Digital optical phase conjugation of fluorescence in turbid tissue

We demonstrate a method for phase conjugating fluorescence. Our method, called reference free digital optical phase conjugation, can conjugate extremely weak, incoherent optical signals. It was used to phase conjugate fluorescent light originating from a bead covered with 0.5 mm of light-scattering tissue. The phase conjugated beam refocuses onto the bead and causes a local increase of over two orders of magnitude in the light intensity. Potential applications are in imaging, optical trapping, and targeted photochemical activation inside turbid tissue

A convergent Born series for solving the inhomogeneous Helmholtz equation in arbitrarily large media

We present a fast method for numerically solving the inhomogeneous Helmholtz equation. Our iterative method is based on the Born series, which we modified to achieve convergence for scattering media of arbitrary size and scattering strength. Compared to pseudospectral time-domain simulations, our modified Born approach is two orders of magnitude faster and nine orders of magnitude more accurate in benchmark tests in 1-dimensional and 2-dimensional systems

A universal matrix-free split preconditioner for the fixed-point iterative solution of non-symmetric linear systems

We present an efficient preconditioner for linear problems $A x=y$. It guarantees monotonic convergence of the memory-efficient fixed-point iteration for all accretive systems of the form $A = L + V$, where $L$ is an approximation of $A$, and the system is scaled so that the discrepancy is bounded with $\lVert V \rVert<1$. In contrast to common splitting preconditioners, our approach is not restricted to any particular splitting. Therefore, the approximate problem can be chosen so that an analytic solution is available to efficiently evaluate the preconditioner. We prove that the only preconditioner with this property has the form $(L+I)(I - V)^{-1}$. This unique form moreover permits the elimination of the forward problem from the preconditioned system, often halving the time required per iteration. We demonstrate and evaluate our approach for wave problems, diffusion problems, and pantograph delay differential equations. With the latter we show how the method extends to general, not necessarily accretive, linear systems.Comment: Rewritten version, includes efficiency comparison with shift preconditioner by Bai et al, which is shown to be a special cas

Blind focusing through strongly scattering media using wavefront shaping with nonlinear feedback

Scattering prevents light from being focused in turbid media. The effect of scattering can be negated through wavefront shaping techniques when a localized form of feedback is available. Even in the absence of such a localized reporter, wavefront shaping can blindly form a single diffraction-limited focus when the feedback response is nonlinear. We developed and experimentally validated a model that accurately describes the statistics of this blind focusing process. We show that maximizing the nonlinear feedback signal does not always result in the formation of a focus. Using our model, we can calculate the minimal requirements to blindly focus light through strongly scattering media.Comment: 13 pages, 4 figure

Scanning a focus through scattering media without using the optical memory effect

Wavefront shaping makes it possible to form a focus through opaque scattering materials. In some cases, this focus may be scanned over a small distance using the optical memory effect. However, in many cases of interest, the optical memory effect has a limited range or is even too small to be measured. In such cases, one often resorts to measuring the full transmission matrix (TM) of the sample to completely control the light transmission. However, this process is time-consuming and may not always be possible. We introduce a new method for focusing and scanning the focus at any arbitrary position behind the medium by measuring only a subset of the transmission matrix, called Sparse Field Focusing (SFF). With SFF, the scan range is not limited to the memory effect and there is no need to measure the full transmission matrix. Our experimental results agree well with our theoretical model. We expect this method will find applications in imaging through scattering media, especially when the optical memory effect range is small.Comment: 5 pages, 3 figures, 1 movie(visualization 1

Model-based wavefront shaping microscopy

Wavefront shaping is increasingly being used in modern microscopy to obtain distortion-free, high-resolution images deep inside inhomogeneous media. Wavefront shaping methods typically rely on the presence of a 'guidestar' in order to find the optimal wavefront to mitigate the scattering of light. However, this condition cannot be satisfied in most biomedical applications. Here, we introduce a novel, guidestar-free wavefront shaping method in which the optimal wavefront is computed using a digital model of the sample. The refractive index model of the sample, that serves as the input for the computation, is constructed in-situ by the microscope itself. In a proof of principle imaging experiment, we demonstrate a large improvement in the two-photon fluorescence signal through a diffuse medium, outperforming the state-of-the-art wavefront shaping techniques by a factor of 21

Optimizing field-of-view of deep-tissue scanning microscopy

For centuries, the optical microscope has been a crucial instrument for new biological findings, as microscopes were the first devices allowing to observe the internal processes of the cell. Unfortunately, this observation requires the use of thin samples, as biological tissue scatters the incoming light, resulting in a blurred image. An ever increasing number of deep-tissue imaging technique have pushed the penetration depth of the optical microscope. Methods such as adaptive optics [1] allow focusing inside biological tissue by correcting for scattering introduced by the sample. However, adaptive optics methods can only correct for image distortions caused by scattering over a single small area (i.e., field-of-view) within tissue. Please click Additional Files below to see the full abstract

Physical key-protected one-time pad

We describe an encrypted communication principle that forms a secure link between two parties without electronically saving either of their keys. Instead, random cryptographic bits are kept safe within the unique mesoscopic randomness of two volumetric scattering materials. We demonstrate how a shared set of patterned optical probes can generate 10 gigabits of statistically verified randomness between a pair of unique 2 mm^3 scattering objects. This shared randomness is used to facilitate information-theoretically secure communication following a modified one-time pad protocol. Benefits of volumetric physical storage over electronic memory include the inability to probe, duplicate or selectively reset any bits without fundamentally altering the entire key space. Our ability to securely couple the randomness contained within two unique physical objects can extend to strengthen hardware required by a variety of cryptographic protocols, which is currently a critically weak link in the security pipeline of our increasingly mobile communication culture

Multimode Fibers for Quantum-Secure Communication

Multimode fibers support a multitude of transverse optical modes. These modes are mixed by the fiber. By complex wavefront shaping through the multimode fiber, we can undo this mixing, making it possible to communicate through the fiber even at very low light levels