Single-shot compressed ultrafast photography: a review

Compressed ultrafast photography (CUP) is a burgeoning single-shot computational imaging technique that provides an imaging speed as high as 10 trillion frames per second and a sequence depth of up to a few hundred frames. This technique synergizes compressed sensing and the streak camera technique to capture nonrepeatable ultrafast transient events with a single shot. With recent unprecedented technical developments and extensions of this methodology, it has been widely used in ultrafast optical imaging and metrology, ultrafast electron diffraction and microscopy, and information security protection. We review the basic principles of CUP, its recent advances in data acquisition and image reconstruction, its fusions with other modalities, and its unique applications in multiple research fields

Dispersive Fourier Transformation for Versatile Microwave Photonics Applications

Abstract: Dispersive Fourier transformation (DFT) maps the broadband spectrum of an ultrashort optical pulse into a time stretched waveform with its intensity profile mirroring the spectrum using chromatic dispersion. Owing to its capability of continuous pulse-by-pulse spectroscopic measurement and manipulation, DFT has become an emerging technique for ultrafast signal generation and processing, and high-throughput real-time measurements, where the speed of traditional optical instruments falls short. In this paper, the principle and implementation methods of DFT are first introduced and the recent development in employing DFT technique for widespread microwave photonics applications are presented, with emphasis on real-time spectroscopy, microwave arbitrary waveform generation, and microwave spectrum sensing. Finally, possible future research directions for DFT-based microwave photonics techniques are discussed as well

Single-shot ultrafast optical imaging

Single-shot ultrafast optical imaging can capture two-dimensional transient scenes in the optical spectral range at ≥100 million frames per second. This rapidly evolving field surpasses conventional pump-probe methods by possessing real-time imaging capability, which is indispensable for recording nonrepeatable and difficult-to-reproduce events and for understanding physical, chemical, and biological mechanisms. In this mini-review, we survey state-of-the-art single-shot ultrafast optical imaging comprehensively. Based on the illumination requirement, we categorized the field into active-detection and passive-detection domains. Depending on the specific image acquisition and reconstruction strategies, these two categories are further divided into a total of six subcategories. Under each subcategory, we describe operating principles, present representative cutting-edge techniques, with a particular emphasis on their methodology and applications, and discuss their advantages and challenges. Finally, we envision prospects for technical advancement in this field

Single-shot compressed ultrafast photography: a review

Compressed ultrafast photography (CUP) is a burgeoning single-shot computational imaging technique that provides an imaging speed as high as 10 trillion frames per second and a sequence depth of up to a few hundred frames. This technique synergizes compressed sensing and the streak camera technique to capture nonrepeatable ultrafast transient events with a single shot. With recent unprecedented technical developments and extensions of this methodology, it has been widely used in ultrafast optical imaging and metrology, ultrafast electron diffraction and microscopy, and information security protection. We review the basic principles of CUP, its recent advances in data acquisition and image reconstruction, its fusions with other modalities, and its unique applications in multiple research fields

Exciton Control in a Room-Temperature Bulk Semiconductor with Coherent Strain Pulses

The coherent manipulation of excitons in bulk semiconductors via the lattice degrees of freedom is key to the development of acousto-optic and acousto-excitonic devices. Wide-bandgap transition metal oxides exhibit strongly bound excitons that are interesting for applications in the deep-ultraviolet, but their properties have remained elusive due to the lack of efficient generation and detection schemes in this spectral range. Here, we perform ultrafast broadband deep-ultraviolet spectroscopy on anatase TiO$_2$ single crystals at room temperature, and reveal a dramatic modulation of the exciton peak amplitude due to coherent acoustic phonons. This modulation is comparable to those of nanostructures where exciton-phonon coupling is enhanced by quantum confinement, and is accompanied by a giant exciton shift of 30-50 meV. We model these results by many-body perturbation theory and show that the deformation potential coupling within the nonlinear regime is the main mechanism for the generation and detection of the coherent acoustic phonons. Our findings pave the way to the design of exciton control schemes in the deep-ultraviolet with propagating strain pulses

Ultrafast Laser Pulse Interaction with Dielectric Materials: Numerical and Experimental Investigations on Ablation and Micromachining

Ultrafast lasers have great capability and flexibility in micromachining of various materials. Due to the involved complicated multi-physical processes, mechanisms during laser-material interaction have not been fully understood. To improve and explore ultrafast laser processing and treatment of dielectric materials, numerical and experimental investigations have been devoted to better understanding the underlying fundamental physics during laser-material interaction and material micromachining. A combined continuum-atomistic model has been developed to investigate thermal and non-thermal (photomechanical) responses of materials to ultrafast laser pulse irradiation. Coexistence of phase explosion and spallation can be observed for a considerably wide range of laser fluences. Phase explosion becomes the primary ablation mechanism with the increase of laser fluence, and spallation can be restrained due to the weakened tensile stress by the generation of recoil pressure from ejection of hot material plume. For dielectric materials, due to the much lower temperature gradient by non-linear absorption, the generated thermal-elastic stress is much weaker than that in non-transparent materials, making spallation less important. Plasma dynamics is studied with respect to ejection directions and velocities based on fluorescence and shadowgraph measurements. The most probable direction (angle) is found insensitive to laser fluence/energy. The plasma expansion velocity is closely related to electron thermal velocity, indicating the significance of thermal ablation in dielectric material decomposition by laser irradiation. A numerical study of ultrafast laser-induced ablation of dielectric materials is presented based on a one-dimensional plasma-temperature model. Plasma dynamics including photoionization, impact ionization and relaxation are considered through a single rate equation. Material decomposition is captured by a temperature-based ablation criterion. Dynamic description of ablation process has been achieved through an improved two-temperature model. Laser-induced ablation threshold, transient optical properties and ablation depth have been investigated with respect to incident fluences and pulse durations. Good agreements are shown between numerical predictions and experimental observations. Fast increase of ablation depth, followed by saturation, can be observed with the increase of laser fluence. Reduction of ablation depth at fluences over 20 J/cm2 is resulted from plasma defocusing effect by air ionization. Thermal accumulation effect can be negligible with repetition rate lower than 1 kHz for fused silica and helps to enhance the ablation depth at 10 kHz (100 pulses) to almost double of that with single pulse. The ablation efficiency decreases with fluence after reaching the peak value at the fluence twice of the ablation threshold. The divergence of tightly focused Gaussian beam in transparent materials has been revealed to significantly affect the ablation process, particularly at high laser fluence. A comprehensive study of ultrafast laser direct drilling in fused silica is performed with a wide range of drilling speeds (20-500 μm/s) and pulse energy (60-480 μJ). Taper-free and uniform channels are drilled with the maximum length over 2000 μm, aspect ratio as high as ~40:1 and excellent sidewall quality (roughness ~0.65 μm) at 270 μJ. The impacts of pulse energy and drilling speeds on channel aspect ratio and quality are studied. Optimal drilling speeds are determined at different pulse energy. The dominating mechanisms of channel early-termination are beam shielding by material modification at excessive laser irradiation for low speed drilling and insufficient laser energy deposition for high speed drilling, respectively. An analytical model is developed to validate these mechanisms. The feasibility of direct drilling high-aspect-ratio and high-quality channels by ultrafast laser in transparent materials is demonstrated

Non-invasive, near-field terahertz imaging of hidden objects using a single pixel detector

Terahertz (THz) imaging has the ability to see through otherwise opaque materials. However, due to the long wavelengths of THz radiation ({\lambda}=300{\mu}m at 1THz), far-field THz imaging techniques are heavily outperformed by optical imaging in regards to the obtained resolution. In this work we demonstrate near-field THz imaging with a single-pixel detector. We project a time-varying optical mask onto a silicon wafer which is used to spatially modulate a pulse of THz radiation. The far-field transmission corresponding to each mask is recorded by a single element detector and this data is used to reconstruct the image of an object placed on the far side of the silicon wafer. We demonstrate a proof of principal application where we image a printed circuit board on the underside of a 115{\mu}m thick silicon wafer with ~100{\mu}m ({\lambda}/4) resolution. With subwavelength resolution and the inherent sensitivity to local conductivity provided by the THz probe frequencies, we show that it is possible to detect fissures in the circuitry wiring of a few microns in size. Imaging systems of this type could have other uses where non-invasive measurement or imaging of concealed structures with high resolution is necessary, such as in semiconductor manufacturing or in bio-imaging

Single-shot ultrafast optical imaging

Single-shot ultrafast optical imaging can capture two-dimensional transient scenes in the optical spectral range at ≥100 million frames per second. This rapidly evolving field surpasses conventional pump-probe methods by possessing real-time imaging capability, which is indispensable for recording nonrepeatable and difficult-to-reproduce events and for understanding physical, chemical, and biological mechanisms. In this mini-review, we survey state-of-the-art single-shot ultrafast optical imaging comprehensively. Based on the illumination requirement, we categorized the field into active-detection and passive-detection domains. Depending on the specific image acquisition and reconstruction strategies, these two categories are further divided into a total of six subcategories. Under each subcategory, we describe operating principles, present representative cutting-edge techniques, with a particular emphasis on their methodology and applications, and discuss their advantages and challenges. Finally, we envision prospects for technical advancement in this field