Since its discovery in 17th-century, optical microscopy has been acting as an indispensable tool in the exploration of the unknown microworld, especially in biomedical fields.

Among different optical imaging techniques, fluorescence microscopy features a fast, non-destructive approach, and it enables us to visualize specific biochemical structures when being tagged with fluorescent dyes, quantum dots, or fluorescent proteins. Of note, with the emergence of photo-switchable fluorescence markers in the past decades, scientists have invented different kinds of super-resolution techniques, which break the optical diffraction limit and improve the resolution of optical microscopes tremendously from 200 nm to tens of nanometers. Currently, super-resolution optical imaging techniques can be mainly divided into two categories: one is based on single-molecule localization, including PALM and STORM; the other is based on point spread function engineering, including STED and SIM.

In aside from fluorescence microscopy, which needs additional fluorescent labeling, it is also desirable to directly observe live cells or tissues in their natural condition. In this direction, people have proposed different kinds of label-free imaging techniques, including but not limited to, second-harmonic generation (SHG) microscopy, coherent anti-Stokes Raman spectroscopy (CARS). In aside of visualizing functional structures selectively, there are also techniques that image morphological structures in global. To cite a few, quantitative phase microscopy (QPM) provides not only phase-contrast images for transparent samples but also quantitative information of 3D profiles or refractive index distributions of the samples. Within this imaging modality, digital holographic microscopy (DHM), transport of intensity equation (TIE), Fourier ptychographic microscopy (FPM), and lens-free on-chip imaging became new paradigms in optical microscopy and were quick to be embraced by the scientific community. Moreover, optical coherent tomography (OCT) allows sub-surface imaging of translucent objects with an axially discriminating capability and in a completely non-invasive manner.

Beyond optical microscopy, spectroscopy approaches allow investigation of composition, physical and electronic structures of matter at atomic, molecular, and macro scales. Important applications arise from biomedical spectroscopy in the areas of tissue analysis and medical imaging. Among these spectroscopic approaches, Raman spectroscopy yields information about intra- and inter-molecular vibrations, which, in turn, provide additional understanding about a reaction. Correlation spectroscopy enables us to quantify the dynamics of biomolecules and the interaction of different biomolecules by tracking the intensity fluctuation when the fluorescently tagged molecules diffuse through a focused light.

Last but not least, computational imaging techniques, including digital holography, phase retrieval, synthetic aperture, along with advanced image processing algorithms, such as compressive sensing, regularized deconvolution, and deep learning technologies brought about new revolutionary changes in optical microscopy, which closely followed the exponential growth in the power of digital computers with image processing capabilities.

In order to facilitate the readers to understand, discuss and communicate the development of optical imaging, spectroscopic techniques, and their applications, in this Research Topic, we aim to collect the research accomplishments to date in the above-mentioned fields. The Research Topic focuses on, but is not limited to:

- New principles and technologies of optical imaging
- Algorithms to improve optical imaging and spectroscopy
- New fluorescent probes
- Molecular spectroscopy including absorption, Raman, FTIR, FT-NIR
- Fluorescence correlation spectroscopy
- Label-free optical imaging techniques
- Computational optical microscopic techniques
- Quantitative phase imaging techniques
- Image processing algorithms, including machine learning, deep learning technology, etc
- Applications of optical microscopic and spectroscopic technologies

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