Focusing and compression of ultrashort pulses through scattering media

Abstract

Abstract:Light scattering in inhomogeneous media induces wavefront distortions which pose an inherent limitation in many optical applications. Examples range from microscopy and nanosurgery to astronomy. In recent years, ongoing efforts have made the correction of spatial distortions possible by wavefront shaping techniques. However, when ultrashort pulses are employed scattering induces temporal distortions which hinder their use in nonlinear processes such as in multiphoton microscopy and quantum control experiments. Here we show that correction of both spatial and temporal distortions can be attained by manipulating only the spatial degrees of freedom of the incident wavefront. Moreover, by optimizing a nonlinear signal the refocused pulse can be shorter than the input pulse. We demonstrate focusing of 100fs pulses through a 1mm thick brain tissue, and 1000-fold enhancement of a localized two-photon fluorescence signal. Our results open up new possibilities for optical manipulation and nonlinear imaging in scattering media. 2The propagation of light in inhomogeneous media results in scattering and distortions of the propagating wavefront. Such distortions limit the effective focusing of optical intensity and degrade imaging quality through disordered or scattering media 1 . The problem of focusing light through inhomogeneous media is even more challenging when ultrashort pulses are considered, as in addition to the spatial distortions scattering also distorts the pulse shape in time [2][3][4][5] . The challenge of correcting the spatial distortions induced by scattering has been in the focus of many recent works [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] . Weak wavefront aberrations, such as those occurring in astronomical observations through the atmosphere, have been efficiently corrected using adaptive optics techniques [6][7][8] . These techniques, however, were considered inadequate for correcting distortions in highly scattering and turbid samples, which lead to diffusive light propagation and result in complex speckle patterns with no simple relation to the incident wavefront 1,20 . Recently, in a pioneering work, Vellekoop et al. have shown that adaptive optimization of the incident wavefront can increase the focused intensity of multiply scattered light by a factor that is roughly equivalent to the number of degrees of control [9][10][11][12] . Using a spatial light modulator (SLM) with 1000 degrees of control enabled a 1000-fold enhancement in the focused brightness after a turbid medium 9 . Following Vellekoop's works, other approaches for determining the optimal corrections were demonstrated either by measurement of the optical transmission matrix 13,14 or the complex-valued relation between spatial modes 15 , or alternatively by directly recording the distorted wavefront using optical phase conjugation 16,17 . These results, however, were only relevant for quasi-continuous light, and in spite of these remarkable achievements in the correction of spatial distortions, no work to date h...

Superpenetration optical microscopy by iterative multiphoton adaptive compensation technique

“…Different from previous work (26)(27)(28)(29)(30)(31), the technique reported here has two different elements, iterative feedback and nonlinearity, and is named iterative multiphoton adaptive compensation technique (IMPACT). Fig.…”

“…Such a scheme was originally developed in the 1970s to focus a laser beam through air turbulence onto a remote target (25). Similar methods have also been explored to focus light through highly turbid media onto a point target or guide star (26)(27)(28)(29). Despite progress in operation speed (26) and efficiency (27), none of the reported methods (26)(27)(28)(29)(30)(31) have been applied to deep tissue microscopy in any practical way.…”

“…Similar methods have also been explored to focus light through highly turbid media onto a point target or guide star (26)(27)(28)(29). Despite progress in operation speed (26) and efficiency (27), none of the reported methods (26)(27)(28)(29)(30)(31) have been applied to deep tissue microscopy in any practical way. The Achilles' heel of these methods is that a point target (size smaller than or comparable to the diffraction limit) or guide star is required to form a diffraction-limited focus inside highly turbid media, a condition rarely satisfied in practical imaging applications.…”

Controlling waves in space and time for imaging and focusing in complex media

“…This is now becoming possible thanks to recent advances on the spatiotemporal control of optical wave fields through random media [65][66][67] . Two of these demonstrations 65,66 relied on the optimization a pulse's peak intensity after it had propagated through a complex medium, either by heterodyne interferometry (Fig. 3c) or by using a two-photon fluorescent material.…”

“…Current experiments [65][66][67] control either the frequency domain or the spatial domain, but not both. An optical TRM will open up an entirely new class of time-reversed optics experiments, yielding new fundamental insights into wave-scattering phenomena [22][23][24]27,121 .…”