Dispersion characteristics of acousto-optic deflector for scanning Gaussian laser beam of femtosecond pulses

High-resolution femtosecond laser beam shaping via digital holography

We propose a method to achieve high-resolution femtosecond laser beam shaping (HR-FLBS) via digital holography based on a digital mirror device (DMD) and optimized optical design. By programming the hologram on the DMD, we captured several patterns such as light spots, decorated letters of "HUST" (Huazhong University of Science and Technology), and a series of concentric annuli whose fine structure approaches the optical diffraction limit. The experimental results confirm that the proposed method can shape the amplitude and phase of a femtosecond laser arbitrarily with high resolution.

Multi-MHz laser-scanning single-cell fluorescence microscopy by spatiotemporally encoded virtual source array

Apart from the spatial resolution enhancement, scaling of temporal resolution, equivalently the imaging throughput, of fluorescence microscopy is of equal importance in advancing cell biology and clinical diagnostics. Yet, this attribute has mostly been overlooked because of the inherent speed limitation of existing imaging strategies. To address the challenge, we employ an all-optical laser-scanning mechanism, enabled by an array of reconfigurable spatiotemporally-encoded virtual sources, to demonstrate ultrafast fluorescence microscopy at line-scan rate as high as 8 MHz. We show that this technique enables high-throughput single-cell microfluidic fluorescence imaging at 75,000 cells/second and high-speed cellular 2D dynamical imaging at 3,000 frames per second, outperforming the state-of-the-art high-speed cameras and the gold-standard laser scanning strategies. Together with its wide compatibility to the existing imaging modalities, this technology could empower new forms of high-throughput and high-speed biological fluorescence microscopy that was once challenged.

Compensation of spatial dispersion of an acousto-optic deflector with a special Keplerian telescope

Compensation of spatial dispersion caused by the acousto-optic deflector (AOD) when using a femtosecond laser is difficult across the whole scanning range of the system, and this is a significant impediment to its use. In conventional methods, the dispersion of the AOD was compensated only when it was at a particular position, while at other positions, the quality of the light beam was reduced. We developed a novel method for compensating the spatial dispersion within the entire scanning range using a special Keplerian telescope. Our experimental results show that the residual dispersion of the AOD is compensated sufficiently, and the focal spots of the laser reach the diffraction limit within a 40-MHz ultrasound bandwidth.

Visualization of brain circuits using two-photon fluorescence micro-optical sectioning tomography

Neural circuits are fundamental for brain functions. However, obtaining long range continuous projections of neurons in the entire brain is still challenging. Here a two-photon fluorescence micro-optical sectioning tomography (2p-fMOST) method is developed for high-throughput, high-resolution visualization of the brain circuits. Two-photon imaging technology is used to obtain high resolution, and acoustical optical deflector (AOD), an inertia-free beam scanner is used to realize fast and prolonged stable imaging. The combination of these techniques with imaging and then sectioning method of a plastic-embedded mouse brain facilitated the acquisition of a three-dimensional data set of a fluorescent mouse brain with a resolution adequate to resolve the spines. In addition, the brain circuit tracing ability is showed by several neurons projecting across different brain regions. Besides brain imaging, 2p-fMOST could be used in many studies that requires sub-micro resolution or micro resolution imaging of a large sample.

Two-photon microscope for multisite microphotolysis of caged neurotransmitters in acute brain slices

We developed a two-photon microscope optimized for physiologically manipulating single neurons through their postsynaptic receptors. The optical layout fulfills the stringent design criteria required for high-speed, high-resolution imaging in scattering brain tissue with minimal photodamage. We detail the practical compensation of spectral and temporal dispersion inherent in fast laser beam scanning with acousto-optic deflectors, as well as a set of biological protocols for visualizing nearly diffraction-limited structures and delivering physiological synaptic stimuli. The microscope clearly resolves dendritic spines and evokes electrophysiological transients in single neurons that are similar to endogenous responses. This system enables the study of multisynaptic integration and will assist our understanding of single neuron function and dendritic computation.

Enhanced dispersion compensation capability of angular elements based on beam expansion

We demonstrate that beam size manipulation plays an important role in dispersion compensation. With expanded beam, the maximal negative group delay dispersion (GDD) provided by angular elements increases by an order of magnitude compared with original beam. Both calculation and experimental results show that a modest 2 x and 4 x expanded beams can improve dispersion compensation capability of prisms or acousto-optical deflectors: the restored minimal pulse width decreases by 50% and the corresponding distance between angular elements is shortened more than 70 cm. These findings will be helpful for designing dispersion compensation schemes for femtosecond pulse laser application systems such as multiphoton microscopy or laser micromachining.