Focusing of light energy inside a scattering medium by enhancing the time-gated multiple light scattering (original) (raw)
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Nature Photonics
The efficient delivery of light energy is a prerequisite for non-invasive imaging and stimulating of target objects embedded deep within a scattering medium. However, injected waves experience random diffusion by multiple light scattering, and only a small fraction reaches the target object. Here we present a method to counteract wave diffusion and to focus multiplescattered waves to the deeply embedded target. To realize this, we experimentally inject light to the reflection eigenchannels of a specific flight time where most of the multiple-scattered waves have interacted with the target object and maximize the intensity of the returning multiple-scattered waves at the selected time. For targets that are too deep to be visible by optical imaging, we demonstrated a more than 10-fold enhancement in light energy delivery in comparison with ordinary wave diffusion cases. This work will lay a foundation for enhancing the working depth of imaging, sensing, and light stimulation.
Light: Science & Applications
Multiple light scattering is considered as the major limitation for deep imaging and focusing in turbid media. In this paper, we present an innovative method to overcome this limitation and enhance the delivery of light energy ultra-deep into turbid media with significant improvement in focusing. Our method is based on a wide-field reflection matrix optical coherence tomography (RM-OCT). The time-reversal decomposition of the RM is calibrated with the Tikhonov regularization parameter in order to get more accurate reversal results deep inside the scattering sample. We propose a concept named model energy matrix, which provides a direct mapping of light energy distribution inside the scattering sample. To the best of our knowledge, it is the first time that a method to measure and quantify the distribution of beam intensity inside a scattering sample is demonstrated. By employing the inversion of RM to find the matched wavefront and shaping with a phase-only spatial light modulator, ...
Focusing through dynamic scattering media
Optics Express
We demonstrate steady-state focusing of coherent light through dynamic scattering media. The phase of an incident beam is controlled both spatially and temporally using a reflective, 1020-segment MEMS spatial light modulator, using a coordinate descent optimization technique. We achieve focal intensity enhancement of between 5 and 400 for dynamic media with speckle decorrelation time constants ranging from 0.4 seconds to 20 seconds. We show that this optimization approach combined with a fast spatial light modulator enables focusing through dynamic media. The capacity to enhance focal intensity despite transmission through dynamic scattering media could enable advancement in biological microscopy and imaging through turbid environments.
Shaped two-photon excitation deep inside scattering tissue
2011
Light is the tool of the 21st century. New photosensitive tools offer the possibility to monitor and control neuronal activity from the sub-cellular to the integrative level. This ongoing revolution has motivated the development of new optical methods for light stimulation. Among them, it has been recently demonstrated that a promising approach is based on the use of wavefront shaping to generate optically confined extended excitation patterns. This was achieved by combining the technique of temporal focusing with different approaches for lateral light shaping including low numerical aperture Gaussian beams, holographic beams and beams created with the generalized phase contrast mthod. What is needed now is a precise characterization of the effect of scattering on hese different methods in order to extend their use for in depth excitation. Here we present a theoretical and experimental study on the effect of scattering on the propagation of wavefront shaped beams. Results from fixed and acute cortical slices show that temporally focused spatial patterns are extremely robust against the effects of scattering and this permits their three-dimensional confinement for depths up to 550 {\mu}m.
Focusing large spectral bandwidths through scattering media
Optics Express, 2019
Wavefront shaping is a powerful method to refocus light through a scattering medium. Its application to large spectral bandwidths or multiple wavelengths refocusing for nonlinear bio-imaging in-depth is however limited by spectral decorrelations. In this work, we demonstrate ways to access a large spectral memory of a refocus in thin scattering media and thick forward-scattering biological tissues. First, we show that the accessible spectral bandwidth through a scattering medium involves an axial spatio-spectral coupling, which can be minimized when working in a confocal geometry. Second, we show that this bandwidth can be further enlarged when working in a broadband excitation regime. These results open important prospects for multispectral nonlinear imaging through scattering media.
Non-invasive imaging through opaque scattering layers
Nature
Non-invasive optical imaging techniques, such as optical coherence tomography, are essential diagnostic tools in many disciplines, from the life sciences to nanotechnology. However, present methods are not able to image through opaque layers that scatter all the incident light. Even a very thin layer of a scattering material can appear opaque and hide any objects behind it. Although great progress has been made recently with methods such as ghost imaging and wavefront shaping, present procedures are still invasive because they require either a detector or a nonlinear material to be placed behind the scattering layer. Here we report an optical method that allows non-invasive imaging of a fluorescent object that is completely hidden behind an opaque scattering layer. We illuminate the object with laser light that has passed through the scattering layer. We scan the angle of incidence of the laser beam and detect the total fluorescence of the object from the front. From the detected si...
Imaging of highly scattering media by spatially modulated pulsed light
Progress in Biomedical Optics and Imaging, 2009
The use of spatially modulated light is finding application in biomedical optics having potential use in imaging and tomography of tissues and small animals. We describe the time-resolved propagation of spatial frequencies in turbid media. We present a setup based on a ps laser source, spatially modulated by a micro-mirror device and a time-gated intensifier. We discuss the relevant information content that can be useful for imaging of tissues, in terms of the spatial Fourier components of the propagating pulse. We demonstrate that high spatial frequencies appear in the early time-gated signal whereas low frequencies persist for longer times and that the combined use of high spatial frequencies and early time gates can be used to improve the resolution in imaging.
Imaging through extreme scattering in extended dynamic media
Optics Letters, 2018
Critical to navigation, situational awareness, and object identification is the ability to image through turbid water and fog. To date, the longest imaging ranges in such environments rely on active illumination and selection of ballistic photons by means of time gating. Here we show that the imaging range can be extended by using time-gated holography in combination with multi-frame processing. Instead of simply summing the intensity of the frames, we use the complex fields retrieved through digital holographic processing and coherently add the frames. We demonstrate imaging through extended bodies of turbid water and fog at one-way attenuation lengths of 13 and 13.6, respectively. Compared to equivalent traditional time-gated systems, gated holography and coherent processing require 20× less laser illumination power for the same imaging range.
Wide-field multiphoton imaging through scattering media without correction
Science Advances
Optical approaches to fluorescent, spectroscopic, and morphological imaging have made exceptional advances in the last decade. Super-resolution imaging and wide-field multiphoton imaging are now underpinning major advances across the biomedical sciences. While the advances have been startling, the key unmet challenge to date in all forms of optical imaging is to penetrate deeper. A number of schemes implement aberration correction or the use of complex photonics to address this need. In contrast, we approach this challenge by implementing a scheme that requires no a priori information about the medium nor its properties. Exploiting temporal focusing and single-pixel detection in our innovative scheme, we obtain wide-field two-photon images through various turbid media including a scattering phantom and tissue reaching a depth of up to seven scattering mean free path lengths. Our results show that it competes favorably with standard point-scanning two-photon imaging, with up to a fiv...