the pulse stretcher detects the pulse propagation in CS2. The CUST system records this ultrafast process and reconstructs it. The results show that the highest frame rate reaches 3.85 THz and the largest frame number is 60. The movements of light flight straightly and with reflection are recorded. In addition, the time-resolved spectral imaging is also demonstrated in experiments. Time-resolved images are reconstructed with a spectral interval of ~0.25 nm, 45 spectral frames, and a temporal resolution of 4 ps.

In chapter VI, a prospection about the CUST system is made. As an all-optical ultrafast imaging system with THz frame rate, tens of frames in a single shot, and a sub-nm spectral resolution, the CUST system can be easily built. Besides, the CUST system also holds great potential for promotion in both temporal and spatial resolution. The CUST system may be applied to observe a broad range of physical or chemical phenomena with high temporal or spectral complexity, such as laser-induced plasma, ultrafast phase changing, and fluorescence or Raman spectrum.

    Research areas

  • Ultrafast imaging, Femtosecond laser, Spectral-temporal coupling, Compressed-sensing