Aberration correction for improving the image quality in STED microscopy using the genetic algorithm

Reference-free in situ rapid regional calibration of phase-only spatial light modulators

Spatial light modulators (SLMs) have become an indispensable element in modern optics for their versatile performance in many applications. Among various types of SLMs, such as digital micromirror devices (DMD), liquid crystal-based phase-only spatial light modulators (LC-SLMs), and deformable mirrors (DM), LC-SLMs are often the method of choice due to their high efficiency, precise phase modulation, and abundant number of effective pixels. In general, for research grade applications, an additional SLM calibration step is required due to fabrication imperfection resulting in non-flat liquid crystal panels and varying phase responses over the SLM area. Here, we demonstrate a straightforward approach for reference-free orthogonal calibration of an arbitrary number of SLM subregions which only requires the same measurement time as global calibration. The proposed method requires minimal optical elements and can be applied to any optical setup as is. As a benchmark performance test, we achieved a 2.2-fold enhancement in correction efficiency for wavefront shaping through scattering media utilizing the calibrated 2160 subregions of the SLM, in comparison with a single global look-up table (LUT).

Multi-color complex spatial light modulation with a single digital micromirror device

Spatial light modulators enabling complex light field manipulation has opened up many opportunities in biomedical imaging, holographic display, and adaptive optics. However, traditional spatial light modulators do not allow multi-color operations simultaneously due to their physical constraints, while multi-color modulations are highly desirable in many applications. To overcome this limitation, we demonstrate a multi-color spatial complex light field modulation with a single binary hologram on digital micromirror devices (DMD). This method combines several neighboring micro-mirror pixels into a giant single superpixel, in which the light field's amplitude and phase can be individually determined by internal pixel combinations, and the dynamic range of phase modulation can exceed 2π for the single wavelength. As a result, this extra phase modulation range offers an additional degree of freedom for independent multi-wavelength light modulation. Based on this scheme, multi-color light modulations have been demonstrated in a 2D plane as well as in multiple 3D holographic planes. Moreover, a dual-colored Airy beam has been realized using the same technique. These results bring complex light modulation into a multi-color regime, paving the way for practical applications in information display, imaging, and optical trapping.

Deep learning-based adaptive optics for light sheet fluorescence microscopy

Light sheet fluorescence microscopy (LSFM) is a high-speed imaging technique that is often used to image intact tissue-cleared specimens with cellular or subcellular resolution. Like other optical imaging systems, LSFM suffers from sample-induced optical aberrations that decrement imaging quality. Optical aberrations become more severe when imaging a few millimeters deep into tissue-cleared specimens, complicating subsequent analyses. Adaptive optics are commonly used to correct sample-induced aberrations using a deformable mirror. However, routinely used sensorless adaptive optics techniques are slow, as they require multiple images of the same region of interest to iteratively estimate the aberrations. In addition to the fading of fluorescent signal, this is a major limitation as thousands of images are required to image a single intact organ even without adaptive optics. Thus, a fast and accurate aberration estimation method is needed. Here, we used deep-learning techniques to estimate sample-induced aberrations from only two images of the same region of interest in cleared tissues. We show that the application of correction using a deformable mirror greatly improves image quality. We also introduce a sampling technique that requires a minimum number of images to train the network. Two conceptually different network architectures are compared; one that shares convolutional features and another that estimates each aberration independently. Overall, we have presented an efficient way to correct aberrations in LSFM and to improve image quality.

Population Control of Upconversion Energy Transfer for Stimulation Emission Depletion Nanoscopy

Upconverting stimulated emission depletion microscopy (U-STED) is emerging as an effective approach for super-resolution imaging due to its significantly low depletion power and its ability to surpass the limitations of the square-root law and achieve higher resolution. Though the compelling performance, a trade-off between the spatial resolution and imaging quality in U-STED has been recognized in restricting the usability due to the low excitation power drove high depletion efficiency. Moreover, it is a burden to search for the right power relying on trial and error as the underpinning mechanism is unknown. Here, a method is proposed that can easily predict the ideal excitation power for high depletion efficiency with the assistance of the non-saturate excitation based on the dynamic cross-relaxation (CR) energy transfer of upconversion nanoparticles. This allows the authors to employ the rate equation model to simulate the populations of each relevant energy state of lanthanides and predict the ideal excitation power for high depletion efficiency. The authors demonstrate that the resolution of STED with the assistance of nonsaturated confocal super-resolution results can easily achieve the highest resolution of sub-40 nm, 1/24^th of the excitation wavelengths. The finding on the CR effect provides opportunities for population control in realizing low-power high-resolution nanoscopy.

Small-molecule fluorogenic probes for mitochondrial nanoscale imaging

Mitochondria are inextricably linked to the development of diseases and cell metabolism disorders. Super-resolution imaging (SRI) is crucial in enhancing our understanding of mitochondrial ultrafine structures and functions. In addition to high-precision instruments, super-resolution microscopy relies heavily on fluorescent materials with unique photophysical properties. Small-molecule fluorogenic probes (SMFPs) have excellent properties that make them ideal for mitochondrial SRI. This paper summarizes recent advances in the field of SMFPs, with a focus on the chemical and spectroscopic properties required for mitochondrial SRI. Finally, we discuss future challenges in this field, including the design principles of SMFPs and nanoscopic techniques.

Aberration Correction to Optimize the Performance of Two-Photon Fluorescence Microscopy Using the Genetic Algorithm

Due to less light scattering and a better signal-to-noise ratio in deep imaging, two-photon fluorescence microscopy (TPFM) has been widely used in biomedical photonics since its advent. However, optical aberrations degrade the performance of TPFM in terms of the signal intensity and the imaging depth and therefore restrict its application. Here, we introduce adaptive optics based on the genetic algorithm to detect the distorted wavefront of the excitation laser beam and then perform aberration correction to optimize the performance of TPFM. By using a spatial light modulator as the wavefront controller, the correction phase is obtained through a signal feedback loop and a process of natural selection. The experimental results show that the signal intensity and imaging depth of TPFM are improved after aberration correction. Finally, the method was applied to two-photon fluorescence lifetime imaging, which helps to improve the signal-to-noise ratio and the accuracy of lifetime analysis. Furthermore, the method can also be implemented in other experiments, such as three-photon microscopy, light-sheet microscopy, and super-resolution microscopy.

Low-Power Two-Color Stimulated Emission Depletion Microscopy for Live Cell Imaging

Stimulated emission depletion (STED) microscopy is a typical laser-scanning super-resolution imaging technology, the emergence of which has opened a new research window for studying the dynamic processes of live biological samples on a nanometer scale. According to the characteristics of STED, a high depletion power is required to obtain a high resolution. However, a high laser power can induce severe phototoxicity and photobleaching, which limits the applications for live cell imaging, especially in two-color STED super-resolution imaging. Therefore, we developed a low-power two-color STED super-resolution microscope with a single supercontinuum white-light laser. Using this system, we achieved low-power two-color super-resolution imaging based on digital enhancement technology. Lateral resolutions of 109 and 78 nm were obtained for mitochondria and microtubules in live cells, respectively, with 0.8 mW depletion power. These results highlight the great potential of the novel digitally enhanced two-color STED microscopy for long-term dynamic imaging of live cells.

Aberration correction in stimulated emission depletion microscopy to increase imaging depth in living brain tissue

Significance: Stimulated emission depletion (STED) microscopy enables nanoscale imaging of live samples, but it requires a specific spatial beam shaping that is highly sensitive to optical aberrations, limiting its depth penetration. Therefore, there is a need for methods to reduce optical aberrations and improve the spatial resolution of STED microscopy inside thick biological tissue. Aim: The aim of our work was to develop and validate a method based on adaptive optics to achieve an a priori correction of spherical aberrations as a function of imaging depth. Approach: We first measured the aberrations in a phantom sample of gold and fluorescent nanoparticles suspended in an agarose gel with a refractive index closely matching living brain tissue. We then used a spatial light modulator to apply corrective phase shifts and validate this calibration approach by imaging neurons in living brain slices. Results: After quantifying the spatial resolution in depth in phantom samples, we demonstrated that the corrections can substantially increase image quality in living brain slices. Specifically, we could measure structures as small as 80 nm at a depth of 90    μ m inside the biological tissue and obtain a 60% signal increase after correction. Conclusion: We propose a simple and robust approach to calibrate and compensate the distortions of the STED beam profile introduced by spherical aberrations with increasing imaging depth and demonstrated that this method offers significant improvements in microscopy performance for nanoscale cellular imaging in live tissue.

Liquid-Crystal-Elastomer-Actuated Reconfigurable Microscale Kirigami Metastructures

Programmable actuation of metastructures with predesigned geometrical configurations has recently drawn significant attention in many applications, such as smart structures, medical devices, soft robotics, prosthetics, and wearable devices. Despite remarkable progress in this field, achieving wireless miniaturized reconfigurable metastructures remains a challenge due to the difficult nature of the fabrication and actuation processes at the micrometer scale. Herein, microscale thermo-responsive reconfigurable metasurfaces using stimuli-responsive liquid crystal elastomers (LCEs) is fabricated as an artificial muscle for reconfiguring the 2D microscale kirigami structures. Such structures are fabricated via two-photon polymerization with sub-micrometer precision. Through rationally designed experiments guided by simulations, the optimal formulation of the LCE artificial muscle is explored and the relationship between shape transformation behaviors and geometrical parameters of the kirigami structures is build. As a proof of concept demonstration, the constructs for temperature-dependent switching and information encryption is applied. Such reconfigurable kirigami metastructures have significant potential for boosting the fundamental small-scale metastructure research and the design and fabrication of wireless functional devices, wearables, and soft robots at the microscale as well.

Deep Penetration Microscopic Imaging with Non-Diffracting Airy Beams

We report a deep penetration microscopic imaging method with a non-diffracting Airy beam. The direct mapping of volume imaging in free space shows that the axial imaging range of the Airy beam is approximately 4 times that of the traditional Gaussian beam along the axial direction while maintaining a narrow lateral width. Benefiting from its non-diffracting property, the microscopic imaging with Airy beam illumination can acquire image structures through turbid medium and capture a volumetric image in a single frame. We demonstrate the penetration ability of the Airy microscopic imaging through a strongly scattering environment with 633 nm and 780 nm lasers. The performances of the volumetric imaging method were evaluated using HeLa cells and isolated mouse kidney tissue. The thick sample was scanned layer by layer in the Gaussian mode, however, in the Airy mode, the three-dimensional (3D) structure information was projected onto a two-dimensional (2D) image, which vastly increased the volume imaging speed. To show the characteristics of the Airy microscope, we performed dynamic volumetric imaging on the isolated mouse kidney tissue with two-photon.

Improving the image quality in STED nanoscopy using frequency spectrum modulation

Since stimulated emission depletion (STED) nanoscopy was invented in 1994, this technique has been widely used in the fields of biomedicine and materials science. According to the imaging principle of STED technology, increasing the power of the depletion laser within a certain threshold can improve the resolution. However, it will cause not only severe photo-damage to the samples and photo-bleaching to the fluorophores but also serious background noise, leading to the degeneration of the quality of STED images. Here we propose a new processing method based on frequency spectrum modulation to improve the quality of STED images, abbreviated as FM-STED. We have demonstrated the performance of FM-STED in improving the signal-to-noise ratio and the resolution using fluorescent beads and biological cells as samples.

Super-resolution Microscopy for Biological Imaging

Studying the ultra-fine structures and functions of the subcellular organelles and exploring the dynamic biological events in depth are the key issues in contemporary biological research. Fluorescence bio-imaging has been used to study cell biology for decades. However, the structures and functions of the subcellular organelles which fall under the diffraction limit are still not explored fully at a nanoscale level. Several super-resolution microscopy (SRM) techniques have been devised over the years which can be utilized to overcome diffraction limit. These techniques have opened a new window in biological research. However, SRM methods are highly vulnerable to the lack of appropriate fluorophores and other sophisticated technical considerations. Therefore, this chapter briefly summarizes the basic principles of various SRM methods which have been frequently utilized in biological imaging. The chapter not only gives an overview of the technical advantages and drawbacks about using different SRM techniques for bio-imaging applications but also briefly articulates the nitty-gritties of selecting a proper fluorescent probe for a specific SRM experiment with biological samples.

Recent advances in luminescent materials for super-resolution imaging via stimulated emission depletion nanoscopy

Recent progress on STED fluorophores for super-resolution imaging and also their characteristics are outlined here, thus providing some guidelines to select proper probes and even develop new materials for super-resolution imaging via STED nanoscopy.

Numerically Enhanced Stimulated Emission Depletion Microscopy with Adaptive Optics for Deep-Tissue Super-Resolved Imaging

In stimulated emission depletion (STED) nanoscopy, the major origin of decreased signal-to-noise ratio within images can be attributed to sample photobleaching and strong optical aberrations. This is due to STED utilizing a high-power depletion laser (increasing the risk of photodamage), while the depletion beam is very sensitive to sample-induced aberrations. Here, we demonstrate a custom-built STED microscope with automated aberration correction that is capable of 3D super-resolution imaging through thick, highly aberrating tissue. We introduce and investigate a state of the art image denoising method by block-matching and collaborative 3D filtering (BM3D) to numerically enhance fine object details otherwise mixed with noise and further enhance the image quality. Numerical denoising provides an increase in the final effective resolution of the STED imaging of 31% using the well established Fourier ring correlation metric. Results achieved through the combination of aberration correction and tailored image processing are experimentally validated through super-resolved 3D imaging of axons in differentiated induced pluripotent stem cells growing under an 80 μm thick layer of tissue with lateral and axial resolution of 204 and 310 nm, respectively.