Ultra-slow Bessel balls improve Raman microscopy

Stimulated Raman scattering (SRS) microscopy is an optical vibrational spectroscopic imaging technique and has emerged as an attractive label-free imaging tool for imaging and characterizing tissues and cells with high biochemical specificity. However, the tightly focused Gaussian excitation beams used in conventional SRS microscopy experience a strong light scattering effect that deteriorates the beam profile when propagating in turbid media due to inhomogeneous refractive indices in tissues, leading to degraded spatial resolution and limited light penetration incapable of volumetric imaging of deep tissues. The use of non-diffracting Bessel beams has emerged as a promising alternative to improve the penetration depth for 3D deep imaging, but raster scanning with Bessel beams can only provide 2D SRS projection images of the sample and thus, depth information is lost. Additionally, existing Bessel beam-based SRS tomography relies on optical beating or optical projection techniques to encode depth information spatially with the acquisition of multiple raw images, which may suffer from sample motion artifacts.

In a new paper published in Light Science & Application, a team of scientists, led by Professor Zhiwei Huang from the Optical Bioimaging Laboratory in the Department of Biomedical Engineering, College of Design and Engineering, National University of Singapore, has developed a novel time-of-flight resolved Bessel light bullet (B²-SRS) for 3D SRS chemical imaging of deeper tissues with high spatial resolution. They designed unique angular dispersion control schemes that simultaneously convert the pump and Stokes beam pulses into ultra-slow Bessel light balls (group velocity (v_g) ~0.1c) which are independently tunable in vg using a single spatial light modulator. More remarkably, they cause the ultra-slow pump and Stokes Bessel light balls to propagate in opposite directions along the axial direction (i.e. pump: v_{g, p}g, S > 0) in the sample; thus, the depth-resolved SRS signal can be immediately detected by controlling the depth at which the two Bessel light balls meet inside the sample by manipulating their relative time of flight without the need for mechanical z-scanning. The described technique will have broad applications for label-free 3D chemical imaging of deep tissues in biological and biomedical systems and beyond. These scientists summarize the working principle of their B²-SRS microscopy:

“We proposed a novel angular dispersion control scheme using axicons and a circularly symmetric modulated SLM along the optical axis, thereby enabling the generation of aberration-free multicolor collinear Bessel light balls for high-resolution bioimaging.”

“The unique B²-SRS technique measures the interaction between the two light pulses with different vg via nonlinear optical processes, the depth information by which the nonlinear interaction takes place can be controlled by their relative velocity or relative time of flight for detection simply using an ordinary photodetector. If the group velocity is v_g“With a resolution of about 0.1 c and a pulse width of about 100 fs, the resolution of time-of-flight-based techniques is expected to be improved by at least three orders of magnitude, down to a micrometer in length, compared to the millimeter-scale resolution of traditional light detection and ranging (LiDAR),” they added.

“The optical sectioning method with generation of ultra-slow counter-propagating Bessel light balls as well as time-of-flight resolved detection invented in B²-SRS is universal and can be easily applied to many other nonlinear optical imaging modalities to significantly advance 3D microscopy imaging in biological and biomedical systems and beyond. Our technique also provides new insights into the characterization of diverse dynamic spatiotemporal wave packets in 4D (e.g., including both space and time) with unprecedented spatiotemporal resolution and spectral information,” the scientists predict.