Abhijit Sanjeev

Bio: Abhijit Sanjeev is an academic researcher from Bar-Ilan University. The author has contributed to research in topics: Wavefront & Scattering. The author has an hindex of 1, co-authored 4 publications receiving 6 citations.

Non-Invasive Imaging Through Scattering Medium by Using a Reverse Response Wavefront Shaping Technique.

Abstract: Fundamental challenge of imaging through a scattering media has been resolved by various approaches in the past two decades. Optical wavefront shaping technique is one such method in which one shapes the wavefront of light entering a scattering media using a wavefront shaper such that it cancels the scattering effect. It has been the most effective technique in focusing light inside a scattering media. Unfortunately, most of these techniques require direct access to the scattering medium or need to know the scattering properties of the medium beforehand. Through the novel scheme presented on this paper, both the illumination module and the detection are on the same side of the inspected object and the imaging process is a real time fast converging operation. We model the scattering medium being a biological tissue as a matrix having mathematical properties matched to the physical and biological aspects of the sample. In our adaptive optics scheme, we aim to estimate the scattering function and thus to encode the intensity of the illuminating laser light source using DMD (Digital Micromirror Device) with an inverse scattering function of the scattering medium, such that after passing its scattering function a focused beam is obtained. We optimize the pattern to be displayed on the DMD using Particle Swarm Algorithm (PSO) which eventually help in retrieving a 1D object hidden behind the media.

Optical reciprocity induced wavefront shaping for axial and lateral shifting of focus through a scattering medium

Abstract: Light propagating along a reversed path experiences the same transmission coefficient as in the forward direction, independent of the path complexity. This is called the optical reciprocity of light, which is valid for not too intense scattering media as well. Hence, by utilizing the reciprocity principle, the proposed novel technique can achieve axially and laterally tunable focus, non-invasively, through a scattering media without a priori knowledge or modeling of its scattering properties. Moreover, the uniqueness of the proposed technique lies in the fact that the illumination and detection are on the same side of the scattering media.

Generation and Manipulation of Superoscillatory Hotspots Using Virtual Fourier Filtering and CTF Shaping.

Abstract: Superoscillation is a technique that is used to produce a spot of light (known as ‘hotspot’) which is smaller than the conventional diffraction limit of a lens and even smaller than the optical wavelength. Over the past few years, several techniques have been realized for the generation of the superoscillatory hotspot. In this article, for the first time to the best of our knowledge, we propose a novel and a more efficient technique for producing superoscillation in microscopic imaging by shaping the Coherent Transfer Function (CTF) of a lens via virtual Fourier filtering followed by a phase retrieval algorithm. We design and realize a phase mask which when placed at the pupil plane of a diffraction-limited lens produces a superoscillatory hotspot with sidelobes properly matched to the field of view (FOV) required in microscopic imaging applications, i.e. hotspot always coexists with huge intense rings known as ‘sidebands’ close to it and hence limiting the FOV. Our technique is also capable of extending the FOV with minimal loss in resolution of the hotspot generated and considerable ratio between the intensity of the hotspot to that of the side lobes while optimizing the obtainable FOV to the requirement of microscopy.

Publisher Correction: Optical reciprocity induced wavefront shaping for axial and lateral shifting of focus through a scattering medium

Abstract: Light propagating along a reversed path experiences the same transmission coefficient as in the forward direction, independent of the path complexity. This is called the optical reciprocity of light, which is valid for not too intense scattering media as well. Hence, by utilizing the reciprocity principle, the proposed novel technique can achieve axially and laterally tunable focus, non-invasively, through a scattering media without a priori knowledge or modeling of its scattering properties. Moreover, the uniqueness of the proposed technique lies in the fact that the illumination and detection are on the same side of the scattering media.

Non-contact optical wavefront shaping for focusing light and high-resolution imaging inside and behind biological scattering medium

Abstract: Optical wavefront shaping is one of the most effective techniques in focusing light inside a scattering medium. Unfortunately, most of these techniques require direct access to the scattering medium or need to know the scattering properties of the medium beforehand. Through our scheme we develop a novel concept in which both the illumination and the detection is on the same side of the inspected object and the imaging process is a real time fast converging operation that does not require to capture large plurality of images. We model the scattering medium being a biological tissue as a Matrix having mathematical properties matched to the physical and biological aspects of the sample. In our adaptive optics scheme, we aim to estimate the scattering function and thus to encode the intensity of the illuminating laser light source using DMD (Digital Micromirror Device) with an inverse scattering function of the scattering medium, such that after passing its scattering function a focused beam is obtained. We optimize the pattern to be displayed on the DMD using Particle Swarm Algorithm (PSO). As first proof of concept we show validation via numerical MATLAB simulations where we obtain a focused spot behind a scattering medium in amplitude modulation scheme.

Optical Eigenmodes; Exploiting the quadratic nature of the energy flux and of scattering interactions

Abstract: We report a mathematically rigorous technique which facilitates the optimization of various optical properties of electromagnetic fields in free space and including scattering interactions. The technique exploits the linearity of electromagnetic fields along with the quadratic nature of the intensity to define specific Optical Eigenmodes (OEi) that are pertinent to the interaction considered. Key applications include the optimization of the size of a focused spot, the transmission through sub-wavelength apertures, and of the optical force acting on microparticles. We verify experimentally the OEi approach by minimising the size of a focused optical field using a superposition of Bessel beams.

Turbidity suppression from the ballistic to the diffusive regime in biological tissues using optical phase conjugation

Abstract: We describe the amplitude and resolution trends of the signals acquired by turbidity suppression through optical phase conjugation (TSOPC) with samples that span the ballistic and diffusive scattering regimes. In these experiments, the light field scattered through a turbid material is written into a hologram, and a time-reversed copy of the light field is played back through the sample. In this manner, the wavefront originally incident on the sample is reconstructed. We examine a range of scattering samples including chicken breast tissue sections of increasing thickness and polyacrylamide tissue-mimicking phantoms with increasing scattering coefficients. Our results indicate that only a small portion of the scattered wavefront (<0.02%) must be collected to reconstruct a TSOPC signal. Provided the sample is highly scattering, all essential angular information is contained within such small portions of the scattered wavefront due to randomization by scattering. A model is fitted to our results, describing the dependence of the TSOPC signal on other measurable values within the system and shedding light on the efficiency of the phase conjugation process. Our results describe the highest level of scattering that has been phase conjugated in biological tissues to date.

Focusing light through scattering media by reinforced hybrid algorithms

Abstract: Light scattering inside disordered media poses a significant challenge to achieve deep depth and high resolution simultaneously in biomedical optical imaging. Wavefront shaping emerged recently as one of the most potential methods to tackle this problem. So far, numerous algorithms have been reported, while each has its own pros and cons. In this article, we exploit a new thought that one algorithm can be reinforced by another complementary algorithm since they effectively compensate each other’s weaknesses, resulting in a more efficient hybrid algorithm. Herein, we introduce a systematical approach named GeneNN (Genetic Neural Network) as a proof of concept. Preliminary light focusing has been achieved by a deep neural network, whose results are fed to a genetic algorithm as an initial condition. The genetic algorithm furthers the optimization, evolving to converge into the global optimum. Experimental results demonstrate that with the proposed GeneNN, optimization speed is almost doubled and wavefront shaping performance can be improved up to 40% over conventional methods. The reinforced hybrid algorithm shows great potential in facilitating various biomedical and optical imaging techniques.

Towards smart optical focusing: deep learning-empowered dynamic wavefront shaping through nonstationary scattering media

Abstract: Optical focusing through scattering media is of great significance yet challenging in lots of scenarios, including biomedical imaging, optical communication, cybersecurity, three-dimensional displays, etc. Wavefront shaping is a promising approach to solve this problem, but most implementations thus far have only dealt with static media, which, however, deviates from realistic applications. Herein, we put forward a deep learning-empowered adaptive framework, which is specifically implemented by a proposed Timely-Focusing-Optical-Transformation-Net (TFOTNet), and it effectively tackles the grand challenge of real-time light focusing and refocusing through time-variant media without complicated computation. The introduction of recursive fine-tuning allows timely focusing recovery, and the adaptive adjustment of hyperparameters of TFOTNet on the basis of medium changing speed efficiently handles the spatiotemporal non-stationarity of the medium. Simulation and experimental results demonstrate that the adaptive recursive algorithm with the proposed network significantly improves light focusing and tracking performance over traditional methods, permitting rapid recovery of an optical focus from degradation. It is believed that the proposed deep learning-empowered framework delivers a promising platform towards smart optical focusing implementations requiring dynamic wavefront control.