In this paper, I describe an overview of special issues for recent progress toward next-generation imaging. The topic of this issue covers imaging cytometry, molecular vibration imaging, computational imaging, electron imaging, ghost imaging, optical phased array, and terahertz wave imaging.

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Imaging flow cytometry (iFCM) combines the spatial resolution of optical microscopes with the fast-sampling capability of flow cytometry. In contrast to recent advances in 2D-iFCMs for both analysis and cell sorting, the throughput and flow-speed in 3D-iFCM were limited. In this review, we overview the field, explain the challenges in speeding up 3D-iFCM, and report the recent developments of two 3D-iFCMs based on an idea of acquiring multiple optical sections of cells in a single frame of a pixel- arrayed camera. The two 3D-iFCM techniques demonstrate a record throughput in cells/sec and a record speed in meter/sec, respectively.

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Vibrational spectroscopy can be applied to label-free microscopy of biological specimens to resolve the disadvantages of fluorescence microscopy. Spontaneous Raman and coherent Raman scattering microscopy have long been used for single-cell imaging, while infrared absorption microscopy has not due to the low spatial resolution and large water absorption background. Recently, mid-infrared photothermal microscopy has demonstrated single-cell infrared absorption imaging with a spatial resolution of the visible light, which is a super-resolution in the context of mid-infrared microscopy. We have invented wide-field mid-infrared photothermal quantitative phase microscopes with various phase imaging techniques.

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Computational imaging is a framework and a research field for innovating imaging systems based on cooperative designs of optics and signal processing. Recent advancements in information science drive this field, and various non-conventional imaging methods have been proposed there. In this article, I introduce our recent research activity related to computational imaging with randomness.

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State-of-the-art electron microscopy instruments allow us to see atoms. Such super-resolution is based on the very short wavelength of accelerated electrons, down to a few picometers. However, the spatial resolution remains far from the diffraction limit of electron waves because of the aberration of the lenses for electron beam imaging. We briefly review the history and recent advances of electron beam imaging and focus on (scanning) transmission electron microscopy. We cover the principle of electron optics, aberration correction, phase-contrast imaging, spectroscopic-analysis techniques as well as light-phase imaging using cathodoluminescence.

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Ghost imaging (GI), which is a statistical imaging method from quantum optics, is positioned as a kind of single pixel imaging, where imaging is performed at a single pixel. Due to its statistical processing, it has excellent properties for imaging in disturbed environments and with low light levels. Unfortunately, it suffers from long measurement times caused by the necessity of multiple illuminations. Because of these problems, many reports have focused on compressive sensing that analytically performs imaging. Recently, the introduction of machine learning is fueling attention on GI. In this paper, we comprehensively explain GI from the origin of quantum optics to its transition and classification with the introduction of machine learning.

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Optical phased arrays (OPAs) integrated on compact semiconductor chips have recently attracted great interest for various imaging applications due to their high-speed non-mechanical beam-forming functionalities. While large-scale OPAs with thousands of optical antennas can be fabricated using the current state-of-art nanofabrication technologies, one inherent problem toward implementing them in actual systems is the difficulty of precisely controlling a massive number of phase shifters. In this article, we introduce our unique approaches to tackle this issue. First, a speckle-based single-pixel imaging scheme is applied to OPA to achieve robust imaging capability without precise tuning of phase controllers. We also show that spatial resolution is enhanced by transmitting the output light from an OPA through a multimode fiber. Finally, ultrahigh-resolution imaging with over 19000 resolvable points is demonstrated by introducing the concept of a non-redundant array to judiciously locate optical antennas on a silicon OPA chip.

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With a single 300-GHz frequency-tunable source and a subharmonic heterodyne receiver, we developed a prototype system for real-time 3D imaging for security gate inspections. We successfully achieved real- time imaging of hazardous materials concealed under the clothing of pedestrians. In this paper, we outline our system and the remained issues to be solved.

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