Rapid acousto-optic focus tuning for improvement of imaging performance in confocal microscopy [Invited]

Enhanced light focusing inside scattering media with shaped ultrasound

Light focusing is the primary enabler of various scientific and industrial processes including laser materials processing and microscopy. However, the scattering of light limits the depth at which current methods can operate inside heterogeneous media such as biological tissue, liquid emulsions, and composite materials. Several approaches have been developed to address this issue, but they typically come at the cost of losing spatial or temporal resolution, or increased invasiveness. Here, we show that ultrasound waves featuring a Bessel-like profile can locally modulate the optical properties of a turbid medium to facilitate light guiding. Supported by wave optics and Monte Carlo simulations, we demonstrate how ultrasound enhances light focusing a factor of 7 compared to conventional methods based on placing optical elements outside the complex medium. Combined with point-by-point scanning, images of samples immersed in turbid media with an optical density up to 15, similar to that of weakly scattering biological tissue, can be reconstructed. The quasi-instantaneous generation of the shaped-ultrasound waves, together with the possibility to use transmission and reflection architectures, can pave the way for the real-time control of light inside living tissue.

Optimization of noncollinear AOTF design for laser beam shaping

Optimization of a wide-angle paratellurite acousto-optic tunable filter (AOTF) is performed for applications in laser beam shaping systems. The AOTF configuration with annular transfer function is analyzed. It is demonstrated that the optimal AOTF design for single-frequency operation as a narrow-band spatial frequency filter is obtained at acoustic propagation angle of 5.6° relative to the [110] axis. The optimal design for maximization of AOTF resolution in multifrequency laser beam shaping operation mode is obtained at acoustic propagation angle of 13.8°.

Diffraction-limited axial scanning in thick biological tissue with an aberration-correcting adaptive lens

Diffraction-limited deep focusing into biological tissue is challenging due to aberrations that lead to a broadening of the focal spot. The diffraction limit can be restored by employing aberration correction for example with a deformable mirror. However, this results in a bulky setup due to the required beam folding. We propose a bi-actuator adaptive lens that simultaneously enables axial scanning and the correction of specimen-induced spherical aberrations with a compact setup. Using the bi-actuator lens in a confocal microscope, we show diffraction-limited axial scanning up to 340 μm deep inside a phantom specimen. The application of this technique to in vivo measurements of zebrafish embryos with reporter-gene-driven fluorescence in a thyroid gland reveals substructures of the thyroid follicles, indicating that the bi-actuator adaptive lens is a meaningful supplement to the existing adaptive optics toolset.